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Coal Blending for Power Stations

AUTHOR: Anne M. Carpenter
DATE: July 1995

Blending of coals of different types at pulverised coal-fired power stations is becoming increasingly common as electric utilities attempt to save costs, meet SO2 emission limits and improve the combustion behaviour of their coals. This report discusses the evaluation and prediction of the behaviour of blends of coals of different types and/or ranks in conventional pulverised coal-fired power stations. It begins with a discussion on why coals are blended, and the methods of blending coals (in stockpiles, in bins and on moving belt conveyors). Current practice for determining the overall quality of blended coals is to use the weighted average of the determined values of the individual coals in the blend. The additivity and non-additivity of the properties and behaviour of blends is a running theme throughout the report. It is the interactions between components of the individual coals in the blend that makes its evaluation inherently complex.

The utilisation of blends can significantly affect several areas of boiler operation which impact overall power plant performance and generating costs. Areas examined include the pulveriser (assessed by the Hardgrove grindability index), combustion behaviour and efficiency (assessed in terms of the ignition, flame stability, reactivity and burnout characteristics), ash deposition behaviour, and SO2 and NOx emissions. The evaluation and prediction of the behaviour of blends using empirical indices, and in bench-scale apparatus progressing through to pilot-scale plant to actual experience in power stations are discussed.