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

Hydrogen content of coke oven gas increases due to cracking of hydrocarbons
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

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Fuels and Combustion
Fuel gases containing hydrocarbons (ie.g. coke oven gas) are not preheated before burning, mainly because

It reduces its calorific value tremendously
There are chances of explosion during preheating
It reduces its flame temperature tremendously
The hydrocarbons crack thereby choking and fouling the heat transfer surface by carbon soot

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