The ash content of coal varies over a wide range. This variation occurs not only in coals from different geographical areas or from different seams in the same region, but also from different part of the same mine.

These variations result primarily from the wide range of conditions that introduced foreign material during or following the formation of coal.

Ash content can also be influence by extraneous mineral matter introduce during the mining operation. Before being sold, some comercial coals are cleaned or washed to remove a portion of what would be labeled ash in the laboratory.

However, the ash content of significance to teh user is the content at the point of use. The values noted below are on the basis.

Most of coal used for power generation has ash content between 6 to 20%. Low value of3 to 4% in bituminous coal are rare and these coals find other commercial uses, particularly in the metallurgical field. On the other hand, some coals may have as content as high as 40%. Many high ash fuel can be sucessfully burned in utility boiler. Their use has increased in areas where they offer an economic advantage.

It is common for design and fuel evaluation purpose to consider ash content on the basis of weight per unit of heat input.

[ Ash (% by weight) / HHV (kJ/kg) ] x 10^3 = [ kg ash / MJ ]


[ Ash (% by weight) / HHV (BTU/lb) ] x 10^4 = [ lb ash / 10^6 BTU ]

Table beside provides proximate analysis for three selected coals. Each coal has a moderate ash content of 9 to 10% by weight.

However on the heat input basis, ash quantity vary significantly.

The lignite in this example would introduce almost three times as much ash compared to the high volatile bituminous coal at the equivalent heat input.

Calculation below is continued from

Boiler load average assumed if we get 90% net capacity factor so boiler energy need average also multiply to 90% (1738MW * 90% = 1564.2MW)


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