Fuels and Combustion
In general, the limit of inflammability/explosion limit of fuel gases is widened by the

Use of pure oxygen for combustion instead of air
Increase in temperature (i.e. preheating)
Increase in gas pressure
All of these

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Fuels and Combustion
Ratio of primary air to secondary air increases with increase in the rank of coal, because the

Ratio of fixed carbon to volatile matter increases
Oxygen content progressively decreases
High rank coals have higher amount of volatile matter
Calorific value of the coal increases

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