Fuels and Combustion
The heat of combustion of a fuel

Is always negative
Can't be known without calculating it
Is always positive
Is equal to the heat of formation

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

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