- Email: [email protected]
A comparison of prairie cordgrass and switchgrass as a biomass for syngas production
Fuel 95 (2012) 573–577
Contents lists available at SciVerse ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
A comparison of prairie cordgrass and switchgrass as a biomass for syngas production Alex Moutsoglou ⇑ Mechanical Engineering Department, South Dakota State University, Box 2219, Brookings, SD 57007, United States
a r t i c l e
i n f o
Article history: Received 5 October 2011 Received in revised form 4 December 2011 Accepted 7 December 2011 Available online 23 December 2011 Keywords: Gasification Prairie cordgrass Switchgrass Simulation CeSFaMB
a b s t r a c t A computational simulation of the gasification of prairie cordgrass (PCG) and switchgrass is conducted to assess their suitability as gasifying fuels in the production of syngas. An Institute of Gas Technology (IGT) bubbling fluidized gasifier is modeled using a commercial code that previously has been documented to simulate this particular reactor exceptionally well. Despite the similarities in the ultimate analyses of the PCG and switchgrass with regards to the amounts of carbon, hydrogen, and oxygen, the composition of the syngas is found to differ notably between the two grasses. The gasification of switchgrass results in significantly higher H2 than that of PCG. The syngas produced from switchgrass also has greater amounts of CO than PCG, but results in very small amounts of methane when compared to PCG. The H2/CO molar ratio of the syngas produced, which is a very important design parameter in the Fischer– Tropsch process, is greater for switchgrass than prairie cordgrass. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Recently, there has been a significant amount of interest in producing fuels from lignocellulosic grasses, as evidenced by the plethora of publications as well as federal funding in this area. In addition to the fast pyrolysis process, biomass can be converted into a synthetic fuel through gasification, a process where biomass is allowed to react with deficient air at high temperatures. The fuel referred to as syngas is primarily made up of hydrogen, carbon monoxide, and methane. Syngas can be burned directly to provide energy or can be converted into liquid fuels via the Fischer–Tropsch polymerization process. In the Fischer–Tropsch (FT) process, carbon monoxide and hydrogen are converted into liquid hydrocarbons using iron and/or cobalt catalysts. The production of syngas is a major cost factor in the operation of an FT reactor. This cost can be as high as 70% of the capital and operating costs of the FT process in making liquid hydrocarbons [1]. It is thus of paramount significance that as much as possible of the reactants CO and H2 are consumed. As illustrated in Moutsoglou and Sunkara [2], the hydrogen to carbon monoxide molar ratio entering the Fischer–Tropsch reactor has a great effect on the selectivity of the hydrocarbon products produced. The study highlighted the importance of matching the molar usage ratio defined from
ðH2 =COÞusage ¼
_ H N_ H N 2in 2out _ CO N_ CO N out in
ð1Þ
with the molar feed ratio H2/CO coming out of the gasifier. It was also found that for the conditions considered in [2], the two ratios match closely for a molar feed ratio H2/CO around 1.9, indicating the maximum utilization of the entering hydrogen and carbon monoxide. Both prairie cordgrass (PCG) and switchgrass are excellent sources for biomass production on wet marginal lands [3]. To assess the performance of prairie cordgrass and switchgrass as biomass sources in producing syngas, a computational study is employed in modeling the gasification of the two lignocellulosic grasses in a pressurized bubbling fluidized bed gasifier built and operated by the Institute of Gas Technology [4]. The commercial code CeSFaMB (Comprehensive Simulator of Fluidized and Moving Bed) [5] is adopted in simulating the gasification in the bubbling fluidized bed reactor. The code has successfully simulated results of experimental data obtained in the IGT reactor. Experimental data and simulation results for the gasification of two different wood pellets are documented in [6]. As can be attested from Tables 9 and 10 in [6], excellent agreement between the experimental data and the simulated results are attained using the code. In the current simulation of the gasification of PCG and switchgrass, reactor conditions as close to those documented in [6] are adopted. Although there are a few published experimental studies on the gasification of switchgrass [7,8], efforts to compare their data with predictions from the computational model failed due to lack of detailed information concerning geometry and operational conditions of these studies. 2. Methods and simulation
⇑ Tel.: +1 605 688 6323; fax: +1 605 688 5878. E-mail address: [email protected] 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.12.016
Two-hundred kilograms of inert sand was used in the IGT reactor as the fluidizing medium. The reactor has a diameter of 0.292 m
574
A. Moutsoglou / Fuel 95 (2012) 573–577
Table 1 Properties of South Dakota prairie cordgrass and Iowa switchgrass. Prairie cordgrass-SD
Switchgrass-IA
Proximate Moisture Volatile Fixed carbon Ash
3.97% 75.44% 15.76% 4.83%
8.4% 73% 13.7% 4.9%
Dry ultimate Carbon Hydrogen Nitrogen Oxygen Sulfur Ash
47.63% 5.81% 0.35% 41.12% 0.06% 5.03%
46.8% 5.1% 0.6% 42.1% 0.1% 5. 3%
HHV dry MJ/kg
18.41
17.88 Fig. 1. Molar percentage of syngas components as a function of mass based air/fuel ratio.
and a total height of 6.147 m and was operated with a bed height of 1.585 m. Steam at 400 °C and 2.23 MPa was introduced as a heating and fluidization agent at a rate of 0.049 kg/s from the main distributor at the bottom of the reactor. Air at 371 °C and 2.23 MPa was injected from the main distributor. Air at 20 °C was also introduced at the intermediate injection point of 0.381 m where the biomass was fed. The biomass entered the reactor at 25 °C at a rate of 0.053 kg/s. The total amount of gasifying air injected varied in order to provide air/fuel ratios for both PCG and switchgrass that corresponded to 20%, 25%, and 30% theoretical air. In each case, the amount of air introduced at the intermediate injection point corresponded to about 34% of the total air added. It is noted that as the theoretical air/fuel ratio of PCG exceeds that of switchgrass, more air is injected for PCG than switchgrass at a ratio of 5.71/5.33. The properties of the prairie cordgrass grown at South Dakota State University and as measured by Hazen Research, Inc., along with those for switchgrass grown in Iowa and provided by Smeenk and Brown [7], are given in Table 1. The higher heating values of the switchgrass were calculated using the empirical correlation of Channiwala and Parikh [9]:
HHV ðkJ=gÞ ¼ 0:34910 C þ 1:1783 H þ 0:1005 S 0:1034 O 0:0151 N 0:0211 A
ð2Þ
where C, H, S, O, N, and A are the corresponding weight percentiles from the ultimate analysis of Table 1. The bulk density of PCG was measured to be 103 kg/m3 while the density of the Iowa switchgrass was reported between 96–128 kg/m3. The CeSFaMB code also required the input of the apparent density of the biomass. In the absence of such data, a published measured pelletized density of switchgrass of 560 kg/m3 was assigned to both grasses. 3. Results and discussion Results from the CeSFaMB code for both prairie cordgrass and switchgrass with air at 20%, 25%, and 30% theoretical air are presented in Figs. 1 and 2. The percent mole fractions of H2, CO, CO2, and CH4 that comprise the major components found in the produced syngas are shown in Fig. 1 as a function of the mass based air/fuel ratio. The gasification of switchgrass consistently produces significantly higher mole fractions of hydrogen than the gasification of prairie cordgrass. This is despite the fact that the amounts of hydrogen in the two grasses are of the same order. The difference in the produced hydrogen gas between the two grasses is made up with more methane gas being produced for prairie cordgrass than switchgrass. The mole fraction of hydrogen for both grasses decrease as the percent theoretical air in the gasifier increases. The mole fraction of methane for PCG also decreases with
Fig. 2. H2/CO molar ratio of syngas as a function of mass based air/fuel ratio.
increasing theoretical air. The methane produced from the gasification of switchgrass is of the order of 1% throughout. The fraction of carbon monoxide produced is found to be independent of the amount of air for the range of air/fuel ratios considered, with switchgrass producing more CO than prairie cordgrass. The carbon dioxide produced from the gasification of prairie cordgrass on the other hand exceeds that from switchgrass, and both decrease with increasing air/fuel ratios for the range considered. The vanishing reaction rates near the top of the freeboard, including the shift reaction provided in the output data, signifies that the mole fractions illustrated in the figure are near equilibrium. The H2/CO molar ratio as a function of the air/fuel ratio is plotted in Fig. 2. The H2/CO ratios follow the trends of Fig. 1, with switchgrass producing H2/CO ratios that exceed those of the prairie cordgrass. As expected, the molar ratios decrease with an increase of the percent theoretical air as hydrogen has a greater affinity to oxygen than carbon monoxide. As the respective amounts of C, H, and O in the two grasses are of the same order as attested from Table 1, the noteworthy differences in the molar concentrations illustrated in Fig. 1, especially for H2, are somewhat surprising. In order to investigate this further, the axial variation of molar fractions and temperatures inside the reactor bed are plotted for both switchgrass and prairie cordgrass for the case of 25% theoretical air in the figures that follow. Figs. 3 and 4 illustrate the axial variation of the molar fractions of O2, CO, and CO2 from the gasification of switchgrass in the two gaseous phases inside the reactor bed: the emulsion and bubbles, respectively. As noted from the two figures, although the oxygen content in the emulsion and bubbles start at the same level, oxygen is
A. Moutsoglou / Fuel 95 (2012) 573–577
consumed faster in the emulsion phase since it is in direct contact with solids and is at higher temperatures near the distributor plate (see Figs. 11 and 12). As the superficial velocity of the gases increases in the bed with increasing bed temperature, the diameter of bubbles increases as bubble size is a strong function of the difference between the actual superficial velocity of the gas and the minimum fluidization velocity. With increasing bubble diameters, heat transfer rates between the emulsion and bubbles decrease, slowing down the consumption of oxygen in the bubbles. Beyond a fluidization condition that establishes the two distinct phases, any excess gas introduced in the reactor is considered to end up in bubbles [10]. Thus, the increase in the oxygen fraction near z = 0.4 m in Fig. 4 is due to the intermediate injection of air at the point where biomass is introduced. Corresponding molar fractions for O2, CO, and CO2 of the gasification of prairie cordgrass are plotted in Figs. 5 and 6 for the emulsion and bubbles, respectively. As can be inferred from the figures, similar trends as those of the gasification of switchgrass are observed. CO2 levels are higher while CO levels are lower for PCG than switchgrass in both emulsion and bubble phases. As a result, the oxygen fraction for PCG is slightly lower than that for switchgrass and is consumed earlier in the bed despite the fact that more air is injected for PCG than switchgrass. Mole fraction variations of H2, CH4, and H2O of the gasification of switchgrass with 25% theoretical air are shown in Figs. 7 and 8 in emulsion and bubbles, respectively. Corresponding illustrations for the gasification of prairie cordgrass are displayed in Figs. 9 and 10. The striking difference among the molar profiles in the figures is the formation of methane in both emulsion and bubble phases in the gasification of prairie cordgrass and the ensuing decrease in hydrogen formation (Figs. 9 and 10). This is attributed to the higher reaction rates for the methanation reaction provided in the output data. Methane formation is minimal for switchgrass, resulting in significantly higher hydrogen mole fractions with the emulsion phase containing higher concentrations. Despite the fact that moisture level in switchgrass is more than twice the level of prairie cordgrass (Table 1), the mole fractions of water are shown to be of the same order between the two grasses in both the emulsion (Figs. 7 and 9) and bubble phases (8 and 10). The temperature profiles as a function of bed height for the gasification of switchgrass and prairie cordgrass are shown in Figs. 11 and 12, respectively. As can be inferred from the two figures, the temperature distribution in the reactor bed for the two grasses is remarkably similar. The sharp peak of the bubble temperature is
Fig. 3. Mole fractions of O2, CO, and CO2 in emulsion for switchgrass with 25% air.
575
Fig. 4. Mole fractions of O2, CO, and CO2 in bubbles for switchgrass with 25% air.
Fig. 5. Mole fractions of O2, CO, and CO2 in emulsion for prairie cordgrass with 25% air.
Fig. 6. Mole fractions of O2, CO, and CO2 in bubbles for prairie cordgrass with 25% air.
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A. Moutsoglou / Fuel 95 (2012) 573–577
Fig. 7. Mole fractions of H2, CH4, and H2O in emulsion for switchgrass with 25% air.
Fig. 10. Mole fractions of H2, CH4, and H2O in bubbles for prairie cordgrass with 25% air.
Fig. 8. Mole fractions of H2, CH4, and H2O in bubbles for switchgrass with 25% air. Fig. 11. Axial temperature variation in reactor bed for switchgrass with 25% air.
Fig. 9. Mole fractions of H2, CH4, and H2O in emulsion for prairie cordgrass with 25% air.
Fig. 12. Axial temperature variation in reactor bed for prairie cordgrass with 25% air.
A. Moutsoglou / Fuel 95 (2012) 573–577
a result of the injection of biomass near that location. No such increase is noted for the emulsion phase. This again is due to the fact that bubbles contain higher oxygen levels than the emulsion phase. The increase in the bubble temperature was also verified for a test case with no intermediate injection of air, and thus cannot be attributed to the intermediate injection of air. As shown in both figures, the average bed temperature at any location is almost that of the emulsion temperature, signifying the small contribution of bubble temperature on the average bed temperature. Finally, a comparison of profiles of the bubble size distributions, bubble rising velocities, and superficial velocities for the gasification of switchgrass and prairie cordgrass did not reveal any noteworthy differences between the two grasses. 4. Conclusions A computational simulation of the gasification of prairie cordgrass and switchgrass in producing syngas is conducted using the CeSFaMB commercial software. Results indicate that the gasification of switchgrass provides higher hydrogen and carbon monoxide molar fractions, as well as higher H2/CO molar ratios than prairie cordgrass. The syngas from the gasification of prairie cordgrass contains higher methane and carbon dioxide levels that those from switchgrass.
577
Acknowledgement This work has been supported through funds provided by the Department of Transportation under Grant DTOS59-07-G-00054. References [1] Steynberg AP, Dry ME, editors. Studies in surface science and catalysis Fischer– Tropsch technology, vol. 152. Elsevier; 2004. [2] Moutsoglou A, Sunkara PP. Fischer–Tropsch synthesis in a fixed bed reactor. Energy & Fuels 2011;25:2242–57. [3] Boe A. Comparison of Prairie cordgrass to switchgrass for biomass on wet soils. In: ASA-CSSA-SSSA international annual meeting, San Antonio, TX; 2011. [4] Evans RJ, Knight RA, Onischak M, Babu SP. Process and environmental assessment of the RENUAS process. In: Symposium on energy from biomass and wastes. Sponsored by Institute of Gas Technology, Washington, DC, April 6–10; 1986. [5] de Souza-Santos ML. CSFMB/CeSFaMB, comprehensive simulator of fluidized and moving bed equipment, Campinas, Brazil; 2011. [6] de Souza-Santos MLA. A new version of CSFB, comprehensive simulator for fluidized bed equipment. Fuel 2007;86:1684–709. [7] Smeenk J, Brown RC. Experience with atmospheric fluidized bed gasification of switchgrass. In: BioEnergy 0 98 conference, Madison, WI, 600-6; 1998. [8] Carpenter DL, Bain RL, Davis RE, Dutta A, Feik CJ, Gaston KR, et al. Pilot-scale gasification of corn stover, switchgrass, wheat straw, and wood: 1. Parametric study and comparison with literature. Ind Eng Chem Res 2010;49:1859–71. [9] Channiwala SA, Parikh PP. A unified correlation for estimating HHV of solid, liquid and gaseous fuels. Fuel 2002;81:1051–63. [10] de Souza-Santos ML. Solid fuels combustion and gasification. Marcel Dekker; 2004.
Contents lists available at SciVerse ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
A comparison of prairie cordgrass and switchgrass as a biomass for syngas production Alex Moutsoglou ⇑ Mechanical Engineering Department, South Dakota State University, Box 2219, Brookings, SD 57007, United States
a r t i c l e
i n f o
Article history: Received 5 October 2011 Received in revised form 4 December 2011 Accepted 7 December 2011 Available online 23 December 2011 Keywords: Gasification Prairie cordgrass Switchgrass Simulation CeSFaMB
a b s t r a c t A computational simulation of the gasification of prairie cordgrass (PCG) and switchgrass is conducted to assess their suitability as gasifying fuels in the production of syngas. An Institute of Gas Technology (IGT) bubbling fluidized gasifier is modeled using a commercial code that previously has been documented to simulate this particular reactor exceptionally well. Despite the similarities in the ultimate analyses of the PCG and switchgrass with regards to the amounts of carbon, hydrogen, and oxygen, the composition of the syngas is found to differ notably between the two grasses. The gasification of switchgrass results in significantly higher H2 than that of PCG. The syngas produced from switchgrass also has greater amounts of CO than PCG, but results in very small amounts of methane when compared to PCG. The H2/CO molar ratio of the syngas produced, which is a very important design parameter in the Fischer– Tropsch process, is greater for switchgrass than prairie cordgrass. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Recently, there has been a significant amount of interest in producing fuels from lignocellulosic grasses, as evidenced by the plethora of publications as well as federal funding in this area. In addition to the fast pyrolysis process, biomass can be converted into a synthetic fuel through gasification, a process where biomass is allowed to react with deficient air at high temperatures. The fuel referred to as syngas is primarily made up of hydrogen, carbon monoxide, and methane. Syngas can be burned directly to provide energy or can be converted into liquid fuels via the Fischer–Tropsch polymerization process. In the Fischer–Tropsch (FT) process, carbon monoxide and hydrogen are converted into liquid hydrocarbons using iron and/or cobalt catalysts. The production of syngas is a major cost factor in the operation of an FT reactor. This cost can be as high as 70% of the capital and operating costs of the FT process in making liquid hydrocarbons [1]. It is thus of paramount significance that as much as possible of the reactants CO and H2 are consumed. As illustrated in Moutsoglou and Sunkara [2], the hydrogen to carbon monoxide molar ratio entering the Fischer–Tropsch reactor has a great effect on the selectivity of the hydrocarbon products produced. The study highlighted the importance of matching the molar usage ratio defined from
ðH2 =COÞusage ¼
_ H N_ H N 2in 2out _ CO N_ CO N out in
ð1Þ
with the molar feed ratio H2/CO coming out of the gasifier. It was also found that for the conditions considered in [2], the two ratios match closely for a molar feed ratio H2/CO around 1.9, indicating the maximum utilization of the entering hydrogen and carbon monoxide. Both prairie cordgrass (PCG) and switchgrass are excellent sources for biomass production on wet marginal lands [3]. To assess the performance of prairie cordgrass and switchgrass as biomass sources in producing syngas, a computational study is employed in modeling the gasification of the two lignocellulosic grasses in a pressurized bubbling fluidized bed gasifier built and operated by the Institute of Gas Technology [4]. The commercial code CeSFaMB (Comprehensive Simulator of Fluidized and Moving Bed) [5] is adopted in simulating the gasification in the bubbling fluidized bed reactor. The code has successfully simulated results of experimental data obtained in the IGT reactor. Experimental data and simulation results for the gasification of two different wood pellets are documented in [6]. As can be attested from Tables 9 and 10 in [6], excellent agreement between the experimental data and the simulated results are attained using the code. In the current simulation of the gasification of PCG and switchgrass, reactor conditions as close to those documented in [6] are adopted. Although there are a few published experimental studies on the gasification of switchgrass [7,8], efforts to compare their data with predictions from the computational model failed due to lack of detailed information concerning geometry and operational conditions of these studies. 2. Methods and simulation
⇑ Tel.: +1 605 688 6323; fax: +1 605 688 5878. E-mail address: [email protected] 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.12.016
Two-hundred kilograms of inert sand was used in the IGT reactor as the fluidizing medium. The reactor has a diameter of 0.292 m
574
A. Moutsoglou / Fuel 95 (2012) 573–577
Table 1 Properties of South Dakota prairie cordgrass and Iowa switchgrass. Prairie cordgrass-SD
Switchgrass-IA
Proximate Moisture Volatile Fixed carbon Ash
3.97% 75.44% 15.76% 4.83%
8.4% 73% 13.7% 4.9%
Dry ultimate Carbon Hydrogen Nitrogen Oxygen Sulfur Ash
47.63% 5.81% 0.35% 41.12% 0.06% 5.03%
46.8% 5.1% 0.6% 42.1% 0.1% 5. 3%
HHV dry MJ/kg
18.41
17.88 Fig. 1. Molar percentage of syngas components as a function of mass based air/fuel ratio.
and a total height of 6.147 m and was operated with a bed height of 1.585 m. Steam at 400 °C and 2.23 MPa was introduced as a heating and fluidization agent at a rate of 0.049 kg/s from the main distributor at the bottom of the reactor. Air at 371 °C and 2.23 MPa was injected from the main distributor. Air at 20 °C was also introduced at the intermediate injection point of 0.381 m where the biomass was fed. The biomass entered the reactor at 25 °C at a rate of 0.053 kg/s. The total amount of gasifying air injected varied in order to provide air/fuel ratios for both PCG and switchgrass that corresponded to 20%, 25%, and 30% theoretical air. In each case, the amount of air introduced at the intermediate injection point corresponded to about 34% of the total air added. It is noted that as the theoretical air/fuel ratio of PCG exceeds that of switchgrass, more air is injected for PCG than switchgrass at a ratio of 5.71/5.33. The properties of the prairie cordgrass grown at South Dakota State University and as measured by Hazen Research, Inc., along with those for switchgrass grown in Iowa and provided by Smeenk and Brown [7], are given in Table 1. The higher heating values of the switchgrass were calculated using the empirical correlation of Channiwala and Parikh [9]:
HHV ðkJ=gÞ ¼ 0:34910 C þ 1:1783 H þ 0:1005 S 0:1034 O 0:0151 N 0:0211 A
ð2Þ
where C, H, S, O, N, and A are the corresponding weight percentiles from the ultimate analysis of Table 1. The bulk density of PCG was measured to be 103 kg/m3 while the density of the Iowa switchgrass was reported between 96–128 kg/m3. The CeSFaMB code also required the input of the apparent density of the biomass. In the absence of such data, a published measured pelletized density of switchgrass of 560 kg/m3 was assigned to both grasses. 3. Results and discussion Results from the CeSFaMB code for both prairie cordgrass and switchgrass with air at 20%, 25%, and 30% theoretical air are presented in Figs. 1 and 2. The percent mole fractions of H2, CO, CO2, and CH4 that comprise the major components found in the produced syngas are shown in Fig. 1 as a function of the mass based air/fuel ratio. The gasification of switchgrass consistently produces significantly higher mole fractions of hydrogen than the gasification of prairie cordgrass. This is despite the fact that the amounts of hydrogen in the two grasses are of the same order. The difference in the produced hydrogen gas between the two grasses is made up with more methane gas being produced for prairie cordgrass than switchgrass. The mole fraction of hydrogen for both grasses decrease as the percent theoretical air in the gasifier increases. The mole fraction of methane for PCG also decreases with
Fig. 2. H2/CO molar ratio of syngas as a function of mass based air/fuel ratio.
increasing theoretical air. The methane produced from the gasification of switchgrass is of the order of 1% throughout. The fraction of carbon monoxide produced is found to be independent of the amount of air for the range of air/fuel ratios considered, with switchgrass producing more CO than prairie cordgrass. The carbon dioxide produced from the gasification of prairie cordgrass on the other hand exceeds that from switchgrass, and both decrease with increasing air/fuel ratios for the range considered. The vanishing reaction rates near the top of the freeboard, including the shift reaction provided in the output data, signifies that the mole fractions illustrated in the figure are near equilibrium. The H2/CO molar ratio as a function of the air/fuel ratio is plotted in Fig. 2. The H2/CO ratios follow the trends of Fig. 1, with switchgrass producing H2/CO ratios that exceed those of the prairie cordgrass. As expected, the molar ratios decrease with an increase of the percent theoretical air as hydrogen has a greater affinity to oxygen than carbon monoxide. As the respective amounts of C, H, and O in the two grasses are of the same order as attested from Table 1, the noteworthy differences in the molar concentrations illustrated in Fig. 1, especially for H2, are somewhat surprising. In order to investigate this further, the axial variation of molar fractions and temperatures inside the reactor bed are plotted for both switchgrass and prairie cordgrass for the case of 25% theoretical air in the figures that follow. Figs. 3 and 4 illustrate the axial variation of the molar fractions of O2, CO, and CO2 from the gasification of switchgrass in the two gaseous phases inside the reactor bed: the emulsion and bubbles, respectively. As noted from the two figures, although the oxygen content in the emulsion and bubbles start at the same level, oxygen is
A. Moutsoglou / Fuel 95 (2012) 573–577
consumed faster in the emulsion phase since it is in direct contact with solids and is at higher temperatures near the distributor plate (see Figs. 11 and 12). As the superficial velocity of the gases increases in the bed with increasing bed temperature, the diameter of bubbles increases as bubble size is a strong function of the difference between the actual superficial velocity of the gas and the minimum fluidization velocity. With increasing bubble diameters, heat transfer rates between the emulsion and bubbles decrease, slowing down the consumption of oxygen in the bubbles. Beyond a fluidization condition that establishes the two distinct phases, any excess gas introduced in the reactor is considered to end up in bubbles [10]. Thus, the increase in the oxygen fraction near z = 0.4 m in Fig. 4 is due to the intermediate injection of air at the point where biomass is introduced. Corresponding molar fractions for O2, CO, and CO2 of the gasification of prairie cordgrass are plotted in Figs. 5 and 6 for the emulsion and bubbles, respectively. As can be inferred from the figures, similar trends as those of the gasification of switchgrass are observed. CO2 levels are higher while CO levels are lower for PCG than switchgrass in both emulsion and bubble phases. As a result, the oxygen fraction for PCG is slightly lower than that for switchgrass and is consumed earlier in the bed despite the fact that more air is injected for PCG than switchgrass. Mole fraction variations of H2, CH4, and H2O of the gasification of switchgrass with 25% theoretical air are shown in Figs. 7 and 8 in emulsion and bubbles, respectively. Corresponding illustrations for the gasification of prairie cordgrass are displayed in Figs. 9 and 10. The striking difference among the molar profiles in the figures is the formation of methane in both emulsion and bubble phases in the gasification of prairie cordgrass and the ensuing decrease in hydrogen formation (Figs. 9 and 10). This is attributed to the higher reaction rates for the methanation reaction provided in the output data. Methane formation is minimal for switchgrass, resulting in significantly higher hydrogen mole fractions with the emulsion phase containing higher concentrations. Despite the fact that moisture level in switchgrass is more than twice the level of prairie cordgrass (Table 1), the mole fractions of water are shown to be of the same order between the two grasses in both the emulsion (Figs. 7 and 9) and bubble phases (8 and 10). The temperature profiles as a function of bed height for the gasification of switchgrass and prairie cordgrass are shown in Figs. 11 and 12, respectively. As can be inferred from the two figures, the temperature distribution in the reactor bed for the two grasses is remarkably similar. The sharp peak of the bubble temperature is
Fig. 3. Mole fractions of O2, CO, and CO2 in emulsion for switchgrass with 25% air.
575
Fig. 4. Mole fractions of O2, CO, and CO2 in bubbles for switchgrass with 25% air.
Fig. 5. Mole fractions of O2, CO, and CO2 in emulsion for prairie cordgrass with 25% air.
Fig. 6. Mole fractions of O2, CO, and CO2 in bubbles for prairie cordgrass with 25% air.
576
A. Moutsoglou / Fuel 95 (2012) 573–577
Fig. 7. Mole fractions of H2, CH4, and H2O in emulsion for switchgrass with 25% air.
Fig. 10. Mole fractions of H2, CH4, and H2O in bubbles for prairie cordgrass with 25% air.
Fig. 8. Mole fractions of H2, CH4, and H2O in bubbles for switchgrass with 25% air. Fig. 11. Axial temperature variation in reactor bed for switchgrass with 25% air.
Fig. 9. Mole fractions of H2, CH4, and H2O in emulsion for prairie cordgrass with 25% air.
Fig. 12. Axial temperature variation in reactor bed for prairie cordgrass with 25% air.
A. Moutsoglou / Fuel 95 (2012) 573–577
a result of the injection of biomass near that location. No such increase is noted for the emulsion phase. This again is due to the fact that bubbles contain higher oxygen levels than the emulsion phase. The increase in the bubble temperature was also verified for a test case with no intermediate injection of air, and thus cannot be attributed to the intermediate injection of air. As shown in both figures, the average bed temperature at any location is almost that of the emulsion temperature, signifying the small contribution of bubble temperature on the average bed temperature. Finally, a comparison of profiles of the bubble size distributions, bubble rising velocities, and superficial velocities for the gasification of switchgrass and prairie cordgrass did not reveal any noteworthy differences between the two grasses. 4. Conclusions A computational simulation of the gasification of prairie cordgrass and switchgrass in producing syngas is conducted using the CeSFaMB commercial software. Results indicate that the gasification of switchgrass provides higher hydrogen and carbon monoxide molar fractions, as well as higher H2/CO molar ratios than prairie cordgrass. The syngas from the gasification of prairie cordgrass contains higher methane and carbon dioxide levels that those from switchgrass.
577
Acknowledgement This work has been supported through funds provided by the Department of Transportation under Grant DTOS59-07-G-00054. References [1] Steynberg AP, Dry ME, editors. Studies in surface science and catalysis Fischer– Tropsch technology, vol. 152. Elsevier; 2004. [2] Moutsoglou A, Sunkara PP. Fischer–Tropsch synthesis in a fixed bed reactor. Energy & Fuels 2011;25:2242–57. [3] Boe A. Comparison of Prairie cordgrass to switchgrass for biomass on wet soils. In: ASA-CSSA-SSSA international annual meeting, San Antonio, TX; 2011. [4] Evans RJ, Knight RA, Onischak M, Babu SP. Process and environmental assessment of the RENUAS process. In: Symposium on energy from biomass and wastes. Sponsored by Institute of Gas Technology, Washington, DC, April 6–10; 1986. [5] de Souza-Santos ML. CSFMB/CeSFaMB, comprehensive simulator of fluidized and moving bed equipment, Campinas, Brazil; 2011. [6] de Souza-Santos MLA. A new version of CSFB, comprehensive simulator for fluidized bed equipment. Fuel 2007;86:1684–709. [7] Smeenk J, Brown RC. Experience with atmospheric fluidized bed gasification of switchgrass. In: BioEnergy 0 98 conference, Madison, WI, 600-6; 1998. [8] Carpenter DL, Bain RL, Davis RE, Dutta A, Feik CJ, Gaston KR, et al. Pilot-scale gasification of corn stover, switchgrass, wheat straw, and wood: 1. Parametric study and comparison with literature. Ind Eng Chem Res 2010;49:1859–71. [9] Channiwala SA, Parikh PP. A unified correlation for estimating HHV of solid, liquid and gaseous fuels. Fuel 2002;81:1051–63. [10] de Souza-Santos ML. Solid fuels combustion and gasification. Marcel Dekker; 2004.