Evaluation of Australian Coals for PFBC

The rise in energy demand world-wide, together with increased concern over the potential environmental impact of emissions to the atmosphere, has created the impetus for technological change. Foremost among the new technologies being developed for coal fired combined cycle power plants is Pressurised Fluidised Bed Combustion (PFBC) which has emerged as the coal technology with lowest capital costs, low environmental emissions and greatest potential as a competitive source of low cost power generation, when using low to medium sulphur coals.

The overall aim of the project is to provide the Australian coal industry with detailed information on the expected performance of a range of Australian coals during PFBC and to compare their performance with world traded coals currently used as feedstocks in prototype and demonstration PFBC plants. Twelve coals were selected for study, seven from Australia (NSW, Queensland and Western Australia) and five international coals. Key coal characteristics which influence combustion efficiency, operational and emissions performance were assessed through a series of laboratory investigations, under conditions simulating PFBC. The economics, process and environmental performance of the coals for advanced combined cycle PFBC plants were predicted using in-house process flow sheet modelling programmes and information gained from the laboratory test programme.

The predicted combustion efficiencies of the Australian and international coals, characterised in terms of volatile yield, coal swelling propensity and char reactivity, were shown to be comparable for a given set of conditions representative of PFBC. The environmental performances of the three Australian coals tested in the fluidised bed reactor were superior to that of the international coal that was tested (Illinois No.6 coal). SO2 emissions from Low Volatile Queensland, Hunter Valley Mine Thermal and WA Collie coals were significantly lower, whilst emissions of NOx were comparable. The sulphur retention efficiency appeared to be partly dependent on the sulphur content of the coal; slightly higher Ca:S molar ratio's being required for low sulphur coals. Differences in N20 emissions observed for the four coals tested appeared to be attributable to the extent of N20 reduction reactions undergone in the laboratory reactor. The influence of coal type on overall N20 emissions from a large scale PFBC plant was thought to be small. The low sulphur and high ash melting characteristics of the Australian coals also offer advantages for PFBC operational performance in the terms of minimising the potential for ash-related operational problems. The requirement to operate at higher Ca:S molar ratios, for environmental compliance or operational requirements, should not impose a restriction on typical operating temperatures for PFBC plants using the Australian coals as feedstocks.

The influence of coal type had a comparatively small impact on process performance in purpose designed PFBC plants for the range of coals studied. The net electrical efficiency calculated for a PFBC plant, based on ABB carbon's P800 module with a GT-140P gas turbine and a supercritical 240 bar, 540/565°C reheat steam cycle with a condenser pressure of 0.02 bar, ranged from 45.4 to 45.8% LHV basis for the twelve study coals. The net power output ranged from 362 to 372.5 MWe. Higher cycle efficiencies were predicted for the coals with low ash, low sulphur, high oxygen contents and high combustion efficiencies. The performance of a PFBC plant also showed some sensitivity to operating parameters influenced by coal type; for example, reducing the stack gas temperature and Ca:S molar ratio, and increasing the bed operating temperature improved the cycle efficiency.

The influence of coal type on generating costs was more evident. The overall specific capital cost for a PFBC plant designed to fire the seven Australian coals ranged from 1553 AUS$/kWe to 1639 AUS$/kWe compared with 1570 to 1695 AUS$/kWe for the international coals. Generating costs ranged from 4.61 to 4.79 AUS¢/kWh for the Australian coals and from 4.64 to 5.10 AUS¢/kWh for the international coals. The requirement to design for high sulphur fuel (5% dry, ash free basis) carries a capital cost premium of 102 AUS$/kWe in order to accommodate the necessary degree of sulphur control. Operating costs were also affected by the use of high sulphur coal due to the increase in sorbent consumption and increase in the residues which incur significant costs associated with ash disposal. The overall result of increasing the coal sulphur content from 0.3 to 5% (dry, ash free basis) was an increase in generating costs of some 0.42 AUSO/kWh. PFBC plants designed for low ash coals, operation at low Ca:S molar ratio's and coals with high specific energy all benefit from a reduction in capital and generating costs.

This study has predicted that the majority of Australian bituminous .coals will make ideal .candidates for PFBC. Low sulphur Australian coals offer significant benefits in terms of capital and operating cost savings and in low environmental emissions. In addition, the high melting characteristics of the Australian coal ashes should minimise the potential for ash-related operational problems.