Fuels and Combustion
A particular coal is said to be free burning when it

Shows little or no fusing action
Gives smokeless burning
Burns completely
None of these

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Fuels and Combustion
Rate of low temperature oxidation of coal due to bad storage conditions

Is accelerated by storage in large heaps with small surface to volume ratio
Does not vary with increase in surface area
Is more for low volatile coal compared to high volatile coal
Decreases with increase in surface area

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

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