Gasification of high ash coal and chars from South African coals
Date
2012-07-06
Authors
Oboirien, Bilainu Obozokhai
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Abstract
This study seeks to explore the behaviour of several different South African
bituminous coals currently used as feed in local power stations and to establish their
technical performance and structural changes in a fluidised bed gasifier. It is
anticipated that this would also assist in optimising gasifier operations for the
gasification of fine high ash coals for power generation. The research was conducted
by correlating the gasification performance of the selected coals and their derived
chars against a range of chemical, physical and optical characteristics including
mineral and maceral (and specifically inertinite) contents after testing in a pilot scale
fluidised bed gasifier. Of specific interest were the changes in chemical
microstructures during the transformation of the various coal macerals to their
relevant chars following gasification. Raman spectroscopy and XRD analyses were
used to examine the chemical carbon structures and the minerals associated in the
coal. The relationship between the organic components and their gasified products
(macerals-to-char) and the inorganic components to their gasified products (mineralsto-
ash) including their physical structure and behaviour was determined by
petrographic analysis.
A higher loss of coal reactivity was obtained from vitrinites-rich coals due to a higher
degree of structural transformation of carbon in the coal. Inertinite-rich coals
experienced a lower loss of coal reactivity and lower degree of structural
transformation even with longer residence time. The structural transformation of the
macerals is due to realignment of the carbon molecules leading to substantial swelling
(enhanced plasticity) in some macerals. Further modification was found to be due to
proximity to melted minerals. Furthermore, the gasification performance of low grade
coals can be optimised by varying the oxygen content used for coal gasification.