Fuels and Combustion
Use of preheated air for combustion of fuel in the furnace, increases the

Calorific value of the fuel
Flame temperature
None of these
Scale losses of the furnace stock

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

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