Alternative Reforming Methods of Primary Tar Released from Gas

Ind. Eng. Chem. Res. 2010, 49, 3577–3584

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Alternative Reforming Methods of Primary Tar Released from Gas Treatment of Biomass at Low Temperature for Development of Pyrolysis/Gasification Process Weerawut Chaiwat, Isao Hasegawa, and Kazuhiro Mae* Department of Chemical Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto, 615-8510, Japan

Primary tars produced from gas treatment in a low-temperature region were confirmed by thermogravimetric analysis of tar-mixed biomass that they would be easily handled by using simple reforming methods. A two-stage reactor was used to investigate the secondary reaction of light tar decomposition. The effect of air on light tar decomposition showed that gas products increased when the upper temperature was increased from 600 to 900 °C. When biomass char pyrolyzed to 900 °C was loaded in the upper reactor, the decomposition of light tar under inert atmosphere occurred by steam reforming to produce fuel gases even at a low temperature of 600 °C. When the upper temperature was increased to 800-900 °C, the yield of light tar significantly decreased and became almost zero because steam reforming and related reactions could proceed to produce CO and CO2 even in the case without biomass char. However, CO could be selectively obtained at the highest yield of 24.0 wt % because of the effect of catalytic char at a high temperature of 900 °C. Finally, alternative methods of light tar reforming are discussed for the development of a biomass gasification process to obtain tar-free gaseous products and/or valuable chemicals. 1. Introduction With respect to the depletion of fossil fuels and the global warming issue, biomass is nowadays focused on as one of the potential renewable resources, due to its abundance and environmentally benign aspect. To convert biomass into an energy form, gasification is one of the promising thermochemical conversion methods. The primary reaction of biomass gasification is endothermic pyrolysis, which releases a large amount of undesired tar products. These tar products have low reactivity and may cause damage in gas turbines and/or gas engines in biomass gasification systems.1-5 Many researchers have studied the reduction of tar released during the gasification process of biomass. Tar removal methods at high temperature (700-900 °C) by using catalysts based on Ni and other transition metals, natural minerals, mesoporous materials, and biomass/coal-derived chars in fixed bed reactors were widely investigated.6-16 The use of biomass chars has some advantages over metal catalysts as a byproduct from gasification because these catalytic chars can be gasified or burned after deactivation. Boroson et al.17 showed that a fraction of newly formed wood pyrolysis tars is very reactive in the presence of fresh wood pyrolysis char at temperatures as low as 400 °C. Abu El-Rub et al.18 compared biomass char with various well-known catalysts such as nickel, silica sand, and raw dolomite as catalytic bed materials in a fixed bed tubular reactor at the temperature range 700-900 °C for the conversion of model tars. Biomass char gave the highest naphthalene conversion among the lowcost catalysts used for tar removal. Hosokai et al.19 studied the decomposition of mono- to tetraaromatics over charcoal at 700-900 °C under steam and H2 atmosphere. They found that naphthalene was completely decomposed at even 750 °C by coking, so-called carbon deposition, rather than steam reforming irrespective of temperature and steam/H2 concentrations if the charcoal has a sufficiently large micropore surface area. However, tar products released at high temperature and mostly used as tar models in previous research were often classified as secondary and tertiary tar compounds such as phenols, benzene, * To whom correspondence should be addressed. E-mail: kaz@ cheme.kyoto-u.ac.jp.

naphthalene, and polyaromatic hydrocarbons (PAHs), which are relatively difficult to decompose.20 In previous work, we found that gas treatment at temperatures lower than 400 °C in the uppermost region of a downdraft gasifier seems to release tar products with low molecular weights, which would be simply converted to other forms of volatile products.21 The combination process of biomass pretreatment and removal of light tar products, therefore, should be further studied for a novel development of a biomass gasification system. In this work, the distribution of light tar products from gas treatment at low temperature was preliminarily examined by using the previously proposed parameter index, the so-called degree of dehydration. Biomass was then mixed with those primary tars to investigate the behavior of pyrolysis under inert and airlike gas atmosphere. We have further investigated the decomposition of light tar obtained during gas treatment at low temperatures below 400 °C by using a two-stage vertical reactor to study the phenomena in the pyrolysis zone of a downdraft gasifier. The secondary reaction of tar with air was investigated by varying the reactor temperatures to study the effect of air on tar decomposition. Pyrolyzed biomass char was then loaded into the upper reactor as the catalyst bed material to study its catalytic effect on light tar decomposition. Plausible reactions, which can occur during tar decomposition, and alternative processes with the combination of pretreatment and selective reforming methods of light tars are finally discussed. 2. Experimental Section 2.1. Preparation of Biomass Sample. 2.1.1. Raw Biomass. Japanese cedar wood was used as a biomass sample because of its abundance in Japan’s forests. A raw sample was pulverized and sieved into 210-500 µm, and then dried in vacuo at 70 °C for 24 h prior to use. The elemental composition of Japanese cedar wood sample consists of 50.6 wt % carbon, 5.9 wt % hydrogen, and 43.5 wt % oxygen. It contains 38.5 wt % lignin component with only 0.07 wt % ash content. 2.1.2. Biomass Mixed with Light Tar from Air Treatment at Low Temperature. In order to investigate the recycle of light tar obtained from air treatment at low temperature,

10.1021/ie901695r  2010 American Chemical Society Published on Web 03/03/2010

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Ind. Eng. Chem. Res., Vol. 49, No. 8, 2010

Figure 1. Schematic diagram of the experimental apparatus with a quartz tube reactor used to produce biomass char. Table 1. Biomass Samples Mixed with Light Tar Obtained from Air Treatment tar-mixed biomass

sample

raw biomass amt after drying [g]

sample amt (wet basis) [g]

moisture content [wt %]

mixed tar in samples (dry basis) [wt %]

biomass to tar mixing ratio (dry basis)

BT1 BT2 BT3

0.8010 0.2074 0.1063

0.8908 0.2736 0.1499

6.77 7.59 7.28

3.31 16.60 21.81

29.2:1 5:1 3.6:1

biomass samples of approximately 0.2 g were treated in a quartz tube reactor (Figure 1) under air atmosphere to 300 °C for several runs, and then their condensed tar products were collected by washing with acetone. Each collected tar was mixed to dried raw samples of approximately 0.8, 0.2, and 0.1 g to obtain different mixing ratios of tar-mixed biomass samples designated as BT1, BT2, and BT3, respectively, as shown in Table 1. Tar-mixed samples were kept at room temperature with a stirrer for 12 h before leaving for acetone evaporation without heating. The moisture content in each sample was determined by comparing the amounts of the tar-mixed samples before and after keeping at 110 °C for 20 min under inert atmosphere. The biomass to tar mixing ratio of each sample then could be calculated as shown in Table 1. Those tar-mixed biomass samples were preliminarily examined by using a thermogravimetric analyzer (Shimadzu, TGA-50) to investigate their pyrolysis behaviors and the effect of air on mixed tar. 2.1.3. Biomass Char. For the preparation of biomass char being used as a simple catalyst in the process investigation of tar decomposition, wood chips of Japanese cedar were pyrolyzed under nitrogen atmosphere from room temperature to 900 °C at a heating rate of 20 K · min-1 in a quartz tube reactor as shown in Figure 1 to produce char of biomass. The preparation was repeatedly carried out to obtain enough chars for use. 2.2. Process of Two-Stage Reaction for Tar Decomposition. A two-stage vertical reactor composed of stainless steel (SUS 316, outer diameter ) 9.5 mm, wall thickness ) 1.0 mm) was used for gas treatment and the tar decomposition process as shown in Figure 2. Pulverized biomass sample of approximately 300 mg was input into the lower reactor, which was 22.4 cm in length. The samples were fixed at the middle of the reactor by using quartz wool. Air treatment was performed by heating the samples in an image furnace from room temperature to 300 °C as shown in Figure 2a, whereas gas treatment with nitrogen was conducted to 340 °C with the same heating rate of 10 K · min-1 as shown in Figure 2b. In the upper reactor of 14.7 cm in length, 100-600 mg of prepared biomass char was well packed by using quartz wool as the catalyst in the upper reactor. The constant temperature of the upper reactor was varied from 600 to 900 °C to investigate the effect of

Figure 2. Schematic diagram of the experimental apparatus of a two-stage SUS reactor used to study the effect of (a) air and (b) pyrolyzed char on light tar decomposition.

temperature on tar decomposition. An experiment without a char packed bed in the upper part was also carried out for comparison. Gas products, then, were analyzed by a gas chromatograph (Shimadzu Co., GC-14A). Moreover, tar decomposition at 600 and 900 °C in the upper reactor filled with the char was repeatedly carried out in order to investigate the deactivation of catalytic biomass char. After the two-stage reactor, volatile products were condensed in the condenser filled with dry ice before the noncondensable gases were collected in the gas bag. The condensable products were carefully washed and collected by using acetone. For the qualitative and quantitative analysis of tar and water, acetone was used as the mobile phase and was fed at 0.5 mL · min-1 to a gel permeation chromatograph (GPC) equipped with a column (Showa Denko, Asahipak GF-310HQ) and a refractive index (RI) detector, to estimate the compositions of tar products. 3. Results and Discussion 3.1. Distributions of the Degree of Dehydration Obtained from Gas Treatment in the Low-Temperature Region. Product distributions of biomass treatments with air and nitrogen in the low-temperature region (240-340 °C) were examined by comparing the distributions of the degree of dehydration, proposed in our previous study,22 with those of cellulose pyrolysis performed at the same heating rate of 10 K · min-1 as shown in Figure 3. For nitrogen treatment, Figure 3a shows that the degree of dehydration during treatment (Xp), calculated


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Figure 4. TG curves of raw biomass and tar-mixed biomass with different mixing ratios when pyrolyzed under helium atmosphere at 10 K · min-1.

Figure 3. Distributions of the degree of dehydration determined from biomass treatment with (a) nitrogen and (b) air, compared with cellulose pyrolysis at 10 K · min-1.

by eq 1 as discussed in our previous study,22 starts to increase from the relatively lower temperature at 240 °C compared to cellulose pyrolysis. Xp )

yield of H2O produced during pyrolysis 16.7 wt %

(1)

The final Xp seemed to be obtained when the temperature reached 320 °C. Moreover, the value of Xp obtained from nitrogen treatment of biomass was much higher than those obtained from cellulose pyrolysis because the determination of Xp in our previous study is based on only the dehydration of cellulose, which has a simple and crystallized structure. Since biomass has a very complex structure with the interaction among cellulose, hemicellulose, and lignin, the maximum amount of releasable water from the biomass structure is not limited to only 16.7 wt % as our previous assumption for the determination of Xp.22 For air treatment as shown in Figure 3b, the value of Xp was extremely higher than those from nitrogen treatment because reactive oxygen in air caused partial oxidation and dehydration to simultaneously release a large amount of water. When focusing on the degree of dehydration in char (Xc), calculated by using the determination in our previous study,22 air-treated biomass showed its maximum at 300 °C, whereas Xc obtained from biomass treated with inert gas gradually increased and seemed to be steady at final Xc. From this, it can be concluded that gas treatment of biomass would not release cross-linked tar when it was treated below 300 °C in air and 340 °C under inert atmosphere. This also indicates that tar produced from gas treatment in the low-temperature region would be easily decomposed by using simple methods to finally obtain tar-free gaseous products. However, a detailed experiment with structural investigation of tar products should be further carried out to validate this assumption in future work. 3.2. Recycle of Light Tar from Air Treatment in the Low-Temperature Region by Mixing with Raw Biomass. Since primary tar released from air treatment at low temperature seems to be easily decomposed, treatment of these light tars with a simple method should be further studied. The recycle of

the light tars by mixing with their raw biomass was preliminarily investigated as one alternative tar treatment method. The mixing ratio of biomass to tar was varied as previously discussed in the Experimental Section. Thermogravimetric analysis was performed to compare the pyrolysis behavior of those tar-mixed biomasses with different mixing ratios. TG curves in Figure 4 show that weight loss of tar-mixed biomass with higher tar content starts at relatively lower temperature compared to pyrolysis of raw biomass. Moreover, the yield of char after pyrolysis to 800 °C was relatively lower when increasing the tar content mixed in raw biomass because the proportion of raw biomass in the tar-mixed sample was less than that of raw biomass. Then, the effect of reactive oxygen on pyrolysis behavior of tar-mixed biomass was further investigated under airlike atmosphere (He + O2) by using TGA. Weight losses of gasification and airlike gas treatment to 340 °C were compared between tar-mixed biomass and raw biomass as shown in Figure 5. TG curves under airlike atmosphere show almost no difference in final char yield after pyrolysis to 800 °C. This phenomenon can be explained as that most of the light tar components mixed in raw biomass can be easily evaporated at a lower temperature range of 150-250 °C before the decomposition of biomass occurs around 250-400 °C without the effect of air. It can be also concluded that no interaction would occur between mixedtar and raw biomass. Consequently, it can be assumed that the deposition of carbon (coking formation) on metal catalysts, which can be normally found during secondary decomposition of heavy tars as discussed in the literature by other researchers, may not occur in the case of those light tars. Then, simple reforming methods of light tar products would be possibly considered for the development of the gasification process. 3.3. Investigation of Light Tar Decomposition without Using Novel Catalysts. According to the confirmation results in previous sections, we found that gas treatment at temperatures lower than 400 °C in the uppermost region of a downdraft gasifier (drying zone) can possibly obtain tar products with low boiling points, which would be simply converted to other forms of volatile products. It, therefore, would be easy to handle those light tars in the next step of the pyrolysis/gasification process. The air-treated char obtained from the drying zone also release less heavy tars at high temperature due to cross-linking in its structure. These product precursors are continuously moved into the pyrolysis zone, where the decomposition of tar occurs at higher temperatures around 600-900 °C as shown in Figure 6. Without additional catalysts, there are three main factors that influence the phenomena of tar decomposition: (i) temperature, (ii) reactive oxygen in air, and (iii) pretreated char itself as previously discussed. Consequently, the effect of the above

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Figure 5. TG curves of tar-mixed biomass (a) BT1, (b) BT2, and (c) BT3 with different mixing ratios when treated under airlike atmosphere at 10 K · min-1.

factors on the decomposition of light tar was then examined to further develop the gasification process to obtain tar-free gaseous products. 3.3.1. Effect of Air on Light Tar Decomposition. The effect of air on the decomposition of light tar obtained from air treatment at low temperature was studied by using the twostage reactor (Figure 3a) as described in the Experimental Section. Figure 7 shows the yield of gaseous products by varying the upper temperature from 600 to 900 °C. The results show that CO2, CO, and hydrocarbon gases gradually increased with an increase in the temperature of the upper reactor. Moreover, the yields of CO2 were significantly higher than those of CO for each experiment run. This indicates that partial oxidation of light tar with air can more proceed to produce a large amount of CO2, whereas steam reforming to produce CO was partially suppressed. Although the total product yields for all experiments were difficult to accurately calculate due to the addition of oxygen content in air and the problem of tar and water collection, tar yields after the upper stage of tar reforming would decrease when the upper temperature was decreased because tar products seem to be converted to gases by mainly partial oxidation and steam reforming. Tar products condensed inside the reactor wall at room temperature after the two-stage reactor can be analyzed by the GPC method as shown in Figure 8. The results show that the elution peaks gradually disappeared with the remaining sharp

peak at the retention time of 20 min (cellobiosan) as the upper temperature increased from 600 to 800 °C. This indicates that tar products obtained after decomposition with air above 700 °C seem to be simply for further handling as previously discussed. Moreover, since the peak of cellobiosan can be only obtained when the upper temperature reaches 800 °C, it can be also concluded that cellobiosan can be selectively extracted as a valuable chemical at this temperature. No peaks were finally obtained when the upper temperature was increased to 900 °C. This indicates that, under air atmosphere, the high temperature at 900 °C may be necessary for the complete decomposition of light tar products obtained from air treatment at low temperature. 3.3.2. Effect of Catalytic Biomass Char on Light Tar Decomposition. The effect of pyrolyzed biomass char on light tar decomposition was then investigated by varying the temperature in the upper reactor. Figure 9 shows the influence of biomass char in the upper part at 600 °C on the product yield distribution under inert gas atmosphere. Nitrogen treatment to 340 °C in the lower reactor without the upper reactor was also conducted for comparison. Volatile products mainly consisted of tar and water with very a small amount of CO2, whereas CO was not obtained at 600 °C under inert atmosphere without the upper reactor. When adding the upper reactor without packed catalysts of biomass char, which was constantly heated at 600 °C, the product yield distribution was also not different from that of no upper reactor. This indicates that the temperature of 600 °C is too low to convert tar products to other forms without catalysts or reactive gas such as air. When approximately 100 mg of biomass char pyrolyzed to 900 °C was loaded in the upper reactor, fuel gases such as CO and hydrocarbon gases were produced with a greater amount of CO2, whereas the amounts of tar and water products were significantly decreased. This shows that tar decomposition by steam reforming was stimulated to proceed by the influence of biomass char even at a low temperature of 600 °C. Then, the amount of biomass char was increased to approximately 600 mg added in the upper reactor to investigate the effect of char amount. The result showed small differences with a slight increase in gaseous products. This indicates that the amount of biomass char has not much influence on tar conversion at this condition because a small amount of initial raw material can release only small amount of gaseous products. However, it can be preliminarily concluded that biomass char influences tar reforming as a catalytic reagent to produce gaseous products even at a low temperature of 600 °C. When the constant temperature of the upper reactor was increased to 800 °C without biomass char, the yield of CO significantly increased to 16.3 wt %, compared to 7.7 wt % obtained from that at 600 °C with added biomass char, as shown in Figure 10. Moreover, light tars obtained from the lower reactor were almost completely decomposed at 800 °C without added catalytic char. When approximately 600 mg of biomass char was added in the upper reactor, the yield of tar finally became zero. Furthermore, the yield of CO slightly increased to 17.2 wt % due to the effect of biomass char, whereas the yield of hydrocarbon gases significantly decreased. Consequently, for the upper reactor at 800 °C, it can be concluded that the temperature effect seems to have more influence on tar decomposition with the steam reforming reaction compared to the catalytic effect of biomass char. When the upper temperature was further increased to 900 °C, without biomass char added in the upper reactor, the product yield distribution seems to rarely show a difference in comparison to those of the upper reactor temperature at 800 °C. This indicates that tar decomposition by the reforming with


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Figure 6. Schematic diagram of tar decomposition in downdraft gasifier.

Figure 7. Gas yields by two-stage reactor with the same lower stage of air treatment to 300 °C, but different upper stages at the fixed temperature varying from 600 to 900 °C.

Figure 8. GPC chromatogram of tar condensed from two-stage reactor with the study on the effect of air on tar decomposition.

Figure 10. Product yields from two-stage reactor with (i, lower part) nitrogen treatment to 340 °C and (ii, upper part) tar decomposition at 800-900 °C.

reactor without biomass char at 900 °C. This indicates that fuel gases, particularly CO, can be selectively obtained by the influence of biomass char on tar decomposition at a higher temperature of 900 °C. For a preliminary evaluation of process feasibility, the enthalpy of reaction (∆Hr) of the combination of thermal treatment with inert gas and tar decomposition with catalytic char at 900 °C, as shown in the reaction eq 2, can be calculated from the gaseous products without tar release obtained by the experiment. The calculated enthalpy of this combination reaction shows that additional heat is required with ∆Hr ) 156 kJ · mol-1. Consequently, the combination process should be further designed to develop the practical gasification process for the production of fuel gas with high heating value. [Cx0Hy0Oz0]biomass f aCO + bCO2 + cH2O + dH2 + [Cx1Hy1Oz1]char

Figure 9. Product yields from two-stage reactor with (i, lower part) nitrogen treatment to 340 °C and (ii, upper part) tar decomposition at 600 °C.

steam released during the treatment period may almost completely proceed at 800 °C. However, when adding biomass char in the upper reactor kept at 900 °C, the yield of CO significantly increased from 16.0 to 24.0 wt %, whereas CO2 yield decreased from 9.3 to 5.2 wt %, compared to those obtained after the upper

(2)

For tar analysis by the GPC method, Figure 11 shows that the distribution of elution peaks obtained from nitrogen treatment without the upper reactor was not rather different from that of the experimental run with the upper reactor kept at 600 °C. However, the amount of tar seems to slightly decrease. This shows an agreement with the results of the product yield distribution shown in Figure 9, that these light tar products might be hardly decomposed at 600 °C without catalysts. When biomass char was added in the upper reactor, broad elution patterns without sharp peaks of the main volatile products (retention time ) 17-23 min) were obtained. This indicates that tar products were partially decomposed by using biomass char as a catalyst reagent even at a low temperature of 600 °C. This also agrees with Figure 9 that gas products, especially CO,

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Figure 11. GPC chromatogram of tar obtained from two-stage reactor with the study on the effect of char on tar decomposition.

Figure 13. Summarizing diagram of the potential factors for light tar decomposition in the pyrolysis zone in downdraft gasifier by the selectivity of gas yields.

Figure 12. Gas yields obtained from two-stage reactor with the upper part packed with biomass char for eight cycles at temperatures of (a) 600 and (b) 900 °C.

were significantly increased when the biomass char was added in the upper reactor. Though some remaining tar products such as cellobiosan and levoglucosan, which show the peaks at 20 and 26 min of retention time, respectively, cannot be completely decomposed at this low temperature of 600 °C, they seem to be simply handled with no trouble of engine damage because of their low boiling points as previously discussed. For higher upper temperatures at 800 and 900 °C, volatile tar products including levoglucosan detectable by the GPC method were not obtained. This indicates that tar products obtained from gas treatment were mostly decomposed by steam reforming, which agrees with the results of the product yield distribution in Figure 9. 3.4. Plausible Mechanism of Light Tar Decomposition over Biomass Catalytic Char. Previous results and discussion confirm that biomass char has a significant effect on tar reforming at the temperatures of 600-900 °C. To investigate the deactivation property of catalytic char, the biomass char loaded in the upper reactor at 600 and 900 °C was reused for eight cycle runs under nitrogen atmosphere. The yields of CO and CO2 did not show a significant decrease, as shown in Figure 12. Gaseous product yields in each cycle slightly fluctuated due to only the experimental inconsistency. This also confirms that the carbon deposition of light tar on biomass char may not occur during the reforming step because no weight difference of biomass char was observed between before and after running through eight cycles. However, further investigation is necessary to clearly understand the mechanism of tar decomposition over biomass char.

Finally, we can summarize the effect of three potential factors on light tar decomposition in the pyrolysis zone in the downdraft gasifier as shown in Figure 13. For the effect of air, both partial oxidation and steam reforming of light tar simultaneously occur and further proceed when the temperature is increased, but partial oxidation plays a more important role than steam reforming to produce CO2 more than CO. Under inert gas atmosphere, the temperature of 600 °C is too low for the reforming of tar by steam released from gas treatment at low temperature to occur without catalysts, but it can almost completely proceed above 800 °C with a high yield of CO. However, tar decomposition by steam reforming can occur even at 600 °C with the presence of catalytic biomass char. Higher selectivity of CO can be obtained at the higher temperature of 900 °C with the effect of biomass char because dry reforming and the gas-shift reaction may proceed at the edge of catalytic char. 3.5. Alternative Tar Reforming Methods in Pyrolysis/ Gasification Process. According to the above discussion on tar reforming methods by the influence of potential factors, the combination process of biomass pretreatment and alternative reforming methods can be illustrated as shown in Figure 14. Through air treatment at low temperature, pretreated char with nonsmoking and high calorific value can be obtained as discussed in our previous study.21 For primary tar products released from air treatment, since it has been confirmed to be considered as light tar with low molecular weight, they can be simply condensed through the heat exchanger with recycle air before feeding to the reforming step. According to the investigations in this work, the reforming of those light tars, therefore, can be alternatively handled through (i) partial oxidation with recycle air at 700-800 °C to selectively obtain valuable chemicals such as cellobiosan and (ii) steam reforming at 900 °C with the catalytic biomass char, partly derived from the pretreatment step, to obtain tar-free fuel gases with a large amount of CO. These combination processes of air treatment and reforming methods of primary tars obtained from pretreatments at low temperature would be alternatively considered to further develop the biomass pyrolysis/gasification process. However, the practical application of each alternative combined process should be further investigated in detail in future work. To evaluate the feasibility of the proposed gasification process, we have preliminarily examined the enthalpies of reactions (∆Hr) obtained from selective processes and heating values of gaseous products compared to those from conventional gasification. The enthalpy of reaction of conventional biomass


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Figure 14. Combination process diagram of biomass pyrolysis/gasification with pretreatments and alternative reforming methods of light tar.

air gasification can be calculated from partial oxidation of biomass to produce CO, CO2, and H2. With the assumption that the production ratio of CO to CO2 (f:g) is 2:1 by mole as shown in eq 3, the conventional gasification of biomass sample used in the experiment is an exothermic reaction with ∆Hr equal to -146 kJ · mol-1. However, since undesirable tar products might release during the practical process, the heating value of gaseous products will be relatively lower than that of the ideal reaction as shown in eq 3. [Cx0Hy0Oz0]biomass or char + eO2 f fCO + gCO2 + hH2

(3) For the first option of partial oxidation, although the heating value of gaseous products was very low due to a large amount of released carbon dioxide, the valuable chemical of cellobiosan can be selectively obtained without additional heat requirement due to the combination of exothermic air treatment and partial oxidation of light tar. For the second option of tar decomposition with catalytic char at 900 °C, the heating value of gas products is relatively higher due to a large amount of CO release, whereas the enthalpy of reaction calculated from the experimental data shows an endothermic reaction as previously discussed (∆Hr ) 156 kJ · mol-1). However, if the endothermic tar decomposition is combined with the exothermic air treatment at 300 °C (∆Hr ) -405 kJ · mol-1 as calculated in our previous study21) and air gasification of pretreated char without tar release (∆Hr ) -250 kJ · mol-1 as calculated with the same assumptions of conventional gasification from eq 3), additional heat for the process will not be required for the production of fuel gases with higher heating value. 4. Conclusions The distribution of the degree of dehydration (Xp) obtained from air and nitrogen treatment was preliminarily compared with those of cellulose pyrolysis. The results showed that gas treatment of biomass would not release cross-linked tar when it was treated not above 300 °C in air and 340 °C under inert atmosphere. Those primary tars were then mixed with raw biomass by varying its mixing ratio to investigate their pyrolysis behavior by using thermogravimetric analysis. TG curves results under inert and airlike atmosphere showed that light tar components can be easily evaporated in the lower temperature range of 150-250 °C without the effect of air before biomass pyrolysis occurs. It, therefore, would be easy to handle those light tars in the next step of the pyrolysis/gasification process. Simple reforming methods of primary tar products were further investigated by considering the related potential factors

in the pyrolysis zone of a downdraft gasifier. The effect of air on light tar decomposition using a two-stage reactor was studied by varying the upper temperature from 600 to 900 °C. The results showed that the yield of gaseous products significantly increased with increasing reforming temperature. The yield of CO2 was much higher than that of CO because the rate of partial oxidation was much higher than the rate of steam reforming. When biomass char pyrolyzed to 900 °C was loaded into the upper reactor, the decomposition of light tar by steam reforming could proceed to produce fuel gases, especially CO, even at the low temperature of 600 °C. When the upper temperature was increased to 800-900 °C, the yield of light tar significantly decreased and became almost zero because steam reforming and related reactions could proceed to produce CO and CO2 even in the case of without biomass char. However, CO could be selectively obtained with the highest yield at 24.0 wt % because of the effect of catalytic char at the high temperature of 900 °C. Alternative methods of light tar reforming were finally discussed for the development of a biomass gasification process to obtain tar-free gaseous products and/or valuable chemicals. Acknowledgment This work was financially supported by the Ministry of Education, Science, Sports and Culture of Japan through the Grant-in-Aid for Scientific Research (A) (Grant 19206083). Literature Cited (1) Klass, D. L. Biomass for Renewable Energy, Fuels, and Chemicals; Academic Press: New York, 1998. (2) Shafizadeh, F. Introduction to Pyrolysis of Biomass. J. Anal. Appl. Pyrolysis 1982, 3, 283. (3) Antal, M. J., Jr.; Va´rhegyi, G. Cellulose Pyrolysis Kinetics: the Current State of Knowledge. Ind. Eng. Chem. Res. 1995, 34, 703. (4) Va´rhegyi, G.; Antal, M. J., Jr.; Jakab, E.; Sza´bo, P. Kinetic Modeling of Biomass Pyrolysis. J. Anal. Appl. Pyrolysis 1997, 42, 73. (5) Blasi, C. D. Comparison of Semi-Global Mechanisms for Primary Pyrolysis of Lignocellulosic Fuels. J. Anal. Appl. Pyrolysis 1998, 47, 43. (6) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. A Review of the Primary Measures for Tar Elimination in Biomass Gasification Process. Biomass Bioenergy 2003, 24, 125. (7) Han, J.; Kim, H. The Reduction and Control Technology of Tar during Biomass Gasification/Pyrolysis: An Overview. Renewable Sustainable Energy ReV. 2008, 12, 397. (8) Corella, J.; Toledo, J. M.; Molina, G. Calculation of the Conditions to Get Less Than 2 g tar/mn3 in a Fluidized Bed Biomass Gasifier. Fuel Process. Technol. 2006, 87, 841. (9) Miyazawa, T.; Kimura, T.; Nishikawa, J.; Kado, S.; Kunimori, K.; Tomishige, K. Catalytic Performance of Supported Ni Catalysts in Partial Oxidation and Steam Reforming of Tar Derived from the Pyrolysis of Wood Biomass. Catal. Today 2006, 115, 254.

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ReceiVed for reView October 29, 2009 ReVised manuscript receiVed February 12, 2010 Accepted February 16, 2010 IE901695R

Alternative Reforming Methods of Primary Tar Released from Gas

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