Coal Gasification in CO2 and Steam: Development of a Steam

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Energy & Fuels 1999, 13, 122-129

Coal Gasification in CO2 and Steam: Development of a Steam Injection Facility for High-Pressure Wire-Mesh Reactors Reinhard C. Messenbo¨ck, Denis R. Dugwell, and Rafael Kandiyoti* Department of Chemical Engineering and Chemical Technology, Imperial College (University of London), London SW7 2BY, U.K. Received June 16, 1998

The development of a steam injection facility and a new control system for steam gasification experiments in a high-pressure wire-mesh reactor have been described. The design is based on preheating the steam path to prevent condensation during experiments. Steam is allowed to contact the coal sample only instants before the temperature of the sample is ramped. Results from experiments in helium, CO2, and steam at pressures up to 30 bar (1000 °C s-1 to 1,000 °C) have been compared for experiments extending to 60 s at peak temperature. The occurrence of a minimum in the conversion vs pressure diagrams, previously observed by numerous researchers, appears to be related to the reactivity of the gasifying agent and the holding time. The minimum, observed at shorter holding times in steam compared to CO2, appears related to the consumption of secondary char deposited during the pyrolysis stage. At 10 and 20 bar, the sample appears to be completely consumed in steam in about 60 s, while after 60 s in CO2, the process appeared to have reached completion at about 92% at 30 bar (∼80% at 10 bar), apparently due to deactivation of char by prolonged exposure to high temperatures. By contrast, chars withdrawn from a pilotplant gasifier fluidized with steam and air appear to be relatively unreactive. Larger particle sizes used in the pilot plant (dp < 3 mm vs 105-152 µm) tend to enhance tar repolymerization and longer exposures at high temperature, required for the consumption of these larger particles, appear to have reduced the reactivity of the pilot-plant chars.

Introduction The wire-mesh (“heated grid”) reactor configuration has served as a valuable tool in determining and understanding solid fuel behavior under pyrolysis, hydropyrolysis, and gasification conditions. The present paper describes the development of a steam injection facility, which extends the use of this type of instrument to gasification in steam, as well as the more usual “noncondensable” gasifying agents. The design forms part of a study to generate realistic bench-scale coal reactivity data relevant to air-blown gasification in fluidized beds. It aims to avoid some of the difficulties previously encountered in bench-scale gasification work. Numerous investigations of coal gasification reactivity have been carried out by pyrolyzing the coal in a separate step, prior to gasification (e.g., refs 1-13). However, there are strong indications that details of the (1) Nozaki, T.; Adschiri, T.; Fujimoto, K. Energy Fuels 1991, 5, 610. (2) Guo, C.; Zhang, L. Fuel 1986, 65, 1364. (3) Haga, T.; Nishiyama, Y. Fuel 1988, 67, 743. (4) Gadsby, J.; Hinshelwood, C. N.; Sykes, K. W. Proc. R. Soc. (London) 1946, A187, 129. (5) Long, F. J.; Sykes, K. W. Proc. R. Soc. (London) 1948, A193, 377. (6) Goring, G. E.; Curran, G. P.; Zielke, C. W.; Gorin, E. Ind. Eng. Chem. 1953, 45, 2586. (7) Goring, G. E.; Curran, G. P.; Tarbox, R. P.; Gorin, E. Ind. Eng. Chem. 1952, 44, 1051. (8) Goring, G. E.; Curran, G. P.; Tarbox, R. P.; Gorin, E. Ind. Eng. Chem. 1952, 44, 1057. (9) Yang, Y.; Watkinson, A. P. Fuel 1994, 73, 1786.

two separate steps significantly affect measured gasification reactivities.7,12,14-17 Chitsera et al.18 observed that at elevated temperatures and pressures, longer soak times during pyrolysis and aging in the atmosphere had negative effects on the reactivity of chars in steam at 40 bar. During steam gasification at 10001300 °C in a pressurized thermogravimetric balance, Peng et al.19 reported that reactivities of in situ generated chars were much greater than those for externally prepared char samples. Finally, the numerous studies where thermogravimetric systems8,14,20-24 have been used would appear to suffer from particle stacking, relatively slow heating rates (up to hundreds of degrees per minute), and difficulties in controlling gas flow (10) Zielke, C. W.; Gorin, E. Ind. Eng. Chem. 1957, 47, 396. (11) Meijer, R.; Kapteijn, F.; Moulijn, J. A. Fuel 1994, 73, 723. (12) Ginter, D. M.; Somorjai, G. A.; Heinemann, H. Energy Fuels 1993, 7, 393. (13) Haga, T.; Sato, M.; Nishiyama, Y. Energy Fuels 1991, 5, 317. (14) Sha, X.; Chen, Y.; Cao, J.; Yang, Y.; Ren, D. Fuel 1990, 69, 656. (15) Mu¨hlen, H.-J.; van Heek, K. H.; Ju¨ntgen, H. Fuel 1986, 65, 591. (16) Alvarez, T.; Fuertes, A. B.; Pis, J. J.; Parra, J. B.; Pajares, J.; Menendez, R. Fuel 1994, 73, 1358. (17) Silveston, P. L. Energy Fuels 1991, 5, 933. (18) Chitsera, C. T.; Mu¨hlen, H.-J.; van Heek, K. H.; Ju¨ntgen, H. Fuel Process. Technol. 1987, 15, 17. (19) Peng, F. F.; Lee, I. C.; Yang, R. Y. K Fuel Process. Technol. 1995, 41, 233. (20) Shufen, L.; Ruizheng, S. Fuel 1994, 73, 413. (21) Mu¨hlen, H.-J.; van Heek, K. H.; Ju¨ntgen, H. Fuel 1985, 64, 944. (22) Kasaoka, S.; Sakata, Y.; Shimada, M. Fuel 1987, 66, 697. (23) Bota, K. B.; Abotsi, G. M. K. Fuel 1994, 73, 1354. (24) Shufen, L.; Xinyan, X. Fuel 1993, 72, 1351.

10.1021/ef980135e CCC: $18.00 © 1999 American Chemical Society Published on Web 12/10/1998

Coal Gasification in CO2 and Steam

patternssso important in suppressing char volatile interactions during pyrolysis. As will be outlined below, the manner in which the pyrolysis step is carried out profoundly affects the structure and reactivity of the char. (During the gasification stage, gas flow patterns in the vicinity of individual particles also determine the magnitude of heat- and mass-transport resistances around coal particles, which significantly affect overall gasification rates.) We have recently compared CO2-gasification reactivities of coal chars prepared by pyrolyzing in a fixed bed, a fluidized bed, and a wire-mesh reactor.25 Pyrolysis and CO2-gasification yields in the fluidized bed and the wire-mesh reactor gave results which were within experimental error. We have observed that pyrolysis followed by gasification gives approximately the same overall conversion as one-step gasification when using the same reactor throughout. However, we have found that results from direct (coal) gasification in any single reactor may differ significantly from conversions in twostep experiments when the pyrolysis and gasification steps were carried out in reactors with different configurations. In particular, misleading results were obtained when the pyrolysis step was carried out in a fixed-bed reactor compared to reactors where the sample is allowed to behave as individual particles (e.g., wiremesh or fluidized bed reactors). Furthermore, if the gasification step is carried out in a high-pressure thermogravimetric balance, with the usual range of relatively slow heating rates, the problem is compounded by loss of char reactivity due to long exposures to high temperatures. There is, therefore, real difficulty in comparing reactivities determined by the many possible methods, even when considering the same original coal sample. Within this framework, the wire-mesh reactor configuration has the advantage of mimicking high heating rates and independent particle behavior, usually encountered in fluidized bed gasifiers. Clearly, one-step gasification is easily achieved, but the instrument needed minor modifications for coal gasification in CO2 (refs 26-28; also see below). However, steam gasification has proved problematic in wire-mesh reactors since the walls of the pressure containment vessel are normally kept cold, irrespective of the temperature reached by the mesh. Only in the commercial version of the apparatus marketed by DMT is the pressure head heated to about 200 °C to allow a small amount of water vapor to be present during the experiment.29 Development of Wire-Mesh Reactors. To put the present reactor development work in context, a brief review of the evolution of wire-mesh reactors has been presented. The review is not meant to be exhaustive but aims to outline major stages achieved in the development of this type of apparatus. The first apparatus of its kind was constructed by Loison and Chauvin.30 Coal was mixed with water into (25) Megaritis, A.; Messenbo¨ck, R. C.; Collot, A.-G.; Zhuo, Y.; Dugwell, D. R.; Kandiyoti, R. Fuel 1998, 77, 1411. (26) Lim, J.-Y. Ph.D. Thesis, University of London, 1997. (27) Lim, J.-Y.; Chatzakis, I. N.; Megaritis, A.; Cai, H.-Y.; Dugwell, D. R.; Kandiyoti, R. Fuel 1997, 76, 1327. (28) Megaritis, A.; Zhuo, Y.-Q.; Messenbo¨ck, R.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 1998, 12, 144. (29) DMT Technical Note, Essen, April 29, 1993. (30) Loison, R.; Chauvin, R. Chim. Ind. Paris 1964, 91, 269.

Energy & Fuels, Vol. 13, No. 1, 1999 123

a paste and pressed onto a single layer of metallic mesh held between two electrodes. One of the electrodes was spring-loaded to take up the thermal expansion of the mesh. Heating was achieved by passing a single electrical pulse from a variable voltage transformer through the sample holding mesh, which also served as the resistance heater. Temperatures were monitored by a Pt/Pt-Rh thermocouple placed at the center of the sample holder. Experiments were restricted to the “heatup” ramp (zero-second holding time) only. Ju¨ntgen and van Heek31 constructed a similar wire-mesh reactor for work under vacuum and linked it to a mass spectrometer. They reported qualitative data on the release of light volatiles during rapid coal pyrolysis. The wire-mesh reactor configuration is best known through the work of Howard and co-workers32-39 and Suuberg and co-workers;40-43 their work up to 1979 has been reviewed in ref 44. These researchers placed the coal sample between two layers of a folded mesh, fixed between relatively massive electrodes to absorb the heat generated by resistance heating. The (early) version built at MIT used direct current for heating the mesh, and the thermocouple was placed between the two layers of mesh (without physical contact) to avoid interference. This reactor was operated at pressures up to 70 bar during hydropyrolysis experiments. Tars were collected by washing internal reactor surfaces with solvent and characterized by size-exclusion chromatography, using tetrahydrofuran as eluent. The first wire-mesh instruments to use feedback control for heating and temperature control and to operate successfully at slow heating rates (e.g., 1 °C s-1), as well as the more usual high heating rates, were described by Hamilton and co-workers45,46 and Williams and co-workers.47 Hamilton’s reactor was only briefly used for recording morphologies of chars formed under a wide range of heating rates (10-1 to 104 °C s-1); the chars were then examined under SEM. In this instrument, a power cut off of 10 ms in every hundred was used for reading the temperature via the thermocouple. (31) Ju¨ntgen, H.; van Heek, K. H. Fuel 1968, 47, 103. (32) Anthony, D. B.; Howard, J. B.; Hottel, H. C.; Meissner, H. P. 15th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1975; p 1303. (33) Anthony, D. B.; Howard, J. B.; Meissner, H. P.; Hottel, H. C. Rev. Sci. Instrum. 1974, 45, 992. (34) Howard, J. B.; Anthony, D. B.; Hottel, H. C. 15th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1975; p 103. (35) Howard, J. B.; Anthony, D. B.; Hottel, H. C.; Meissner, H. P. Fuel 1976, 55, 121. (36) Howard, J. B.; Anthony, D. B. AIChe J. 1976, 22, 625. (37) Suuberg, E. M.; Peters, W. A.; Howard, J. B. Ind. Eng. Chem. PDD 1978, 17, 37. (38) Suuberg, E. M.; Peters, W. A.; Howard, J. B. 17th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1978; p 117. (39) Suuberg, E. M.; Peters, W. A.; Howard, J. B. Fuel 1980, 59, 405. (40) Unger, P. E.; Suuberg, E. M. Fuel 1984, 63, 606. (41) Suuberg, E. M.; Unger, P. E.; Lilly, W. D. Fuel 1985, 64, 956. (42) Suuberg, E. M.; Unger, P. E. 18th Symposium (International) on Combustion; The Combustion Institute, Pittsburgh, PA, 1981; p 1203. (43) Unger, P. E.; Suuberg, E. M. Prep. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1983, 28, 278. (44) Howard, J. B. In Chemistry of Coal Utilization; Elliot, M. A., Ed.; John Wiley & Sons: New York, 1981; p 665. (45) Hamilton, L. H.; Ayling, A. B.; Shiboaka, M. Fuel 1979, 58, 873. (46) Hamilton, L. H. Fuel 1980, 59, 112. (47) Desypris, J.; Murdoch, P.; Williams, A. Fuel 1982, 61, 807.

124 Energy & Fuels, Vol. 13, No. 1, 1999

In Canada, Stangeby and Sears48-50 developed a configuration with a lateral sweep gas flow of 3 cm s-1 to remove volatiles from the heated zone. They used pressures up to 100 bar in hydrogasification. The reactor constructed by Arendt and van Heek51,52 used an alternating high-frequency (10 kHz) heating current and analogue feedback control; the system operated above 210 °C s-1; the construction of the electrodes allowed experiments at up to 2 s holding time, and hydropyrolysis experiments were carried out at pressures up to 100 bar. Niksa and co-workers53-55 used a DC “operational power supply” powered from mains to deliver a constant direct current, as well as an optional constant voltage available from batteries. A single thermocouple was installed slightly above the sample holder, and a gas stream was allowed to sweep parallel to the mesh; one electrode was spring-loaded to absorb thermal expansion of the mesh on heating. The instrument was subsequently used for tar-yield determinations at up to 25 bar of hydrogen.56 Freihault and coworkers also developed an atmospheric pressure instrument where the thermocouple was welded to the wire mesh; reported tar yields suggested that secondary cracking of tars could not be altogether avoided.57-59 Two reactors were constructed at Imperial College for vacuum/atmospheric pressure and high-pressure operation.60-63 The design combined the use of water-cooled electrodes (to avoid heating of electrodes, particularly during slow heating experiments) and a flow of sweep gas through the mesh, for removing volatiles away from the reaction zone. One of the electrodes was springloaded for taking up the thermal expansion of the mesh, which was heated by an alternating current. Thermocouple readings were taken during interruptions in the heating current. On-line feedback control allowed operation over a wide range of heating rates (1-1000 °C s-1). During atmospheric pressure operation, the carrier gas stream flowing through the mesh was directed into a liquid nitrogen cooled trap for condensing and capturing evolved tars. Changes in tar and total volatile yields as a function of heating rate could be determined accurately. Subsequent developments allowed quantitative recovery and characterization of tars evolved during vacuum and atmospheric pressure pyrolysis.64,65 Cai66 later extended the temperature range (48) Stangeby, P. C.; Sears, P. L. Final Report to Canadian Department of Energy, Mines and Resources, 1978. (49) Stangeby, P. C.; Sears, P. L. Fuel 1981, 60, 131. (50) Stangeby, P. C.; Sears, P. L. Fuel 1981, 60, 125. (51) Arendt, P. Ph.D. Thesis, University of Aachen, Germany, 1980. (52) Arendt, P.; van Heek, K.-H. Fuel 1981, 60, 779. (53) Niksa, S. J. Ph.D. Thesis, Princeton University, 1982. (54) Niksa, S. J.; Russel, W. B.; Saville, D. A. Fuel 1982, 61, 1207. (55) Niksa, S. J.; Heyd, L. E.; Russel, W. B.; Saville, D. A. 20th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1984; p 1445. (56) Bautista, J. R.; Russel, W. B.; Saville, D. A. Ind. Eng. Chem. Fundam. 1986, 25, 536. (57) Freihaut, J. D.; Seery, D. J.; Zabielski, M. F. 19th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1982; p 1159. (58) Freihaut, J. D.; Seery, D. J.; Proscia, W. M. Energy Fuels 1989, 3, 692. (59) Freihaut, J. D.; Proscia, W. M. Energy Fuels 1989, 3, 625. (60) Gibbins-Matham, J. R.; Kandiyoti, R. Energy Fuels 1988, 2, 505. (61) Gibbins, J. R.; Kandiyoti, R. Energy Fuels 1989, 3, 670. (62) Gibbins, J. R.; Kandiyoti, R. Fuel 1989, 68, 895. (63) Gibbins-Matham, J. R.; King, R. A. V.; Wood, R. J.; Kandiyoti, R. Rev. Sci. Instrum. 1989, 60, 1129. (64) Li, C.-Z.; Bartle, K. D.; Kandiyoti, R. Fuel 1993, 72, 3. (65) Li, C.-Z.; Bartle, K. D.; Kandiyoti, R. Fuel 1993, 72, 1459.

Messenbo¨ ck et al.

of this instrument to 1500 °C by using Pt-Pt/Rh thermocouples and a molybdenum mesh. However, when the analogous reactor was operated at high pressure, the gas sweep through the mesh caused temperature instabilities.61 Subsequent work showed this to be due to the combined effect of turbulence coupled to higher heat-transfer rates from the mesh to the pressurized gas.67,68 A flow-smoothing cell based on a bank of tubes, introduced underneath the sample holder, served to suppress turbulence and stabilize the gas flow through the mesh. The modification allowed sufficient temperature stability for a stream of gas to be passedsas in the atmosphericpressure instrumentsthrough the sample holding mesh for accurate determinations of tar yields at pressures up to 160 bar68 and over the 1-1000 °C s-1 range. This apparatus provides the basis of the wire-mesh instrument with steam injection described below. In parallel work, Howard and co-workers69,70 have constructed a wire-mesh reactor where volatiles were drawn away from the heated mesh by suction tubing via glass funnels and subsequently quantified. This reactor was powered by alternating current, avoiding problems of interference between heating current and temperature readings. Tars were collected on aluminum foils covering the funnels and on filters in the gas suction lines above and below the mesh. Data were reported for atmospheric pyrolysis experiments in helium, with heating rates ranging from 10 to 20 000 °C s-1. Numerous groups have thus developed wire-meshtype reactors to study the pyrolytic behavior of solid fuels. Some of these groups have extended the use of this reactor configuration to higher pressures, e.g., Howard and co-workers at MIT,39,69 van Heek and coworkers at Bergbau Forschung52 (now DMT), Stangeby and Sears,49,50 Niksa and co-workers at Princeton University,54,55 and the present group at Imperial College.68 Initially all five groups aimed to investigate aspects of hydropyrolysis and hydrogasification. In this paper, we present modifications to the design of the high-pressure wire-mesh reactor, which allowed gasification experiments to be carried out in CO2 and steam. The steam injection system and changes to the control system and experimental procedures have been described. Results of experiments carried out in CO2 and steam (up to 30 bar, 1000 °C) using a range of holding times will be presented and compared. The gasification and combustion reactivities of residual chars and scanning electron photomicrographs will be presented in a separate paper.71 Experimental Section Reactor Development. The operating principle of the wire-mesh configuration is well-known, and some features of the present reactor have already been published.68 Briefly, (66) Cai, H.-Y. Ph.D. Thesis, University of London, 1994. (67) Guëll, A. J. Ph.D. Thesis, University of London, 1993. (68) Guëll, A. J.; Kandiyoti, R. Energy Fuels 1993, 7, 943. (69) Griffin, T. P.; Howard, J. B.; Peters, W. A. Energy Fuels 1993, 7, 297. (70) Howard, J. B.; Peters, W. A.; Darivakis, G. S. Energy Fuels 1994, 8, 1024. (71) Messenbo¨ck, R. C.; Dugwell, D. R.; Kandiyoti, R. Fuel 1998, in press.



Figure 2. New design for the flow of cooling water through the mesh support plate.

Figure 1. Design of the wire-mesh high-pressure reactor: (1) gas exit; (2) quartz bell; (3) electrode clamps; (4) mesh support plate; (5) current supply; (6) sinter disk; (7) support plate stands, hollow to allow water flow; (8) copper seals; (9) gas inlet; (10) base plate; (11) throw-over sealing ring; (12) flowsmoothing cell; (13) spring, hollow to allow water flow; (14) flame trap matrix; (15) pressure bell; (16) mesh. the sample is placed as a monolayer between two layers of a folded mesh (Figure 1; 16), which is resistance-heated between water-cooled electrodes (3). One electrode is spring-loaded to take up thermal expansion by the mesh; stainless steel tubes (13) serve, at once, as springs and current and water carriers. Gas enters through the bottom (9) and is directed through a flow-smoothing section (6, 14) to a circular hole in the supporting plate (Figure 1; 4). To monitor lateral temperature differences, two thermocouples are placed in the sample holding section of the mesh. The bank of tubes (14) initially installed to reduce gas turbulence and enable adequate temperature control at the mesh, at high pressures,68 has subsequently been replaced with a flame trap matrix, which reduces the characteristic flow diameter still further and allows stable operation. A thin layer of mica provides electrical insulation between the mesh (16) and the support plate; the latter was water-cooled to remove heat from parts of the mesh not swept through nor cooled by the gas stream. By sweeping through the sample holding part of the mesh, the gas stream serves to remove evolving volatiles away from the reaction zone into a quartz bell (2), the offtake tube (1), and liquid nitrogen (or ice) cooled, packed tubular steel tar traps (not shown). The present on-line data acquisition, temperature control, and heating system is based on a 486DX machine with Turbo Pascal 6.01 as the operating language. Details of the basic data acquisition, control, and safety monitoring systems have been described elsewhere.72,73 The thermocouple sampling frequency has been increased from 50 to 100 Hz, allowing close control of the temperature and the current passing through (72) Xu, B.; Dix, M.; Kandiyoti, R. Rev. Sci. Instrum. 1995, 66, 3966. (73) Cai, H.-Y.; Megaritis, A.; Messenbo¨ck, R.; Dix, M.; Dugwell, D. R.; Kandiyoti, R. Fuel 1998, 77, 1273.

the mesh. The new control system allows preprogramming of virtually arbitrary time-temperature histories. Operation can be switched simply between K-type and S-type thermocouples using four data acquisition channels. Adjustments to increase the sampling frequency of the temperature to 200 or 400 Hz have been provided. Power is adjusted by on-line PID control, and parameters for operation in helium and in CO2, at heating rates between 1 and 10 000 °C s-1, may be called up. S-type thermocouples were used during high-pressure CO2 gasification and all experiments in steam, while less durable K-type thermocouples were retained for use during analogous pyrolysis experiments and atmospheric-pressure CO2-gasification runs. Modifications for Operation in CO2 and Steam. The wire-mesh instruments described above were originally designed for operating mostly in helium and hydrogen. CO2 and steam have higher densities and heat capacities and much lower thermal conductivities. To attain a given temperature, operation with these gases, therefore, requires greater power input into the mesh. During operation with CO2, this combination of conditions led to overheating in parts of the mesh not cooled by the sweep gas stream. The thin layer of mica providing electrical insulation between the mesh and the support plate (between 4 and 16 in Figure 1) melted frequently, causing short circuits and destroying the sample holder. While CO2 gasification at 1000 °C could still be undertaken below 30 bar and holding times shorter than 10 s (ref 27), design changes were required to ensure reliable operation over a wider range of parameters. Figure 2 presents the new design of the support plate, with a circular pattern for the flow of cooling water, to suppress local overheating in parts of the wire mesh not swept by the gas stream, particularly of parts furthest away from the watercooled electrodes. In addition, the mica insulation was replaced by a 2 mm thick sheet of ceramic (Macor Machinable Ceramics; Goodfellows, U.K.), drilled through to open a 30 mm hole to mirror the shape of the support plate. These changes have allowed reliable runs with holding times up to 60 s at pressures up to 30 bar in CO2 and steam. Severe problems were anticipated during operation with steam. The first requirement for carrying out a steam gasification experiment is the absence of condensation inside the reactor vessel, where all components except the mesh itself are normally kept at ambient temperature. Furthermore, any condensation on the mesh would lead to difficulties with the electrical resistance heating circuit. Reproducible steam flow rates, with even distribution across the reaction zone and reliable performance over a range of steam/sweep-gas ratios, while operating between 1 and 30 bar, were the other design requirements.


Figure 3. Steam injection unit and the wire-mesh reactor: (1) water reservoir; (2) filter; (3) displacement pump; (4) steam heater; (5), (6) on/off valves; (7) steam bypass collector; (8) sweep gas; (9) safety valve; (10) flow control valve; (11) gas heater; (12) nonreturn valve; (13) mixing point of steam and gas; (14) wire-mesh reactor; (15) countercurrent condenser; (16) flow control valve; (17) water collector; (18) cold trap; (19, 20) flowmeter; (21) transformer; (22) watchdog. P ) pressure, T ) temperature, C ) controller, I ) indicator; (s) tubing, (-‚-) thermocouple lines, (-‚‚) electric current lines. Preliminary work26 indicated that preventing steam condensation by preheating the flow path of the sweep gas stream, including the mesh itself, would be feasible. A new gas inlet, comprising a flow distributor [two sinters separated by a settling chamber (6)] and a stainless steel housing (12) were constructed along the lines of the existing design but with Teflon seals and other temperature-sensitive components replaced with others made of copper; the plastic insulation of the thermocouple wires was replaced with drawn glass capillaries. Figure 3 presents a schematic diagram of the steam injection unit. The design was based on introducing the minimum amount of steam into the reaction chamber during a given run. Before an experiment, the steam path including the flowsmoothing section and the mesh itself were preheated by a stream of helium and the steam was allowed to contact the coal sample only instants before the temperature of the sample was ramped. Helium Flow. The “dry” sweep gas (8, only He in this paper) flowed through a heater (11, Thermospeed 308-412) and a nonreturn valve (12) to prevent steam from entering the gas line during operation. Prior to steam injection, the thermocouple (13) was used to determine the sweep gas temperature. Steam Flow. While the steam path was preheated with helium, water was pumped by positive displacement (3, Magnus P4000/D) from a 2-L reservoir (1) through a filter (2, Fisons mobile-phase filter) into the steam generator (4). A [Thermospeed] pressure indicator (PI) was placed between the pump and the steam generator. The latter consists of a 1 in. i.d. AISI 316 tube filled with 3-5 mm glass beads, heated by a resistance heater [Electrothermal Engineering HC503, HC504], and insulated by ceramic fiber to reduce heat loss. The temperature on the outside of the tube was monitored by

Messenbo¨ ck et al. a thermocouple and fed to a temperature controller [Thermospeed Lae MTW11 on/off controller] preset to (usually) 370 °C. Before triggering a run, the on/off valve at the exit (5) was kept closed and the steam was continuously discarded through a bypass line and an on/off valve (6) to a reservoir (7). The temperature of the steam-helium mixture was independently monitored at the exit of the generator at the point where the two steams were mixed (13); the mixture temperature was usually close to the steam temperature. Experimental Procedure. During operation, the steam and helium heaters were set at 300 °C, with typical steam and helium temperatures of about 280 and 250 °C, respectively. The temperature of the mixture was found to vary between 260 and 270 °C, well above the condensation temperature of steam at 20 bar (212.4 °C), the maximum steam pressure used in this study. Before an experiment, the temperature of the mesh did not normally exceed 150 °C, the large thermal mass of the electrodes and support plate absorbing most of the heat input. Prior to initiating a run, therefore, the mesh was electrically preheated to 300 °C to prevent condensation of the steam. Preheating to 300 °C for short periods before a run does not appear to alter the sample behavior significantly. A pyrolysis run was carried out using the same temperature sequence as a steam-gasification run (but with no steam). At 1 bar and 10 s, the yields and the combustion reactivity of the char residue were similar to those from a pyrolysis experiment where the temperature had been directly ramped from ambient to 1000 °C. Before introducing the steam, the reactor pressure was set by adjusting the pressure let-down valve (16). As the steam passing through the reactor was condensed (15) before reaching the flow control valve (16), the overall flow rate did not change significantly when steam was introduced into the system. The volumetric contribution of condensed water and water vapor carried by the saturated He to the overall flow through the valve was less than 5%. Controlling the reaction pressure by presetting the let-down valve before switching-in the steam did not, therefore, prove problematic. Samples were heated at 1000 °C s-1 to 1000 °C. The hold time at 1000 °C was varied between 0 and 60 s and the pressure from 1 to 30 bar. In pyrolysis and CO2-gasification experiments, the sweep gas was passed over the mesh at 0.1 m s-1. In steam gasification experiments, an 80:20 steam: helium mixture was used and the helium flow rate was set at 0.02 m s-1, with a total gas flow through the sample holder of about 0.1 m s-1. The overall objective was to keep the gas velocity through the mesh constant; hence, the mass flow rate of the gas was allowed to change with pressure. To carry out a steam gasification run, the steam and the temperature ramp were switched on simultaneously. Figure 4 presents a typical time-temperature plot from an experiment with steam injection, showing temperature traces from the two thermocouples together with the average temperature (used as the control signal) and the plot of the power input level. Temperature stability during steam injection was found to be comparable to levels of stability observed during experiments with any of the dry gases. At the exit of the reactor, the steam/gas mixture was cooled under pressure (15). The water was then collected in a cold trap (17) cooled by liquid nitrogen (18); the dry gas flow was measured by rotameters (19 and 20, for two different flow ranges) installed downstream of the condensers. Total volatile yields were determined by determining the difference in sample weight before and after experiments.61,62 Sample. The sample used in this preliminary stage of the study was Daw Mill coal, a U.K. noncaking coal: vitrinite 66% (v/v, mmf), exinite 13% (v/v, mmf), inertinite 21% (v/v, mmf); the average vitrinite reflectance of the sample was 0.6. The proximate analysis of the sample was moisture 6.1% (a.d. (as determined)), ash 4.4% (a.d.), fixed carbon 53.8% (a.d.),



Figure 6. Daw Mill CO2 gasification total volatile yields (1000 °C s-1, 1000 °C, CO2).

Figure 4. Typical time-temperature plot during a steam run with the new control system.

Figure 5. Daw Mill pyrolysis total volatile yields (1000 °C s-1, 1000 °C, helium). volatile matter 39.9% (daf). The elemental composition was as follows: carbon 80.1% (daf), hydrogen 4.7% (daf), nitrogen 1.3% (daf), organic sulfur 1.12% (db), sulfate sulfur < 0.1% (db), pyritic sulfur 0.28% (db), oxygen 11.5% (daf). Samples were dried for 18 h in a vacuum at 50 °C and stored under nitrogen. Typically, the amount of sample used for an experiment was between 5 and 7 mg. The particle size range was 106-150 µm.

Results and Discussion Figure 5 presents total volatile yields from the pyrolysis of Daw Mill coal under He pressures between 1 and 30 bar for 0, 10, and 60 s holding times at peak temperature (1000 °C). The main observable effect was the expected decrease of yields with pressure and a small increase in sample weight loss between 0 and 10 sec; the effect is well-known from the earlier pyrolysis work of Howard et al. (e.g., cf., ref 44). Differences between 10 and 60 s were within experimental error ((1%). Changes in sample weight loss over the pressure range (1-30 bar) were no greater than about 8%. Figure 6 presents total volatile yields from CO2gasification experiments at 1000 °C, showing changes in conversion as a function of pressure (1-30 bar) at holding times of 0, 10, 20, and 60 s. The similarity

between the curve for zero-holding and data from pyrolysis is clear, and events during heatup appear to be nearly totally confined to devolatilization: some gasification (perhaps 1-3%) appears to take place during heatup, but differences were small. At the other extreme of the hold-time range (60 s), a sharp increase in weight loss was observed between 1 and 10 bar; conversions at 30 bar were found to be as high as 92%. For long hold-time experiments, the same total volatile yields have been confirmed by a fluidized bed reactor.25 However, apart from hydropyrolysis results already referred to above, there appears to be no gasificationrelated work carried out in wire-mesh-type reactors to compare with our data. Comparable time resolution is not available in the vast amount of published data on the basis of TGA instruments. Figure 6 also shows that results from 10 s holding experiments traced a minimum in the vicinity of 10 bar. Similar trends have been observed within the context of hydropyrolysis/hydrogasification experiments. As first discussed by Howard et al.,44 at the lower pressures the physical suppression of devolatilization (by the external pressure) appears to cause loss of volatile matter while at higher pressures the reactivity of the gas appears to enhance increasing sample weight loss. Judging by the zero-hold-time results, the suppression of devolatilization due to pressure appears to take place (almost totally) before the end of the heatup period. We have shown elsewhere that the effect is due mainly to suppression of tar evolution.68 At this heating rate, the gasification process appears to go through several successive stages involving volatile suppression (mainly by tar repolymerization), gasification of the (relatively inert74) secondary char, followed by direct gasification of char residue. However, the results in Figure 6 show that observing a minimum and increasing yields depend not only on the pressure (related to the reactivity of the gas and the char) but also on the holding time at the given peak temperature. Experiments with 20 s holding did not show a minimum, suggesting that the secondary-char layer had already been consumed at the end of the 20 s period and that significant gasification of the char particle had taken place. Changes in weight loss as a function of hold time may be obtained from cross-plotting the data in Figure 6. At (74) Guëll, A. J.; Cai, H.-Y.; Dugwell, D. R.; Kandiyoti, R. Fuel Process. Technol. 1993, 36, 259.


Messenbo¨ ck et al.

Figure 7. Daw Mill steam gasification total volatile yields (1000 °C s-1, 1000 °C, steam).

Figure 9. Daw Mill pyrolysis and gasification total volatile yields (1000 °C s-1, 1000 °C, 10 s).



the higher pressures, much of the weight loss appears to take place during the early stages of the experiment (over 15% during the first 10 sec at 30 bar). Figure 7 presents changes in sample weight loss during steam gasification as a function of holding times up to 60 s at three different pressures (1, 10, and 20 bar). Differences between yields at 1 and 10 bar were large, qualitatively reflecting the results for CO2 gasification. Differences between 0 and 10 s holding were again large, particularly at the higher pressures. At 20 bar, gasification of the sample (105-152 µm) was virtually completed after 20 s, leaving nothing but small ash particles on the sample holder. In Figure 8, results from 0s holding experiments from gasification in CO2 and steam have been compared with data from pyrolysis: at atmospheric pressure, a difference of about 4% was observed between the pyrolysis and gasification results. A small amount of gasification during heatup may, therefore, be inferred. Parallel sharp drops in sample weight loss were observed between 1 and 10 bar. In He and CO2, the decline was monotonic over the experimental range. However, above 10 bar, volatile yields in steam traced a minimum similar to the one observed earlier for 10 s holding under CO2 (Figure 6). The effect appears associated with the greater reactivity of steam under similar conditions. With 10 s holding in steam (Figure 9), the minimum appears to have disappeared; clearly, the observation of a minimum depends on a delicate balance between (gas and solid) reactivities, pressures, and holding times. Figure 9 also shows the (expected) sharp difference in reactivity between steam and CO2. The earlier release


of volatiles in steam would be expected to have implications on bed stability in fluidized bed gasifiers. Figure 10 (20 s holding) shows that results for the CO2 experiments (single points only) lie on a straight line, while yields in steam gasification approach total gasification at 20 bar. After 60 sec (Figure 11), gasification in steam appears to be complete at the higher pressures. Conversions in CO2 also reached high levels: the high yields observed at 30 bar were reproducible. However, it may be observed that after 60 s, gasification in CO2 appeared to have reached completion. At 10 bar (Figure 12), a similar effect may be observed at 60 s at a lower conversion level (approximately 80%), apparently due to the deactivation of the char by prolonged exposure to high temperature.


Figure 12. Daw Mill pyrolysis and gasification total volatile yields (1000 °C s-1, 1000 °C, 10 bar).

Summary and Conclusions The development of a steam injection facility has been described, extending the use of a high-pressure wiremesh reactor to gasification in steam as well as the more usual noncondensable gasifying agents. The new design is based on preheating the steam path, including the flow-smoothing section and the mesh itself; the steam is allowed to contact the coal sample only instants before the temperature of the sample is ramped. Sample weight loss in steam gasification has been determined with a repeatability of less than (1%, similar to the experiments in dry gases. Results of experiments carried out in helium, CO2, and steam (up to 30 bar, 1000 °C) using holding times between 0 and 60 s have been compared. (1) The evolution of gasification conversions vs pressure diagrams (CO2 and steam) has been presented for


several holding times between 0 and 60 s at 1000 °C. These data have shown that the occurrence of a minimum in conversion vs pressure diagrams, previously observed by numerous researchers, appears to be related to the reactivity of the gasifying agent and the holding time. Compared to experiments in CO2, the minimum was observed at shorter holding times in the presence of (more reactive) steam. The minimum appears to be related to deposition and eventual consumption of the secondary char deposited during the pyrolysis stage; at longer hold times, the secondary-char layer appears to be consumed and conversions, apparently due to direct gasification of the char particle, then increase monotonically. (2) In both CO2 and steam, only a small amount of gasification was observed during heatup. At 10 and 20 bar, the sample appears to be completely consumed in steam in about 60 s. However, it was observed that after 60 s in CO2, the process appeared to have reached completion around 92%, apparently due to the deactivation of the char by prolonged exposure to high temperature. At 10 bar, a similar effect may be observed at 60 s at a lower conversion level (approximately 80%). Acknowledgment. Support for this work by the European Union under Contract Nos. JOF3/CT95/0018 and ECSC 7220-ED/075 and the British Coal Utilization Research Association (BCURA)/Department of Trade and Industry (DTI) under Contract No. B38 is gratefully acknowledged. EF980135E

Coal Gasification in CO2 and Steam: Development of a Steam

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