What are the functions of primary air and secondary air in a pulverized coal fired boiler?

（1）Primary air is used for transportation of fine coal particle from coal mill to boiler floor at different elevations as per requirement.

（2）Primary air is used for preheating of moisturized coal into the coal mill so that minimum energy is required for combustion of the same.

（3）As we all know that only 23% oxygen is available in the air by weight. For combustion of huge quantity of coal inside the boiler very high quantity of oxygen is required. This oxygen requirement will be completed by secondary air which is supplied by FORCED DRAFT fans through ducts.

（4）This secondary air is coming via air preheater hence its outlet temperature increases this will help in combustion and low heat input is required for combustion of coal.