Energy Fuels 2010, 24, 76–83 Published on Web 08/14/2009
: DOI:10.1021/ef9005109
In-Situ Reforming of Tar from the Rapid Pyrolysis of a Brown Coal over Char† )
Toru Matsuhara,‡ Sou Hosokai,‡ Koyo Norinaga,§ Koichi Matsuoka, Chun-Zhu Li,^ and Jun-ichiro Hayashi*,‡ Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, 816-8580, Japan, §Center for Advanced Energy Conversion Materials, Hokkaido University, Sapporo, 060-8628, National Institute of Advanced Industrial Science and Technology, Tsukuba, 305-8569, Japan, and ^Curtin Centre for Advanced Energy Science and Engineering, Curtin University of Technology, GPO Box U1987, Perth, Australia )
‡
Received May 22, 2009. Revised Manuscript Received July 21, 2009
Reforming of nascent tar from the rapid pyrolysis of a brown coal over char prepared from the same coal was studied at 750-900 C. The reforming was very rapid and extensive, allowing only benzene (0.02% on a coal C basis), naphthalene (0.001%), and phenanthrene (0.0001%) to escape from the char bed at an empty-bed gas residence time of less than 170 ms and 900 C, respectively. Reforming even at 750 C converted 96% of heavy tar (boiling point temperature >336 C) into noncondensable gases and coke deposit over the char. Decreasing conversion of the tar into coke with increasing temperature suggested that the tar was reformed in a sequence of coking and steam gasification of the coke rather than direct steam reforming over the char. The reforming at 900 C gave a negative coke yield due to progress of coke/ char gasification faster than the coke deposition. Results of this work thus showed a possibility of complete tar reforming by intensification of contact between the char and volatiles even in the absence of a catalyst.
process, that is, combustion of a minimum amount of residual char.1,2 The heat of combustion is effectively transferred to heat carrier solid circulating between the endothermic and exothermic reactors, which is then recuperated into chemical energy in the latter reactor. Bubbling fluidized bed is a candidate of the endothermic reactor in which the heat carrier also plays a role of fluidizing medium, while another bubbling fluidized bed or riser (lifter) can be employed for the char combustor or partial combustor. One of the requirements is a substantially high carbon conversion of coal into syngas by the reforming/gasification, in other words, the amount of char to be burned must be minimized as far as the heat of combustion is enough to drive the endothermic processes. Pyrolysis, steam reforming of volatiles, and steam gasification of char, if operated at temperatures of 700-850 C in a singe reactor such as a fluidized bed reactor, will not give so high coal conversion as required for realizing thermal balance between endothermic and exothermic reactors,2 mainly due to strong chemical interaction between the char and volatiles. Steam gasification of char at temperatures lower than 900 C is considerably inhibited or even terminated by radicals, such as hydrogen ones formed from thermal cracking of the volatiles,3 and these species seem to be much stronger inhibitors of steam gasification than H2. Hydrogen radicals also penetrate into the carbon matrix of char, inducing rearrangement of aromatic ring systems and thereby reducing the char reactivity.4-6 Moreover, hydrogen radicals from the volatiles also promote volatilization of alkali and alkaline earth metallic (AAEM) species that play catalytic roles in the steam
1. Introduction Integrated coal-gasification combined cycles (IGCC) and those with fuel cells (IGFC) produce electricity with higher efficiency than conventional combustion-based power generation, although further improvement of the efficiency is required for minimizing CO2 emission regardless of whether the power generation is combined with a CO2 capture/storage technology or not. In both IGCC and IGFC, the greatest loss of exergy occurs neither in the fuel cell nor gas/steam turbines but in the gasification process, more exactly saying, the exothermic partial combustion of coal. Introduction of endothermic steam and/or CO2 gasification is hence essential for reducing the exergy loss, but it inevitably brings about reduction in the temperature and then the rate of coal conversion, resulting in difficulty in complete coal conversion within a limited size of reactor or gas/solid residence time. Very recently, a novel coal gasification system was proposed.1 In this system, heat exhaust from the fuel cell and/or gas turbine is transferred to high-temperature pressurized steam, recycled to the gasifier, and then recuperated into chemical energy of syngas through steam gasification. A process simulation predicted electrical efficiency (at generating end) as high as 70% for a particular type of IGFC, assuming complete gasification at temperatures of 700-850 C.1 A potential way of low temperature gasification is to apply two-stage gasifiers that isolate endothermic reaction processes, that is, pyrolysis of coal, steam reforming of volatiles, and steam gasification of char; and from an exothermic † Presented at the 2009 Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies. *To whom all correspondence should be addressed. E-mail:
[email protected]. (1) Tsutsumi, A. Clean Coal Technol. J. (in Japanese) 2004, 11, 17–22. (2) Hayashi, J.; Hosokai, S.; Sonoyama, N. Process Safety Environ. Protect. 2006, 84 (B6), 409–419.
r 2009 American Chemical Society
(3) Bazardorj, B.; Sonoyama, N.; Hayashi, J.-i.; Hosokai, S.; Li, C.-Z.; Chiba, T. Fuel 2006, 85, 340–349. (4) Wu, H.; Hayashi, J.-i.; Chiba, T.; Takarada, T.; Li, C.-Z. Fuel 2004, 83, 23–30. (5) Wu, H.; Li, X.; Hayashi, J.-i.; Chiba, T.; Li, C.-Z. Fuel 2005, 84, 1221–1228. (6) Li, X.; Wu, H.; Hayashi, J.-i.; Li, C.-Z. Fuel 2004, 83, 1273–1279.
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: DOI:10.1021/ef9005109
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gasification of char. Thus the volatiles-char interaction has significant negative effects on the progress of steam gasification of char. The steam gasification of char, if allowed, would preferably be performed in the absence of volatiles, that is, in that of volatiles-char interaction. The volatiles-char interaction (VCI) also causes deposition of carbonaceous matter (coke) from the volatiles onto the char.10-12 The coke deposition has a direct and negative impact on the steam gasification. But, from another point of view, the coke deposition could be an effective way to decompose tar that must be removed from the syngas, preferably within the gasifier/reformer. Coke deposition from aromatic compounds13-18 and tar from a biomass gasifier19 onto the surface of carbonized solids seems to be very fast. The present authors studied rapid pyrolysis of brown coals in drop-tube reactors10,11 and found simultaneous progress of in situ reforming of tar and steam gasification of char at 900 C within a few to several seconds. It was also shown that inherent AAEM species played an essential catalytic role on the char surface contributing to fast progress of such simultaneous processes as above.11,20 There have so far been a number of reports on catalytic reforming of tar from the pyrolysis of coal and biomass. However, it may be hard to develop a practical catalytic system that is free from poisoning by Cl/S-containing species and volatile AAEM species, deactivation by coking, and even accumulation of ash/char fines. Rapid and complete decomposition of tar over char, if it is possible, will lead to proposal of reactor systems that enable both elimination and intensification of VCI. A potential system would be a fast solid circulating system consisting of three reactors; a downer, a bubbling fluidized bed, and a riser (lifter) through which fluidizing medium and char circulate. Pulverized coal is mixed with hot circulating solids and rapidly pyrolyzed in the downer. Tar is decomposed in situ into syngas and/or coke over the nascent and recycled chars in a very short time. The char is separated from the syngas, sent to the fluidized bed together with the fluidizing medium, and gasified there in the absence of VCI. The combustor accepts the char and the fluidizing medium, burn a portion of the char generating heat of combustion and transferring it to the medium, and then recycle the solids to the downer exhausting the flue gas. In this work, reforming of nascent volatiles from the rapid pyrolysis of a brown coal over char from the same
coal was studied. The main purpose was to examine potentiality of simultaneous pyrolysis and tar elimination in such a reactor as assumed above. 2. Experimental Section 2.1. Coal and Char Samples. Loy Yang brown coal from the Latrobe Valley, Victoria, Australia, was employed as the starting coal sample. The coal was dried and pulverized in the same way as in our previous study.20 The elemental composition of the coal was as follows: C, 67.4; H, 4.85; N, 0.67; S, 0.25; O, 26.8 (by difference) wt % on a dry-and-ash-free basis (daf), and Na, 0.13; Ca, 0.03; Mg, 0.06 wt % on a dry basis. A fraction of the coal with particle sizes of 0.5-1.0 mm was collected by sieving and used. A char sample was prepared by pyrolyzing the coal in an atmospheric fluidized bed reactor.18 The coal was heated up to 800 C at a rate of 10 C min-1 and further heated at 800 C for 30 min while fluidized by a flow of N2 (purity >99.9995 vol %). No fluidizing medium (solid diluent) such as silica sand was used. The char was pulverized to collect a fraction with particles sizes of 0.35-0.50 mm, and the fraction was used as the bed material for the reforming of tar. The elemental composition of the char was as follows: C, 95.0; H, 1.36; N, 0.62 wt % daf. The char is hereafter referred to as LYC. 2.2. Pyrolysis and Reforming. A two-stage reactor similar to that used in our previous study16 was employed for investigating in situ reforming of volatiles from the rapid pyrolysis of the coal over LYC and also gas-phase thermal cracking of the volatiles. Figure 1 shows the schematic diagram of the experimental apparatus and details of the two-stage reactor. The reactor consisted of the pyrolyzer and reformer. Coal particles were fed into the pyrolyzer at a rate of 0.1 g min-1 together with N2 (purity >99.9995 vol %) at a flow rate that was 0.8 NL min-1. The feeding period was 40 min. The pyrolyzer was made of a thimble of SUS316 wire mesh (mesh opening; 0.037 mm) that played a role of gas-solid separator that fed the nascent volatiles to the reformer, leaving the char particles inside the pyrolyzer. The pyrolysis temperature was fixed at 600 C, which was high enough to complete the evolution of tar from the pyrolysis of the coal and also low enough to avoid significant cracking of the tar in the gas phase.11,21-23 Accumulation of char particles inside the pyrolyzer was inevitable. It was, however, believed that such accumulation minimally influenced the composition of the volatiles to be fed into the reformer, because the nascent volatiles were swept away of the pyrolyzer through the wire mesh without traveling through the char bed that was formed at the thimble bottom. In fact, the tar yield from the pyrolysis, 25.2% on a coal carbon basis, was in good agreement with that from the pyrolysis of the same coal as used in the present work in a Curie-point pyrolyzer.11 The reformer was made of a transparent quartz glass tube with an inner diameter of 20 mm. Its bottom was closed by a quartz flit for supporting the fixed bed of LYC with a height and mass of 30 mm and about 3.7 g, respectively. The temperature of the LYC bed was 750, 800, 850, or 900 C. The empty-bed residence time of gas within the bed was calculated as 160190 ms from the flow rate of N2. The residence time was in practice slightly shorter by about 10% due to gas formation and steam from the coal moisture. The reforming at a temperature X C will be denoted by XR (e.g., 750R). Separately from the reforming with LYC, the gas-phase thermal cracking of the volatiles was performed at 600 or 900 C in the absence of LYC (i.e., in empty reformer). The gas-phase pyrolysis will be denoted by 600P or 900P. The abovementioned tar yield (25.2%) was that from 600P.
(7) Quyn, D. M.; Wu, H.; Li, C.-Z. Fuel 2002, 81, 143. (8) Quyn, D. M.; Wu, H.; Bhattacharya, S. P.; Li, C.-Z. Fuel 2002, 81, 151. (9) Wu, H.; Quyn, D. M.; Li, C.-Z. Fuel 2002, 81, 1033. (10) Hayashi, J.-i.; Takahashi, H.; Iwatsuki, M.; Essaki, K.; Tsutsumi, A.; Chiba, T. Fuel 2000, 79, 439–447. (11) Hayashi, J.-i.; Iwatsuki, M.; Morishita, M.; Tsutsumi, A.; Li, C.-Z.; Chiba, T. Fuel 2002, 81, 1977–1987. (12) Li, X.; Wu, H.; Hayashi, J.-i.; Li, C.-Z. Fuel 2004, 83, 1273–1279. (13) Griffiths, D. M. L.; Mainhood, J. S. R. Fuel 1967, 46, 167–76. (14) Abu El-Rub, Z. Y.; Bramer, E. A.; Brem, G. Ind. Eng. Chem. Res. 2004, 43, 6911–6919. (15) Hosokai, H.; Kumabe, K.; Ohshita, M.; Norinaga, K.; Li, C.-Z.; Hayashi, J.-i. Fuel 2008, 87, 2914–2922. (16) Hosokai, S.; Hayashi, J.-i.; Shimada, T.; Kobayashi, Y.; Kuramoto, K.; Li, C.-Z.; Chiba, T. Chem. Eng. Res. Des. 2005, 83 (A9), 1093–1102. (17) Abu El-Rub, Z.; Bramer, E. A.; Brem, G. Fuel 2008, 87, 2243– 2252. (18) Sugawa, M; Hosokai, S.; Norinaga, K.; Li, C.-Z.; Hayashi, J.-i. Ind. Eng. Chem. Res. 2008, 47, 5346–5352. (19) Abu El-Rub, Z. Biomass Char as an In-situ Catalyst for Tar Removal in Gasification Systems; Thermal Engineering Lab., University Twente: Enschede, The Netherlands, 2008. (20) Masek, O.; Sonoyama, O.; Ohtsubo, E.; Hosokai, S.; Li, C.-Z.; Chiba, T.; Hayashi, J.-i. Fuel Process. Technol. 2007, 88, 179–185.
(21) Li, C.-Z.; Sathe, C.; Kershaw, J.-R.; Pang, Y. Fuel 2000, 79, 427– 438. (22) Hayashi, J.-i.; Takahashi, H.; Doi, S.; Kumagai, H.; Chiba, T.; Yoshida, T. Energy Fuels 2000, 14, 400–408. (23) Jamil, K.; Hayashi, J.-i.; Li, C.-Z. Fuel 2004, 83, 833–843.
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Figure 1. Schematic diagram of experimental apparatus and two-stage reactor for coal pyrolysis and in situ reforming of volatiles over char.
number of aromatic rings per molecule and type of condensation.24 The analysis detected benzene, naphthalene, acenaphthylene, fluorene, phenanthrene, anthracene, pyrene, fluoranthene, chrysene, perylene, benzo[a]anthracene, and benzo[ghi]perylene. Each of these compounds was detected with a photodiode array (PDA) UV/vis detector and identified through confirmation of agreement in the retention time between the tar sample and a standard sample and checking of the UV/vis spectrum of the tar sample against that from the standard sample over a wavelength range from 200 to 600 nm. The mass of LYC increased or decreased during the reforming due to net progress of carbon deposition from the volatiles (coking) or that of steam gasification, respectively. The extent of the coking or gasification was determined from the difference in the amount of carbon between the LYC just before coal feeding and spent LYC. There occurred gas release during heating the bed of LYC to prescribed temperature and subsequent heating at the temperature (before the coal feeding). The amount of the released gases;H2, CO, CO, and CH4;were quantified and considered in calculating the extent of the coking or gasification. LYC samples were subjected to measurement of N2 adsorption isotherms at -196 C. A general Langmuir model and a density functional theory (DFT) were applied to calculation of specific surface area and pore volume, respectively.15
2.3. Product Analyses and Quantification. A thimble filter made of silica fibers and a condenser were equipped in series at the reactor downstream. The temperatures of the filter and condenser were maintained at 200 and -73 C, respectively. A portion of condensables was deposited onto the filter while the other was collected in the condenser quantitatively. The noncondensable gases were allowed to pass through the filter/ condenser, collected in a gasbag, and then analyzed by gas chromatography. Intermittent gas sampling and analysis were also done for investigating time-dependent changes in H2 flow rate. The char from the rapid pyrolysis and LYC were recovered after cooling the reactor to ambient temperature. Details of the product recovery, identification and quantification were reported elsewhere.11,18 In the present work, the organic condensables were classified to two groups, light tar and heavy tar. The light tar was defined as the compounds with normal boiling point temperatures (bp) lower than 336 C (i.e., that of phenanthrene), whereas the heavy tar was those with bp of 336 C or higher. It was confirmed that the light tar was collected in the condenser exclusively and dissolved into methanol. The constituents of the light tar were quantified by gas chromatography. The heavy tar was recovered from the filter and also condenser by washing them with tetrahydrofuran (THF), and its yield was determined on a coal carbon basis from its total mass and carbon contents after removal of THF. The heavy tar was analyzed by HPLC in a size exclusion (gel permeation) mode with two different analytical columns connected in series (Showa Denko K.K.; Shodex KF801 and KF802.5) and THF as the mobile phase. In addition to the product recovery as mentioned above, the product gas was also sampled intermittently for investigating time-dependent change in the activity of LYC from those in the yields of aromatic hydrocarbons. The sampling of tar vapor was made using a syringe that had been charged with a known volume of n-heptane. This solvent was used for absorbing aromatic hydrocarbons quantitatively. Every solution of the tar was analyzed by a normal phase HPLC with an analytical column packed with Inertsil NH2 (GL Sciences Inc.; particle size, 5 μm) and n-heptane as the mobile phase. This HPLC allowed aromatic hydrocarbons with no oxygen or nitrogen functionalities to elute and separated them according to the
3. Results and Discussion 3.1. Effect of Reforming Temperature on the Fate of Tar. Figure 2 shows yields of the light and heavy tars in a cumulative manner. The total tar yield from 600P was 25.2% on the coal carbon basis, and it was equivalent with that from the rapid pyrolysis of the same coal in a Curie-point pyrolyzer,10,11 in which the secondary pyrolysis of the nascent volatiles was minimized. This result showed complete pyrolysis in terms of tar evolution and insignificant progress of gas-phase pyrolysis of the nascent volatiles in the pyrolyzer and empty reformer, respectively. The reforming over LYC converted the tar to significant degrees over the temperature range examined. The heavy tar (24) Hayashi, J.-i.; Kawakami, T.; Taniguchi, T.; Kusakabe, K.; Morooka, S. Energy Fuels 1993, 7, 57–66.
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Figure 4. H2 yields. Run IDs are indicated in the same manner as in Figure 2.
Figure 2. Yields of light and heavy tars. Conditions: 600P, gas-phase cracking of nascent volatiles at 600 C; 750R, 800R, 850R, and 900R, reforming over char at 750, 800, 850, and 900 C, respectively.
Figure 5. Coke yields. Run IDs are indicated in the same manner as in Figure 2. Figure 3. Yields of COx and GHC. Run IDs are indicated in the same manner as in Figure 2.
750R was as much as 11.7%, and it accounted for 46% of the tar from 600P. Coke was thus a major product of the tar reforming at 750 C. The coke yield decreased with increasing reforming temperature and finally became negative at 900 C. This was due to progress of steam gasification of LYC and/or coke from the tar that was faster than the coke formation in terms of carbon. It was believed that steam from the moisture of the coal (about 11 mol-H2O/100-molcoal-C) and that formed from the pyrolysis were involved in the gas from the pyrolyzer. According to our previous studies on the rapid pyrolysis of the same Loy Yang coal as used in this work, it was estimated that H2O yield from the pyrolysis was about 10 mol-H2O per 100-mol-coal-C.10,11 Although consumption of steam was not measured in this work, it was estimated from changes in the product composition by changing the reforming temperature. The increase in COx yield by raising the reforming temperature from 750 to 800, 850, or 900 C (see Figure 3) resulted in more steam consumption, because oxygen involved in neither the tar nor coke was important. ΔYi was defined as the difference in the yield of product i between 800R, 850R, or 900R and 750R. ΔYH2 (unit; mol-H2/100-mol-coal-C) was then given as a function of ΔYi of the carbon-containing products from the nascent volatiles. R β ΔYH2 ≈ΔYCO þΔYCO2 - ΔYtar - ΔYcoke 2 2 1 - 4ΔYCH4 þ2ΔYC2 H4 þ3ΔYC2 H6 þ2ΔYC3 H6 2 8 ð1Þ þ ΔYC3 H8 þ2ΔYC4 H8 3
yield was decreased from 23.1 to 0.9% by changing the condition from 600P to 750R, while the light tar yield was slightly decreased. Though not shown in this figure, 900P gave light and heavy tar yields of 1.9 and 5.7%, respectively. The reforming over LYC at 750 C was thus more effective than the gas-phase cracking at 900 C. Both the light and heavy tar yields decreased with increasing reforming temperature. The light tar from 900R consisted of only benzene (yield; 0.023%) and naphthalene (0.0015%). 900R also allowed only 0.03% of the heavy tar to survive in the LYC bed. This heavy tar yield was measured by a combustion method due to the very small amount of the recovered heavy tar (
