Energy Fuels 2010, 24, 3108–3118 Published on Web 04/23/2010
: DOI:10.1021/ef100204s
Gasification Characteristics of Coal/Biomass Blend in a Dual Circulating Fluidized Bed Reactor Myung Won Seo,† Jeong Hoi Goo,† Sang Done Kim,*,† See Hoon Lee,‡ and Young Chan Choi‡ †
Department of Chemical and Biomolecular Engineering, Energy and Environmental Research Center, Korea Advanced Institute of Science and Technology (KAIST), 373-1, Guseong-dong, Yuseong-gu, Daejeon, 305-701, and ‡Gasification Research Center, Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea Received February 23, 2010. Revised Manuscript Received April 2, 2010
The effects of reaction temperature (750-900 °C), steam/fuel ratio (0.5-0.8), and biomass ratio (0, 0.25, 0.5, 0.75, 1) on the gasification characteristics have been determined in a dual circulating fluidized bed reactor (combustor, 0.04 m 0.11 m 4.5 m high; gasifier, 0.04 m 0.285 m 2.13 m high). Indonesian Tinto sub-bituminous coal and Quercus acutissima sawdust were used as the coal and biomass, respectively. The product gas yield, carbon conversion, and cold gas efficiency from gasification of biomass are higher than those of coal with increasing temperature and steam/fuel ratio. After pyrolysis, surface area, pore volume, and micropores of coal/biomass blend char increase. The maximum increase in gas yield can be obtained with a biomass ratio of 0.5 at the given reaction temperature. Calorific values of the product gas are 9.89-11.15 MJ/m3 with the coal, 12.10-13.19 MJ/m3 with the biomass, and 13.77-14.39 MJ/m3 with the coal/biomass blends at 800 °C. The synergistic effects on the basis of calorific value and cold-gas efficiency are pronounced with the coal/biomass blends.
overall efficiency.6 Biomass with a high H/C ratio can act as a H2 donor to increase the product gas yield from gasification. Furthermore, the use of coal with biomass can provide stable gasification conditions and prevent biomass seasonal shortage.7 Studies on the cogasification of coal and biomass are summarized in Table 1. All of these experiments were performed in bench- or pilot-scales. Pan et al.8 presented a guideline for the biomass ratio of 20% for low-grade coal and 40% for refuse coal. The addition of biomass to coal increases the product gas heating value and carbon conversion. McLendon et al.9 observed the transport property improvement of the coal/biomass mixture in a pressurized fluidized bed. Kumabe et al.10 found the maximum conversion of the water-gas shift reaction at a biomass blend ratio of 50% in a downdraft fixed bed. Lapuerta et al.11 reported that the producer gas quality and gasification efficiency increase exponentially with increasing biomass ratio in the coalcoke/biomass blend. Fermoso et al.12 found that carbon
1. Introduction Because of the current high energy price, the gasification technology to convert solid fuels to the product gases (H2 þ CO) for the integrated gasification combined cycle (IGCC), coal-to-liquid (CTL), chemical synthetic processes is under active development in many countries.1 Among the solid fuels, coal and biomass are widely used for gasification feedstock. While the utilization of coal is causing serious environmental pollution and carbon dioxide emissions, biomass is environmentally friendly and renewable for its carbon dioxide neutrality when utilized for energy production.2 However, biomass has lower calorific values and density than those of coal, which leads to an increase in the gasifier size with high transportation and storage costs.3 The additive fuel such as coal is needed to supply a heat source for the biomass gasification. Co-utilization of coal and biomass for energy production can provide the economical and environmental benefits.4 It cannot only reduce air pollutants such as NOx, SOx, and volatile organic compounds (VOCs)5 but also improve gasification reactivity and the
(7) Pinto, F.; Franco, C.; Neto, A. R.; Tavares, C.; Dias, M.; Gulyurtlu, I.; Cabrita, I. Effect of Experimental Conditions on Cogasification of Coal, Biomass and Plastics Wastes with Air/steam Mixtures in a Fluidized Bed System. Fuel 2003, 82, 1967–1976. (8) Pan, Y. G.; Velo, E.; Roca, X.; Manya, J. J.; Puigjaner, L. Fluidized-bed Co-gasification of Residual Biomass/poor Coal Blends for Fuel Gas Production. Fuel 2000, 79, 1317–1326. (9) McLenon, T. R.; Lui, A. P.; Pineault, R. L.; Beer, S. K.; Richardson, S. W. High-pressure Co-gasification of Coal and Biomass in a Fluidized bed. Biomass Bioenergy 2004, 26, 377–388. (10) Kumabe, K.; Hanaoka, T.; Fujimoto, S.; Minowa, T.; Sakanishi, K. Co-gasification of Woody Biomass and Coal with Air and Steam. Fuel 2007, 86, 684–689. (11) Lapuerta, M.; Hernandez, J. J.; Pazo, A.; Lopez, J. Gasification and Co-gasification of Biomass Wastes: Effects of the Biomass Origin and the Gasifier Operating Conditions. Fuel Process. Technol. 2008, 89, 828–837. (12) Fermoso, J.; Arias, B.; Plaza, M. G.; Rubiera, F.; Pis, J. J.; Garcia-Pena, F.; Casero, P. High-pressure Co-gasification of Coal with Biomass and Petroleum Coke. Fuel Process. Technol. 2009, 90, 926–932.
*To whom correspondence should be addressed. Telephone: þ82-42350-3913. Fax: þ82-42-350-3910. Email:
[email protected]. (1) Corella, J.; Toledo, J. M.; Molina, G. A Review on Dual FluidizedBed Biomass Gasifier. Ind. Eng. Chem. Res. 2007, 46, 6831–6839. (2) Zhu, W.; Song, W.; Lin, W. Catalytic Gasification of Char from Co-pyrolysis of Coal and Biomass. Fuel Process. Technol. 2008, 89, 890–896. (3) McKendry, P. Energy Production from Biomass (Part 1): Overview of Biomass. Bioresour. Technol. 2002, 83, 37–46. (4) Haykiri-Acma, H.; Yaman, S. Synergy in Devolatilization Characteristics of Lignite and Hazelnut Shell during Co-pyrolysis. Fuel 2007, 86, 373–380. (5) Jones, J. M.; Kubaki, M.; Kubica, K.; Ross, A. B.; Williams, A. Devolatilization Characteristics of Coal and Biomass blends. J. Anal. Appl. Pyrolysis 2005, 74, 502–511. (6) Brown, R. C.; Liu, Q.; Norton, G. Catalytic Effects Observed during the Co-gasification of Coal and Switchgrass. Biomass Bioenergy 2000, 18, 499–506. r 2010 American Chemical Society
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Table 1. Summary of Previous Coal/Biomass Gasification Studies author 8
temperature (°C)/ pressure (atm)
reactor/fuel
biomass ratio (%)
steam/fuel ratio
Pan et al.
fluidized bed/2 types of coal þ pine chips
840-910/ambient
air/steam mixture (unknown)
0, 20, 40, 60, 80, 100
Pinto et al.7
fluidized bed/60% coal þ 20% pine þ 20% PE waste
750-890/ambient
steam flow = 2-5 kg/h O2/steam ratio = 0.02 - 0.28
20
McLendon et al.9 pressurized fluidized bed/ sub bituminous and bituminous coal þ sawdust
∼900/30
unknown
15, 25, 35
Kumabe et al.10
downdraft fixed bed/Mulia coal þ Japanese cedar
900/ambient
air/fuel ratio = 0.5 steam/fuel ratio = 3
0, 30, 50, 60, 80, 100
Lapuerta et al.11
circulating flow/forestry, agricultural waste, industry wastes þ coalcoke
800-1400/ambient
fuel/air ratio = 1.0 -6.0 air gasification
0, 10, 30, 50, 70, 90
Fermoso et al.12
pressurized fluidized bed/ bituminous coal þ petcoke þ 3 types of biomass (almond shell, olive stones, eucalyptus) fluidized bed/sub-bituminous coal þ 3 types of biomass (rice husk, sawdust, coffee husk)
850-1000/-20
mixture of steam (40-85 vol %) and O2 (2-15 vol %), an inert flow of N2
0, 5, 10
800-850/ambient
steam/fuel ratio = 0.1-0.8 air/fuel ratio =2.0- 3.0
6, 15
Velez et al.13
remark For no less than 20% of biomass for low-grade coal and 40% for refuse coal, product gas LHV and carbon conversion from coal increase. With increasing temperature, further hydrocarbon cracking reaction results in increasing H2 and decreasing tar and hydrocarbon content. Synergy not observed for gasification reaction. Transport property of mixture greatly improved compared to coal. With biomass ratio = 0.5, extent of shift reaction becomes maximum and induces synergy. Cold gas efficiency: 65-85% Producer gas quality increases and gasification efficiency increases exponentially with increasing biomass ratio in the mixture. With increasing biomass ratio, H2, CO yield increases and H2/ CO ratio decreases. Carbon conversion and cold gas efficiency increases. Although efficiency decreases with increasing biomass ratio, the advantage from CO2 reduction is dominant.
Table 2. Proximate, Ultimate, Ash Analysis and Calorific Value of the Samples samples
Indonesian Tinto coal
moisture volatile matter fixed carbon ash C H Oa N S calorific value (MJ/kg)
Quercus acutissima sawdust
Proximate Analysis (wt %, As Received) 3.12 34.64 50.86 11.38
2.29 77.01 19.94 0.76
Ultimate Analysis (wt %, daf) 74.6 4.72 19.33 1.07 0.28 28.01
48.89 6.44 44.22 0.13 0.32 19.88
ash analysis (wt %)
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
K2O
Na2O
coal sawdust
41.40 60.60
17.79 16.40
28.21 5.98
6.36 10.93
2.90 0
1.62 2.36
2.50 2.93
0.24 3.99
a
Calculated by difference.
of the medium calorific value gas (12-18 MJ/Nm3) by separating the combustion and gasification zones in which steam is used as a gasifying agent.14 The bed material (silica sand) heated by the generated heat from the combustion reaction in the combustor is transported to the endothermic gasifier to maintain the operating temperature. On the basis of the reaction kinetics of combustion and gasification, the
conversion, cold gas efficiency increases and H2/CO yield ratio decreases with increasing biomass ratio. Most of the cogasification studies were conducted in conventional fluidized beds at atmospheric7,8,13 and pressurized9,12 conditions. Conventional fluidized bed gasifier by using air/steam as a gasifying agent produces the low calorific value gas (4-6 MJ/Nm3) due to nitrogen dilution. Whereas, a dual fluidized bed gasification technology enables production
(14) Hofbauer, H. Scale up of Fluidized Bed Gasifiers from Laboratory Scale to Commercial Plants: Steam Gasification of Solid Biomass in a Dual Fluidized Bed System. Proceedings of the 19th International Conference on Fluidized Bed Combustion, Vienna, Austria, May 2006.
(13) Velez, J. F.; Chejne, F.; Valdes, C. F.; Emery, E. J.; Londono, C. A. Co-gasification of Colombian Coal and Biomass in Fluidized Bed: An Experimental Study. Fuel 2009, 88, 424–430.
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Figure 1. Schematic diagram of a dual circulating fluidized bed reactor: (1) preheater, (2) combustor (riser), (3) cyclone, (4) downcomer, (5) gasifier (bubbling fluidized bed), (6) loop-seal, (7) air box, (8) drain, (9) steam generator, (10) dust filter, (11) condenser, (12) collector, (13) settler, (14) I.D. fan, and (15) suction pump.
combustor is operated in the fast fluidized bed whereas the gasifier is operated in the bubbling fluidized bed.15 The solid circulation rate between the two reactors is controlled by nonmechanical valves (loop-seal, seal-pot) which connect the gasifier and the combustor. The synthesis gas (H2 þ CO) without nitrogen dilution can be obtained from the bubbling fluidized bed gasifier.
experimental variables
operational range
feed rate (kg/h) temperature (°C) steam/fuel ratio (-) fluidizing gas velocity (m/s) biomass blend ratio (-)
6.17-10.3 750, 800, 850, 900 0.5-0.8 0.16-0.26 0, 0.25, 0.5, 0.75, 1
(15) Murakami, T.; Xu, G.; Suda, T.; Matsuzawa, Y.; Tani, H.; Fujimori, T. Some Process Fundamentals of Biomass Gasification in Dual Fluidized Bed. Fuel 2007, 86, 244–255.
In the present study, a dual circulating fluidized bed reactor (combustor, 0.04 m 0.11 m 4.5 m high; gasifier, 0.04 m 0.285 m 2.13 m high) was designed and constructed
Table 3. Experimental Variables and Operational Range
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Figure 2. Effect of reaction temperature on product gas yields from pyrolysis. Table 4. Correlation of Product Gas Yield from Coal and Biomass Pyrolysis y = aT þ b y: gas yield to fuel ratio on a dry ash-free basis (kg/h), T: temperature (K) gas composition H2 CO CH4 CO2
coal biomass coal biomass coal biomass coal biomass
a ( 10-4)
b
1.685 1.624 40.715 21.564 1.773 3.745 -3.584 1.203
-0.03 -0.021 -0.703 -0.16 0.013 -0.035 0.143 0.114
for gasification of coal, biomass, and their blends. The effects of reaction temperature (750-900 °C), steam/fuel ratio (0.5-0.8), and biomass ratio (0, 0.25, 0.5, 0.75, 1) on the gasification characteristics have been determined in the dual circulating fluidized bed reactor. 2. Experimental Section
Figure 3. Effect of reaction temperature on carbon conversion from pyrolysis.
2.1. Materials. Indonesian Tinto sub-bituminous coal and Quercus acutissima sawdust were used. Since Quercus acutissima is widely used in building, pulp, and shipping industries, its demand and supply in Korea is high. The produced Quercus acutissima sawdust is relatively cheap and suitable for the biomass feedstock. The proximate, ultimate, ash analyses and calorific value of the samples are given in Table 2. The particle size of coal and biomass are, respectively, 0-1 mm (dp = 348 μm) and 0-2 mm (dp = 1438 μm). The density of coal and biomass are 1406 and 384 kg/m3, respectively. The bed materials were sand particles with a density of 2466 kg/m3 and a mean diameter (dp) of 276 μm. The minimum fluidization velocity (Umf) of sand particle is determined to be 0.085 m/s at room temperature and 0.043 m/s at 800 °C. 2.2. Apparatus and Procedure. Gasification experiments were carried out in a dual circulating fluidized bed reactor
(combustor, 0.04 m 0.11 m 4.5 m high; gasifier, 0.04 m 0.285 m 2.13 m high) as shown in Figure 1. The apparatus consists of air preheaters, a combustor (riser), two cyclones, a downcomer, a screw feeder, a gasifier (bubbling fluidized bed), and a loop-seal. The combustor, gasifier, and loop-seal have rectangular shape for easy scale-up and maintenance. To enhance heat transfer and lessen heat loss between the two beds, the combustor was attached to the side of gasifier. Compressed air was fed into the combustor through a filter, pressure regulators, and a flow meter into an air box (0.04 m 0.11 m 0.20 m high). A distributor plate with eight bubble caps (4 holes 3.5 mm i.d.) was placed between the air box and the combustor. The entrained particles from the combustor were collected by a cyclone and transported to the gasifier through a downcomer. The exit part from the combustor to cyclone was angled 30° downward to 3111
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Figure 4. Effect of reaction temperature on (a) total gas yield and (b) each product gas yield from the gasifier.
injection point was located at the vertical section (l/d = 2.5) of the loop-seal to improve solids circulation. Temperature distributions were measured by thermocouples in the combustor, cyclone, feeder, gasifier, and loop-seal. Pressure taps were mounted flush with the wall of the column and connected to the manometers and pressure transducers. The obtained temperature distribution and pressure drop signals were stored in a personal computer through a data acquisition system. The whole reactor was insulated with Kaowool to prevent heat loss to surroundings. Also, ash-drain ports were installed at the bottom of the loop-seal. At the beginning of the experiments, only air was fed into the combustor until the bed temperature reached 450-500 °C. Thereafter, the electric heater was turned off and coal or biomass was fed into the reactor. When a desired reaction temperature was reached, steam was introduced into the gasifier. The product gas
aid smooth circulation of solid to the gasifier. Flue gas from the combustor was discharged through a settler and I.D. fan. The coal, biomass, and their blends were injected into the downcomer through a screw-type feeder. Steam was introduced into the air box (0.04 m 0.285 m 0.15 m high) of the gasifier through two pipe spargers and distributed through 12 bubble caps (4 holes1.5 mm i.d.). The solid particles in the gasifier were recycled into the combustor through a loop-seal (0.175 m 0.11 m 0.37 m high) to regulate solid circulation rate by aeration. Air was injected into the loop-seal at three locations; the recycle chamber, supply chamber, and vertical section. The air box (0.178 m 0.11 m 0.20 m high) was divided into a recycle and supply chamber located at the bottom of the loop-seal. Then air was distributed through four (recycle chamber) and nine (supply chamber) bubble caps (4 holes 1 mm i.d.) in the distributor plate. The aeration 3112
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Figure 5. Effect of reaction temperature on carbon conversion in the gasifier and in the combustor.
from the gasifier was sampled when the gasifier operation reached at steady state. Then it was cooled through condensers and analyzed by using gas chromatography (HP GC 5890 series II) with a thermal conductivity detector and columns of 60/80 Carboxen 1000. To calculate the amount of product gas and its composition from the gasifier, helium gas (3 L/min) as a tracer was injected into the gasifier. Flue gas (CO2, CO) from the combustor was also sampled and analyzed by using a nondispersive infrared (NDIR) analyzer (Fuji Electric System Co.). The experimental variables and their operational ranges are shown in Table 3. To determine the interaction between the coal and biomass, structural changes of coal, biomass, and their blends were analyzed. The structural changes such as surface area and pore property of the char after pyrolysis were determined by the BET method (Tristar 3000, Micromeritics). Figure 6. Effect of reaction temperature on the (a) calorific value of the product gas and (b) cold gas efficiency.
3. Results and Discussion 3.1. Gasification Characteristics of Coal and Biomass. 3.1.1. Pyrolysis Characteristics of Coal and Biomass. Since the gasification involves pyrolysis and steam gasification, the pyrolysis characteristics of coal and biomass were determined in the gasifier of the dual circulating fluidized bed reactor. In the pyrolysis operation, only nitrogen gas was introduced into the gasifier. Pyrolysis is known to act mainly on gasification of fuels having high volatile contents.16 Different pyrolysis characteristics of coal and biomass can be expected based on their different proximate and ultimate analyses (Table 2). The effect of reaction temperature (750900 °C) on the product gas yield from coal and biomass pyrolysis is shown in Figure 2. The yield of H2, CO, and CH4 components increase but CO2 yield remains constant with increasing temperature. The correlation of product gas yield with temperature from coal and biomass pyrolysis is shown in Table 4. Compared to CO and CO2, H2 and CH4 yields increase more rapidly with increasing temperature as can be seen in Table 4. Lee et al.16 proposed correlation of gas yields and temperature for Australian sub-bituminous coal. Although a and b values are not applicable to this study, increasing trends of product gas yields accord well with
previous coal pyrolysis studies.16,17 A large amount of O2 and H2 in biomass may induce higher yields of CO, CO2, and H2 from biomass pyrolysis. The effect of reaction temperature on carbon conversion of coal and biomass pyrolysis is shown in Figure 3. The carbon conversion is defined as carbon content in the product gas divided by the carbon content in the feed coal or biomass.18 With the temperature increased, carbon conversion increases from 0.08 to 0.15 with the coal and from 0.32 to 0.42 with the biomass. Carbon conversion of biomass is higher around 0.24-0.27 than that of coal since volatile content in biomass (77.01%) is much higher than that of coal (34.64%). This is because the biomass has relatively weak ether bonds (bond energy = 380-420 kJ/mol) than aromatic ring bonds (bond energy = 1000 kJ/mol) of coal.19 Different pyrolysis characteristics of coal and biomass are expected to affect different product gas compositions, gas yield, and carbon conversion of coal and biomass gasification in the dual circulating fluidized bed reactor. (17) Kim, Y. J.; Lee, S. H.; Kim, S. D. Coal Gasification Characteristics in a Downer Reactor. Fuel 2001, 80, 1915–1922. (18) Kim, Y. J.; Lee, J. M.; Kim, S. D. Coal Gasification Characteristics in an Internally Circulating Fluidized Bed with Draught tube. Fuel 1997, 76, 1067–1073. (19) Zhang, L.; Xu, S.; Zhoo, W.; Lin, S. Co-pyrolysis of Biomass and Coal in a Free Fall Reactor. Fuel 2007, 86, 353–359.
(16) Lee, J. M.; Kim, Y. J.; Lee, W. J.; Kim, S. D. Coal-gasification Kinetics Derived from Pyrolysis in a Fluidized Bed Reactor. Energy 1998, 23, 475–488.
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Figure 7. Effect of steam/fuel ratio on the (a) total gas yield and (b) each product gas yield from the gasifier.
determined to be 4.8 m/s at 800 °C by the emptying time method.21 To operate a constant solid circulation rate in the dual circulating fluidized bed, gas velocity to the combustor (Ug,c) should be above Utr,22 so that it was maintained at 4.8-5.0 m/s at the operating temperature of 750-900 °C. The total and each product gas yields from coal and biomass
3.1.2. Effect of Temperature. Since the main gasification reactions are endothermic in nature, the reaction temperature is one of the most important experimental parameters affecting the performance of the gasifier. The reaction temperature in the gasifier (bubbling fluidized bed) was varied from 750 to 900 °C, and that is a limited temperature for the ash fusion and production of undesirable gases such as NOx.20 The transport velocity (Utr), a transition velocity between the turbulent and fast fluidization flow regimes, was
(21) Goo, J. H.; Seo, M. W.; Park, D. K.; Kim, S. D.; Lee, S. H.; Lee, J. G.; Song, B. H. Hydrodynamic Properties in a Cold-Model Dual Fluidized Bed Gasifier. J. Chem. Eng. Jpn. 2007, 41, 686–690. (22) Goo, J. H.; Seo, M. W.; Kim, S. D.; Song, B. H. Effects of Temperature and Particle Size on Minimum Fluidization and Transport Velocities in a Dual Fluidized Beds. Proceedings of the 20th International Conference on Fluidized Bed Combustion, Xian, China, May 2009.
(20) Foong, S. K.; Lim, C. J.; Watkinson, A. P. Spouted Bed Gasification of Western Canadian Coals. Can. J. Chem. Eng. 1980, 58, 84–92.
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Seo et al. Table 5. Structural Characteristics of Raw and Char Samples of Coal, Biomass, and Coal/Biomass Blend coal raw
char
biomass raw
char
surface area (m2/g) 6.95 83.86 0.29 170.38 pore volume 102 1.55 3.01 0.07 1.28 (cm3/g) average pore size (A˚) 141.7 82.2 628.1 331.3
biomass ratio = 0.5 raw
char
2.20 0.86
208.13 3.11
366.8
43.8
gasification at different temperatures are shown in Figure 4. With the reaction temperature increased, the product gas yield increases due to the increase in initial pyrolysis rate,23 increase in cracking of heavier hydrocarbon/tars, and increase in endothermic char gasification reaction. The product gas yield from pyrolysis, char gasification, and steam reforming of tars in the gasification region increases from 0.27 to 0.40 m3/kg with coal and 0.71 to 0.80 m3/kg with biomass. The total product gas yield from biomass is higher (around 0.4 m3/kg) than that from coal due to the higher pyrolysis yield of biomass. The yields of H2, CO, and CO2 from gasification increase due to the increase of tar and hydrocarbon decomposition, whereas CH4 yield from coal gasification decreases slightly.17 Higher yield of CH4 is obtained compared to that in other types of gasifier because only steam was used as a gasifying agent.20,24 Each contribution of pyrolysis, steam gasification, and combustion for carbon conversion can be expressed as follows: ð1Þ Xc, t ¼ Xc, p þ Xc, g þ Xc, c
Figure 8. Effect of steam/fuel ratio on carbon conversion in the gasifier and in the combustor.
where Xc,t is total carbon conversion obtained in the combustor and gasifier, Xc,p is carbon conversion from pyrolysis, Xc,g is carbon conversion from steam gasification, and Xc,c is carbon conversion from the combustor. The effect of reaction temperature on the carbon conversion is shown in Figure 5. As can be seen, the total carbon conversion (Xc,t) increases with increasing temperature from 0.62 to 0.72 with coal and from 0.96 to 0.99 with biomass. The carbon conversion of coal is lower than that of biomass because the gasification temperature in the present study is lower than that in coal gasification.25 The carbon conversion of biomass in the gasifier (Xc,p þ Xc,g) reaches up to 0.80 at 900 °C so that most carbon conversion takes place in the gasifier. The carbon conversion from the combustor (Xc,c) decreases from 0.30 to 0.19 with increasing temperature. On the other hand, carbon conversion of coal in the gasifier (Xc,p þ Xc,g) ranges from 0.32 to 0.41, indicating that a large portion of the unreacted carbon fractions (Xc,c) is combusted from 0.28 to 0.31 in the combustor. The carbon conversion from steam gasification (Xc,g) maintains from 0.24 to 0.28 with coal and from 0.35 to 0.38 with biomass. Calorific values of the product gas and cold gas efficiency in the present and previous studies17,24,26,27 as a function of (24) Lee, W. J.; Kim, S. D.; Song, B. H. Steam Gasification of Coal with Salt Mixture of Potassium and Nickel in a Fluidized Bed Reactor. Korean J. Chem. Eng. 2001, 18, 640–645. (25) Harris, D. J.; Roberts, D. G.; Henderson, D. G. Gasification Behaviour of Australian Coals at High Temperature and Pressure. Fuel 2006, 85, 134–142. (26) Lee, J. M.; Kim, Y. J.; Kim, S. D. Catalytic Coal Gasification in an Internally Circulating Fluidized Bed Reactor With Draft Tube. Appl. Therm. Eng. 1998, 18, 1013–1024. (27) Pfeifer, C.; Rauch, R.; Hofbauer, H. In-bed Catalytic Tar Reduction in a Dual Fluidized Bed Biomass Steam Gasifier. Ind. Eng. Chem. Res. 2004, 43, 1634–1640.
Figure 9. Effect of steam/fuel ratio on (a) calorific value of the product gas and (b) cold gas efficiency.
(23) Herguido, J.; Corella., J.; Conzales-Saiz, J. Steam Gasification of Lignocellulosic Residues in a Fluidized Bed at a Small Pilot Scale. Ind. Eng. Chem. Res. 1992, 31, 1274–1282.
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0.20 with coal and from 0.38 to 0.45 with biomass, due to the increase in product gas yield. 3.1.3. Effect of Steam/Fuel Ratio. The effect of the steam/ fuel mass ratio into the gasifier on the total and each product gas yield at 800 °C is shown in Figure 7. At the feed rate of 6.17 kg/h, gas velocity in the gasifier (Ug,g) was varied from 0.16 to 0.26 m/s by varying the steam flow rate. The product gas yield increases from 0.31 to 0.40 m3/kg with coal and from 0.70 to 0.76 m3/kg with biomass (Figure 7a). With an increase in the steam/fuel ratio, the secondary water-gas (C þ 2H2O T CO2 þ 2H2) and water-gas shift reactions (CO þ H2O T CO2 þ H2) are enhanced to increase CO2 and H2 production. However with Ug,g increased, the residence time of steam in the gasifier decreases, resulting in decomposition of hydrocarbon and consequent reduction in CO and CH4 production (Figure 7b). The maximum product gas yield can be obtained at the steam/fuel ratio around 0.7 as found previously.28 The effect of the steam/fuel ratio on carbon conversion is shown in Figure 8. As can be seen, with the steam/fuel ratio increased, the total carbon conversion (Xc,t) maintains from 0.61 to 0.67 with coal and from 0.94 to 0.96 with biomass. Carbon conversion of coal and biomass in the gasifier (Xc,p þ Xc,g) maintains from 0.37 to 0.39 and 0.71 to 0.73, respectively. Compared with the reaction temperature, the effect of the steam/fuel ratio on carbon conversion is marginal since a decrease in the CO yield compensates for the increase in the CO2 yield. As observed in Figure 8, most of carbon content in biomass is converted in the gasifier because conversion of biomass takes place within a short reaction time compared to coal. Calorific values of the product gas and cold gas efficiency in the present and previous studies17,24,26,27 with an increasing steam/fuel ratio are shown in Figure 9. With CO2, H2 gas yields increased and CO, CH4 gas yields decreased, calorific values of the product gas slightly
temperature are shown in Figure 6. Calorific value of the product gas slightly decreases with increasing temperature due to the decrease in the amount of hydrocarbons. These values are much higher than those in a conventional fluidized bed24 using air as an oxidant and similar with the downer reactor17 and the internally circulating fluidized bed (ICFB) with an orifice type draft tube26 using only steam as a gasifying agent. The calorific value of the product gas is 12.10-13.19 MJ/m3 with biomass, which is higher than that of coal (9.89-11.15 MJ/m3), due to a higher product gas yield of CH4 (55.5 MJ/kg) and CO (10.9 MJ/kg) as can be seen in Figure 4b. The calorific value of biomass is similar to that in the dual fluidized bed gasifier of wood pellet as a feedstock.27 The cold-gas efficiency increases from 0.11 to
Figure 10. Effect of biomass ratio on the total product gas yield from the gasifier.
Figure 11. Effect of biomass ratio on each product gas yield from the gasifier.
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Figure 12. Effect of biomass ratio on (a) carbon conversion in a gasifier and in a combustor and (b) calorific value of the product gas and cold gas efficiency at 800 °C.
3.2.1. Char Properties of Coal/Biomass Blend. The surface properties of the raw and char product of coal, biomass, and biomass ratio of 0.5 are presented in Table 5. The char samples were collected after pyrolysis under an isothermal condition (800 °C). After pyrolysis, the char product has more surface area and pore volume, whereas the average pore size decreases. The increasing rate of surface area after pyrolysis is higher in the biomass (583 times) than that of coal (12 times) due to the higher volatile matter content and loose structure of the biomass. The largest surface area (208.13 m2/g) and the highest pore volume (311 cm3/g) are exhibited at the biomass ratio of 0.5. Synergistic effects such as higher reactivity for further gasification reaction after pyrolysis and effective surfaces for condensation and repolymerization of tar29 can be expected with a large surface area and pore volume. As the micropore ratio in the pore structure increases, the average pore size decreases with pyrolysis (Table 5). This micropore structure can enhance the gasification reactivity since the reaction takes place on active carbon sites in micropores.
decrease with an increasing steam/fuel ratio as found previously.17,26,27 The calorific value of the product gas decreases from 11.11 to 10.82 MJ/m3 with coal and from 13.02 to 12.80 MJ/m3 with biomass. The cold-gas efficiency increases from 0.12 to 0.16 with coal and from 0.37 to 0.40 with biomass. With the maximum product gas yield, the maximum cold-gas efficiency can be obtained at the steam/fuel ratio of 0.7. 3.2. Gasification Characteristics of Coal/Biomass Blends. The biomass ratio is defined as the ratio of the sawdust feed rate to the total feed rate (sawdust þ coal) on the carbon basis as follows:10 sawdust feed rate ðmolcarbon =hÞ ð2Þ biomass ratio ¼ total feed rate ðmolcarbon =hÞ To determine the interaction between coal and the biomass, the structural changes of the coal/biomass blends were analyzed by using a BET method. (28) Franco, C.; Pinto, F.; Gulyurtlu, I.; Cabrita, I. The Study of Reactions Influencing the Biomass Steam Gasification Process. Fuel 2003, 82, 835–842.
(29) Tyler, R. J. Flash Pyrolysis of Coals. Devolatilitation of Bituminous Coals in a Small Fluidized-bed Reactor. Fuel 1980, 59, 218–226.
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ratio (0.75). The optimum biomass blend ratio can be determined to be 0.5 as found previously.10 Furthermore; these synergistic reactions can be enhanced with larger surface area and pore volume in a coal/biomass ratio of 0.5. The effect of biomass ratio on carbon conversion, cold gas efficiency, and calorific value at 800 °C is shown in Figure 12. With an increase of the biomass ratio, the total carbon conversion (Xc,t) increases from 0.67 to 0.96 and carbon conversion in the gasifier (Xc,p þ Xc,g) increases from 0.39 to 0.72. While the calorific value of the product gas ranges from 11.15 (coal) to 13.0 MJ/m3 (biomass), the calorific values of coal/biomass blends are found to be from 13.77 to 14.39 MJ/ m3. The maximum calorific value (14.39 MJ/m3) is obtained with the maximum yield of combustible gases (H2, CO, CH4) at the biomass ratio of 0.5. Also, the cold-gas efficiency exhibits its highest value (0.45) with the biomass ratio of 0.5. The cold-gas efficiency of coal and biomass are found to be 0.15 and 0.41, respectively.
3.2.2. Gasification Characteristics of Coal/Biomass Blend. The gasification characteristics of coal/biomass blends are determined at a steam/fuel ratio of 0.7 and at the gasifier temperature of 750-900 °C. With an increase in the biomass ratio, the total product gas yield increases from 0.27 to 0.4 m3/kg (750 °C) and 0.4 to 0.8 m3/kg (900 °C). If there is no interaction between the coal and biomass, the product gas yield is expected to increase with increasing biomass ratio proportionally. However, quadratic curves of the product gas yield with an increase in the biomass ratio can be seen in Figure 10. Hydrogen radicals transfer from the biomass to coal and additional decomposition of coal might induce higher product gas yield.19 Increases in the product gas yield are remarkable at a biomass ratio of 0.5 with all the operating temperature. With an increase in the biomass ratio, H2, CH4, CO, and CO2 gas yields increase as can be seen in Figure 11. The obtained H2, CH4, CO gas yields exhibit their maximum values at a biomass ratio of 0.5 or 0.75. The increasing rate of CO product yield is about 10 times higher than that of H2, which indicates the H2/CO ratio decreases with increasing the biomass ratio as found previously in cogasification of Mulia coal/Japanese cedar10 and bituminous coal/pet coke blend.13 CH4 yield increases by increasing the secondary tar craking30 with reactive agent such as water and hydrogen radical, which are major components of biomass volatiles. A large amount of oxygen from biomass pyrolysis is thought to react with carbon from coal to induce partial oxidation and produce a higher yield of CO with coal/biomass blends. However, such an increase in CO2 yield with coal/biomass is not observed that increases linearly with an increasing biomass ratio. Increases in H2, CH4, CO gas yields are pronounced at a biomass ratio of 0.5. This is due to lower H2 radicals from biomass at a lower biomass ratio (0.25) and lower residual char product from coal at a higher biomass
4. Conclusions The effects of reaction temperature (750-900 °C), steam/ fuel ratio (0.5-0.8), and biomass/blend ratio (0, 0.25, 0.5, 0.75, 1) on the gasification characteristics have been determined in a dual circulating fluidized bed reactor. Product gas yield, carbon conversion, and cold gas efficiency increase with increasing temperature and steam/fuel ratio in biomass gasification which are higher than those from coal gasification. Synergistic effect on gas yields is observed with a larger surface area, pore volume, and micropore at a biomass ratio of 0.5. The calorific values of the product gas at 800 °C are 9.89-11.15 MJ/m3 with the coal, 12.10-13.19 MJ/m3 with the biomass, and 13.77-14.39 MJ/m3 with the coal/biomass blend. The maximum cold-gas efficiency is 0.45 with the biomass ratio of 0.5. Acknowledgment. This work was supported by the CTL project of Korea Institute of Energy Research and the Brain Korea 21 graduate program.
(30) Sonobe, T.; Worasuwannarak, N.; Pipatmanomai, S. Synergies in co-pyrolysis of Thai lignite and corncob. Fuel Process. Technol. 2008, 89, 1371–1378.
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