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Zero-Emission Carbon Concept (ZECA): Equipment Commissioning and Extents of the Reaction with Hydrogen and Steam Lu Gao, Nigel Paterson,* Denis Dugwell, and Rafael Kandiyoti Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom ReceiVed September 6, 2007. ReVised Manuscript ReceiVed October 31, 2007
A high-pressure wire-mesh reactor has been modified to investigate the reactions underlying the zero-emission carbon concept (ZECA) process. This is a novel power generation concept that involves producing hydrogen from coal. The first step involves reacting coal in a steam-hydrogen mixture to form primarily CH4. The product gas is then steam-reformed and shifted to maximize the H2 content. CO2 is removed by the carbonation of CaO, to give a nearly pure stream of H2. The CaCO3 is calcined in a separate reactor to release the CO2 stream for storage. This paper outlines initial results from experiments simulating the carbon-hydrogen and carbon-steam reactions taking place in the gasifier. Experiments were carried out under inert gas (pyrolysis) and under reactive gas atmospheres. Extents of the reaction were calculated by subtracting the weight loss during pyrolysis from conversions in a reactive atmosphere under otherwise the same experimental conditions. Therefore, data presented in this way show the impact on the level of conversion caused by the presence of H2. It has not been possible to directly measure the amount of CH4 formed with this type of reactor. Initial results have shown that under 7 MPa of pure hydrogen, at temperatures between 750 and 1050 °C, and with a 10 s hold time, between 15 and 25% (w/w, daf) of Daw Mill coal (U.K.) reacted with H2. Rates of the reaction were rapid during the pyrolytic stage, the first few seconds of the test, but then rapidly declined to low values. Preliminary tests have been performed to assess the impact of the reaction with steam under the ZECA conditions. The data indicate extents of the reaction that are greater than those achieved in H2 alone, so that, when the two reactants are present together, moderate to high overall conversions were achieved with the bituminous coal.
Introduction There are serious concerns about the impact of the increasing concentration of CO2 in the atmosphere of the earth on the world climate and the potential for an enhancement of the greenhouse effect. On a worldwide basis, the use of fossil fuels (particularly coal and oil) is increasing, with large increases in usage in countries with developing and transitional economies, in addition to projected increases in already industrialized countries. Although, direct, conclusive evidence linking increased CO2 concentrations to observed global warming is lacking, there is a growing consensus that it is sensible to attempt to control emission rates of a range of pollutants (CO2, NOx, and SOx). Europe and the U.S.A. are independently moving toward stabilizing emission rates (particularly CO2) at lower than current emission levels, such that their contribution to the world atmosphere does not cause enhanced warming. In the case of the debate on CO2 emissions, this amounts to a “least-regrets” policy. Present trends toward increased energy demand and its ramifications on the CO2 climate-change debate have given rise to a wide range of proposed technical solutions. These include process routes that focus on the optimization of existing technologies as well as the development of new concepts. Advanced combustion technologies and various types of gasification systems are being promoted to minimize the impact of coal use on the environment. One strand of development
involves routes that are generically termed “near zero-emission technologies”. From the vantage point of developed economies, the strategy envisages a significant future for coal, “provided ways can be found materially to reduce its carbon emissions”.1 The concept of zero emissions embraces the capture and disposal of CO2 and other polluting species, such as particulates, mercury, sulfur, nitrogen, and organic compounds. The only releases to the atmosphere would be N2 and water vapor. This objective can be achieved by the gasification of coal to produce an impure fuel gas (i.e., containing N2, H2, H2O, CO, CO2, CH4, H2S, etc.). This gas is then subjected to further downstream processing to maximize the concentration of H2 and remove the environmentally sensitive gases by a combination of absorption and catalytic decomposition. The H2 may be used to generate power in fuel cells or gas turbines. However, the pollutant capture systems impose a thermal penalty on the overall process and reduce the overall efficiency. When coal gasification is adapted to produce a stream of H2, with the sequestration of CO2, there is an efficiency penalty of about 20%. In other words, more coal has to be used to generate the same amount of power from a zeroemission process than for a conventional gasification plant. There is therefore an incentive to promote ways of raising the efficiency of other parts of the process to compensate (at least in part) for this CO2 removal penalty. The gasification systems that have been developed to date use steam gasification to produce either fuel or synthesis gases.
* To whom correspondence should be addressed. Fax: 00-44-2075945604. E-mail:
[email protected].
(1) Our Energy Future—Creating a Low Carbon Economy. Her Majesty’s Stationery Office (HMSO), Cm5761, February, 2003.
10.1021/ef700534m CCC: $40.75 2008 American Chemical Society Published on Web 11/30/2007
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Figure 1. Basic flow diagram of the ZECA process.
The enthalpy required for the process to run is produced by introducing O2 with the steam.2 Alternatively, air-blown gasification can be used, but the calorific value of the fuel gas is low because of the nitrogen allowed by the use of air.3 Many of these processes have found limited commercial applications, in Europe, the U.S.A., India, and China, primarily for making synthetic chemicals (methanol, ammonia, etc.) and for power generation. The climate-change debate and associated process constraints have encouraged the development of more novel process concepts, which can potentially raise the efficiency during power generation. Aspects of one potential process have been studied at Imperial College London and the University of Cambridge in the U.K. The process, known as the zero-emission carbon concept (ZECA), was first proposed by researchers at the Los Alamos National Laboratory (LANL) and Louisiana State University in the U.S.A. It is generally referred to as the LANL ZEC technology.4 The work falls within the Memorandum of Understanding between the U.K. and U.S.A. to cooperate on aspects of clean-coal research. ZECA. The process is shown as a block diagram in Figure 1. It involves the gasification of coal in H2 (hydrogasification) and is designed to avoid using O2 for combustion in the provision of process energy. This integrated process route has not been studied experimentally, although different parts of the concept have been tested (e.g., hydrogasification and CaO-driven steam reforming/shifting of CH4). However, an independent study by Nexant, Inc. (a Bechtel-affiliated company)5 has concluded that the concept is potentially viable and efficient and that experimental studies are needed to determine reaction conditions, prior to developing a detailed process flow sheet. Coal is gasified in H2, at a pressure of approximately 7 MPa, to produce CH4 via the methanation reaction (1) C + 2H2 ) CH4 (methanation reaction)
∆H ) -75 kJ/mol (1)
Reaction 1 is exothermic, and steam would be added to moderate the temperature to approximately 900 °C. Next, CH4 is steamreformed to produce H2 (reaction 2), and the water gas shift (2) Gasification of Solid and Liquid Fuels for Power Generation. Technology Status Report 008, Department of Trade and Industry (DTI), December, 1998. (3) Dawes, S. G.; Mordecai, M.; Brown, D.; Burnard, G. K. The Air Blown Gasification Cycle. Proceedings of the 13th International Conference on FBC, Orlando, FL, May, 1995. (4) Solutions for the 21st Century, Zero Emissions Technologies for Fossil Fuels. OECD/IEA Technology Status Report, May, 2002. (5) Ruby, J.; Johnson, A.; Ziock, H.; Lackner, K. Proceedings of the 27th International Technical Conference on Coal Utilization and Fuel Systems, Clearwater, FL, March, 2002; pp 767–778.
reaction (reaction 3) is used to maximize the H2 content of the gas CH4 + H2O(g) h CO + 3H2 (reforming reaction) ∆H ) +206 kJ/mol (2) CO + H2O(g) h CO2 + H2 (shift reaction) ∆H ) -41 kJ/mol (3) CaO + CO2 h CaCO3 (caronation reaction) ∆H ) -179 kJ/mol (4) Overall, the H2-producing reactions are endothermic. The heat requirement is provided by reaction 1 and by the carbonation of CaO (reaction 4). The latter reaction provides the major portion of the energy and serves to remove the CO2 from the reaction mixture. The resulting shift of the equilibrium in reaction 3 ensures that the H2 concentration is maximized and provides the means to collect a concentrated stream of CO2. Reaction temperatures may be limited by the melting properties of the coal and its mineral matter, the need to avoid the formation of eutectic liquid phases6 in the solid sorbents used to remove CO2, and the equilibrium-controlled inability of CaO to bind CO2 at temperatures above 900 °C at a CO2 partial pressure of less than 0.1 MPa. To ensure effective energy use, reactions 2-4 need to be conducted in a single reactor. Overall, the scheme results in the formation of an extra 2 mol of H2, for each 2 mol of H2 used in the methanation stage. The H2 for the methanation is recycled from the H2 stream remaining after the further processing of the gas. The bound CO2 is subsequently released as a concentrated stream for sequestration by heating CaCO3. Overall, the same amount of CO2 is formed with both ZEC and conventional technologies; however, the ZEC process potentially has a higher efficiency, because losses are avoided by the integration of the various process steps. It also isolates the CO2 as a nearly pure stream for storage, by use of the carbonation/calcination cycle. The ZECA Corporation has funded some initial studies of the hydrogasification properties of several U.S. coals.7 The work was performed at the Gas Technology Institute, using an isothermal test procedure in a high-pressure thermogravimetric analyzer (PTGA). The tests were performed at pressures approaching 7 MPa and temperatures around 900 °C, using atmospheres of pure H2 and steam/H2 (50:50). Fuels used were Illinois No. 6 (bituminous) coal, Antelope (sub-bituminous) coal, (6) Paterson, N.; Elphick, S.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2001, 15, 894–902. (7) Ziock, H.-J.; Anthony, E. J.; Brosha, E. L.; Garzon, F. H.; Guthrie, G. D.; Johnson, A. A.; Kramer, A.; Lackner, K. S.; Lau, F.; Mukundan, R.; Nawaz, M.; Robison, T. W.; Roop, B.; Ruby, J.; Smith, B. F.; Wang, J. Progress in the Development of Zero Emission Coal Technologies. Proceedings of the 28th International Technical Conference on Coal Utilization and Fuel Systems, Clearwater, FL, 2003.
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Table 1. Times for 50% Reaction in Hydrogen and the Steam/ Hydrogen Mixturea time for 50% carbon conversion (min) coal
temperature (°C)
H2 (100%)
steam/H2 (50:50)
Illinois No. 6 Antelope Saskatchewan
871 875 875
45 68 7
8.3 3.7
a
Pressure ) 6.6 MPa.
and Saskatchewen lignite. The bituminous and sub-bituminous coals were partially devolatilized prior to the PTGA tests, so that the hydrogasification of the char could be studied. Some results are shown in Table 1, and these illustrate the main findings of the work. The above data have been taken from the figures in the reference and are expressed as the time taken for half of the base carbon to react. It seems that the reaction is relatively slow in pure H2 with the bituminous and sub-bituminous coals. The lignite reacts at an appreciably higher rate. The carbon reacts at a much higher rate in the steam/H2 mixture, but even in this atmosphere, many minutes are needed to react half of the material. The data obtained give a useful insight into the extent of the reaction; however, the impact of the conditions of the partial devolatilization of the bituminous and sub-bituminous coals on the reactivity is not certain and is likely to have influenced the reactivity. The details of the conceptual plant have not yet been developed, pending the collection of further data relating to the reaction processes occurring in the various unit processes. The current project aims to provide key aspects of the missing data. Imperial College is investigating the hydrogasification/pyrolysis/ steam gasification reactions, and Cambridge University is studying the reactions in the reforming/shift conversion/ carbonization stages. In the gasifier, the rate of methane formation is promoted by high H2 pressures and high temperatures, although the equilibrium concentration of CH4 is greater at lower temperatures.8 Reaction 1 has not been studied in detail under conditions appropriate for H2 production in a ZEC-like process. In addition, the relative rates of gasification in the presence of hydrogen and steam (together) at high pressure are not known and need to be established. These data would enable the evaluation of conceptual mass and energy balances for beginning to design the process. The parallel project at Cambridge University is investigating the carbonation/calcination reactions of limestone9 and their integration with the shift conversion and steam reforming stages of the process. Studies on hydropyrolysis and hydrogasification were conducted in the early 1990s in this laboratory,10 to investigate the potential of the reactions to form useful liquids from coal. This work involved equipment development to enable tests to be performed at high pressures with the direct capture of evolved tars. Studies were performed in atmospheres of pure H2 or He, and the effects of total pressure and the heating rate were studied at temperatures up to 850 °C. With a heating rate of 1000 K s-1, the tar yield in H2 was observed to decrease with an increasing pressure, with an accompanying rise in the total (8) Elliot, M. A. Chemistry of Coal Utilization (Supplementary Volume); Wiley: New York, 1981; Chapter 23. (9) Fennell, P. F.; Pacciani, R.; Dennis, J. S.; Davidson, J. F.; Hayhurst, A. N. The Effects of Repeated Cycles of Calcination and Carbonation on a Variety of Different Limestones, as Measured in a Hot Fluidized Bed of Sand. Energy Fuels 2007, 21, 2072–2081. (10) Guell, A. J.; Kandiyoti, R. Energy Fuels 1993, 7, 943–952.
volatile yield; i.e., a higher proportion of gas and low boiling point organics were formed at the higher pressures. Trends in tar and volatile yields were measured as a function of the reaction conditions. The current work represents an extension of this earlier work to higher temperatures and the addition of steam to hydrogen upon the extent of the reaction and tar formation. In this paper, further development of the equipment is described for combined steam/H2 injection, at temperatures up to 1050 °C. Data on the effects of pressure, temperature, heating rate, incoming reagent gas composition, and hold time (residence time) at the peak temperature upon the extent of conversion by H2 and steam are given. Experimental Section Wire-Mesh Reactor. The experiments described below have been carried out in a high-pressure wire-mesh reactor (WMR), modified for H2/steam injection, at temperatures up to 1050 °C and 8 MPa. This is a reactor configuration that has proven very versatile and has been used for a variety of pyrolysis-, gasification-, and combustion-related experiments. Originally conceived by Loison and Chauvin,11 the wire-mesh reactor configuration was adapted by Jüntgen and van Heek12 for work under vacuum and connected to a mass spectrometer. This reactor configuration is best known through the early work of Howard and co-workers and Suuberg and co-workers.13 Coal pyrolysis and hydropyrolysis literature up to 1979 and the work by Howard and co-workers have been exhaustively reviewed.14 Another wire-mesh instrument was constructed at Bergbau-Forschung for operation under elevated pressures.15 Niksa et al.16 constructed a high-pressure wire-mesh cell at Princeton University. Further work by researchers at Massachusetts Institute of Technology (MIT) has also been reported,17 and some equipment development was introduced in later work.18 A fuller account of the development of wire-mesh reactors including distinguishing features of the various instruments has recently been given by Kandiyoti et al.19 Figure 2 presents the schematic diagram of the high-pressure WMR developed at Imperial College London, as configured more recently. The reactor features water-cooled electrodes, a wider span of heating rates and larger mesh than most applications, and a stream of sweep gas designed to carry evolving volatiles away from the (11) Loison, R.; Chauvin, R. Chim. Ind. Paris 1964, 91, 269; Translation by University of Sheffield (DJB/WBD) May, 1964; National Coal Board, Coal Research Establishment Library (September, 1964). (12) Jüntgen, H.; van Heek, K. H. Fuel 1968, 47, 103. (13) (a) Suuberg, E. M.; Unger, P. E. 18th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1981; 1203. (b) Suuberg, E. M.; Unger, P. E.; Lilly, W. D. Fuel 1985, 64, 956. (c) Unger, P. E.; Suuberg, E. M. Fuel 1984, 63, 606. (d) Unger, P. E.; Suuberg, E. M. Prepr. Pap.-Am. Chem. Soc., DiV. Fuel Chem. 1983, 28, 278. (14) Howard, J. B. Chemistry of Coal Utilization (Second Supplementary Volume); Elliott, M. A., Ed.; Wiley: New York, 1981; Chapter 12. (15) Arendt, P. Ph.D. Thesis, University of Aachen, Germany, 1980. (b) Arendt, P.; van Heek, K.-H. Fuel 1981, 60, 779. (16) (a) Niksa, S. J.; Russel, W. B.; Saville, D. A Fuel 1982, 61, 1207. (b) Niksa, S. J.; Russel, W. B.; Saville, D. A. 19th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1982; 1151. (c) Niksa, S. J.; Heyd, L. E.; Russel, W. B.; Saville, D. A. 20th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1984; 1445. (17) (a) Fong, W. S.; Khalil, Y. F.; Peters, W. A.; Howard, J. B. Fuel 1986, 65, 195. (b) Fong, W. S.; Peters, W. A.; Howard, J. B. Fuel 1986, 65, 251. (18) Howard, J. B.; Peters, W. A.; Derivakis, G. S. Energy Fuels 1994, 8, 1024. (19) Kandiyoti, R.; Herod, A. A.; Bartle, K. D. Solid Fuels and HeaVy Hydrocarbon Liquids: Thermal Characterization and Analysis; Elsevier Science Publishing: Amsterdam, The Netherlands, 2006; ISBN: 0-08044486-5; cf. Chapters 3 and 4. (20) Messenböck, R.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 1999, 13, 122.
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Figure 2. High-pressure WMR.20 (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 seal, (9) gas inlet, (10) base plate, (11) throw over sealing ring, (12) flow smoothing cell, (13) spring, hollow to allow water flow, (14) wound corrugated tube, (15) pressure bell, (16) mesh, and (17) line to pressure gauge.
heated reaction zone into a liquid-nitrogen-cooled quench section. This reactor system has been extensively used and modified for multiple applications.21–28 The present application of the WMR necessitated a return to the original hydropyrolysis configuration, modified to accept steam injection. A previously developed steam injection system constructed around the high-pressure WMR29 was modified for higher pressure operation.30 The system is based on the evaporation of a measured flow rate of water in a tube heated to 100 °C above the boiling point of water at the required test pressure (boiling point of water is 280 °C at 7 MPa). The whole gas flow path of the steam is preheated, using electrical heating tapes, to avoid recondensation, before it reaches the mesh. In addition, any gas that is to be mixed with the steam is also preheated to avoid condensation upon mixing. A schematic diagram of the equipment is given in Figure 3. The test program required the reactor to operate at pressures up to 8 MPa, in atmospheres containing pure He, H2, steam, or combinations of these gases. Calculations were performed to check the pressure/temperature integrity of the pipe work, and (21) Gibbins, J. R.; Kandiyoti, R. Energy Fuels 1988, 2, 505. (22) Gibbins, J. R.; Kandiyoti, R. Energy Fuels 1989, 3, 670. (23) Güell, A. J.; Kandiyoti, R. Energy Fuels 1993, 7, 943. (24) (a) Messenböck, R. C.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 1999, 19, 122. (b) Messenböck, R.; Dugwell, D. R.; Kandiyoti, R. Fuel 1999, 78, 781. (25) Peralta, D.; Paterson, N.; Dugwell, D.; Kandiyoti, R. Energy Fuels 2005, 19, 532. (26) Wang, B.; Li, X.; Xu, S.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2005, 19, 2006. (27) Wu, L.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2006, 20, 2572. (28) Wu, L.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2007, 19 (4), 2325–2334. (29) Messenböck, R.; Dugwell, D. R.; Kandiyoti, R. Fuel 1999, 78, 781– 793. (30) Guell, A. Development of a High Pressure Wire Mesh Reactor for the Direct Determination of Tar and Total Volatile Yields: Applications to Coal and Biomass Pyrolysis and Hydropyrolysis. Ph.D. Thesis, University of London, London, U.K., 1993.
Gao et al. rigorous pressure testing was performed before any tests were initiated. As an additional safety precaution, a H2 analyzer was used to monitor the atmosphere in the vicinity of the WMR pressure shell. An accurately weighed 5–6 mg sample is used for each test and is evenly distributed in the folded wire-mesh sample holder. The mesh is held between water-cooled electrodes and serves as a resistance heater. The fuel size is in the range of 105–150 µm and is determined by the aperture size of the mesh. The brass support plate under the mesh is lined with a 2 mm thick alumina sheet to prevent electrical contact between the sample holder and the support plate. A stream of gas is used to sweep the sample holding part of the mesh, carrying evolving volatiles away from the sample holder; this helps to minimize the interactions between volatiles, the char, and the mesh. This is an important advantage of the wire mesh technique, because it enables the assessment of the behavior of a single layer of particles. It avoids complications caused by the occurrence of secondary reactions between the char and volatiles, which can occur in other types of laboratory-scale reactors (e.g., fixed and fluidized beds). These can influence the reactivity of the char through the deposition of secondary carbon, and this can confuse the evaluation of data. The weight change (called the total volatile yield) of the mesh and sample during a test is the measure of the extent of the reaction. Total volatile yields can be determined with a repeatability of (1%. During experiments using an inert sweep gas, only pyrolysis reactions can occur and the weight loss shows the extent of loss of the fuel volatiles under the conditions used. When a reactive gas is used (H2, CO2, steam, diluted air, or mixtures), the weight change is due to the combined weight changes (nearly) successively caused by pyrolysis followed by the reaction with the sweep gas. Therefore, to obtain the weight change because of the reaction, two experiments must be performed: one under inert conditions to measure the weight change by pyrolysis and the other in the reactive atmosphere, under otherwise the same conditions. The pyrolysis result is subtracted from the result in the reactive atmosphere to obtain the weight change because of the reaction. The weight loss measured during a test shows how much the original sample converted to gases and condensable liquids (tars). The tar can be estimated by passing the exit gas from the wire mesh through a specially designed cold trap (cooled in liquid N2). The collected material can be weighed and characterized using a variety of methods (e.g., gas chromatography, mass spectrometry, and size-exclusion chromatography). It was not possible to measure the composition of the gas produced during tests in the WMR, because the concentrations of individual product gases will have been too low for reliable analysis. It is assumed that by deducting the result obtained during pyrolysis in He from a result obtained in the presence of H2, under otherwise similar conditions, then it is the impact of the presence of H2 that is being measured. Therefore, the value obtained shows both the extent of hydrogasification and any influence on the pyrolysis process. The difference between the total loss and the weight of tar gives an indication of the amount of gas that has been produced. Measurements of the amount of tar formed have not been performed in this phase of the work but are included in the next part of the study. The experimental procedure is relatively simple but exacting, because weight changes of 1–2 mg must be determined to within 1–2% reproducibility. The key to success in using a WMR is rigorous training and validation of the performance that a particular operator can achieve with the equipment. The procedure used at Imperial College is that the training is conducted using a laboratory standard coal sample, whose performance under a standard set of conditions in the equipment has been established by many previous workers. Of particular importance is the weighing procedure: a calibrated five-figure balance must be used, which is kept in a dry, draft-free enclosure. Other key factors include mesh preparation, sample placement on the mesh, and the method used to weigh the residue. Careful sample preparation is also important to avoid sample losses from the mesh: samples in the range of 105–150 µm are used with commercially available mesh. A fresh thermocouple pair is used for each experiment (using 0.05 mm wires), and these
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Figure 3. Schematic diagram of the steam injection system. (1) Water reservoir, (2) displacement pump, (3) steam generator, (4 and 5) on/off valves, (6) steam bypass collector, (7) gas cylinder, (8) pressure relief valve, (9) flow control valve, (10) gas heater, (11) nonreturn valve, (12) mixing point of steam and gas, (13) pressure transducer, and (14) transformer. P, pressure; T, temperature; C, control; solid lines, tubing; and dash-dot lines, thermocouple lines. Table 2. Fuel Analysisa
a
Table 3. Repeatability of Test Dataa
analysis
Daw Mill coal
moisture (% as analyzed) ash (% as analyzed) volatile matter (% db) C (%, daf) H (%, daf) N (%, daf) S (%, daf) O (%, daf) (by difference)
4.1 6.9 33.7 72.7 4.7 1.1 1.1 20.4
db ) dry basis. daf ) dry ash-free basis.
are tied through mesh apertures to ensure good thermal contact. In this project, particular attention had to be given to the preheating of the mesh and inlet lines for work using steam at high pressure. In addition, the mesh width had to be reduced in the part containing the thermocouples, to avoid overheating and mesh melting in parts that were not cooled. The WMR has been shown to be an extremely versatile and valuable tool for simulating the conditions in developing and commercial-scale coal use processes. It has been operated at temperatures up to 2000 °C, pressures up to 7 MPa, heating rates up to 5000 °C s-1, residence (hold) times up to 60 s, and in atmospheres containing H2, CO2, steam, N2, He, O2, and combinations of these gases. A heating rate of 1000 °C s-1 was used in the majority of the tests reported here. Fuel Samples. The analysis of Daw Mill coal (typical U.K. power station coal) used in this part of the study is given in Table 2.
Results and Discussion Repeatability of the WMR Test. Multiple tests, in He (0.1 MPa, 700 °C), showed that a repeatability of (2% (actual) was
total volatile yield in H2 (100%) (%, daf) hold time (s)
750 °C
950 °C
1050 °C
0
35.5 34.0 35.0
46.4 44.8 45.5
10
51.1 49.8 50.8
44.5 45.2 46.0 45.2 54.3 56.4 56.6
a
61.4 60.0 60.5
Pressure ) 7 MPa. Heating rate ) 1000 °C s-1.
achieved. This value was used as the acceptance criterion for tests at 7 MPa. Each test condition at 7 MPa was performed in triplicate, and the results were accepted, if the repeatability was within the (2% limit. Table 3 shows several sets of data, which show good agreement between individual test results that were achieved during tests with 100% H2. Pyrolysis of Daw Mill Coal under the “ZECA Conditions”. Figure 4 shows pyrolysis data obtained at pressures up to 7 MPa, with a hold time of 5 s, at 750 °C. The data show that the volatile release by pyrolysis decreased as the pressure was raised, with the effect diminishing at the higher pressures in the range studied. This effect is due to the increased external pressure, which retards the release of volatiles from the particles, and this increases the extent of intraparticle pyrolysis. This results in the deposition of secondary char (soot-like material) within the pores of the particles, which reduces their reactivity, until it has been removed by the reaction.
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Figure 4. Effect of pressure on the extent of pyrolysis. Heating rate, 1000 °C s-1; hold time, 5 s; temperature 750 °C. Figure 6. Effect of the hold time on total volatile yield during the hydropyrolysis/hydrogasification of Daw Mill coal at 850 and 950 °C. heating rate, 1000 °C s-1; pressure, 7 MPa.
Figure 5. Effect of the hold time on the extent of pyrolysis. Pressure, 7 MPa; temperature, 850 °C. Table 4. Effect of Temperature on the Extent of Pyrolysisa
a
temperature (°C)
total volatile yield (%, daf)
700 900 950
33.6 35.2 35.5
Pressure ) 7 MPa. Hold time ) 5 s. Heating rate ) 1000 °C s-1.
Figure 5 shows the effect of the hold time on the extent of pyrolysis at 7 MPa and 850 °C. The data show that pyrolysis was completed within the heating time to 850 °C (0 s hold time). There was no further change, within the repeatability limits of the test ((2%, actual), as the hold time was increased to 30 s. This is the expected trend, and it is well-established that pyrolysis is rapid under the conditions used and would be nearly complete by the time the peak temperature is reached.31 The effect of temperature on the extent of pyrolysis is shown in Table 4. The data show a minor increase in yield between 700 and 900 °C, with no further change at 950 °C. The data obtained in He have mapped out the effects of temperature, pressure, and hold time on the extent of pyrolysis. These data have been used to remove the effect of this reaction on the total volatile yield during tests with H2, steam, and H2/ steam mixtures. Hydropyrolysis and Hydrogasification of Daw Mill Coal. A preliminary suite of tests to measure the total volatile yield during tests with 100% H2 were performed, using Daw Mill coal as feedstock. The results are shown in Figure 6. Each data point shown on the graphs is the average of at least three determinations: the repeatability was (1.5%. The data show the total volatile yields (i.e., weight loss because of the combination of pyrolysis and hydrogasification) with peak temperatures of 850 and 950 °C. It is apparent that the zero second holding data at both temperatures are significantly higher (31) Pipatmanomai, S.; Herod, A. A.; Morgan, T. J.; Paterson, N. R.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2004, 18, 68–76.
than the pure pyrolysis data obtained in He (see Figure 5). This shows the impact of H2 during the pyrolysis stage of the process (i.e., the hydropyrolysis reactions). The result is consistent with H2 stabilizing free radicals formed during pyrolysis and favoring their release as volatiles. In the absence of H2, these species are more likely to repolymerize, harden, and form deposits as secondary char, within the structure of the primary char. Previous work evaluating the effect of hydrogen during heatup to 700 °C (cf. ref 9) had shown relatively small differences with simple pyrolysis. When the results are taken together, they suggest the reactivity of hydrogen to become more significant between 700 and 850 °C. After the pyrolysis/hydropyrolysis stage, the increase in the extent of the reaction is lower and the rate of weight loss declines with an increasing hold time. In this period of the tests, pyrolysis will have been completed and additional weight loss appears because of char hydrogasification. The char reaction rate is clearly much lower than that of the initial pyrolysis/ hydropyrolysis stage. Beyond a hold time of 10 s, the extent of the reaction is low and must be limited by the low reactivity of the char.32 In another project, the reactivity of the char has been measured under pyrolysis and air-blown gasifier conditions (similar temperatures to here but lower pressure and no H2 present). In that work, a significant decline in the reactivity of the char was observed over the range of residence times studied in this project, at temperatures between 900 and 1000 °C.33 Figure 7 shows the extent of the reaction with H2, as a function of the hold time and temperature. The values shown have been derived from the data shown in Figure 6 by subtracting the proportion of the total volatile yield attributed to the release of products of pyrolysis. Additional data for tests at 750 and 1050 °C are also given. The 0 s hold time data indicate the extent of the enhancement of the pyrolysis reactions caused by the presence of H2. At 750 °C, no effect is evident in these data; however, the extent of hydropyrolysis (at 0 s hold) increases with temperature up to 950 °C. It then stabilizes (because the values at 950 and 1050 °C are the same), which suggests that a limiting value for the extent of hydropyrolysis has been reached. At longer hold times, the data reflect the trends shown in Figure 6 but show the extent of the reaction of the char with H2 (i.e., hydrogasification). The data at 850 and 950 (32) Megaritis, A.; Zhuo, Y.-Q.; Messenbock, R.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 1998, 12, 144–151. (33) Cousins, A.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2006, 20, 699–704.
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Energy & Fuels, Vol. 22, No. 1, 2008 469
Figure 7. Extent of hydrogasification as a function of the hold time and temperature.
Figure 8. Extent of gasification by steam.
Table 5. Effect of the Heating Rate on the Combined Extents of Pyrolysis, Hydropyrolysis, and Hydrogasificationa total volatile yield (%, daf)
a
temperature (°C)
1000 °C s-1
5000 °C s-1
800 900
51.0 53.6
50.0 53.2
Pressure ) 7 MPa. Hold time ) 10 s. Flow gas ) 100% H2.
°C show an increase over the initial 10 s hold time, but the rate of rise progressively decreases to a low value. This increase must be due to hydrogasification, and the decreasing rate probably reflects the aging of the char. The data show that the maximum extent of the reaction with H2 (i.e., hydropyrolysis and hydrogasification) equates to 25% or slightly less of the fuel sample (daf basis) at either 950 °C and a 30 s hold time or 1050 °C and a 10 s hold time. A test was performed with a 50% H2/50% He mixture, at 950 °C and a 10 s hold time. The extent of the reaction with hydrogen was 4% (actual) lower than measured with 100% H2. This test was performed to enable an estimation of the extent of the reaction, assuming additivity, in a 50:50 H2/steam mixture, for a comparison with an experimentally measured value. The above tests were performed with a heating rate of 1000 °C s-1. Tests were also performed at a rate of 5000 °C s-1, to assess whether increasing the heatıng rate had an influence on the extent of conversion. The results in Table 5 show that there is no significant difference between the results obtained at the two heating rates. In our earlier studies,10 performed with a peak temperature of 700 °C and heating rates of 1 and 1000 °C s-1, a more complex relationship between the effects of an increasing heating rate and pressure on volatile yield was identified. It was observed that, at 0.1 MPa, the volatile yield increased slightly, with an increasing heating rate. As the pressure was increased, the effect of increasing the heating rate on the yield changed: at 1 MPa, it had virtually no impact, whereas at 7 MPa, it declined with an increasing rate. The change in behavior with pressure and the heating rate must reflect the combined effects of the time/temperature histories of the particles at the different heating rates and the effect of the increased resistance to the release of volatiles from the particles at the higher pressures. The present study has shown that at high heating rates (above 1000 °C s-1) and at high pressure (7 MPa) the volatile yield has reached a steady value, showing that any effects of time/ temperature must have stabilized. Steam Gasification of Daw Mill Coal. Figure 8 shows the effect of the temperature on the extent of conversion using a 50% steam/50% He mixture. The extent of conversion by pyrolysis has been removed from the results of the test; therefore, the data show the extent of the reaction caused by
Figure 9. Extent of conversion in a 50:50 H2/steam mixture.
the steam. Tests were performed with 50% steam in preparation for tests with H2/steam mixtures. Using 100% steam was not practical (or necessary) in this program of tests. The data show that, at 700 °C, the extent of the reaction with steam does not change with an increasing hold time, which suggests that the extent of the reaction is negligible. The values measured at low hold times were higher than the total volatile yield in He (35%, i.e., pyrolysis only), and this would be consistent with steam having an impact on the pyrolysis of the coal. At 850 and 950 °C, the total volatile yield with a 1 s hold is similar to the value at 700 °C, which suggests negligible steam gasification under this condition. However, with a 10 s hold, the extent of the reaction has increased to 16.5% at 950 °C (actual). The extent of the reaction with steam is slightly lower (by about 5% actual) than measured with 100% H2, at 950 °C, 7 MPa, and a 10 s hold time. Preliminary Tests with H2/Steam Mixtures. Figure 9 presents data obtained using a 50:50 H2/steam mixture. Hydrogen is there to convert C to CH4, and steam is present to gasify C (to CO and H2). In a large-scale plant, the H2/steam ratio would be used to control the temperature of the gasifier. With the most intense conditions tested thus far with the steam/H2 mixture (10 s hold, at 950 °C), the total conversion was 78.6% with the bituminous coal. The individual extents of conversion by H2 and steam under these conditions were 20.0 and 16.5%, respectively, which together with the pyrolysis conversion (35.5%) gives a total calculated conversion of 72.0%. This is lower than the measured value (by 6.6%, actual) and suggests a positive influence of the individual reactions on each other. Further studies are planned to widen the range of test conditions, to investigate the impact of the fuel type on the
470 Energy & Fuels, Vol. 22, No. 1, 2008
conversion during tests with H2, steam, and steam/H2 mixtures and to measure the amount of tar formed under the different conditions. Conclusions The WMR has been successfully adapted to enable reactions to be studied under conditions envisaged for the ZECA process. Data on the effects of temperature, pressure, and residence time on the extents of pyrolysis (in He) have been obtained. These results are being used to remove the effect of this reaction on the data obtained during tests in H2, steam, and H2/steam mixtures. Tests in pure H2 with a bituminous coal have revealed
Gao et al.
extents of hydrogasification between 15 and 25%, in the temperature range of 750-1050 °C. The extent of conversion in steam was slightly lower than in H2 under similar reactor conditions. Tests with a mixture of H2/steam have indicated that the conversion with the mixture was higher than that calculated from the results of tests with the individual gases. Acknowledgment. The authors acknowledge the funding received from the Engineering and Physical Sciences Research Council (EPSRC) and the Department of Trade and Industry (DTI) (contract number C/07/00379/00/00) for this project. EF700534M
