Heat of combustion, also known as enthalpy of combustion, heating value, calorific value and heat of combustion, is the amount of heat produced by a complete combustion process, usually expressed in kcal/kg or joules/kg.
Heat of combustion is determined by testing the fuel in a calorimeter, which accurately measures the amount of energy released by burning the fuel in a controlled environment. Generally, the greater the heat of combustion, the larger the quantity of energy available from the burning of a given quantity of fuel.
The heat of combustion of coal varies widely according to the type of coal it is, the region in which it is mined, and by the percentage of impurities and other elements it contains. Generally speaking, the higher the heat of combustion, the more valuable and efficient the coal is for burning for energy.
The heat of combustion of coal is usually expressed in megajoules (MJ) per kilogram (kg). A MJ is equal to one million joules of energy, while one kilogram is equivalent to one thousand grams. The heat of combustion per kilogram (MJ/kg) accurately measures the energy released from a single unit of coal (1kg) when burned completely.
Typically, the heat of combustion of coal ranges from roughly 25 MJ/kg to over 35 MJ/kg, with the average around 29 MJ/kg. This makes coal one of the more valuable and cost effective sources of energy available for electricity generation.
Based on the historical records, bituminous coal, the more efficient type found in most countries’ major coal fields, typically has a heating value of 29–31 MJ/kg. However, the calorific value of lignite, the lower grade coal, has been known to average as low as 14–15 MJ/kg.
The calorific values of coal depend upon its ultimate chemical composition, and also upon the quality of its preparation. This can vary slightly depending on the chemical makeup of the coal and the methods of its preparation.
The physical shape and size of the particles, their resistivity to crushing, their porosity, and the temperature of their combustion will also cause variations in the amount of heat released.
Despite the inherent differences between coal varieties, most items of equipment must be designed to handle the worst case. So, when selecting a fuel source, the rating of the highest heat of combustion should be used.
The heat of combustion of coal is a key factor in deciding its choice as a fuel for electricity generating stations. As a result, power plants are making active efforts to increase the calorific value of their fuel sources as much as possible, so as to ensure efficiency.
While the heat of combustion of coal is typically higher than that of its counterparts such as oil and natural gas, it shouldn’t be taken for granted when considering the production of electricity. Carefully consider the calorific value of the coal before using it for electricity production, to ensure maximum efficiency for the entire process.
