Modeling chemical and physical processes of wood and biomass pyrolysis

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1 Progress in Energy and Combustion Science 34 (2008) Modeling chemical and physical processes of wood and biomass pyrolysis Colomba Di Blasi Dipartimento di Ingegneria Chimica, Università degli Studi di Napoli Federico II, P.le V. Tecchio, Napoli, Italy Received 5 February 2005; accepted 7 December 2006 Available online 23 April 2007 Abstract This review reports the state of the art in modeling chemical and physical processes of wood and biomass pyrolysis. Chemical kinetics are critically discussed in relation to primary reactions, described by one- and multi-component (or one- and multi-stage) mechanisms, and secondary reactions of tar cracking and polymerization. A mention is also made of distributed activation energy models and detailed mechanisms which try to take into account the formation of single gaseous or liquid (tar) species. Different approaches used in the transport models are presented at both the level of single particle and reactor, together with the main achievements of numerical simulations. Finally, critical issues which require further investigation are indicated. r 2007 Elsevier Ltd. All rights reserved. Keywords: Pyrolysis; Wood; Biomass; Chemical kinetics; Transport models Contents 1. Introduction Chemical kinetics of biomass pyrolysis Measurements of primary pyrolysis rates One-component mechanisms of primary pyrolysis Multi-component devolatilization mechanisms Multi-component mechanisms of primary pyrolysis Secondary reactions Outline of multi-step mechanisms of cellulose pyrolysis Distributed activation energy (DAE) models Conclusions and further developments Transport models of biomass particle pyrolysis Transport models with volumetric decomposition rates Intra-particle transport phenomena External heat transfer coefficients Extra-particle processes Unreacted-core-shrinking models Simulation results Experimental validation Empirical correlations and apparent kinetics Conclusions and further developments Tel.: ; fax: address: diblasi@unina.it /$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi: /j.pecs

2 48 ARTICLE IN PRESS C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) Models of pyrolysis reactors Fixed-bed reactors Fast pyrolysis reactors Conclusions and further developments Acknowledgments References Introduction Sustainable heat and power generation from biomass are at the center of scientific and industrial interest owing to the increasing awareness about the continuous diminution in the availability of fossil fuels and the higher sensibility toward environment preservation from pollutants generated by conventional energetic systems. Biomass is a term for all organic material that stems from plants [1], with wood as the main representative. From the chemical point of view, it is a composite material, constituted by a mixture of hemicellulose, cellulose, lignin and extractives [1 5], with proportion and chemical structure affected by variety. Inorganic matter (ash) is also part of the biomass composition, with a content ranging from less than 1% in wood to 15% in herbaceous biomass and feedstocks [6] and up to 25% in some agricultural residues [7]. Potassium, calcium, sodium, silicon, phosphorus and magnesium are the main ash constituents [8 11]. Chlorine is also found in significant concentrations in herbaceous biomass [10,11]. The classification techniques developed for coal, that is, proximate and ultimate analyses, are also applied for the characterization of biomass fuels. These are generally low in carbon (roughly between 40% and 50% on dry, ash-free basis) and high in volatile matter and oxygen [1,12,13], which result in low calorific values. A significant advantage of biomass with respect to coal is that the contents of nitrogen and especially sulfur are low. From the physical point of view, biomass presents a complex structure, accurately characterized for hardwoods and softwoods [3], which gives rise to anisotropic properties [14 16]. In this case, the thermal conductivity across and tangential to the grain direction is approximately onethird that along the grain, whereas the permeability to gas flow across the wood grain is much lower (up to a factor of 10 4 ) than that along the other two directions. Moreover, for conditions of practical interest, biomass always present a non-negligible content of moisture. In freshly harvested wood, moisture can exist in three forms: water vapor in the pores, capillary or free (liquid) water in the pores and hygroscopic or bound water in the solid structure [16]. The water vapor can be assumed to be in the same potential energy state as it would exist outside the solid. Neglecting the capillary-water attractive force, the same assumption holds for capillary water. The bound water consists of water molecules absorbed into the cellulose molecule by hydrogen bounding at the hydroxyl locations. When all available sites are occupied with water molecules, the medium is at the fiber saturation point (FSP). For many types of wood, this contribution can reach about 30% of the dry weight. Pyrolysis, consisting of solid thermal degradation in the absence of oxidizing agents, is a possible thermochemical conversion route, resulting in the production of a huge number of chemical compounds. However, for engineering applications, reaction products are often lumped into three groups: permanent gases, a pyrolytic liquid (bio-oil/tar) and char [17], or simply into volatiles and char. They result from both primary decomposition of the solid fuel and secondary reactions of volatile condensable organic products into low-molecular weight gases and char, as they are transported through the particle and the reaction environment. Numerous factors affect the pyrolysis rate and the yields, composition and properties of the product classes. Temperature, pressure and heating rate are the chief operating parameters. In addition, biomass properties (chemical composition, ash content and composition, particle size and shape, density, moisture content, etc.) also play an important role. Permanent gases comprise CO 2, CO, CH 4 and lower amounts of H 2 and C 2 hydrocarbons (for instance, see [18 29]). The composition of the liquid is highly dependent on the severity of the thermal treatment, that is, temperature and residence of the tar vapors in the hot reaction environment [30 37], and the presence of char [38]. Primary vapors (oxygenates) are associated with reaction temperatures below K, followed by hydrocarbon or secondary tars for temperatures up to 1123 K and aromatic or tertiary tars above K. The composition increases in the order of mixed oxygenated compounds, phenolic eters, alkyl phenolycs, hetherocyclic ethers, polyaromatic hydrocarbons (PAH), and large PAH [30,31]. Liquid products also contain an appreciable proportion of water originated from both the moisture content of the solid fuel and the decomposition reactions. Reviews are available on the operating conditions and the reactor configurations which maximize the yields of condensable products [39 43] or char [38,44,45] from biomass pyrolysis. In the former case, the conversion process is indicated as fast pyrolysis and, after cooling and condensation, bio-oil is obtained. This is a renewable fuel, which can be easily stored and transported and can also be used for the production of chemicals [46]. Following the definition of fast pyrolysis given in [43], specific for the maximization of the liquid products, the following conditions should be met: (a) high heating and heat transfer rates at the reaction zone, (b) primary conversion temperature of

3 C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) about 770 K and vapor phase temperature of K, (c) short residence time of products in the vapor phase (below 2 s), (d) rapid cooling of the vapor-phase products to obtain pyrolysis liquid. Elimination of points (c) and (d), through an increased activity of secondary reactions of tar vapors, favor the formation of gaseous (cracking) and/or solid (polymerization) species. Conversely, the elimination of point (a) is the key feature for the so-called conventional (or slow) pyrolysis, which produce comparable yields of the three lumped classes of products. The char product is useful as a renewable fuel or for other applications, such as metal reductant, soil amender and the production of activated carbon and biocarbon electrodes [38]. As a measure of the efficiency of the carbonization process of a given feedstock, a comparison is suggested between the fixed-carbon yield of char and the theoretical yield of carbon, predicted by thermochemical equilibrium calculations. Slow heating rates coupled with a low-temperature peak, high pressures (also in the presence of relatively high heating rates, typically 1 2 K/s) and long residence times of the vapor phase products within the reaction environment are indicated [38] as the chief conditions for maximizing the yields of this product. Despite the low values, a key constraint in the thermal conversion of biomass arises from the presence of nitrogen. Nitrogen containing species, released during biomass pyrolysis, include [47] hydrogen cyanide (HCN), ammonia (NH 3 ) and isocyanuric acid (HNCO). The first two compounds are quantitatively more important and their relative share is affected by the thermal conditions established during conversion (temperature and particle size) and the fuel type [47,48]. At low temperatures, a large portion of the nitrogen is retained in the char and NH 3 is the main gaseous nitrogeneous species. At high temperatures, more nitrogen is released in the gas phase with HCN becoming the most important product. Hence, two main routes have been identified: one leading to NH 3 and char nitrogen and the other leading to volatile cyclic amides which, following cracking, give rise to HCN and HNCO. High amounts of inorganic constituents, especially potassium, in some residues and herbaceous biomass often contribute to adverse impact on the different elements of the conversion systems through fouling, slagging and, in the case of fluidized-bed reactors, bed agglomeration [49]. Chlorine is a facilitator of alkali volatilization and, like sulfur, is an important contributor to corrosion, metal wastage and pollution [49]. It is not possible [50,51] to separate the chlorine products into either the char residue or the volatile products during pyrolysis. Indeed, for some herbaceous biomass and residues, about 20 50% is released at temperatures between 573 and 673 K, whereas about 30 60% is still retained by the char up to temperatures of 1173 K [50]. Water washing (leaching) is a mean to eliminate alkali metals from herbaceous biomass, thus improving the behavior during thermal treatments [10,52]. Technical enhancements in the contribution of biomass to commercial energy needs are focused on improving both the efficiency and environmental impacts of conversion processes. Large-scale development and optimization require mathematical modeling which, allowing quantitative representation of various phenomena, is a powerful tool for process design, prediction of reactor performances, understanding of pollutants evolution, analysis of process transients and examination of strategies for effective control. Thermo-chemical conversion of biomass in practical systems results from a strong interaction between chemical and physical processes at the levels of both the single particle and the reaction environment. Thus, in this review the current state of the art of wood and biomass pyrolysis models is analyzed in relation first to chemical kinetics and then to transport phenomena. It is worth noting that a large part of the models and results reviewed here may also be of interest in fire safety science where pyrolysis of lignocellulosic fuels is an important step for the initiation and growth of both forest and building fires. 2. Chemical kinetics of biomass pyrolysis Pyrolysis kinetics, coupled with the description of transport phenomena, produce advanced computational tools for the design and optimization of chemical reactors applied for thermochemical conversion of wood and biomass. Also, the knowledge of the fuel reactivity is needed for the formulation of simple design and scaling rules. Several reviews on the chemical kinetics of lignocellulosic materials are already available, which include those of Refs. [17,53 59]. In this section, after a brief presentation of the problems encountered in carrying out measurements of weight loss under a pure kinetic control, the literature results on the chemical kinetics of wood and biomass are reviewed, giving special consideration to work recently published or not previously examined. Wood/ biomass components are not explicitly considered, apart from a brief outline of the most recent findings about cellulose pyrolysis, with special emphasis on the formation rates of the main decomposition products. It should be mentioned that the mechanisms and kinetic constants for the decomposition of xylan (representative of hardwood hemicelluloses) and lignin are summarized in the literature review reported in the studies [60,61], respectively Measurements of primary pyrolysis rates On an indicative basis, in thermogravimetry (slow heating rates for a sufficiently small mass of the sample, so that a kinetic control is established), primary degradation of biomass starts at about 500 K, fast rates are attained at about 573 K and the process is practically terminated at K [55,62,63]. Weight loss results from the activity of numerous reactions. Therefore, thermogravimetric curves, measured for dynamic or static (isothermal) conditions, are useful simply for the formulation of global or semi-global mechanisms, which can include the effects of reaction parameters and sample

4 50 ARTICLE IN PRESS C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) properties. Several studies (for instance, see [64 67]) suggest that primary decomposition rates of biomass can be modeled taking into account the thermal behavior of the main components and their relative contribution in the chemical composition. For heating rates at sufficiently slow or moderate temperatures, several zones appear in the weight loss curves, which can be associated with component dynamics. Indeed, hemicelluloses decompose at K, cellulose at K, whereas lignin decomposes gradually over the temperature range of K [63]. As the heating rate is increased, given that the range of the degradation temperatures of components is relatively narrow, the different peaks in the degradation rate tend to merge and the characteristic process temperatures tend to become progressively higher. Furthermore, if temperatures are sufficiently high, significant degradation rates are simultaneously attained by all the components. The term pseudo-component is more appropriate as it is impossible to avoid overlap between the different components in the measured weight loss curves. In other words, although for each zone a main contributor can be identified as hemicellulose, cellulose and lignin, respectively, the simultaneous participation of the other components cannot be avoided with an extent that depends on the biomass characteristics and the severity of the conversion conditions. On the other hand, results obtained from the analysis of single components cannot be directly applied to biomass because of chemical and physical alterations introduced in the separation procedure, the impossibility to reproduce component interactions and the presence of ash [55,67]. Ash constituents, especially potassium, sodium and calcium, act as catalysts for the decomposition process and favor char formation [52,68]. As chlorine and potassium in biomass are water soluble, they can largely be removed through leaching, thus mitigating their impact on the high-temperature conversion devices [10,11]. In reality, water or mild acid washing also introduce significant modifications in the biomass decomposition characteristics [52,69 73]. In particular, this procedure is effective for separating and sharpening the peaks of the rate curves, thus facilitating, for instance, in situ investigation of cellulose and hemicellulose decomposition kinetics. An increase in reaction temperature and amount of volatile products is also observed. Despite the numerous weight loss measurements available in the literature, a systematic classification of biomass fuels, based on thermogravimetric analysis, and general mechanisms to interpret such measurements are not available. Indeed, the differences in the experimental apparatus and conditions and the lack of reference materials make this task extremely difficult. Quantitative differences between thermogravimetric characteristics are caused by several factors which, in addition to the wood or biomass species, include, even for the same sample, the geographical origin, the age or the specific part of the plant [3,73]. A difficulty in kinetic analysis also exists in separating the effects of chemistry and transport phenomena. One of the key points, in relation to intrusion of heat and mass transfer processes in kinetic analysis, is the sample size/ mass during pyrolysis which cause spatial gradients of temperature (a process taking place under non-negligible effects of internal heat transfer) or significant differences of temperatures between the sample and the controlling thermocouple, especially when these are not in close contact (non-negligible external heat transfer resistance). The latter is the most common case given that, in the experiments motivated by kinetic analysis, usually the sample mass is small and the rate/temperature of heating is kept at low levels. Owing to sample thermal inertia and/or reaction energetics, significant differences, often indicated as thermal lag [74], may be established between the sample and the controlling (external) thermocouple. These effects, extensively discussed in previous reviews [53,55], are quite high for cellulose [72,74 78] as a consequence of the strong endothermicity of the decomposition process when pressures are low and mass transfer resistance negligible. The most visible effects of such a drawback are a shift of the mass loss peak to higher temperatures and an increase in the yields of char as the sample mass is augmented. In contrast with the pyrolysis of cellulose, the sample mass is shown to have a negligible influence on the position of the maximum rate of mass loss and the yield of char for straw and washed straw [72]. This result is likely to hold also for other biomasses and stems mainly from lower effects of reaction energetics. Several studies [79 82] clearly show that exothermicity in biomass pyrolysis is associated with the formation of char. It can be understood that, owing to the higher yields of char generated from biomass in comparison with cellulose, the positive and negative effects in the reaction heats tend to become nearly equivalent. Char is generated from both primary and secondary reactions [38]. In the latter case, it is a coke derived from the decomposition of organic vapors (tars) on the carbonaceous solid, which acts as a catalyst. Beneficial effects are reported of both prolonged vapor-phase residence times and increased concentrations of vapors on the carbonization chemistry [80]. Under pressure, tarry vapors have a smaller specific volume, so that their intraparticle residence time is prolonged, favoring their decomposition, as they escape the biomass particle. Also the concentration (partial pressure) of tarry vapors is higher, thus increasing the decomposition reaction rate. Hence, pressure and flow rate [80] and, in general, mass transfer limitations [80,81] are key parameters for both the yields of char and the exothermicity of the pyrolysis process. Indeed, a linear relation between heat of pyrolysis reactions and char yields is found [80 82]. In quantitative terms, for the exothermic formation of char, values are reported of 3.6 [80], 2 [81] and [82] kj/g of char formed. The reactions forming tar precursors are estimated to be nearly thermo neutral [81] and the main enthalpy sink is attributed to tar evaporation [80,81], which is favored by

5 C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) low pressures and high flow rates. The endothermicity of tar formation is estimated to be comprised between 0.9 and 1.3 kj/g of tar formed [82]. Finally, it should also be noted that, for the conditions of thermal analysis, the processes of lignin and cellulose decomposition are considered [83 85] to be globally exothermic and endothermic, respectively. Another important aspect to be taken into account, when investigating primary decomposition, is that the effects of secondary reactions should be kept at a minimum. Although it is not always possible to separate the activity of the two classes of reactions, small size/mass of the sample is highly advisable because of the consequent reduction in the residence times of the vapor phase products in the hot reaction environment. A further key parameter in this matter is the reaction temperature which, to limit the activity of secondary reactions of tar cracking, should be maintained at least below 773 K [18 22,27,28,30]. When coupled with the description of transport phenomena, chemical kinetics should be able to predict: (1) conversion time, and (2) product distribution, as the operating conditions are varied. Associated with these global parameters, details about the degradation dynamics are also obtained. A simplified description of primary decomposition processes, usually adopted for isothermal conditions or fast heating rates, is based on a onecomponent (or one-stage) reaction process. In this case, weight loss curves are often associated with additional measurements concerning the yields of the three product classes, in order to evaluate the related formation rates. Multi-component (or multi-stage) reaction mechanisms are also proposed where each reaction takes into account the dynamics of several zones or pseudo-components in the measured curves of weight loss. Devolatilization reactions are essentially considered, with only a very few exceptions where both devolatilization and charring are included. The kinetic models make use of an Arrhenius dependence on temperature, thus introducing the parameters activation energy and pre-exponential factor, and a linear or powerlaw dependence on the component mass fraction, which may lead to additional parameters (the exponents). Contrary to the case of coal, models based on a Gaussian distribution of activation energies are scarce. A mention should also be made of isolated applications of peculiar models, not discussed in detail here, which include the deactivation model proposed in [86] for beech wood and a few agricultural residues, the use of artificial neural network methods for cellulose and lignin decomposition [87] and other treatments as discussed in [58] One-component mechanisms of primary pyrolysis As already observed, biomass weight loss curves, obtained under dynamic or isothermal conditions, present different reaction zones mainly corresponding to component decomposition, which tend to merge as the heating conditions become more severe. Hence, when fast heating rates or high temperatures are established, the majority of kinetic mechanisms consists of a single or three parallel reactions for the formation of the main product classes (one-stage or one-component mechanisms) following the proposal by Shafizadeh and Chin [88] for wood (Fig. 1). The separate formation rates of different product classes introduced by the reaction mechanism of Fig. 1 may be questionable from the point of view of analytical chemistry [53,54]. However, as previously observed [17,89,90], the comparable activation energies of the three reactions do not allow the selectivity to be displaced toward only one of the products. For negligible activity of secondary reactions, product distribution from cellulose pyrolysis carried out at atmospheric pressure (in accordance with the mechanism proposed by Bradbury et al. [91]) indicates that both char and gas yields decrease as the reaction temperature is increased (that is, their formation is linked) whereas, in primary wood pyrolysis, both liquid and gas yields continuously increase at the expense of char [18 22, 27,29]. It can be postulated that, during fast heating, given that holocellulose is converted mainly into liquids, the other two product classes (gases and char) are mainly due to lignin degradation. Hence, at low temperatures, on a global basis, there is a competition between liquid (holocellulose degradation) and char (lignin degradation) formation, with the former becoming successively more favored. At high temperatures, gas formation rates tend to increase, owing to the predominance of devolatilization (versus charring) rates of lignin decomposition. Table 1 and Fig. 2 present a summary of the onecomponent mechanisms of wood/biomass pyrolysis proposed on the basis of experiments carried out under isothermal or fast heating rate conditions. At a first glance, it appears that the activation energy of the global reaction rate (k) presents widely variable values, roughly comprised between 56 and 174 kj/mol. This can be the result of the different heating conditions established in the experimental devices, which include tube furnaces, entrained and fluid bed reactors, screen heaters, drop tubes and classical thermogravimetry, the different sample characteristics (size or mass and wood/biomass variety) and the mathematical treatment of the experimental data. A more careful examination of the data, taking into account the temperature range investigated, produces to the following main groups: (a1) high-temperature data (up to 1400 K) with E ¼ kj/mol [93,94,98]; (a2) low-temperature data (below K) with E ¼ kj/mol [92,95,96,99]; Fig. 1. One-component mechanism of primary wood pyrolysis proposed by Shafizadeh and Chin [88] (activation energies are expressed as cal/mol).

6 52 ARTICLE IN PRESS C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) Table 1 Kinetic constants for one-component mechanisms of wood/biomass pyrolysis (same data as in Fig. 1) Author (Ref.) Feedstock (variety, size, mass) Experimental system T r Reaction mechanism Kinetic constants: E (kj/mol), A (s 1 ) Thurner and Mann [92] Oak, 650 mm Isothermal tube furnace K dy dt ¼ ky k ¼ k C þ k G þ k L k ¼ exp( 106.5/RT) k L +k G ¼ exp( 106.5/RT) k C ¼ exp( 106.5/RT) k L ¼ exp( 112.7/RT) k G ¼ exp( 88.6/RT) Gorton and Knight [93] Hardwood, mm Isothermal entrained-flow reactor K dy dt ¼ ky k ¼ exp( 89.52/RT) Ward and Brashlaw [64] Wild cherry Isothermal tube furnace K dy dt ¼ kðy Y C1Þ Y C1 ¼ 0:25 0:30 k ¼ exp( 173.7/RT) Nunn et al. [94] Sweet gum, hardwood, mm, 100 mg Screen heater (1000 K/min) K dy V ¼ kðy V1 Y V Þ dt Y V1 ¼ 0:93 k L +k G ¼ exp( 69/RT) Chan et al. [14] dy dt ¼ ky k ¼ k C þ k T þ k G þ k W Font et al. [95] Almond shells, mm, 2 mg Pyroprobe K dy dt ¼ ky k C ¼ exp( 121/RT) k T ¼ exp( 133/RT) k G ¼ exp( 140/RT) k W ¼ exp( 92.1/RT) k ¼ exp( 108/RT) k C ¼ exp( 73/RT) k L ¼ exp( 119/RT) k G ¼ exp( 139/RT) Samolada and Vasalos [96] Fir wood, mm, 2 g Isothermal batch fluid-bed K dy V ¼ kðy V1 Y V Þ dt Y V1 ¼ 0:005 k G +k L ¼ exp( 94/RT) Wagenaar et al. [97] Reina et al. [98] Pine, mm TGA K dy dt ¼ ky k ¼ exp( 150/RT) Drop tube K k C ¼ exp( 125/RT) k ¼ k C þ k G þ k L k L ¼ exp( 149/RT) k G ¼ exp( 177/RT) Forest waste, p1000 mm, 25 mg Isothermal TGA K dy dt ¼ kðy Y C1Þ; Y C1 ffi 0:25 k ¼ exp( /RT) K dy dt ¼ kðy Y C1Þ; Y C1 p0:15 k ¼ exp( 91.53/RT) Di Blasi and Branca [99] Beech, o80 mm, 9 mg Tube furnace K dy dt ¼ ky (a) k ¼ exp( 95.4/RT) (b) k ¼ exp( 141/RT) k L +k G ¼ exp( 149/RT) k C ¼ exp( 112/RT) k G ¼ exp( 153/RT) k L ¼ exp( 148/RT) (a3) low-temperature data (below K) with E ¼ kj/mol [64,97 99]. In the presence of non-negligible heat and mass transfer limitations, the analysis and interpretation of measurements traduce in apparent kinetics, characterized by activation energies much lower than the true values and lower rates than those determined in classical thermogravimetry [55]. This appears to be the case of the kinetic constants estimated by means of the high-temperature data of the group a1 (for instance, screen heaters or entrained bed reactors), which may be affected by significant heat/ mass transfer intrusions. The kinetic models of Refs. [93,94] are essentially proposed as correlations for specific

7 C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) ln k [95] [97] [93] [94] [99b] [92] [96] [99a] [64] [98] Char Yield [wt%] Ref. [14] ; [64] ; [92] ; [97] ; [98] ; [99] /T [K -1 ] Fig. 2. Arrhenius plot for the global decomposition rate of wood/biomass based on one-component mechanisms (see Table 1 for kinetic parameter values) T [K] Fig. 3. Predicted (lines) and measured (symbols) of char yields on dependence of the reaction temperature (kinetic control and onecomponent mechanisms). experiments. The actual particle temperatures are unknown in [93], whereas screen heater measurements [94] are affected by significant practical problems [100]. In this apparatus, it is difficult to achieve a good control of the sample temperature, the recorded value is strongly affected by thermocouple positioning and the effects of reaction energetics are not taken into account. Finally, the low values of the kinetic constant predicted with the model of Ref. [98] in the high-temperature range ( K, not shown) can again be due to a lack of kinetic control. The impossibility to get high temperatures without going through the low-temperature region also excludes a change in the reaction mechanism. These considerations lead Antal [54] to write that accurate estimates of high-temperature biomass pyrolysis reaction rates can be best obtained by extrapolating low-temperature kinetic data applicable to the reaction pathways of interest at high temperatures. However, it can be noted that, for low-temperature data, both a low-activation energy group (a2) and a highactivation energy group (a3) of kinetic constants exist. In order to estimate the global rate constant, k, for the isothermal process described by the three reactions of Fig. 1, two different treatments can be used [99], though assuming in both cases that only the central part of the weight loss curves, the most important from the quantitative point of view, has to be described. That is, the mass conservation equations can be integrated over the entire duration (time) of the process, or specifically over the time corresponding to the central part of the weight loss curve. In fact, zones with different slopes in the Arrhenius plot make evident the existence of several sequential reaction steps. Then, the usual Arrhenius plot and a least-square analysis give the activation energy and the pre-exponential factor of the global pyrolysis kinetics. It is found [99] that two sets of kinetic constants can be obtained (Table 1: E ¼ 95.4 kj/mol (model a), and E ¼ kj/mol (model b)) for the same group of isothermal experiments. Fig. 2 shows that both sets are comprised in the range of literature values. The kinetic constants of the model of Ref. [99] (model a) and those of Refs. [92,96] are roughly the same. The kinetic constants are also roughly the same for the model of Ref. [99] (model b)) and those of Refs. [97,98] (Ref. [98] for a temperature range of , forest waste). The results of Ref. [99] for the models (a) and (b), plotted in Fig. 2, indicate that the mathematical treatment of the central part of the same weight loss curves affects significantly the activation energies. At low temperatures, the model (a) predicts degradation rates faster than those of the model (b) but fails to predict the fast increase of the wood degradation rate with temperature, which is due mainly to the activity of components in the central part of the isothermal weight loss curves. Hence, the model (b) and those of group a3, introduced above, appear to be more appropriate than the model (a) (and those of group a2) for predicting the behavior of chemical reactors in practical applications. The kinetic constants, plotted in Fig. 2, are inversely proportional to the characteristic (chemical) times of wood pyrolysis. If product yields are among the desired model outputs, the kinetics for the formation rates of the different product classes should be estimated. For isothermal data, this requires the knowledge of the corresponding yields for each reaction stage of the mechanism [89,99]. Given that accurate measurements of the volatile fractions cannot be accomplished for the small sample quantity used in thermogravimetric analysis, and laboratory scale reactors only allow the total final yields of products to be obtained, alternative formulations are difficult to accomplish with respect to a one-component mechanism of wood or biomass primary degradation including the product formation rates such as in the proposal of Fig. 1. The kinetic constants for the formation rates of char, gas and liquids (or for volatiles and char), where available, are

8 54 C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) Yields [wt%] Liquid Gas Ref. [14] T [K] [92] [97] [99] Fig. 4. Predicted liquid and gas yields on dependence of the reaction temperature (kinetic control and one-component mechanisms). also listed in Table 1. The corresponding predictions of the product yields for wood/biomass, obtained under kinetic conditions and following [99], are summarized in Fig. 3 (char) and Fig. 4 (gas and liquids) where, for completeness, the experimental data are also included. In all cases, the measured char yields decrease as the temperature is increased. Good agreement (Fig. 3) is observed between predictions and measurements [64,98,99] for low temperatures. The Thurner and Mann [92] mechanism does not reproduce the experimental trends even from the qualitative point of view, whereas quantitative differences are high between the other mechanisms. In particular, only the kinetic constants of Refs. [97,99] predict a strong influence of temperature (quantitative differences are probably due to the use of softwood and hardwoods, respectively). In other cases, the char yields remain high (20 30%). Consequently, the related kinetics are not applicable for fast pyrolysis, where typical char yields are below 15%. For liquid and gas formation, differences between kinetics (Fig. 4) exist from both the qualitative and the quantitative point of view. Kinetic constants of Refs. [14,99], in qualitative agreement with experimental measurements (for instance, [18 22,25 29]), indicate that both liquid and gas yields increase with temperature. The gas yields attain about the same values, but liquid yields are higher in [99] (lower char yield). The other two sets of data do not give predictions in qualitative agreement with experimental observation. Indeed, the gas [92] or the liquid [97] yields decrease as the temperature is increased (though, in the latter case, this is hardly evident). It is difficult to explain the differences between the proposed kinetics, but the use of thick particles [92] clearly gives rise to heat and mass transfer limitations, which appear as high char yields. Other critical aspects are the very narrow range of temperatures investigated, the evaluation of product yields at high temperature [97], when secondary reaction activity is not negligible, and the absence of temperature measurement/control [92,97]. Finally, the results may be affected by the wood or biomass species (for instance, at low temperatures, very high yields of char are predicted [90] in the case of almond shells [95]) Multi-component devolatilization mechanisms The majority of multi-component mechanisms simply consist of devolatilization reactions, which can be applied to predict only the rate of weight loss, provided that the total amount of matter to be released in the gas/vapor phase is already known (assigned or measured). Dynamic experiments are generally carried out by means of classical thermogravimetry. The most used mechanisms usually comprise parallel reactions (Fig. 5) for the decomposition of the volatile fractions of pseudo-components, although consecutive reactions can also be applied [101], owing to significant overlap between the different evolution times. In the former case, each pseudo-component, whose volatile fraction is among the model parameters, acts as if there were no interactions. The number of pseudo-components or zones, in the majority of the cases, is three and again coincides with hemicellulose, cellulose and lignin (threecomponent devolatilization mechanisms). In a few cases, the contribution of extractives or more than one reaction stage in the decomposition of components, especially hemicellulose and lignin, are also taken into account. An important aspect is represented by the mathematical treatment of the experimental data to formulate reaction mechanisms and to estimate the related kinetic parameters. The use is recommended [59,101] of differential (versus integral) measurements because the details of the devolatilization process are better shown. Furthermore, linear forms of the mass conservation equations, usually combined with analytical methods for the evaluation of the kinetic constants, may present serious drawbacks deriving from data manipulation and applicability limited to single measurements [59]. Instead, numerical solutions of the conservation equations coupled with minimization methods of objective functions, adequately defined, are advised [57,59]. The three-component mechanism with linear or nonlinear dependence on species concentrations, for the volatile fractions of the pseudo-components hemicellulose, cellulose and lignin, is widely applied [55,57,69,70, 73, ] to describe dynamic thermogravimetric curves of wood/biomass devolatilization. In several cases [55,57,69,73,104,105,107] dynamic measurements and the corresponding kinetic analyses examine one heating rate only, generally below 10 K/min. Process simulations show that the pseudo-components hemicellulose and cellulose decompose independently of one another, the former Fig. 5. Multi-component devolatilization mechanisms (C i is the volatile fraction of the ith pseudo-component).

9 C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) dy/dt x 10 3 [s -1 ] hemicellulose cellulose lignin T [K] Fig. 6. Comparison between the observed (symbols) and simulated differential curves (solid line) for beech wood heated at 5 K/min, by means of a three-component devolatilization model (parameters estimated for a single heating rate [107]). Lines with various styles denote the predicted volatile evolution from the different pseudo-components. associated with the shoulder and the latter with the peak of the rate curves, whereas the lignin pseudo-component decomposes slowly over a very broad range of temperatures. An example is provided in Fig. 6, for beech wood and a heating rate of 5 K/min [107]. The agreement between the kinetic parameters, estimated by means of differential curves, is acceptable. Activation energies vary between 80 and 116 kj/mol for hemicellulose, kj/ mol for cellulose, and kj/mol for lignin. Furthermore, the component contributions, expressed as percent of the total mass fraction, are roughly 20 30% for hemicellulose, 28 38% for cellulose and 10 15% for lignin. They are obtained as a result of the optimization procedure (based on total mass conservation, the complement to 100 is represented by char). Compared with the case of one-component reaction mechanisms, the use of classical thermogravimetric systems with very slow heating rates and the application of numerical methods for parameter estimations certainly contribute to reduce the differences between the estimated values of the kinetic constants. The effects of the highly heterogeneous material, however, still remain and general mechanisms with a wide range of applicability are not available. Attempts in this direction are the results presented in [107], in relation to nine wood species whose chemical composition is comprised within the widest range indicated for the hardwood and softwood categories, and in [73] for chestnut, which presents large deviations in the chemical composition compared with standard hardwood categories. It is reported that, at least for one slow heating rate, the same set of activation energies can be applied in all cases. The differences in the characteristic reaction temperatures and the yields of char between species are taken into account by pre-exponential factors and stoichiometric coefficients. However, in the case of chestnut, the accuracy in the predictions is acceptable only for engineering applications. The same considerations apply for the effects of sample geographical origin and pretreatments (hot water or acetone extraction) [73]. When the kinetic constants are estimated by means of one experiment only, compensation effects [110,111] are not avoided, that is, the possibility of different couples of pre-exponential factor and activation energy to describe reasonably well the same weight loss curve. Indeed, only one set of data can predict the behavior of the material at several heating rates, consisting of the displacement of the weight loss curves toward successively shorter times for thermal conditions successively more severe [110]. Numerical simulations [74,77], based on simple models ignoring spatial gradients and for the conditions (sample size and heating rate) typical of thermal analysis, show the existence of a thermal lag between the sample and the external (heating) temperature. It is a consequence of the endothermicity of biomass decomposition and has been related by Narayan and Antal [74] to compensation effects. If the thermal lag is non-negligible, estimated values of the apparent activation energy and pre-exponential factor will be less that their true (intrinsic) values. Conesa et al. [59],in their literature review, also agree with this explanation. Hence, in order to determine the intrinsic reaction kinetics, such effects should be avoided. It is suggested that [59] the fitting of various runs performed in different conditions (different heating rates, or different temperature program in general) at the same time, using the same kinetic model and parameters, could be a method valid for distinguishing an actual model and for solving the compensation effect. A model involving a change in kinetic constants with the heating rate and/or the extension of the reaction can only be considered a correlation model very far from the actual kinetics. The inclusion of several heating rates, especially the higher values, in kinetic analysis of wood/biomass devolatilization is also important from the practical side. Indeed, the fuel particles in industrial systems usually experience widely variable heating rates, which often are higher than those typical of thermal analysis. In some cases, thermogravimetric curves of biomass decomposition are measured at several heating rates but the kinetic analysis is incomplete as no effort is made to produce a general kinetic model applicable for the different thermal conditions (for instance, see [66] for rice husks). The separate analysis of each curve, based on analytical solutions, usually produce sets of kinetic parameters highly variable with the heating rate. This result can be partly attributed to non-negligible effects of transport phenomena, especially at the higher heating rates, and partly to the simplified method for extracting kinetic parameters from the measured curves. In any case, such kinetics should not be incorporated in transport models for the description of practical conversion systems. The simultaneous evaluation of multiple thermogravimetric curves for different heating rates is examined in several studies, which include 2 25 K/min for olive stones and almond shells [102], K/min for untreated and

10 56 ARTICLE IN PRESS C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) water washed rice husks [103], K/min for waterwashed beech wood [106] and 5 20 K/min for waste wood and other residues [108]. Apart from the different fuels and pre-treatments examined, the comparison between the results is difficult owing to power-law dependence on the mass fraction [102,106,108], the use of integral data [103,106] and the selection of widely different values of the component fractions. The overall trend is that the kinetic evaluation of cellulose devolatilization remains roughly unchanged (activation energies of kj/ mol, which compare well with single-curve results), but higher activation energies of hemicellulose degradation are reported ( kj/mol). As for the reaction of the third pseudo-component, activation energies of 36 kj/mol [103], associated with a strong dependence of the corresponding pre-exponential factor on the heating rate, kj/mol [102,106], and kj/mol [108] are given (in [106], the devolatilization of this component takes place along the low-temperature region typical of hemicellulose). The recent study by Branca et al. [109] tries to explain the differences in the kinetic constants when the threecomponent mechanism, with rates linearly dependent on the volatile content, is applied for the interpretation of thermogravimetric curves obtained at several heating rates (range K/min). It is found that the set of activation energies estimated in [107] (100, 236 and 46 kj/mol, respectively) and representative of values obtained when only one differential thermogravimetric curve is processed, gives rise to very high deviations between predicted and measured rate curves. The agreement is highly improved by a new set of data consisting of activation energies of 147, 193 and 181 kj/mol, respectively, for the pseudo-components hemicellulose, cellulose and lignin. An example of the predicted dynamics is shown in Fig. 7 for beech wood and a heating rate of 5 K/min. A comparison with the kinetics determined with the use of one heating rate only (Fig. 6) shows that the overlap is reduced between the devolatilization rates of the three pseudo-components whose chemical composition is also modified. The amount of volatiles released in the second (cellulose) stage is increased (46 49% versus 38 44%) at the expenses of that associated with the first (hemicellulose) stage (about 18 24% versus 23 30%). Furthermore, instead of a slow decomposition rate over a broad range of temperatures, the activity of the third reaction (lignin devolatilization) is mainly explicated along the high-temperature (tail) region of the weight loss curves. Sometimes the three-component mechanism is modified to include additional steps for improving the accuracy of the predictions, as in [112] for isothermal data, and [107,113,114] for dynamic data. In [112] wood (pine, chestnut and pine bark), devolatilization is modeled by means of six parallel reactions corresponding to different volatile fractions. The first two, quantitatively more important, are associated with hemicellulose and cellulose, respectively (the corresponding activation energies of 83 and 146 kj/mol are in good agreement with the values -dy/dt x 10 3 [s -1 ] hemicellulose cellulose lignin T [K] Fig. 7. Comparison between the observed (symbols) and simulated differential curves (solid line) for beech wood heated at 5 K/min, by means of a three-component devolatilization model (parameters estimated for heating rates between 5 and 104 K/min [109]). Lines with various styles denote the predicted volatile evolution from the different pseudocomponents. reported for single dynamic curves of the weight loss rate). The other components are assumed to correspond mainly to parts of the lignin macromolecule (activation energies between 60 and 130 kj/mol). Two additional reactions, in the low-temperature region (below 553 K) associated with extractive decomposition, are considered in [107] for both hardwood and softwood species. In [113], thermogravimetric curves of wood (hornbeam, walnut, pine) are interpreted using five parallel reactions, with a power-law dependence on the volatile mass fraction, with parameters dependent on the wood species. Two fractions are considered for both the hemicellulose (activation energies in the ranges and kj/mol, respectively) and cellulose (activation energies and kj/mol, respectively) pseudocomponents and one for lignin (activation energies of kj/mol), though it is pointed out that the evaluations for the small fraction of cellulose, characterized by a low activation energy, are uncertain. Reaction orders are comprised between 0.8 and 1.8. It can be noted that this study again confirms high activation energies for the decomposition of the cellulose component. It is also found [114] that, to obtain accurate predictions of the devolatilization rates of wood (poplar, black locust and willow) at different heating programs (heating rates between 20 and 40 K/min), it is necessary to use at least six or four reactions depending on the assumption of a linear or a power-law dependence on the volatile fraction. This is an indirect confirmation of the large inaccuracies introduced in multiple-curve predictions by the set of kinetic parameters of the three-component mechanism as estimated for single curves. The use of additional reactions or parameters, compared with previous slow heating rates analysis, is justified [114] with the consideration that a wider range of experimental conditions reveals more of

11 C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) the chemical inhomogeneities of the biomass components. The pseudo-component cellulose is still characterized by a quite high value of the activation energy (188 kj/mol), whereas a wide range of activation energies is proposed for the remaining reactions ( kj/mol). Although the differences in the kinetics of wood/biomass devolatilization may be a consequence of model features and procedure of parameter estimation (in addition to the chemical properties of the samples), a round robin with eight participants [115], carried out to evaluate the chemical kinetics of Avicel PH 105 microcrystalline cellulose, clearly puts into evidence the existence of some flaws in the temperature measurements even for standard thermogravimetric systems. It shows that, for low heating rates (5 K/min), the agreement is acceptable (scatter in the temperature measurements of about 17 K and char yields between 4% and 10.9%) and, in terms of kinetic parameters, the differences can be accounted for by some uncertainty on the pre-exponential factor of the one-step first-order reaction (devolatilization). At higher heating rates (40 K/min), the impact of thermal lag is larger and appears as reduced values of activation energies and preexponential factors. The authors also point out that the scatter in the experimental data and the instrumental errors well represent the performances of the state-of-the-art instruments, so that they are inclined to acknowledge the fact that biomass pyrolysis kinetics are inherently difficult to study by any technique, and these difficulties contribute significant uncertainties in the understanding of the phenomena Multi-component mechanisms of primary pyrolysis A very few multi-component (or multi-stage) mechanisms of wood/biomass pyrolysis are available, that is, mechanisms for predicting the formation rates and the yields of reaction products or solid- and gas-phase intermediates. Examples based on series reactions, which try to take into account the presence of several zones in the isothermal weight loss curves, are given in [116,117] for straw and beech wood, respectively. The mechanism of wood pyrolysis, proposed by Miller and Bellan [118], also belongs to this category, which incorporates multi-step mechanisms for the main components and uses the superposition of the different contributions, taking into account chemical composition. It can be considered a reexamination of the mechanisms originally proposed in [64,65]. A three-stage series mechanism (Fig. 8) is proposed in [116,117], which takes into account the competitive formation of classes of compounds belonging to either the gas (vapor) or the solid phase. However, kinetic parameters are estimated only for each global reaction (simultaneous charring and devolatilization processes) of the three stages. In the case of beech wood, estimations are carried out in the temperature range K (k 1 ), K (k 2 ) and K (k 3 ), for three zones clearly Fig. 8. The multi-component pyrolysis mechanism proposed in [117] (A is the fuel, B and D are reaction intermediates). visible in the isothermal weight loss curves and corresponding to the main pseudo-components introduced in Fig. 8. The inclusion of low-temperature ( K) data in the Arrhenius plot does not result in significant changes in the kinetic parameters for the central part of the reaction zone (an activation energy of 143 kj/mol against 141 kj/mol previously estimated [99] for temperatures above 573 K), which are also close to those of other isothermal analyses [64,97,98,112]. These values are, however, lower than those estimated for the devolatilization mechanisms based on dynamic measurements. As for the hemicellulose stage, the activation energy (76 kj/mol) reported in [117] is in the range of those obtained from the evaluation of single dynamic curves and the isothermal analysis in [112]. The low activation energy for the third (lignin) stage also agrees with the results obtained from evaluation of single dynamic curves. In the light of the criticism raised by the recent analysis by Branca et al. [109] about the kinetic constants of the three-component devolatilization mechanism estimated by the use of single curves (also taking into account possible flaws in the thermogravimetric measurements [115]), it can be argued that the kinetic constants estimated by means of isothermal curves also produce poorly accurate predictions. Moreover, the consecutive reactions of Fig. 8 suffer from a higher overlap between the degradation rates of the main components compared with slow heating rate dynamic analyses. On the other hand, if the formation rate of the product classes for each stage should be determined, the corresponding final yields should be measured. However, this is not possible because, as already pointed out, only integral data concerning complete conversion can be measured from chemical reactors. A multi-component mechanism, taking into account the decomposition rate of hemicellulose, cellulose and lignin, is proposed in [118], as already in [64 65]. The model in not based on a specific set of experiments but relies upon a reexamination of literature data. It is assumed that the mechanism of cellulose pyrolysis by Bradbury et al. [91] is also applicable for the other two main components (Table 2). The reaction rates present the usual Arrhenius dependence on temperature and are first order in the reactant mass fraction. The depolymerization step does not introduce any change in the chemical composition but it is suggested to modify the physical properties, for instance, porosity. The kinetic constants estimated by Bradbury et al. [91] are used for cellulose. The corresponding initial estimates for the activation step of the hemicellulose and lignin components are derived from [64]. The cellulose parameters reported in [64] are also assigned for the other

12 58 ARTICLE IN PRESS C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) Table 2 Multi-component mechanism and kinetic constants for wood pyrolysis based on the contribution of the three main components [118] TAR k 2 C k 1 A k 3 ν C CHAR+(1- ν C )GAS C Kinetic constants: A (s 1 ), E (kj/mol) Cellulose Hemicellulose Lignin k exp( 242.4/RT) exp( 186.7/RT) exp( 107.6/RT) k exp( 196.5/RT) exp( 202.4/RT) exp( 143.8/RT) k exp( 150.5/RT) exp( 145.7/RT) exp( 111.4/RT) n c steps of the hemicellulose mechanism whereas those for the lignin component are derived from [65] disregarding the power-law dependence for the solid mass fraction. The final parameter values are estimated so as to get the best fit with the char yields obtained in [65,119] for beech wood (TGA tests executed with heating rates of 5 80 K/min) and lignin (isothermal tests for temperatures in the range K for small-sized particles) and in [21] for small-sized maple wood particles (isothermal tests in a fluidized-bed reactor with temperatures in the range K). Extractive and ash contents are incorporated in the hemicellulose component. The resulting set of kinetic parameters is reported in Table 2. Comparison with other experiments is made a posteriori without any readjustment of the parameters. Good qualitative agreement is attained with the expected behavior of the three components for the range of pyrolysis temperatures and both location and magnitude of the peak rates. The predicted char yields decrease as the temperature is increased for all the components: the largest values are obtained for lignin, the minimum for cellulose and hemicellulose yields are bounded by the former two. A slight delay in the beginning of the devolatilization process occurs but it is related to a region of small weight loss. Although not extensively tested, this remains one of the few attempts to produce a general mechanism of biomass pyrolysis Secondary reactions At high temperatures and given sufficiently long residence times, secondary reactions of primary tar vapors also become active [30,120,121]. These alter both the yields and composition of the wood/biomass pyrolysis products. They may occur in the pores of the particles, while undergoing primary degradation, homogeneously in the Fig. 9. A global mechanism for the secondary reactions of vapor-phase tarry species as proposed by Antal [120,121]. vapor phase and heterogeneously over the char surfaces and the extra-particle surfaces. The latter aspect is not considered here, but extensive research on biomass gasification confirm the catalytic effects exerted by different materials on the cracking of tarry components (see, for instance, the reviews [122,123]). Secondary reactions of tar vapors are classified as homogeneous and heterogeneous and include processes such as cracking, partial oxidation, re-polymerization and condensation [36]. The complex chemical composition of tarry products would require a huge number of chemical reactions to describe the details of the transformations. However, despite the quantitative understanding about the chemical composition of this class of products, the most cited mechanism simply consists of two competing reactions [120,121] as reported in Fig. 9. The reactive volatile matter is assumed to be consumed by two competitive reactions leading to the formation of permanent gases and a refractory condensable material. The existence of the second reaction is inferred from the gas yield data, which display an asymptotic behavior (after residence times of about 5 s) that is strongly dependent on temperature. Higher temperatures result in dramatic increases in the asymptotic yields of all the light permanent gases produced. The temperature-dependent asymptotes require the existence of the second reaction in order to explain the disappearance of carbon atoms in the gas phase when the gas phase temperature is reduced. However, the kinetic

13 C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) models usually neglect this competition because either the temperatures are sufficiently high so that the amount of refractory tar formed is small or a devolatilization process is considered (similar to devolatilization versus pyrolysis for the mechanisms of primary reactions). In the former case, the rate of tar cracking is generally described by a global reaction, with a rate linearly dependent on the mass concentration of the vapor-phase tar (r V ) and the usual Arrhenius dependence on temperature. In alternative, the cracking rate is linearly dependent only on the reactive fraction of the primary tar (r VT ). In a few cases, the existence of tar fractions with different reactivities is explicitly acknowledged. The competition between the chemical paths of gas and refractory tar formation (Fig. 9) has important implications from the point of view of process development. The thermal stability of tars for temperatures below 773 K [18 22,27,28,30] is a key issue in the fast pyrolysis processes aimed at bio-oil production, as extensively discussed in several technology reviews [39 43]. The kinetics of secondary tar reactions is also of paramount importance in biomass gasification. The amount of tar produced and its composition depend on the type of gasifier and the process conditions. In principle, producer gas with a low tar content can be obtained if a high-temperature zone can be created where the volatile products of pyrolysis are forced to reside sufficiently long to undergo secondary gasification. However, the discover of a refractory tar product [120,121] of secondary reactions has motivated extensive research activities on catalytic pyrolysis for the vapor phase products which, as anticipated, have been reviewed. Compared with primary reactions, secondary reactions are less investigated and evaluations of the kinetic constants are essentially available only for the cracking process. This information, together with the range of experimental conditions, is summarized in Table 3. Fig. 10 provides the corresponding Arrhenius plot. It can be seen that the estimated activation energies vary between 66 and 123 kj/mol though, for the majority of the studies, the range is narrower and roughly corresponds to kj/ mol, with the exception of cellulose tars [120]. It is plausible that the comparable values of the cracking rates, reported in all cases, are the result of the high simplification represented by the global one-step reaction applied to model the cracking process and the assumption of ideal plug-flow behavior for the gas/vapor phase. Indeed, experimental conditions (thermal and fluid-dynamic conditions of primary and secondary degradation), sample characteristics (biomass/wood, size, shape, ash and moisture content), and mathematical treatments of the data are highly different among authors. Experiments consider either one reactor [124,127, 129,133] for both primary and secondary reactions, or two reactors (or zones) in series [36,120,121,125, 128,132,135]. The variety of experimental conditions established during primary pyrolysis (nominal heating rates from a few to thousands K/min with final temperatures between K) results in different composition (and reactivity) of tarry species. In other words, the activity of secondary reactions is also explicated, at different extent, during the process of primary decomposition of the starting material. Moreover, apart from the cases where continuous systems are used [36,127], the composition of the vapor stream undergoing secondary reactions is also highly variable with time and may be affected by the location and type (total gas or single gaseous species, liquid yields, etc.) of measurements [135]. In addition to the differences in the temperature and residence times of the vapor phase, the presence of reactive species, such as steam (also from primary decomposition), and char may have an important impact on the tar decomposition rates. For instance, char freshly formed is reported [136] to cause the heterogeneous conversion of about 14% of the primary tar product. The catalytic role exerted by charcoal on tar conversion is also recognized in biomass gasification. To exploit this feature, peculiar reactor design schemes have been proposed, such as for the downdraft gasifier, modified to include internal recycle of vapor-phase tars over the glowing char and combustion of pyrolysis gases [137], and two-stage fixed-bed gasifiers [138]. Also, the catalytic action exerted by charcoal on tar conversion is confirmed for supercritical water gasification of biomass [139]. Given that the presence of specific compounds in the pyrolysis liquids is directly related to the decomposition of the main constituents of wood/biomass and also affected by the chemistry of the ligninic fraction [37], it can be understood that differences in the reactivity are also caused by the type of feedstock. Different woods [36,120,121,125, 127,128,133,135] are examined together with cellulose [120,121], biomass [129,134], municipal solid waste (MSW) [124], refuse-derived fuel (RDF) [131] and lignin [132]. Differences in the rate expressions also produce variations in the estimated values of the kinetic parameters. As anticipated, the global cracking rate is assumed to be linearly dependent on the mass concentration of the vaporphase tar (r V ). Exceptions are represented by the studies of Refs. [36,125,128]. Boroson et al. [128] assume that it is linearly dependent only on the reactive fraction of the primary tar (r VT ) which, for the experimental conditions investigated and sweet gum hardwood, corresponds to about 94%. The same kinetic equation and activation energy reported in [128] are also used in [140] for predicting the decomposition of beech wood tar in a thermogravimetric system and a muffle furnace. The reactive fraction of tar is, however, lowered to 78% and the pre-exponential factor is modified to become A ¼ s 1. Tar vapors are also assumed to consist of two [124] or three [125] fractions with different reactivities. Garcia et al. [124] use either one or two parallel reactions with rates linear in the total tar concentration. It is found that the latter mechanism is associated with significantly higher values of the activation energies ( kj/mol). More-

14 60 Table 3 Kinetic constants for the global reaction of tar cracking, where r V and r VT are the total and reactive mass concentration of vapor-phase tar, respectively (same data as in Fig. 10) Author (Ref.) Reactor; atmosphere Primary pyrolysis: material; heating conditions Secondary pyrolysis: temperature; vapor residence time Kinetic law A (s 1 ); E (kj/mol) Antal [120,121] Tubular, two zone, batch reactor; steam mg of cellulose, cherry wood, yellow pine powder; 773 K K; s r ¼ exp( 204/RT)r T ; (cellulose); E (cherry wood) ¼ 98.6; E (yellow pine) ¼ 101 Diebold [126] Continuous vortex reactor+tubular Softwood sawdust; 898 K K; o1s r ¼ exp( 87.6/RT)r T reactor; steam Liden et al. (1988) [127] Continuous, bubbling fluidized-bed mm thick poplar; K K; s r ¼ exp( 107.5/RT)r T reactor; nitrogen Boroson et al. [128] Series-connected, tubular reactors; helium 20 mm deep bed of sweetgum hardwood powder; 0.2 K/s up to 723 K K; s R ¼ exp( 93.3/RT)r TV ; y NR ¼ 6% Font et al. [129] Batch, fluidized-bed reactor; nitrogen 3 4 g of almond shells, K r ¼ exp( 110.1/RT)r T mm thick; K Graham et al. [130] Continuous ultrapyrolysis reactor; Cellulose powder; K K; s r ¼ exp( 100.8/RT)r TV nitrogen Cozzani et al. [131] Two-zone tubular reactor; helium 15 g of milled RDF; K K; 6 22 s r ¼ exp( 102.3/RT)r TV Garcia et al. [124] Batch, fluidized bed reactor; nitrogen g of MSW; K; r ¼ exp( 99.5/RT)r T K o5 s Caballero et al. [132] Pyroprobe 1000+tubular reactor packed with quartz particles; helium 1 mg of Kraft lignin (Ecalyptus wood) powder; 300 K/s up to 973 K Lede [133] Cyclone reactor; mixture of helium/ mm thick beech wood particles; argon and steam K Fagbemi et al. [134] Tubular reactor+packed-bed (metallic g of wood, straw, coconut shells rings) reactor; helium powder; K Rath and Staudinger [125] TGA+tubular quartz reactor; nitrogen 500 mg of spruce wood (0.5 1 mm thick particles); 5 K/min up to 1323 K Morf et al. [36] Baumlin et al. [135] Continuous fixed-bed reactor+tubular reactor; nitrogen Tubular reactor+perfectly stirred reactor; argon K; r ¼ exp( 84.7/RT)r T K; s r ¼ exp( 123.5/RT)r T K; s r ¼ 4.34 exp( 23.4/RT)r T K; s r I ¼ exp( 66.3/ RT)r T(I) ; r II ¼ exp( 109/ RT)r T(II) ; y NR ¼ 22% K; o0.2 s r ¼ exp( 76.6/RT)r T mm thick spruce and fir particles; 653 K 1 g of beech wood sawdust; 820 K K; s r ¼ exp( 59/RT)r T C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) ARTICLE IN PRESS

15 C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) lnk [36] [135] [124] [133] [129] [125] [134] [126] [130] [125 ] [128] [120] -4 [132] [131] /T [K -1 ] [127] Fig. 10. Arrhenius plot for the global reaction of vapor-phase tar cracking (see Table 3 for the values of the kinetic parameters). over, for the specific experimental conditions, around 70 80% of primary tar generates permanent gases, whereas the remaining fraction is a refractory matter or char, a confirmation of varying reactivities among the tar fractions. From the mathematical point of view, when a multistep process is fitted by a single step reaction model, the estimated activation energy is lower than the intrinsic (actual) values of the different steps. However, although in [125] primary tar is assumed to consist of two reactive and one unreactive fraction, kinetic evaluation for the two parallel reactions still produces low values of the activation energies (Table 3). A competition between the reactions leading to the formation of permanent gases and a refractory tar is explicitly postulated in [120,121] where an evaluation is also provided of the kinetic parameters for the polymerization process. Little work has been made on the kinetics of the reactions undergone by primary products of wood/biomass pyrolysis. Levoglucosan has often been considered [79 81,141] as a model compound for tar. The results of a comprehensive study on the influences of the gas flow at various pressures on the yields of char generated from cellulose are summarized [80] by a conceptual mechanism, which includes both primary and secondary decomposition. Low-temperature paths are evidenced (formation of anhydrocellulose and char), which are not of great importance because associated with slow heating rates, that are uneconomical for practical applications [38]. The most important results concern the pathway associated with the formation of levoglucosan, favored by fast heating, and its evolution. At high flow rates and pressures below 1 MPa, an endothermic zone is observed and associated with levoglucosan evaporation. However, at low flow rate, the endothermic zone is replaced by a strong exothermic zone, attributed to decomposition reactions with formation of gas and carbonized levoglucosan. The decomposition of this compound occurs via two Table 4 Mechanism and kinetic constants for 5-hydroxymethylfurfural, levoglucosan and hydroxyacetaldehyde proposed in [141] Primary! k 1 Secondary! k 2 Tertiary E 1 (kj/mol) E 2 (kj/mol) A 1 (s 1 ) A 2 (s 1 ) 5-Hydroxymethyl furfural Levoglucosan Hydroxyacetaldehyde competing reactions affected by the heating rate (high values of these favor gas formation). Kinetic constants for the different reaction paths are not available, so this mechanism has never been incorporated in transport models. Information is also available [141] on the global kinetics for the formation of primary, secondary and tertiary products from the decomposition 5-hydroxymethylfurfural (5-HMF), levoglucosan and hydroxyacetaldehyde. Measurements are carried out by means of a tubular gas phase pyrolysis reactor coupled to a molecular-beam mass spectrometer (MBMS). Multi-variate data analysis is used for the estimation of kinetic constants for the reaction mechanism formulated by analysis of the temporal profiles of the pyrolysis products. For 5-HMF and levoglucosan, two sequential reactions are proposed (Table 4) for describing primary, secondary and tertiary products (primary components are the evaporated reactants and their fragment ions). Products from hydroxyacetaldehyde (Table 4) only require a one step reaction. Recent investigations [ ] on the low-temperature devolatilization (evaporation and cracking) of pyrolysis liquids produced from different fuels and variable heating conditions confirm the importance of polymerization versus cracking reactions. For fast pyrolysis liquids and a devolatilization process carried out under the conditions of thermal analysis, secondary char retains about the half of the initial carbon content of the liquid. Moreover, high yields are obtained especially for liquids produced from cellulose, indicating the important role played by sugars and not only by the products of lignin decomposition. However, while the global devolatilization kinetics is provided [144,145], further experimentation is still need for an evaluation of the secondary char formation rate Outline of multi-step mechanisms of cellulose pyrolysis Given its large share in wood and biomass composition, cellulose pyrolysis is the subject of a significant number of studies. Reviews [53 56] are also proposed. The most recent modifications are discussed here of the mechanism proposed by Bradbury et al. [91] extended to include the reaction of tar cracking (Fig. 11), which is often coupled with transport equations. The first step, not detected by thermogravimetric systems and associated with the forma-

16 62 ARTICLE IN PRESS C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) tion of a molten-phase intermediate (active cellulose), is the subject of research studies [146]. Recent experiments [147] carried out for very short residence times (35 75 ms) show that it is possible to selectively depolymerize pure cellulose powder, which passes through a molten phase, to give significant yields of anhydro-oligosaccarides in the range C2 C7. The active cellulose decomposes via two competitive reactions associated with weight loss. The first is a depolymerization process, favored at high temperatures (high activation energy, Bradbury et al. [91]), and leading to the formation of volatile species (following the studies carried out by Shafizadeh et al. [148] and Shafizadeh [149], mainly levoglucosan), which may also undergo secondary reactions. The second pathway, favored at low temperatures, foresees the formation of char, carbon dioxide and steam. Extensive analyses on the products of cellulose pyrolysis and their dependence on temperature [ ] support the speculation that hydroxyacetaldehyde is a product of primary decomposition, specifically through a process of ring scission which becomes progressively more important at high temperatures. Furthermore, it is noted that when large amounts of hydroxyacetaldehyde are formed, the formation of char (favored at low temperatures and/or by the presence of ionic substances) is reduced. As a result of these studies, Piskorz et al. [151] propose a modified version (Fig. 12) of the mechanism by Bradbury et al. [91]. The initial stage takes into account the competitive formation of char (with carbon dioxide and steam), which is again favored by low temperatures, and a rapid Fig. 11. The mechanism of cellulose pyrolysis proposed by Bradbury et al. [91]. reduction in the degree of polymerization associated with the formation of active cellulose. Successively, a further competition is established between two pathways. The first is ring fragmentation (decarbonylation, dehydration) with the formation of hydroxyacetaldehyde (and other products including formic acid, acetic acid, glyoxal, methylglyoxal, etc.), which is favored by high temperatures and catalyzed by metallic compounds. The low-temperature process, favored by the absence of impurities in the substrate, is depolymerization by transglycosylation with the formation of levoglucosan, cellobiosan (and other sugars, such as glucose, fructose) in high yields. Possible decomposition of cellobiosan is indicated as the major route for formaldehyde production. More recent studies [ ] largely confirm the findings summarized by the conceptual mechanism by Piskorz et al. [151] and contribute in the determination of the kinetic constants for the formation of some compounds. Baniasz et al. [156,157] measure the release curves of formaldehyde, hydroxyacetaldehyde, carbon monoxide and carbon dioxide during the rapid pyrolysis of cellulose in the temperature range K. The yields of formaldehyde, hydroxyacetaldehyde and carbon monoxide are observed to increase as the reaction conditions are made more severe, whereas the yields of carbon dioxide decrease. Independent mechanisms are proposed for the interpretation of the different dynamics, but the estimation of the kinetic constants points out that inter-relations exist between the examined products. The proposed mechanism is reported in Fig. 13 together with the kinetic constants. The pathway leading to intermediates is the limiting step for the formation of formaldehyde and carbon monoxide, which are indicated as products of secondary reactions. Levoglucosan is considered the major contributor (with carbon dioxide) among the competing species for the formation of hydroxyacetaldehyde. The difference, with respect to the mechanism proposed by Piskorz et al. [151], Fig. 12. The mechanism of cellulose pyrolysis proposed by Piskorz et al. [151].

17 C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) Fig. 13. The mechanism of cellulose pyrolysis proposed by Banyasz et al. [156,157] (activation energies are expressed as cal/mol). is that the formation of hydroxyacetaldehyde occurs through the formation of reaction intermediates Distributed activation energy (DAE) models The chemical complexity of both the biomass and the related pyrolysis products motivate the introduction of kinetic models based on kinetic laws different from those presented above. A few cases, not considered in [58] and specifically formulated for biomass fuels, are examined here [128, ]. A DAE model is always proposed starting from treatments already used for coal. This approach avoids the low values of the activation energies which result when a single-step reaction is applied to fit a temperature dependence that arises from the occurrence of different reactions in different temperature intervals [128]. Chen et al. [159] combine a functional group (FG) model for gas evolution and a statistical depolymerization, vaporization and crosslinking (DVC) model for tar and char formation. The evolution of each FG is described by a first-order Arrhenius reaction with a DAE of width s. Measurements are made using thermogravimetric analysis coupled with Fourier transform infrared spectroscopy (TG-FTIR) for cellulose, wood and some biomasses. Compared with coal, the number of FGs is lower and, apart from methane, each gaseous species (tar, CO, CO 2, H 2 O) evolves in the form of a single peak. The estimated values of the average activation energies are of the order of kj/mol (in the case of cellulose up to 318 kj/mol). The agreement is good between measurements and predictions for moderate heating rates, whereas it becomes poor when the screen heater measurements by Nunn et al. [94] are considered. The same modeling approach and comparable values of the activation energies are reported in [160] for wood and Miscanthus pyrolysis, but the analysis is extended to 15 species, including problematic compounds such as HCN and HNCO (the release rate of nitrogen compounds, in biomass combustor models, is usually assumed to be directly proportional to the rate of solid devolatilization, although thermogravimetric measurements indicate a temporal lag between the two processes [48]), and makes use of one to three FGs for each species. Difficulties are again found for the correct predictions of the gaseous species release curves for different heating rates, probably in consequence of a lack of competition between tar and gas formation in the proposed model (comparable values of the kinetic parameters). Rostami et al. [161] modify the DAE model to facilitate its coupling with mathematical descriptions of transport phenomena. Although special care [160] may be taken in reducing the activity of secondary reactions, the results of the kinetic analyses discussed above combine the activity of both primary and secondary reactions in the estimated parameters. Specific for the secondary reactions is the DAE model proposed by Boroson et al. [128], to take into account the complex chemistry of tarry products and to extend the range of applicability of the proposed kinetics. This treatment assumes that each species is the result of a large number of independent parallel first-order reactions with invariant pre-exponential factor and activation energies described by a continuous distribution function. The estimation procedure assumes a constant pre-exponential factor (10 13 s 1 ) and produces an activation energy of 234 kj/mol (standard deviation 21 kj/mol and unreactive tar fraction 4.77%), which is about 2.5 higher than that of the single reaction previously considered. A DAE model is also used in [162] to evaluate the kinetics of tar cracking, based on measurements carried out with a fluidized-bed reactor with variable freeboard height (approximated by a plug-flow reactor) for cellulose, MSW and birch wood pyrolysis (particles of mm, temperatures in the range K and vapor residence times of s). However, the average activation energies (81 (cellulose), 73 (birch wood) and 89 (MSW) kj/mol) are much lower that than those reported by Boroson et al. [128] Conclusions and further developments One-component or multi-component mechanisms of primary pyrolysis have been proposed based on the analysis of experimental data on wood/biomass pyrolysis, obtained for isothermal conditions or fast heating rates. One-component mechanisms generally consist of three parallel reactions, as proposed by Shafizadeh and Chin [88], for the formation of the three classes of pyrolysis products: char, gas and tars (or liquids). The activation

18 64 ARTICLE IN PRESS C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) energies of the global pyrolysis rate lie within two separate ranges: and kj/mol, respectively. It appears that the former range of low values results from either severe heat and mass transfer limitations in the experiments, owing to high temperatures/heating rates, or simplifications in the reaction mechanism, that is, the use of a single global reaction for the description of the entire curves of weight loss. The second set of high activation energies is produced from measurements carried out under moderate thermal conditions and the description of the central part of the weight loss curves only, where the contribution of the cellulosic component is predominant. In this case a fast increase of the decomposition rate with temperature, as observed from the measurements, is predicted and therefore this set of data appears to be more appropriate for inclusion in transport models for the simulation of particle and/or reactor dynamics. The kinetic parameters for the formation rates of the three products classes from wood/biomass pyrolysis (onecomponent mechanisms) have also been estimated. In some cases even the qualitative trends, on dependence of temperature, are not predicted and the final char yields remain high, presumably again as a consequence of transport phenomena control or too low reaction temperatures. Only in two cases [97,99], a strong dependence of the yields of products on temperature is predicted and thus these kinetic parameters appear to be suitable, in principle, also for modeling fast pyrolysis processes. Another critical point of high-temperature experiments is that the yields of products, needed in combination with weight loss measurements for parameter estimation, may result from the activity of both primary and secondary degradation. The advantage of one-component pyrolysis mechanisms is that, when coupled with transport equations, both the yields of products and the decomposition rate (conversion time) can be predicted. However, the assumption of onecomponent behavior for composite fuels, such as wood and biomass, unavoidably produces inaccuracies in the details of the decomposition rates (and conversion time). The few multi-component mechanisms of biomass/wood pyrolysis, based on the description of different zones in the isothermal weight loss curves (for instance, [117]) or on the combination of multi-step mechanisms for the decomposition of the pseudo-components hemicellulose, cellulose and lignin (treatment proposed in [118]), have been formulated with the scope of improving these aspects. However, in the first approach, the product formation rates cannot be predicted for the different stages given that product yields can be measured only on an integral basis. In the second case, although the combination of multi-step kinetics for components is capable of predicting both conversion times and yields of products, the general validity of this approach is not supported by reliable and extensive experimentation and, on the other hand, the difficulties and inaccuracies associated with the use of component degradation rates are well known to the researches of this field. Multi-component mechanisms, for the large majority, simply describe the devolatilization process (the global devolatilization rate on dependence of time). That is, the final char yields should be known or assigned a priori and product distribution cannot be predicted. Usually, three parallel, first-order reactions in the amount of volatiles released from the pseudo-components hemicellulose, cellulose and lignin are considered. The analysis of single dynamic thermogravimetric curves assumes that hemicellulose and cellulose are associated with the shoulder and the peak of the rate curves, respectively, whereas lignin decomposes slowly over a very broad range of temperatures. The activation energies vary between 80 and 116 kj/ mol for hemicellulose, kj/mol for cellulose, and kj/mol for lignin. Analyses based on the simultaneous evaluation of thermogravimetric curves, obtained for several heating rates, needed to avoid compensation effects in parameter estimation, confirm only the kinetics for the cellulosic components. Although the comparison between different results is difficult owing to variations in the experimental conditions, mathematical treatment of the data, nature of the fuel and possible flaws in the measurements, it appears that the heating rate effects, when assuming first-order reactions, result in higher activation energies for the devolatilization of the pseudocomponents hemicellulose (147 kj/mol) and lignin (181 kj/ mol) [109]. Furthermore, the activity of the lignin devolatilization reaction is mainly explicated along the high-temperature (tail) region of the weight loss curves. Devolatilization mechanisms are also available where additional reactions are introduced, to describe volatile formation from minor components, such as extractives, or to take into account different steps in the volatile release from the chief biomass components, or power-law dependence on the mass fractions are assumed. It can be understood that the accuracy in the predictions of the weight loss characteristics is improved as the number of model parameters is increased. However, simplicity is always desired for the global reaction mechanisms especially in the view of inclusion in transport models. Secondary degradation of tar products has been observed [120,121] to take place according to two competitive reactions for the formation of permanent gases (cracking) and refractory condensable materials which, depending on pressure, temperature and flow rate conditions, may also lead, in addition to further gas, to (secondary) char formation. The large majority of kinetic studies disregards this competition and simply assumes a global cracking rate, proposing a convergence toward activation energies roughly comprised between 80 and 100 kj/mol. In a few cases, the recognition of different tar fractions with variable reactivity and the use of two or three reactions to describe the cracking process give rise to higher activation energies. Levoglucosan has often been used as a model compound (more recently also 5-HMF and hydroxyacetaldehyde) of tar to investigate secondary reaction chemistry. However, the understanding of the

19 C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) reaction paths of refractory tar and secondary char formation is still only qualitative. The chemical characterization of the gas and vapor phase products generated from pyrolysis of cellulose has also been used to formulate conceptual mechanisms for the formation of single products (levoglucosan, hydroxyacetaldehyde, hydroxy-2-propanone), starting from the proposal by Bradbury et al. [91]. The main findings, summarized by Piskorz et al. [151], are supported by more recent experiments and preliminary evaluations of the rate constants [ ]. However, no quantitative information is available for wood or biomass, even with reference to the main components of the pyrolysis liquid, for instance water, which is an important parameter for the quality of this product. Moreover, only a few attempts have also been made to develop DAE models. Although the performances of the mechanisms for primary and secondary pyrolysis can be fully evaluated after combination with the transport equations to describe applications at a laboratory or industrial scale, it can be concluded that, even for a kinetic control, only in a very few cases the correct trends are predicted. The understanding of the pyrolysis kinetics is essentially qualitative (for instance, in relation to the influences of some variables, such as, ash content/composition, pressure, gaseous environment and gas flow rate). The large scatter in the predictions is caused by the fuel type, the mathematical treatment of the data and possible errors in the measurements. Hence, extrapolations to conditions different from those of the experiments is highly questionable. Following these conclusive remarks on the biomass pyrolysis kinetics, aspects which deserve further investigation and/or consideration are related to the formulation of both (a) lumped-reaction mechanisms and (b) detailed reaction mechanisms taking into into account the dynamics of single-product species, valid for widely variable thermal conditions and biomass/wood varieties. While lumped kinetics, referred to product classes (either char, gas and liquid or, simply, volatiles and char), can be very useful for the design of different reactor types capable of achieving, for instance, fast pyrolysis or flash carbonization, detailed kinetics are needed for selecting optimal operating conditions apt to maximize the yields of specific compounds. Further efforts in both directions are highly desirable. To improve kinetic models, experimental data should be produced on the quantitative aspects of pyrolysis product yields and composition and their dependence on sample properties and conversion conditions. As for the lumped-reaction mechanisms, although the degree of detail may be determined by the final application, the validity of one-component versus multi-component mechanisms for the quantitative prediction of both the conversion time and decomposition rates should be assessed. Another aspect of great importance concerns the extension of multi-component mechanisms to predict product distribution. As for detailed mechanisms, significant experimental effort is needed to produce quantitative data on the chief tar components and their dependence on the reaction conditions and the wood/biomass species. This could provide the basis for the successive formulation of kinetics for tar component formation, for improving the quality of fast pyrolysis oils, and destruction, for the optimization of the gasification processes. The influences of the heating rate on the kinetic constants of the multi-component mechanisms or, in other words, the verification of the absence of compensation effects in the kinetic constants currently available are only partially addressed. Moreover, it is often pointed out that kinetic analysis does not use thermal conditions comparable with those of practical applications. This is a very critical issue. Indeed, to produce experimental data representative of the intrinsic kinetics of primary reactions at high temperatures, adequate consideration should be given to the separation between chemistry and transport effects, on one side, and between primary and secondary reaction processes, on the other. In reality, for correct measurement of high-temperature data, there could be a need to introduce innovative concepts in experiment design. Another aspect, often overlooked despite of the clear evidence shown by accurate investigations in this sector, is the presence of errors in the measurements even when carried out with commercial thermogravimetry. Kinetic analysis of pyrolysis/devolatilization processes has been mainly focused on wood species and, in a few cases, on agricultural residues. Thus, the search of general reaction schemes applicable for the different varieties of biomass fuels has not yet been extensively pursued. Given the wide variability among the chemistry of the different species and the influences of sample origin, this issue is of particular practical importance. The application of mathematical modeling of conversion systems also requires sub-models for the release of volatile nitrogen species and other minor compounds, such as chlorine, during pyrolysis. Simplified treatments, in transport models of biomass combustors, usually assume that these compounds are released together with gases and tar vapors. Further effort is needed on these aspects as results of experimental investigations have not yet been interpreted by kinetic models. 3. Transport models of biomass particle pyrolysis In practical conversion systems, wood or biomass conversion takes place as a result of a strong interaction between chemistry and transport phenomena at the level of both the single particles and the reaction environment. When exposed in a high-temperature environment, the particle is initially interested by transient heat conduction. Then the process of moisture evaporation occurs, which is highly endothermic. Depending on the initial moisture content, capillary flow of free water through the voids, bound water diffusion and convective and diffusive transport of water vapor are controlling. Successively, the already dried portion of the particle, in the neighborhood

20 66 ARTICLE IN PRESS C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) of the heated surface, undergoes thermal degradation. When all the volatile species are removed from the solid, a char layer is formed. Therefore, the following spatial zones appear during the process transients: an inert char layer, a pyrolysis region, a drying region and the virgin moist solid. Water vapor and volatile pyrolysis products, partly leave the particle flowing across the heat-exposed surface. A fraction may also migrate toward low-temperature regions, where re-condensation may occur. The flow of products, owing to larger permeabilities, mainly occurs toward the heated surfaces and, because of the high temperature, secondary reactions of tar degradation may occur. In addition to heat, momentum and mass transfer, changes in the physical structure of the reacting solid are observed with the development of a network of cracks in the already pyrolyzed region, surface regression, internal shrinkage and/or swelling and, in some cases, (primary) fragmentation. The reaction environment affects the particle pyrolysis, essentially through the rate of external heating, and also the final yields of products. Indeed, secondary reactions of primary tar vapors also take place outside the wood/biomass particle given temperature and residence times sufficiently high. The most important achievements in transport models of wood/biomass pyrolysis, which couple the chemical kinetics with mathematical descriptions of physical processes are reviewed in [16,17] and, with special emphasis on material flammability characteristics, in [163]. Papers published afterwards, which include innovative features in relation to model development or present significant improvements in the understanding of wood pyrolysis processes, are reviewed here. Transport models for single wood particles/logs assume that the porosity is fine and uniformly distributed so that the material is a homogeneous medium, where gas and solid are in good thermal contact. The detailed models couple kinetics of primary and secondary reactions for the three classes of lumped products with the description of the relevant physical processes. Transport models are also available consisting of more simplified kinetics with descriptions of transport phenomena using different degrees of approximation. In contrast with the assumption of volumetric decomposition rates, a further category of simplified models uses the unreacted-core-shrinking treatment, where decomposition takes place at an infinitely thin surface with either infinite- or finite-rate kinetics. Finally, the empirical formulae for the conversion time and the parameters of apparent global kinetics, useful for the design of conversion units, are briefly examined. The numerical simulation of biomass pyrolysis, often controlled by intra-particle heat conduction, usually does not present specific computational problems. Standard numerical methods, for instance finite-difference schemes which, in addition to the properties of convergence, consistence and accuracy, guarantee the absence of phase and amplitude errors (causing unphysical oscillations or instabilities), also preserve conservation of mass, momentum and energy of the discretized version of the conservation equations. Critical conditions, which require a careful selection of time and space steps, may be shown by fast pyrolysis, where gradients of temperature, chemical species and flow velocity become very high Transport models with volumetric decomposition rates Detailed models couple mechanisms of primary and secondary reactions for the three main product classes (liquids, gas and char) with the conservation equations of mass, momentum and energy. This model category includes the work presented in [77,118, ]. The onecomponent mechanism of primary wood degradation, based on three parallel reactions for the formation of primary pyrolysis products, as proposed by Shafizadeh and Chin [88], is generally used in the transport models where predictions of product yields are of interest. The kinetic constants estimated by Thurner and Mann [92] are used in [166,178,179,181], those by Chan et al. [14] in [16,170, 172,176], those by Di Blasi and Branca [99] in [180], those by Wagenaar et al. [97] in [177]. A comparison is provided in [166] between the kinetic constants estimated in [14,92,95], whereas different description of chemical kinetics are also considered in [173]. A three-component mechanism, accounting for the competitive formation of tar and linked gas and char, is used in [167]. Given its important contribution in wood composition, cellulose is considered as the starting material in several cases [77,165,167,168,171], where the chemical kinetics are described as in Bradbury et al. [91]. Secondary reactions of tar cracking are generally described according to the kinetics by Liden et al. [127], but several rate constants are compared in [180]. Tar polymerization is modeled in [167,178,179,181] with kinetic constants as in [182]. Associated with primary and secondary degradation, enthalpy variation takes place. Janse et al. [177] assume that the formations of the primary products from wood pyrolysis are endothermic processes, whereas in [118,167,174,180] primary char formation is an exothermic process and tar formation/vaporization is an endothermic process. Secondary tar cracking is always modeled as a weakly exothermic process. In the Bradbury et al. [91] mechanism, the formation of the active intermediate, which is however not rate-limiting, is described as an isothermal process [77,165,168]. Some transport models make use of simplified kinetics of devolatilization or decomposition without secondary reactions. These include the works of Refs. [85, ]. A one-step pyrolysis reaction is used with assigned yields of volatile and solid products and guessed values of the kinetic parameters in [183,184,186,188,190] or with different sets of kinetic constants referred to the cellulosic component in [192]. Two parallel reactions for the formation of volatile species and char are proposed in [191], with parameters previously determined [194] and representative of apparent kinetics. Bilbao et al. [85,189]

21 C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) use kinetic constants for the devolatilization process extracted from thermogravimetric analysis [195,196]. These foresee a low- and a high-temperature step, with a demarcation temperature of 563 K. The first endothermic step, associated mainly with cellulose devolatilization, presents a kinetic constant independent of temperature but affected by the heating rate. The second exothermic step is related to lignin decomposition and presents the usual Arrhenius dependence on temperature. Saastamoinen and Richard [187] use a global decomposition reaction, where the yield of char is assigned by means of an empirical correlation to account for the temperature effects. Modifications in the kinetic law would require, as pointed out by the authors, a congruent evaluation of the kinetic parameters. However, the estimations provided by Nunn et al. [94] are used. Boutin et al. [193] disregard char formation in the mechanism by Bradbury et al. [91] of cellulose pyrolysis, to describe pellets behavior under fast external heat transfer rates Intra-particle transport phenomena In the description of intra-particle transport phenomena, the majority of the models [77, , , ,184,185,187,189, ] uses a one-dimensional system with the following assumptions: local thermal equilibrium, perfect gases, negligible kinetic and potential energy and replacement of internal energy with enthalpy, negligible enthalpy flux due to species diffusion and body forces. Sometimes diffusive transport of species within the pores and accumulation of energy in the gas phase [178,179,181,185,190] and pressure variations [185,187,189, 190] are neglected. The most advanced models take into account the following physical processes [77,118, , 183,184,188]: heat transfer by convection, conduction and radiation, convective transport of volatile species, gas pressure and velocity variations. The latter variables are described by means of the Darcy law, except in [118,167], where the conservation equations for the gas phase momentum are written assuming that the gaseous mixture flows through individual channels (pores) within the particle. Convective transport of enthalpy is simplified in [189] by means of a global term at the particle surface using a finite-difference balance, to express the boundary condition. The assumption of one-dimensional system is removed in a few cases [85,171,175,183,184,186,188,191]. These are simplified models with the exception of the model presented in [171] which couples the kinetics of primary and secondary reactions with the description of the chief heat, mass and momentum transfer processes. Thermal conductivity and permeability along the two directions are assigned so as to reproduce those parallel and perpendicular to wood fibers, respectively. The same features in the description of physical processes are presented by the models developed in [183,184,188]. Bilbao et al. [85] consider a two-dimensional unsteady equation for enthalpy convection and conduction, formulated for constantvolume (spherical) particle at a constant pressure. In other cases, a simple unsteady two-dimensional heat conduction equation is introduced [186,191]. To take into account the effects of wood anisotropy on heat transfer, a simplified description of convective transport along the wood fibers is also proposed in [175] in conjunction with a comprehensive description of the processes for the perpendicular direction. Volume variation is modeled in relation to the shrinkage of the particle [166,172,173,175, ,191]. A general model is proposed in [166] and used in [172,173,175,180]. Equations describing the time evolution for the volume occupied by the solid and the gas are written. It is assumed that the volume occupied by the solid decreases linearly with the wood mass and increases with the char mass, by a chosen shrinkage factor, a, as devolatilization takes place. Hence, the volume occupied by the gas is made by two contributions, the first due to the initial volume occupied by volatile species and the second by the fraction, b, of volume left by the solid as a consequence of the devolatilization process. In order to account for possible structural changes during devolatilization, the initial volume of volatiles may also vary linearly with the composition of the degrading medium, from an initial value, determined by the initial solid porosity, to a final value taken as a fraction, g, of the initial one. The parameters a, b, g, which vary from 0 (total disintegration of the particle) to 1 (no shrinkage), should be assigned. In [178,181], shrinkage is defined as the ratio between the current and the original size of the computational cell of the one-dimensional integration domain. It is assumed to vary linearly with the composition of the solid and the final size of the sample should be assigned. In the twodimensional cylindrical domain modeled in [191], particle shrinkage is made to occur simply by varying the size of the elementary control volumes of the discretized integration domain in proportion to the averaged conversion, without modifications in the formulation of the conservation equations. Three models are proposed corresponding to uniform shrinkage, shrinking shell and shrinking cylinder, which produce the shrinkage factors along the radial and axial directions. The models differ in the way they average the conversion. The final shrinkage is always calculated by means of an empirical formula obtained considering data for cylindrical particles ( mg) exposed to furnace temperatures of K. Bharadwaj et al. [192] examine the effects of particle shrinkage according to two limit approximations corresponding to constant volume (no shrinkage) and constant density (shrinkage proportional to the mass of volatile products). The model equations do not include any term related to volume variations and thus it is likely that variations in the grid size are used to describe this process. Volume variation should also be accounted for in the case of ablative pyrolysis. Di Blasi [168] describes char ablation and the chief transport phenomena through the porous solid and the molten layer (adjacent to the hot plate) whose properties and size vary during the thermal

22 68 ARTICLE IN PRESS C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) degradation. The early stages of cellulose pellet pyrolysis subjected to image furnace heating (high, controlled heat flux intensities delivered for assigned period of time) are modeled in [193] by describing heat conduction through the solid and the liquid phase and neglecting char formation. Models of high-temperature evaporation of moisture in wood are available. In some cases, a detailed description of these processes is made [197], also coupled [16,170,172, 173,175] with the description of the main processes of wood pyrolysis. The mathematical formulation is based on the conservation equations for enthalpy, mass and momentum for the solid, the liquid and the gas phase. Phenomena of moisture transport include water vapor convection and diffusion and capillary water convection in the pores of the particle, and bound water diffusion in the solid wood. Momentum transfer, for the liquid and the gas phase, is described according to the multi-phase Darcy law. Local thermodynamic equilibrium is assumed, with a sorption isotherm that couples the moisture contents in the solid and the gas phase. In simplified models of moisture evaporation, the transport phenomena of liquid-phase water and vapor diffusion are assumed to be negligible [85,179, , 189]. In[183,184] the moisture evaporation rate is derived from the saturation vapor pressure, whereas Bilbao et al. [189] adopt the treatment originally proposed in [198]. The drying rate is controlled by heat supply and takes place at the normal boiling point of water or, for low moisture contents, at a temperature as empirically determined. In this way, the vapor pressure depression is treated as a rise in the moisture boiling point, defined as the temperature at which moisture is in equilibrium with water vapor at atmospheric pressure. An Arrhenius law temperature dependence of the moisture evaporation rate is proposed in several cases [179,186,199]. In[179], moisture evaporation begins only for temperature above 368K and the corresponding rate is approximated by a first-order Arrhenius law. The activation energy is the same as proposed in [14] whereas the corresponding pre-exponential factor is increased by a factor of 10 4 to provide a plateau between 373 and 393 K in the case of thick wood. Re-condensation of water vapor is modeled assuming supersaturated state conditions with a condensation rate proportional, through an empirical parameter, to the flow rate. In [199], the pre-exponential factor of Ref. [14] is again modified but re-condensation is not taken into account. This approach allows for a finite thickness of the evaporating region and also eliminates the complications in the numerical solution [197], due to the presence of the unknown moisture evaporation rate with an empirical expression for the vapor pressure, instead of an evolution equation or a simple production term. The main qualitative features of the process are also retained, that is, the temperatures rises continuously into and out of a drying plateau [179]. Moisture evaporation takes place at an infinitely thin front at constant temperature in the transport models of wood pyrolysis proposed in [85,187,192]. The process is controlled by heat transfer, that is, the heat transferred at the front is entirely used for moisture evaporation. The evaporation temperature is assumed to coincide with the normal boiling point of water or with a close value. In particular, in [192], when locally (at a certain control volume of the computation grid) attained, this temperature is assumed to remain constant as long as the moisture content is different from zero. The steady-state formulation of the enthalpy conservation equation is used to evaluate the evaporation rate. This treatment avoids the numerical complications due to the assumption of an infinitely thin front of moisture evaporation at constant temperature. On the other hand, it does not introduce any special complication in unreacted-core-shrinking models of wood pyrolysis, as discussed in the following. An important aspect of transport models is represented by the description of physical properties and their variations during the conversion process. A comprehensive review is given by Gronli [16] of correlations and values of the thermal conductivities, gas permeability, specific heats and other properties of several wood species and char. In the majority of transport models, thermal conductivity, mass diffusivity and permeability are assumed to vary linearly with conversion between the values for the virgin solid and the char (for instance, [16,77, ]). Effective values are used for the thermal conductivity, which also includes a radiative contribution dependent on temperature. In a few cases, correlations explicitly incorporating a temperature dependence are used [183,184]. Specific heats for the virgin solid and the pyrolysis products, and dynamic viscosity are assumed to vary with the temperature [170, ,183,184] or to remain constant. Modeling results of inert char heating [200] show that a better agreement between predicted and measured temperature profiles is obtained when a constant value of the thermal diffusivity is used. A possible compensation is suggested between the simultaneous increase in the specific heat and the effective thermal conductivity with temperature. The void fraction is assumed to vary with the solid density [178,179,181], the conversion [77, , ] or to remain constant [192]. Primary fragmentation of wood particles is attributed [201,202] to the pressure build-up, when the rate of volatiles production internally is faster than their escape rate through the pores in the charred wood. A mechanical model for this process is available [202]. The heating rate, the rate and heat of reactions and the thermophysical properties are indicated as the factors responsible for the details of structural behavior. At both low and high temperatures, the pressure peaks before the center temperature exceeds the external temperature. Then, particle break-up suddenly occurs and the pressure drops quickly to the ambient value. It is worth noting that fragmentation of coal particles, fed to fluidized-bed combustors, is also concentrated around the end of the devolatilization regime [203]. It is plausible that, despite the slow devolatilization

23 C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) rates established for the inner core of the particles, the outward flux rates of volatile products are even slower, owing to the presence of a thick char layer, creating in this way the conditions for the pressure to increase. On the other hand, as overpressures attain a maximum at near complete conversion, it can be postulated that for a large part of the conversion process the particles are not highly affected by structural failure. On the contrary, the effects of primary fragmentation are important for the subsequent oxidation of char as reported for both coal [203] and wood [204,205] External heat transfer coefficients The boundary conditions, at the particle surface, can be used to describe the effects of external heating rates established in different reactor configurations. However, in several cases, the transport models are applied to simulate conversion of particles exposed to hot gases and/or radiative heating, where external conditions are assigned so as to describe bench-scale systems without reference to any specific conversion unit. More precisely, the majority of the models [16,85, ,169,170,172, 173,175,176,178,179,181] examines the conventional pyrolysis of particles or logs exposed to thermal radiation. To evaluate the Biot number, a radiative external heat transfer coefficient is introduced [178,179,181] as h rad ¼ fs (T s +T N )(T s 2 +T N 2 ), where the surface temperature T N is a function of time. However, evaluations can be made for steady conditions (char heating) assuming, for simplicity, that T s and T N are coincident. This leads to maximum heat transfer coefficients between 80 and 230 W/m 2 for temperatures in the range K. Convective heat transfer is comparatively much smaller and described by coefficients of the order of 5 W/m 2 or using the Ranz Marshall correlation [206]. In a few cases [85,173,183,184], the measured temperatures are used as boundary conditions at the solid surface. Several applications deal with the fast pyrolysis of cellulose or wood, that is ablative pyrolysis of cellulose pellets subjected to a high-pressure contact against a hot spinning disk [168] or to concentrated radiation [193], and conversion in fluidized-bed reactors of cellulose pellets [174] and wood particles [177,180]. The conditions typically established in pulverized coal/biomass burners are investigated in [192], where millimeter-sized biomass particles are exposed to rapid heating and high temperatures. This last feature introduces a significant difference with fast pyrolysis systems. As the external heat transfer coefficient is the most important parameter for the design of pyrolysis reactors, especially for fast pyrolysis processes, specific investigations are available for ablative pyrolysis [207,208], the Pyrovac process [209], the rotating cone reactor [210], not to mention the numerous studies for fluidized-bed reactors (for instance, [206,211]). For contact (ablative) pyrolysis the global external heat transfer coefficient is found to be proportional to the applied pressure [207,208], with values in the range W/m 2 K as simulated in [168] by the transport model for cellulose pellets pressed against a hot spinning disk. The Pyrovac process is based on a moving and stirred bed reactor using an eutectic mixture of high-temperature molten salts as heat carrier and a sophisticated agitation device to enhance heat transfer between the feedstock and the heating plate. During pyrolysis, the feedstock is heated under vacuum to a temperature of about 770 K. The range of estimated values [209] for the heat transfer coefficient is W/m 2 K (versus 5 30 W/m 2 K of static bed reactors). Agitation speed, properties of the bed and particle movement are the chief parameters affecting the heat transfer process. This model has not yet been coupled with the description of transport phenomena and chemical reaction of vacuum pyrolysis of biomass. The external heat transfer coefficient for particles flowing along the surface of a rotating cone reactor is evaluated in [210]. For biomass particles (without sand) the values are in the range W/m 2 K and an improvement up to 1500 W/m 2 K is reported when sand is also supplied. The rotating cone frequency (variations in the contributions of gas-phase convection and wall heat transfer rate) and size of the particle (variations in the particle flow pattern over the conical surface) are the key variables. Fluidized-bed reactors allow for very high heat transfer rates between the gas and the solid, as a result of the high surface area of the particle phase. It is widely recognized that, for large particles, convective heat transfer between the two phases may become controlling [212] and that the low biomass thermal conductivity introduces significant internal heat transfer limitations even for very small particle sizes [174,180]. Therefore, isothermal conversion at the temperature of the surrounding environment and chemical reaction control are not established in practical situations. Numerous correlations are proposed for the heat transfer coefficient between fluidizing gas and beds of uniform particles [206] and for fixed tubes of diameter much larger than the bed particles [206,212]. These are of interest for thermochemical conversion processes, given that the active particles are larger (and with different density) than the inert bed particles. Also, correlations for heat transfer to large mobile particles in fluidized beds are reported especially in relation to coal conversion (reviews are reported in [211,212]), based on the assumption that they reside only in the emulsion phase. In reality, the bedto-surface heat transfer is an unsteady process and instantaneous measurements show sharply varying values, suggesting that the object surface is being bathed alternately by gas bubbles (low values) and emulsion packets (high values) [206]. The unsteadiness of the heating process is enhanced for active particles as they are not fixed. Active particles show a circulation pattern which causes heat transfer with the emulsion phase and at a certain extent with the bubble phase. In addition, the average contact

24 70 ARTICLE IN PRESS C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) time with the emulsion phase (and the heat transfer rate) during ascent is different in comparison with that during descent. The model for the average heat transfer coefficient developed by Agarwal [211] takes into account all the features listed above and is used to simulate the fast pyrolysis of cellulose and wood in a fluidized bed [174,180]. It expresses the overall heat transfer coefficient, h, as the sum of three components, h pc, the particle convective component during ascent (a) and descent (d), the gas convective component, h gc, and the bubble component h bub, averaged according to the probabilities of the particle to reside in the emulsion phase during one circulation or to be rising during one circulation, p and p 0, respectively. The assumptions and the resulting equations, based the two-phase flow theory, are the same as those in the original paper [211]. The model predictions [211] compare well with experimental data for the average heat transfer coefficient, although improvements could be probably obtained by taking into account the variation of the bubble parameters within the bed and with a more accurate description of the movement of the particle. In general, the gas convective component is of importance only for beds of large group B and group D materials [212], while for group A and small group B powder the particle convective contribution dominates. In addition to the Agarwal model [211], two further models are considered to describe cellulose or wood pyrolysis in a fluidized-bed reactor [174,180]: the Ranz - Marshall correlation which is representative of the convective heating of a single particle, and the whole bed coefficient, corresponding to a bed of particles heated by a hot gas stream [206]. For the wood particle characteristics typical of fuidized-bed reactors (particle half thickness between 0.1 and 5 mm and bed temperature of 800 K), the following range of values for the global heat transfer coefficient is evaluated [180]: W/m 2 K (Ranz Marshall correlation), W/m 2 K (whole-bed correlation), W/m 2 K (the Agarwal model). Janse et al. [177] use a global heat transfer coefficient equal to 500 W/m 2 K. Furthermore, the assumption of an infinitely fast external heat transfer rate is also examined for comparison purposes [177,189]. Finally, effects of the efflux of volatiles generated as a consequence of the degradation process on the heat transfer to the particle are taken into account through the introduction of a correction factor. In Bharadwaj et al. [192], the particle surface exchanges heat with the external environment through convection and radiation. A global mass transfer coefficient is also used for the volatile species equations. Empirical correlations are applied for such parameters suitable for small-sized particles and the thermal conditions of commercial coalfired boilers Extra-particle processes Extra-particle processes are not taken into account except for the simplified treatments proposed for an updraft gasifier [185], a fluidized-bed reactor [174,180] and an entrained-flow reactor [192]. Also, Miller and Bellan [118,167] describe extra-particle processes to simulate conversion in an infinite domain. In the simplified model for the pyrolysis of a moist wood particle in an updraft gasifier [185], the effects of the reaction environment are taken into account by means of the boundary conditions at the particle surface where heating occurs essentially by convection. The external temperature is obtained from the energy equation taking into account the heat transported by the gas flow and the heat exchanged between the gas and the particle surface. The temperature of the gas at the inlet section of the drying/pyrolysis zone of the gasifier should be assigned. The effects of particle shrinkage are incorporated only in the calculation of the height of the reaction zone. Extra-particle processes for fluidized bed reactors [174,180] are described according to the following assumptions: (1) all the particles experience the same thermal (and conversion) history, (2) the organic fraction of liquids produced undergo extra-particle cracking according to an apparent residence time as defined by Scott and Piskorz [18,19] and Scott et al. [20,21], or to a residence time evaluated with reference to the expanded bed height and the bubble velocity, (3) secondary reactions occur at the bed temperature. The model of Ref. [192], for conversion in an entrained flow, considers a particle traveling through a one-dimensional plug flow reactor, where the assigned temperature and the velocity profiles are used to evaluate the external heat and mass transfer coefficients. For the spherical geometry considered in [118,167], a fictitious sphere is introduced whose radius is from 5 to 10 times larger than that of the particle. These are a measure of the distance from the sample at which the computational boundary must be placed in order to correctly simulate particle pyrolysis in an infinite domain. The conservation equations are valid for the entire integration domain which takes into account the spatial variation in the medium properties. However, the schematization of a particle undergoing pyrolysis in an infinite domain does not reproduce the experimental conditions established either at laboratory scale or in practical chemical reactors Unreacted-core-shrinking models Another approach used in wood pyrolysis modeling is based on the use of the unreacted-core shrinking approximation usually associated with the assumptions of no moisture content and one-dimensional system [ ]. Degradation is described by a one-step reaction, where the ratio between the yields of char and volatiles (composition resulting from the activity of both primary and secondary reactions) is constant. The reaction takes place at an infinitely thin surface which propagates from the surface of the particle toward the center. It is the moving boundary between the completely charred and the virgin solid

25 C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) (unreacted-core-shrinking approximation), where the solid properties (density, porosity, thermal conductivity, etc.) vary from the initial virgin wood values to the final char values. Both infinite-rate kinetics [ ], that is, a constant pyrolysis temperature, and finite-rate kinetics [213, ], based on the usual Arrhenius law and a linear dependence on the solid density and the surface area, are considered. The thickness of the reaction zone is proportional to l DTq c [213] (l is the thermal conductivity, DT a characteristic temperature difference across the reaction front and q c the heat flux reaching the reaction front). As l and DT are determined by the nature of the fuel (l is a fuel property and DT is of the order of 200 K, given wood pyrolysis temperatures between 550 and 750 K [99]), high applied external heat fluxes appear to be a condition necessary for the validity of this treatment. In the dynamic evolution of the process, different stages exist, which should be properly modeled. The sample, initially at ambient conditions, is exposed to radiative/ convective heating. Hence, a pre-heating stage occurs, with negligible activity of the degradation reactions and an extension of the unreacted core (and integration domain) coincident with the sample thickness. This is followed by the reacting stage, where the unreacted shrinking core is surrounded by a char layer. The passage from the preheating to the reacting stage is assumed to occur, for instance [218] when the thickness of the char layer equals a small assigned value. From here on, the integration domain is divided into two zones, the char layer and the unreacted core. The special techniques needed to track infinitely thin fronts [221] may become computationally complicated and expensive. On the contrary, a uniform temperature profile for the unreacted core associated with a quasi-steady char zone [213] or the use of integral solution methods for the heat conduction problem [ ] are particularly attractive because they simplify the mathematical model from partial to ordinary differential equations. In the integral method, the temperature profile is assumed to be a known function of the spatial coordinate, chosen so as to satisfy the boundary conditions. It is then substituted into the enthalpy equation which, upon integration with respect to the space variable, reduces to an ordinary differential equation. Different treatments, including polynomial and exponential profiles, are reported in the literature. However, a quadratic profile is observed to describe well not only several moving boundary problems but also experimental data obtained for standard fire resistance test conditions and wood particle conversion [218]. In this case, a parabolic profile is used for the one-dimensional unsteady enthalpy equation written for either the char layer or the unreacted core. Some of the unreacted-shrinking-core models are proposed for fire safety engineering, thus there are several specific features and limitations which do not allow their straightforward application in the biomass thermochemical conversion sector. These include: the geometry of the solid (a slab [215], generally with infinite thickness [214, 216,217]), the completely absent [213,214] or very limited [215] experimental verification, the lack of sensitivity analysis on model assumptions and parameters. In contrast, the model presented in [218,220] is specifically developed for biomass thermochemical conversion and thus takes into account the effects of finite sample size. Aside from a more accurate description of pyrolysis processes (convective, conductive and radiative heat transfer, finite-rate kinetics), it uses realistic input data and is experimentally validated in relation to mass loss characteristics and conversion times. The unreacted-shrinking-core model, originally developed for dry wood [218,220] is also modified to include the effects of moisture evaporation [219]. This process is described assuming that (1) evaporation takes place at an infinitely thin, constant-temperature front, which separates the moist from the dry region; (2) the evaporation temperature at the drying front coincides with the normal boiling point of water; (3) the moist core of the particle is at ambient (initial) temperature; (4) the heat flux at the drying front is applied exclusively for raising the surface temperature, from the initial to the evaporation value, and sustaining the endothermic evaporation process Simulation results Detailed particle models are extensively applied to simulate the effects of sample properties and heating conditions on the pyrolysis characteristics. More specifically, parametric analysis is focused on the role of model assumptions [77,85,165,175,192], physical properties, such as particle size, initial moisture content, shrinkage, transport parameters, and operating conditions [164, ]. Simplified models, based on either volumetric reaction rates or a reaction located at an infinitely thin surface, are also extensively used for numerical simulation. In the latter case, attention is mainly focused [218,219] on mass loss characteristics, as these are the key variables of interest when considering the coupling of particle and reactor models. On the other hand, it cannot be expected that this simplification can provide quantitatively accurate temperature and concentration profiles. The main results are discussed below for both conventional and fast pyrolysis. The dynamics of wood particle/log conversion are examined by all the transport models cited above. The qualitative features for one-dimensional systems remain the same as already discussed in a previous review [17]. Numerical simulation of two-dimensional particle dynamics [171,183,184] shows that the propagation of pressure and reaction fronts from the heat-exposed surface toward the inner core of the particle is highly affected by the anisotropic structure of wood. These features clearly appear from the example of process dynamics shown in Figs. 14A D (temperature color maps), Fig. 15A D (pressure color maps) and Fig. 16A D (vector velocity

26 72 ARTICLE IN PRESS C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) Fig. 14. Color maps of temperature for a particle (half thickness equal to 5 mm) exposed to an external temperature of 900 K as simulated in [171] for times of 31 s (A), 63 s (B), 93 s (C) and 125 s (D). field) as simulated in [171] for the cross section of a particle exposed to a high-temperature environment. In this case, a comparison is also made [171] between the total heat transferred to the virgin solid (conduction minus convection) along and across the grain. Despite the lower thermal conductivities, owing the concomitant slower convective transport (lower gas permeabilities), the largest contribution is that across the solid grain. The role played by convective heat transport becomes successively less important as the particle size is increased. A comparison between the two-dimensional and the one-dimensional simulations shows that the multi-dimensional structure of the reaction zone affects not only the details of sample conversion dynamics, but also global parameters, such as conversion time and product distribution. On the average, the process is faster and the volatiles yields larger for the two-dimensional configuration. Further results of the comparison between the two-dimensional model [171] and a one-dimensional pure heat conduction model are presented in [222,223]. Conversion regimes are defined for particles exposed to radiative heating, based on either limit values for the total solid residue [77] or the Biot number [181]: the pure kinetic regime (chemical time much longer than heat transfer times), the thermally thin regime (internal heat transfer rates much faster than external heat transfer rates), the thermally thick regime (comparable values for the internal and external rates of heat transfer) and thermal wave regime (internal heat transfer rates much slower than external heat transfer rates). In the presence of moisture [181], drying and pyrolysis occur in series (thermally thin regime), slightly overlap (thermally thick regime) or occur simultaneously through a large part of the conversion process (thermal wave regime). In general, the overlap between the two processes is enhanced as successively higher initial moisture contents are considered [181,187]. Given that the separation between char and virgin solid, on one side, and between drying and pyrolysis, on the other, is absent or scarce, the initial moisture content and shrinkage essentially affect only the conversion time for the first two regimes. The effects are comparatively much higher for the thermal wave regime, where the impact on the product yields is also important. These results are also confirmed by the simulations obtained with an unreacted-shrinking-core model [219], as shown by Fig. 17A, where the thickness of the dry wood layer is plotted as a function of time for several particle sizes and two initial moisture contents. In all cases, it initially increases to attain a maximum, as consequence of the faster propagation speed of the evaporation front with respect to the decomposition front. Then, as moisture evaporation terminates and the decomposition front moves

27 C. Di Blasi / Progress in Energy and Combustion Science 34 (2008) Fig. 15. Color maps of gas pressure (atm) for a particle pyrolysis as simulated in [171] for the same conditions and times of Fig. 14. toward the particle center, it rapidly goes to zero. The dry wood layer also becomes successively thinner as the particle size or the initial moisture content are increased, that is, moisture evaporation becomes more strongly coupled with wood decomposition. For the strongest coupling (initial moisture contents of 50% and particle radius of 100 mm), the dry wood layer only varies between about 5 and 20% of the initial particle radius (R 0 ). This finding supports the assumption of a single infinitely thin front where both moisture evaporation and wood decomposition take place. This simplification is clearly not applicable for thin particles (for instance, the case of R 0 ¼ 20 mm), where the maximum thickness of the dry wood layer may attain values of about 50% of the initial particle radius. For thick particles (R 0 ¼ 100 mm), the assumption of a single evaporation/decomposition front becomes progressively less accurate as the heating conditions are made less severe, as shown in Fig. 17B (the dry wood layer as a function of time for several heat fluxes and two moisture levels). Model results can also be used to construct maps [77,181], where the main features (regime) of the pyrolysis process can be read, for instance, on dependence of sample size and final temperature. However, it should be noted that they are valid only for the specific material properties, mode of external heating and reaction kinetics used in the numerical simulations. Extensive simulations are also used to evaluate the effects of the initial moisture contents on the conventional pyrolysis of wood based on descriptions of the evaporation process more accurate [16,167,176,183,184,197] than the first-order Arrhenius law or the infinitely thin evaporation front at constant temperature examined above. In all cases it is found that moisture evaporation occurs at temperatures near to the normal boiling point of water. Water vapor is driven toward the cooler zones and condenses. This is a local process which takes place over a distance of a few millimiters in the zones with temperatures around 373 K. For the conditions of interest in thermochemical conversion (external temperatures well above the boiling point of water) it is found [197] that liquid phase processes are not controlling. Moreover, the thickness of the evaporation zone is relatively thin, gas overpressures in the wet region are significant and gas phase convective transport is important. These results and the consideration that the parameters for the drying process are determined for conditions quite different from those of interest in practical systems [178] support the application of simplified models of moisture evaporation during wood pyrolysis. The role of some model assumptions is discussed for conditions of internal heat transfer control using detailed transport models. It is shown that, for permeability values typical of cellulosic materials, the assumption of constant

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