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Gasification of pelletized renewable fuel for clean energy production
Fuel 90 (2011) 3352–3358
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Gasification of pelletized renewable fuel for clean energy production A. Lickrastina ⇑, I. Barmina, V. Suzdalenko, M. Zake Institute of Physics, University of Latvia, 32 Miera str., Salaspils, LV-2169, Latvia
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Article history: Received 3 February 2011 Received in revised form 18 May 2011 Accepted 1 June 2011 Available online 6 July 2011 Keywords: Biomass mixture Gasifier Products composition
a b s t r a c t The main aim of the study was to develop and investigate a small-scale experimental gasification technique for the effective thermal decomposition of pelletized renewable fuels (wood sawdust, wheat straw). The technical solution of the biomass gasifier for gasification of renewable fuels presents a downdraft gasifier with controllable additional heat energy supply to the biomass using the radial propane flame injection into the bottom part of the biomass layer. From the kinetic study of the mass conversion rate of pelletized biomass and variations of the composition of produced gas it is concluded that the process of biomass gasification is strongly influenced by the amount of additional heat energy and air supply into the biomass. The results of experimental measurements of the composition of produced gas have shown that under the conditions of the sub-stoichiometric air supply into the layer of pelletized wood biomass (a < 0.3) increasing additional heat energy supply in a range from 60 kJ up to 130 kJ leads to an enhanced mass loss of pelletized biomass and enhanced formation of volatiles (CO, H2) in the flaming pyrolysis zone. For the wood biomass the average content of CO in the products can be increased from 73 g/m3 up to 97 g/m3, while the average content of H2 increases from 4.7 g/m3 up to 6.2 g/m3. Similar variations of the composition of products are observed during the enhanced gasification of the wheat straw. At constant rate of additional heat energy supply and the sub-stoichiometric combustion conditions (a 0.17 0.30), a faster thermal decomposition of the pelletized biomass and larger average amount of the produced volatiles (CO, H2) can be obtained by increasing the air supply rate from 0.27 to 0.43 g/s, determining the variations of air-to-fuel ratio in a range from 1.3 up to 1.6. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays different types of biomass are recognized and accepted to be potential carbon neutral sources for energy production, which replace fossil fuels. At the same time, the utilization of biomass as an energy source in the energy production is limited due to its dissimilar structure and variations of the elemental composition and moisture content for different biomass types. A more effective, clean and controllable energy production can be obtained by converting the raw biomass into a more concentrated and compact fuel with a higher heating value and controllable fuel characteristics. It could be done by providing pelletization of the biomass as well as by converting the biomass into a liquid or a gaseous fuel under the preliminary defined process conditions. A welldeveloped technology of biomass conversion into a gaseous fuel is the biomass gasification, i.e. an incomplete combustion with thermal decomposition of the biomass resulting in the production of combustible volatiles (CO, H2, CH4, CxHy) and solid residues (char). The low calorific value (LCV) gases produced during the gasification and thermochemical conversion of the biomass are used ⇑ Corresponding author. Tel.: +371 29475762; fax: +371 67901214. E-mail address: [email protected] (A. Lickrastina). 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.06.003
to fire boilers (directly) and kilns, and generate power in ICEs and gas turbines. Basically, gasification systems consist of a gasifier unit, a purification system and energy converters, i.e. different types of burners or engines. According to the design of the gasifier and type of fuels used, the existing gasifiers are classified in several categories: updraft, downdraft, twin-fire, cross-draft, stratified, downdraft fixed bed, fluidized bed gasifiers [1–7]. Some researchers [7–11] have examined the thermochemical conversion of different types of biomass along with complex experimental research and simulation of chemical and physical processes, such as vaporization, devolatilization, volatile secondary reactions and char oxidation, coupled with the transport phenomena. Depending on the operation conditions and values of the controlled parameters (temperature, elemental composition of biomass, etc.), the processes of thermochemical conversion may exhibit different behaviors. The previous study has shown that the rate of biomass gasification, determining the composition of volatiles, is very temperature-sensitive – the increased temperature during the gasification of the biomass enhances the formation of combustible gases with a more complete thermal conversion of the biomass [12– 15]. At a temperature below 900 K, the complete gasification does not occur, gasification reactions are rate-controlled in a temperature range of 900–1200 K, while at T > 1200 K the rate of the
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Fig. 1. Digital image (a) and principal scheme (b) of the downdraft gasifier: 1 – grate; 2 – propane flame flow inlet; 3 – air supply nozzle; 4 – facility to control the thickness of the granulated biomass layer in the gasifier; 5 – orifice to insert a thermocouple into the gasification zone (T1); 6 – orifice to insert a thermocouple into the fuel gas flow (T2); 7 – orifice to place a gas sampling device; 8 – fuel gas flow channel sections; 9 – gas outlet.
biomass gasification reactions is highly sensitive to the variations of heat/mass transfer rates and to the rates of heat and mass supply into the system. The increased heat loss has a significant effect on the biomass conversion ratio determining the incomplete conversion near the extinction limit when the temperature is not sufficient to provide the complete conversion. In accordance with [7], the peak temperature depends on the air mass flux in the system and it can be increased when the air mass flux increases. Besides the gasifier temperature, the residence time also has a strong influence on the biomass conversion. The current study is aimed to convert the pelletized biomass (wood sawdust and wheat straw pellets) into a gaseous fuel considering the effect of variations of the operation conditions (additional heat energy and air supply rates) on the kinetics of the biomass gasification and on the composition of the produced fuel gas. 2. Experimental The pilot-scale downdraft gasification setup for the controllable process of pelletized renewable fuel (wood sawdust, wheat straw) gasification is shown in Fig. 1a and b. The experimental setup consists of two main sections. The first section is a downdraft gasifier, where discrete doses of the pelletized renewable fuel (180 g) are loaded from the upper part of the gasifier on the steel grate located at the bottom of the gasifier (1). The average height of the pelletized biomass layer is L = 110– 120 mm and the average mass density reaches 1.7–1.8 g/mm. The propane burner with the stoichiometric propane and air supply into the burner is used as an external heat energy source, which allows to control the amount and duration of the additional heat energy supply into the bottom part of the gasifier to initiate gasification of the pelletized biomass and to ensure control of the operating conditions (2). In the recent study, variations of the additional heat energy supply are provided by varying the stoichiometric propane supply rate from 0.7 up to 0.9 l/min and air supply rates in the swirl burner from 17.5 up to 23.0 l/min with the corre-
lating increase of the additional heat energy supply rate by the propane flame flow from 1 kJ/s to 1.3 kJ/s. The duration of the additional heat energy supply into the biomass is varied from 40 s to 100 s. The swirled air flow is supplied from the upper part of the gasifier using two tangential air nozzles (3) located above the layer of the pelletized biomass. The average rate of air supply into the gasifier is varied from 0.27 g/s to 0.45 g/s, determining the formation of the fuel-rich conditions (a 0.2 0.3) during the biomass gasification. In order to calculate the biomass consumption rate, timedependent variations of the biomass level in the gasifier during the pelletized biomass gasification are performed by a test facility consisting of a moving rod with a pointer (4). The variations of the biomass level in the gasifier are controlled with a time interval of 100 s. At constant cross-section area of the gasifier (S 31.15 cm2), fixed average mass density of the biomass (q 0.59 g/cm3) and the average value of m/L at the initial stage of the biomass gasification ranging from 18.5 g/cm to 19.0 g/cm the mass consumption rate (dm/dt) is estimated from the measurements of the time-dependent variations of the biomass level in the gasifier (dL/dt) and can be expressed as a linear dependence on the variations of the
Table 1 Main characteristics of the pelletized biomass. Main characteristics of pelletized biomass
Wood pellets
Wheat straw pellets
Proximate analysis Moisture content, % Volatiles, % Ash content, % HHV, kWh/kg
7.9 81.3 2.1 4.96
9.4 71.3 3.7 4.6
Ultimate analysis Carbon content, % Hydrogen content, % Nitrogen, % Sulphur content, %
45.0 6.0 0.1 0.010
49.7 5.3 1.1 0.1321
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and evaluated according to the corresponding standards [23] and are presented in Table 1. Biomass was pelletized using a laboratory pellet mill KAHL 14175. The water and ash content were determined according to CEN/TS 14774-1 and CEN/TS 14775, correspondingly. C, N H, S in total was determined using the Analysis System Vario Macro CHNS. Higher heating values (HHV) of pellets were determined by burning sample in the calorimetric bomb according to ISO1928. Combustion sulfur content (Scomb) was determining according to ISO 334-75. The wheat straw pellets have lower volatiles content (Table 1) thus leading to the lower fuel gas production during the gasification process. The higher ash or mineral content leads to larger amount of residues after gasification. The lower volatiles content together with higher carbon content of wheat straw pellets lead to more lasting char combustion stage comparing to wood pellets.
3. Results and discussion
Fig. 2. Variations of the normalized mass loss of wood pellets (a and b) and wheat straw pellets (c and d) by varying the additional heat supply (a and c) and air supply (b and d) into the gasifier.
biomass level in the gasifier: dm/dt dL/dt. The temperature inside the reaction zone of the gasifier is controlled using a thermocouple inserted into the gas flow through the orifice at the distance L = 50 mm above the grate (5). The second section of the device is a gas flow channel (8), through which the products of gasification (CO, H2, CO2, N2 and H2O) move to the channel outlet. The gas flow channel is equipped with a gas sampling port (7), which is located at the distance L = 170 mm from the fuel gas outlet, to provide on-line measurements of the fuel gas composition at different stages of the biomass gasification using a gas analyzer Testo 350XL. For the additional qualitative control of the fuel gas composition the infrared spectra of gaseous products are measured using an infrared spectrometer FTIR. Local measurements of the fuel gas flow temperature (T2) are taken out at the distance L = 160 mm from the fuel gas outlet using a Pt/Pt-Rh (10%) thermocouple, which can be inserted into the diagnostic sections through orifice 6 (see Fig. 1), and are processed using the PC-20TR software. The main characteristics of the pelletized biomass are measured
The biomass gasification in this study is represented by an incomplete combustion of discrete dozes of pelletized biomass, where thermal decomposition of the biomass is provided by means of external heat supply from the propane flame flow and swirling air supply under sub-stoichiometric conditions (Fig. 1). The external additional heat supply assures the necessary energy to initiate the thermal decomposition of the biomass constituents. The relative proportion of hemicellulose, cellulose or lignin to the total mass of all three biomass constituents varies depending on the type of biomass and determining the characteristics and the process of thermal decomposition [16]. However, all three constituents require an additional heat supply for temperatures up to 450 K as strong endothermic reactions dominate determining dehumidification and initiating thermal degradation. The results of the experimental study on the mass consumption rate during the biomass gasification evidence that the biomass gasification process and the production of combustible volatiles with the correlating mass conversion (m/m0) of the pelletized biomass is a complex process, involving a number of reactions, and influenced by many factors, such as the heating rate, residence time of reactions and the composition of biomass. Hence, the biomass gasification is highly influenced by variations of the additional heat energy supply into the biomass and by the air supply rate into the gasifier, determining the equivalence ratio of air supply into the biomass. The effect of the operation conditions on the time-dependent variations of mass conversion for pelletized wood biomass and wheat straw is illustrated in Fig. 2a–d. The time-dependent variations of normalized mass conversion show that there are three main stages during the gasification of the pelletized biomass. The first stage is heating and drying of the pellets, the next stage involves thermal decomposition of the biomass, while the final stage is char combustion/reduction of the pelletized biomass. The stage of heating and drying of the pelletized biomass is relatively slow, and the average mass conversion rate of the pellets at this stage varies from 0.05 to 0.07 g/s with an average mass loss of 10–12%. If compared with the mass conversion of wood pellets (Fig. 2a), wheat straw pellets show a faster transition (already around 400 K) from endothermic reactions of drying/heating to exothermic [17] biomass devolatilization (Fig. 2c). As follows from Fig. 2a and c, the endothermic biomass drying/heating is very sensitive to variations of the additional heat energy supply by the propane flame flow into the bottom part of the gasifier: increasing the additional heat energy supply from 60 kJ up to 129 kJ promotes a faster transition from the stage of biomass drying/heating to the second active stage of biomass devolatilization. During this active
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Fig. 3. Infrared absorption spectrum of the produced fuel gas during the gasification of the pelletized wood biomass.
stage of biomass gasification, the average mass conversion rate for the pelletized biomass increases to 0.2–0.3 g/s. The biomass conversion at this stage, in the absence of oxygen/air, is a consequence of the thermal decomposition of hemicellulose and cellulose, which occurs in the temperature range 450–700 K, as well as the slow and lasting decomposition of lignin at temperatures from 450 K to 1200 K [18–20] leading to the formation of solid, liquid and gaseous products. Volatiles released during primary decomposition of the biomass are unstable and they break up into a large number of gaseous species and tar. Supplying sub-stoichiometric (a 0.2 0.3) air results in a partial combustion of the volatile pyrolysis products that enables the conversion of the biomass into a combustible gas. At this stage of biomass conversion, an increased additional heat energy supply into the gasifier promotes an enhanced thermal decomposition and partial combustion of the volatiles with the correlating increase of the average mass loss rate, a faster transition to the final stage of char combustion/reduction and decreases the total time of the biomass gasification (Fig. 2a and c). Due to the lower amount of volatiles (Table 1), wheat straw pellets exhibit a more pronounced and longer char gasification stage, comprising about 50–55% of the total biomass gasification time (Fig. 2a and c). With the constant additional heat energy supply into the gasifier, the faster transition from the heating/drying stage to the devolatilization stage with the correlating decrease of the total time of the pelletized biomass gasification can be obtained by increasing the sub-stoichiometric air–fuel ratio in the gasifier from 0.19 up to 0.30 (Fig. 2b and d). The duration of the wood pellets heating/ drying phase decreases from 670 s to 450 s, while the duration of the char gasification stage decreases from 1200 s to 600 s, indicating a faster combustion of charcoal. Similar variations in duration of the heating/drying phase are detected for wheat straw pellets, where the heating/drying phase has reduced from 480 s to 270 s. Less pronounced variations for wheat straw pellets are observed at the char conversion stage (Fig. 2d). Theoretically, the air–fuel ratio needed to assure the complete combustion of volatiles emitted from the biomass varies 6.0:1–6.5:1, with the final products being CO2 and H2O, whereas at gasification, when the combustion of volatiles takes place under the sub-stoichiometric conditions, the recommended air–fuel ratio is 1.5:1–1.8:1 [21]. With reference to [5], the optimum gasification conditions for the biomass can be achieved when the air–fuel ratio reaches 2.78 and the thermal decomposition of the biomass, resulting in the formation of CO, CO2, H2, H2O, can be approximately expressed as:
CH1:4 O0:6 þ 0:4O2 þ heat ! 0:7CO þ 0:3CO2 þ 0:6H2 þ 0:1H2
ð1Þ
Under the given conditions, when the average biomass conversion rate during thermal decomposition is about 0.17–0.26 g/s (Fig. 2)
and the equivalence ratio of the air supply has increased from 0.19 to 0.30, the biomass gasification develops at the average air– fuel ratio 1.20–1.95. Therefore, for the current study, the thermal decomposition of the biomass at the devolatilization stage develops with the air–fuel ratio recommended in [21] and gradually approaches the optimum gasification conditions recommended in [5]. The thermal degradation of the biomass with the partial combustion of the volatiles under the given conditions results in an intensive release of CO, CO2, H2, H2O and traces of CH4 confirmed by FTIR measurements of the infrared absorption spectra of the produced fuel gas (Fig. 3), showing the appearance of absorption bands of CO2 in the spectrum range 2329 cm1–2372 cm1, absorption bands of CO within 1970–2200 cm1, a weak absorption band of methane at 3015 cm1 and the absorption of water, which appears within 3600–3800 cm1 (Fig. 3). The experimental study of the effect of additional heat energy supply on the time-dependent variations of the rates of CO and H2 production that are measured on-line using the gas analyzer TESTO, has shown that wood pellets begin to decompose when the temperature of the flaming pyrolysis zone reaches 450 K, promoting an intensive release and ignition of the volatiles with the correlating increase of the temperature in the flaming pyrolysis zone up to 1400 K. As follows from Fig. 4, the most intensive production of CO and H2 can be obtained at the primary stage of wood fuel devolatilization, while the most intensive production of NO is observed at the final stage of gasification during the char reduction stage. The regression analysis of the variations of the average mass fraction of the main combustible volatiles under the varying operation conditions has shown that the increased heat energy supply into the wood biomass from 60 kJ up to 130 kJ leads to linear dependence (R2 0.94 0.99) of the average mass fraction of the produced volatiles on the applied heat energy, increasing the average content of CO in the products from 73 g/m3 up to 97 g/m3, while the average content of H2 increases from 4.7 g/m3 up to 6.2 g/m3. The kinetic study of the correlation between the composition of the produced fuel gas and the temperature in the flaming pyrolysis zone has shown that the substoichiometric air supply into the layer of wood pellets (a 0.3) leads to an enhanced formation of volatiles in the pyrolysis zone and to an intensive heat consumption, which assures the temperature decrease to the minimum value (Figs. 4 and 5). Such correlation shows that the heat consumption rate at the endothermic stages of wood fuel heating/drying and thermal degradation of the biomass constituents exceeds the heat release rate determined by the substoichiometric gasification of the biomass and, hence, a higher heat energy supply into the biomass is needed to accomplish the endothermic processes of biomass pyrolysis. With reference to [22], the average heat energy supply into the biomass must be increased to 1.6–2.0 kJ/g, comprising 6–10% of the heat of the dry wood biomass combustion.
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Fig. 4. Effect of the additional heat energy supply on the time-dependent variations of CO, H2 and NO production and on the flame temperature in the flaming pyrolysis zone during the thermal decomposition of wood pellets.
Unlike wood pellets, the thermal decomposition of wheat straw pellets with a smaller content of volatiles (Table 1) develops with a less intensive formation of CO and H2 during the devolatilization and with a more intensive release of the volatiles at the final stage of char conversion (Fig. 5). The increased heat energy supply into the layer of wheat straw pellets from 60 kJ up to 130 kJ leads to increasing content of CO in the products from 64 g/m3 up to 89 g/m3, while content of H2 can be increased from 5.0 g/m3 up to 6.6 g/m3. As follows from Fig. 5d, wheat straw pellets start to decompose when the temperature of the flaming pyrolysis zone reaches 430–450 K. At the low rate of the additional heat energy supply into the biomass (1kJ/s), the devolatilization of wheat straw pellets develops with intensive heat consumption from the reaction zone and with instability of the wheat straw gasification and temperature pulsations. The kinetic study of the temperature has shown that the process can be stabilized by increasing the additional heat en-
Fig. 5. Effect of the additional heat energy supply on the time-dependent variations of CO, H2 and NO production and on the flame temperature in the flaming pyrolysis zone during the thermal decomposition of wheat straw pellets.
ergy supply to 129 kJ that provides a faster gasification of wheat straw pellets with the correlating increase of the temperature in the pyrolysis zone to 1000 K, leading to the increase of the content of the main volatiles (CO, H2) and to a slight increase of the content of NO in the products (Fig. 5a–d). The formation of a relatively high temperature level in the flaming pyrolysis zone for wheat straw pellets is observed at the end stage of gasification, when the rise of the peak flame temperature to 1200 K supports the increase of the reaction rates of char conversion, resulting in the enhanced formation of CO and H2 [11]:
C þ 0:5O2 ! CO
122:9 kJ=mol
ð2Þ
C þ CO2 ! 2CO 160:5 kJ=mol
ð3Þ
C þ H2 O ! CO þ H2
ð4Þ
118:3 kJ=mol
Measurements of the mass loss rates for the biomass pellets at the constant additional heat energy supply (Fig. 2) have shown that the
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Fig. 6. Effect of the air supply rate on the formation of main products and on the temperature of the flaming pyrolysis zone during the thermal decomposition and gasification of wheat straw pellets.
increased sub-stoichiometric air–fuel ratio 0.19–0.30 results in an enhanced transition from the heating/drying phase to the devolatilization stage of the biomass pellets gasification with the enhanced formation of the volatiles. For wood pellets, the average mass fraction of CO in the products has increased from 53 g/m3 at a 0.20 to 70 g/m3 at a 0.29 assisting to a faster gasification. A similar correlation between the increase of the average values of the main combustibles in the products and the air supply rates is observed for wheat straw pellets (Fig. 6). As follows from Fig. 6, the increased air supply rate results in a faster gasification of wheat straw and a faster ignition of the volatiles with the pronounced increase of the temperature to its peak value (1300 K). A more pronounced increase of the CO mass fraction is observed at the final stage of char conversion, while the enhanced release of hydrogen is observed during the devolatilization of wheat straw pellets when the peak mass fraction of H2 in the products increases from 4.30 g/m3 for the air supply rate 0.27 g/s to 0.74 g/m3 for the air supply rate 0.43 g/s with a nearly constant air–fuel ratio (a 0.26).
Fig. 7. Effect of the operation conditions on the formation of ash and tar during the gasification of the biomass pellets and on the average temperature in the flaming pyrolysis zone (T1) and products (T2).
One of the major problems dealing with the biomass gasification is the formation of ash and tar [11]. Tar is a complex mixture of condensable hydrocarbons, which block and foul gasification facilities, providing variations of the operation parameters (temperature, equivalence ratio, residence time) and requiring complex cleaning inside the gasifier. The results of the experimental study on the influence of operation conditions on the biomass gasification process show that the variation of the rates of air and heat supply into the gasifier directly influences not only the formation of tars, but it also affects the formation of ash in the bottom part of the gasifier. In accordance with [24] the ash content is substantially influenced by temperature, determining substantial mass loss of ash at temperatures over 900 K that can be related to the thermal decomposition of carbonates. The performed study shows that the rapid reduction of the mass of ash at the bottom part of the gasifier can be achieved by varying the operation conditions (Fig. 7).
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As follows from Fig. 7a and b, with the constant rate of additional heat supply into the gasifier, the amount of tar and ash can be reduced by increasing the air supply rate with the correlating increase of the temperature of the produced gas (T2). Similar variations of the tar and ash production are observed when increasing the additional heat supply from the propane flame flow into the bottom part of the gasifier (Fig. 7c and d), which shows the increase of the average temperature of the pyrolysis zone and produced fuel gas with the correlating drop in mass of ash and tar. 4. Conclusions In this study, the conceptive design of the downdraft gasifier for the gasification of pelletized biomass (wood, wheat straw) has been developed under controllable operation conditions: a controllable heat energy supply into the bottom part of the gasifier and a controllable sub-stoichiometric swirling air supply into the upper part of the gasifier, determining the control of the rate of thermal decomposition of biomass and the control of the composition of the produced fuel gas. The results show that the biomass consumption is a complex multistep process involving biomass heating/drying, thermal decomposition/gasification and char gasification stages, which are influenced by the composition of the biomass and by the external heat energy and air supply rates into the gasifier, determining the correlating variations of the temperature in the flaming pyrolysis zone and the composition of the products. The correlation analysis of the biomass consumption rate, composition of the produced volatiles and the temperature of the flaming pyrolysis zone shows that the development of endothermic processes of heating/drying and thermal decomposition of biomass under the conditions of sub-stoichiometric air supply leads to an intensive heat consumption, which contributes to the temperature decrease in the pyrolysis zone by limiting the production of the volatiles, as the exothermic reactions during the sub-stoichiometric gasification of the biomass are not sufficient to sustain the endothermic reactions of the biomass pyrolysis. The thermal decomposition of the biomass has been stabilized by increasing the additional heat energy supply into the gasifier, with correlating increase of the average mass fraction of the produced volatiles (CO, H2) in the products, while decreases the average mass of the produced ash and tars. In general, the external heat supply to the layer of biomass pellets leads to a faster thermal decomposition and a higher volatile production depending on the source of biomass. For wood pellets, the maximum rate of the volatile production with the peak value of CO and H2 in the produced fuel gas has been observed at the stage of wood pyrolysis/gasification, while for wheat straw pellets, the maximum rate of the CO and H2 production has been detected at the final stage of char gasification. With the constant rate of additional heat energy supply and the sub-stoichiometric combustion conditions in the flaming pyrolysis zone (a 0.17 0.30), a faster thermal decomposition of the pelletized biomass and a larger average amount of the produced volatiles (CO, H2) can be obtained by increasing the air supply rate from 0.27 g/s to 0.43 g/s, determining the air–fuel ratio 1.3–1.6, which gradually approaches the optimum conditions of air supply for the biomass gasification, as recommended in [5]. It should be noted that the increase of additional heat energy and sub-stoichiometric air supply into the gasifier contributes to the enhanced production of NO at the thermal decomposition of biomass by limiting the production of NO at the char gasification stage. Acknowledgments The authors would like to acknowledge the financial support from the European Regional Development Funding 2.1.1.1. ‘‘Sup-
port to Science and Research’’, Nr. 2010/0241/2DP/2.1.1.1.0/10/ APIA/VIAA/006 and the financial support of Latvian Grant V7638. References [1] Walawander W, Chern SM, Fan LT. Fundamentals of thermo-chemical biomass conversion. New-York: Elsevier Applied Science; 1985. p. 911–22. [2] Anil Rajvanshi K. Biomass gasification. In Yogi Goswami D, editor. Alternative energy in agriculture, vol. II. CRC Press; 1986. p. 83–102. [3] Chandrakant Turare. Biomass gasification. ARTES institute, University of Flensburg, Germany; 1997. [4] Williams RH, Larson ED. Biomass gasifier gas turbine power generating technology. Biomass Bioenergy 1996;10(2–3):149–66. [5] Handbook of Biomass Downdraft Gasifier Engine Systems, U.S Solar Energy Research Institute, SERI/SP-271-3022; 1988. pp. 1–148 . [6] Reeda TB, Waltb R, Ellisc S, Dasd A, Deutche S. Superficial velocity – the key to downdraft gasification. In Proceedings of the 4th biomass conference of the Americas. Oakland, CA; 1999. p. 1–8. [7] Dasappa S, Paul PJ, Mukunda HS, Shrinivasa U. Wood-char Gasification experiments and analysis on single particles and packed beds. In: 21th Symposium (International) on combustion/the combustion institute, vol. 27(1); 1998. p. 1335–42. [8] Wang Yiqun, Yan Lifeng. CFD studies on biomass thermochemical conversion. Int J Mol Sci 2008;9(6):1108–30. . [9] Di Blasi C. Modeling chemical and physical processes of wood and biomass pyrolysis. Progr Energy Combust Sci 2008;34(1):47–90. [10] Sadakata M, Takahashi K, Saito M, Takeshi S. Production of gas fuel and char from wood, lignin and holocellulose by carbonization. Fuel 1987;66(12):1667–71. [11] Cao Y, Wang Y, Rieley J, Pan W. A novel biomass air gasification process for producing tar-free higher heating value fuel gas. Fuel Process Technol 2006;87(4):343–53. [12] Scott DS, Piscorz J, Bergougnou MA, Graham R, Overend RP. The role of temperature in fast pyrolysis of cellulose and wood. Indust Eng Chem Res 1988;27(1):8–11. [13] Harris D, Roberts D, Henderson D. Gasification behavior of Australian coals at high temperature and pressure. Fuel 2006;85(2):134–42. [14] Elliot DC, Sealock LJ. Low temperature gasification of biomass under pressure. In: Overend RP, Milne TA, Mudge KL, editors. Fundamentals of thermochemical biomass conversion. London, UK: Elsevier Applied Science Publishers; 1985. p. 937–51. [15] Beaumont O, Schwob Y. Influence of physical and chemical parameters on wood pyrolysis. Ind Eng Chem Process Des Dev 1984;23(4):637–64. [16] Strezov V, Moghtaderi B, Lucas JA. Thermal study of decomposition of selected biomass sample. J Therm Anal Calori 2003;72(3):1041–8. [17] Thermal degradation of wood components: A review of literature. U.S.D.A Forest service research paper FPL 13, May 1970. US Department of Agriculture Forest Service Forest Products Laboratory Madison, Wis. [18] Gasparovic L, Korenova Z, Jelemensky L. Kinetic study of wood chips decomposition by TGA. In: Proceedings of the 36th international conference of SSCHE. Tatranske Matliare, Slovakia; May 25–29 2009. [19] Lv Gao-jin, Wu Shu-bin, Lou Rui. Kinetic study of the thermal decomposition of hemicellulose isolated from corn stalk. Hemicellul Pyrol BioRes 2010;5(2):1281–91. . [20] Paul de Wild. Innovative thermochemical conversion of biomass for valueadded chemicals. ECN biomass, coal and environmental research biorefineries and bio-based products. RAI, Amsterdam; March 15–16, 2010 . [21] Biomass gasification. Heat & Power, Combustion Gasification & Propulsion Laboratory, Department of Aerospace Engineering (IISc) Bangalore, India . [22] Handbook of biomass downdraft gasifier engine systems. Solar Energy Research Institute, US Department of Energy; 1988. p. 1–140. [23] Arshanitsa A, Barmina I, Andersone A, Telysheva G, Zake M. Processing and complex research of the main characteristics of pelletized lignocellulosic materials for clean and effective energy production. Int Scient Colloq Modell Mater Process, Riga 2010;16–7. [24] Misra Mahendra K., Ragland Kenneth W, Baker Andrew I. Wood and ash composition as a function of furnace temperature. Madison, USA: Department of Mechanical Engineering, University of Wisconsin-Madison; 1992. p. 1–14 .
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Fuel journal homepage: www.elsevier.com/locate/fuel
Gasification of pelletized renewable fuel for clean energy production A. Lickrastina ⇑, I. Barmina, V. Suzdalenko, M. Zake Institute of Physics, University of Latvia, 32 Miera str., Salaspils, LV-2169, Latvia
a r t i c l e
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Article history: Received 3 February 2011 Received in revised form 18 May 2011 Accepted 1 June 2011 Available online 6 July 2011 Keywords: Biomass mixture Gasifier Products composition
a b s t r a c t The main aim of the study was to develop and investigate a small-scale experimental gasification technique for the effective thermal decomposition of pelletized renewable fuels (wood sawdust, wheat straw). The technical solution of the biomass gasifier for gasification of renewable fuels presents a downdraft gasifier with controllable additional heat energy supply to the biomass using the radial propane flame injection into the bottom part of the biomass layer. From the kinetic study of the mass conversion rate of pelletized biomass and variations of the composition of produced gas it is concluded that the process of biomass gasification is strongly influenced by the amount of additional heat energy and air supply into the biomass. The results of experimental measurements of the composition of produced gas have shown that under the conditions of the sub-stoichiometric air supply into the layer of pelletized wood biomass (a < 0.3) increasing additional heat energy supply in a range from 60 kJ up to 130 kJ leads to an enhanced mass loss of pelletized biomass and enhanced formation of volatiles (CO, H2) in the flaming pyrolysis zone. For the wood biomass the average content of CO in the products can be increased from 73 g/m3 up to 97 g/m3, while the average content of H2 increases from 4.7 g/m3 up to 6.2 g/m3. Similar variations of the composition of products are observed during the enhanced gasification of the wheat straw. At constant rate of additional heat energy supply and the sub-stoichiometric combustion conditions (a 0.17 0.30), a faster thermal decomposition of the pelletized biomass and larger average amount of the produced volatiles (CO, H2) can be obtained by increasing the air supply rate from 0.27 to 0.43 g/s, determining the variations of air-to-fuel ratio in a range from 1.3 up to 1.6. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays different types of biomass are recognized and accepted to be potential carbon neutral sources for energy production, which replace fossil fuels. At the same time, the utilization of biomass as an energy source in the energy production is limited due to its dissimilar structure and variations of the elemental composition and moisture content for different biomass types. A more effective, clean and controllable energy production can be obtained by converting the raw biomass into a more concentrated and compact fuel with a higher heating value and controllable fuel characteristics. It could be done by providing pelletization of the biomass as well as by converting the biomass into a liquid or a gaseous fuel under the preliminary defined process conditions. A welldeveloped technology of biomass conversion into a gaseous fuel is the biomass gasification, i.e. an incomplete combustion with thermal decomposition of the biomass resulting in the production of combustible volatiles (CO, H2, CH4, CxHy) and solid residues (char). The low calorific value (LCV) gases produced during the gasification and thermochemical conversion of the biomass are used ⇑ Corresponding author. Tel.: +371 29475762; fax: +371 67901214. E-mail address: [email protected] (A. Lickrastina). 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.06.003
to fire boilers (directly) and kilns, and generate power in ICEs and gas turbines. Basically, gasification systems consist of a gasifier unit, a purification system and energy converters, i.e. different types of burners or engines. According to the design of the gasifier and type of fuels used, the existing gasifiers are classified in several categories: updraft, downdraft, twin-fire, cross-draft, stratified, downdraft fixed bed, fluidized bed gasifiers [1–7]. Some researchers [7–11] have examined the thermochemical conversion of different types of biomass along with complex experimental research and simulation of chemical and physical processes, such as vaporization, devolatilization, volatile secondary reactions and char oxidation, coupled with the transport phenomena. Depending on the operation conditions and values of the controlled parameters (temperature, elemental composition of biomass, etc.), the processes of thermochemical conversion may exhibit different behaviors. The previous study has shown that the rate of biomass gasification, determining the composition of volatiles, is very temperature-sensitive – the increased temperature during the gasification of the biomass enhances the formation of combustible gases with a more complete thermal conversion of the biomass [12– 15]. At a temperature below 900 K, the complete gasification does not occur, gasification reactions are rate-controlled in a temperature range of 900–1200 K, while at T > 1200 K the rate of the
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Fig. 1. Digital image (a) and principal scheme (b) of the downdraft gasifier: 1 – grate; 2 – propane flame flow inlet; 3 – air supply nozzle; 4 – facility to control the thickness of the granulated biomass layer in the gasifier; 5 – orifice to insert a thermocouple into the gasification zone (T1); 6 – orifice to insert a thermocouple into the fuel gas flow (T2); 7 – orifice to place a gas sampling device; 8 – fuel gas flow channel sections; 9 – gas outlet.
biomass gasification reactions is highly sensitive to the variations of heat/mass transfer rates and to the rates of heat and mass supply into the system. The increased heat loss has a significant effect on the biomass conversion ratio determining the incomplete conversion near the extinction limit when the temperature is not sufficient to provide the complete conversion. In accordance with [7], the peak temperature depends on the air mass flux in the system and it can be increased when the air mass flux increases. Besides the gasifier temperature, the residence time also has a strong influence on the biomass conversion. The current study is aimed to convert the pelletized biomass (wood sawdust and wheat straw pellets) into a gaseous fuel considering the effect of variations of the operation conditions (additional heat energy and air supply rates) on the kinetics of the biomass gasification and on the composition of the produced fuel gas. 2. Experimental The pilot-scale downdraft gasification setup for the controllable process of pelletized renewable fuel (wood sawdust, wheat straw) gasification is shown in Fig. 1a and b. The experimental setup consists of two main sections. The first section is a downdraft gasifier, where discrete doses of the pelletized renewable fuel (180 g) are loaded from the upper part of the gasifier on the steel grate located at the bottom of the gasifier (1). The average height of the pelletized biomass layer is L = 110– 120 mm and the average mass density reaches 1.7–1.8 g/mm. The propane burner with the stoichiometric propane and air supply into the burner is used as an external heat energy source, which allows to control the amount and duration of the additional heat energy supply into the bottom part of the gasifier to initiate gasification of the pelletized biomass and to ensure control of the operating conditions (2). In the recent study, variations of the additional heat energy supply are provided by varying the stoichiometric propane supply rate from 0.7 up to 0.9 l/min and air supply rates in the swirl burner from 17.5 up to 23.0 l/min with the corre-
lating increase of the additional heat energy supply rate by the propane flame flow from 1 kJ/s to 1.3 kJ/s. The duration of the additional heat energy supply into the biomass is varied from 40 s to 100 s. The swirled air flow is supplied from the upper part of the gasifier using two tangential air nozzles (3) located above the layer of the pelletized biomass. The average rate of air supply into the gasifier is varied from 0.27 g/s to 0.45 g/s, determining the formation of the fuel-rich conditions (a 0.2 0.3) during the biomass gasification. In order to calculate the biomass consumption rate, timedependent variations of the biomass level in the gasifier during the pelletized biomass gasification are performed by a test facility consisting of a moving rod with a pointer (4). The variations of the biomass level in the gasifier are controlled with a time interval of 100 s. At constant cross-section area of the gasifier (S 31.15 cm2), fixed average mass density of the biomass (q 0.59 g/cm3) and the average value of m/L at the initial stage of the biomass gasification ranging from 18.5 g/cm to 19.0 g/cm the mass consumption rate (dm/dt) is estimated from the measurements of the time-dependent variations of the biomass level in the gasifier (dL/dt) and can be expressed as a linear dependence on the variations of the
Table 1 Main characteristics of the pelletized biomass. Main characteristics of pelletized biomass
Wood pellets
Wheat straw pellets
Proximate analysis Moisture content, % Volatiles, % Ash content, % HHV, kWh/kg
7.9 81.3 2.1 4.96
9.4 71.3 3.7 4.6
Ultimate analysis Carbon content, % Hydrogen content, % Nitrogen, % Sulphur content, %
45.0 6.0 0.1 0.010
49.7 5.3 1.1 0.1321
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and evaluated according to the corresponding standards [23] and are presented in Table 1. Biomass was pelletized using a laboratory pellet mill KAHL 14175. The water and ash content were determined according to CEN/TS 14774-1 and CEN/TS 14775, correspondingly. C, N H, S in total was determined using the Analysis System Vario Macro CHNS. Higher heating values (HHV) of pellets were determined by burning sample in the calorimetric bomb according to ISO1928. Combustion sulfur content (Scomb) was determining according to ISO 334-75. The wheat straw pellets have lower volatiles content (Table 1) thus leading to the lower fuel gas production during the gasification process. The higher ash or mineral content leads to larger amount of residues after gasification. The lower volatiles content together with higher carbon content of wheat straw pellets lead to more lasting char combustion stage comparing to wood pellets.
3. Results and discussion
Fig. 2. Variations of the normalized mass loss of wood pellets (a and b) and wheat straw pellets (c and d) by varying the additional heat supply (a and c) and air supply (b and d) into the gasifier.
biomass level in the gasifier: dm/dt dL/dt. The temperature inside the reaction zone of the gasifier is controlled using a thermocouple inserted into the gas flow through the orifice at the distance L = 50 mm above the grate (5). The second section of the device is a gas flow channel (8), through which the products of gasification (CO, H2, CO2, N2 and H2O) move to the channel outlet. The gas flow channel is equipped with a gas sampling port (7), which is located at the distance L = 170 mm from the fuel gas outlet, to provide on-line measurements of the fuel gas composition at different stages of the biomass gasification using a gas analyzer Testo 350XL. For the additional qualitative control of the fuel gas composition the infrared spectra of gaseous products are measured using an infrared spectrometer FTIR. Local measurements of the fuel gas flow temperature (T2) are taken out at the distance L = 160 mm from the fuel gas outlet using a Pt/Pt-Rh (10%) thermocouple, which can be inserted into the diagnostic sections through orifice 6 (see Fig. 1), and are processed using the PC-20TR software. The main characteristics of the pelletized biomass are measured
The biomass gasification in this study is represented by an incomplete combustion of discrete dozes of pelletized biomass, where thermal decomposition of the biomass is provided by means of external heat supply from the propane flame flow and swirling air supply under sub-stoichiometric conditions (Fig. 1). The external additional heat supply assures the necessary energy to initiate the thermal decomposition of the biomass constituents. The relative proportion of hemicellulose, cellulose or lignin to the total mass of all three biomass constituents varies depending on the type of biomass and determining the characteristics and the process of thermal decomposition [16]. However, all three constituents require an additional heat supply for temperatures up to 450 K as strong endothermic reactions dominate determining dehumidification and initiating thermal degradation. The results of the experimental study on the mass consumption rate during the biomass gasification evidence that the biomass gasification process and the production of combustible volatiles with the correlating mass conversion (m/m0) of the pelletized biomass is a complex process, involving a number of reactions, and influenced by many factors, such as the heating rate, residence time of reactions and the composition of biomass. Hence, the biomass gasification is highly influenced by variations of the additional heat energy supply into the biomass and by the air supply rate into the gasifier, determining the equivalence ratio of air supply into the biomass. The effect of the operation conditions on the time-dependent variations of mass conversion for pelletized wood biomass and wheat straw is illustrated in Fig. 2a–d. The time-dependent variations of normalized mass conversion show that there are three main stages during the gasification of the pelletized biomass. The first stage is heating and drying of the pellets, the next stage involves thermal decomposition of the biomass, while the final stage is char combustion/reduction of the pelletized biomass. The stage of heating and drying of the pelletized biomass is relatively slow, and the average mass conversion rate of the pellets at this stage varies from 0.05 to 0.07 g/s with an average mass loss of 10–12%. If compared with the mass conversion of wood pellets (Fig. 2a), wheat straw pellets show a faster transition (already around 400 K) from endothermic reactions of drying/heating to exothermic [17] biomass devolatilization (Fig. 2c). As follows from Fig. 2a and c, the endothermic biomass drying/heating is very sensitive to variations of the additional heat energy supply by the propane flame flow into the bottom part of the gasifier: increasing the additional heat energy supply from 60 kJ up to 129 kJ promotes a faster transition from the stage of biomass drying/heating to the second active stage of biomass devolatilization. During this active
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Fig. 3. Infrared absorption spectrum of the produced fuel gas during the gasification of the pelletized wood biomass.
stage of biomass gasification, the average mass conversion rate for the pelletized biomass increases to 0.2–0.3 g/s. The biomass conversion at this stage, in the absence of oxygen/air, is a consequence of the thermal decomposition of hemicellulose and cellulose, which occurs in the temperature range 450–700 K, as well as the slow and lasting decomposition of lignin at temperatures from 450 K to 1200 K [18–20] leading to the formation of solid, liquid and gaseous products. Volatiles released during primary decomposition of the biomass are unstable and they break up into a large number of gaseous species and tar. Supplying sub-stoichiometric (a 0.2 0.3) air results in a partial combustion of the volatile pyrolysis products that enables the conversion of the biomass into a combustible gas. At this stage of biomass conversion, an increased additional heat energy supply into the gasifier promotes an enhanced thermal decomposition and partial combustion of the volatiles with the correlating increase of the average mass loss rate, a faster transition to the final stage of char combustion/reduction and decreases the total time of the biomass gasification (Fig. 2a and c). Due to the lower amount of volatiles (Table 1), wheat straw pellets exhibit a more pronounced and longer char gasification stage, comprising about 50–55% of the total biomass gasification time (Fig. 2a and c). With the constant additional heat energy supply into the gasifier, the faster transition from the heating/drying stage to the devolatilization stage with the correlating decrease of the total time of the pelletized biomass gasification can be obtained by increasing the sub-stoichiometric air–fuel ratio in the gasifier from 0.19 up to 0.30 (Fig. 2b and d). The duration of the wood pellets heating/ drying phase decreases from 670 s to 450 s, while the duration of the char gasification stage decreases from 1200 s to 600 s, indicating a faster combustion of charcoal. Similar variations in duration of the heating/drying phase are detected for wheat straw pellets, where the heating/drying phase has reduced from 480 s to 270 s. Less pronounced variations for wheat straw pellets are observed at the char conversion stage (Fig. 2d). Theoretically, the air–fuel ratio needed to assure the complete combustion of volatiles emitted from the biomass varies 6.0:1–6.5:1, with the final products being CO2 and H2O, whereas at gasification, when the combustion of volatiles takes place under the sub-stoichiometric conditions, the recommended air–fuel ratio is 1.5:1–1.8:1 [21]. With reference to [5], the optimum gasification conditions for the biomass can be achieved when the air–fuel ratio reaches 2.78 and the thermal decomposition of the biomass, resulting in the formation of CO, CO2, H2, H2O, can be approximately expressed as:
CH1:4 O0:6 þ 0:4O2 þ heat ! 0:7CO þ 0:3CO2 þ 0:6H2 þ 0:1H2
ð1Þ
Under the given conditions, when the average biomass conversion rate during thermal decomposition is about 0.17–0.26 g/s (Fig. 2)
and the equivalence ratio of the air supply has increased from 0.19 to 0.30, the biomass gasification develops at the average air– fuel ratio 1.20–1.95. Therefore, for the current study, the thermal decomposition of the biomass at the devolatilization stage develops with the air–fuel ratio recommended in [21] and gradually approaches the optimum gasification conditions recommended in [5]. The thermal degradation of the biomass with the partial combustion of the volatiles under the given conditions results in an intensive release of CO, CO2, H2, H2O and traces of CH4 confirmed by FTIR measurements of the infrared absorption spectra of the produced fuel gas (Fig. 3), showing the appearance of absorption bands of CO2 in the spectrum range 2329 cm1–2372 cm1, absorption bands of CO within 1970–2200 cm1, a weak absorption band of methane at 3015 cm1 and the absorption of water, which appears within 3600–3800 cm1 (Fig. 3). The experimental study of the effect of additional heat energy supply on the time-dependent variations of the rates of CO and H2 production that are measured on-line using the gas analyzer TESTO, has shown that wood pellets begin to decompose when the temperature of the flaming pyrolysis zone reaches 450 K, promoting an intensive release and ignition of the volatiles with the correlating increase of the temperature in the flaming pyrolysis zone up to 1400 K. As follows from Fig. 4, the most intensive production of CO and H2 can be obtained at the primary stage of wood fuel devolatilization, while the most intensive production of NO is observed at the final stage of gasification during the char reduction stage. The regression analysis of the variations of the average mass fraction of the main combustible volatiles under the varying operation conditions has shown that the increased heat energy supply into the wood biomass from 60 kJ up to 130 kJ leads to linear dependence (R2 0.94 0.99) of the average mass fraction of the produced volatiles on the applied heat energy, increasing the average content of CO in the products from 73 g/m3 up to 97 g/m3, while the average content of H2 increases from 4.7 g/m3 up to 6.2 g/m3. The kinetic study of the correlation between the composition of the produced fuel gas and the temperature in the flaming pyrolysis zone has shown that the substoichiometric air supply into the layer of wood pellets (a 0.3) leads to an enhanced formation of volatiles in the pyrolysis zone and to an intensive heat consumption, which assures the temperature decrease to the minimum value (Figs. 4 and 5). Such correlation shows that the heat consumption rate at the endothermic stages of wood fuel heating/drying and thermal degradation of the biomass constituents exceeds the heat release rate determined by the substoichiometric gasification of the biomass and, hence, a higher heat energy supply into the biomass is needed to accomplish the endothermic processes of biomass pyrolysis. With reference to [22], the average heat energy supply into the biomass must be increased to 1.6–2.0 kJ/g, comprising 6–10% of the heat of the dry wood biomass combustion.
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Fig. 4. Effect of the additional heat energy supply on the time-dependent variations of CO, H2 and NO production and on the flame temperature in the flaming pyrolysis zone during the thermal decomposition of wood pellets.
Unlike wood pellets, the thermal decomposition of wheat straw pellets with a smaller content of volatiles (Table 1) develops with a less intensive formation of CO and H2 during the devolatilization and with a more intensive release of the volatiles at the final stage of char conversion (Fig. 5). The increased heat energy supply into the layer of wheat straw pellets from 60 kJ up to 130 kJ leads to increasing content of CO in the products from 64 g/m3 up to 89 g/m3, while content of H2 can be increased from 5.0 g/m3 up to 6.6 g/m3. As follows from Fig. 5d, wheat straw pellets start to decompose when the temperature of the flaming pyrolysis zone reaches 430–450 K. At the low rate of the additional heat energy supply into the biomass (1kJ/s), the devolatilization of wheat straw pellets develops with intensive heat consumption from the reaction zone and with instability of the wheat straw gasification and temperature pulsations. The kinetic study of the temperature has shown that the process can be stabilized by increasing the additional heat en-
Fig. 5. Effect of the additional heat energy supply on the time-dependent variations of CO, H2 and NO production and on the flame temperature in the flaming pyrolysis zone during the thermal decomposition of wheat straw pellets.
ergy supply to 129 kJ that provides a faster gasification of wheat straw pellets with the correlating increase of the temperature in the pyrolysis zone to 1000 K, leading to the increase of the content of the main volatiles (CO, H2) and to a slight increase of the content of NO in the products (Fig. 5a–d). The formation of a relatively high temperature level in the flaming pyrolysis zone for wheat straw pellets is observed at the end stage of gasification, when the rise of the peak flame temperature to 1200 K supports the increase of the reaction rates of char conversion, resulting in the enhanced formation of CO and H2 [11]:
C þ 0:5O2 ! CO
122:9 kJ=mol
ð2Þ
C þ CO2 ! 2CO 160:5 kJ=mol
ð3Þ
C þ H2 O ! CO þ H2
ð4Þ
118:3 kJ=mol
Measurements of the mass loss rates for the biomass pellets at the constant additional heat energy supply (Fig. 2) have shown that the
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Fig. 6. Effect of the air supply rate on the formation of main products and on the temperature of the flaming pyrolysis zone during the thermal decomposition and gasification of wheat straw pellets.
increased sub-stoichiometric air–fuel ratio 0.19–0.30 results in an enhanced transition from the heating/drying phase to the devolatilization stage of the biomass pellets gasification with the enhanced formation of the volatiles. For wood pellets, the average mass fraction of CO in the products has increased from 53 g/m3 at a 0.20 to 70 g/m3 at a 0.29 assisting to a faster gasification. A similar correlation between the increase of the average values of the main combustibles in the products and the air supply rates is observed for wheat straw pellets (Fig. 6). As follows from Fig. 6, the increased air supply rate results in a faster gasification of wheat straw and a faster ignition of the volatiles with the pronounced increase of the temperature to its peak value (1300 K). A more pronounced increase of the CO mass fraction is observed at the final stage of char conversion, while the enhanced release of hydrogen is observed during the devolatilization of wheat straw pellets when the peak mass fraction of H2 in the products increases from 4.30 g/m3 for the air supply rate 0.27 g/s to 0.74 g/m3 for the air supply rate 0.43 g/s with a nearly constant air–fuel ratio (a 0.26).
Fig. 7. Effect of the operation conditions on the formation of ash and tar during the gasification of the biomass pellets and on the average temperature in the flaming pyrolysis zone (T1) and products (T2).
One of the major problems dealing with the biomass gasification is the formation of ash and tar [11]. Tar is a complex mixture of condensable hydrocarbons, which block and foul gasification facilities, providing variations of the operation parameters (temperature, equivalence ratio, residence time) and requiring complex cleaning inside the gasifier. The results of the experimental study on the influence of operation conditions on the biomass gasification process show that the variation of the rates of air and heat supply into the gasifier directly influences not only the formation of tars, but it also affects the formation of ash in the bottom part of the gasifier. In accordance with [24] the ash content is substantially influenced by temperature, determining substantial mass loss of ash at temperatures over 900 K that can be related to the thermal decomposition of carbonates. The performed study shows that the rapid reduction of the mass of ash at the bottom part of the gasifier can be achieved by varying the operation conditions (Fig. 7).
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As follows from Fig. 7a and b, with the constant rate of additional heat supply into the gasifier, the amount of tar and ash can be reduced by increasing the air supply rate with the correlating increase of the temperature of the produced gas (T2). Similar variations of the tar and ash production are observed when increasing the additional heat supply from the propane flame flow into the bottom part of the gasifier (Fig. 7c and d), which shows the increase of the average temperature of the pyrolysis zone and produced fuel gas with the correlating drop in mass of ash and tar. 4. Conclusions In this study, the conceptive design of the downdraft gasifier for the gasification of pelletized biomass (wood, wheat straw) has been developed under controllable operation conditions: a controllable heat energy supply into the bottom part of the gasifier and a controllable sub-stoichiometric swirling air supply into the upper part of the gasifier, determining the control of the rate of thermal decomposition of biomass and the control of the composition of the produced fuel gas. The results show that the biomass consumption is a complex multistep process involving biomass heating/drying, thermal decomposition/gasification and char gasification stages, which are influenced by the composition of the biomass and by the external heat energy and air supply rates into the gasifier, determining the correlating variations of the temperature in the flaming pyrolysis zone and the composition of the products. The correlation analysis of the biomass consumption rate, composition of the produced volatiles and the temperature of the flaming pyrolysis zone shows that the development of endothermic processes of heating/drying and thermal decomposition of biomass under the conditions of sub-stoichiometric air supply leads to an intensive heat consumption, which contributes to the temperature decrease in the pyrolysis zone by limiting the production of the volatiles, as the exothermic reactions during the sub-stoichiometric gasification of the biomass are not sufficient to sustain the endothermic reactions of the biomass pyrolysis. The thermal decomposition of the biomass has been stabilized by increasing the additional heat energy supply into the gasifier, with correlating increase of the average mass fraction of the produced volatiles (CO, H2) in the products, while decreases the average mass of the produced ash and tars. In general, the external heat supply to the layer of biomass pellets leads to a faster thermal decomposition and a higher volatile production depending on the source of biomass. For wood pellets, the maximum rate of the volatile production with the peak value of CO and H2 in the produced fuel gas has been observed at the stage of wood pyrolysis/gasification, while for wheat straw pellets, the maximum rate of the CO and H2 production has been detected at the final stage of char gasification. With the constant rate of additional heat energy supply and the sub-stoichiometric combustion conditions in the flaming pyrolysis zone (a 0.17 0.30), a faster thermal decomposition of the pelletized biomass and a larger average amount of the produced volatiles (CO, H2) can be obtained by increasing the air supply rate from 0.27 g/s to 0.43 g/s, determining the air–fuel ratio 1.3–1.6, which gradually approaches the optimum conditions of air supply for the biomass gasification, as recommended in [5]. It should be noted that the increase of additional heat energy and sub-stoichiometric air supply into the gasifier contributes to the enhanced production of NO at the thermal decomposition of biomass by limiting the production of NO at the char gasification stage. Acknowledgments The authors would like to acknowledge the financial support from the European Regional Development Funding 2.1.1.1. ‘‘Sup-
port to Science and Research’’, Nr. 2010/0241/2DP/2.1.1.1.0/10/ APIA/VIAA/006 and the financial support of Latvian Grant V7638. References [1] Walawander W, Chern SM, Fan LT. Fundamentals of thermo-chemical biomass conversion. New-York: Elsevier Applied Science; 1985. p. 911–22. [2] Anil Rajvanshi K. Biomass gasification. In Yogi Goswami D, editor. Alternative energy in agriculture, vol. II. CRC Press; 1986. p. 83–102. [3] Chandrakant Turare. Biomass gasification. ARTES institute, University of Flensburg, Germany; 1997