Energy & Fuels 2000, 14, 483-489
483
Char Gasification with O2, CO2, and H2O: Effects of Pressure on Intrinsic Reaction Kinetics D. G. Roberts†,‡ and D. J. Harris*,† CSIRO Division of Energy Technology and Cooperative Research Centre for Black Coal Utilisation, and Department of Chemical Engineering, The University of Newcastle, Australia Received September 2, 1999
Measurements of the intrinsic reaction rates of two Australian coal chars (made under laboratory conditions) with O2, CO2, and H2O at increased pressures (up to 30 atm) have been made using a pressurized thermogravimetric analyzer (TGA). It was found that the reaction order in CO2 and H2O was not constant over the pressure range investigatedsvarying from 0.5 to 0.8 at atmospheric pressure and decreasing at pressures above approximately 10 atm. The apparent reaction order in oxygen was less affected by pressure over the range 1 to 16 atm. Char surface area after reaction at higher pressures was generally greater than that after reaction at lower pressures. This resulted in a reduced effect of pressure on the intrinsic rates at 10% conversion. Activation energies for all three reactions were not significantly affected by an increase in reaction pressure. The intrinsic rate data obtained in this work were used to estimate the high-temperature reactivity of the chars using a basic knowledge of the pore structure of the samples. This provided a framework within which coal behavior under high-temperature gasification conditions can be predicted using intrinsic rate data and a knowledge of the structure of the char.
Introduction The reaction of carbons with oxygen, carbon dioxide, and steam is important in a variety of applications including power generation using coal gasification and the smelting of iron and steel. Recently, due to economic and environmental constraints, efforts have been increased to improve the efficiency of coal, and indeed all energy conversion processes. As a result, advanced coal utilization technologies have been developed that operate at increased efficiencies compared to traditional pulverized-fuel combustion methods. Such technologies include power generation from integrated gasification combined cycle (IGCC) and pressurized fluidized bed (PFB) combustion and gasification. The increased use of pulverized-coal injection (PCI) in blast furnaces is being driven by the need to reduce coke consumption and thereby reduce costs and emissions from coke oven plants. There is a need to investigate coal reactivity under conditions relevant to such applications. The ratedetermining step in the gasification process is the series of heterogeneous reactions between O2, CO2, and H2O and the char that remains after the initial, rapid pyrolysis of the coal. The reaction rate of the char therefore has a strong influence on the conversion efficiency that can be attained under process conditions. Low-temperature measurementssmade in the absence of any limitations from mass transfer to the surface of * Author to whom correspondence should be addressed at P.O. Box 883, Kenmore, Qld, Australia 4069. Fax: +61 7 3212 4606. E-mail:
[email protected]. † CSIRO Division of Energy Technology and Cooperative Research Centre for Black Coal Utilisation. ‡ The University of Newcastle.
or within the pore structure of the particlescan be used to calculate intrinsic char reaction rates that are required for the development and application of predictive gasification models. The reactivity of coals with gasification and combustion media has been the focus of numerous studies, mostly at atmospheric pressure1-7 with a few at increased pressures.8-11 In many cases studies performed at increased pressure have not approached the issue with the objective of determining fundamental rate data that can easily be transferred to practical applications through process and mechanistic models. Instead, these investigations mostly measured rates under simulated process conditions where significant pore-diffusion limitations or product gas inhibitions were present. As a (1) Dutta, S.; Wen, C. Y. Ind. Eng. Chem. Process Des. Dev. 1977, 16, 31-36. (2) Dutta, S.; Wen, C. Y.; Belt, R. J. Ind. Eng. Chem. Process Des. Dev. 1977, 16, 20-30. (3) Laurendeau, N. M. Prog. Energy Combust. Sci. 1978, 4, 221270. (4) Smith, I. W. Fuel 1978, 57, 409-414. (5) Smith, I. W. The Combustion Rates of Coal Chars: A Review. In Proceedings of the Nineteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1982; pp 1045-1065. (6) Harris, D. J.; Smith, I. W. Intrinsic Reactivity of Coke and Char to Carbon Dioxide. In Proceedings of the 197th ACS National Meeting, 1989; pp 94-101. (7) Harris, D. J.; Smith, I. W. Intrinsic Reactivity of Petroleum Coke and Brown Coal Char to Carbon Dioxide, Steam and Oxygen. In Proceedings of the Twenty-Third Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1990; pp 1185-1190. (8) Turnbull, E.; Kossakowski, E. R.; Davidson, J. F.; Hopes, R. B.; Blackshaw, H. W.; Goodyer, P. T. Y. Chem. Eng. Res. Des. 1984, 62, 223-234. (9) Mu¨hlen, H.-J.; van Heek, K. H.; Ju¨ntgen, H. Fuel 1985, 64, 944949. (10) Monson, C. R.; Germane, G. J.; Blackham, A. U.; Smoot, L. D. Combust. Flame 1995, 100, 669-683. (11) MacNeil, S.; Basu, P. Fuel 1998, 77, 269-275.
10.1021/ef9901894 CCC: $19.00 © 2000 American Chemical Society Published on Web 02/18/2000
484 Energy & Fuels, Vol. 14, No. 2, 2000
Roberts and Harris
Table 1. Analyses of the Coal and Char Samples Used in This Papera ultimate analysis (daf)b
proximate analysis coal D char D coal Y char Y a
moist. (%)
ash (%)
vol. (%)
fixed C (%)
C (%)
H (%)
N (%)
S (%)
O (%)
3.4 0.2 1.5 0.2
6.6 14.0 10.4 10.8
38.6
51.4
8.8
79.3
82.9 98.6 90.6 98.8
5.95 0.17 3.64 0.10
1.83 0.92 1.87 1.01
0.88 0.62 0.67 0.63
8.4
