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Decomposition and gasification of pyrolysis volatiles from pine wood through a bed of hot char
Fuel 90 (2011) 1041–1048
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Decomposition and gasification of pyrolysis volatiles from pine wood through a bed of hot char Qingsong Sun a, Sang Yu a, Fuchen Wang b, Jie Wang a,⇑ a b
Department of Chemical Engineering for Energy, East China University of Science and Technology, 130# Meilong Road, Shanghai 200237, PR China Key Laboratory of Coal Gasification of Ministry of Education, East China University of Science and Technology, 130# Meilong Road, Shanghai 200237, PR China
a r t i c l e
i n f o
Article history: Received 24 December 2009 Received in revised form 9 December 2010 Accepted 18 December 2010 Available online 1 January 2011 Keywords: Pine wood Pyrolysis Gasification Catalytic tar cracking Char
a b s t r a c t Pine wood was pyrolyzed in a fixed bed reactor at a heating rate of 10 °C and a final temperature of 700 °C, and the resultant volatiles were allowed to be secondarily cracked through a tubular reactor in a temperature range of 500–700 °C with and without packing a bed of char. The thermal effect and the catalytic effect of char on the cracking of tar were investigated. An attempt was made to deconvolute the intermingled contributions of the char-catalyzed tar cracking and the char gasification to the yields of gaseous and liquid products. It was found that the wood char (charcoal) was catalytically active for the tar cracking at 500–600 °C, while at 650–700 °C, the thermal effect became a dominant mode of the tar cracking. Above 600 °C, the autogenerated steam gasified the charcoal, resulting in a marked increase in the yield of gaseous product and a significant change in the gas composition. An anthracite char (A-char), a bituminous coal char (B-char), a lignite char (L-char) and graphite also behaved with catalytic activities towards the tar cracking at lower temperature, but only L-char showed reactivity for gasification at higher temperature. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Thermochemical conversion of biomass as a renewable resource to fuel gas via pyrolysis and gasification has been drawing considerable attention in recent years [1–4], owing to the rapid resource depletion and pressing environmental issues arising from huge consumption of fossil fuels. A great deal of work has been done to develop new technologies of biomass pyrolysis and gasification [5–7]. It is well known that one of the major problems encountered in pyrolysis and gasification processes is the tar formation, which reduces the efficiency of gas production and interferes with the equipment operation. Various methods have thereby been investigated to eliminate or reduce the tar formation [8,9]. The cracking of tar downstream from pyrolyzer or gasifier offers an effective approach to minimizing the concentration of tar in gas. In the case of homogenous cracking of tar (without catalyst), however, some refractory tar is hard to eliminate, so that a high temperature is required [8,10]. Brandt et al. [11] claimed that the temperature and residence time were 1250 °C and 0.5 s, respectively, in order that a sufficient extent of tar decomposition was guaranteed. Heterogeneous or catalytic cracking of tar is advantageous in efficient conversion and regulating gas composition. Hence it has been the subject of numerous studies. Various ⇑ Corresponding author. Tel./fax: +81 21 64252853. E-mail address: [email protected] (J. Wang). 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.12.015
catalysts, such as dolomites [12], olivines [12–14], alkali and alkaline earth salts [15–17], Fe or Ni-based catalysts [18–21], composite catalysts [22,23], have demonstrated their effectiveness. However, the many have a drawback of catalytic deactivation caused by coke deposition on the surfaces [24–26]; composite catalysts can prevent deactivation, but the cost is relatively high [22]. Char-induced or -catalyzed cracking of tar is an attractive research topic with three aspects of consideration. First, one way to reduce the tar formation can be implemented inside gasifier by modifying its configurations and process parameters, and this entails the information on the interactions between tar and char. Second, an alternative way to remove tar is by thermal cracking of tar in a separated reactor using char as a catalyst, which has advantages over common mineral catalysts: (1) char is produced in the pyrolysis and gasification system; (2) char surface can be refreshed via char gasification to avoid deactivation; (3) char can be loaded with dispersed metallic catalysts, and this would greatly enhance the catalytic capability; and (4) it is feasible to regulate or upgrade gas composition via char gasification. Third, biomass usually contains high moisture, and the dewatering process loses large energy [27]. The char-catalyzed tar cracking may save this energy expenditure by exploiting the autogenerated steam directly for char gasification. In the past decades, there have been a few interesting studies pertaining to the char-catalyzed cracking of tar [28–33]. Boroson et al. [28] found that heterogeneous or char-induced conversion
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pyrolysis reactor include non-condensable gases and condensable liquid products (including water and tar), and the tar consisted of a number of oxygenated compounds characteristic of the degradation products of holocellulose and lignin constituents in wood. As a consequence, not only the cracking of tar but also the reaction between char and steam is provoked when the volatiles flow over a char bed in the second reactor depending on the cracking conditions. This study has tried to clarify their individual contributions to the yields of gaseous and liquid products.
of fresh wood pyrolysis tars (subtraction from thermal conversion) was significant but essentially constant at 14 ± 7 wt.% of tar in a temperature range of 400–600 °C and space times from 2.5 to 100 ms. Abu El-Rub et al. [31] compared biomass char to some common inorganic catalysts for the cracking of phenol and naphthalene in a temperature range of 700–900 °C, and they observed that biomass char was almost as catalytically active as nickel and dolomite. Gilbert et al. [33] investigated the conversion of pyrolysis vapors through a char bed with varying parameters including bed lengths (0–450 mm), temperatures (500–800 °C), particle sizes (10 and 15 mm) and nitrogen purge rates (1.84–14.70 mm/s). To the contrary, they observed that the char-catalytic effect was marginal, and the thermal effect was the main mode of tar decomposition. In this study, we investigate the behaviors of char-catalyzed cracking of biomass tar using a two-stage fixed bed reactor, based on our previous study on the behaviors of biomass pyrolysis in the first fixed bed reactor [34]. The nascent volatiles from the first
2. Experimental 2.1. Biomass sample and char preparation The biomass sample used in this study was the wood sawdust of Chinese white pine, which was supplied by a local wood processing factory. The sample was screened to the particle sizes of less than 1 mm, and dried at 105 °C for 6 h prior to experiment, and then stored in a sealed desiccator. The dried wood sample contained 0.35% ash. Elemental analysis showed that the ash-free sample contained 47.6% C, 6.1% H, 0.1% N, <0.05% S and 46.2% O (by difference). The charcoal was prepared by pyrolyzing the wood sample at 750 °C for 30 min in a heater under an air-isolated condition. Three other chars, called A-char, B-char, and L-char, were prepared in the same manner, using an anthracite sample, a bituminous coal sample and a lignite sample, respectively. All the char samples were ground to the particle sizes of 0.075–0.15 mm. The proximate and ultimate analyses of the char samples, together with the ash compositions, are tabulated in Table 1. The char samples were stored in the sealed desiccator, and the storage duration was not beyond one week in all the experiments. For comparison, a commercial reagent of powdery graphite (particle size, <0.030 mm) was used in the experiment.
Table 1 The proximate and ultimate analyses of the char samples as well as the ash compositions. A-char
B-char
L-char
Proximate analysis (%, dry basis) Ash 2.1 Volatile 6.9 Fixed carbon 91.0
Charcoal
27.2 3.4 69.4
12.1 4.3 83.6
32.8 6.1 61.1
Ultimate analysis (%, daf. basis) C 96.0 H 0.9 N 0.2 S <0.05 O (by diff.) 2.9
97.0 1.4 0.7 0.8 0.1
91.2 1.4 1.5 0.06 5.9
88.2 1.2 0.9 1.5 8.2
Ash composition (%) SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3 Others
55.7 30.1 3.5 3.4 0.3 0.6 1.3 2.9 2.2
52.1 30.8 6.7 3.3 0.4 Trace 1.2 2.3 2.2
28.1 17.3 8.0 25.3 3.9 0.2 1.1 14.2 1.9
– – – – – – – – –
2.2. Pyrolysis apparatus and procedures Fig. 1 is the diagram of the experimental apparatus. Two tubular reactors, both of which were made of stainless steel with a length
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Fig. 1. The apparatus of two-stage fixed bed reactors. (1) Argon bomb; (2) air bomb; (3) pressure control valve; (4) flow rate meter; (5) adsorbent column; (6) ceramic pellets; (7) electric furnace; (8) tubular reactors; (9) thermocouple; (10) biomass sample; (11) tape heaters; (12), char bed; (13) electric furnace; (14) ice-water trap; (15–17) temperature controlling instruments.
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2.3. Analysis methods The major gases including H2, CO, CH4 and CO2 were determined on a gas chromatograph (Aglient 6820) equipped with a thermal conductivity detector (TCD) and a packed column (S.S.
3 m 1/800 OD Unibeads C 80/100) using argon as a carrier gas. The compounds of liquid products were characterized using GC/MS (PerkinElmer clams 500), equipped with a HP-5 capillary column. Seven typical liquid compounds were quantitatively determined on a gas chromatograph (Agilen-6820), equipped with a flame ionization detector (FID) and HP-5 capillary column, by an external calibration method. The seven compounds included hydroxyl propanone, furan methanol, phenol, methyl-phenol, guaiacol, 2, 4-dimethyl-phenol, and D-allose. Elemental analysis (C, H and N) was carried out on an elemental analyzer (Elementar Vario EL III). Sulfur analysis was accomplished on a Coulomb sulfur analyzer (CLS-2). The metallic composition of ash was determined by X-ray fluorescence spectrometry (Shimadzu, XRF-1800).
3. Results and discussion 3.1. Effects of charcoal 3.1.1. Overall distribution of products To differentiate between the thermal effect and the charcatalytic effect on the secondary cracking of tar, experiments were conducted with and without packing charcoal in the second reactor. For briefness, the temperature used for biomass pyrolysis in the first reactor is called the pyrolysis temperature, and the cracking temperature refers to that used for tar cracking in the second reactor. Fig. 2 shows the effect of different cracking temperatures on the yields of gaseous, liquid and char products produced over the total pyrolysis process with and without charcoal. The yield of gas product is the summed amounts of four major gases (H2, CO, CH4 and CO2). The yields of gaseous product, total liquid product and char are the averages obtained by the duplicate or triplicate experiments, and their respective variations are displayed on the bars. The liquid product is divided into the NEV-Tar and EV-Liquid, and their yields are also displayed. In the absence of char, the yield of liquid product was as high as 59% (including 25% of EV-Liquid and 34% of NEV-Tar) at the
100
Yield (wt.%, dry biomass)
of 620 mm and an inside diameter of 23 mm, were vertically set in series. The reactors were heated independently by two electric furnaces. The first reactor in the lower part was used for slow pyrolysis of biomass. Ceramic pellets were packed in this reactor to support the biomass sample. In each run, a 2.2 g wood sample was wrapped in a stainless steel wire mesh, and then placed on the support layer within a uniform temperature zone. A K-type thermocouple was made to contact with the sample wrap for measuring the pyrolysis temperature. The second reactor in the upper part was used for cracking of tar. A 1.3 g char sample was held in a wire mesh wrap, and then tightly inserted in the center of this reactor. The height of the char bed was about 30 mm. Another K-type thermocouple was made to contact with the sample wrap for measuring the cracking temperature. The argon gas from the pressurized cylinder was de-moisturized through a silica gel/molecular sieve adsorbent column, and then purged upward from the first reactor to the second reactor, with the flow rate of 150 mL/min. Prior to the beginning of biomass pyrolysis in the first reactor, the char sample in the second reactor was pre-heated to the predetermined temperature, and it was kept at the final temperature for a period of time (about 20 min) until no gases were released from the char sample. Then, the first reactor was heated up at a rate of 10 °C/min till a final temperature of 700 °C. During the heat-up, the volatiles produced in the first reactor were conveyed through the hot char in the second reactor and subjected to the isothermal cracking. To suppress the undesired condensation of tar vapors in the connectors, the off-gas lines and the joints between two reactors were maintained at 260 °C with tape heaters. Experiments of tar cracking were also carried out without packing char in the second reactor by way of contrast. The residence time of purging gas through the void spaces of the char bed was approximately 2.2 s, and the corresponding residence time without packing char was 5.0 s. The liquid product including aqueous and oily phases was condensed in two U-type glass traps immersed in the ice-water baths. The glass traps were carefully weighed before and after each experiment to determine the total liquid yield (including water and tar). To fractionate the liquid product, evaporation experiments were conducted by heating two product-attached traps at 105 °C under atmospheric pressure for 6 h until constant weights of the traps were reached. The tarry fraction retained in the traps after evaporation is named the non-evaporated tar (NEV-Tar), which mainly includes heavy tar; the liquid fraction evaporated from the traps is named the evaporated liquid product (EV-Liquid), which includes water and some light organic compounds. When the liquid sample was used for gas chromatography (GC) and gas chromatography mass spectrometry (GC–MS) analyses, the traps together with the connected tubes were dismantled, and the liquid product in them was dissolved with acetone under ultrasonic stirring. The gases were collected in the sequence of gas bags behind the vapor condensation at an interval of five minutes. Four major gases including H2, CO, CH4 and CO2 were quantitatively determined. The char samples in the first reactor and in the second reactor were collected separately after the reactors were cooled down, and then gravimetrically determined. The yield of char refers to the yield of the solid product remaining in the first reactor after biomass pyrolysis. Care was taken to clean up the reactors after finishing each pyrolysis experiment.
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650
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700
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With charcoal
Cracking temperature (oC) Char
NEV-Tar
EV-Liquid
Gas
Fig. 2. Yields of gaseous, liquid and char products obtained at different cracking temperatures with and without wood char (charcoal). Final pyrolysis temperature, 700 °C. The liquid product was fractionated to the non-evaporated tar (NEV-Tar) and the evaporated liquid (EV-Liquid).
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fractionation experiment. It was revealed that in the case of no catalyst, a part of heavy tar was thermally converted mainly to light liquid compounds (including water) rather than permanent gases at lower temperatures (500–550 °C); the substantial cracking of tar to gases occurred at higher temperatures (above 600 °C). In the presence of the charcoal, the yields of liquid product at the cracking temperatures of 500, 550, 600, 650 and 700 °C were, respectively, 13%, 13%, 10%, 8% and 11% lower than the corresponding data in the absence of char, in conjunction with increases in the yield of gaseous product. At 500–600 °C, the yields of NEV-Tar decreased from 18–20% without charcoal to 6–11% with charcoal, and the yields of EV-Liquid simultaneously decreased to different degrees. The results clearly indicated that at 500–600 °C, the charcoal intensely facilitated the decomposition of both heavy tar and light tar to gases. At 650–700 °C, the differences in the yields of NEV-Tar
cracking temperature of 300 °C, with the yield of gaseous product of 15%. In this circumstance, the cracking reactions of the nascent liquid products derived from the wood pyrolysis in the first reactor were negligible. At the cracking temperatures of 500–550 °C, the yields of liquid product were only slightly lower than the yield obtained at 300 °C, and the yields of gaseous products were also close to the yield obtained at 300 °C; nevertheless, the yields of NEV-Tar were appreciably lower than the yield obtained at 300 °C. With elevating cracking temperature up to 600–700 °C, the thermal treatment resulted in a marked reduction in the yield of liquid product and a marked increase in the yields of gaseous product. As an example, the yield of liquid product dropped from 59% at 300 °C to 35% at 700 °C, corresponding to an increase in the yield of gaseous product from 15% to 43%. Moreover, only 5% of liquid product obtained at 650–700 °C remained non-evaporated in the
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Pyrolysis temperature (oC) Fig. 3. Cumulative yields of major gases at different pyrolysis temperatures with varying cracking temperatures without char (a) or with charcoal (b). Cracking temperature: h-, 300 °C; –N–, 500 °C; –s–, 550 °C; –j–, 600 °C; q, 650 °C; –.–, 700 °C.
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3.1.3. Liquid products To further illustrate the effect of the charcoal on the decomposition of tar, GC–MS was used for characterizing organic compounds in the liquid products. Seven compounds were quantitatively determined, as described in the Experimental section. Other compounds were semi-quantitatively estimated according to the ratios of their peak areas to the peak area of one of the compounds quantified. In the interest of clarity, the main liquid compounds are grouped as carboxylic acids (acetic acid, etc.), cyclopentenones (methyl-cyclopentenone, hydroxyl-cyclopentenone, etc.), ketones (hydroxylpropanone, cyclohetanone, etc.), aldehydes (acetamido acetaldehyde, methyl-furan aldehyde, etc.), furans (dimethyl furan, furfural, furan methanol, benzofuran, etc.), alcohols (cyclopropyl carbinol, phenyl-methoxyethanol, etc.), saccharides (dianhydro-D-glucopyranose, anhydro-D-mannose, anhydro-D-galactose, anhydro-D-altroside, anhydro-D-talopyranose, D-allose, etc.), guaiacols (guaiacol, methyl-guaiacol, ethyl-guaiacol, vinyl-guaiacol, prophenyl-guaiacol, etc.), esters (ethanediol monoacetate, oxo-propanoic acid methylester, etc.), phenols (phenol, methyl-phenol, ethyl-phenol, etc.), and polyaromatic hydrocarbons (naphthalene, anthracene, fluorene, etc.). The yields of the grouped compounds produced from five typical cracking experiments are shown in Fig. 4. The primary liquid products obtained at the cracking temperature of 300 °C were abundant with a variety of oxygenated compounds, but no polyaromatic
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3.1.2. Gaseous products Fig. 3 shows the cumulative yields of four major gases (H2, CO, CH4 and CO2) with elevating pyrolysis temperature and varying cracking temperatures in two circumstances (with and without charcoal), where the scales of vertical axis for H2 and CO2 are under three times magnification in the case of no catalysis, relative to those in the case of using charcoal. The results obtained at the cracking temperature of 300 °C were representative of no secondary reactions. In this case, CO and CO2 were released noticeably in the pyrolysis temperature range of 200–450 °C. It was previously observed that most liquid products were also formed in this temperature range [34]. The cracking temperature exerted profound influences on the release patterns of gases whether charcoal was present or not. In either case, changes predominantly occurred in a pyrolysis temperature range of 200–450 °C, because most volatiles were formed in this temperature range. In the absence of char, the cumulative yields of H2, CO, CH4 and CO2 over the total pyrolysis process (from room temperature to 700 °C) were increased by a factor of 2.4, 3.2, 3.0, and 1.2, respectively, with the cracking temperature rising from 300 °C to 700 °C. It was indicated that the yield of CO2 was less varied by thermal effect. This was probably because CO2 was dissociated from fragile oxygenated groups like carboxyl group, and such decarboxylation reactions were kinetically less resistant than the reactions of dehydrogenation, decarbonylation and demethanation. It should be emphasized that in the case of no catalyst, the enhancements in the formation of CO and CH4 were slight at the cracking temperatures below 600 °C, and only at the temperatures exceeding 600 °C did the enhancements become striking. The analogous turning point for the formation of H2 was 650 °C. This result was consistent with the observation that the thermal cracking of liquid products to gases significantly occurred at temperatures above 600 °C, as shown in Fig. 2. The release patterns of four gases with the charcoal showed some salient alterations as compared to those without char. First, the presence of charcoal accelerated the formation of all four major gases at lower cracking temperatures (500–600 °C). As an example, at the cracking temperature of 550 °C, the cumulative yields of H2, CO, CH4 and CO2 over the total pyrolysis process in the presence of charcoal were 2.1, 1.3, 1.4 and 1.5 times higher than those in the absence of charcoal, respectively. Second, at higher cracking temperatures, four gases behaved differently in their yield changes influenced by charcoal. At a cracking temperature of 700 °C, the cumulative yields of H2 and CO2 obtained with charcoal were 3.9 and 2.9 times as high as those without charcoal, whereas the cumulative yields of CO and CH4 obtained with charcoal were only 1.2 and 1.1 times as high as those obtained without charcoal. This difference between the gases was attributable to twofold effects. One was the effect of char on the tar cracking, which was weakened relative to the increased thermal effect at higher cracking temperature. The yields of CO and CH4 obtained with and without
charcoal were thus approached at higher cracking temperature. Another was the autogenerated steam gasification of charcoal, which became intense with increasing cracking temperature, resulting in the enhancements in the yields of H2 and CO2. Third, although the effect of char dominated in the pyrolysis temperature range of 200–450 °C, it was discernable that in the pyrolysis temperature of higher than 450 °C, subtly more CH4 was formed in the case of using charcoal. This implied that the charcoal facilitated the decomposition of guaiacols, because the main liquid products produced from the pyrolysis of wood at temperature above 450 °C were guaiacols, a lignin degradation product [34].
Yield (mg/g-dry biomas)
obtained with and without char were insignificant, since the yields of NEV-Tar produced only by the thermal cracking were already small; however, the presence of charcoal lowered the yields of EV-Liquid. It should be pointed out that the reductions in the yields of EV-Liquid at 650–700 °C were mainly ascribed to the loss of the autogenerated steam by its reaction to gasifying charcoal. This point will be further clarified in a later section. The mass closure exceeding 100% at 650–700 °C also suggested the occurrence of charcoal gasification at these temperatures. Incidentally, in all experiments performed in the absence of char, the sum of the gaseous, liquid and char yields gave the mass closures of 92–99%, and no case was found of the mass closure over 100%. It should be noted herein that the influence of the wire mesh on the tar cracking was observed to be insignificant.
no char 300°C
no char charcoal 500°C 500°C
no char charcoal 600°C 600°C
Fig. 4. Yields of the grouped compounds in the liquid products obtained with and without charcoal at different cracking temperatures. Final pyrolysis temperature, 700 °C.
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hydrocarbons (PAHs) were present in it. The enrichment of saccharides in the primary liquid product suggested a mild degradation of hemicellulose and cellulose by the wood pyrolysis in the first reactor [34]. In the absence of charcoal, the thermal cracking at 500 °C led to some noticeable changes in the yields of liquid compounds as compared to the primary liquid product: (1) ketones, aldehydes, alcohols, saccharides completely disappeared; (2) carboxylic acids, furans, guaiacols and esters substantially decreased; (3) cyclopentenones slightly decreased; and (4) some PAHs were newly formed. As the cracking temperature was elevated to 600 °C (without charcoal), carboxylic acids and guaiacols completely disappeared; cyclopentenones substantially decreased; phenols trended to increase. Comparison of the yields of liquid compounds obtained with and without charcoal revealed that the charcoal promoted the decomposition of liquid compounds at the cracking temperatures of both 500 °C and 600 °C, but at 500 °C, the effect was relatively great. It could be seen that the presence of charcoal led to a remarkable decrease in the yields of carboxylic acids, cyclopentenones, furans, guaiacols and esters at 500 °C. We also analyzed the compounds in the liquid product obtained at the cracking temperature of 700 °C with charcoal (not shown). It was observed that the yield of phenols was lower than the corresponding result obtained at 600 °C, and some larger molecular PAHs such as fluorene and anthracene were detected at 700 °C while they are not discernable at 600 °C. Incidentally, the summed yields of the organic compounds determined by GC–MS was approximately 8%-dry biomass for the primary liquid product, much smaller than the yield of NEV-Tar (34%), indicating that GC–MS was limited to determination of a part of liquid compounds.
3.2. Effects of various carbon samples In addition to charcoal, graphite and three coal chars (A-char, B-char and L-char) were used to examine their effects on the secondary reactions of the volatiles. An equal amount of carbon sample was filled in the cracking reactor, and the heights of the
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3.3. Deconvolution of catalytic tar cracking and char gasification Char has two intermingled effects: the char-catalyzed tar cracking and the char gasification. Therefore, it is strongly desired to quantify their individual contributions to the yields of gaseous and liquid products. One method for quantifying the char gasification is by examining the changes of the char sample used for tar cracking. In our tests, however, the weight variations of the char samples obtained before and after experiment were not meaningful to measure the extent of char gasification. An alternative method is proposed to quantify the char gasification as follows. As illustrated in Fig. 6, the molar ratio of H2/(2CO2 + CO) in the gas produced in the case of only thermal cracking was virtually constant with the cracking temperature, whereas in the case of char-catalyzed cracking, the molar ratio of H2/(2CO2 + CO) was considerably varied depending on the cracking temperature. The gas produced only by the thermal cracking at 700 °C had a H2/ (2CO2 + CO) molar ratio of 0.24. The gasification of the charcoal with the autogenerated steam was experimentally observed to yield a gas with H2/(2CO2 + CO) molar ratio of 1.37, larger than a theoretical value deduced from the reactions of pure carbon with steam, as the charcoal contained some hydrogen. Taking the overall contribution of the tar cracking and the char gasification to the yield of gas as 1, according to the H2/(2CO2 + CO) molar ratio (R) of
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carbon sample beds were close in all experiments. Fig. 5 shows the yields of gas, liquid and char produced with the four carbon samples at cracking temperatures of 550 °C and 700 °C. The data for the charcoal are listed together for contrast. It was seen that all four carbon samples lowered the yields of liquid product at 550 °C, and L-char had relatively strong effect. The yields of NEVTar were 16%, 19%, 14% and 13%, respectively, for graphite, A-char, B-char and L-char, all lower than the corresponding yield obtained without carbon sample (20%). At 700 °C, the yield of liquid produced with L-char was appreciably lower than that obtained without carbon sample, in contrast to the results for graphite, A-char and B-char; the yields of NEV-Tar were near to zero irrespective of carbon samples. We examined the cumulative yields of four major gases produced with four carbon samples. At the cracking temperature of 700 °C, the yields of H2 and CO2 obtained with L-char were appreciably higher than those obtained with graphite, A-char and B-char; this result was similar to that obtained with charcoal.
Without char With charcoal 0,6
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Fig. 5. Effect of graphite and different chars on the yield of gaseous, liquid and char products at the cracking temperatures of 550 °C (the left part) and 700 °C (the right part). Final pyrolysis temperature, 700 °C. The liquid product was fractionated to the non-evaporated tar (NEV-Tar) and the evaporated liquid (EV-Liquid).
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Cracking temperature (oC) Fig. 6. Changes in the molar ratio of H2/(2CO2 + CO) with cracking temperature in two cases with charcoal (d) and without charcoal (h). Final pyrolysis temperature, 700 °C.
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the gas produced with the charcoal at a cracking temperature, we can then determine the contribution of the char gasification (Xg) in terms of the equation:
1:37 X g þ 0:24ð1 X g Þ ¼ R The observed values of R at the cracking temperatures of 500, 550, 600, 650 and 700 °C in the case of using charcoal were 0.23, 0.31, 0.39, 0.48 and 0.54, respectively, as shown in Fig. 6. Therefore the values of Xg calculated using the above equation were 0, 0.06, 0.13, 0.21 and 0.27, respectively. It was clear that the char gasification contributed to the gas yield to a greater extent at higher cracking temperature. On the other hand, the char gasification converted a fraction of the autogenerated steam to non-condensable gases. Thus, there were two effects of charcoal, the promoted tar cracking and the consumption of water for the char gasification, which led to a reduction in the yield of liquid product. From the value of Xg and the yields of CO2 and CO, we can then estimate a reduction in the liquid yield attributed to the amount of water consumed for the char gasification. Fig. 7 shows the separated contributions of the catalyzed tar cracking and the char gasification to the differences between the yields of liquid product obtained without and with charcoal at varying cracking temperatures. It could be seen that the reductions in the yields of liquid product were predominantly or mainly due to the catalyzed tar cracking at 500–600 °C, whereas those obtained at 650 and 700 °C essentially equaled to the amounts of water consumpted for the char gasification. It became quite clear that at lower temperatures (500–600 °C), the catalytic effect of charcoal on tar cracking was a main cause for the increased yield of gas at the expense of the reduced yield of liquid product; at higher temperature, as the catalytic tar cracking effect was masked by the thermal effect, the char gasification was principally responsible for an increase in the yield of gas accompanied by exploiting a part of the autogenerated steam for the char gasification. If four major gases are considered to calculate the composition of the gas accumulated over the total pyrolysis process, the gas produced by thermal cracking at 700 °C without char was composed of 19% H2, 51% CO, 16% CH4 and 14% CO2; in contrast, the corresponding gas obtained with the charcoal contained 39% H2, 31% CO, 9% CH4 and 22% CO2 (all vol.%). It was indicated that the gasification of char with the autogenerated steam could significantly alter the gas composition. As stated above, we observed that the catalytic effect of charcoal on the tar cracking was noticeable at 500–600 °C, but it 15 by catalytic tar cracking by char gasification
Difference (%)
12
9
6
3
0 500
550
600
650
700
Cracking temperature (oC) Fig. 7. Separated contributions of the catalytic tar cracking and the char gasification to the differences between the yields of liquid product obtained without charcoal and with charcoal at varying cracking temperatures. Final pyrolysis temperature, 700 °C.
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became obscure at higher temperatures (650–700 °C) on account of the intensified thermal effect. Herein we wish to compare our observation with the literature. Boroson et al. [28] positively observed the promoted cracking of biomass tar at 400–600 °C, but no data were available at higher temperature. Abu El-Rub et al. [31] confirmed the catalytic effect of char on decomposition of phenol and naphthalene at 700–900 °C. However, Gilbert et al. [33] reported the insignificant influence of char in a temperature range of 500–800 °C. By comparison of our result with Refs. [28,31], we assume that the catalytic effect of char may depend largely on the components of the primary tar. For thermally less stable oxygenated tar such as saccharides, furans and guaiacols, char is catalytically active at lower temperature, while for thermally more stable tar such as phenols and naphthalenes, only at higher temperature does char behave actively. In comparison of our result with Ref. [33], there remains an unknown discrepancy. Gilbert et al. [33] thought that the char was probably inactivated under an inert gas stream, leading to a suppressed catalytic effect of charcoal in their experiment. However, we also used argon as an inert purging gas. In a similar manner, we calculated the individual contributions of the char-catalyzed cracking of tar and the char gasification to the yields of gaseous and liquid products in the case of using other carbon samples. It was observed that the gasification was insignificant for graphite, A-char and B-char, but significant for L-char at the cracking temperature of 700 °C. This result was in agreement with a general order of the gasification reactivity: lignite > bituminous coal > anthracite > graphite. In addition, calculation showed that at the cracking temperature of 700 °C, the reductions in the yields of liquid product caused by L-char were completely attributable to the gasification of L-char with the autogenerated steam, similar to the case of the charcoal.
4. Conclusions (1) The thermal cracking of tar at lower temperatures (500–600 °C) resulted in the conversion of part of heavy tar mainly to light liquid compounds (including water), and the thermal cracking of tar became intense at higher temperatures (650–700 °C) with marked formation of gases. The charcoal catalytically promoted the cracking of tar to gases at lower temperatures (500–600 °C); at higher temperatures (650–700 °C), the catalytic effect was nullified by the intensified thermal effect. (2) GC–MS analysis showed that the primary liquid product consisted of a number of oxygenated compounds, which were representative of a mild thermal degradation of wood constituents. The charcoal promoted the decomposition of most oxygenated compounds present in the primary liquid product at the cracking temperatures of 500 and 600 °C. (3) The autogenerated steam gasified the charcoal at higher cracking temperatures (650–700 °C), resulting in an increase in the yield of gaseous product and a change in gas composition. This reaction was responsible for the reduction in the yield of liquid product caused by using charcoal at higher cracking temperature. (4) Apart from the charcoal, all other carbon samples including graphite, A-char, B-char, and L-char showed the catalytic effect, to different degrees, on the tar cracking at lower cracking temperature, and comparatively L-char had a high activity. L-char also behaved with high reactivity for the gasification with the autogenerated steam at higher cracking temperature, like the charcoal but unlike graphite, A-char and B-char.
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Acknowledgements This study is sponsored by the Shanghai Pujiang Program (Project no.: 07pj14028) and the Program for Changjiang Scholars and Innovative Research Team in University in China (IRT0620). The authors thank associate professor F. Wang, associate professor J.Q. Cao and Mr. Z.Y. Wang for their assistance in the research. The authors appreciate two anonymous reviewers for their invaluable advice in the revision of the manuscript. References [1] Goyal HB, Seal D, Saxena RC. Bio-fuels from thermochemical conversion of renewable resources: a review. Renew Sust Energy Rev 2008;12:504–17. [2] Ni M, Leung YC, Leung MKH, Sumathy K. An overview of hydrogen production from biomass. Fuel Process Technol 2006;87:461–72. [3] Ferdous D, Dalai AK, Bej SK, Thring RW, Bakhshi NN. Production of H2 and medium Btu gas via pyrolysis of lignins in a fixed-bed reactor. Fuel Process Technol 2001;70:9–26. [4] Liu DW, Liu DP, Zeng RJ, Angelidaki I. Hydrogen and methane production from household solid waste in the two-stage fermentation process. Water Res 2006;40:2230–6. [5] Prins MJ, Ptasinski KJ, Janssen FJJG. More efficient biomass gasification via torrefaction. Energy 2006;31:3458–70. [6] Leung DYC, Yin XL, Wu CZ. A review on the development and commercialization of biomass gasification technologies in China. Renew Sust Energy Rev 2004;8:565–80. [7] Guéhenneux G, Baussand P, Brothier M, Poletiko C, Boissonnet G. Energy production from biomass pyrolysis: a new coefficient of pyrolytic valorisation. Fuel 2005;84:733–9. [8] Devi L, Ptasinski KJ, Janssen FJJG. A review of the primary measures for tar elimination in biomass gasification processes. Biomass Bioenergy 2003;24:125–40. [9] Han J, Kim H. The reduction and control technology of tar during biomass gasification/pyrolysis: an overview. Renew Sust Energy Rev 2008;12:397–416. [10] Li XT, Grace JR, Lim CJ, Watkinson AP, Chen HP, Kim JR. Biomass gasification in a circulating fluidized bed. Biomass Bioenergy 2004;26:171–93. [11] Brandt P, Henriksen U. Decomposition of tar in gas from updraft gasifier by thermal cracking. In: Kyritsis S, Beenackers AACM, Helm P, Crassi A, Chiaramonti D. editors, Proceedings of the 1st world conference on biomass for energy and industry, 5–9 June. James & James (Science Publishers) Ltd.: Spain; 2000. [12] Devi L, Ptasinski KJ, Janssen FJJG, van Paasen SVB, Bergman PCA, Kiel JHA. Catalytic decomposition of biomass tars: use of dolomite and untreated olivine. Renew Energy 2005;30:565–87. [13] Devi L, Craje M, Thune P, Ptasinski KJ, Janssen FJJG. Olivine as tar removal catalyst for biomass gasifiers: catalyst characterization. Appl Catal A Gen 2005;294:68–79.
[14] Pütün E, Uzun BB, Pütün AE. Rapid pyrolysis of olive residue. 2. Effect of catalytic upgrading of pyrolysis vapors in a two-stage fixed-bed reactor. Energy Fuel 2009;23:2248–58. [15] Adam J, Blazso M, Meszaros E, Stocker M, Nilsen MH, Bouzga A, et al. Pyrolysis of biomass in the presence of Al–MCM-41 type catalysts. Fuel 2005;84:1494–502. [16] Garcı´a XA, Alarcon NA, Gordon AL. Steam gasification of tars using a CaO catalyst. Fuel Process Technol 1999;58:83–102. [17] Wang ZY, Wang F, Cao JQ, Wang J. Pyrolysis of pine wood in a slowly heating fixed-bed reactor: potassium carbonate versus calcium hydroxide as a catalyst. Fuel Process Technol 2010;91:942–50. [18] Uddin MA, Tsuda H, Wu SJ, Sasaoka E. Catalytic decomposition of biomass tar with iron oxide catalyst. Fuel 2008;87:451–9. [19] Zhang RQ, Brown RC, Suby A, Cummer K. Catalytic destruction of tar in biomass derived producer gas. Energy Convers Manage 2004;45:995–1014. [20] Li C, Hirabayashi D, Suzuki K. Development of new nickel based catalyst for biomass tar steam reforming producing H2-rich syngas. Fuel Process Technol 2009;90:790–6. [21] Li H, Xu Q, Xue H, Yan Y. Catalytic reforming of the aqueous phase derived from fast-pyrolysis of biomass. Renew Energy 2009;34:2872–7. [22] Ammendola P, Lisi L, Piriou B, Rruppolo G. Rh-perovskite catalysts for conversion of tar from biomass pyrolysis. Chem Eng J 2009;154:361–8. [23] Miyazawa T, Kimura T, Nishikawa J, Kunimori K, Tomishige K. Catalytic properties of Rh/CeO2/SiO2 for synthesis gas production from biomass by catalytic partial oxidation of tar. Sci Technol Adv Mater 2005;6(6):604–14. [24] Kuramoto K, Matsuoka K, Murakami T, Takagi H, Nanba T, Suzuki Y, et al. Cracking and coking behaviors of nascent volatiles derived from flash pyrolysis of woody biomass over mesoporous fluidized-bed material. Ind Eng Chem Res 2009;48:2851–60. [25] Rapagnà S, Jand N, Foscolo PU. Catalytic gasification of biomass to produce hydrogen rich gas. Int J Hydrogen Energy 1998;7(23):551–7. [26] Baker EG, Mudge LK, Brown MD. Steam gasification of biomass with nickel secondary catalysts. Ind Eng Chem Res 1987;26:1335–9. [27] Hu G, Huang H, Li Y. Hydrogen-rich gas production from pyrolysis of biomass in an autogenerated steam atmosphere. Energy Fuel 2009;23:1748–53. [28] Boroson ML, Howard JB, Longwell JP, Peters WA. Heterogeneous cracking of wood pyrolysis tars over fresh wood char surfaces. Energy Fuel 1989;3:735–40. [29] Brandt P, Larsen E, Henriksen U. High tar reduction in a two-stage gasifier. Energy Fuel 2000;14:816–9. [30] Seshadri KS, Shamsi A. Effects of temperature, pressure, and carrier gas on the cracking of coal tar over a char dolomite mixture and calcined dolomite in a fixed-bed reactor. Ind Eng Chem Res 1998;37:3830–7. [31] Abu El-Rub Z, Bramer EA, Brem G. Experimental comparison of biomass chars with other catalysts for tar reduction. Fuel 2008;87:2243–52. [32] Chembukulam SK, Dandge AS, Rao NLK, Seshagiri K, Vaidyeswaran R. Smokeless fuel from carbonized sawdust. Ind Eng Chem Prod Res Dev (United States) 1981;20(4):714–9. [33] Gilbert P, Ryu C, Sharifi V, Swithenbank J. Tar reduction in pyrolysis vapours from biomass over a hot char bed. Bioresour Technol 2009;100:6045–51. [34] Wang ZY, Cao JQ, Wang J. Pyrolytic characteristics of pine wood in a slowly heating and gas sweeping fixed-bed reactor. J Anal Appl Pyrol 2009;84:179–84.
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Decomposition and gasification of pyrolysis volatiles from pine wood through a bed of hot char Qingsong Sun a, Sang Yu a, Fuchen Wang b, Jie Wang a,⇑ a b
Department of Chemical Engineering for Energy, East China University of Science and Technology, 130# Meilong Road, Shanghai 200237, PR China Key Laboratory of Coal Gasification of Ministry of Education, East China University of Science and Technology, 130# Meilong Road, Shanghai 200237, PR China
a r t i c l e
i n f o
Article history: Received 24 December 2009 Received in revised form 9 December 2010 Accepted 18 December 2010 Available online 1 January 2011 Keywords: Pine wood Pyrolysis Gasification Catalytic tar cracking Char
a b s t r a c t Pine wood was pyrolyzed in a fixed bed reactor at a heating rate of 10 °C and a final temperature of 700 °C, and the resultant volatiles were allowed to be secondarily cracked through a tubular reactor in a temperature range of 500–700 °C with and without packing a bed of char. The thermal effect and the catalytic effect of char on the cracking of tar were investigated. An attempt was made to deconvolute the intermingled contributions of the char-catalyzed tar cracking and the char gasification to the yields of gaseous and liquid products. It was found that the wood char (charcoal) was catalytically active for the tar cracking at 500–600 °C, while at 650–700 °C, the thermal effect became a dominant mode of the tar cracking. Above 600 °C, the autogenerated steam gasified the charcoal, resulting in a marked increase in the yield of gaseous product and a significant change in the gas composition. An anthracite char (A-char), a bituminous coal char (B-char), a lignite char (L-char) and graphite also behaved with catalytic activities towards the tar cracking at lower temperature, but only L-char showed reactivity for gasification at higher temperature. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Thermochemical conversion of biomass as a renewable resource to fuel gas via pyrolysis and gasification has been drawing considerable attention in recent years [1–4], owing to the rapid resource depletion and pressing environmental issues arising from huge consumption of fossil fuels. A great deal of work has been done to develop new technologies of biomass pyrolysis and gasification [5–7]. It is well known that one of the major problems encountered in pyrolysis and gasification processes is the tar formation, which reduces the efficiency of gas production and interferes with the equipment operation. Various methods have thereby been investigated to eliminate or reduce the tar formation [8,9]. The cracking of tar downstream from pyrolyzer or gasifier offers an effective approach to minimizing the concentration of tar in gas. In the case of homogenous cracking of tar (without catalyst), however, some refractory tar is hard to eliminate, so that a high temperature is required [8,10]. Brandt et al. [11] claimed that the temperature and residence time were 1250 °C and 0.5 s, respectively, in order that a sufficient extent of tar decomposition was guaranteed. Heterogeneous or catalytic cracking of tar is advantageous in efficient conversion and regulating gas composition. Hence it has been the subject of numerous studies. Various ⇑ Corresponding author. Tel./fax: +81 21 64252853. E-mail address: [email protected] (J. Wang). 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.12.015
catalysts, such as dolomites [12], olivines [12–14], alkali and alkaline earth salts [15–17], Fe or Ni-based catalysts [18–21], composite catalysts [22,23], have demonstrated their effectiveness. However, the many have a drawback of catalytic deactivation caused by coke deposition on the surfaces [24–26]; composite catalysts can prevent deactivation, but the cost is relatively high [22]. Char-induced or -catalyzed cracking of tar is an attractive research topic with three aspects of consideration. First, one way to reduce the tar formation can be implemented inside gasifier by modifying its configurations and process parameters, and this entails the information on the interactions between tar and char. Second, an alternative way to remove tar is by thermal cracking of tar in a separated reactor using char as a catalyst, which has advantages over common mineral catalysts: (1) char is produced in the pyrolysis and gasification system; (2) char surface can be refreshed via char gasification to avoid deactivation; (3) char can be loaded with dispersed metallic catalysts, and this would greatly enhance the catalytic capability; and (4) it is feasible to regulate or upgrade gas composition via char gasification. Third, biomass usually contains high moisture, and the dewatering process loses large energy [27]. The char-catalyzed tar cracking may save this energy expenditure by exploiting the autogenerated steam directly for char gasification. In the past decades, there have been a few interesting studies pertaining to the char-catalyzed cracking of tar [28–33]. Boroson et al. [28] found that heterogeneous or char-induced conversion
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pyrolysis reactor include non-condensable gases and condensable liquid products (including water and tar), and the tar consisted of a number of oxygenated compounds characteristic of the degradation products of holocellulose and lignin constituents in wood. As a consequence, not only the cracking of tar but also the reaction between char and steam is provoked when the volatiles flow over a char bed in the second reactor depending on the cracking conditions. This study has tried to clarify their individual contributions to the yields of gaseous and liquid products.
of fresh wood pyrolysis tars (subtraction from thermal conversion) was significant but essentially constant at 14 ± 7 wt.% of tar in a temperature range of 400–600 °C and space times from 2.5 to 100 ms. Abu El-Rub et al. [31] compared biomass char to some common inorganic catalysts for the cracking of phenol and naphthalene in a temperature range of 700–900 °C, and they observed that biomass char was almost as catalytically active as nickel and dolomite. Gilbert et al. [33] investigated the conversion of pyrolysis vapors through a char bed with varying parameters including bed lengths (0–450 mm), temperatures (500–800 °C), particle sizes (10 and 15 mm) and nitrogen purge rates (1.84–14.70 mm/s). To the contrary, they observed that the char-catalytic effect was marginal, and the thermal effect was the main mode of tar decomposition. In this study, we investigate the behaviors of char-catalyzed cracking of biomass tar using a two-stage fixed bed reactor, based on our previous study on the behaviors of biomass pyrolysis in the first fixed bed reactor [34]. The nascent volatiles from the first
2. Experimental 2.1. Biomass sample and char preparation The biomass sample used in this study was the wood sawdust of Chinese white pine, which was supplied by a local wood processing factory. The sample was screened to the particle sizes of less than 1 mm, and dried at 105 °C for 6 h prior to experiment, and then stored in a sealed desiccator. The dried wood sample contained 0.35% ash. Elemental analysis showed that the ash-free sample contained 47.6% C, 6.1% H, 0.1% N, <0.05% S and 46.2% O (by difference). The charcoal was prepared by pyrolyzing the wood sample at 750 °C for 30 min in a heater under an air-isolated condition. Three other chars, called A-char, B-char, and L-char, were prepared in the same manner, using an anthracite sample, a bituminous coal sample and a lignite sample, respectively. All the char samples were ground to the particle sizes of 0.075–0.15 mm. The proximate and ultimate analyses of the char samples, together with the ash compositions, are tabulated in Table 1. The char samples were stored in the sealed desiccator, and the storage duration was not beyond one week in all the experiments. For comparison, a commercial reagent of powdery graphite (particle size, <0.030 mm) was used in the experiment.
Table 1 The proximate and ultimate analyses of the char samples as well as the ash compositions. A-char
B-char
L-char
Proximate analysis (%, dry basis) Ash 2.1 Volatile 6.9 Fixed carbon 91.0
Charcoal
27.2 3.4 69.4
12.1 4.3 83.6
32.8 6.1 61.1
Ultimate analysis (%, daf. basis) C 96.0 H 0.9 N 0.2 S <0.05 O (by diff.) 2.9
97.0 1.4 0.7 0.8 0.1
91.2 1.4 1.5 0.06 5.9
88.2 1.2 0.9 1.5 8.2
Ash composition (%) SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3 Others
55.7 30.1 3.5 3.4 0.3 0.6 1.3 2.9 2.2
52.1 30.8 6.7 3.3 0.4 Trace 1.2 2.3 2.2
28.1 17.3 8.0 25.3 3.9 0.2 1.1 14.2 1.9
– – – – – – – – –
2.2. Pyrolysis apparatus and procedures Fig. 1 is the diagram of the experimental apparatus. Two tubular reactors, both of which were made of stainless steel with a length
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Fig. 1. The apparatus of two-stage fixed bed reactors. (1) Argon bomb; (2) air bomb; (3) pressure control valve; (4) flow rate meter; (5) adsorbent column; (6) ceramic pellets; (7) electric furnace; (8) tubular reactors; (9) thermocouple; (10) biomass sample; (11) tape heaters; (12), char bed; (13) electric furnace; (14) ice-water trap; (15–17) temperature controlling instruments.
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2.3. Analysis methods The major gases including H2, CO, CH4 and CO2 were determined on a gas chromatograph (Aglient 6820) equipped with a thermal conductivity detector (TCD) and a packed column (S.S.
3 m 1/800 OD Unibeads C 80/100) using argon as a carrier gas. The compounds of liquid products were characterized using GC/MS (PerkinElmer clams 500), equipped with a HP-5 capillary column. Seven typical liquid compounds were quantitatively determined on a gas chromatograph (Agilen-6820), equipped with a flame ionization detector (FID) and HP-5 capillary column, by an external calibration method. The seven compounds included hydroxyl propanone, furan methanol, phenol, methyl-phenol, guaiacol, 2, 4-dimethyl-phenol, and D-allose. Elemental analysis (C, H and N) was carried out on an elemental analyzer (Elementar Vario EL III). Sulfur analysis was accomplished on a Coulomb sulfur analyzer (CLS-2). The metallic composition of ash was determined by X-ray fluorescence spectrometry (Shimadzu, XRF-1800).
3. Results and discussion 3.1. Effects of charcoal 3.1.1. Overall distribution of products To differentiate between the thermal effect and the charcatalytic effect on the secondary cracking of tar, experiments were conducted with and without packing charcoal in the second reactor. For briefness, the temperature used for biomass pyrolysis in the first reactor is called the pyrolysis temperature, and the cracking temperature refers to that used for tar cracking in the second reactor. Fig. 2 shows the effect of different cracking temperatures on the yields of gaseous, liquid and char products produced over the total pyrolysis process with and without charcoal. The yield of gas product is the summed amounts of four major gases (H2, CO, CH4 and CO2). The yields of gaseous product, total liquid product and char are the averages obtained by the duplicate or triplicate experiments, and their respective variations are displayed on the bars. The liquid product is divided into the NEV-Tar and EV-Liquid, and their yields are also displayed. In the absence of char, the yield of liquid product was as high as 59% (including 25% of EV-Liquid and 34% of NEV-Tar) at the
100
Yield (wt.%, dry biomass)
of 620 mm and an inside diameter of 23 mm, were vertically set in series. The reactors were heated independently by two electric furnaces. The first reactor in the lower part was used for slow pyrolysis of biomass. Ceramic pellets were packed in this reactor to support the biomass sample. In each run, a 2.2 g wood sample was wrapped in a stainless steel wire mesh, and then placed on the support layer within a uniform temperature zone. A K-type thermocouple was made to contact with the sample wrap for measuring the pyrolysis temperature. The second reactor in the upper part was used for cracking of tar. A 1.3 g char sample was held in a wire mesh wrap, and then tightly inserted in the center of this reactor. The height of the char bed was about 30 mm. Another K-type thermocouple was made to contact with the sample wrap for measuring the cracking temperature. The argon gas from the pressurized cylinder was de-moisturized through a silica gel/molecular sieve adsorbent column, and then purged upward from the first reactor to the second reactor, with the flow rate of 150 mL/min. Prior to the beginning of biomass pyrolysis in the first reactor, the char sample in the second reactor was pre-heated to the predetermined temperature, and it was kept at the final temperature for a period of time (about 20 min) until no gases were released from the char sample. Then, the first reactor was heated up at a rate of 10 °C/min till a final temperature of 700 °C. During the heat-up, the volatiles produced in the first reactor were conveyed through the hot char in the second reactor and subjected to the isothermal cracking. To suppress the undesired condensation of tar vapors in the connectors, the off-gas lines and the joints between two reactors were maintained at 260 °C with tape heaters. Experiments of tar cracking were also carried out without packing char in the second reactor by way of contrast. The residence time of purging gas through the void spaces of the char bed was approximately 2.2 s, and the corresponding residence time without packing char was 5.0 s. The liquid product including aqueous and oily phases was condensed in two U-type glass traps immersed in the ice-water baths. The glass traps were carefully weighed before and after each experiment to determine the total liquid yield (including water and tar). To fractionate the liquid product, evaporation experiments were conducted by heating two product-attached traps at 105 °C under atmospheric pressure for 6 h until constant weights of the traps were reached. The tarry fraction retained in the traps after evaporation is named the non-evaporated tar (NEV-Tar), which mainly includes heavy tar; the liquid fraction evaporated from the traps is named the evaporated liquid product (EV-Liquid), which includes water and some light organic compounds. When the liquid sample was used for gas chromatography (GC) and gas chromatography mass spectrometry (GC–MS) analyses, the traps together with the connected tubes were dismantled, and the liquid product in them was dissolved with acetone under ultrasonic stirring. The gases were collected in the sequence of gas bags behind the vapor condensation at an interval of five minutes. Four major gases including H2, CO, CH4 and CO2 were quantitatively determined. The char samples in the first reactor and in the second reactor were collected separately after the reactors were cooled down, and then gravimetrically determined. The yield of char refers to the yield of the solid product remaining in the first reactor after biomass pyrolysis. Care was taken to clean up the reactors after finishing each pyrolysis experiment.
80
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650
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700
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Cracking temperature (oC) Char
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Gas
Fig. 2. Yields of gaseous, liquid and char products obtained at different cracking temperatures with and without wood char (charcoal). Final pyrolysis temperature, 700 °C. The liquid product was fractionated to the non-evaporated tar (NEV-Tar) and the evaporated liquid (EV-Liquid).
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fractionation experiment. It was revealed that in the case of no catalyst, a part of heavy tar was thermally converted mainly to light liquid compounds (including water) rather than permanent gases at lower temperatures (500–550 °C); the substantial cracking of tar to gases occurred at higher temperatures (above 600 °C). In the presence of the charcoal, the yields of liquid product at the cracking temperatures of 500, 550, 600, 650 and 700 °C were, respectively, 13%, 13%, 10%, 8% and 11% lower than the corresponding data in the absence of char, in conjunction with increases in the yield of gaseous product. At 500–600 °C, the yields of NEV-Tar decreased from 18–20% without charcoal to 6–11% with charcoal, and the yields of EV-Liquid simultaneously decreased to different degrees. The results clearly indicated that at 500–600 °C, the charcoal intensely facilitated the decomposition of both heavy tar and light tar to gases. At 650–700 °C, the differences in the yields of NEV-Tar
cracking temperature of 300 °C, with the yield of gaseous product of 15%. In this circumstance, the cracking reactions of the nascent liquid products derived from the wood pyrolysis in the first reactor were negligible. At the cracking temperatures of 500–550 °C, the yields of liquid product were only slightly lower than the yield obtained at 300 °C, and the yields of gaseous products were also close to the yield obtained at 300 °C; nevertheless, the yields of NEV-Tar were appreciably lower than the yield obtained at 300 °C. With elevating cracking temperature up to 600–700 °C, the thermal treatment resulted in a marked reduction in the yield of liquid product and a marked increase in the yields of gaseous product. As an example, the yield of liquid product dropped from 59% at 300 °C to 35% at 700 °C, corresponding to an increase in the yield of gaseous product from 15% to 43%. Moreover, only 5% of liquid product obtained at 650–700 °C remained non-evaporated in the
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Pyrolysis temperature (oC) Fig. 3. Cumulative yields of major gases at different pyrolysis temperatures with varying cracking temperatures without char (a) or with charcoal (b). Cracking temperature: h-, 300 °C; –N–, 500 °C; –s–, 550 °C; –j–, 600 °C; q, 650 °C; –.–, 700 °C.
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3.1.3. Liquid products To further illustrate the effect of the charcoal on the decomposition of tar, GC–MS was used for characterizing organic compounds in the liquid products. Seven compounds were quantitatively determined, as described in the Experimental section. Other compounds were semi-quantitatively estimated according to the ratios of their peak areas to the peak area of one of the compounds quantified. In the interest of clarity, the main liquid compounds are grouped as carboxylic acids (acetic acid, etc.), cyclopentenones (methyl-cyclopentenone, hydroxyl-cyclopentenone, etc.), ketones (hydroxylpropanone, cyclohetanone, etc.), aldehydes (acetamido acetaldehyde, methyl-furan aldehyde, etc.), furans (dimethyl furan, furfural, furan methanol, benzofuran, etc.), alcohols (cyclopropyl carbinol, phenyl-methoxyethanol, etc.), saccharides (dianhydro-D-glucopyranose, anhydro-D-mannose, anhydro-D-galactose, anhydro-D-altroside, anhydro-D-talopyranose, D-allose, etc.), guaiacols (guaiacol, methyl-guaiacol, ethyl-guaiacol, vinyl-guaiacol, prophenyl-guaiacol, etc.), esters (ethanediol monoacetate, oxo-propanoic acid methylester, etc.), phenols (phenol, methyl-phenol, ethyl-phenol, etc.), and polyaromatic hydrocarbons (naphthalene, anthracene, fluorene, etc.). The yields of the grouped compounds produced from five typical cracking experiments are shown in Fig. 4. The primary liquid products obtained at the cracking temperature of 300 °C were abundant with a variety of oxygenated compounds, but no polyaromatic
25
20
15
10
5 00
0000 00 0
0000 0000
00
0
0
PA Hs
cy
a clo cids pe nt ne en s ke to ne s al de hy de s fu ra ns al co ho sa ls cc ha rid es gu ai ac ol s es te rs ph en ol s
0
-o
3.1.2. Gaseous products Fig. 3 shows the cumulative yields of four major gases (H2, CO, CH4 and CO2) with elevating pyrolysis temperature and varying cracking temperatures in two circumstances (with and without charcoal), where the scales of vertical axis for H2 and CO2 are under three times magnification in the case of no catalysis, relative to those in the case of using charcoal. The results obtained at the cracking temperature of 300 °C were representative of no secondary reactions. In this case, CO and CO2 were released noticeably in the pyrolysis temperature range of 200–450 °C. It was previously observed that most liquid products were also formed in this temperature range [34]. The cracking temperature exerted profound influences on the release patterns of gases whether charcoal was present or not. In either case, changes predominantly occurred in a pyrolysis temperature range of 200–450 °C, because most volatiles were formed in this temperature range. In the absence of char, the cumulative yields of H2, CO, CH4 and CO2 over the total pyrolysis process (from room temperature to 700 °C) were increased by a factor of 2.4, 3.2, 3.0, and 1.2, respectively, with the cracking temperature rising from 300 °C to 700 °C. It was indicated that the yield of CO2 was less varied by thermal effect. This was probably because CO2 was dissociated from fragile oxygenated groups like carboxyl group, and such decarboxylation reactions were kinetically less resistant than the reactions of dehydrogenation, decarbonylation and demethanation. It should be emphasized that in the case of no catalyst, the enhancements in the formation of CO and CH4 were slight at the cracking temperatures below 600 °C, and only at the temperatures exceeding 600 °C did the enhancements become striking. The analogous turning point for the formation of H2 was 650 °C. This result was consistent with the observation that the thermal cracking of liquid products to gases significantly occurred at temperatures above 600 °C, as shown in Fig. 2. The release patterns of four gases with the charcoal showed some salient alterations as compared to those without char. First, the presence of charcoal accelerated the formation of all four major gases at lower cracking temperatures (500–600 °C). As an example, at the cracking temperature of 550 °C, the cumulative yields of H2, CO, CH4 and CO2 over the total pyrolysis process in the presence of charcoal were 2.1, 1.3, 1.4 and 1.5 times higher than those in the absence of charcoal, respectively. Second, at higher cracking temperatures, four gases behaved differently in their yield changes influenced by charcoal. At a cracking temperature of 700 °C, the cumulative yields of H2 and CO2 obtained with charcoal were 3.9 and 2.9 times as high as those without charcoal, whereas the cumulative yields of CO and CH4 obtained with charcoal were only 1.2 and 1.1 times as high as those obtained without charcoal. This difference between the gases was attributable to twofold effects. One was the effect of char on the tar cracking, which was weakened relative to the increased thermal effect at higher cracking temperature. The yields of CO and CH4 obtained with and without
charcoal were thus approached at higher cracking temperature. Another was the autogenerated steam gasification of charcoal, which became intense with increasing cracking temperature, resulting in the enhancements in the yields of H2 and CO2. Third, although the effect of char dominated in the pyrolysis temperature range of 200–450 °C, it was discernable that in the pyrolysis temperature of higher than 450 °C, subtly more CH4 was formed in the case of using charcoal. This implied that the charcoal facilitated the decomposition of guaiacols, because the main liquid products produced from the pyrolysis of wood at temperature above 450 °C were guaiacols, a lignin degradation product [34].
Yield (mg/g-dry biomas)
obtained with and without char were insignificant, since the yields of NEV-Tar produced only by the thermal cracking were already small; however, the presence of charcoal lowered the yields of EV-Liquid. It should be pointed out that the reductions in the yields of EV-Liquid at 650–700 °C were mainly ascribed to the loss of the autogenerated steam by its reaction to gasifying charcoal. This point will be further clarified in a later section. The mass closure exceeding 100% at 650–700 °C also suggested the occurrence of charcoal gasification at these temperatures. Incidentally, in all experiments performed in the absence of char, the sum of the gaseous, liquid and char yields gave the mass closures of 92–99%, and no case was found of the mass closure over 100%. It should be noted herein that the influence of the wire mesh on the tar cracking was observed to be insignificant.
no char 300°C
no char charcoal 500°C 500°C
no char charcoal 600°C 600°C
Fig. 4. Yields of the grouped compounds in the liquid products obtained with and without charcoal at different cracking temperatures. Final pyrolysis temperature, 700 °C.
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hydrocarbons (PAHs) were present in it. The enrichment of saccharides in the primary liquid product suggested a mild degradation of hemicellulose and cellulose by the wood pyrolysis in the first reactor [34]. In the absence of charcoal, the thermal cracking at 500 °C led to some noticeable changes in the yields of liquid compounds as compared to the primary liquid product: (1) ketones, aldehydes, alcohols, saccharides completely disappeared; (2) carboxylic acids, furans, guaiacols and esters substantially decreased; (3) cyclopentenones slightly decreased; and (4) some PAHs were newly formed. As the cracking temperature was elevated to 600 °C (without charcoal), carboxylic acids and guaiacols completely disappeared; cyclopentenones substantially decreased; phenols trended to increase. Comparison of the yields of liquid compounds obtained with and without charcoal revealed that the charcoal promoted the decomposition of liquid compounds at the cracking temperatures of both 500 °C and 600 °C, but at 500 °C, the effect was relatively great. It could be seen that the presence of charcoal led to a remarkable decrease in the yields of carboxylic acids, cyclopentenones, furans, guaiacols and esters at 500 °C. We also analyzed the compounds in the liquid product obtained at the cracking temperature of 700 °C with charcoal (not shown). It was observed that the yield of phenols was lower than the corresponding result obtained at 600 °C, and some larger molecular PAHs such as fluorene and anthracene were detected at 700 °C while they are not discernable at 600 °C. Incidentally, the summed yields of the organic compounds determined by GC–MS was approximately 8%-dry biomass for the primary liquid product, much smaller than the yield of NEV-Tar (34%), indicating that GC–MS was limited to determination of a part of liquid compounds.
3.2. Effects of various carbon samples In addition to charcoal, graphite and three coal chars (A-char, B-char and L-char) were used to examine their effects on the secondary reactions of the volatiles. An equal amount of carbon sample was filled in the cracking reactor, and the heights of the
80
3.3. Deconvolution of catalytic tar cracking and char gasification Char has two intermingled effects: the char-catalyzed tar cracking and the char gasification. Therefore, it is strongly desired to quantify their individual contributions to the yields of gaseous and liquid products. One method for quantifying the char gasification is by examining the changes of the char sample used for tar cracking. In our tests, however, the weight variations of the char samples obtained before and after experiment were not meaningful to measure the extent of char gasification. An alternative method is proposed to quantify the char gasification as follows. As illustrated in Fig. 6, the molar ratio of H2/(2CO2 + CO) in the gas produced in the case of only thermal cracking was virtually constant with the cracking temperature, whereas in the case of char-catalyzed cracking, the molar ratio of H2/(2CO2 + CO) was considerably varied depending on the cracking temperature. The gas produced only by the thermal cracking at 700 °C had a H2/ (2CO2 + CO) molar ratio of 0.24. The gasification of the charcoal with the autogenerated steam was experimentally observed to yield a gas with H2/(2CO2 + CO) molar ratio of 1.37, larger than a theoretical value deduced from the reactions of pure carbon with steam, as the charcoal contained some hydrogen. Taking the overall contribution of the tar cracking and the char gasification to the yield of gas as 1, according to the H2/(2CO2 + CO) molar ratio (R) of
0,8
60
40
20
Char
NEV-Tar
EV-Liquid
r
Char coal
700 oC
L-ch a
ar
B-ch ar
A-ch
har
Grap hite
No c
Char coal
ar
550 oC
L-ch ar
B-ch
hite
A-ch ar
Grap
No c
har
0
Molar ratio of H2 / (2CO2+CO)
Yield (wt.%, dry biomass)
100
carbon sample beds were close in all experiments. Fig. 5 shows the yields of gas, liquid and char produced with the four carbon samples at cracking temperatures of 550 °C and 700 °C. The data for the charcoal are listed together for contrast. It was seen that all four carbon samples lowered the yields of liquid product at 550 °C, and L-char had relatively strong effect. The yields of NEVTar were 16%, 19%, 14% and 13%, respectively, for graphite, A-char, B-char and L-char, all lower than the corresponding yield obtained without carbon sample (20%). At 700 °C, the yield of liquid produced with L-char was appreciably lower than that obtained without carbon sample, in contrast to the results for graphite, A-char and B-char; the yields of NEV-Tar were near to zero irrespective of carbon samples. We examined the cumulative yields of four major gases produced with four carbon samples. At the cracking temperature of 700 °C, the yields of H2 and CO2 obtained with L-char were appreciably higher than those obtained with graphite, A-char and B-char; this result was similar to that obtained with charcoal.
Without char With charcoal 0,6
0,4
0,2
0,0 Gas
Fig. 5. Effect of graphite and different chars on the yield of gaseous, liquid and char products at the cracking temperatures of 550 °C (the left part) and 700 °C (the right part). Final pyrolysis temperature, 700 °C. The liquid product was fractionated to the non-evaporated tar (NEV-Tar) and the evaporated liquid (EV-Liquid).
300
400
500
600
700
Cracking temperature (oC) Fig. 6. Changes in the molar ratio of H2/(2CO2 + CO) with cracking temperature in two cases with charcoal (d) and without charcoal (h). Final pyrolysis temperature, 700 °C.
Q. Sun et al. / Fuel 90 (2011) 1041–1048
the gas produced with the charcoal at a cracking temperature, we can then determine the contribution of the char gasification (Xg) in terms of the equation:
1:37 X g þ 0:24ð1 X g Þ ¼ R The observed values of R at the cracking temperatures of 500, 550, 600, 650 and 700 °C in the case of using charcoal were 0.23, 0.31, 0.39, 0.48 and 0.54, respectively, as shown in Fig. 6. Therefore the values of Xg calculated using the above equation were 0, 0.06, 0.13, 0.21 and 0.27, respectively. It was clear that the char gasification contributed to the gas yield to a greater extent at higher cracking temperature. On the other hand, the char gasification converted a fraction of the autogenerated steam to non-condensable gases. Thus, there were two effects of charcoal, the promoted tar cracking and the consumption of water for the char gasification, which led to a reduction in the yield of liquid product. From the value of Xg and the yields of CO2 and CO, we can then estimate a reduction in the liquid yield attributed to the amount of water consumed for the char gasification. Fig. 7 shows the separated contributions of the catalyzed tar cracking and the char gasification to the differences between the yields of liquid product obtained without and with charcoal at varying cracking temperatures. It could be seen that the reductions in the yields of liquid product were predominantly or mainly due to the catalyzed tar cracking at 500–600 °C, whereas those obtained at 650 and 700 °C essentially equaled to the amounts of water consumpted for the char gasification. It became quite clear that at lower temperatures (500–600 °C), the catalytic effect of charcoal on tar cracking was a main cause for the increased yield of gas at the expense of the reduced yield of liquid product; at higher temperature, as the catalytic tar cracking effect was masked by the thermal effect, the char gasification was principally responsible for an increase in the yield of gas accompanied by exploiting a part of the autogenerated steam for the char gasification. If four major gases are considered to calculate the composition of the gas accumulated over the total pyrolysis process, the gas produced by thermal cracking at 700 °C without char was composed of 19% H2, 51% CO, 16% CH4 and 14% CO2; in contrast, the corresponding gas obtained with the charcoal contained 39% H2, 31% CO, 9% CH4 and 22% CO2 (all vol.%). It was indicated that the gasification of char with the autogenerated steam could significantly alter the gas composition. As stated above, we observed that the catalytic effect of charcoal on the tar cracking was noticeable at 500–600 °C, but it 15 by catalytic tar cracking by char gasification
Difference (%)
12
9
6
3
0 500
550
600
650
700
Cracking temperature (oC) Fig. 7. Separated contributions of the catalytic tar cracking and the char gasification to the differences between the yields of liquid product obtained without charcoal and with charcoal at varying cracking temperatures. Final pyrolysis temperature, 700 °C.
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became obscure at higher temperatures (650–700 °C) on account of the intensified thermal effect. Herein we wish to compare our observation with the literature. Boroson et al. [28] positively observed the promoted cracking of biomass tar at 400–600 °C, but no data were available at higher temperature. Abu El-Rub et al. [31] confirmed the catalytic effect of char on decomposition of phenol and naphthalene at 700–900 °C. However, Gilbert et al. [33] reported the insignificant influence of char in a temperature range of 500–800 °C. By comparison of our result with Refs. [28,31], we assume that the catalytic effect of char may depend largely on the components of the primary tar. For thermally less stable oxygenated tar such as saccharides, furans and guaiacols, char is catalytically active at lower temperature, while for thermally more stable tar such as phenols and naphthalenes, only at higher temperature does char behave actively. In comparison of our result with Ref. [33], there remains an unknown discrepancy. Gilbert et al. [33] thought that the char was probably inactivated under an inert gas stream, leading to a suppressed catalytic effect of charcoal in their experiment. However, we also used argon as an inert purging gas. In a similar manner, we calculated the individual contributions of the char-catalyzed cracking of tar and the char gasification to the yields of gaseous and liquid products in the case of using other carbon samples. It was observed that the gasification was insignificant for graphite, A-char and B-char, but significant for L-char at the cracking temperature of 700 °C. This result was in agreement with a general order of the gasification reactivity: lignite > bituminous coal > anthracite > graphite. In addition, calculation showed that at the cracking temperature of 700 °C, the reductions in the yields of liquid product caused by L-char were completely attributable to the gasification of L-char with the autogenerated steam, similar to the case of the charcoal.
4. Conclusions (1) The thermal cracking of tar at lower temperatures (500–600 °C) resulted in the conversion of part of heavy tar mainly to light liquid compounds (including water), and the thermal cracking of tar became intense at higher temperatures (650–700 °C) with marked formation of gases. The charcoal catalytically promoted the cracking of tar to gases at lower temperatures (500–600 °C); at higher temperatures (650–700 °C), the catalytic effect was nullified by the intensified thermal effect. (2) GC–MS analysis showed that the primary liquid product consisted of a number of oxygenated compounds, which were representative of a mild thermal degradation of wood constituents. The charcoal promoted the decomposition of most oxygenated compounds present in the primary liquid product at the cracking temperatures of 500 and 600 °C. (3) The autogenerated steam gasified the charcoal at higher cracking temperatures (650–700 °C), resulting in an increase in the yield of gaseous product and a change in gas composition. This reaction was responsible for the reduction in the yield of liquid product caused by using charcoal at higher cracking temperature. (4) Apart from the charcoal, all other carbon samples including graphite, A-char, B-char, and L-char showed the catalytic effect, to different degrees, on the tar cracking at lower cracking temperature, and comparatively L-char had a high activity. L-char also behaved with high reactivity for the gasification with the autogenerated steam at higher cracking temperature, like the charcoal but unlike graphite, A-char and B-char.
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