Fuels and Combustion
The function of secondary combustion air is to

Pneumatically convey the pulverised coal
Completely burn the volatile matter
None of these
Burn the lumpy coal

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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 flame temperature tremendously
It reduces its calorific value tremendously
There are chances of explosion during preheating

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