coal selection criteria for industrial pfbc firing project 3.2 - CCSD

ABSTRACT
“Coal Selection Criteria for Industrial PFBC Firing”
Pressurized Fluidized Bed Combustion (PFBC) is one of the clean coal technologies.
There are several PFBC plants operating all over the world. As this technology is
relatively new, some problems were encountered during the plants’ operation. These
include combustion inefficiency, bed agglomeration, cyclone clogging, filter blockage,
gas turbine and in-bed heat exchanger tube erosion and corrosion. In this report, we have
focussed only on those aspects of the problems which were coal-related, since those
aspects affect coal selection for PFBC.
Combustion inefficiency was mainly caused by unburnt char elutriation from the bed. For
Australian export coals, it was found that unburnt char elutriation was related to the ratio
of Telovitrinite : Inertinite. For a wider range of coal rank, there was generally a decrease
in combustion efficiency with increasing rank, but this generalisation did not always
predict coal performance in commercial PFBC plants. Hence, petrographic analysis is
preferred for bituminous and sub-bituminous coals. A Telovitrinite : Inertinite ratio <
0.200 is recommended for satisfactory PFBC performance.
Low ash fusion temperature generated agglomeration. Despite their high combustion
efficiencies, low rank coals contain high alkali that caused agglomeration problems. Two
of the Japanese commercial plants firing Australian export coals specify < 7% Fe2O3 in
the coal ash and one also specifies an ash fusion temperature > 1200 o C.
During combustion, iron contained in the coals was oxidized and decomposed, causing
fouling and deposit formation. Low iron content coals were recommended to be used to
minimize deposit formation.
Two solutions to filter blockage problems were to use ash for maintaining bed inventory
and to use coals with high Al2O3 and SiO2 contents in their ash, which agglomerated to
larger ash particles. The recommended method to overcome filter blockage is to allow
larger particles into the filter which form a layer of cake on the filter surface instead of
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