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

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

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Fuels and Combustion
Calorific value of

Light paraffinic fuel oils is lower than that of equivalent olefins
N-paraffin is lower than that of iso-paraffins of the same compounds
Light paraffinic fuel oils is higher than that of equivalent olefins
Light paraffinic fuel oils is equal to that of equivalent olefins

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