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Steam Gasification of Biomass-Derived Char for the Production of Carbon Monoxide-Rich Synthesis Gas S. T. Chaudhari, S. K. Bej, N. N. Bakhshi, and A. K. Dalai* Catalysis and Chemical Reaction Engineering Laboratory, Department of Chemical Engineering, University of Saskatchewan, Saskatoon, S7N 5C9, Canada Received December 4, 2000. Revised Manuscript Received March 2, 2001
There is growing interest in the conversion of biomass and related materials into gaseous and liquid fuels. During fast pyrolysis of biomass in a fluidized-bed reactor, 15% of biomass is converted to char whereas 70% is converted to liquid and the rest to gas. In the present work, a systematic study has been conducted to explore the possibilities of using biomass-derived char for the production of various types of gaseous fuels and synthesis gas through pyrolysis and steam gasification in a tubular reactor. The pyrolysis experiments were carried out in the presence of nitrogen ( flow rate of 20 mL/min) in the temperature range of 700-800 °C. Steam gasification experiments were carried out in the temperature range of 650-800 °C, steam flow rate of 2.515 g/h/g of char. The reaction time was varied from 0.5 to 2.0 h. It has been found that a combination of lower steam flow rate (about 2.5 g/h/g of char), lower temperature (about 700 °C), along with lower reaction time (about 0.5 h) produces synthesis gas having a molar ratio of H2/ CO favorable for Fischer-Tropsch synthesis. On the other hand, synthesis gas having very high H2/CO molar ratio (about 5-6) can be produced via the steam gasification reaction at a temperature of 800 °C, steam flow rate of about 10 g/h/g of char and a reaction time of 0.5 h. The heating value of the product gas at different process conditions has also been calculated.
Introduction The gradual shortage of oil reserves has created considerable interest in using alternative source of energies, which are renewable in nature. For example, the renewable energy technologies (RET) of the European commission has the target of doubling their contribution from the present 5.6% to about 12% in the future.1 Among all the renewable energy sources, biomass represents the highest potential and will play a vital role in near future.1 Two approaches, namely pyrolysis and gasification of biomass, have been attempted to convert biomass into useable form of energy.2-5 The pyrolysis process is generally carried out by subjecting the biomass to high temperature under inert or oxygen-deficient atmosphere. The fast pyrolysis of biomass generally gives three products, viz., gas, biooil (or bio-fuel), and char. On the other hand, in a gasification process, the solid fuels are completely converted (except the ashes in the feed) to gaseous products having various compositions. Because of the production of cleaner gaseous fuel as well as almost complete conversion of biomass, the gasification process * Corresponding author. Tel: 306-966-4771. Fax: 306-966-4777. E-mail:
[email protected]. (1) Maniatis, K.; Millich, E. Biomass Bioenergy 1998, 15 (3), 195. (2) Pindoria, R. V.; Megaritis, A.; Messenbock, R. C.; Dugwell, D. R.; Kandiyoti, R. Fuel 1998, 77 (11), 1247. (3) Chen, G.; Yu, Q.; Sjostrom, K. J. Anal. Appl. Pyrolysis 1997, 4041, 491. (4) Bakhshi, N. N.; Dalai, A. K.; Thring, R. W. ACS Meeting, Annaheim, CA, 1999. (5) Iqbal, M.; Dalai, A. K.; Thring, R. W.; Bakhshi, N. N. 33rd Intersociety Engineering Conference on Energy Conversion, Colorado, Springs, CO, 1998.
in converting biomass into energy is becoming attractive day by day.3 Some reasonable amount of char is produced during the fast pyrolysis of biomass. This char is highly reactive and can be gasified with gasifying agents such as steam, CO2, oxygen, and hydrogen to gaseous fuels. The char produced during the pyrolysis of biomass, can also be used separately for various applications.6 Recently, there is lot of interest in utilizing the pyrolysis-derived char for steam gasification to produce gaseous fuel.3,7-11 For example, Chen et al.3 have studied the reactivity of char derived from the pyrolysis of Birch wood. They have reported that the reaction rates of the char were strongly affected by the time-temperature history of char formation. It has also been observed that a rapid heating of the raw material gave a char which possesses higher activity in the presence of either carbon dioxide and/or steam as compared to the char formed under slow heating rate. Similarly, Kumar and Gupta7 have studied the influence of carbonization conditions on the gasification of acacia and eucalyptus biomass-derived chars by carbon dioxide. They have reported that both reactivity and activation energy for the gasification of biomass -derived chars were strongly influenced by the carbonization (6) Kumar, M.; Gupta, R. C. Energy Sources 1998, 20 (7), 575. (7) Kumar, M.; Gupta, R. C. Fuel 1994, 73, 1992. (8) Kojima, T.; Assavadakorn, A.; Furusawa, T. Fuel Process. Technol. 1993, 36, 201. (9) Standish, N.; TanJung, A. Fuel 1988, 67, 666. (10) Chen, G.; Sjostrom, K.; Bjornbom, Ind. Eng. Chem. Res. 1992, 31, 2764. (11) Slaghuis, J. H.; Van der Walt, T. J. Fuel 1991, 70, 831.
10.1021/ef000278c CCC: $20.00 © 2001 American Chemical Society Published on Web 04/13/2001
Steam Gasification of Biomass-Derived Char
conditions employed during their preparation and wood type. Slow carbonization led to the production of wood chars having lower reactivities. Kojima et al.8 have studied the gasification kinetics of sawdust char with steam. They have derived the overall gasification rate of the char in a continuous bed with various residence times and conversions. Ross and Fikis12 have studied the gasification reactions of char and modified chars produced from Jack and Pine Bark. The product gases contained CO, CO2, CH4, and H2. They have observed that above the char preparation temperature (i.e., 350 °C), the yields of gaseous products increased with temperature until 550-600 °C when the production of CH4 and CO decreased. Thus, it is evident from the above discussion that the composition of the gas produced during steam gasification of char depends on the inherent nature of the char and the process conditions employed during steam gasification. Depending on the composition of the gas produced, it can be used for a variety of purposes. For example, if the gas is rich in hydrogen content, it can be utilized in fuel cell units for electricity production. Similarly, if the ratio of carbon monoxide and hydrogen is ∼1:2, it can be used as a feedstock for Fischer-Tropsch synthesis. Also, if the gas is enriched with CH4, it can be used as a heating fuel. But, a particular set of process condition during steam gasification may not be optimum for producing gases having different applications. To our knowledge, there is hardly any publication in the literature providing information on different optimum conditions for producing gases for different applications. Hence, there is a need for a systematic study in this direction. The present study has, therefore, been focused with a view to optimize the process conditions for producing gases from biomass-derived char for different uses. As will be evident from the results of this study that simply by adjusting the process conditions, the compositions of the gas can be tuned for specific applications. Experimental Section The biomass-derived char (BDC) used in the present investigation was obtained from ENSYN Technologies Inc., Ottawa, Ontario. It was produced during the fast pyrolysis of biomass using their RTI process (Resource Transforms International Ltd.). Its elemental composition was: C ) 76.38%, H ) 1.01%, N ) 0.22%, and O ) 32.39% (by difference). Its ash content was 3.1 wt % and BET surface area was 0.3 m2/g. The pyrolysis and steam gasification experiments were carried out in a down flow reactor. A schematic diagram of the experimental set up used for the pyrolysis and steam gasification of BDC is shown in Figure 1. It consists of a preheater to vaporize water into steam, an inconell reactor (450 mm long and 11 mm i.d.) with temperature controller, a metering pump (to feed desired amount of water), a liquid collection trap, and a gas collection system. The reactor temperature was controlled by a temperature controller and was measured with a thermocouple placed in the center of the char bed. The procedure for a typical steam gasification experiment is described below. A mixture of 1 g of BDC and an equal amount of quartz chips (of 2-3 mm mesh size) was held on a (12) Ross, R. A.; Fikis, D. V. Can. J. Chem. Eng. 1980, 58, 230.
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Figure 1. Schematic diagram of the experimental set up for steam gasification of char (A: Water pump; B: Furnace; C: Reactor; D: Condenser; E: Liquid product collector; F: Cold trap; G: Gas collector tank containing brine solution). quartz wool plug, which was placed on a supporting perforated disk inside the reactor. The top of the char sample was also covered by another plug of quartz wool. About 3 g of quartz chips (2-3 mm size) was kept on top of this quartz wool for uniform distribution of steam through the char bed. After loading these materials in the reactor, it was weighed and then placed in the furnace. The system was flushed with helium gas for about 30 min. The furnace and the preheater were then turned on. When the reactor temperature reached 150 °C, the water pump was started and the flow was adjusted at the desired level. The formation of gas started when the reactor temperature reached around 300 °C. The products leaving the reactor were passed through a water condenser and the gas product was collected over saturated brine solution in the gas collection system. The condensate was mostly unreacted water. It took about 50-60 min to reach the desired reaction temperature. The steam gasification reaction was continued for 30 min after the attainment of the desired reaction temperature. Then heating of the furnace and the preheater was stopped. The water pump was turned off and the reactor cooled to the ambient temperature. Then, the reactor was removed and weighed to determine the amount of unconverted char. The product gas was analyzed for its composition. The gas analysis was done using two gas chromatographs (Carle GC 500 and Hewlett-Packard 5890). The hydrocarbons were analyzed with a FID using a combination of packed and capillary columns (Stabilwax, 30 m long and 0.25 mm i.d.). Hydrogen, CO, and CO2 were analyzed with a TCD using Chromosorb 102 column (1.8 m length and 3.175 mm i.d.). Temperature programming of the oven was used for analysis of hydrocarbons and permanent gases.
Results and Discussion In the present study, experiments were conducted on the pyrolysis and steam gasification of BDC. The results are compared and possible explanations are given for the role of steam during gasification. The present investigation was also conducted at different process parameters such as steam flow rate (2.5-15 g/h/g of char), reaction temperature (650-800 °C) and time (15120 min) on the gas yield and its composition during steam gasification of a biomass-derived char to determine optimum conditions for producing gases having different applications. Pyrolysis of BDC. The effect of temperature (700800 °C) on the pyrolysis of BDC was determined in the presence of nitrogen flow (20 mL/min). The results on the conversion of BDC, production of gas, and gas composition are shown in Figures 2 and 3.
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Figure 2. Effect of temperature on conversion of char and volume of gas produced in pyrolysis of char.
Figure 3. Effect of steam flow rate on conversion and total volume of gas formed in steam gasification of char at 700 °C. Table 1. Effect of Temperature on Product Gas Composition and H2/CO Ratio in Pyrolysis of Char at N2 Flow Rate of 20 mL/min. product gas composition, mol % temp., oC
conversion, wt %
H2
CH4
CO2
C2+
CO+H2
H2/CO ratio
700 750 800
20 30 40
20.7 21.7 19.0
51.5 50.3 61.8
14.0 11.1 9.5
2.33 1.80 1.85
32.2 36.8 26.9
1.78 1.44 2.41
Figure 2 shows that during the pyrolysis of BDC, char conversion was increased from 20 to 40% with an increase in temperature from 700 to 800 °C. The production of total gas also increased from 19.8 to 23.5 L/100 g of char when the temperature was increased from 700 to 800 °C (see Figure 2). It can be seen from Table 1 that H2 + CO production was 37 mol % and maximum at 750 °C whereas a high H2/CO ratio of 2.4 was observed at 800 °C. It was observed that the production of hydrogen and C2+ hydrocarbons was almost the same at ∼20 mol % and ∼2 mol %, respectively, for all the three temperatures. However, the formation of CO2 decreased in the product gas while that of methane increased with an increase in temperature from 700 to 800 °C. Steam Gasification of BDC. Effect of Steam Flow Rate. The effects of steam flow rate on the conversion of BDC, total volume of gas produced, and its composi-
Figure 4. Effect of steam flow rate on product gas composition and H2/CO ratio in steam gasification of char at 700 °C.
tion were studied by changing the steam flow rate from 2.5 to 15 g/h/g of BDC and maintaining the reaction temperature constant at 700 °C. Figure 3 shows the effect of steam flow rate on char conversion and total gas produced. It is observed that conversion increases from 20 to 72% with an increase in steam flow rate from 0 to 10 g/h/g of char when the reactor was operated for 30 min at 700 °C. As a result, the volume of gas produced increased from 19.8 to 146.5 L/100 g char with increasing steam flow rate from 0 to 10 g/h/g of char. This is quite expected since steam being one of the reactants, increasing its amount in the reaction leads to higher conversion as well as higher gas production. Beyond the flow rate of 10 g/h/g of char, its effect on both conversion and gas production was negligible. The effect of steam flow rate on product gas composition is plotted in Figure 4. The Figure shows that up to a steam flow rate of about 5 g/h/g of char, the percentage of methane, carbon monoxide, and carbon dioxide decreased while that of hydrogen increased. Steam being the major source of hydrogen during steam gasification, an increase in steam flow rate resulted in higher production of hydrogen. Due to higher production of H2, the product gas volume also increased (see Figure 3). Carbon monoxide may be formed either by partial oxidation of char and hydrocarbons or by steam reforming of hydrocarbons. It is quiet well-known that the proportion of CO in synthesis gas becomes low when it is produced by the steam reforming of methane. With the increase in steam flow rate, the steam reforming of methane became significant and as a result, the proportion of CO is decreased. The reduction in the concentration of carbon monoxide also caused low production of carbon dioxide.The composition of the gas is almost independent of the steam flow rate beyond 5 g/h/g of char, possibly because of the attainment of equilibrium in gas composition. Figure 4 also shows that the production of synthesis gas (CO + H2) increases from 32 to 59 mol % with increase in steam flow rate from 0 to 5 g/h/g of char and then levels off. This figure shows the variation in H2/ CO ratio in the product gas with steam flow rate. It is noted from the figure that this ratio increases from 1 to 5 by changing the steam flow from 2.5 to 5.0 g/h/g of
Steam Gasification of Biomass-Derived Char
Figure 5. Effect of steam flow rate on Btu/Scf and Btu of gas/ 100 g char at 700 °C.
char and the remains constant. Also, Figure 4 indicates that use of high steam flow rate at 10 g/h/g of char produces synthesis gas suitable for hydrogen production. It is well-known that synthesis gases having different levels of H2/CO ratios are suitable for different applications. For example, synthesis gas having H2/CO molar ratio in the higher range is desirable for producing hydrogen for ammonia synthesis. Also, this gas can be used to produce pure hydrogen for fuel cell applications. The production of synthesis gas having higher levels of H2/CO ratio is generally not a problem. Besides the high level of H2/CO produced during the commercial steam gasification, this ratio is increased further during the water gas shift reaction for the removal of CO. On the other hand, it is generally rather costly to produce synthesis gas having lower H2/CO molar ratio (in the range of 1-2).13 Synthesis gas having CO and H2 composition in this range is highly desirable as feedstock for Fischer-Tropsch synthesis for producing transportation fuels. It is indicated from Figure 4 that such a composition having H2/CO molar ratio of about 1.0 could be obtained by carrying out the experiments at a very low flow rate of steam (about 2.5 g/h/g of char). Thus, by controlling the input of steam, which is the predominant source of hydrogen, the molar ratio of H2/CO in the synthesis gas during steam reforming of biomass derived char can be adjusted to the desirable value. Besides the above-mentioned applications, the product gas may be directly used as a fuel. Figure 5 shows the heating value (per unit volume of gas) as well as the total energy of the gas obtained from 100 g of char as a function of steam flow rate. The figure shows that gas having heating value in the range of 395 Btu/Scf is produced at low flow rate of steam (2.5 g/h/g of char) compared to a fuel having 666 Btu/Scf during pyrolysis. However, as the volume of gas produced at this low steam flow rate is less, the energy content of the gas is also lower about 1400 Btu/100 g of char (see Figure 3). As the flow rate of steam is increased to 10 g/h/g of char, the heating value per unit volume of gas decreased due to its lower methane content. However, the energy content of the total gas increased because of higher gas formation at higher flow rate of steam (see Figure 3). Beyond this flow rate of steam, the heating value as well as the energy content of the total gas remained constant (13) Wender, I. Fuel Process. Technol. 1996, 48, 189.
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Figure 6. Effect of temperature on conversion of char and volume of gas produced at a constant steam flow rate of 10 g/h/g of char.
Figure 7. Effect of temperature on product gas composition and H2/CO ratio at steam flow rate of 10 g/h/g of char.
since there was no change in gas formation and its composition beyond this steam flow rate. Effect of Temperature. It can be seen from the above discussion that a low steam flow rate produces gas with a low H2/CO ratio (suitable for Fischer-Tropsch synthesis) while a high flow rate of steam favors the production of gas suitable for hydrogen production. Therefore, the effect of temperature has been studied at two different levels of steam flow rates viz. 2.5 g/h/g of char and 10 g/h/g of char. Effect of Temperature at High Steam Flow Rate. The effect of temperature for the steam flow rate of 10 g/h/g of char has been studied by changing it from 650 to 800 °C. The char conversion and gas production are plotted in Figure 6. As expected, both conversion and total gas production increased continuously with increase in temperature. Over 90% conversion of biomass-derived char was achieved producing gas about 350 L per 100 g of char at 800 °C. Figure 7 shows the effects of reaction temperature on the product gas composition. It is observed from the figure that the concentration of methane decreased slowly from 12 to 10 mol % with increasing temperature from 650 to 800 °C because of increasing hydrogen production at higher temperatures. As a result, the hydrogen concentration increased from 48 to 56 mol % for increasing temperature in the above range. The change in the concentration of carbon monoxide and
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Chaudhari et al. Table 2. Effect of Temperature on Product Gas Composition at Steam Flow Rate of 2.5 g/h/g of Char product gas composition, mol % temp., conversion, oC wt % 650 700 750 800
Figure 8. Effect of temperature on total Btu and Btu/Scf in char gasification at steam flow rate of 10 g/h/g of char.
Figure 9. Effect of temperature on conversion and total volume of gas produced at a constant steam flow rate of 2.5 g/h/g of char.
carbon dioxide with temperature was not significant. Figure 7 also shows that as the temperature is increased from 650 to 700 °C, the H2/CO molar ratio increases very sharply from about 4 to 6. Higher molar ratio of about 6 was obtained at all temperatures beyond 700 °C. Thus, the use of high reaction temperature (about 800 °C) along with a steam flow rate of 10 g/h/g of char gave very high conversion of char, thus producing synthesis gas highly suitable for hydrogen production. Figure 8 shows that the heating value of the product gas drops from 350 to 310 Btu/Scf with an increase in reaction temperature from 650 to 800 °C. On the other hand, for a similar increment in reaction temperature the energy content of the total gas showed significant increase i.e., from 1200 Btu to 3700 Btu per 100 g of char. This was due to the higher rate of gas production at higher temperature. Effect of Temperature at Low Steam Flow Rate. The effects of temperature at a low steam flow rate of 2.5 g/h/g of char were also studied by varying it from 650 to 800 °C. The effects of temperature on conversion and gas production are shown in Figure 9. It can be seen from the figure that at lower range of temperature (650 to 700 °C), the increase in conversion with increase in temperature is very small, indicating lower reactivity of biomass-derived char at these temperature and lower steam flow rate. But as the temperature was increased beyond 700 °C, the conversion of char was enhanced significantly. For example, the conversion was increased from 34 to 89% for a change in temperature from 700
29 33 63 87
H2
CH4
CO2
C2+
25.1 15.3 24.6 15.9 27.1 19.3 30.7 2.6 51.2 10.5 24.9 1.1 51.6 9.9 23.1 0.8
CO + H2
H2/CO ratio
44.2 47.4 63.5 66.2
1.31 1.33 4.16 3.53
to 800 °C. A similar trend was also observed in the behavior of gas production with change in temperature. The change in gas composition with an increase in reaction temperature is shown in Table 2. The results show that the concentration of methane increases from 15 to 19 mol % with a temperature change from 650 °C to 700 °C, then decreases from 19 to 10 mol % for a temperature increase from 700 to 750 °C (see Table 2). Beyond this temperature, the concentration of methane remained unchanged. This interesting behavior of methane may possibly be explained by assuming it an intermediate product. At lower temperature (about 650 °C), its rate of formation was very low. At 700 °C, its rate of formation increased. At this temperature, the secondary reactions of methane such as steam reforming, partial oxidation were comparatively lower than its rate of formation and hence, a net increase in its concentration was observed. At 750 °C, the secondary decomposition of methane became more dominating than its formation. Beyond 750 °C, the equilibrium composition of the gas was attained. But the trend in the change in concentration of higher hydrocarbons (C2+) was slightly different. C2+ hydrocarbons, being more reactive than methane, decomposed via secondary reactions at temperatures below 700 °C. Beyond 700 °C, the rate of secondary reactions of C2+ hydrocarbons leveled off because of very low reaction rate due to low concentration of these hydrocarbons. The concentration of CO remained almost constant around 20 mol % at lower range of temperature (up to 700 °C) but decreased sharply to about 12 mol % for higher range of temperature (∼750 to 800 °C) because of the dominance of the secondary reactions of methane at higher temperature. The behavior of carbon dioxide was somewhat similar to that of methane. The concentration of CO2 increased to some extent for a temperature change of 650 °C to 700 °C due to its formation from the complete combustion of C2+ hydrocarbons. As the temperature was increased from 700 to 750 °C, the concentration of CO2 decreased, which could be due to its further reaction with carbonaceous material to form CO. The concentration of CO2 then slowly equilibrated in the temperature range of 800 °C. The effect of reaction temperature at lower flow rate of steam on the heating value and the energy content of the total gas produced are shown in Figure 10. Gas having high heating value of about 650 Btu/Scf was formed at lower temperature. However, the energy content of the total gas produced per 100 g of char is low because of lower gas formation. The reverse situation was observed at higher temperature. Effect of Reaction Time. It is evident from the above discussion that steam gasification of char at a low steam flow rate of 2.5 g/h/g of char and at lower temperature (650-700 °C) could produce synthesis gas having very
Steam Gasification of Biomass-Derived Char
Figure 10. Effect of temperature on total Btu value and Btu/ Scf in char gasification at steam flow rate of 2.5 g/h/g of char.
Figure 11. Effect of reaction time on conversion of char and volume of gas produced at a constant temperature of 700 °C and steam flow rate of 2.5 g/h/g of char.
low molar ratio (∼1.3) of H2/CO, which is suitable for Fischer-Tropsch synthesis. However, the char conversion and the product gas volume generated under these conditions were comparatively lower. It is also evident from the above results that both conversion and gas production could be increased by increasing temperature but the ratio of H2/CO increased drastically. Thus, a logical thinking would be to explore whether the conversion and gas production could be enhanced by allowing the reaction to proceed for a longer time at lower ranges of temperatures and steam flow rates. Further experiments were, therefore, performed to study the effect of reaction time (in the range of 0.5 to 2 h) on BDC conversion, gas formation, and its composition at a constant reaction temperature of 700 °C and a steam flow rate of 2.5 g/h/g of char. The results are shown in Figures 11 and 12. It is clear from Figure 11 that the conversion of char could be increased at a sharp rate from 33 to 65% by increasing the reaction time from 0.5 to 1.5 h. The gas production could also be increased from 63 to 132 L per 100 g of char. But the conversion as well as gas formation leveled off beyond a reaction time of 1.5 h. It can be inferred from these results that different materials present in char react with steam at different rates. The highly reactive materials require lesser reaction time and get converted within 1.5 h of reaction at 700 °C. However, the remaining unconverted material does not react even for a reaction time of 2 h.
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Figure 12. Effect of reaction time on product gas composition at 700 °C and a steam flow rate of 2.5 g/h/g of char.
The effect of reaction time on the composition of total product gas formed is shown in Figure 12. It is observed that the concentration of CO, CO2, methane, and higher hydrocarbons (C2+) decreased with increase in reaction time from 0.5 to 1 h and then remained somewhat constant with further increase in reaction time. On the other hand, the concentration of hydrogen increased initially up to 1 h and then leveled off. As a result, the molar ratio of H2/CO increased from 1.3 to about 5.8 for a change in reaction time from 0.5 to 1 h. Beyond this reaction time, the ratio was obtained in the range of 5-6. Thus, the easily reactive materials, which reacted at an early stage of reaction, produced CO-rich synthesis gas, while the materials that have lower reactivity such as methane produced a H2-rich synthesis gas. It can be concluded that the production of synthesis gas for Fischer-Tropsch applications has to be carried out at lower temperature, lower steam flow rate, and lower reaction time. The results also indicate that under these conditions it is difficult to achieve the complete conversion of char. However, the remaining unconverted material can perhaps find other applications such as activated carbon. The unconverted material can also be possibly consumed by increasing the reaction temperature and producing gaseous fuel for other applications. Comparison between Pyrolysis and Steam Gasification of BDC. If we compare the results of pyrolysis and steam gasification of BDC, it is observed that the conversion and total product gas formed is much more in the case of steam gasification (see Figures 2 and 9). This indicates that steam plays a significant role in the gasification of BDC. The comparison in product gas composition shows that the formation of H2 is more when the steam was used in the gasification (see Tables 1 and 2). The formation of more H2 could be because of the contribution of hydrogen from steam. As a result of the higher amount of hydrogen, the production of synthesis gas (CO + H2) is also more in case of steam gasification. It is interesting to note from Tables 1 and 2 that the concentration of methane is more in case of pyrolysis of BDC. In this case, the intermediate product (methane) does not react with inert nitrogen and hence, its higher concentration is observed. On the other hand, during steam gasification, methane undergoes secondary reaction with steam. As a result, its concentration decreases during steam gasification.
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Conclusion The following conclusions could be drawn from the present study. (i) There is significant difference in the conversion and total gas formation and its composition during pyrolysis and steam gasification of biomass-derived char. (ii) A steam flow rate of 2.5 g/h/g of BDC and temperature 650-700 °C are suitable for the production
Chaudhari et al.
of synthesis gas having lower (1.33) ratio of H2/CO. (iii) A steam flow rate of around 10 g/h/g of BDC and temperature in the range of 800 °C give higher (about 6) ratio of H2/CO. Also, these conditions are suitable to convert 90% of BDC producing 350 L of gas per 100 g of BDC. EF000278C
