Fuels and Combustion
A coal with high ash content is undesirable, as

All of these
The ash in molten condition gets absorbed in the pores of the refractory lining of the furnace and causes its spalling due to different co-efficient of expansion/contraction of the refractory and the
It is abrasive to the coal pulveriser (ie.g. ball mill) and the combustion chamber
The ash retains the sulphur & phosphorus and thus affects the quality of products in metallurgical furnace apart from increasing the slag volume. Besides, it may fuse and stick to the boiler tubes

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Fuels and Combustion
With increase in the temperature of carbonisation of coal

Calorific value of the coke oven gas decreases due to cracking of hydrocarbons which is not compensated by increase in CO & H? content
All of these
Methane content in the coke oven gas decreases and carbon monoxide content increases
Hydrogen content of coke oven gas increases due to cracking of hydrocarbons

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