Catalytic Gasification of Sawdust Derived from Various Biomass

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Energy & Fuels 2003, 17, 1456-1463

Catalytic Gasification of Sawdust Derived from Various Biomass A. K. Dalai,*,† E. Sasaoka,‡ H. Hikita,‡ and D. Ferdous† Department of Chemical Engineering, Catalysis and Chemical Reactor Engineering Laboratories, University of Saskatchewan, Saskatoon, SK, S7N 5C9, Canada, and Department of Environmental Chemistry and Materials, Okayama University, 3-1-1 Tsushima, Okayama, 700-8530 Japan Received February 25, 2003. Revised Manuscript Received July 18, 2003

A systematic study is conducted for the steam gasification of biomass materials (cellulose, Cedar, and Aspen) using temperature-programmed gasification (TPG) and constant-temperature gasification (CTG) methods in order to produce H2-rich gas. The performance of catalyst (CaO) was also studied by varying the catalyst loading from 0 to 8.9 wt % during TPG and CTG processes. The TPG and CTG experiments showed that the use of CaO as a catalyst reduced the maximum gasification temperature by ∼150 °C. Also, the rate of H2 and cumulative H2 productions were increased with the impregnation of CaO in cellulose, Cedar, and Aspen during TPG and CTG processes. In TPG, the rate of production of H2 was increased from 0.21 to 0.38 cm3 (STP)/min/ (0.04 g of sample) when 5.5 wt % CaO was impregnated in cellulose. Higher CaO loading of 8.9 wt % did not improve H2 production. In CTG, the rate of H2 production and cumulative production of H2 increased from 0.18 to 0.31 cm3 (STP)/min and from 11 to 14 cm3 (STP)/(0.04 g of sample) when 5.5 wt % CaO was impregnated in cellulose. The rate of production and cumulative production of H2 from Cedar and Aspen were significantly higher than those from cellulose for catalytic as well as for noncatalytic TPG and CTG processes. Total fuel yield, H2, and carbon yields were also significantly increased with the impregnation of CaO in cellulose, Cedar, and Aspen.

Introduction Biomass such as wood, agricultural wastes, etc., is a renewable material containing appreciable quantities of hydrogen, oxygen, and carbon.1 Biomass is a preferred renewable resource of energy and can be used as a fuel, which is CO2 neutral. Therefore, the development of a highly efficient utilization technology using biomass is very important for improvement of our environment. The utilization of wood as a fuel in circulating fluid beds is going on in various parts of the world. The production of hydrogen and medium Btu gas from biomass has been investigated in the past in laboratories and pilot-plant scales in a variety of reactors. The product (gas, char, tar, water-soluble organics) distribution and the gas composition (H2, CO, CO2, CH4, and C2+) depend on many variables, such as type of reactor, temperature, the steam-to-carbon ratio, the space time in the gasified bed, the type of reactant material, moisture and ash content, and also on the particle size of the reactant material. In the past, the steam gasification of biomass has been mainly performed with or without catalyst in fluidizedbed reactors.2-7 Other types of reactor configuration * Corresponding author. Tel: (306) 966-4771. Fax: (306) 966-4777. E-mail: [email protected]. † University of Saskatchewan. ‡ Okayama University. (1) Maschio, G.; Lucchesi, A.; Stoppato, G. Biores. Technol. 1994, 48, 119.

were also used for the steam gasification of biomass materials such as fixed-bed,8,9 fluidized- and fixed-bed in series,10-12 and continuous stirrer tank reactor.13 Antal9 studied the effect of temperature, residence time, and pressure on product compositions during the steam gasification reaction of biomass in a fixed-bed reactor in the temperature range of 500-750 °C. Increasing temperatures and residence times resulted in increased CH4 formation. Maximum H2 production (18 mol %) was observed at 750 °C. At temperatures above 650 °C, the steam gasification reaction proceeded quickly and generated increasing amounts of hydrocar(2) Raman, K. P.; Walaander, P. W.; Fan, T. L. Ind. Eng. Chem. Process Des. Dev. 1980, 19, 623. (3) Walaender, W. P.; Hoveland, D. A.; Fan L. T. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 813. (4) Singh, S. K.; Walawender, W. P.; Fan, L. T. Wood Fiber Sci. 1986, 18, 327. (5) Baker, E. G.; Mudge, L. K.; Brown, M. B. Ind. Eng. Chem. Res. 1987, 26, 1335. (6) Prasad, B. V. R. K.; Kuester, J. L. Ind. Eng. Chem. Res. 1988, 27, 304. (7) Coralla, J.; Aznar, M. P.; Delegado, J.; Aldea, E. Ind. Eng. Chem. Res. 1991, 30, 2252. (8) Naushkin, M. Y.; Zhorov, M. Y.; Nikanorova, P. L. Solid Fuel Chem. 1998, 22, 136. (9) Antal, J. M. Biomass as a Non Fossil Fuel Source; KLASS, 1981. (10) Caballero, A. M.; Aznar, P. M.; Gil, J.; Martin, A. J.; Frances, E.; Corella, J. Ind. Eng. Chem. Res. 1997, 36, 5227. (11) Delgado, J.; Azner, M. P.; Corella, J. Ind. Eng. Chem. Res. 1997, 36, 1535. (12) Rapagna, S.; Jand, N.; Foscolo, U. P. Int. J. Hydrogen Energy 1998, 23, 551. (13) Minowa, T.; Inoue, S. Renewable Energy 1999, 16, 1114.

10.1021/ef030037f CCC: $25.00 © 2003 American Chemical Society Published on Web 09/25/2003

Catalytic Gasification of Biomass Sawdust

bons. It was observed that increased pressure inhibited the gasification process. Corella et al.14 made possible improvements of steam gasification of biomass using two reactors in series, where the first reactor was a simple fluidized reactor and the second reactor was a fixed or fluidized catalytic bed reactor. High H2 and CH4 productions were observed using dolomite as a catalyst. Naushkin et al.8 studied the catalytic [(Ni)(Fe)/Al2O3] conversion of aqueous suspensions of plant biomass (wood waste, straw, haulm, etc.) with steam at 700800 °C in a fixed-bed reactor. They reported the production of H2, CO, CH4, and CO2. Olivares et al.15 studied the gasification of pine chips in a fluidized bed. In their work, they studied the gasification reaction using a fixed bed of dolomite (CaO‚ MgO) at different concentrations. The catalyst bed temperature was varied from 795 to 835 °C. They reported a significant increase in H2 production when the dolomite content of the catalyst was increased from 0 to 30 wt %, but the H2 content in the gasifier exit gas leveled off with a further increase in dolomite concentration. Ora et al.16 also gasified biomass-derived products using different types of dolomite in a fixed-bed rector and reported a significant increase in H2 production. They had mainly studied the influence of the pore structure and chemical composition of dolomite on its tar elimination capacity. They reported the same tar elimination activity of all dolomites, even though their pore volume and pore diameter were different. Caballero et al.10 and Aznar et al.17 studied the effects of temperature on the gasification of small chips of pine wood in two reactors in series using oxygen with steam. The first one was a fluidized-bed reactor and the second one was the catalytic fixed-bed reactor containing a Nibased catalyst. The H2 and CO contents in the product gas were increased by 4-14 and 1-8 vol %, respectively, and no catalyst deactivation was found in 48 h of timeon-stream tests when the catalyst temperature was relatively high (780-830 °C). Rapagna et al.12 also investigated the catalytic gasification of biomass (almond shells) for the production of hydrogen in a dual-reactor system in series, where the first one was a fluidized-bed gasifier and the second one was a catalytic fixed-bed reactor with a commercial nickel catalyst to produce a hydrogen-rich gas. The conditions in the fluidized-bed reactor were kept constant and the temperatures of the fixed bed were in the range of 665-830 °C. The product gas consisted mainly of H2, CO, CO2, and CH4. Maximum hydrogen production of 62.2 mol % was obtained at 665 °C. From the literature, it is observed that most of the research studies on the catalytic gasification of biomass have been conducted using two reactors in series using Ni-based and dolomite catalysts. Research related to the (14) Corella, J.; Aznar, P. M.; Herguido, J.; Gonza´lez-Saı´z, J.; Delegado, J.; Iglesias, I. J.; Alday, J. F.; Rodrı´guez-Trujillo, L. J. Proceedings of the Euroforum-New Energies Congress, 1988, 3, October 24-28, Germany. (15) Olivares, A.; Aznar, P. M.; Caballero, A. M.; Gil, J.; Frances, E.; Corella, J. Ind. Eng. Chem. Res. 1997, 36, 5220. (16) Orı´o, A.; Corella, J.; Narva´ez, I. Ind. Eng. Chem. Res. 1997, 36, 3800. (17) Azner, P. M.; Caballero, A. M.; Gil, J.; Martin, A. J.; Corella, J. Ind. Eng. Chem. Res. 1998, 37, 2668.

Energy & Fuels, Vol. 17, No. 6, 2003 1457

Figure 1. Schematic diagram of experimental setup for noncatalytic and catalytic gasification of biomass.

biomass gasification using CaO is not available. The biomass gasification by impregnating catalyst in the biomass is also scarce. Furthermore, no work has been done before on the comparison of performance of temperature-programmed gasification (TPG) and constanttemperature gasification (CTG) processes. So, the objective of this research is to study TPG and CTG processes with and without CaO to produce H2-rich gas from industrial waste wood such as sawdusts from cellulose, Cedar, and Aspen. Experimental Section Material and Sample Preparation. Sawdust powders derived from Cedar (Japan) and Aspen (Canada) wood (average diameter: 0.2 mm), and cellulose obtained from MakaraiTesque Inc., Kyoto, Japan, were used in this research. Calcium oxide was obtained from Wako Pure Chemical Industries Ltd., Osaka, Japan. All experiments were performed in the Department of Environmental Chemistry and Materials at Okayama University, Japan. Pure CaO was used in order to avoid the interference of different impurities on the catalytic activity of CaO. CaO-supported samples were prepared by a vaporization-impregnation method. The desired amount of CaO powder was dissolved in 1 cm3 of water in a round flask. A 100 mg sample of dry sawdust or cellulose sample was added to this solution and mixed. The mixture was dried using a rotary evaporator at 40 °C and then dried at 110 °C overnight in N2. Apparatus and Procedure. The experiments were carried out using a down-flow packed-bed tubular reactor system equipped with a programmable temperature controller under atmospheric pressure (see Figure 1). The microreactor consisted of a quartz tube of 0.5 cm i.d., in which 0.04 g of sample was packed. The gasification experiments were carried out by two methods. One of them is a temperature-programmed gasification (TPG) under a constant heating rate of 3 °C/min. In this case, a mixture of H2O (30%) and N2 was fed into the reactor at 20 cm3(STP)/min. After TPG up to 850 °C, the reaction temperature was kept at 850 °C for 1 h. Another procedure involved a constant-temperature gasification (CTG) process at 850 °C. The heating rate up to 850 °C under N2 flow was set at 3 °C/min and 30 °C/min. At 850 °C, a mixture of H2O (30%) in N2 was fed into the reactor at 20 cm3 (STP)/ min. In both cases, the outlet gas from the reactor was collected using a sample bag. The compositions of gaseous products containing H2, CO, and CH4, produced during TPG or CTG

1458 Energy & Fuels, Vol. 17, No. 6, 2003

Dalai et al.

Table 1. Carbon, Hydrogen, Oxygen, and Nitrogen Analyses of Feed Materials mol % sample

C

H

Oa

N

cellulose Cedar wood (Japan) Aspen (Canada)

27.6 29.8 30.2

47.8 48.5 47.4

24.6 21.6 22.4

0.04 0.05 0

a

Calculated value.

Figure 2. Evolution of H2, CO, and CH4 during TPG of cellulose. were measured using two GCs. One of the two GCs was equipped with an MS 13X-packed column for H2 analysis (carrier gas, Ar). Another GC was also equipped with an MS 13X-packed column for CO analysis (carrier gas, He) and a Porapak QS-packed column for CO2 analysis (carrier gas, He). The total amount of each gas evolved (H2, CO, CH4, and CO2) in each experiment was calculated from the product gas compositions and total volume of gas collected.

Results and Discussion Noncatalytic Gasification of Cellulose. The carbon, hydrogen, oxygen, and nitrogen contents of the samples are given in Table 1, indicating large amounts of C and H and low nitrogen content in these samples. The rate of production of H2, CO, and CH4 with temperature during temperature-programmed gasification (TPG) of the cellulose sample is given in Figure 2. It shows that, during the course of heating, CO was evolved first in the range of 250 to 400 °C, and a small amount CH4 evolution started from ∼400 °C and reached maximum at ∼500 °C, and then H2 evolution started. The H2 production increased monotonically up to 850 °C and was accompanied with CO generation. The results in Figure 2 indicate that the gasification of biomass is a complex process involving several physical and chemical steps. The mechanism of steam gasification of biomass is suggested by Antal et al.18 and Raman et al.2 This mechanism can be used for the steam gasification of cellulose:

cellulose + heat ) CO + CO2 + CH4 + other hydrocarbons + organic + oxygenated compounds + charcoal (carbon) (1) If the temperature is sufficiently high, the following reactions can occur: (18) Antal, M. J.; Edwards, H. I.; Friedman, H. L.; Rogers, F. E. Project Report to APA, W. W. Liberik, Project Officer, Cincinnati, OH, 1978.

CH4 + H2O f CO + 3H2

(2)

CH4 + CO2 f 2CO + 2H2

(3)

CO + H2O f CO2 + H2

(4)

C + CO2 f 2CO

(5)

At temperatures above 700 °C, the following reactions occur:

CS + H2O f CO + H2

(6)

CS + 2H2O f CO2 + 2H2

(7)

2CS + 2H2O f CH4 + CO2

(8)

CS + CO2 f 2CO

(9)

Figure 2 shows that maximum production of CO for the low-temperature range of 250-400 °C is probably because of the pyrolysis of cellulose (see eq 1). However, the CH4 production at ∼400 °C could be due to primary decomposition of cellulose. In addition, the production of H2 and more CO at ∼500 °C suggests that cellulosechar gasification with H2O is more effective at temperatures > 500 °C. The rate of production of H2 and CO increased systematically from 0.02 to 0.21 and from 0.01 to 0.06 cm3 (STP)/min/(0.04 g of sample), respectively, when the temperature was increased from 600 to 850 °C. The data imply that the production of these gases would increase from 0.5 to 5.25 and from 0.25 to 1.5 cm3 (STP)/min/ (g of sample) with the increase in temperature from 600 to 850 °C. In both cases, H2 generation was 3.7 times higher than that of CO at 850 °C. The increase in production of CO and H2 with temperature is probably because of reactions 2-9. In this temperature range, the production of CH4 decreased because of reactions 2 and 3. Figure 2 also shows that, when the reaction was continued at 850 °C for 60 min, the rates of production of H2, CO, and CH4 were decreased. Corella et al.7 also reported an increase in H2 and a decrease in CH4 production and, contrary to our data, a decrease in CO production when the temperature was increased from 650 to 750 °C for the steam gasification of cellulose waste in a fluidized-bed reactor. Catalytic Gasification of Cellulose. The purpose of using catalyst includes: (1) cracking of tar, if any; (2) methanation of gas to increase methane content in gas; (3) to enhance steam reforming and water gas shift reactions in order to produce H2-rich gas.19 Catalytic gasification of cellulose was carried out using two CaO-impregnated samples containing 5.5 and 8.9 wt % of CaO in the temperature range of 250 to 850 °C and was continued at 850 °C for another 1 h. It is known that CaO converts to Ca(OH)2 quickly in the presence of moisture. However, Ca(OH)2 is stable only up to 540 °C,20 after which it decomposes to CaO. Due to continuous supply of moisture, Ca(OH)2 may also be partly present beyond 540 °C. However, some Ca(OH)2 can convert to CaCO3 by reacting with CO2 produced (19) Ferdous, D.; Dalai, K. A.; Bej, K. S.; Thring, W. R. Can. J. Chem. Eng. 2001, 79, 913. (20) Ullmann, F. Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; 1985; 15.



Figure 3. Evolution of H2 and CO during TPG of CaOimpregnated cellulose.

during reaction. Since CaO and Ca(OH)2 are interconvertible, this paper discusses only the effects of CaO on the production of H2, CO, CO2, CH4, and total fuel gas from various biomass products. The effect of CaO loading on the rate of production of H2 and CO at various temperatures is given in Figure 3. Figures 2 and 3 show that the production of H2 increased significantly with the impregnation of CaO in cellulose. The maximum rate of H2 production increased from 0.21 to 0.38 cm3 (STP)/min/(0.04 g of sample) when 5.5 wt % CaO was impregnated in cellulose. It is evident from these two figures that the temperature for the maximum rate of production of H2 and CO decreased from 825 to 675 °C when CaO was impregnated in cellulose. Furthermore, the additive CaO drastically suppressed the high-temperature CO evolution and also suppressed somewhat the low-temperature CO evolution. The decrease in CO production and the increase in H2 production occurred because of the increase in water gas shift reaction (reaction 4) in the presence of CaO. However, the increase in CaO concentration from 5.5 to 8.9 wt % did not have any significant effect on the rate of production of H2 and CO (see Figure 3). The cumulative production of H2, CO, CH4, and CO2 from cellulose during catalytic and noncatalytic gasification is given in Figure 4. This figure shows that impregnation of CaO in cellulose caused a significant increase in H2 production and decrease in CO production. For example, the cumulative production of H2 increased from 14 to 17.5 cm3 (STP)/(0.04 g of sample) whereas that of CO decreased from 3.75 to 1.67 cm3 (STP)/(0.04 g of sample). This is in agreement with those reported in Figures 2 and 3 and in agreement with those reported by Olivares et al.15 In their work, they reported an increase in H2 production and decrease in CO production due to the water gas shift reaction during the gasification of biomass-derived product using dolomite (CaO‚MgO). Because of the decrease in production of CO, the ratio of H2 to CO was also drastically increased by the impregnation with CaO. For example, the H2/CO ratios for noncatalytic and catalytic (for 5.5 wt % CaO) processes were 2.8 and 11.1, respectively. An increase in the H2/CO ratio for the catalytic process indicates that the product gas obtained from the catalytic process can be used in fuel cells as well as to

Figure 4. Cumulative production of H2, CO, CH4, and CO2 during TPG of CaO-impregnated cellulose.

produce liquid fuel using a Fischer-Tropch synthesis process Figure 4 shows significant increase in CO2 production with the impregnation with CaO. This can be explained on the basis that addition of CaO caused an increase in the water gas shift reaction and carbon-steam reforming reaction (reactions 4 and 8), thus increasing CO2 production. In contrast, Olivares et al.15 reported no change in CO2 content in the exit gas when dolomite was used. Figure 4 also shows a small increase in CH4 production with the increase in CaO impregnation in cellulose. It is probably the initial pyrolysis of the cellulose and the carbon reforming reaction (reactions 1 and 8) that lead to the formation of CH4. Furthermore, from the results shown in Figures 3 and 4, it is also concluded that 5.5 wt % CaO is enough to catalyze the gasification of cellulose with H2O to produce H2 and CO. However, an increase in CaO impregnation caused a slight increase in CH4 production, which is an important component to produce higher-heating-value gas. Corella et al.14 had reported a significant increase in H2 and CH4 production during gasification of biomass when dolomite was used as a catalyst. For the evaluation of the utilization of cellulose, fuel yield was calculated on the basis of the following equation:

yield of total fuel gas ) (MH2 + MCO + 4MCH4)/ (2MC + 1/2MH - MO) (10) The yield of the total fuel gas was based on the hydrogen production ability. In eq 10, MC, MH, and MO (mole) were calculated from the molar composition of the cellulose, and MH2, MCO, and MCH4 were the total molar amounts of the evolved gas. Yields of hydrogen and carbon also were calculated using the following equations:

yield of hydrogen ) MH2/(2MC + 1/2MH - MO) (11) yield of carbon ) (MCO2 + MCO + MCH4)/MC (12) The difference between their yields and the amounts


Dalai et al.

Figure 5. Yields of total fuel gas, H2, and carbon during TPG of CaO-impregnated cellulose.

present in the feed gives the amount unconverted which remained in the reactor. The yields of total fuel, H2, and C during TPG of CaOimpregnated cellulose are given in Figure 5, which shows that the yield of total fuel gas was increased by ∼5%, and the yields of H2 and carbon were also increased by ∼23% and ∼10%, respectively, by the impregnation with CaO. From the experimental results using the cellulose and the CaO-loaded cellulose, it may be concluded that CaO catalyzes the steam gasification of cellulose, increases carbon and H2 content in fuel gas, and also increases conversion of CO to H2. Catalytic TPG of Cedar and Aspen Sawdust. TPG of Cedar and Aspen were carried out in order to compare their gasification reactivities with that of cellulose. Unfortunately, the CaO loading in these samples could not be maintained at the same value as in case of cellulose by our preparation method. Reactions was carried out at two different CaO loadings. The rates of H2 production during the TPG of the samples prepared from Cedar and Aspen dust as well as cellulose are given in Figure 6. This figure shows that the gasification of both the Cedar and Aspen sawdusts increased by the addition of CaO and the trend of rate of H2 evolution from these samples was similar to that of the cellulose. Figure 6 also shows that the rates of production of H2 from Cedar and Aspen were comparatively higher than that from cellulose both without and with CaO. For example, at 850 °C the rate of production of H2 from cellulose in the absence of CaO was 0.21 cm3 (STP)/min/(0.04 g of sample), whereas that from Cedar and Aspen were 0.30 and 0.25 cm3 (STP)/min/(0.04 g of sample), respectively. The increase in H2 production from Cedar and Aspen over that from cellulose is probably because of higher amounts of C and H2 in Cedar and Aspen materials (see Table 1). The rates of CO production as a function of temperature from cellulose, Cedar wood, and Aspen for noncatalytic and catalytic TPG are given in Figure 7. The rate of CO evolution from the wood dusts shows trends similar to that from the cellulose. But the evolution of CO from Cedar was somewhat higher than that from cellulose and Aspen. For example, for the temperature range of 600 to 850 °C, the maximum rate of evolution

Figure 6. Evolution of H2 during TPG of CaO-impregnated cellulose, Cedar, and Aspen.

of CO from Cedar was 0.08 cm3 (STP)/min/(0.04 g of sample), whereas that from cellulose and Aspen were 0.05 and 0.045 cm3 (STP)/min/(0.04 g of sample), respectively. It is probably because of the different rates of reactions such as shown in eqs 2, 3, 6, 7, and 9 for cellulose, Cedar wood, and Aspen. The CO evolution from Cedar and Aspen for catalytic TPG shows trends similar to that from cellulose (Figure 7). For the temperature range of 250 to 400 °C, the CO evolution from cellulose and Cedar decreased with the impregnation with CaO. For example, the rate of CO production decreased from 0.05 to 0.03 and from 0.03 to 0.025 cm3 (STP)/min/(0.04 g of samples) for cellulose and Cedar, respectively, with the increase in CaO loading from 0 to 5.5 wt %. On the other hand, impregnation with CaO caused an increase in rate of CO production from Aspen at a lower temperature range. However, the temperature for the higher rate of CO production decreased from 850 to 675 °C in all three cases due to impregnation with CaO. The cumulative productions of H2, CO, CH4, and CO2 as a function of CaO in Cedar and Aspen are given in Figure 8. It indicates that H2 production increased with the increase in CaO loading in both cases. But the increase in H2 production from Cedar is higher than that from Aspen for both the catalytic and noncatalytic



Figure 9. Yields of total fuel gas, H2, and carbon during TPG of CaO-impregnated Cedar and Aspen.

Figure 7. Evolution of CO during TPG of CaO-impregnated cellulose, Cedar, and Aspen.

Figure 8. Cumulative productions of H2, CO, CH4, and CO2 during TPG of CaO-impregnated Cedar and Aspen.

processes. For example, for the noncatalytic process the production of H2 from Cedar was 24 cm3 (STP)/(0.04 g of sample), whereas that from Aspen was 21 cm3 (STP)/ (0.04 g of sample). The impregnation with CaO caused

a significant increase in H2 production from Cedar wood. For example, the production of H2 increased from 24 to 29 cm3 (STP)/(0.04 g of sample) with its impregnation with 2.4 wt % CaO, whereas in the case of Aspen the production increased from 21 to 23 cm3 (STP)/(0.04 g of sample). It is probably due to somewhat different catalyst loading as well as to different feed properties. Figure 8 also shows that the increase in CaO loading from 2.4 to 3.7 wt % did not have a significant effect on the production of H2 from Cedar wood, indicating that 2.4 wt % CaO loading is sufficient for the maximum H2 production from this material. Figure 8 shows that the production of H2 slightly increased with the increase in CaO loading from 3.9 to 4.3 wt % for TPG of Aspen. However, the impregnation with CaO caused a decrease in CO and an increase in CO2 production in both cases. However, there was a small increase in CH4 production with the impregnation by CaO in Cedar and Aspen. The yields of total fuel, H2, and C from Cedar and Aspen are given in Figure 9. This figure shows that total fuel yield increased significantly with the impregnation with CaO in Cedar wood and Aspen. For example, it increased from 72 to 78% for Cedar wood for CaO impregnation of 2.4 wt % and increased from 58 to 66% for Aspen for CaO impregnation of 3.7 wt %. However, further increase in CaO content caused no significant effect on fuel yield. Similar trends were observed for the yields of H2 and C for catalytic TPG of Cedar and Aspen. It may be noted that the yields of total fuel, H2, and C from Cedar wood were significantly higher that those from Aspen.


Dalai et al.

Figure 10. Evolution of H2 during CTG of CaO-impregnated cellulose.

Figure 11. Cumulative productions of H2, CO, CH4, and CO2 during CTG of CaO-impregnated cellulose, Cedar, and Aspen at the heating rate of 30 °C/min.

The total fuel yields from the samples with CaO and without CaO are in the following orders:

Cedar wood > Aspen g cellulose without CaO (13) Cedar wood > Aspen > cellulose with CaO (14) The carbon yields from the samples with CaO and without CaO are in the following order:

Cedar wood > Aspen > cellulose

(15)

In cases of gasification of Cedar and Aspen, the total fuel yields were improved by the additive CaO. Catalytic CTG of Cellulose, Cedar, and Aspen Sawdust. The effect of time on stream (TOS) on the production of H2 was similar for catalytic as well as for noncatalytic CTG of cellulose (see Figure 10). In this case, the reaction was conducted at 850 °C for 180 min. It is observed that the rate of hydrogen production decreased from 0.18 to 0.00 cm3 (STP)/min/(0.04 g of sample) when TOS was increased from 15 to 135 min. CaO impregnation enhanced H2 production especially at TOS < 55 min. There was no significant change in H2 production rate in the CTG process when the catalyst loading was increased from 5.5 to 8.9 wt %. The cumulative production of H2, CO, CH4, and CO2 as a function of catalyst loading for CTG of cellulose, Cedar, and Aspen are given in Figure 11. This figure shows a trend similar to that of the TPG process. But the cumulative productions of CO and H2 obtained from CTG were somewhat less than from the TPG process.

Figure 12. Yields of total fuel gas, C, and H2 during CTG of CaO-impregnated cellulose, Cedar, and Aspen at the heating rate of 3 and 30 °C/min.

For example, when the CaO loading was increased from 0 to 2.4 wt %, the cumulative production of H2 for Cedar for CTG and TPG processes increased from 19 to 26 and from 24 to 29 cm3 (STP)/min/(0.04 g of sample), respectively (see Figures 8 and 11). A similar trend was observed for the cumulative production of CO (see Figures 8 and 11). Also, a decrease in cumulative productions of H2 and CO was observed in CTG compared to that in TPG or the gasification of cellulose (Figures 4, 8, and 11). However, an increase in cumulative production of H2 and CO was observed in case of CTG as well as in the case of TPG for the gasification of cellulose, Cedar wood, and Aspen. The yield of total fuel gas, H2, and C as a function of CaO loading from CTG of cellulose, Cedar, and Aspen at 850 °C for two different heating rates of 3 and 30 °C/min are given in Figure 12. From this figure it is observed that the yield of total fuel as well as H2 and C from cellulose increased with the increase in heating rate. This trend is similar to those obtained by Ferdous


et al.21 In their work, they reported the increase in total gas and H2 production for the gasification of Kraft lignin in a fixed-bed reactor when the heating rate was increased from 5 to 15 °C/min. Figure 12 also shows that at the heating rate of 3 °C/min, addition of 5.5 wt % CaO to cellulose caused an increase in yields of total fuel, H2, and C from 36 to 38%, from 26 to 32%, and from 21 to 25%, respectively, which leveled off with the further increase in CaO loading. Similar trends were also observed for the yields of total fuel, H2, and C when CTG was carried out at a heating rate of 30 °C/min. The trends for the yield of total fuel, H2, and C are different for Cedar and Aspen (Figure 12). For example, an increase in heating rate caused the decrease in yields of total fuel, H2, and C for the CTG of Cedar and Aspen. However, in both cases the yield increased with the impregnation with CaO. It is observed that Cedar is more reactive than Aspen and cellulose, and cellulose is least reactive. In comparison, TPG is favorable for high yields of total fuel, H2, and C in comparison to CTG. Figure 12 also shows that the additive CaO improved the yields of the gases. Particularly, in the case of the Aspen samples, the yields were considerably improved. The differences in the effects of the CaO impregnation on the C, H2, and fuel gas yields may be induced by the differences in the structures and compositions of the samples. The improvement of the yields suggested that (21) Ferdous, D.; Dalai, K. A.; Bej, K. S.; Thring, W. R.; Bakhshi N. N. Fuel Process. Technol. 2001, 70, 9.


the additive CaO contributed to the carbonization of the volatile matter of the samples and enhanced water gas shift reactions producing more H2. Conclusions The rate of production of H2 in the TPG process increased from 0.21 to 0.38 cm3 (STP)/min/(0.04 g of sample) when 5.5 wt % CaO was impregnated in cellulose. Further increase in CaO loading to 8.9 wt % did not have any significant effect on the rate of H2 production. The rate of production of H2 and cumulative H2 production from Cedar and Aspen were also increased with impregnation with CaO and were significantly higher than those from cellulose for catalytic as well as for noncatalytic TPG processes. TPG experiments showed that the use of CaO as a catalyst for all three biomass materials reduced the maximum gasification temperature by ∼150 °C. The cumulative production of H2 obtained from Cedar and Aspen was higher than that of cellulose for the CTG process. Total fuel yield was also significantly increased with the impregnation of CaO in cellulose, Cedar wood, and Aspen. During TPG and CTG processes, the total fuel yield was increased by ∼5 and 5.5 wt %, respectively, when CaO was added. Impregnation with 5.5 wt % CaO in cellulose caused an increase in H2/CO ratio from 2.8 to 11.1, which indicates that the product gas obtained from the catalytic process can be used in fuel cells as well as to produce liquid fuel using a Fischer-Tropsch synthesis process. EF030037F

Catalytic Gasification of Sawdust Derived from Various Biomass

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