Fuels and Combustion
A carbonaceous fuel (containing no H₂ or hydrocarbons) is burnt and the resulting flue gas contains 21% CO₂. It means that

Complete combustion of fuel has taken place
21% excess air has been used for combustion
No excess air has been used for combustion
21% excess oxygen has been used for combustion

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

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
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
All of these
It is abrasive to the coal pulveriser (ie.g. ball mill) and the combustion chamber

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