Cracking and Coking Behaviors of Nascent Volatiles Derived from

Ind. Eng. Chem. Res. 2009, 48, 2851–2860

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Cracking and Coking Behaviors of Nascent Volatiles Derived from Flash Pyrolysis of Woody Biomass over Mesoporous Fluidized-Bed Material Koji Kuramoto,*,† Koichi Matsuoka,† Takahiro Murakami,† Hideyuki Takagi,† Tetsuya Nanba,† Yoshizo Suzuki,† Sou Hosokai,‡ and Jun-ichiro Hayashi‡ National Institute of AdVanced Industrial Science and Technology (AIST), Onogawa 16-1, Tsukuba, Ibaraki 305-8569, Japan, and Center for AdVanced Research of Energy ConVersion Material (CAREM), Hokkaido UniVersity, N13-W8 Kita-ku, Sapporo 060-8628, Japan

To increase the calorific value of the gaseous product from the woody biomass gasification process, a circulating dual-bubbling fluidized-bed gasification system based on the concept of physical separation of the combustion zone from the gasification zone has been proposed. To cope with the difficulty in control of the tar behavior, a catalytic porous γ-alumina has been considered as a bed material for the above process, which circulates between the biomass pyrolysis/steam gasification zone and the char/coke combustion zone, capturing tar vapor as coke on its surface catalytically in the gasifier. Preliminary process simulation clarified the minimum amount of carbonaceous portion (char and coke) in the biomass that must be burnt in the combustor for a thermally self-sustained process operation, suggesting that the coke derived from tar deposited over γ-alumina should be distributed to combustion and gasification zones by controlling the extent of the steam gasification of coke. To develop a method to control the tar conversion during pyrolysis and gasification, we examined the effects of porous γ-alumina on the tar behavior in the secondary reaction in a two-stage bubbling fluidizedbed reactor. Both gasification tests and Brunauer-Emmett-Teller analyses revealed that porous γ-alumina was markedly catalytic toward coking and dehydrogenation of the tar vapor, until a substantial amount of coke had deposited on the catalyst, thus occluding pores. 1. Introduction “Biomass” is a term that describes any plant-derived organic matter available on a renewable basis, such as crop residues, wood wastes and residues, animal wastes, municipal wastes, and dedicated energy crops and trees. Strategic introduction and expansion of the renewable biomass energy are of growing concern to produce an array of energy-related products such as electricity; liquid, solid, and gaseous fuels; heat; and chemicals. Toward this end, biomass gasification is recognized as an important technology for converting the energy contained in most biomass feedstocks into oils or clean gaseous fuels such as hydrogen and syngas (a mixture of carbon monoxide and hydrogen).1-4 For stable and efficient production of clean and useful gas from biomass resources, a circulating dual-bubbling fluidizedbed biomass gasification process has been developed.5-10 This gasification process is shown schematically in Figure 1. The main components of the process are a steam-blown gasifier and an air-blown combustor. In general, the pulverized biomass particles fed into the gasifier undergo rapid heating and are pyrolyzed. The resulting volatile matter, consisting of condensable organic compounds (so-called tar) and gases released during rapid pyrolysis, is converted into light gases such as H2, CO, CO2, CH4, C2H4, and C2H6 through thermal cracking and steam gasification. Both the pyrolysis of biomass and the subsequent thermal cracking and steam gasification of volatiles and char are highly endothermic reactions, so an adequate thermal energy input is required to maintain the temperature of the gasifier. In the conventional steam gasification of hydrocarbons, a portion of the supplied biomass is burnt with air or oxygen in the gasifier * To whom correspondence should be addressed. Tel.: +81 29 861 8076. Fax: +81 11 706 6850. E-mail: [email protected]. † AIST. ‡ Hokkaido University.

(partial oxidation) to maintain the temperature of the reactor as well as the gasification rate.11 For solid fuels such as coal and biomass, however, the oxygen supplied to the gasifier tends to consume the reactive volatile gas and tar in the reactor, and the remaining less-reactive carbonaceous material (char) must then be gasified with steam. Consequently, the rate of gasification decreases, as does the extent of carbon conversion to calorific gas.12,13 In addition, the gas produced from steam gasification with partial oxidation contains CO2 and N2, which decrease the calorific value of the produced gas. As an alternative means of biomass gasification, a circulating dual-bubbling fluidized-bed biomass gasification process is proposed herein. In this system, less-reactive carbonaceous reminders such as char and coke generated by pyrolysis in the gasifier are transported to a combustor and burnt with air to produce heat. The heat generated in the combustor is transported to the gasifier by an external circulation of hot-bed material between the combustor and gasifier, thus supplying the heat required for the endothermic gasification. Our previous studies14 lead us to believe that the physical separation of exothermic combustion from endothermic gasification afforded by this process, along with the transportation of thermal energy from the combustor to the gasifier by external solid circulation between the two chambers, makes this approach promising to minimize the reduction in the calorific value of produced gas and to enhance the extent of total carbon conversion. When fed into a fluidized-bed gasifier, biomass particles undergo rapid pyrolysis and yield nascent products such as incondensable gases (H2, CO, CO2, and light hydrocarbons), H2O, tar, and char. Tar is a major product of pyrolysis and gasification of biomass (e.g., the tar yield for flash pyrolysis at 773 K frequently exceeds 50% on a carbon basis, whereas the char yield for the same process is less than 20-30%) and condenses in reduced-temperature regions of the reactors, causing a decrease in the process efficiency as well as other

10.1021/ie800760s CCC: $40.75  2009 American Chemical Society Published on Web 02/23/2009

2852 Ind. Eng. Chem. Res., Vol. 48, No. 6, 2009

Figure 1. Schematic illustration of the proposed biomass gasification process (a circulating dual-bubbling fluidized-bed system).

practical problems such as plugging and fouling of the process equipment. Therefore, the concentration of tar at the exit of the gasifier should be kept as low as possible for efficient and stable process operation. In addition, especially for thermally selfsustained operation of the circulating dual-bubbling fluidizedbed biomass gasification process, the demand for thermal energy for endothermic gasification and the supply of energy produced from exothermic char (and coke) combustion should be appropriately balanced by controlling the extent of conversion of tar into gas, and the production of char and coke, as well as their subsequent gasification in the gasifier. To achieve this energy balance, researchers have extensively investigated in-bed tar elimination by means of cracking catalysts with novel metal species, such as Ni-based catalysts.15-19 This approach can promote the reduction of tar in fluidizedbed reactors at relatively low temperatures to yield gas, a part of which could be used as a fuel in the combustor. However, such highly active catalysts are often markedly deactivated by carbon deposition over the active sites of the catalysts or by poisoning by sulfonated and chlorinated gases. Therefore, in this study we investigated another potential refractory bed material, namely, mesoporous γ-alumina particles. γ-Alumina exhibits a significant ability to capture tar produced from pyrolysis of the biomass and convert it rapidly into coke over the mesoporous γ-alumina surface.20-22 Hosokai et al.14 systematically examined the thermal cracking and steam reforming of nascent volatiles derived from the flash pyrolysis of pulverized biomass samples and found that γ-alumina and cokecoated γ-alumina exhibit multiple catalytic functions in capturing, coking, and reforming tar heavier than naphthalene. These effects could be attributed to the highly developed mesoporous structure and intrinsic catalytic properties of γ-alumina. Additionally, Matsuoka et al. examined the pyrolysis and steam gasification behavior of biomass in the presence of γ-alumina and observed the progress of steam reforming of coke deposited onto γ-alumina, as well as the catalytic contribution of active

sites of the resultant char surface to the subsequent water-gas shift reaction.23 Therefore, to further the development of the proposed fluidized-bed reactor process, we believe that an optimum amount of char and tar-derived coke can be fed to the combustor if the conversion of tar into coke and gas can be controlled appropriately by mesoporous γ-alumina particles. We have thus investigated the fundamental characteristics of secondary inhomogeneous reactions, such as thermal cracking and coking of nascent volatiles, over the porous materials at elevated temperatures. In this study, we first examine heat and mass flows in the proposed process using a commercial process simulator to clarify the minimum requirement of the feed of the carbonaceous portion (char and coke) for combustion at different operating conditions of the gasifier for the thermally self-sustained operation of the whole process. Then, the characteristics of the secondary reaction of nascent volatiles derived from the rapid pyrolysis of the biomass over the porous bed material were experimentally examined by means of a twostage bubbling fluidized-bed reactor. 2. Simulation: Preliminary Thermodynamic Analysis of Heat and Mass Flows in a Circulating Dual-Bubbling Fluidized-Bed Gasification Process For preliminary thermodynamic analysis, an Aspen Plus commercial process simulator was used to draw a block diagram (Figure 2) for the circulating dual-bubbling fluidized-bed biomass gasification process based on the proposed concepts of thermal recuperation from combustion of char and coke to gasification of volatiles. Through this simulation, changes in the distribution of chemical energy contained in the biomass to gasifier and combustor with operating variables such as the gasification temperature and partial pressure of steam in the gasifier were examined. The purpose of this numerical thermodynamic analysis was to clarify the possible operating conditions for the gasifier and combustor for thermally self-sustained

Ind. Eng. Chem. Res., Vol. 48, No. 6, 2009 2853

Figure 2. Schematic diagram of the proposed process flow for numerical process simulation. Table 1. Properties of Solid Fuel Used in Simulation ultimate analysis, wt % (dry basis) proximate analysis, wt % (dry basis) C

H

N

O

volatile

fixed carbon

ash

50.47

6.03

0.22

43.06

81.5

18.28

0.22

process operation. In this simulation, the gasification process was split into two steps: pyrolysis in the pyrolyzer and gasification in the gasifier. Here, basic chemical and physical properties of solid fuel such as heat of combustion, heat of formation, and heat capacity were estimated from ultimate and proximate analyses of the fuel by means of an optional general coal enthalpy model prepared in Aspen. Properties of the biomass used in this calculation are shown in Table 1, and sulfides were not taken into account in this simulation. All moisture contained in the biomass feedstock [assumed biomass feed rate: 100 kg h-1 (dry base)] was first removed in the fuel drying unit. Ash and char located downstream from the pyrolyzer were separated from volatiles in the ash separation and char separation units, respectively. The volatiles and steam were eventually introduced into the gasifier. For simplification, the formation of tar was not taken into account in the present analysis. For the gasifier, a Gibbs reactor model, which is based on Gibbs free energy minimization, was applied to calculate the chemical equilibrium composition, accounting for the gaseous products of CH4, CO, CO2, H2, H2O, and solid carbon (char). The char, which was assumed to contain carbon only, was introduced into the combustor and burnt completely at 1223 K. In the gasifier, the volatiles supplied were converted to gas under steam/carbon molar ratios (S/C) that were varied between 0.6 and 2.3. Here, S/C was defined as the molar ratio of the amount of steam supplied to the gasifier as the gasifying agent to the amount of carbon contained in the volatiles supplied to the gasifier. In the present analysis, the amount of char to be introduced into the combustor was determined by calculating the heat duty, in which the enthalpy produced from char combustion was made equal to the enthalpy needed for the gasification of volatiles. Under this heat duty, the minimum requirement in the amount of char that must be burnt in the

combustor to make the gasification process thermally selfsustained was examined at varying gasification temperatures and S/C ratios. 3. Experimental Section 3.1. Two-Stage Fluidized-Bed Reactor for the Investigation of the Secondary Reaction of Nascent Volatiles Derived from Flash Pyrolysis of Woody Biomass. A two-stage fluidized-bed reactor was used to study the secondary reaction of nascent volatiles derived from the rapid pyrolysis of pulverized woody biomass over different porous bed materials. A schematic diagram of the experimental setup is shown in Figure 3. The quartz glass reactor consisted of outer (lower part) and inner (upper part) fluidizing columns (30 mm i.d.) for flash pyrolysis of the biomass and the secondary reaction of nascent volatiles, respectively. The temperature of each zone was maintained at prescribed values by means of an electric furnace. For each experimental run, porous or nonporous particles were loaded on the distributor in the upper secondary reaction zone, forming a static bed height of 5 or 10 mm. Pulverized biomass sawdust (350-500 µm particle size) was continuously fed into the lower pyrolysis zone with a screw feeder, and N2 (at a flow rate of ca. 900 cm3 min-1) was used as an inert carrier gas. In order to observe a fundamental chemical interaction between the porous bed material and tar vapor as well as tar sorption characteristics of the bed materials, no steam was injected in the present experiments. Notably, the feed rate of the biomass sawdust fluctuated intermittently, and sometimes gradually varied with time, because electrostatic adhesion among biomass particles changed the packing state of the biomass feedstock in the hopper. Thus, the metallic parts in the hopper-screw feeder system were grounded during experiments to minimize electrostatic adhesion. To confirm that biomass feeding occurred steadily during experiments, variations in the concentration of the gaseous products and in the gas flow rate at the exit of the reactor were monitored over time with a Q-mass filter and a flowmeter, respectively. The biomass sample was pyrolyzed at a fixed temperature of 773 K in the lower pyrolysis zone. The


Figure 3. Schematic diagram of the experimental apparatus (two-stage fluidized-bed reactor).

nascent volatiles (gas and tar) released in the primary pyrolysis zone were immediately swept away with an upward N2 flow and then introduced into the upper secondary gasification zone. Thus, the chemical interaction between tar and char particles was considered to be suppressed during pyrolysis and gasification, as reported previously.24,25 Incondensable gaseous products were introduced to a microgas chromatograph (micro-GC, M200, Agilent Technologies, Tokyo, Japan) equipped with a thermal conductivity detector, and the amounts of H2, CO, CO2, CH4, C2H4, and C2H6 were quantified. Two tar traps (kept at ca. 200 K) and thimble filters were connected in series at the exit of the reactor to capture tar. Tar captured by the tar traps and filters was recovered as much as possible by washing with acetone (Wako Pure Chemical Industries, Ltd., Osaka, Japan) and vaporizing the acetone with an evaporator. The recovered tar was then dried at 353 K under vacuum for 8 h and weighed. Notably, complete carbon recovery could not be achieved: especially, recovery of the tar was difficult. The reason is that a portion of the tar that might have the broadness of the composition and contain some low-boiling tar would be vaporized during the drying process. Thus, we estimated that the yield of tar for different experimental conditions was by difference. The amount of carbon deposited on the porous or nonporous bed material loaded in the secondary reaction zone during the experiments was estimated by thermogravimetric analysis (Thermoplus TG8120, Rigaku Corp., Tokyo, Japan): coke accumulated on the bed material was burnt with oxygen, and the amount of the deposited carbon was estimated from measurements of transient changes in CO and CO2 concentra-

tions with time in conjunction with the flow rate of the flue gas at the exit of a thermogravimetric analyzer. Brunauer-EmmettTeller (BET) analysis using an adsorbate of N2 (N2-BET; BELSORP18, BEL Japan, Inc., Osaka, Japan) was used to characterize the surface structure of the bed material such as the specific pore volume (Vp), specific surface area (ap), and pore radius distribution because these physical factors can influence the rate and capacity of tar reduction, as demonstrated previously for fluid catalytic cracking and heavy oil cracking.25,26 In particular, we monitored the surface structure of the material during tar exposure by observing changes in the specific surface area and pore size distribution. The acidity of the surface of the bed materials was characterized by a temperature-programmed desorption method using an adsorbate of NH3 (NH3TPD; TPD-1-AT, Bel Japan, Inc., Osaka, Japan). 3.2. Materials. γ-Alumina, porous silica gel, and silica sand each were used as a bed material in the secondary reaction zone. The γ-alumina particles were purchased from Mizusawa Industrial Chemicals Co, Ltd., Tokyo, Japan. Their average particle size was 200 µm, their specific surface area was 230 m2.g-1, their pore volume was 0.47 mL g-1, and their pore size was 4.7 nm. The γ-alumina particles were spherical, hollow, and refractory at temperatures as high as 1273 K. NH3-TPD tests confirmed that γ-alumina had an acidic surface with acidity as strong as that of H-YZ and H-ZSM-5 zeolites (Figure 4), though the density of γ-alumina acidic sites was less than the densities of these zeolites. Porous silica gel (specific surface area ) 483 m2 g-1 and pore volume ) 0.75 mL g-1; Wako Pure Chemical


Figure 4. NH3-TPD analysis for different acidic catalysts (fresh γ-alumina, H-YZ, and H-ZSM5).

Figure 6. Effects of the gasification temperature and bed material on the yields of gas, tar, coke, and char (on the basis of carbon in the biomass).

Figure 5. Changes in FC,comb/FC,total at different S/C ratios and gasification temperatures.

Industries, Ltd., Osaka, Japan) and nonporous silica sand were also tested in the present investigation. The biomass sample was pulverized oak sawdust (the results of ultimate analysis [daf]: C, 49.8 wt %; H, 6.2 wt %; N, 0.3 wt %; O, 43.7 wt %) with particle diameters of 350-500 µm. 4. Results and Discussion 4.1. Thermodynamic Analysis for the Proposed DualBubbling Fluidized-Bed Biomass Gasification Process. Using the simulation, we obtained a minimum fraction (FC,comb/FC,total) of carbon to be burnt in the combustor to the total quantity of carbon fed to the process for the thermally self-sustained operation of the process at different gasification temperatures (Tgasifier) and S/C ratios, where FC,comb and FC,total is defined as the molar flow rate of carbon (as char) supplied to the combustor and the dual-bubbling fluidized-bed system, respectively. The results are shown in Figure 5. In the calculation presented here, the temperature of char combustion (Tcombustor) was fixed at 1223 K. The FC,comb/FC,total value for gasification at Tgasifier ) 873 K and S/C ) 0.67 was about 30% and monotonically increased up to 42% when the S/C ratio was increased to 2.3. In addition, the increase in Tgasifier caused an apparent increase in FC,comb/ FC,total. These results clearly indicate that the thermal energy required to maintain the temperature of endothermic gasification

increased with increasing S/C ratio or Tgasifier. Though not shown here, the so-called cold gas efficiency, which is defined as the ratio of the lower heating value of the product gas to the higher heating value of the biomass decreased with increasing S/C and Tgasifier because of the increase in the energy demand to generate high-temperature steam. From a kinetic point of view, the higher S/C and Tgasifier are both preferable to promote the steam gasification of volatiles, while the gasification of the biomass at relatively lower S/C and temperature is preferable for thermal efficiency of the whole gasification process. For the thermally self-sustained operation of the proposed gasification process, we must appropriately distribute the energy contained in the biomass feedstock for endothermic gasification and exothermic combustion processes. For instance, the maximum yield of char for the present gasification temperature range (Tgasifier ) 873-1023 K) would be less than 30%, which is obviously an insufficient amount for the thermally self-sustained operation of the gasification process. Thus, the ability to control the fate of nascent tar is essential to further the development of the proposed process. In general, the yield of tar from thermal decomposition of the biomass is strongly governed by the temperature: the lower the gasification temperature, the higher the tar yield. An appropriate extent of tar reduction or tar capture at relatively lower gasification temperatures can be achieved by means of a secondary activator, such as catalysts or sorbents. In the following sections, we discuss our experimental results on the secondary reactions of nascent volatiles over porous bed materials in the two-stage fluidized-bed reactor. 4.2. Gasification and Coking Characteristics of Nascent Volatiles over Contact with Different Bed Materials. Figure 6 summarizes the effects of temperature and bed material (silica sand, porous silica gel, and γ-alumina) on the secondary reaction of nascent volatiles observed in continuous pyrolysis/gasification tests using the two-stage fluidized-bed reactor, in which the yields of products (gas, tar, and coke) in the secondary reaction zone are compared in terms of the basis of carbon in the biomass over the different bed materials. Note that the yield of tar is estimated by the following calculation: yield of tar [%, on the basis of carbon contained in the biomass supplied to the reactor]


Figure 7. Variation with time of the yield of gaseous products and carbon conversion to gas (Xc) over different bed materials in the secondary gasification zone: (a) porous silica gel; (b) γ-alumina.

) 100 - yieldgas+coke+char. The primary pyrolysis temperature was maintained at 773 K for all cases, and all of the bar graphs include the char yield in the primary pyrolysis zone. Here, coke is defined as the carbon deposited onto the surface of the bed material. For the case of silica sand, which is a noncatalytic bed material for the gasification of volatiles, a significant amount of tar (more than 50% on the basis of carbon in biomass supplied) was produced, and very little coke formation (less than 1%) was observed on the silica sand. The gas yield increased from 24 to 50% with an increase in the temperature from 873 to 973 K in the secondary reaction zone because the thermal cracking of tar is promoted to some extent owing to the temperature increase. The use of porous silica gel resulted in 52 and 61% reductions in the tar yield at 873 and 973 K, respectively, compared to the case of silica sand. In addition, the presence of porous silica gel resulted in a slight increase in the gas yield at 873 K and in coke yields of 15 and 10% at 873 and 973 K, respectively. When γ-alumina was used in the secondary gasification zone, we observed a urther increase in the yield of coke and almost all tar was converted into coke at 973 K, although there was no quantitative difference in the gas yield between porous silica gel and γ-alumina. Thus, this result suggests that γ-alumina obviously enhanced the conversion of tar derived in the primary pyrolysis into coke, rather than carboncontaining gases, through chemical interaction with γ-alumina. We also compared changes in the yield of gaseous products and conversion of carbon in the biomass to carbon-containing gases with varying biomass feeding periods during the continuous pyrolysis/gasification tests for the bed of porous silica gel (Figure 7a) and γ-alumina particles (Figure 7b). As shown in Figure 7a, when the porous silica gel was used (static bed height of 5 mm), CO, CO2, H2, CH4, C2H4, and C2H6 were observed at the exit of the reactor during the continuous pyrolysis/ gasification test. At the steady state, CO was dominantly formed; its yield accounted for ca. 17 mol/100 mol of carbon, and the extent of carbon conversion to gases (Xc) was about 32% for both bed materials. Also, for the porous silica gel, the yield of H2 was less than 2 mol/100 mol of carbon. In contrast, when γ-alumina was used (Figure 7b), the yield of H2 increased to 5 mol/100 mol of carbon, although the yields of CO, CO2, CH4, C2H4, and C2H6 were almost the same as those observed over

Figure 8. Variation with time of the yield of gaseous products and carbon conversion to gas (Xc) in the presence of γ-alumina in the secondary gasification zone (bed height of γ-alumina ) 10 mm).

porous silica gel. When the amount of γ-alumina was doubled (static bed height of ca. 10 mm), as shown in Figure 8, little change in the yields of CO, CO2, CH4, C2H4, and C2H6 or in the carbon conversion to gas was observed, whereas the yield of H2 increased. These results suggest that the increase in the yield of H2 is not attributable to typical homogeneous reactions, such as the water-gas shift (CO + H2O ) H2 + CO2) or the carbon-water reaction (e.g., C + H2O ) H2 + CO). One possible pathway for the observed increase in H2 is coking of the deposited tar (heavy tar) and the dehydrogenation of deposited organic compounds over γ-alumina.27 As mentioned above, we measured the acidity of γ-alumina through NH3-TPD analysis and found that γ-alumina had an acidic surface with acidity as strong as the acidities of H-YZ and H-ZSM-5 zeolites. Thus, we conclude that the acid sites of γ-alumina selectively abstracted carbon in the deposited tar and eventually cleaved


Figure 9. Variation with the time of gaseous products and carbon conversion to gas (Xc) during repetitive tar-exposure tests using γ-alumina at a secondary gasification temperature of 873 K.

the H-C bonds of volatile organic compounds at elevated temperatures, resulting in the promotion of coking of the nascent tar on the γ-alumina surface. As reported previously,28 aromatic hydrocarbons might be decomposed into coke over the cokecoated surface of γ-alumina because coke on the surface of γ-alumina particles can enhance a “charring reaction”. This autocatalytic effect caused by coke formation on γ-alumina might also be a reaction path by which the yield of H2 increased.14 From a practical point of view, particularly for our biomass gasification process, the bed material should maintain its physical constitution as well as its catalytic activity for reliable tar reduction. Thus, we investigated the durability of the catalytic activity by means of a long-term tar-exposure test at different temperatures in the secondary gasification zone. The results obtained at secondary reaction temperatures of 873 and 973 K are shown in Figures 9 and 10, respectively. In these figures, the range of the biomass-to-alumina mass ratio, MBM/Mcatalyst, is indicated. Notably, the soot formed in the lower pyrolysis zone accumulated on the gas distributor (sintered plate made of quartz glass) of the upper secondary reaction zone during the long-term tar-exposure test and eventually clogged the distributor when the feeding period exceeded about 30-40 min, and thus we were unable to continue the experiment. Therefore, each run was restricted to a biomass feeding period of 30-40 min, and the soot accumulated over the distributor was removed at the conclusion of each prescribed period. The durability of γ-alumina was investigated by repeating this test under the same operating conditions, using the spent γ-alumina particles. As shown in Figure 9, the composition of the produced gases and carbon conversion to gas remained unchanged for the range of MBM/Mcatalyst from 0 to 8.43, and the yield of H2 started decreasing when the feeding period exceeded about 10 min in RUN2. An increase in the secondary reaction temperature from 873 to 973 K (see Figure 10) resulted in an increase in the yields of CO, CO2, CH4, and C2H4 and in carbon conversion. As can be seen in Figure 10, the yield of H2 reached its maximum value of ca. 23 mol/100 mol of carbon at the beginning of the biomassfeeding period and then decreased gradually with exposure duration. As the yield of H2 varied, however, the yields of other gaseous products such as CH4 and other carbon-containing gases

Figure 10. Variation with the time of gaseous products and carbon conversion to gas (Xc) during repetitive tar-exposure tests using γ-alumina at a secondary gasification temperature of 973 K.

Figure 11. Amount of carbon deposition over the porous bed materials at different biomass-to catalysts ratios, MBM/Mcatalyst.

did not vary at all. These results suggest that the acidic sites might not have been active at 873 K (Figure 9), causing the observed limited dehydrogenation of deposited tar on the alumina, whereas a significant amount of H2 was produced through the enhanced coking of tar at 973 K (Figure 10). A further increase of the tar-exposure duration resulted in a gradual decrease in the yield of H2. In Figure 11, the amount of coke deposited over the porous silica gel during tar-exposure tests at 873 K was plotted as a function of MBM/Mcatalyst and was compared with the tar-exposure data obtained for γ-alumina at different temperatures. These plots confirm that γ-alumina had a greater ability to capture tar on its surface than did silica gel, although the specific surface area and the pore volume of fresh porous silica gel are about twice those of γ-alumina. These results also suggest that the rate of carbon deposition on γ-alumina was faster for MBM/Mcatalyst values less than about 6.0 and then decreased slightly as the tar-exposure duration was increased. These trends of carbon deposition/accumulation were


Figure 12. BJH pore radius distributions, specific pore volumes (Vp), and specific surface areas (ap) for (a) fresh γ-alumina, (b) spent γ-alumina (MBM/ Mcatalyst ) 5.68), and (c) spent γ-alumina (MBM/Mcatalyst ) 12.87).

qualitatively the same for secondary reaction temperatures of 873 and 973 K. This observation suggests that the temperature did not significantly influence the amount of coke deposited on γ-alumina in the present temperature range, though more condensed coke will be formed at higher temperatures because of the enhancement of coking/hydrogenation over alumina. The decreases in the yield of H2 and in the rate of coke accumulation during the continuous pyrolysis/gasification test might have been responsible for the observed changes in the surface properties of γ-alumina as coke formed on its surface. Changes in the pore size distribution of the spent γ-alumina following different tarexposure durations were examined by means of N2-BET analysis (Figure 12). As shown in Figure 12a, a single-peaked Barrett-Joyner-Halenda (BJH) desorption pore size distribution centered at 3.5-4.0 nm was obtained for fresh γ-alumina. As the surface was further brought into contact with tar, as shown in Figure 12b,c, the pores with radii larger than 4 nm were preferentially filled by coke, and alumina subjected to longterm tar exposure eventually exhibited a sharp single-peaked pore size distribution centered at 1.96 nm. These results suggest that the formation of coke occurred in the larger pores and that the relatively smaller pores with radii of less than 2 nm were not as interactive with the tar vapor, because these pores were less accessible to the tar vapor. To summarize our findings and discussion, the plausible phenomenon describing the interaction of the alumina surface with the tar vapor is schematically drawn in Figure 13. In the presence of mesoporous γ-alumina in the secondary reaction zone, almost all tar vapor derived in the primary flash pyrolysis is trapped over the acidic surface of alumina at 973 K. We found that a higher yield of H2 was obtained with γ-alumina than with porous silica gel, although the yields and composition of carboncontaining gases were almost the same for both silica gel and γ-alumina. From this result, we conclude that the tar deposited onto the surface of γ-alumina underwent coking, producing the higher yield of H2 observed in the secondary reaction. Another plausible mechanism to explain the increase in the H2 yield is the cracking of tar caused by the autocatalytic effects of coke over the surface of γ-alumina, as described in the literature.29 Also, the results of N2-BET analysis suggested the occurrence of pore shrinkage:30 the pores with radii larger than 4 nm were preferentially filled by coke, and the relatively smaller pores with radii of less than 2 nm remained unfilled, because they were less accessible to the tar vapor. We conclude that coke accumulation, as well as the lack of diffusion of tar vapor to

the small pores, caused reduced chemical interaction of tar with the acidic surface of γ-alumina, thus decreasing the extent of coking and dehydrogenation. As suggested by the simulation results, tar capture and appropriate control of the extent of the gasification of the tarderived coke deposited on γ-alumina enabled the process to achieve efficient and thermally self-sustained operation. Concerning tar reforming characteristics, Hosokai et al.14 examined the fate of heavy tar over coke-coated alumina and found that the overall activity of coke-coated alumina is sufficiently high as to keep the heavy tar yield at 4 nm), whereas smaller pores with radii of less than 2 nm were not likely interactive with the tar vapor, because these pores were less accessible to the tar vapor. Literature Cited (1) Czernik, S.; Bridgwater, A. V. Overview of Application of Biomass Fast Pyrolysis Oil. Energy Fuels 2004, 18, 590–598. (2) Bridgwater, A. V. Renewable fuels and chemicals by thermal processing of biomass. Chem. Eng. J. 2003, 91, 87–102.

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ReceiVed for reView May 12, 2008 ReVised manuscript receiVed December 5, 2008 Accepted December 16, 2008 IE800760S

Cracking and Coking Behaviors of Nascent Volatiles Derived from

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