The kinetics of steam gasification of South African coals.

Abstract

The prime objective of a current research project at the University of Natal is to develop a novel autothermal fluidised bed coal gasifier which is capable of efficiently producing synthesis quality gas (rich in hydrogen and carbon monoxide) from discard of duff coal resources using air and steam as the reactant gases. The development of this gasifier was initially motivated to utilise the ever increasing supply of discard coal in South Africa which represents a significant potential source of energy and currently poses severe environmental pollution hazards caused by spontaneous combustion and wind erosion of the discard coal dumps. Recently, however, the gasifier has been considered for the conversion of more general coal resources in an Integrated Coal Gasification Combined Cycle process (IGCC) for the production of electricity. The knowledge of the kinetics of steam gasification of local coal resources is of vital importance to the design of this gasifier. However, no such kinetic data are available of which the author is aware. This thesis presents the following contributions to the overall knowledge of the gasifier (a) The development of a micro reactor to measure the rate of reaction of the steam gasification of coal-char at temperatures of up to l000oC and pressures up to 5 bar absolute; (b) Kinetic studies using the microreactor on the steam gasification of coal-chars derived from Bosjesspruit and Transvaal Navigation coal samples. The following principal results were obtained with Bosjesspruit coal-char : The rate of steam-char gasification is very sensitive to variations in the temperature of reaction in the range 840°C to 920°C. Neither the rate of steam-char gasification nor the product gas composition are affected by the steam partial pressure in the range 1.8 to 4.8 bar absolute; The concentrations of the H2 and CH4 components of the product gas stream rapidly approached their respective equilibrium compositions, whereas the concentrations of CO and CO2 gradually approach their respective equilibrium compositions during gasification at a rate which is typical of the stoichiometry of the Boudouard reaction. The average product gas composition is independent of the temperature of reaction in the range 840°C to 920°C and is approximately 49% H2, 32% CO, 17% CO2 and 2% CH4 on a molar basis; The steam gasification kinetic data are well described by a fundamental Arrhenius-type volumetric reaction model at (c) temperatures of up to 920°C. The value of the activation energy for the reaction is 146 kJ/gmol, which indicates that the gasification kinetics are controlled by the rates of the chemical reactions (ie. C + H2O = CO + H2 and C + CO2 = 2CO) at temperatures up to 920o C; There are no major differences between the kinetics measured for Bosjesspruit coal-char and those reported in the literature for foreign coal-chars. The experimental results obtained for the steam gasification of char derived from Transvaal Navigation coal show that the concentrations of both the Hz and the CH4 in the product gas stream rapidly attain their respective equilibrium values and remain approximately constant throughout gasification, whereas the concentrations of CO and CO2 gradually approach their respective equilibrium values during the course of gasification and almost attain equilibrium concentrations as the conversion of carbon nears completion. The rate of steam gasification of this char is therefore also controlled by the rate of the Boudouard reaction. The mathematical development of a steady-state, one-dimensional compartment model of the gasifier. The model is also presented in the form of a Fortran 77 computer program which is designed to run on a personal computer. The program is capable of simultaneously solving the overall material and energy balances of the gasifier to a tolerance of l% within 15 minutes when using a microprocessor which operates at 10 Mhz. (d) The gasifier simulation program is currently being used in the design of a pilot scale gasifier which is intended to demonstrate the capability of the process on a continous basis of operation. (e) Experimentation on the air-steam gasification of Bosjesspruit coal using a mini-pilot scale gasifier. These experiments have successfully demonstrated the feasibility of the production of a gas stream which is rich in hydrogen and carbon monoxide. The composition of the product gas stream compares well with the predictions of the simulation model of the gasifier.