coal selection criteria for industrial pfbc firing project 3.2 - CCSD

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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 Page 2
“Coal Selection Criteria for Industrial PFBC Firing” penetrating into it. Cyclone plugging was due to the same properties of coal which caused sticky ash material. Gas turbine blades erosion was due to fine quartz particles while corrosion is due to fine ash particles and corrosive compounds of sulfur, alkali and alkaline earth elements contained in the coals. To reduce erosion and corrosion it was recommended to use coals with low quartz, sulfur and alkali contents. Another part of PFBC plant which experienced erosion and corrosion is the in-bed heat exchanger tubes. In preventing such erosion and corrosion, at low temperature it is important to apply thermal spray coatings. For high temperature, the tube materials should have sufficient erosion and corrosion resistance due to the formation of hard oxide scale on the surfaces. Pollutant emissions need to be regulated to achieve sustainable environment control. These emissions were mainly influenced by the operating conditions rather than the coal properties. Page 3
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Page 5 and 6: LIST OF TABLES “Coal Selection Cr
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coal selection criteria for industrial pfbc firing project 3.2 - CCSD

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