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Pyrolysis and CO2 Gasification of Chinese Coals in a High-Pressure Wire-Mesh Reactor under Conditions Relevant to Entrained-Flow Gasification David Peralta, Nigel Paterson,* Denis Dugwell, and Rafael Kandiyoti Department of Chemical Engineering and Chemical Technology, Imperial College London, London, SW7 2AZ, United Kingdom Received September 15, 2004. Revised Manuscript Received December 13, 2004
A high-pressure wire-mesh reactor has been successfully re-commissioned to operate at high temperatures (up to 2000 °C) and pressures (up to 30 bara) for characterization of coal samples under conditions relevant to entrained-flow gasification. A sample of coal supplied by the Thermal Power Research Institute (China) has been tested. Pyrolysis yields increased as the temperature increased and decreased as the pressure increased. The extent of gasification at low temperature (1000 °C for 1 s) must have been low, because the weight losses in CO2 and helium did not differ greatly. Nevertheless, the gasification yields at high temperature (1500 °C and 1 s) were significant, and near-complete conversion of the sample occurred in CO2. Non-isothermal thermogravimetric analysis of residual char clearly showed an increased thermal deactivation with increased temperature and pressure. The coupled use of the high-pressure wire-mesh reactor and the thermogravimetric analyzer has potential as a laboratory tool to assess the reactivity of coals to be used in entrained-flow gasifiers.
Introduction Currently, coal provides over 23% of the world’s total primary energy and generates ∼38% of the world’s electricity.1 The use of coal for power generation is projected to increase by 60% by 2030, especially in countries with developing economies, such as China and India.2 However, increased use of coal results in several issues of serious environmental concern, including increased release of fossil-derived carbon dioxide (CO2), particulates, and trace elements (with mercury being of the greatest concern). Cleaner coal technologies (CCTs) have been developed and demonstrated to minimize the impact of these pollutant emissions. O2/ steam-blown entrained-flow gasifiers have been successfully demonstrated in integrated gasification combined cycle (IGCC) projects in Europe and the United States.3,4 Combustion of part of the fuel with the O2 provides the process heat. The fuel gas CV is generated by a combination of pyrolysis and gasification reactions (by steam and CO2). This promising, efficient, clean coal technology converts coal to a gas mixture that contains high proportions of carbon monoxide (CO) and hydrogen (H2), with the ash components being effectively removed from the reactor as saleable slag. Coal is injected either * Author to whom correspondence should be addressed. Telephone: 44 207 594 5581. Fax: 44 207 594 5604. E-mail address: n.paterson@ ic.ac.uk. (1) Key World Energy Statistics; International Energy Agency: Paris, 2003. (2) World Energy Outlook 2002; International Energy Agency: Paris, 2002. (3) Bee´r, J. M. Prog. Energy Combust. Sci. 2000, 26, 301-327. (4) Trapp. W. Eastman Coal Gasification: 19 Years of Reliable Operation; Paper presented at the 5th European Gasification Conference, Amsterdam, The Netherlands, April 8-10, 2002.
as dried pulverized particles (dry feeding system) or as a coal/water slurry (wet feeding system). In the dry feeding system, additional steam is injected to moderate the reaction temperature in the gasifier. In contrast, the wet feeding system always generates steam in situ from the flash vaporization of water at the very high temperatures inside the reactor. This difference has a direct effect on the temperature range at which the gasifiers operate: 1700-2000 °C for the dry feeding system and 1500-1800 °C for the wet feeding system. Both systems operate at a pressure of typically 20-30 bara, depending on the requirements of the gas turbine that is used to generate power from the energy content of the gas. There are plans to develop a coal-fired IGCC in China based on entrained-flow technology. The actual design, which has not been fully developed yet, is likely to be based on a dry fed gasifier, because of the potentially higher cold gas efficiency, compared to a slurry fed system. A collaborative project has been completed between Imperial College London (in the United Kingdom) and the Thermal Power Research Institute (in China) to investigate a suite of Chinese coals for use in dry feed, entrained-flow gasifiers and to gain technical information on the use of selected Chinese coals under relevant conditions. The work forms part of the ongoing UK/ China initiative on clean coal technology, which was formalized by a Memorandum of Understanding signed in September 1998. In this paper, the further development of a highpressure wire-mesh reactor is described, which enables it to operate at temperatures of up to 2000 °C and pressures up to 30 bara for the characterization of coal
10.1021/ef049762w CCC: $30.25 © 2005 American Chemical Society Published on Web 01/28/2005
Pyrolysis and CO2 Gasification of Chinese Coals
Figure 1. Schematic diagram of the wire-mesh reactor.
samples under conditions relevant to entrained-flow gasification. The development and commissioning are described together with the results of initial trials on a coal sample from China. Previously, this type of reactor has been used to investigate the reactivity of coals under conditions that simulate those in fluidized-bed gasifiers, operating at temperatures approaching 1000 °C.5 Only one study has been found in the literature on the assessment of the performance of coals under laboratory-scale conditions that simulate conditions in entrained flow gasifiers. This used a drop tube reactor at temperatures up to 1600 °C.6 Most of the studies that are reported in the literature concern pilot-scale and modeling studies of entrained flow gasifiers.7,8 Experimental Section Equipment. The High-Pressure Wire-Mesh Reactor. A schematic diagram of the apparatus is shown in Figure 1. The main features of the wire-mesh reactor (WMR) and the specific characteristics of the high-pressure version have been described elsewhere.9-11 Briefly, the WMR is a laboratory-scale device used to heat a small amount of pulverized solid particles (6 mg typically) under computer-controlled conditions. A variety of gases can be used to study processes such as pyrolysis (helium or nitrogen), gasification (CO2 or steam), and combustion (air or oxygen). Process yields are determined from the weight difference of the sample before and after testing (5) Messenbo¨ck, R.; Dugwell, D. R.; Kandiyoti, R. CO2 and Steam Gasification in a High-Pressure Wire-Mesh Reactor: The Reactivity of Daw Mill Coal and Combustion Reactivity of Its Chars. Fuel 1999, 78, 781-793. (6) Mamori, T.; Sagimori, K.; Baba, A.; Yatani, T.; Yasumi, T.; Inoue, H. Basic Study of Entrained Flow Gasification (in Jpn.). Souken Houkoku 1998, 55, 91-97. (ISSN: 0285-6697.) (7) Hara, S.; Inumaru, J.; Ashizawa, M.; Ichikawa, K. A Study on Gasification Reactivity of Pressurised Two-Stage Entrained Flow Gasifier. JSME Int. J., Ser. B 2002, 45, 518-522. (8) Brown, B.; Smot, L. D.; Smith, P. J.; Hedman, P. O. Measurement and Prediction of Entrained Flow Gasification Processes. AlChE J. 1988, 34, 435-446. (9) Cai, H.-Y.; Megaritis, A.; Messenbock, R.; Dix, M.; Dugwell, D. R.; Kandiyoti, R. Pyrolysis of Coal Maceral Concentrates under pfCombustion Conditions (I): Changes in Volatile Release and Char Combustibility as a Function of Rank. Fuel 1998, 77, 1273-1282. (10) Gu¨ell, A. J.; Kandiyoti, R. Development of a Gas-Sweep Facility for the Direct Capture of Pyrolysis Tars in a Variable Heating Rate High-Pressure Wire-Mesh Reactor. Energy Fuels 1993, 7, 943-952. (11) Messenbo¨ck, R. C.; Dugwell, D. R.; Kandiyoti, R. Coal Gasification in CO2 and Steam: Development of a Steam Injection Facility for High-Pressure Wire-Mesh Reactors. Energy Fuels 1999, 13, 122-130.
Energy & Fuels, Vol. 19, No. 2, 2005 533 and is reported on a dry, ash-free (daf) basis. The high-pressure wire-mesh reactor at Imperial College London has the following characteristics: (1) A continuous flow of process gas, to minimize secondary cracking or repolymerization reactions. (2) Efficient recovery and quantification of residual char and tar for further analysis. (3) Operation under a wide range of heating rates, temperatures and pressures. Modification of the Wire-Mesh Reactor. The high-pressure WMR was modified as part of this project, so that it could operate at temperatures up to 2000 °C and pressures up to 30 bara. A pair of high-temperature thermocouples was installed in the reactor. The conductor combinations of these thermocouples (type D) were 97% tungsten with 3% rhenium (positive pole) and 75% tungsten with 25% rhenium (negative pole). This pair can be used at temperatures up to 2400 °C. The material is known to harden at high temperature; however, this did not cause any problems in this work. A fresh thermocouple was constructed for each test. New ports for this set of thermocouples were installed in the reactor controllers, together with the corresponding calibration curve. The thermocouples were tested at different peak temperatures (1000, 1500, and 2000 °C), using helium as the sweeping gas. In these tests, no coal was present on the mesh. The holding time at peak temperature was reduced with increasing peak temperature, to protect the integrity of the molybdenum mesh. An insulating sheet made of alumina was used to prevent the mesh from contacting the support plate and avoid a short circuit; this was used successfully. Previously, a material called “Macor” was used; however, this material broke apart after a limited number of tests. A molybdenum mesh was used to withstand the very high temperatures that were used in this work. In helium, the molybdenum mesh did not suffer any physical damage, even at the highest temperature of 2000 °C. However, in CO2, the mesh became very brittle at 2000 °C and the rates of reaction were very high; consequently, in this atmosphere, the upper temperature was limited to 1500 °C. This is the first time that this type of apparatus has been operated under such extreme conditions. The experiments under these conditions were difficult to conduct; however, their successful completion reflects the considerable potential of the technique for studies under these conditions and the practical skills of the operators. Thermogravimetric Analysis. Chars produced in the WMR tests were collected, and their relative reactivity was assessed using thermogravimetric analysis (TGA). The weight loss with time was measured, and the indicator of reactivity (the halflife) was the time required for 50% of the initial sample mass (on a daf basis) to be lost. Longer reaction times indicated lessreactive samples. A Perkin-Elmer model TGA 7 (Series 1020) analyzer was used. The nitrogen and air flow rates were each set at 40 mL/min. Samples were initially placed in the balance pan at 50 °C for 5 min for weight stabilization in a nitrogen atmosphere. The furnace then was heated at 50 °C/min to 400 °C and again kept at this temperature for 5 min for weight stabilization. After this period, nitrogen was replaced with air to start the combustion process. The furnace was then heated at 15 °C/min to 900 °C and kept at this temperature for 5 min to ensure complete combustion of the sample. However, the test was stopped at the time when the sample was completely consumed. The results are reported as a plot of sample weight versus time from the time of switching from nitrogen to air. Coal Sample. Yanzhuo Beishu coal, supplied by TPRI, was used in the commissioning trials. It was sent to TES (Bretby), Ltd., for initial crushing to a size of 1000 °C. The continual increase at temperatures up to 2000 °C shows that other mechanisms also contributed to the weight loss in the tests at very high temperatures. One possible explanation is that further thermal cracking of the organic structure occurred at the higher temperatures. However, it is also possible that, at these temperatures, a portion of the inorganic material became unstable and was vaporized. Any material released in this way would
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Figure 3. Temperature profile for test at 2000 °C and 30 bara, under pyrolysis conditions.
Figure 4. Total volatiles released at different peak temperatures and pressures from Yanzhuo Beishu coal.
be quantified with the organic volatiles. The data in Figure 4 also show that the pyrolysis yields decreased as the pressure increased. This has been observed in previous work and is explained by the effect of the greater resistance exerted by the sweeping gas at the higher pressures on the escaping volatiles. This causes an increase in the residence time within the particles, where repolymerization reactions occur and carbon is reincorporated into the forming char. Gasification Tests. Yanzhuo Beishu coal was gasified with CO2 at temperatures up to 1500 °C and pressures up to 20 bara. The hold times and heating rates were the same as those used for the tests in helium. Under these conditions, the measured weight loss is caused by both pyrolysis and gasification (in CO2) reactions. In CO2, the maximum operating conditions are limited by the embrittlement of the mesh, which makes it difficult to remove the char sample for weighing without contamination with broken wires from the mesh. A temperature of 1500 °C is approximately the maximum temperature that can be used, with CO2 as the sweep gas, without incurring this problem. The time-temperature profile for a gasification test (not shown here) is similar to that shown in Figure 3 (for a pyrolysis test), except that the peak temperature was 1500 °C. Figure 5 shows the data obtained during this suite of tests. At 1000 °C, the yields were slightly larger than the pyrolysis-alone yields under the same experimental conditions. This effect was slightly more pro-
Figure 5. Total mass loss on gasification of Yanzhuo Beishu coal in CO2.
nounced at high pressure. This indicates a low extent of gasification under these conditions. However, at 1500 °C, there was near-complete conversion (on a daf basis) at all pressures, which shows that the extent of gasification increases rapidly between the two temperatures used. Thermogravimetric Analysis. The TGA profiles of pyrolysis chars produced in the WMR at pressures of 2.5-30 bara and temperatures of 1000-2000 °C have been measured. An example of the TGA data (obtained at 1500 °C) is shown in Figure 6. The half-life (i.e., the time required to combust half of the organic char material) has been estimated from these data, and the values are shown in Table 4. These data show that the half-life increased with pyrolysis temperature, i.e., the char became less reactive. This must have been caused by thermal annealing, which occurs when chars are exposed to very high temperatures. This work shows that the annealing is rapid and occurs well within 1 s at the temperatures needed for entrained-flow gasifiers. An effect of pressure on char reactivity was also measurable at 1000 °C but was less significant than the effect of temperature. However, the effect of pressure became negligible at 2000 °C. The thermogravimetric profiles of gasification chars produced at 1000 °C are shown in Figure 7, with the corresponding half-lives being shown in Table 5. The data show that the gasification chars were slightly less reactive than the corresponding pyrolysis chars and a similar effect of increasing pressure was apparent.
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Figure 6. Thermogravimetric analysis (TGA) profiles of Yanzhuo Beishu chars produced by pyrolysis in helium at 1500 °C.
Figure 7. TGA profiles of Yanzhuo Beishu gasification chars produced at 1000 °C. Table 4. Thermogravimetric Half-Life of Yanzhuo Beishu Pyrolysis Chars Thermogravimetric Half-Life (min) temperature (°C)
2.5 bara
10 bara
20 bara
30 bara
1000 1500 2000
11.6 17.9 19.2
11.7 17.8 19.9
12.2 18.8 20.0
14.0 18.4 20.2
Table 5. Thermogravimetric Half-Life of Yanzhuo Beishu Gasification Chars Thermogravimetric Half-Life (min) temperature (°C)
2.5 bara
10 bara
20 bara
1000
11.8
12.2
13.9
Because of the high conversions in CO2 at 1500 °C, it was not possible to recover enough char to perform a thermogravimetric test on any of these chars. Conclusions A high-pressure wire-mesh reactor (WMR) has been successfully re-commissioned for operation at high tem-
peratures (up to 2000 °C in helium and 1500 °C in CO2) and pressures (up to 30 bara). It is the first time that this type of apparatus has been operated at such high temperatures. The modified reactor has been used to pyrolyze and gasify a sample of coal from China. Pyrolysis yields increased as the temperature increased and decreased as the pressure increased. Gasification yields at 1000 °C did not differ significantly from the pyrolysis yields under the same operating conditions for a reaction time of 1 s. Nevertheless, the gasification yields were very high at 1500 °C and showed a small but measurable increase with increasing pressure. Thermogravimetric analysis (TGA) of both pyrolysis chars and gasification chars showed deactivation with increasing process temperature and pressure. However, it has been shown that the gasification rates at high temperature (1500 °C) are fast enough to gasify a large part of the coal (in some cases, all of it) and that they are enhanced at higher pressure. The coupled use of the high-pressure WMR and TGA has proved to be a useful laboratory tool to characterize coals to be used in entrained-flow gasifiers.
Pyrolysis and CO2 Gasification of Chinese Coals
Acknowledgment. The authors would like to express their thanks to the Department of Trade and Industry (DTI, UK) and the Ministry of Science and Technology (MOST, China) for the funding received under Contract No. C/07/00298/00/00. D.P. would like
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to acknowledge the extended permission from the Consejo Nacional de Ciencia y Tecnologı´a (CONACYT, Mexico) to conduct this work. EF049762W
