The kinetics of steam gasification of South African coals.
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Date
1990
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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.
Description
Thesis (Ph.D.)-University of Natal, Durban, 1990.
Keywords
Coal Gasification--South Africa., Fluidized reactors., Coal Gasification--Equipment and supplies., Theses--Chemical engineering.