All coals have a significant content of ash-forming inorganic material that cannot be economically removed before combustion. This amount can range from below 3% in a clean low-ash feedstock, to over 40% in some low-grade coals. Most commonly, the ash-forming material represents between 10% and 25% of the feed coal. This Chapter reviews coal ash and the detrimental effects that it can have on combustion equipment through the formation of unwanted deposits. Following a brief overview of the types of problem that can be encountered in coal-fired boilers of all types, the Chapter provides an insight into the mineral matter that forms the basis for ash, and into the analytical methods available for the identification of the mineral species contained in it. A section on the methods of ash formation in coal-fired boilers is followed by a review of the transformations undergone by inorganic mineral particles during the combustion processes, and the Module then looks at ash deposition. Slagging and fouling can occur in all types of coal-combustion equipment, and the next three sections focus on potential ash problems in pulverised-coal, fluidised-bed and gasification systems. The Chapter includes a section on recent innovations in the development of more accurate predictive systems for identifying coals with a particular risk of causing ash deposition problems, and concludes with a review of testwork procedures available for identifying deposit accumulations and for investigating the suitability of specific coals for individual combustion systems.

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