Fly ash from thermal power plants - waste management and overview

COAL-based thermal power plants have been a major source of power generation in India, where 75% of the total power obtained is from coal-based thermal power plants. The coal reserve of India is about 200 billion tonnes (bt) and its annual production reaches 250 million tonnes (mt) approximately. About 70% of this is used in the power sector. In India, unlike in most of the developed countries, ash content in the coal used for power generation is 30–40%. High ash coal means more wear and tear of the plant and machinery, low thermal efficiency of the boiler, slogging, choking and scaling of the furnace and most serious of them all, generation of a large amount of fly ash. India ranks fourth in the world in the production of coal ash as by-product waste after USSR, USA and China, in that order. Fly ash is defined in Cement and Concrete Terminology (ACI Committee 116) as the ‘finely divided residue resulting from the combustion of ground or powdered coal, which is transported from the fire box through the boiler by flue gases’. Fly ash is fine glass powder, the particles of which are generally spherical in shape and range in size from 0.5 to 100 μm. Fly ash is classified into two types according to the type of coal used. Anthracite and bituminous coal produces fly ash classified as class F. Class C fly ash is produced by burning lignite or sub-bituminous coal. Class C fly ash has self-cementing properties. The estimated thermal power generation, coal consumption and ash generation in India is given in Tables 1 and 2.