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Burning Coal in Power Plants – Calorific Value and Moisture

written by: johnzactruba • edited by: Lamar Stonecypher • updated: 10/20/2009

Coal is the primary fuel for producing Electricity. Some of the characteristics of coal have profound influence on the day to day working and economics of the power plant. This article discus two of the important characteristics – Calorific Value and Moisture.

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    Calorific Value or Heating Value

    This is the most important parameter that determines the economics of the power plant operation.

    • It indicates the amount of heat that is released when the coal is burned. The Calorific Value varies on the geographical age, formation, ranking and location of the coal mines. It is expressed as kJ/kg in the SI unit system. Power plant coals have a Calorific Value in the range of 9500 kJ/kg to 27000 kJ/ kg.

    The calorific value is expressed in two different ways on account the moisture in the coal. Coal contains moisture. When coal burns the moisture in coal evaporates taking away some heat of combustion which is not available for our use.

    • When we say Gross Calorific Value or Higher Heating Value it is the total heat released when burning the coal.
    • When we say Nett Calorific Value or Lower Heating Value it is the heat energy available after reducing the loss due to moisture.

    The Heating Value determines how much fuel is required in the power plant. Higher the Calorific Value lesser the amount of the coal required per unit of Electricity. Higher Calorific value also means the cost of the coal is higher but is offset by the lower cost of logistics, storage and ash disposal.

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    The coal when mined contains moisture. The moisture is in two forms. First is the inherent moisture which is entrapped within the structure of the coal. Second is the external moisture that is outside of the coal structure. The amount of moisture depends again on the geographical age, location and condition in the mines. A part of this moisture can easily evaporate in atmospheric conditions during its transfer from the mines, storage at the power plant and finally feeding to the boiler in the power plant. Depending on where and when you determine the moisture, values will be different for the same of coal.

    The amount of moisture determines how much of heating is to be done to dry the coal before it is burned in the boiler.

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    Reporting Coal Properties

    Moisture in coal is expressed as % by weight. So the change in the moisture content changes the proportion of the other coal constituents and the Calorific Value.

    • ‘As Received’ coal, is the coal received in the power plant premises. The payment to the coal companies are normally made based on the ‘As Received’ coal properties.
    • ‘As Fired’ coal is the coal entering the boiler system. The performance of the boiler and power plant is based on the ‘As Fired’ coal properties.
    • ‘Air Dried’ coal is what is used in the laboratory for analysis. This coal is dried in atmosphere and has the lowest amount of moisture. Laboratory results are reported as ‘Air Dried’ coal properties.

    The difference between the above three conditions is the proportion of the Moisture. The Calorific Value and other coal constituents analysed in the laboratory on ‘Air Dried’ basis is converted to ‘As received’ or ‘As Fired’ basis proportional to the moisture content.