Research Proposal. Supervisors: Submitted to The University of Adelaide

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1 Research Proposal THOMAS KIRCH Supervisors: PAUL R. MEDWELL CRISTIAN H. BIRZER PHILIP J. VAN EYK Submitted to The University of Adelaide Faculty of Engineering, Computer and Mathematical Sciences School of Mechanical Engineering South Australia 5005 Australia August 5, 2016

2 1. Project Title Understanding small-scale biomass gasification and combustion 2. Project Summary Domestic size biomass cookstoves have received little scientific research interest and still have a very low efficiency, while releasing large amounts of pollutant emissions. The topic of this proposed research is to gain a deeper understanding of the gasification and combustion processes in top-lit up-draft (TLUD) cookstoves. By establishing a mathematical model of the ongoing processes inside the fuel bed, followed by an extensive experimental campaign targeting processes inside the fuel bed as well as the flame front, further insights can be achieved and the initial model can be extended and validated. This could enable optimisation of future TLUD designs. 3. Project Details 3.1 Introductory Background Energy consumption in private households in developing countries is still primarily based on biomass fuels. This concerns approximately 2.7 billion people [1] who rely on traditional methods which have typically very low efficiency and produce harmful emissions through incomplete combustion. This results in worldwide, approximately 4.3 million premature deaths each year from cooking-related illnesses caused by household air pollution [2]. Household air pollution from solid fuels is a major contributor to the burden of disease, especially for women, for whom it is second only to high blood pressure, as a cause of premature deaths [3]; and children, for whom it is responsible for 50% of premature death s under five years of age [2]. Improved cookstoves, such as the TLUD, have the potential of far reaching impact an this current situation. Top-lit up-draft cookstoves are characterised by a combination of the thermochemical conversion process of gasification and subsequent combustion of the gaseous and liquid products. Limited scientific research is available concerning the processes in TLUD cookstoves. Most previous research has focused on the influence of the air supply and different fuels in multiple stove designs. These tests were mostly carried out to determine the efficiency and the amount of released pollutant emissions of the specific stove design [4]. To establish measures that could lead to a consistent reduction of pollutant emissions from TLUD cookstoves deeper insights into the ongoing gasification and combustion processes would be beneficial. The proposed research will focus on advancing the fundamental knowledge of ongoing processes inside the fuel bed and the flame front. This will be addressed via mathematical modelling as well as experimental techniques. 2

3 3.2. LITERATURE REVIEW 3 A mathematical model detailing the ongoing process inside the fuel bed could provide a powerful tool for the design of an effective experimental campaign as well as yield further insights into the ongoing processes. Experimental investigations will address the evolution of gasification products within the fuel stack as well as the influence of feedstock composition on the yields of gasification products. In addition to studying the processes inside the fuel bed the effect of varying compositions of gasification products on subsequent combustion reactions and the release of emissions will be investigated. Within the experimental studies the model will be extended, validated and used to aid, explain and further contribute to the different study objectives. 3.2 Literature Review Solid biomass fuels provide the largest part of renewable resources to date [5] and are mostly used in private households for cooking and heating. Still there has been limited research on appropriate and technologically advanced cooking systems. A deeper understanding of the ongoing processes in such systems is needed for the optimisation of future designs. Highly efficient cooking systems in combination with programs for their dissemination could provide a far-reaching impact on the environment and the quality of life therein [6]. In order to achieve substantial health benefits, cleaner burning cookstoves than are currently in widespread use are needed [7, 8]. One type of cookstove that has been recognised as potentially able to achieve this goal are gasifier stoves [9]. These use the thermochemical process of gasification to transform the fuel into combustible gases and burn them separately in time and location [10]. Gasifier cookstoves can be distinguished by the direction of the gasification air. Available designs of cookstoves use either updraft, downdraft or inverted downdraft, also called top-lit up-draft (TLUD), flow [4] Solid Biomass Fuel Combustion All plants, plant-derived materials and livestock manures are termed biomass. Biomass is a lignocellulose material, consisting mainly of cellulose, hemicellulose and lignin. These constitute of organic materials like carbohydrates, fats and proteins. Minerals such as sodium, phosphorus, calcium and iron are also present in biomass, mostly in small amounts. If biomass is used as a fuel it is either classified by its lignocellulose composition, but more often by its chemical constituents and the atomic ratios of hydrogen (H) and carbon (C) to oxygen (O). A comparison of different fuels is presented in the van Krevelen diagram in Figure 3.1. Generally with the age of a fuel the energy content increases, while the atomic ratios decrease [11]. The combustion reaction, neglecting mineral components, of a typical biomass can be written as: CH 1.44 O λ (O N 2 ) intermediates(c, CO, H 2, CO 2, C x H y, etc.) CO H 2 O + (λ 1)O λ N 2 (3.1)

4 4 Figure 3.1: Biomass classification [11]. This reaction provides an energy release of 439 kj/kmol and is visualised in Figure 3.2, with λ being the ratio of the provided air-to-fuel ratio over the stoichiometric air-to-fuel ratio. The products of this process consist of those of incomplete combustion (CO, C x H y, polycyclic aromatic hydrocarbons (PAH), tar, soot, unburnt carbon, H 2, HCN, NH 3, and N 2 O), those of complete combustion (NO x -NO and NO 2 -, CO 2, and H 2 O) and ash (KCl, SO 2, HCl, Cu, Pb, Zn, Cd, etc.)[12]. Biomass has generally a high volatile content, around 80 wg-%, placing a focus on related processes. After ignition the gaseous components will be released from the fuel in the pyrolysis process, while the temperature increases. The pyrolysis process determines the amount and physical characteristics of the remaining char. Furthermore the reaction rate of the released volatile compounds is much greater than that of the solid char particles and is important for flame ignition and stability [13]. Figure 3.2: Two stage biomass combustion, adopted from Nussbaumer et al., 2003 [12].

5 3.2. LITERATURE REVIEW 5 A distinguishing characteristic of the TLUD stove is the separation of gasification, the pyrolysis and partial oxidation of volatile compounds, and combustion, the complete oxidation of the fuel. This is achieved by providing sufficient primary air to sustain the pyrolysis and devolatilisation process, through partial oxidation of volatile compounds. Subsequently excess secondary air is provided for complete combustion. The lack of knowledge of the ongoing processes as well as their interconnection results in insufficient control and efficiency, with a great potential for optimisation TLUD To operate as a TLUD, the stove is filled in batches and lit from the top. Firstly, the top layer of biomass is ignited, by a kindling material, before a pyrolytic front forms, which moves downwards, opposite to the gas flow, through the fuel-stack, as illustrated in Figure 3.3. Three phases have been identified, (1) a lighting phase, in which the fuel is ignited, (2) a steady state-phase, where a steady flow of volatile compounds is released and burned at the secondary air inlet, and (3) the char phase, in which either the remaining char is gasified releasing mainly CO or the process is quenched. In the current project a focus will be placed on the lighting and steady-state phase, with separate research on processes inside the fuel bed and in the flame. Figure 3.3: Schematic illustration of the combustion process in TLUD cookstoves Lighting Phase In the lighting phase, a kindling material is spread on top of the fuel stack, twigs, leaves or other small material is used in regular cookstoves. In laboratory studies it is common to use more high value fuel like kerosene or ethanol. After the kindling material is lit the lighting phase lasts until the top layer of the fuel stack is heated such that a pyrolysis front forms.

6 6 Wood, mustard stalks and kerosene were tested as kindling materials and it was observed that CO peaks increase with lower calorific values of the kindling materials [14]. Uncertainty in the existing results is exacerbated by the influence of different standardised tests and kindling materials on the performance of TLUD cookstoves. Arora et al., 2014, assessed different test protocols, and determined that, for given conditions, the emissions factors, (primarily of CO and PM), varied leading to differences in the cookstove performance [14]. A water boiling test (WBT) [15] study of four forced draft TLUDs showed that, of the total PM 2.5 emissions, of the lighting and cooking phases combined, a considerable proportion was released in the lighting phase. This phase accounted for 44.9 ± 5.9%, for medium power, and 33.9 ± 3.1%, for high power operation, of the total emissions [16]. Keeping in mind that the lighting phase is generally much shorter than the cooking phases, it becomes evident that this phase contributes significantly to the overall emissions. Further understanding of the ongoing processes in this phase is necessary to ensure that low emissions of incomplete combustion can be achieved in future stove designs. Insights of the interaction between different kindling materials and the fuel stack, as well as of optimal conditions for complete combustion could aid in achieving this goal Steady-State Phase In the steady-state phase, the heat released from the top layer of the fuel stack causes the next layer to pyrolyse, which means that volatile matter is released from the fuel in an inert atmosphere [17]. This process is called a migrating pyrolytic front [18], because cold reactants are separated from the high-temperature reaction products by a relatively thin area in which the reactions take place [19]. This front moves, in relation to the amount of primary air, down the fuel-stack [20]. The pyrolysis products are liquids (water, heavier hydrocarbons, and tars), gases (such as CO, CO 2, or CH 4 ) and solid char [21]. The endothermic pyrolytic front is sustained by simultaneous exothermic gasification reactions, in which the pyrolysis products can partially oxidise with the primary air (CO, CO 2, H 2 and lesser quantities of hydrocarbon gases) [22]. In inverted downdraft gasifiers, heavier hydrocarbons and the liquid tar can crack into lighter components as they move through the high temperature zone of the char layer [20, 23]. This process is highly complex, in part due to the thin char layer [24], and therefore the scientific understanding of these reactions in TLUD stoves is currently limited. The combustible pyrolysis products leave the fuel-stack at temperatures of approximately 600 C [25], mainly composed of carbon monoxide, hydrogen, methane and some heavier hydrocarbons [26]. Once these gases reach the secondary air inlet, they are mixed with air and can be combusted if an ignition source is present. As a result of gas-combustion, compared with other cookstoves, gasifier stoves have been shown to produce low CO and particulate matter (PM) emissions under laboratory conditions [27, 28]. It has been shown that variations in the stove geometry and the utilized fuel have a significant impact on the stove s performance [16, 29, 30]. The heat transfer, to a vessel on the stove, has been observed to be a strong function of the vessel diameter, while swirl of secondary air has a negligible impact [24]. Laboratory studies have also shown that using forced air increases

7 3.2. LITERATURE REVIEW 7 the heat transfer to a vessel on top of the stove and reduces products of incomplete combustion, foremost CO and PM, compared to other cookstoves [31, 32, 33, 34]. On the contrary, it has also been found that forced air stoves, though more energy efficient, are not necessarily more emission efficient [35] and although emitting relatively less PM 2.5 mass the number of ultra-fine particles can increase [27]. The presented studies have evaluated specific designs and analysed their performance while performing certain cooking tasks. In order to understand the ongoing processes in TLUD stoves more research into the ongoing fundamental combustion processes is needed. The whole process inside a TLUD cookstove can be seen as a combination of a packed bed autothermal gasifier and subsequent gas-phase combustion of the released products. Therefore a transfer knowledge from previously intensively studied systems needs to be considered. Then the application and extension of fundamental knowledge for this particular system in combination with extensive research on the influence of different, highly complex, biomass fuels is needed to enable the design of more efficient systems Char Phase After the pyrolysis process, char remains as a solid product. The char yield is mainly dependent on the superficial velocity, which is determined by the gas flow over the cross sectional area [20] and the moisture content of the biomass [36, 37]. This char can be further gasified and combusted in the stove or, if no further air is supplied and the oxidation process is quenched, it can be collected. If collected, the char can be used as either fuel or as a soil amendment (termed biochar). When using it as a soil amendment the whole process could be seen as a mechanism for carbon sequestration [29, 38]. If the quenching process is not conducted early enough, the char can continue to burn, producing high levels of CO, as well as ash. The proposed study will mainly focus on the ongoing processes in the lighting as well as the steady-state phases. Therefore the process will be terminated after the steady-state phase, allowing insights into the quenching process as well as the composition and the structure of the remaining char. The gasification and combustion of char will be of less interest, since post-process analysis of the remaining char can provide valuable information Air Supply As with any combustion device, the supply of oxygen is highly important in TLUD stoves. Air can be provided either by air via natural draft, which is dependent on the buoyancy force at the air inlet, or with the assistance of a fan (or compressed air) which creates a forced airflow. The influence of different kindling materials on the cookstove performance of natural and forced draft stoves has previously been reported [14]. It was observed that the CO emissions were lower with forced air, compared to natural draft [14]. An investigation of the influence of the air supply on the fuel lighting could yield further insights into the ongoing processes. The migrating pyrolysis front propagation, and thus the fuel conversion as well as the heat release are controlled by the supply of PA, which will be explained in detail in Section It has been shown that forced air can lead to a reduction of emissions of incomplete combustion from cookstoves [31, 32, 33]. Furthermore in stoves which facilitate forced draft, the air supply for the PA and SA is generally

8 8 delivered through a single fan, where the flow to each of the two inlets into the combustion chamber is dependent on the stove geometry [18]. The relationship of the two flows to one another, and even the overall air flow, are mostly unknown. Identifying a relationship of the SA to the PA flow, could provide further means to reduce pollutant emissions. Laboratory studies have shown that using forced air increases the heat transfer to a vessel on top of the stove and reduces products of incomplete combustion, foremost CO and PM, compared to other cookstoves [31, 32, 33, 34]. On the contrary, it has also been found that forced air stoves, though more energy efficient, are not necessarily more emission efficient [35] and although emitting relatively less PM 2.5 mass, the number of ultra-fine particles can increase [27]. There is widespread belief that forced air leads to greater mixing and thus increased performance[37]. As a result there is an increasing number of TLUD cookstove designs featuring force draft. However, there is a paucity of scientific understanding of such systems and the effect on emissions. In particular, the influence of controlled primary and secondary air flows as well as their relative location to one another is poorly understood Biomass Gasification in Packed Beds The ongoing processes inside the fuel bed in TLUD cookstoves can be compared to those in packed bed gasification systems. An extensive amount of research has been published previously on such technology [21, 39, 40]. A significant difference is that most research on gasification has been done with continuously fed gasifiers, while the TLUD is a batch fed system. Here only the most relevant findings related to TLUD cookstoves are discussed Influence of the Primary Air Supply As in any combustion device the air supply has a defining influence on ongoing processes. In gasification systems it determines the fuel consumption and product gas yield, as the air to fuel ratio self regulates at a given air flow rate [41]. Gasifier systems are also labelled according to fuel and air flow direction. The TLUD configuration can be compared to inverted downdraft, downdraft and updraft packed bed gasifiers depending on the configuration. Three regimes have been recognised with increased primary air flow in such packed bed gasifiers, as presented in Figure 3.4: (1) an oxygen limited regime, in which the combustion chamber temperature rises with a higher air flow [42] and which is divided in two subregimes, one where oxygen is not fully and one where it is completely consumed [43]; (2) a reaction limited regime; and (3) a regime where the process is cooled by convection and finally quenched [44]. TLUD cookstoves operate in the oxygen limited, or sub-stoichiometric, regime. To achieve this regime the primary air mass flow rate (ξ) needs to be lower than ξ 1, typical values for biomass are: 0.15 ξ kg m 2 s 1. The corresponding ignition mass flux (ν) for different biomass fuels has been found to be approximately 0.04 ν 0.10 kg m 2 s 1 [43, 45, 46, 44]. In addition to the dependence of the ignition mass flux on the primary air supply, also a dependence on the fuel moisture content has been observed [46, 45]. Limited research is available on systems which provide gasification, by limiting

9 3.2. LITERATURE REVIEW 9 Figure 3.4: Fuel combustion behaviour in downdraft gasification [44] the primary air supply, and subsequent combustion, with a secondary air supply, in such close proximity as in TLUD cookstoves. A modified commercial biomass combustion boiler has been studied and it has been found that the primary air ratio (the relationship between primary air and the ignition mass flux) as well as the fuel moisture content has significant influence on the gasification and combustion products [47]. This study also showed that, when increasing the primary air ratio, initially the heating value of the product gas rises until an ideal value and then drops, if further increased. Further research to establish the relation between the primary air and the ignition mass flux of different fuels in TLUD stoves and its influence on subsequent gasification and combustion products could yield additional insights Gasification Products During gasification the complex molecules of the solid biomass fuel are broken into smaller ones in the endothermic thermochemical pyrolysis process, which is sustained by subsequent partial combustion of the released products. The final products of gasification are classified in the three different aggregates: (1) liquid tars, heavier hydrocarbons and water; (2) solid char (mostly carbon and ash); and (3) a wide variety of gases (CO 2, H 2 O, C 2 H 2, C 2 H 4, C 2 H 6, C 6 H 6, etc.). Many factors, such as the heating rate, the final temperature and the type of initial biomass fuel, influence the relative amounts of received products [21]. The advantages of commercial scale inverted downdraft gasification is that a large amount tars formed are being consumed within the process and the feedstock ash remains in the char after gasification [48]. To establish the relative amounts of products from gasification, the extent of consumption of produced tars and an extension of current understanding of the processes leading to the final products will be sought for in the proposed research.

10 10 Syngas The gaseous products, released from gasification of solid fuels, is termed syngas. Syngas is a mixture of mainly CO, H 2, CO 2, CH 4, low molecular weight hydrocarbons and N 2 [49, 50]. In a TLUD stove concentrations of 8-13% CO, 18-20% CO 2, % CH 4, % O 2 and % H 2, have been measured on a volumetric basis and fuel wood consumption of g min 1, during the steady-state phase [24]. An improvement of the syngas composition was recognised for increased fuel consumption rates. The syngas composition is dependent on many influencing parameters. Apart from the fuel composition and the moisture content, the fuel size has also been found to have a significant impact on certain gas species yields [51]. Additional research is needed to investigate the influence of different feedstock, apart from wood, multiple shapes and fuel sizes on the syngas composition. If the syngas composition is known deductions about the ongoing processes inside the fuel bed and predictions on the secondary combustion front can be made. Liquid products The liquid gasification products are composed of mainly water, tar and heavier hydrocarbons. Many definitions of tar exist up to date. The problem is merely that there seems to be no universal definition of tars but also that the amount and composition of tars is extremely dependent on the trapping and measuring technique used [52, 53]. Different tar definitions need to recognised. Kiel et al., 2004, defines tars as all organic compounds with a molecular weight larger than benzene (excluding soot and char) and divides them into five subclasses [54]. Milne et al., 2004, uses a more general definition: The organics, produced under thermal or partialoxidation regimes (gasification) of any organic material, are called tars and are generally assumed to be largely aromatic [53]. In this definition tars are further devided into: (1) primary products, derived from cellulose (such as levoglucosan, hydroxy- acetaldehyde, and furfurals), from hemicellulose and from lignin (methoxyphenols); (2) secondary products, characterised by phenolics and olefins; (3) alkyl tertiary products, including methyl derivatives of aromatics (such as methyl acenaphthylene, methylnaphthalene, toluene, and indene); and (4) condensed tertiary products (such as benzene, naphthalene, acenaphthylene, anthracene/phenanthrene and pyrene) [55, 53]. The definition of Milne et al., 1998, will be applied throughout this document because it seems to be the most commonly used. Previous studies have shown that downdraft gasifiers produce almost exclusively condensible tertiary, aromatic tars [53, 55] in concentrations of approximately 1 g Nm 3 [53]. Very limited information is known about tar formation and evolution in biomass gasification. Zwart et al., 2009, described that empirical studies are almost exclusively carried out with model compounds and none have been performed on thermal cracking of tars from actual biomass gasification plants [52]. Therefore it needs to be determined if the generalisation, of only low concentrations of tertiary tars, is applicable to small-scale devices, such as TLUD

11 3.2. LITERATURE REVIEW 11 cookstoves, and further insights into the tar formation and evolution processes need to be sought. Tars from Biomass Gasification The release of tars from biomass gasification systems has been recorded previously. A recent study performed on a top-lit up-draft gasifier, using pine wood, presented tar yields between 6.2 and 93.5 g m 3, much higher as previously proposed. This research also found that lower tar yields were achieved with a higher fuel moisture content, higher temperatures and ignition mass flux, while higher tar yields were experienced with a higher fuel bulk density [51]. In a commercialscale downdraft gasifier, operated with wood pellets, the tar yield was found to be 0.34 to 0.68 g Nm 3 [56]. Furthermore mainly tertiary tar species (Toluene, Naphthalene, Styrene, Indene, Phenol) were detected, as presented in Figure 3.5, suggesting that primary and secondary cracking is efficient in this kind of system [56]. These findings for a commercial-scale gasifier are supported by another recent study gasifying Casuarina woodships and coconut shell where mainly light aromatic compounds were detected, with 70% of which were benzene and toluene [57]. Comparing the tar yield of the commercial downdraft to the TLUD gasifier illustrates that generally accepted guidelines for gasifiers can not directly be assumed for small-scale systems. Further research in necessary to gain insights into the ongoing processes and to analyse which factors influence tar evolution. Figure 3.5: Tar compounds in the syngas from downdraft gasification of wood pellets [56]. Catalytic Cracking of Tars with Char In TLUD cookstoves release pyrolysis tars move through the hot char layer, because the air flow and the pyrolysis front propagation directions are inverted. It has been shown that cracking, secondary decomposition reactions, of tars released from biomass pyrolysis is catalysed by char particles [58, 21, 59, 60], with extensive reviews having been published elsewhere [61, 62]. It has been suggested that this processes is limited in TLUD cookstoves, because of only a thin layer of char

12 12 [24]. This suggestion is supported by higher tar yields, than expected from literature, having been measured from a TLUD gasifier [51]. The question is therefore, to what extent does the char and the char layer thickness influence the amount and composition of released tars from the gasification process in TLUD stoves? Char Char is the solid carbonaceous product, including ash, of biomass gasification [63] and is called biochar if used for environmental management purposes [64]. Its composition varies depending on the composition of the feedstock and the intensity of the pyrolysis process [63]. Especially biochar has the potential to be used as a nutrient recycler, soil conditioner, income generator, waste management system, and agent for carbon sequestration [63]. In the proposed research the char will be collected, primarily for potential analyses of its structure and composition in regard to possible catalytic influences on the evolution of gasification products Emissions of Combustion of Gasification Products The products of combustion will consist of emissions of complete combustion (CO 2, H 2 O and small amounts of NO x -NO and NO 2 -), but also of emissions of incomplete combustion (CO, C x H y, polycyclic aromatic hydrocarbons (PAH), tar, soot, unburnt carbon, H 2, HCN, NH 3, and N 2 O), as explained in Section [12]. Time, temperature and turbulence are the most important requirements to achieve complete combustion [12]. Since complete combustion is desired, the focus of emissions testing are the products of incomplete combustion, mainly CO and particulate matter (PM). Particulate matter (PM), and the more general term smoke, include all nongaseous material released from a combustion system like ash, inorganic aerosols, PAH, tar particles, small particles of fragmented char and soot [65]. Within these constituents the ones that are relevant to the present research are carbonaceous products of incomplete combustion such as PAH, tar particles and soot. The term soot describes carbonaceous particulates that form from gas-phase processes [13] or even all combustion-generated carbonaceous aerosol [66]. This definition of soot includes black carbon(bc), which is a strong light-absorbing constituent [66]. The effect of BC emissions on climate forcing have been considered to be second most important, after CO [67], and biomass combustion was estimated to contribute 20% of all BC in the atmosphere [66]. These significant impacts on the climate, as well as the adverse human health effects, described previously, emphasise the need for further research to reduce these emissions from biomass cookstoves. Soot forms in conditions of between 1500 and 2500 K in a time-frame of a few milliseconds [68], usually close to the main combustion zone and hydrogen rich and oxygen poor atmosphere [69]. Biomass soot is considered to form via two main pathways from PAHs, which are seen as the principal precursor [70]: (1) through hydrogen abstraction and carbon addition (Haca) [69]; and (2) the reaction with cyclopentadienyl (CPDyl) moieties [71]. An extensive review on the soot formation through PAH has been published previously [72]. Measuring PAHs could, if possible, yield valuable insights into the soot formation processes.

13 3.2. LITERATURE REVIEW 13 In Table 3.1 the guidelines for testing cookstoves have been developed in a international workshop agreement by the ISO to be classified Tier 4, highest performing [73], and by the WHO emissions rates from household fuel combustion are presented. These guidelines set the target for current stove development. Table 3.1: Guidelines for testing cookstoves by the ISO (High Power (HP) and Low Power (LP) and guidelines for emissions rates from household fuel combustion by the WHO. HP CO LP CO Indoor CO HP PM LP PM Indoor PM (gmj 1 d ) (gmin 1 L 1 ) (gmin 1 ) (mgmj 1 d ) (mgmin 1 L 1 ) (mgmin 1 ) ISO Tier WHO unvented vented Multiple TLUD stoves have been tested using these guidelines [14, 16, 30, 36, 74], though each study focused on different aspects and tested different designs none were able to meet the PM targets and only few the CO targets. This highlights that further understanding of the PM formation in TLUD cookstoves is necessary to achieve the set goals. The approach in most of these studies [14, 16, 30, 74] was to test different designs of stoves that have been based on previous designs, or with slight alterations, to test if different designs or alterations could yield a higher efficiency and less emissions of incomplete combustion, while performing certain tasks. In the proposed study a research furnace, analogous to a TLUD cookstove will be used, independent of any cooking task, to be able to focus on the ongoing combustion processes Modelling Mathematical modelling has been used for a long time to study processes which can not be measured, find optimum parameters for existing systems or establish new designs [75]. Its effective use relies on an in-depth understanding of the ongoing processes and a sophisticated algorithm that can reproduce measured values [75]. Such a model could be extremely helpful to better understand and to complement experimental results as well as to optimise the testing procedure and avoid redundant tests [76]. A model of TLUD stove will need to incorporate both reaction kinetics and reactor hydrodynamics. For such a gasifier system this requires knowledge of bed hydrodynamics, mass and energy balances, gas, tar, and char yields at certain operation conditions, as well as physical mixing processes inside the reactor [77]. Literature can provide most of these requirements. Extensive reviews have been published on the chemical and physical of processes of biomass pyrolysis [78] as well as of combustion of hydrocarbon fuels [79].

14 Counter Current Gasifier Steady-state Models The major challenge in biomass gasification modelling is that many non-linear phenomena, such as a large amount of different species combustion reactions, turbulence, radiative heat transfer and many more need to be considered [4]. Di Blasi et al. have presented multiple works on this subject over the last years [80, 81, 82, 83, 84, 85], including one extensive review [78]. The most recent model focusses on the effects of double air entry, throughout the fuel bed, in a downdraft gasifier, but seems to be a comprehensive study representing the ongoing processes inside the fuel bed [85]. Included in the model is moisture evaporation and condensation, finite-rate kinetics of wood devolatilization, primary tar degradation, heterogeneous gasification and combustion of char, combustion of volatile species, steam reforming of methane and refractory tar, finite-rate gas-phase water gas shift, extra-particle mass transfer resistances, heat and mass transfer across the bed, absence of thermal equilibrium, solid- and gas-phase heat transfer with the reactor walls, radiative heat transfer through the porous bed, and variable solid and gas flow rates [85]. The main difference to a TLUD stove is that a TLUD is batch fed, which has to be considered while designing the model and will influence many of the modelled parameters. A fixed bed on a grate system has been modelled like a batch-fed system, similar to a TLUD [86]. In this model the movement of solid, caused by the shrinking and decomposing particles, is accounted for by a volume factor that relates a volume element to its initial volume, leading to a change in volume equalling a change in vertical direction [86]. These presented studies show that models of similar systems have been established previously with satisfying results. Adapting previous findings to be able to correctly represent the ongoing processes in TLUD cookstoves will still be a challenging task. In combination with a detailed measuring program a robust model could present a powerful tool for investigating, optimising and designing TLUD cookstoves. 3.3 Aims and Objectives The overall goal of this project is to gain a deeper understanding of the gasification processes of small-scale autothermal gasification and the influence on the subsequent combustion of gasification products, in TLUD cookstoves. This goal will be addressed through the following aims and objectives: 1. Aim: Gain a deeper theoretical understanding of the ongoing physical and chemical processes inside the fuel bed. Objective: Consolidate knowledge from other gasification and combustion systems to establish a mathematical model based on current knowledge. Motivation: A deeper understanding of the ongoing processes will be established through a model that includes thermodynamics as well as hydrodynamics inside the fuel bed of the system, as explained in Section

15 3.3. AIMS AND OBJECTIVES 15 The preliminary model, validated only on literature, can then aid in designing necessary parameters for the laboratory studies, conducted on a TLUD research furnace. Task: Develop a simplified model of a TLUD cookstove. 2. Aim: Identify the impact of char on the gasification product composition downstream of the migrating pyrolysis front. Objective: Study the influence of the char layer thickness above the pyrolysis front on the gasification products. Motivation: The second aim and objective is the start of an extensive experimental campaign. The focus will be placed on the ongoing processes inside the fuel bed. The hypothesis that will be addressed here is if the char layer above the migrating pyrolysis front has a noticeable influence on the composition of the gasification products. If that is the case, as expected from the literature presented in Section , the extent and outcome of this influence needs to be quantified and analysed. Also the newly gained information will be introduced in the established model, which will be validated and refined on the basis of the experimental results. This can lead to an iterative process of advancing the model and using this computational tool to extend and optimise the experimental campaign. Task: Conduct an experimental campaign in which the char bed thickness changes and measure the gasification products. Update and validate the model. 3. Aim: Generate further understanding of the impact of fuel type and composition on the gasification process in a TLUD. Objective: Investigate how different fuels and fuel compositions effect the gasification process. Motivation: Different fuels with varying compositions and constituents will greatly influence the gasification process, as explained in Section Thus the third aim and objective can be investigated by introducing a limited number of different kinds of fuels. This gap in literature, where wood is the preferred biomass fuel, is especially relevant to be able to design suitable gasification and combustion devices. The selection of fuels will be based on the availability in target regions for cookstove, therefore agricultural residues (like rice husks) or animal manure will be considered. The result of this study can then be incorporated into the model. Task: Test with changing fuel properties and measure the gasification products. Update and validate the model.

16 16 4. Aim: Achieve deeper insights into the influences of the air supply on the combustion process. Objective: Examine how variations of the air supplies influence on the combustion process. Motivation: In Sections and , it has been shown that the air supply has significant influence on the gasification and combustion process. Further research could yield valuable insights into this influence and investigate correlations between the air supply conditions and emissions of incomplete combustion. Measurements and findings can be integrated into the model. Task: Experiment with natural draft, forced primary, forced secondary air and different locations of the air supply to determine their influence on the combustion products. Update and validate the model. 5. Aim: Establish the interrelationship between the gasification and the combustion processes. Objective: Analyse previously measured data of combustion products and extend these with further measurements of certain parameters. Motivation: The fifth aim and objective will complete the experimental campaign. Here the focus will be placed on examining the ongoing processes inside the secondary flame front, where the gasification products are combusted. Little work has previously been done on this kind of flame front, as presented in Section In biomass gasification devices, where the conditions in the fuel bed are similar, the gas is usually cleaned from tars and other undesirable products, before further use or combustion. Conversely in biomass combustion devices the conditions inside the fuel bed and thus also at a secondary air port are different and hardly comparable. This investigation on the products of combustion of different compositions of gasification products could yield valuable insights into the ongoing processes in such flames. Incorporating the findings from this aim into the model can extend the model from representing the processes inside the fuel bed to also cover the secondary combustion. Task: Examine the influence of different products of gasification on the release of emissions from the combustion system. Update and validate the model. All of the aims and objectives focused on in this research have been chosen to gain deeper insights into biomass gasification and combustion. They will be studied in detail through the application in a TLUD research furnace. The simplicity of the furnace design and the approach of achieving a fundamental understanding will enable wide ranging relevance and impact of this research, not only in the small field of cookstoves, but in biomass gasification and combustion.

17 3.4. METHODS Methods To achieve the stated aims and objectives a combined modelling and experimental research approach will be used. First of all a model will be established based on the literature, which can be refined through measurements conducted on a TLUD research furnace. An extensive experimental campaign will be conducted to receive further insights into the ongoing process in the fuel bed, as well as in the subsequent combustion TLUD Mathematical Model The model aims to incorporate an in depth understanding of the ongoing physical and of the chemical processes. This model will be based on similar established work such as that of Di Blasi et al., 2013, [85], van Eyk et al., 2016 [87], Mandl et al., 2010 [88], and Tinaut et al., 2008 [89]. Model parameters will be adapted to fit the TLUD system. The model will be written as a mathematical model using the finite differences method in the MATLAB software. Preliminary assumptions will be gas and solid flow in plug flow regime, momentum transfer between the gas and the solid phase negligible, there is no spatial intra-phase temperature and concentration gradients as well as uniform particle size [90]. Continuity equations will be considered for all relevant species. Submodels will be included to be able to address gasification processes such as moisture evaporation, devolatilisation and pyrolysis, gas-phase reactions, tar degradation, heterogeneous reactions and mass and heat transfer. Table 3.1 presents various submodels that have been successfully used previously in similar systems to TLUD cookstoves which will be considered in this study. The validation of the preliminary model will be based on measurements from literature. This will include measurements from published models [89, 100], gasifier systems [43, 51, 107, 108] and TlUD stoves [24, 37] TLUD Experiments An extensive experimental campaign will be conducted testing a custom made research furnace, analogous to a TLUD cookstove, in a newly constructed testing facility. The proposed parameters to be tested as well as the necessary equipment are presented in Table 3.2. Temperature Temperature measurements will be conducted along the fuel bed and inside the flame front. Along the fuel bed thermocouples, of N- or K-type, will be mounted every 30 mm and one will be placed above the secondary air inlet. This will allow local temperatures to be measured as well as the calculation of the velocity of migrating pyrolysis front. A detailed temperature profile can also yield valuable information about ongoing processes inside the fuel bed and the flame front. Mass flux A new research furnace is currently being constructed in a way that its weight will be supported by the surrounding structure, while a bottom plate and the a fuel grate are independently placed on top of either a load cell or a weighing scale. The fuel will be weighed before being placed on top of the fuel grate to account

18 18 Table 3.2: Possible submodels to describe certain model parameters. Process Specific Description Ref. Moisture evaporation Diffusion limited, neglection blowing effects [91] Pyrolysis One step global reaction [88] Considering pyrolysis as a combination of [92] reactions of wood to gas, tar and char, and tar to gas and char Gas-phase reactions CO combustion Oxidation, one step rate equation best data [93] correlation under conditions in which equilibrium limitations are negligible To account for high temperatures a CO 2 revese [94] reaction can be added, fitted to reproduce equilibrium constants for CO-CO 2 -O 2 systems Or with CO/CO 2 equilibrium reverse reaction [95] Tar degradation CH O one step combustion reaction [96] CH 4 combustion One step reaction [97] Water-gas shift One step reaction [97, 98, 99] Tar and CH 4 Steam reforming [97] Tar degradation First order kinetics with respect to the [100] tar concentration Tar conversion as a function of the residence [101] time of gas throughout the packing. Heterogeneous reactions Unreacted-core Model development [102, 98] -shrinking Application for wood degradations [103] Heat transfer Solid/gas heat and mass transfer [104] Thermal conductivity and viscosity of gas [105] Effective thermal conductivities, including a [106] radiative contribution Batch process Movement of solid, caused by the shrinking and [86] decomposing particles, is accounted for by a volume factor that relates a volume element to its initial volume, leading to a change in volume equalling a change in vertical direction Table 3.3: Experimental parameters and equipment. Location Parameter Equipment Furnace Input Primary- and-secondary air Rotameters Fuel weight Weighing scale Fuel composition Proximate- and ultimate analysis Inside the fuel bed Fuel mass loss Weighing scale Temperature profile Thermocouples Gasification products Tar content Laboratory weighing scale Tar composition GC-MS Gas composition Testo 350XL, MRU, Gas meter (NH 3, SO 2, H 2 S, NO, NO 2, and total VOCs) Post-combustion Gas composition Testo 350XL, MRU, Gas meter (NH 3, SO 2, H 2 S, NO, NO 2, and total VOCs) Particulate matter Dust Track II mass fraction and particle counter, Particle sizer Particulate matter composition GC-MS Char yield Weighing scale

19 3.5. CONTRIBUTION TO THE DISCIPLINE 19 for loading effects generated by interaction between the fuel and the reactor walls. During experiments the the mass will be continuously recorded to be able to determine the mass flux and thus the fuel consumption in the furnace. By weighing and analysing the char after the experiment, the release of combustible products throughout the experiment can be calculated. Gasification products Gasification products will be extracted from the system above the fuel bed and before the secondary air inlet. These will run through a cooling trap, either filled with wool or a solvent, where tars will condense to be weighed and if possible analysed for their composition after each experiment. In succession the remaining gas-phase products will be analysed in a MRU Vario Plus gas analyser. This analyser uses electrochemical sensors to measure O 2, NO, NO 2, SO 2 and H 2 S, and infrared (NDIR) sensors to measure CO, CO 2, hydrocarbons (CH 4 and C 3 H 8 ) and H 2. The MRU analyser will be primarily used for gasification products, but could also be used to measure the combustion products. Additionally it might be possible to use a further gas meter which can measure NH 3, SO 2, H 2 S, NO, NO 2, and total VOC emissions. Combustion products The extraction port for measurement of the combustion products will be inside the ducting of the emissions extraction system. The primary gas analyser to measure combustion products is a Testo 350XL which uses electrochemical sensors to measure O 2, NO x, CO, CO 2, and SO 2 concentrations. Additional it might be possible to use a further gas meter which can measure NH 3, SO 2, H 2 S, NO, NO 2, and total VOC emissions. A Dust Track II will be used to measure aerosol concentrations corresponding to PM 1, PM 2.5 and PM 10 size fractions as well as real-time mass concentrations. Furthermore a particle-sizer will be used to measure the particulate mass distribution between 10 µm and 1 µm. 3.5 Contribution to the Discipline The contribution of the proposed research to the discipline could be manifold. The overall goal that could be achieved is to enable the design of more efficient and less polluting TLUD cookstoves. By targeting three different focus areas, the process input, the ongoing processes inside the fuel bed and the ongoing processes in the flame, as well as their interconnection could achieve a broad expansion of current understanding. This understanding can subsequently be used for the design of optimised TLUD cookstoves. The main contribution to the discipline will be that all findings will be published in peer reviewed scientific journals. Valuable findings should be generated concerning the: char structure, composition and most importantly catalytic influence; evolution of tars; composition of gasification products; and influence of air supply.

20 The impact on TLUD design could be provided by: the validated model which will enable the study of many parameters that would be to time consuming or difficult to perform, such as air supply locations, and present a useful tool in the design process; by the discussion of length vs diameter - high positive influence of char layer on gasification vs no significant influence; enabling easier design of TLUD cookstoves for different geographical regions (different local fuels); establishing the location and amount of the air supply to achieve low pollutant emissions; and providing knowledge about the gasification and combustion conditions needed to achieve low emissions. 4. Budget All necessary equipment for the project will be provided by the School of Mechanical Engineering of the University of Adelaide and is either already available or has been allocated to different funds. Measuring equipment, including the mentioned MRU and Testo gas analysers, the Dust Trak particulate matter meter, weighing scales, thermocouples and thermcouple readers are present in the University s Thebarton laboratories. The testing facilities, which have been designed in accordance with an international standard currently in development, are in the progress of construction, financed through other funds. The project fund will be used for small additions and repairs of laboratory equipment, which can only be approximated at this time. The analysis of the composition of chars and tars, can be conducted within the University of Adelaide but will cost approximately $ 30, per sample. Not more than $ should be allocated for maintenance the equipment and $ for the analysis of samples. Further funds allocated to this project by the University of Adelaide, in the amount of $ 3.000, will be used for the attendance of national and international conferences. 5. Research Plan, Timeline and Contents of Thesis The plan of this research is to achieve a thesis by publication. Multiple publications addressing the objectives stated in Section 3.3 are being sought. The planned timeline can be found in the Gantt in Appendix A.1. The first six months of this research project have been dedicated to a detailed literature review on the subject, with the most important findings being presented in this research proposal. The next step will be to specifically study and adapt literature on modelling of the ongoing processes in TLUD cookstoves, to establish a preliminary model. It is planned that this model will be finalised within the first year of the candidature. 20