In order to burn volatile gases from soft coal more efficiently, more of what type of air should be supplied?

To burn volatile gases from soft coal more efficiently, the supply of overfire air is crucial. Overfire air is introduced into the combustion zone above the primary combustion area, allowing for better mixing with the volatile gases that are released during the combustion of coal. When these gases rise into the upper regions of the combustion chamber, they need adequate oxygen for complete combustion. The addition of overfire air helps achieve this by providing the necessary oxygen at a stage in the combustion process where it is most effective, thus promoting a more thorough burning of the volatiles and reducing emissions of unburned hydrocarbons or carbon monoxide.

In contrast, primary air is mixed with the coal at the stoker or burner and is mainly responsible for the initial combustion of the coal particles. Secondary air is introduced into the combustion process to ensure complete combustion of gases, but its function is more about supporting the initial fire rather than fully combusting the volatiles. Fume air doesn’t have a specific role in optimizing coal combustion as typically understood in this context.