Examination of the Low-Temperature Region in a Downdraft Gasifier

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Ind. Eng. Chem. Res. 2009, 48, 8934–8943

Examination of the Low-Temperature Region in a Downdraft Gasifier for the Pyrolysis Product Analysis of Biomass Air Gasification Weerawut Chaiwat, Isao Hasegawa, and Kazuhiro Mae* Department of Chemical Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan

Treatment of biomass in nitrogen and air at 240-340 °C was conducted in order to examine the low-temperature region in a downdraft gasifier by analyzing the treated precursors and product distribution. Gas-treated precursors were then pyrolyzed in flash mode at 764 °C for further analysis. Overall, the tar yield decreased from approximately 50 wt % to less than 20 wt % upon oxidation of the sample at a very low heating rate to 260-300 °C in air. Moreover, tar evolution was almost completely suppressed during the subsequent flash pyrolysis. This indicates that the structure of the treated precursors was gradually changed to suppress tar release through cross-linking reactions and partial oxidation. From elemental analysis of the precursors treated with air, it was also estimated that dehydration and partial oxidation proceeded simultaneously. The results indicate that the release of tar products such as dimers can be partially suppressed by air treatment at low temperature. 1. Introduction With respect to the depletion of fossil fuels and the global warming issues, biomass has become a focus as a potential renewable energy resource, because it is abundant and environmentally benign. To convert biomass into a usable form of energy, gasification is one of the most promising thermochemical conversion methods. The main objective of biomass gasification is to produce electrical energy and thermal power by optimizing the energy conversion of raw biomass into combustible gaseous products, such as CO, H2, and CH4, known as producer gas, in a gasifier.1-5 Two major types of gasifiers, fixed-bed and fluidized-bed reactors, are generally applied for biomass gasification. Fixed-bed gasifiers are widely used and studied because of their simplicity in construction and operation. The reaction distribution regions in a fixed-bed reactor are different depending on the type of gasifier design, namely, updraft (countercurrent) and downdraft (concurrent), as shown in Figure 1a and b, respectively. According to the research on biomass gasification using both downdraft gasifiers6-10 and updraft gasifiers,11,12 the gas composition by volume is mainly CO (20-30%), H2 (5-15%), CH4 (1-3%), and CO2 (5-15%). Moreover, it has been clarified that downdraft gasifiers produce relatively lower amounts of undesirable tar products, which have low reactivity and cause damage to gas turbines and/or gas engines in biomass gasification systems. Consequently, the downdraft gasifier would be a reasonable selection for further development of reactor designs. However, the tar concentrations obtained in most downdraft gasifiers still exceed the limit for internal combustion (IC) engines, because the primary reaction of biomass gasification is endothermic pyrolysis, which releases a large amount of tar products.6-13 A schematic diagram of a conventional downdraft gasifier is shown in Figure 1b. Previous investigations have reported that pyrolysis, combustion, and reduction take place in separate regions at high temperature in a downdraft gasifier, and the uppermost zone of the gasifier is often considered as a drying zone because of its low temperature. Biomass fuel is usually fed from the open top of the gasifier, whereas air is typically fed into the gasifier at a position between the combustion and reduction zones, as shown in Figure 1b.13 * To whom correspondence should be addressed. E-mail: kaz@ cheme.kyoto-u.ac.jp.

However, the reaction distribution regions with appropriate temperatures in the gasifier, including the position of inlet air, should be reconsidered to increase efficiency and obtain tarfree gaseous products. Consequently, the uppermost zone in the downdraft gasifier was the focus of this work for gas treatment at low temperature in order to investigate the effect of temperature and gasifying agent, particularly air, on biomass precursor structures and pyrolysis products. Several researchers have investigated the thermal gas treatment of coal and biomass at low temperatures of less than 400 °C. Graff and Brandes studied the pretreatment of Illinois No. 6 coal with 50 atm subcritical steam at 320-360 °C for 30 min.14,15 Their results showed an increase in tar and volatile yields when pretreated coal was pyrolyzed at 650-800 °C. Steam pretreatment under these conditions leads to an increase in hydroxyl groups in the coal structure and the cleavage of cross-linking, which increases tar yield. Miura et al. reported the thermal treatment of low-rank coal at 70-150 °C for 1 h, followed by pyrolysis in flash mode at 3000 °C · s-1 with a Curie-point pyrolyzer.16 For Taiheiyo and Baiduri coals, the results showed that the total volatile and tar yields increased by approximately 3-4 wt % compared to untreated samples, which indicates the thermal breakage of hydrogen bonds at low temperature to suppress cross-linking reactions and produce more tar products. Hayashi et al. investigated the thermal pretreatment of Yallourn brown coal at 200-400 °C for 1 h and flash pyrolysis at 764 °C (3000 °C · s-1) in a Curie-point pyrolyzer.17 The results showed that the tar yield increased, whereas the char yield decreased for thermal treatment at temperatures less than 300 °C. On the other hand, the tar yield decreased and the char yield increased for thermal treatment above 300 °C. Zeng et al. studied both thermal and steam pretreatment of Loy Yang Brown Coal at low temperature.18,19 The yield of tar decreased, whereas the char yield increased when the coals were thermally pretreated in a helium atmosphere above 250 °C and then pyrolyzed in a Curie-point pyrolyzer at 900 °C. When coals that had been pretreated with steam above 250 °C for 30 min were pyrolyzed, the overall tar yield decreased to less than that for helium pretreatment. This is because the hydrolysis reaction among weak bonds such as ester or ether bonds proceeds during steam pretreatment, which

10.1021/ie900264n CCC: $40.75  2009 American Chemical Society Published on Web 09/11/2009

Ind. Eng. Chem. Res., Vol. 48, No. 19, 2009

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Figure 1. Schematic diagrams of conventional (a) updraft- and (b) downdraft-type gasifiers.

enhances cross-linking reactions. For thermal treatment in air, Worasuwannarak et al. studied the effect of preoxidation at low temperature (250-330 °C) for various coals.20 After the samples had been carbonized at a heating rate of 10 °C · min-1 to 900 °C, the char and gas yields increased, whereas the yields of tar and methane decreased because of cross-linking reactions. Gas thermal treatment with steam or air at low temperature might reduce tar production during pyrolysis and/or gasification processes. However, air treatment of biomass samples at low temperature has not been sufficiently studied with analysis of their treated precursors and tar products. Therefore, the appropriate design of gasifier, including the position of air or reactive gas inlet and the temperature profile in each reaction zone inside the reactor, should be further investigated for the development of gasification systems. In this article, we compare the product distributions and precursor structures of biomass treated with nitrogen and air at low temperature. The treated precursors were subsequently pyrolyzed in flash mode to investigate the distributions of their pyrolysis products. Moreover, heating patterns during air treatment, particularly heating rate and holding time, were adjusted to examine the effect of reaction time during air treatment. Tar products obtained from different air treatments were then preliminarily compared and analyzed. Finally, the reaction behavior and pyrolyzed products in the uppermost zone of a downdraft gasifier were clarified to assist in the design of the most appropriate conditions in the gasifier. 2. Experimental Section 2.1. Samples. Japanese cedar wood was used as a biomass sample because of its abundance in Japan. Raw samples were crushed and sieved to a size of 210-500 µm and then dried in vacuo at 70 °C for 24 h prior to use. The elemental composition of Japanese cedar wood is 50.6 wt % C, 5.9 wt % H, and 43.5 wt % O. It contains a 38.5 wt % lignin component with an ash content of only 0.07 wt %. The ash content yield was calculated from the weight loss and char yield obtained during pyrolysis of samples using a thermogravimetric (TG) analyzer (Shimadzu, TGA-50). For lignin determination, hydrolysis was carried out using concentrated acid. A 0.5-mg dried sample was mixed with 7.5 mL of 72 wt % sulfuric acid. After the solution had been kept at 20 °C for 4 h, it was diluted with 280 mL of distilled water. The diluted solution was further maintained at 120 °C

Figure 2. Schematic diagram of the experimental apparatus with a quartz tube reactor used to perform gas treatment at low temperature.

for 1 h to complete hydrolysis. After being washed with hot water, the hydrolyzed sample was carefully collected using 1.0µm membrane paper filter. The hydrolyzed sample was dried in vacuo for 24 h prior to quantitative analysis. Finally, the yield of lignin was calculated from the ash content and the yield of extracted components by acid hydrolysis. 2.2. Gas Treatment under Different Conditions. A quartz tube reactor with a temperature controller was used to conduct gas treatment experiments, as schematically shown in Figure 2. Samples of 50-100 mg each were placed on a ceramic tray in the middle of the reactor in contact with a temperature probe used to control the temperature distribution. The samples were treated in air and nitrogen at a flow rate of 50 mL · min-1. After being held at 110 °C for 20 min to evaporate moisture, the samples were heated to 240, 280, 300, 320, and 340 °C at a heating rate of 10 °C · min-1 and were then immediately cooled after reaching the treatment temperature. Gas products during treatment were collected in a gas bag and were then analyzed using gas chromatography (GC; Shimadzu Co., GC-14A). The pretreated precursors were dried in vacuo at 70 °C for 24 h prior to analysis with an elemental combustion system (Costech Instruments Co., ECS-4010) for ultimate analysis, Fourier transform infrared spectroscopy (FTIR; JEOL, JIR-SPX60), and X-ray diffraction (XRD; Rigaku, MultiFlex) for structural analysis of the solid residues. Various air treatments were also carried out by adjusting the air composition flow rate, temperature, and heating pattern for comparison of the obtained products. Air treatment at 300 °C was performed with different air concentrations and flow rates. Moreover, the heating pattern during air treatment was adjusted

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Figure 3. Product yields for nitrogen and air treatment at 240-340 °C (a-1 and b-1) and subsequent flash pyrolysis at 764 °C (a-2 and b-2), based on untreated samples.

by decreasing the treatment temperature and heating rate and also increasing the holding time at the treatment temperature to investigate the most suitable conditions for the suppression of tar production. The air flow rate used in the experiment was decreased to 40 mL · min-1 (for air treatment to 300 °C at heating rates of 0.5 and 1 °C · min-1) and 20 mL · min-1 (for air treatment to 260 °C with a holding time of 30-60 min and a heating rate of 0.2 °C · min-1), because of the limitation of the gas bag volume. Air treatment at low temperature and subsequent pyrolysis using a TG analyzer (Shimadzu, TGA-50) were also carried out to confirm the effects of air on biomass pyrolysis at low temperature. Raw biomass samples of 1.5-3.0 mg were heated from room temperature to 110 °C in an inert atmosphere (He) and then held at this temperature for 20 min to vaporize moisture in the samples. The samples were treated under an airlike atmosphere (He + O2) at a heating rate of 10 °C · min-1 to the air treatment temperatures of 240, 280, 300, 320, 340, 380, and 400 °C. The conditions were then immediately converted from air-treatment mode to an inert atmosphere at the air treatment temperatures, and the samples were continuously pyrolyzed at the same heating rate up to 800 °C. The samples were completely combusted with air at 800 °C to determine their carbon conversions and char yields. Air gasification and pyrolysis with inert gas (nonair treatment) from 110 to 800 °C at a heating rate of 10 °C · min-1 were also performed for comparison. 2.3. Flash Pyrolysis. Flash pyrolysis of solid precursors treated with nitrogen and air was performed at 764 °C at a heating rate of 3000 °C · s-1 using a Curie-point pyrolyzer (CPP; Japan Analytical Ind., JHP-2S), and the pyrolysis products were then analyzed. Samples of 0.5-1.0 mg were weighed and wrapped in metal foils with different Curie points to indicate their pyrolysis temperatures. Gaseous products were continu-

ously analyzed using a gas chromatograph (Shimadzu, GC-14A) directly connected to the CPP. 2.4. Tar Analysis. After the gas treatment process, condensed tar products that adhered to the inner tube wall of the reactor outlet were washed and collected using acetone. For the tar analysis, acetone was used as the mobile phase, fed at 0.5 mL · min-1 to a gel permeation chromatograph equipped with a column (Showa Denko, Asahipak GF-310HQ) heated at a constant temperature of 40 °C and a refractive index (RI) detector, to estimate the composition of tar products. 3. Results and Discussion 3.1. Comparison of Air and Inert-Gas Treatment at Low Temperature. 3.1.1. Influence of Gas Treatment Temperatures on Product Yield Distribution. Parts a and b of Figure 3 show the product yields of biomass samples treated under nitrogen and air atmospheres, respectively, at 240, 280, 300, 320, and 340 °C with a heating rate of 10 °C · min-1 and then pyrolyzed in flash mode at 764 °C. The graphs of product yields based on untreated samples can be separated into two stages: (i) gas treatment (Figure 3a-1 and b-1) and (ii) flash pyrolysis (Figure 3a-2 and b-2). The yield of treated solid precursors obtained by gas treatment is the total yield of pyrolysis products obtained by CPP. Gas products such as CO2, CO, and hydrocarbons obtained during gas treatment and flash pyrolysis were determined by GC. Because the yield of water released during gas treatment was difficult to determine accurately, tar and water components were calculated from the elemental balance of the carbon and hydrogen contents in all products, including the measured yields of pretreated solid and gaseous products, mainly CO, CO2, and hydrocarbons. The calculation of tar and water products was based on the assumption that, during gas treatment at low temperature, tar products were mostly due to the decomposition of cellulose and


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Figure 4. TG curves for Japanese cedar wood samples treated with He + O2 gas at 240-380 °C and then heated under an inert gas atmosphere to 800 °C at a heating rate of 10 °C · min-1: (a) temperature range of 200-800 °C and (b) enlarged scale of 250-450 °C.

Figure 5. FTIR spectra of biomass precursors from gas treatment at 240-340 °C in (a) nitrogen and (b) air.

hemicellulose components in the biomass structure whereas most of the lignin composition became solid residues and gas with insignificant tar yield because of its aromatic-ring structure, which is difficult to volatilize at low temperature. For product distributions from nitrogen treatment (Figure 3a1), the yields of tar and gas products increased with increasing treatment temperature, whereas the solid yield gradually decreased as a result of the decomposition of the biomass samples. During flash pyrolysis of the solid precursors treated with nitrogen at 764 °C (Figure 3a-2), the yields of tar and gas products decreased with increasing treatment temperature. However, the overall tar yields obtained by both gas treatment and flash pyrolysis were not significantly different, at approximately 50 wt % for all different nitrogen treatment temperatures. The char yield after flash pyrolysis increased slightly with treatment temperature, possibly because slow heating to higher temperature during the gas treatment period causes more cross-linking within the structure of the treated biomass. For air treatment, the total product yield based on the untreated sample exceeded 1.0, because of the additional oxygen content in air during the treatment, as shown in Figure 3b. CO and CO2 increased with increasing treatment temperature, whereas the overall tar yields obtained from both air treatment and flash pyrolysis were approximately 10-15 wt % lower than those for nitrogen treatment in Figure 3a. In particular, tar released during air treatment was suppressed when the temperature increased above 300 °C. The yield of tar with air treatment at 300-340 °C was almost the same as that shown in Figure 3b-1. Moreover, with air treatment above 300 °C, tar was hardly produced during subsequent pyrolysis, as shown in Figure 3b2. This indicates that partial oxidation at low temperature promoted the production of CO and CO2, whereas tar evolution was terminated by the influence of oxygen in the air. The structure of solid precursors treated with air at low temperature

Figure 6. XRD patterns of biomass precursors from gas treatment at 240-340 °C in (a) nitrogen and (b) air.

seems to change into a form such as a cross-linked structure from which it is difficult to release tar products during subsequent pyrolysis at higher temperature. However, the overall product distribution with air treatment at 240 °C was almost identical to that for nitrogen treatment, because the decomposition of biomass was still in the initial stage at this relatively low temperature. Moreover, TG analysis was used to confirm the effect of air treatment at low temperature on the pyrolysis rate. TG curves for air treatment at different temperatures with subsequent pyrolysis to 800 °C including gasification and pyrolysis (nontreatment) curves are shown in Figure 4a. The curves are enlarged in the range of 250-450 °C (Figure 4b) for clarity. Air treatment at 240 °C resulted in almost the same curve as obtained for nontreatment pyrolysis, because the decomposition of biomass at a heating rate of 10 °C · min-1 was not promoted at this low temperature. During the pyrolysis step at 240-350 °C, the decomposition curves for the air-treatment samples were shifted to lower temperature compared to that for nontreatment pyrolysis, which indicates that dehydration and/or partial oxidation to produce CO and CO2 could proceed by the influence of active oxygen in the air. Moreover, the pyrolysis characteristics between the treatment temperatures at 500 °C were

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Table 1. Elemental Analysis of Japanese Cedar Wood Sample Treated with Nitrogen and Air in the Low-Temperature Region elemental component (wt/wt) C

H

elemental ratio (mol/mol)

O

N

H/C

O/C

treatment temperature (°C)

N2

air

N2

air

N2

air

N2

air

N2

air

N2

air

240 280 300 320 340

50.49 57.99 65.62 72.67 74.16

51.77 61.88 64.25 62.08 63.71

5.74 5.45 5.06 4.69 4.49

5.61 3.60 2.40 1.85 1.77

43.63 36.39 29.09 22.32 20.94

42.49 34.16 32.91 35.59 34.40

0.14 0.17 0.24 0.32 0.41

0.13 0.36 0.45 0.47 0.11

1.364 1.127 0.925 0.775 0.726

1.399 1.364 1.127 0.925 0.775

0.648 0.471 0.332 0.230 0.212

0.645 0.648 0.471 0.332 0.230

different. In particular, the yield of treated solid obtained during air treatment at 300-340 °C was slightly higher than that for nontreatment pyrolysis. This can be explained by the progress of cross-linking reactions during air treatment. On the other hand, for air treatment at temperatures above 350 °C, the char yields after pyrolysis to 800 °C were lower than those of the nontreatment samples, because the gasification step appeared to start from 350 °C. Through these results from TG analysis, it was concluded that the structure of the biomass was changed by cross-linking reactions with the simple method of air treatment at low temperature, resulting in increased char yields. 3.1.2. Analysis of Solid Precursors Treated with Gas at Various Temperatures. To compare and further analyze the structure of solid precursors treated with nitrogen and air at low temperature, FTIR and XRD analyses were conducted, the results of which are given in Figures 5 and 6, respectively. Absorption bands between 3000 and 3600 cm-1 in the FTIR spectra are typically ascribed to hydroxyl groups (OH), whereas that at around 1700 cm-1 is usually due to carbonyl groups (CO) in ketones and aldehydes, as well as carboxyl groups that are dependent on cross-linkage and hydrate formation. Moreover, the absorption bands between 2800 and 3000 cm-1 can be assigned to a stretching vibration of aliphatic group C-H bonds. The FTIR spectrum of the residue obtained from nitrogen treatment at 280 °C (Figure 5a) had the same pattern as that treated with nitrogen gas at 240 °C, whereas the OH peak intensity gradually decreased when the samples were treated at higher temperatures of 300-340 °C. Following treatment with air at 240-300 °C (Figure 5b), the spectra showed a relative decrease in the intensity of absorptions around 3200 and 3600 cm-1, as well asa small increase in the intensity of the absorption at 1700 cm-1. This indicates that more cross-linking reactions, such as dehydration, proceed to produce water through air treatment. In addition, the amount of both intramolecular and intermolecular hydrogen bonds and glucose products released by the dissociation of glycosidic bonds decreased, as a result of the influence of the air treatment. However, the spectra of the residues from gas treatment at 300-340 °C showed almost the same intensities for the absorptions around 3200 and 3600 cm-1, but with lower intensities compared to those of the nitrogen-treatment residues at the same treatment temperature. This indicates that dehydration to produce water was promoted by the influence of air and was then terminated above 300 °C. Moreover, the intensity of the absorption bands between 2800 and 3000 cm-1 (C-H bonds in aliphatic groups) significantly decreased with increasing air-treatment temperature and almost disappeared when the treatment temperature reached 300 °C. In contrast, for nitrogen treatment, this absorption band showed little decrease in intensity up to a treatment temperature of 320 °C and then a slight decrease in intensity when it was treated at 340 °C. The decrease in C-H bonds in aliphatic groups in the biomass structure treated with air also confirms that partial oxidation of aliphatic hydrocarbons proceeded to produce gases such as CO and CO2. This is consistent with the product yields of air treatment, as shown in Figure 3a-1.

XRD patterns of the samples treated with nitrogen and air showed peak intensities at 2θ ) 20-25°, as shown in Figure 6. In the case of nitrogen treatment (Figure 6a), the peak intensities continuously decreased with increasing treatment temperature from 280 to 340 °C, which indicates that the crystallinity of the residue precursors gradually decreased. The sample structure then became almost amorphous at 340 °C, because the glycosidic bonds could be randomly decomposed to produce tar when heated at higher temperature. This is consistent with the yield of tar, which gradually increased with increasing temperature (Figure 3a-1). For treatment in air at 280-300 °C (Figure 6b), the peak intensities were relatively decreased compared with those of nitrogen treatment. The peaks almost disappeared for the samples treated in air at 320-340 °C, because cross-linked bonds were also randomly formed in the structure upon air treatment. In addition, ultimate analyses of the solid precursors treated with nitrogen and air were also compared to thoroughly investigate structural changes and reaction mechanisms during gas treatment at low temperature. Pretreated precursors were randomly selected at least three times for examination of the elemental composition to confirm homogeneity. The results showed insignificant differences in elemental composition for all samples. The average percentages of each elemental component by weight and the molar ratios for each gas treatment at different temperatures are reported in Table 1. For nitrogen treatment, as the treatment temperature increased, the carbon content remaining in the precursor structure increased from approximately 50 wt % at 240 °C to more than 70 wt % at 320-340 °C, but the oxygen content significantly decreased to approximately 20 wt % at 340 °C. The hydrogen content showed a small decrease with increasing treatment temperature. For air treatment, the carbon content increased, whereas the oxygen content decreased with increased treatment temperature to 300 °C. However, the percentage carbon and oxygen contents in the treated precursors were almost constant at approximately 63 and 34 wt %, respectively. The hydrogen content decreased from more than 5 to less than 2 wt % when the temperature was increased from 240 to 340 °C. For more detailed investigations, the relationship between the O/C and H/C elemental molar ratios was plotted to estimate a possible reaction mechanism during gas treatment at low temperature, as shown in Figure 7.

Figure 7. Relationship between O/C and H/C elemental molar ratios of samples treated with nitrogen and air in the low-temperature region.


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Figure 8. Product yields (based on untreated samples) of samples subjected to (a) air treatment to 300 °C and (b) subsequent flash pyrolysis at 764 °C under various air flow rates (10, 40, and 50 mL · min-1) and oxygen concentrations (2.1, 10.5, and 21.0 wt %).

If the slope of such a graph, which shows the O/H ratio, were equal to 0.5, then only the dehydration reaction would mainly occur, with the formation of water (O/H ) 0.5 for H2O production). However, the slope obtained in Figure 7 was approximately 0.67 or 2:3 for nitrogen treatment. Therefore, it was estimated that not only was dehydration proceeding, but light tar products containing OH groups such as aromatic alcohols might also be released during biomass treatment at low temperature (O/H ) 2:3 for production of both H2O and OH). On the other hand, the O/H slope for air treatment decreased to approximately 0.23 (less than 1:4), which indicates that the loss of methyl groups (CHx) in the biomass structure by partial oxidation with air seems to increase the proportion of hydrogen content in the O/H ratio, which is consistent with the FTIR results. Consequently, dehydration (H2O production) and partial oxidation (CHx loss) could occur simultaneously during air treatment, whereas tar evolution could be suppressed, as discussed previously. 3.2. Effects of Air Treatment under Various Conditions on Product Yield Distribution. 3.2.1. Influence of Air Concentration and Flow Rate. Air influences the biomass structure and product distribution during treatment at low temperature, especially for treatment temperatures from 300 °C, as discussed in previous section; therefore, treatments with varying oxygen concentrations and flow rates of air to 300 °C were conducted to further investigate the effect of the oxygen ratio in air on the treatment products. The maximum air flow rate used for the treatment was 50 mL · min-1, because biomass samples could be partially burned out with high flow rates above 50 mL · min-1 in the experimental setup used. Therefore, the air flow rates were decreased to 40, 20, and 10 mL · min-1 for comparison. The maximum oxygen concentration was fixed at an oxygen content of 21.0 wt % as with air in the atmosphere, and the oxygen concentration was subsequently diluted to 16.8, 10.5, 4.2, and 2.1 wt %. Air was mixed with pure nitrogen gas at fixed ratios for low oxygen concentrations and the diluted air was then passed through a static mixer (Noritake) before being allowed to flow into the reactor. The product distributions resulting from gas treatment and pyrolysis at various oxygen concentrations and flow rates are shown in Figure 8. Lower yields of CO and CO2 and higher overall tar yields were obtained with decreasing oxygen concentration. The results also showed the same tendency with lower air flow rates, because of the relative reduction in the contact surface area of treated solid for the reaction promoted by reactive oxygen. The product distribution for air treatment with extremely low oxygen concentrations of 2.1 and 4.2 wt % and low flow rate at 10

mL · min-1 reached that for nontreatment, because the oxygen content became too low to be effective for reactions such as partial oxidation. 3.2.2. Influence of Reaction Time during Variation of the Air Treatment Heating Rate and Holding Time. Tar evolution can be considerably suppressed by air treatment at low temperature, particularly at 300-340 °C, with the yield of tar decreasing to approximately 35 wt %. However, this amount of tar production is still considered to be rather high and can easily damage gas turbines or gas engines in gasification systems. Therefore, the heating patterns of air treatment were adjusted by decreasing the heating rate and treatment temperature and increasing the holding time of the air treatment to investigate their effects on product yields, especially the amount of released tar. Air treatment to 300 °C with a flow rate of 40 mL · min-1 and low heating rates of 1 and 0.5 °C · min-1 was performed for comparison with the heating rate of 10 °C · min-1. The treatment period was then increased from 19 min to 3 h 10 min and 6 h 20 min, by decreasing the heating rate from 10 °C · min-1 to 1 and 0.5 °C · min-1, respectively. The product distributions, as shown in Figure 9a-1, indicate that the tar yields during air treatment decreased to approximately 20-30 wt % when the samples were subjected to a very low heating rate, whereas the CO, CO2, and water yields significantly increased. Moreover, the tar yield released during flash pyrolysis at 764 °C (Figure 9a-2) reached almost zero for air treatment at very low heating rates, especially at 0.5 °C · min-1. This indicates that, when biomass samples are gradually heated in the low-temperature region, the structure of the solid precursors treated with air might randomly produce more cross-linking. Therefore, tar is hardly produced during subsequent pyrolysis at higher temperature, even though more gas products, including water, are produced as a result of the aliphatic cracking of hydrocarbons. Furthermore, an increase in the holding time of the treatment period and a decrease in the heating rate were considered as condition adjustments for air treatment to 260 °C, because slow heating with air at low temperatures less than 300 °C might also suppress tar evolution and produce cross-linked char. For different holding times, the same yields of tar products were obtained during air treatment, as shown in Figure 9b-1. The yields of tar products released during subsequent pyrolysis decreased when the temperature was held at 260 °C for 30 min and 1 h during air treatment, whereas the gas products increased, because partial oxidation can proceed further with longer reaction times, as shown in Figure 9b-2. For a decrease in the

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Figure 9. Product yields (based on untreated samples) for samples subjected to air treatment at (a-1) 300 °C with various heating rates (0.5, 1.0, and 10.0 mL · min-1) and at (b-1) 260 °C with various heating rates (0.2 and 10.0 mL · min-1) and holding times (0 min, 30 min, and 1 h) and subsequent flash pyrolysis of the treated precursors at 764 °C (a-2 and b-2). Table 2. Experimental Conditions and Enthalpies of Reaction for Gas Treatment Using Various Heating Patterns in the Low-Temperature Region total treatment period run

gas flow rate (mL · min-1)

1 2 3 4 5 6 7 8

N2 50 air 10 air 10 + N2 40 air 50 air 40 air 20 air 40 air 20

enthalpy of reaction at 298 K (kJ)

treatment heating rate heating time to holding time at gas treatment at released tar decomposition temperature (°C) (°C · min-1) treatment temperature treatment temperature low temperature (eq 1) by steam reforming (eq 2) 300 300 300 300 300 260 300 260

10.0 10.0 10.0 10.0 1.0 10.0 0.5 0.2

19 min 19 min 19 min 19 min 3 h 10 min 15 min 6 h 20 min 12 h 30 min

heating rate to 0.2 °C · min-1 (treatment period ) 12 h 30 min), the tar yield during air treatment at 260 °C decreased significantly to 18.4 wt %, and almost no tar was produced during subsequent pyrolysis at high temperature. It can be concluded that tar evolution by biomass decomposition is significantly suppressed when samples are gradually treated with air at low temperature (260-300 °C). Moreover, very few tar products are released during subsequent pyrolysis at high temperature with very low heating rates (0.2-1 °C · min-1) and long holding times (1 h) during air treatment at low temperature. 3.2.3. Verification of the Combustion Problem during Air Treatment at Low Temperature. The various conditions for gas treatment with nitrogen and air and their enthalpy of reaction are summarized in Table 2 (runs 1-8). The chemical reaction for gas treatment is expressed by eq 1, excluding oxygen gas for nitrogen treatment. The enthalpy of reaction at 298 K can be calculated from the elemental balance and enthalpy of formation for each product. The enthalpies of gas treatments as shown in Table 2 indicate that nitrogen treatment, so-called

1h -

48 -135 -81 -405 -578 -530 -777 -742

298 273 256 217 204 167 146 125

pyrolysis, is an endothermic reaction, whereas air treatment under more severe conditions is more exothermic in the range of 100-800 kJ, because of CO and CO2 release. However, the amount of heat released during air treatment at low temperature is significantly lower than the enthalpy of combustion (eq 2) of 1771 kJ. This result is consistent with the value obtained from partial oxidation of general biomass and confirms that the combustion reaction does not occur. Table 2 also shows that tar decomposition by steam reforming (eq 3) is endothermic. [Cx0Hy0Oz0]biomass + a[O2] f bCO + cCO2 + dH2O + [Cx1Hy1Oz1]precursor + [Cx2Hy2Oz2]tar at low temp

(1)

[Cx0Hy0Oz0]biomass + eO2 f f CO2 + gH2O

(2)

[Cx2Hy2Oz2]tar at low temp + hH2O f iCO + jH2

(3)

The TG analysis results exhibited the same tendency in the char yields obtained by air treatment using the experimental


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Figure 10. Determination of Ea using the distributed activation energy model (DAEM) with (a) the V/V* vs T relationship measured at a ) 5, 10, and 20 °C · min-1 and (b) Arrhenius plots of a/T2 vs 1/T at selected values of V/V*.

apparatus. The air flow rate was further increased from 50 to 75 and 100 mL · min-1 to observe the effects of diffusion. TG curves with increased air flow rates showed no differences, so that it was concluded that air treatment at low temperature can be considered as a rate-controlling reaction, not a diffusionlimited reaction. To confirm that no combustion reaction occurred during air treatment at low temperature, the activation energies (Ea) were calculated using the distributed activation energy model (DAEM) by the method developed by Miura et al.21,22 Three sets of air-treatment biomass samples with different heating rates of 5, 10, and 20 °C · min-1 were studied by TG analysis. Figure 10a shows the relationship between V/V* and temperature for biomass air treatment, where V is the total volatiles evolved over time and V* is the effective volatile content of the biomass. The result indicates two reaction steps: (i) pyrolysis under air atmosphere at low temperature and (ii) air gasification at high temperature. The Ea values, calculated from the slopes of the Arrhenius plots in Figure 10b, were found to be 310-340 kJ · mol-1 for V/V* < 0.6 (pyrolysis under air atmosphere) and decreased to 230-310 kJ · mol-1 at higher temperature with V/V* at greater than 0.6 (air gasification). These results are in agreement with the Ea value of 120-400 kJ · mol-1 obtained from the general pyrolysis of biomass, whereas the reported values for Ea obtained for biomass char combustion are relatively lower, in the range of 70-120 kJ · mol-1.23,24 This large difference in Ea between combustion and air treatment confirms that the combustion reaction did not occur in our experiments. However, to avoid problems of the occurrence of combustion and the presence inhomogeneous pretreated precursors, each experimental run of gas treatment at low temperature and subsequent flash pyrolysis was carried out repeatedly. The samples used in the experiments were small amounts of wellsieved material, so that the distribution of product yields was quite similar for all runs. This indicates that the experimental apparatus developed for gas treatment under the designed conditions at low temperature successfully obtained homogeneous pretreated products without undefined mass and heat transfer, varying temperature distribution, or combustion problems. 3.2.4. Analysis of Tar Products Obtained during Gas Treatment under Different Conditions. The yields of tar produced during both gas treatment and flash pyrolysis under the various conditions listed in Table 2 are summarized in Figure 11. The overall tar yield decreased from approximately 50 wt % for nitrogen treatment (run 1) to less than 20 wt % for air treatment to 260 °C with a heating rate of 0.2 °C · min-1 (run 8). Moreover, the tar yield obtained during flash pyrolysis at 764 °C decreased from 10 to less than 5 wt % upon air treatment, and it reached almost zero for air treatment at 260-300 °C with heating rates of 0.2-1 °C · min-1 (runs 5, 7, and 8).

Figure 11. Overall yields of tar products released during gas treatment and subsequent flash pyrolysis.

Tar produced during gas treatment under various heating patterns was analyzed using gel permeation chromatography (GPC). Because the retention peak distribution of the GPC elution patterns showed no significant difference among the tar products treated with air at low temperature (260-340 °C), as shown in Figure 12a, the elution patterns of tar released during nitrogen and air treatment at 300 °C were chosen for comparison, as shown in Figure 12b. The retention peaks at 19-20 and 22 min, which are presumed to be due to dimers, are clearly evident for nitrogen treatment (run 1), whereas they are almost absent for air treatment (run 4). This indicates that the release of tar, especially in the form of dimers, is partially suppressed by the formation of cross-linked precursors and gas products through the influence of air during treatment at low temperature. However, cellobiosan, which is one of the main tar products that can still be released during air treatment at low temperature, is indicated by the retention peak at 21 min. Although the release of all tar products cannot be suppressed at low temperature, these tars seem to be simply decomposition products of the reforming reaction with steam/air or catalysts at higher temperature, compared to the higher-molecular-weight tars released at high temperature. 3.3. Proposed Reaction Mechanism for Biomass Gas Treatment at Low Temperature. From the results of the product yield distributions and structural analyses of the treated solids, a schematic diagram of the change in the biomass structure with temperature during gas treatment at low heating rate (

Examination of the Low-Temperature Region in a Downdraft Gasifier

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