Proximate analysis indicates the percentage by weight of the fixed carbon, volatiles, ash, and moisture content in coal. The amounts of fixed carbon and volatile combustible matter directly contribute to the heating value of coal. Fixed carbon acts as a main heat generator. High volatile matter content indicates easy ignition of fuel. The ash content is important in the design of the furnace grate, combustion volume and pollution control equipment, and ash handling systems of a furnace.
Fixed Carbon: Fixed carbon is the solid fuel left in the furnace after volatile matter is distilled off. It consists of carbon but also contains some hydrogen, oxygen, sulphur, and nitrogen not driven off with the gases. Fixed carbon gives a rough estimate of heating value of coal.
Volatile Matter: Volatile matters are the methane, hydrocarbons, hydrogen and carbon monoxide, and incombustible gases like carbon dioxide and nitrogen found in coal. Thus, the volatile matter is an index of the gaseous fuels present. Typical range of volatile matter is 20–35%. Volatile matter proportionately increases flame length, helps in easier ignition of coal, sets minimum limit on the furnace height and volume, influences secondary air requirement and distribution aspects, and also influences secondary oil support.
Ash Content: Ash is an impurity that does not burn. It ranges from 5 to 40%. Ash reduces handling and burning capacity, increases handling costs, affects combustion efficiency and boiler efficiency, and causes clinkering and slagging.
Moisture Content: Moisture in coal must be transported, handled, and stored. Since it replaces combustible matter, it decreases the heat content per kg of coal. It ranges from 0.5 to 10%. Moisture increases heat loss due to evaporation and superheating of vapour.
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