Investigation of the cigar burner combustion system for baled biomass

Transcription

1 biomass and bioenergy 58 (2013) 10e19 Available online at ScienceDirect Investigation of the cigar burner combustion system for baled biomass B.S. Repic a, *, D.V. Dakic b, A.M. Eric a, D.M. Djurovic a, A.D. Marinkovic a, S.Dj. Nemoda a a University of Belgrade, Vinca Institute of Nuclear Sciences, Laboratory for Thermal Engineering and Energy, P.O. Box 522, Belgrade, Serbia b University of Belgrade, Innovation Center, Faculty of Mechanical Engineering, Kraljice Marije 16, Belgrade, Serbia article info Article history: Received 14 June 2011 Received in revised form 20 August 2013 Accepted 11 October 2013 Available online 26 October 2013 Keywords: Biomass Combustion Boiler Straw Cigar burner Modelling abstract Biomass is deemed to be the main source of renewable energy in Serbia. Over the past couple of years, considerable efforts have been made to develop a technology which would enable biomass bales of various sizes and shapes to be used for energy production. A hot water boiler with cigarette type of combustion was constructed and used in the experimental investigation of biomass combustion phenomena. During the experiments performed, numerous parameters were measured: flue gas temperature, water temperature at the boiler inlet and outlet, while O 2,CO 2, CO, SO 2, and NO x content in the flue gas was measured at the boiler outlet. Experiments were performed with biomass feed rate of 0.12 kg/s, mean boiler output of 1.56 MW and mean excess air coefficient of 2.1. During the steady state boiler operation, exhaust gas temperature was measured to be around 150 e160 C and obtained CO and NO x emission rates were found to be quite acceptable. In addition, combustion of biomass bales in cigar burners was modelled by the means of appropriate numerical simulation. A good agreement between experimental and numerical results was obtained. ª 2013 Elsevier Ltd. All rights reserved. 1. Introduction In 2011, the average annual energy consumption in the Republic of Serbia was around 15 million tons oil equivalent (Mtoe), out of which 7.4 Mtoe represents the net consumption and 3 Mtoe is electricity consumption. Energy generated by coal combustion accounts for 7.9 Mtoe (52%) of the total annual energy consumption, 4 Mtoe (27%) is provided by liquid fuel combustion, 2.1 Mtoe (14%) by natural gas combustion, while 1 Mtoe (7%) is provided by hydropower utilization [1]. Serbia s potential in renewable energy sources equals approximately 4.3 Mtoe/year (2.7 Mtoe biomass, 0.6 Mtoe small hydropower, 0.2 Mtoe geothermal energy, 0.6 Mtoe solar energy and 0.2 Mtoe wind energy). The specified biomass energy potential mainly represents the potential of agricultural biomass, which accounts for 60% of total biomass potential, while the remaining 40% is the potential of forest biomass [2]. Agricultural biomass is mostly available as biomass bales. There are only few boilers and furnaces burning biomass bales in Serbia, all characterized by very poor performance. The main cause of this situation appears to be the absence of * Corresponding author. Tel.: þ ; fax: þ address: brepic@vinca.rs (B.S. Repic) /$ e see front matter ª 2013 Elsevier Ltd. All rights reserved.

2 biomass and bioenergy 58 (2013) 10e19 11 properly organized energy market, as well as insufficient number of biomass energy consumers. Boilers and furnaces combusting biomass bales can be manufactured with a rated output ranging from 0.1 to 10 MW. The use of biomass bales for energy production does not require large investments associated with fuel preparation nor it consumes significant amount of energy per kg of biomass bales used. Use of straw for energy production is associated with different logistic problems (collection, preparation for transport, transport and storage). The problems are less significant and easier to handle if biomass is used in energy generation facilities located in the vicinity of the biomass collection sites. The use of renewable energy sources is becoming more and more important [3], mainly due to continuously increasing prices of fossil fuels, resource depletion and global attempts to achieve maximum feasible CO 2 emission reduction [4]. The use of biomass fuel has significant environmental benefits since biomass combustion is associated with no net increase in atmospheric CO 2. In this manner, the use of biomass can play a crucial role in the achievement of the Kyoto Protocol goals, accepted by the Republic of Serbia as well. Therefore, great efforts have been made to develop a technology which would enable biomass bales of various sizes and shapes to be used for energy production [5]. 2. Biomass-to-energy conversion technologies Technologies enabling biomass use for energy generation are mainly dependant on biomass characteristics. Different biomass conversion technologies available on the market include [6,7]: fixed-bed combustion, combustion on the grate, combustion in dust burners, fluidized bed combustion and gasification. Literature review [8] provides detail description of all combustion technologies mentioned, including the associated pros and cons. Basic advantages of available biomass-to-energy conversion technologies include the long history of utilization for the combustion of different fossil fuels, extensive range of equipment suppliers etc. Main disadvantages include the fact that biomass combustion technologies have been shortly in use, as well as inflexibility of combustion technologies with respect to the biomass varieties combusted and lack of wellproven technologies for agricultural biomass combustion [4]. Although the use of forest biomass for energy production is deemed quite simple, utilization of agricultural biomass faces a lot of challenges. One of the main disadvantages is a tendency of agricultural biomass ash to melt. For the combustion of biomass bales two technologies are currently used. The first is based on whole-bale combustion in the combustion chamber, while the second considers combustion of biomass bales in cigar burners. The first technology is associated with poor combustion control, while the other one can provide better process control. The cigar firing technology provides better quality of the combustion process, resulting in lower pollutant emissions and increased plant efficiency. Cigar burner combustion system is also recommended by expert committees of the European Union as the most suitable technology for the combustion of baled agricultural residues [6,9]. Cigar firing technology developed in Denmark [10,11] has been designed exclusively for the combustion of straw bales and is deemed suitable for the combustion of whole-crop bales. A cigar-burner combustion facilities also have been constructed in Schkoelen, Germany (3.15 MWth) and Duernkurt, Austria (2.18 MWth). A cigar firing combustion system is expected to exhibit the following advantageous features: a) combustion of whole bales and whole energy crops; b) compact combustor design; c) short start up period, good loadfollowing performance; d) profitable operation of smaller facilities (down to 1 MWth); e) division of combustion from the heat recovery system, usable not only for the provision of steam (for heat generation or CHP), but also as a hot gas generator in industrial drying applications. Cigar burner combustion system promises a more competitive use of renewable for green heat and power generation as well as their use in various industrial applications. Possible disadvantages of cigar burner combustion system include: a) a need for a smart and sophisticated process control system; b) thermal attacks on the metal in combustion chamber. 3. Materials and methods Grain production provides large quantities of straw residues, which, in some cases, may exceed by up to three times the amount of grain produced [2]. There are two basic types of agricultural bales produced: small bales (usually cm square bales) and large bales (usually ø cm cylindrical bales or cm square bales). Each form and size of biomass bales is associated with certain advantages and disadvantages with respect to the baling rates, prices of baling presses, transportation and storage providing means, stowed position for transport or storage, etc. The research investigation described herein was focused on developing a cigar burner combustion system suitable for the bales combustion of various sizes and shapes and their utilization for energy production Experimental facility The initial set of analyses carried out in the research investigation conducted focused on the combustion of small bales in cigar burners. For that purpose, an experimental, 75 kwth hot water boiler was designed and constructed [12]. The furnace was built entirely out of an insulating material providing favourable biomass combustion conditions. In order to properly determine required design parameters and provided a good basis for development industrial scale straw-fired facilities appropriate tests were conducted. Combustion of rolled ø m bales in cigar burners was analyzed in the next investigation phase. In order to assess the combustion quality and obtain data needed for proper design of the hot water boiler, a 1 MWth demonstration furnace was constructed and tested [5]. A pilot plant capable of burning large m bales was designed and built as a result of the specified investigation

3 12 biomass and bioenergy 58 (2013) 10e19 Fig. 1 e The scheme of the demonstrating boiler. efforts. A 1.5 MWth industrial-scale hot water boiler (Fig. 1) was constructed and installed in the Agricultural Corporation Belgrade for heating 1 ha of vegetable greenhouses. Baled straw (Fig. 1, item no. 1) is fed using a piston feeder (3) through a water-cooled inlet (2) into the combustion chamber (12). Reciprocating feeder is powered by a suitable hydraulic system (5). The primary air for combustion is provided through primary air fan (4) and it is divided into two parts. One part is introduced just below and above the bale and the second part of the storage of ash (7). This air is carried out and burning unburnt particles, in its own layer of ash that falls through water-cooled grate (11). The ashes are using a screw extractor (6) derives from the system thus maintaining the desired height of fluidized bed. Secondary air is introduced into the combustion process through a water-cooled inlet (8), directly into the combustion zone of coke residue. This editorial has the ability to partially translatory and rotational movement. With translatory movement of the editorial made the secondary air furnace power regulation and rotational movement of removal of ash and partially burnt biomass from the top bale. Secondary air fan is provided with secondary air (9), and translational and rotational movement of the editorial with the transmission mechanism (10). Flue gases created in the primary combustion chamber after the turn entering the secondary and tertiary chamber in which gas is made burning phase, followed by flametube smoke (13) entering the first and second drafts of the heat exchanger (14) and (15). On leaving the heat exchanger, cooled flue gases entering the first cyclone filter (16), and then into the chimney (17). Additional new design idea represents the provision of a 100 m 3, thermally insulated, heat storage vessel i.e. heat accumulator (21). The specified item was introduced to provide better system response to the varying heat demand of heated greenhouses. The boiler is operated and controlled by a SCADA-based control system, via a computer Equipment used In order to obtain relevant data on the operation of the facility considered, the following parameters were measured: fuel consumption, air flow rate, flue gas and water temperatures, flue gas composition at the boiler outlet (content of O 2,CO 2, CO, SO 2,NO x and particulate matter) and under pressure in the furnace. Fuel consumption was determined indirectly, by measuring the amount of fuel fed into the combustion chamber and fuel burning time in the boiler furnace. The mass of straw bales used in the experiments conducted was measured by the means of 0.01 kg resolution digital scales. Air flow rates were measured several times during the experiments performed, utilizing Alnor AXD563 MicroManometer Kit equipped with Pitot probe. The accuracy of the instrument was rated to þ/ 1% of the reading. Flue gas, air and water temperatures were determined by the means of CreAl type K calibrated thermocouple probes, positioned in appropriately selected measurement points. The probes were connected to Keithley 2700 Digital Multimeter and Data Acquisition System. Gas sampling was performed by the means of a gas sampling probe, positioned close the furnace exit, and M&C PSS-5 gas conditioner, equipped with a heated hose. Flue gas composition was determined by the means of Servomex 4900C1 multi-component gas analyzer. Particulate emissions were measured by the means of appropriately selected equipment, in accordance with ISO Experimental procedure Experimental investigation was conducted to analyze biomass combustion in the demonstration boiler constructed. Investigation started with biomass sample preparation. An appropriate bale loader was utilized for handling the large biomass bales, weighing 250 kg in average. Based on the previously performed air fan calibration, carried out with the use of built-in AC drives, fresh air flow rate was adjusted in accordance with the biomass combustion process requirements. In addition, frequency inverter was used to adjust ventilation flue gas flow rate in order to achieve the furnace under pressure of 100 Pa. The aforementioned parameters were measured during the stable plant operation Selection of experimental parameters Biomass feed rates and fresh air flow rates were selected in such a way as to ensure that related values of boiler thermal output, mean furnace heat release rate and excess air

4 biomass and bioenergy 58 (2013) 10e19 13 coefficient were similar to the respective values obtained by calculation. The boiler thermal output was targeted to reach 1.50 MW, the mean furnace heat release per unit area was targeted to be about 811 kw/m 2, the mean specific volume furnace heat release to be about 169 kw/m 3 and excess air coefficient to be around 2.1 (Table 1). The excess air coefficient represents the ratio of the air flow rate injected into the furnace to the air flow rate theoretically required for complete combustion. Two sets of experiments were performed, namely experiments with soybean straw and experiments with rapeseed straw Base boiler parameters Excess air coefficient was determined based on the fuel proximate and ultimate analyses and flue gas composition measurements: l ¼ O 2 Boiler efficiency can be calculated based on the enthalpy of the fuel entering the boiler and enthalpy of flue gas exiting the boiler. In the analysis performed, heat losses to the environment and enthalpy of the air entering the boiler were considered negligible. h ¼ H in H ou H in ¼ 1 H ou H in ¼ 1 m gasc pgas T ou m fu H lfu c pgas e specific heat capacity of combustion products (kj/ (kgk)), m fu e fuel feed rate (kg/s), H lfu e lower heating value of the fuel (kj/kg), T ou e flue gas exit temperature (K). Mass flow rate of combustion products can be calculated based on the known excess air coefficient: m gas ¼ m fu 1 þ llmin g ash where: L min e minimum amount of air needed for the complete (stoichiometric) combustion of 1 kg of fuel (kg/kg), g ash e mass fraction of ash content in fuel. Minimal quantity of air needed for fuel combustion can be calculated using the following expression: 8 3 L min ¼ g C þ 8g H þ g S g O 0:231 where g C, g H, g S and g O represent mass fractions of carbon, hydrogen, sulphur and oxygen in the fuel, respectively. Boiler thermal output P (MW) can be calculated based on the known boiler efficiency and utilizing the following expression: P ¼ m fu H lfu h where: h e boiler efficiency (%), H in e flow of enthalpies at the boiler entrance (kj/kg), H ou e enthalpy of flue gas exiting the boiler (kj/kg), m gas e flow rate of combustion products (kg/s), Table 1 e Boiler and test parameters. Parameter Value Fuel bales Straw bales (soybean, rapeseed) Feeding system Hydraulic operated Bales size (length*width*height) (m) 0.7*1.2*2.0 Bale volume (m 3 ) 1.68 Bale weight e average (kg/bale) 235 Bale head area (m 2 ) 0.84 Average fuel consumption (kg/s) Fuel heat input (MW) 1.5 Heat load per bale head area (MW/m 2 ) 1.79 Furnace size (width*depth*height) (m) 1.49*1.24*4.8 Furnace cross-section (m 2 ) 1.85 Furnace volume (m 3 ) 8.87 Mean heat release rate per 811 unit area (kw/m 2 ) Heat release rate in furnace (kw/m 3 ) 169 Primary air flow (kg/s) Secondary air flow (kg/s) Tertiary air flow (kg/s) Total air flow rate (kg/s) Air excess coefficient measured 1.38e2.35 at the furnace exit l ( ) Start up time (s) 1500e Results and discussion 4.1. Biomass characteristics Experimental investigation of biomass combustion was carried out over a long period of time. Combustion of large bales of soybean, rapeseed, wheat and maize (cornstalks) straw had been analyzed. Proximate analysis of biomass varieties used in the tests conducted is presented in Table 2. Fuel quality was analyzed in a national accredited laboratory certified for fuel quality testing. Moisture and ash content determined in each of the fuel types examined were found to vary significantly. Contents of combustible and volatile matters were determined to be quite high. Heating values of biomass samples were determined to be quite high and close to heating value of coal. Ash melting temperatures, determined in accordance with [13], were observed to differ between the ranges of biomass samples analyzed. Ash melting temperatures determined for the case of soybean and rapeseed straw combustion was found to be quite high. On the other hand, ash melting temperatures associated with wheat and maize straw combustion were found to be rather low, indicating that problems associated with ash particle adherence to the furnace walls may be encountered during combustion of these biomass varieties. One of the very important parameters of any biomass combustion process is the ash shrinkage (sintering) starting temperature when ash starts to exhibit a tendency towards slagging. This temperature is defined as when the area of the test piece falls below 95% of the original test piece area at

5 14 biomass and bioenergy 58 (2013) 10e19 Table 2 e Proximate analyze of agricultural biomass (dry base). Value Units Type of straw Soybean Rapeseed Wheat Maize Total moisture % 8.35e e e e9.79 Ash % 3.92e e e e7.22 Sulphur total % 0.17e e e e0.30 Sulphur in ash % 0.07e e e e0.09 Sulphur combustible % 0.07e e e e0.22 Char % 18.60e e e e23.48 Fixed carbon % 14.06e e e e17.95 Volatile matter % 69.06e e e e80.67 Combustible matter % 83.95e e e e97.23 Lower heating value MJ/kg 16.4e e e e17.9 Ash melting temperatures Shrinkage starting temperature C Deformation temperature C Hemisphere temperature C Flow temperature C C [13]. In a facility with no liquid slag removal system, temperatures developed in the combustion chamber must not exceed values that might lead to ash slagging. One of the particularly unfavourable features associated with agricultural biomass combustion considers the fact that ashes produced by biomass combustion exhibit a strong slagging tendency. This means that sintering temperature of ash residue produced by agricultural biomass combustion is much lower than the respective temperature associated with combustion of forest biomass [5]. This phenomenon is not accidental but rather the result of artificial fertilizer use in the cultivation of agricultural crops. The use of artificial fertilizers entails increased levels of potassium in the mineral-storing parts of the plant [14] (which convert to ash). On the other hand, one of the main components contained in the mineral parts of the plant is SiO 2 e quartz sand. Melting temperature of SiO 2 equals 1400e1500 C, but with an addition of potassium or sodium the temperature can be lowered to below 800 C. This is also the main cause of the pronounced ash slagging tendency associated with agricultural biomass combustion in power plants or other combustion systems. Experimental investigation included several tests carried out with various agro-biomass samples. Experiments performed with soybean and rapeseed straw were deemed successful, since they had clearly demonstrated that the type of straw utilized in the experiments was suitable for use in the cigar burner combustion systems. Biomass feed rates used during the experiments equalled 0.12 kg/s (432 kg/h) in average. The boiler thermal output varied from 1.32 to 1.59 MW (Fig. 2) and regulated by fuel feed rate and air flow rate regulation, whereby the temperature of the flue gas was kept constant. Significant problem encountered during the tests concerned a considerable variation in the moisture content in biomass bales. A portion of the biomass bales was stored in the open, covered with thick plastic sheet and exposed to atmospheric precipitation. Such situation occasionally resulted in completely wet biomass bales. Tests performed with several biomass samples showed that the moisture content in such inadequately stored bales varied from 11.83% to 68.98%. It was deemed reasonable to assume that the moisture content in combustible bales should not exceed 25%. For that reason the bales with high moisture content were not used in the tests performed Measurement results and discussion Fig. 2 e The boiler thermal power. In order to obtain relevant data on the boiler operation, the following parameters were measured: flue gas temperature (in the burn-out zone t 2, at the furnace exit t 3, in the lower section of the furnace t 4, at the furnace bottom t 5 ); water temperature (at the boiler outlet and inlet); flue gas composition at the boiler outlet. Flue gas temperatures measured during one operating regime are presented in Fig. 3. Vertical lines in the figure represent a moment when new biomass bale was fed into the furnace. It can be observed that flue gas temperature in the furnace varied from 700 C to 1000 C in the burn-out zone, reaching 250 C at the furnace bottom and 160 C at the boiler

6 biomass and bioenergy 58 (2013) 10e19 15 Fig. 3 e Temperature of the flue gases. Fig. 5 e Concentration of the CO, NO x and SO 2 in exit flue gases. exit. Based on the temperature histories obtained, it is concluded that combustion of soy and rapeseed straw in the boiler constructed was satisfactory. Temperatures achieved in the combustion zone were mainly below 900 C, which is considered safe with respect to the ash sintering problems. Soy straw shrinkage starting temperature was determined to be 1185 C, while the rapeseed straw sintering temperature was lower and equalled 1100 C(Table 2). On the other hand, working temperatures achieved were high enough to ensure high quality of combustion process. Measured contents of O 2,CO 2, CO and NO x are presented in Figs. 4e6. Measurements have shown that O 2 content varied in range 10e16%, depending on the combustion regime. Fig. 4 shows O 2 and CO 2 contents, determined during one sequence of measurements performed under stable operating conditions. Serbian regulations [15] recommend that biomass fired furnaces and boilers should be operated in such a way as to enable requirements related to the CO and NO x emission limitations to be met. Measured CO emissions were observed to vary over a wide range (Fig. 5). When stable operating regime was established, CO emissions were lower than the respective emission limit values (for furnaces and boilers with rated power output of 1e50 MWth, CO emission limit value is set to 250 mg/m 3, calculated for 11% O 2 ). NO x emission was (Fig. 6), in most cases examined, below the 500 mg/m 3 emission limit. Particulate matter emissions measured at the boiler exit were quite high, in the order of 500 mg/m 3. The main reason for such high particulate emissions lies in the fact that the industrial boiler used has not yet been equipped with electrostatic precipitator, meaning that the flue gas cleaning was performed only in the high-efficiency cyclone. Investigation of boiler performance provided all parameters (primary and secondary air flow rates, temperatures in the furnace, at the heat exchanger inlet/outlet and at the boiler outlet) needed for calculation and design of higher Fig. 4 e Concentration of O 2 and CO 2 in exit flue gases. Fig. 6 e Gas emission in exit flue gases products related to 11% O 2.

7 16 biomass and bioenergy 58 (2013) 10e19 Table 3 e Ash melting behaviour for wheat straw. Value Additives Without Shrinkage starting temperature C Deformation temperature C Hemisphere temperature C Flow temperature C Ash content at 550 C % capacity boiler intended to be used for the combustion of biomass bales Effect of additives Additives are substances that have a capacity to lower sintering tendency of ash deposits and reduce corrosion problems caused by alkaline ash components. That can be achieved by converting the alkaline substances into less harmful compounds [16]. A change in the ash characteristics achieved with the use of additives is accomplished through an increase in the ash melting temperature. Additives react with KCl and NaCl, forming KeNa compounds with relatively high melting points and HCl which is released into the flue gas. HCl remains in the gaseous phase all the way through the boiler and is ultimately released to the stack and into to the environment, together with the flue gas. In this way chlorine is removed from the sediments, hence preventing the corrosion provoking temperature distribution. Additives are therefore added in order to bind the gaseous alkaline compounds and form less harmful compounds. The resultant compounds are generally less volatile and are usually formed as coarse ash particles which remain at the bottom section of the boiler throughout the entire combustion process. Effectiveness of additives depends on several factors [16]: a) additive particle size distribution e smaller additive particles provide larger surface area available for the process reactions; b) reaction temperature and time; c) additive composition (active additive component); d) stoichiometry (sufficient amount of additives). Additives can be divided into several groups: additives containing Ca, P, S, Al or aluminium silicate. Additives are selected from substances which are easily handled, produce no toxic residues and are deemed effective. There are various kinds of mineral additives, such as sand (0.11e0.15 mm), chalk, kaolin (clay), rolovite, bentonite, aluminium sulphate, dolomite, calcite etc. All these additives have been used as alkali binding agents or for preventing chemical reactions with elements causing the formation of eutectic mixtures [17]. Some of the additives (dicalcium phosphate, monocalcium phosphate) are flame suppressors and therefore not suitable in cigar combustion. Potential additive impact on the biomass combustion process was examined in order to determine ash melting temperatures of biomass samples containing 5% by weight of selected additive. Biomass sample was mixed with the indicated amount of additive and the mixture produced was heated to 550 C in order to obtain the ash to be examined. Investigation was carried out with six different clay and sand additives: 1) bentonite, 2) clay K.M. Krusik, 3) clay Cirinac, 4) kaolin Bz, 5) sandy clay Plocnik, 6) clay K.M.Cirinac. With respect to the analyzed sample of wheat straw, which is generally characterized by low ash melting temperature, practically all additives were found to have caused an increase in the ash melting temperature of the sample. However, some of the additives used, such as kaolin and sandy clay additive, were associated with particularly significant ash melting temperature increase (Table 3). With respect to the maize sample analyzed, it was observed that clay additives (no. 3 and 6) had caused a significant increase in the related ash melting temperatures. In the case of rapeseed ash, characterized by high ash melting temperature, the use of each additive analyzed resulted in decreased ash melting temperature. Performed investigation has demonstrated that the use of additives allows safe and reliable biomass combustion, even of biomass varieties characterized by low ash melting temperature, such as wheat straw and maize. 5. Mathematical modelling of straw bale combustion Described boiler has been in operation for quite a while and has been quite successfully used for soy been straw combustion. However, in order to examine potential limitations and improve the overall boiler operation, primarily with respect to the combustion processes occurring in the furnace, additional investigations were deemed necessary. In addition to the performed experimental investigations, numerical simulation of the processes occurring in the facility examined was deemed to be quite useful for better understanding this new and not yet fully explored biomass combustion technology. Numerical simulation was also expected to indicate possible boiler manufacturing flaws and to provide opportunity to improve boiler performance with respect to process efficiency and environmental indicators. In addition, numerical calculations may, in some cases, replace quite expensive experiments and enable quick, economical and comprehensive data acquisition needed for the process performance improvements. Numerical simulation of the considered facility involves modelling of momentum, heat and mass transfer processes occurring during combustion of biomass bales, which are, based on their structure, considered as porous media. It is therefore understood that combustion processes occurring in the boiler examined needed to be modelled both with respect to processes occurring in porous media (biomass bale) as well

8 biomass and bioenergy 58 (2013) 10e19 17 Fig. 8 e Temperature profile in furnace. Fig. 7 e Domain mathematical model. as processes occurring in the fluid media (the area in the furnace surrounding the biomass bale). Based on the literature review of theoretical studies conducted [18e20] and also taking into account the type of phenomena needed to be modelled, respective momentum, heat and mass transfer equations have been defined in order to accurately describe changes in the three primary parameters (velocity, concentration and temperature). Certain assumptions and simplifications have been adopted in order to eliminate the factors that were deemed to have very little effect on the processes analyzed and that would only contribute to the increased model complexity. The first assumption made was that two-dimensional and stationary model, with constant position of the combustion front, was able to accurately describe the facility and processes analyzed. Porous layer i.e. the biomass fuel, was assumed to be continuously fed into the combustion chamber, whereby all processes were considered stationary due to the fact that the biomass feed rate was equal to the velocity of moisture, volatile and coke residue separation. Fuel feeding process was simulated by volumetric sources of different fuel components present in the porous zone (moisture, volatiles and coke residue), defined based on the known boiler heat output. Porous medium was deemed homogeneous and characterized by uniform thermal and flow properties (porosity, permeability, thermal conductivity) across the porous layer. The specified thermal and flow properties were determined experimentally. The model assumed homogeneous combustion occurring both in the porous and the fluid medium, with Arrhenius equation utilized to describe the constant rate of chemical reactions. In addition, it was assumed that volatile components of the soybean residue comprised propane, carbon dioxide and water vapour [21]. Energy conservation in the porous medium was modelled by solid-and-liquid-phase-temperaturebalance model i.e. by one-equation, single-phase model. A system of partial differential equations comprising the mathematical model used for modelling the combustion of baled soybean residue was solved using the commercial finite volume code FLUENT Pressureevelocity fields coupling were achieved using the SIMPLE algorithm, while discretization of equations was performed with utilization of a first order upstream scheme. Fig. 7 presents the domain of the mathematical model developed, with boundary conditions shown in Table 4. A temperature profile obtained by the mathematical model used to simulate the biomass combustion in cigar burners is shown in Fig. 8. Comparison of the combustion modelling results with the results obtained from the experiments is presented in Figs. 9e11. The figures show comparison of numerical and experimental results obtained at the outlet of the Table 4 e Boundary conditions on the front sections of the model. Unit Mass flow, [kg/s] Mass flow of fuel, [kg/s] Inlet 1 Inlet 2 Inlet 3 Inlet 4 Value Fig. 9 e Temperature at the model outlet.

9 18 biomass and bioenergy 58 (2013) 10e19 Fig. 10 e Concentrations of O 2 and CO 2 at the model outlet. considered flow domain, without showing the characteristic parameters inside the simulation domain. A good agreement between numerical and experimental results was obtained. Outlet temperature obtained using the model developed (Fig. 9) was 7 K higher than the experimentally measured value. In addition, calculated NO concentration (Fig. 11) was 10 ppm higher than the value measured. Carbon monoxide concentration (Fig. 11) obtained by process modelling was about 20 ppm higher than the value measured. Based on everything that has been said, it is concluded that developed mathematical model quite accurately describes the transport phenomena occurring in porous and fluid media during the combustion of baled agricultural biomass in cigar burners. combustion of soybean and rapeseed straw samples was investigated. Obtained results indicate that developed combustion technology was very convenient for combustion of biomass varieties characterized by high ash melting temperatures. For biomass varieties characterized by lower ash melting temperature, such as wheat and maize straw, the use of fuel additives was found to be necessary in order to increase the ash melting temperature. Effects of six different fuel additives on the combustion process were examined, leading to the conclusion that clay or sand additives were very useful in providing favourable conditions for the combustion of biomass varieties used. Developed model enabled the effect of fuel moisture content on the temperature distribution in the furnace to be analyzed, as well as related emissions of harmful combustion products into the environment. Performed investigation has demonstrated that high combustion temperatures can be achieved in furnaces used for the combustion of agricultural biomass and that associated CO and NO x emission levels are lower than the regulatory emission limit values defined by Serbian legislation. Acknowledgements This work was supported by the Ministry of Education, Science and Technological Development of Serbia, through the project III42011 Development and improvement of technologies for energy efficient and environmentally sound use of several types of agricultural and forest biomass and possible utilization for cogeneration. references 6. Conclusions Combustion of agricultural biomass was analyzed with respect to the cigar burner combustion technology, suitable for the whole-bale combustion. Developed technology was tested in the 1.5 MW industrial hot water boiler, where Fig. 11 e Concentration of CO and NO at the model outlet. [1] [2] Dodic S,Zekic V, Rodic V, Tica N, Dodic J, Popov S. Situation and perspectives of waste biomass application as energy source in Serbia. Renew Sust Energy Rev 2010;14(9):3171e7. [3] De Wit M, Faaij A. European biomass resource potential and costs. Biomass Bioenergy 2010;34(2):188e202. [4] Saidur R, Abdelaziz EA, Demirbas A, Hossain MS, Mekhilef S. A review on biomass as fuel for boilers. Renew Sust Energy Rev 2011;15(5):2262e89. [5] Mladenovic R, Dakic D, Eric A, Mladenovic M, Paprika M, Repic B. The boiler concept for combustion of large soya straw bales. Energy 2009;34(5):715e23. [6] Obernberger I. Decentralized biomass combustion: state of the art and future development. Biomass Bioenergy 1998;14(1):33e56. [7] Nieminen M, Veijonen K. Bioenergy technologies applied in EU countries. Project report PRO2/P2015/05, VTT Finland; [8] Quaak P, Knoef H, Stassen H. Energy from biomass e a review of combustion and gasification technologies. World Bank technical paper No Washington, USA: Energy Series; [9] Kavalov B, Peteves SD. Bioheat applications in the European Union: an analysis and perspective for European Commission, Directorate-General Joint Research Centre. Institute for Energy; [10] Marutzky R, Seeger K. Energie aus holz und anderer biomasse [Energy from wood und other biomasses]. DRW Verlag Weinbrenner; 1999 [in German].

10 biomass and bioenergy 58 (2013) 10e19 19 [11] Jorgensen K, Meier E, Madsen H. Modern biomass boilers. In: First world conference on biomass for energy and industry. Seville: Spain; June [12] Repic B, Dakic D, Djurovic D, Eric A. Development of a boiler for small straw bales combustion. In: Nathwani J, Ng Artie, editors. Paths to sustainable energy p. 647e64. India. [13] CEN/TS (ICS ). Solid biofuels e method for the determination of ash melting behaviour e part 1: characteristic temperatures method, European committee for standardization; September [14] Cuiping L, Chuangzhi W, Yanyongjie Haitao H. Chemical elemental characteristics of biomass fuels in China. Biomass Bioenergy 2004;27(2):119e30. [15] Regulation on the emission limit values of the pollutants in the air. Official Gazette of Republic of Serbia. No. 71; [16] Bafwer L, Ronnback M, Leckner B, Claesson F, Tullin C. Particle emission from combustion of oat grain and its potential reduction by addition of limestone or kaolin. Fuel Process Technol 2009;90(3):353e9. [17] Vamvuka D, Zografos D, Alevizos G. Control methods for migrating biomass ash-related problems in fluidized beds. Bioresource Technol 2008;99(9):3534e44. [18] Miltner M, Makaruk A, Harasek M, Friedl A. CFD-modelling for the combustion of solid baled biomass. In: Fifth international conference on CFD in the process industries CSIRO December Melbourne, Australia. [19] Miltner M, Miltner A, Harasek M, Friedl A. Process simulation and CFD calculations for the development of an innovative baled biomass-fired combustion chamber. Appl Therm Eng 2007;27(7):1138e43. [20] Bech N, Wolff L, Germann L. Mathematical modeling of straw bale combustion in cigar burners. Energ Fuel 1996;10(2):276e83. [21] Eric A. Thermo mechanical processes during baled soya residue combustion in pushing furnace. PhD Thesis. University of Belgrade, Faculty of Mechanical Engineering; 2010.