1. From the Angle of the dense-phase zone burning more fully , it is considered that the bigger primary air is much better,  at the same time the good fluidization can be ensured and the possibility of burning accident can be reduced.
2. From the perspective of reducing oxycarbide and fly ash carbon content, the bigger secondary air is much better, but too big secondary air will affect the flue gas flow.
3. Firstly enough combustion share of the dense-phase zone shall be ensured, because the fuel for lower temperature combustion and circulating ash both enters from dense-phase zone; To a certain extent, dense-phase zone is the heat source of the boiler, how much heat will generate will generate in the dense-phase zone heat will directly affect the whole combustion and heat transfer inside the boiler. but enough secondary air shall be available in order to reduce the generation of oxycarbide, to strengthen the combustion of upper part of the hearth and to reduce the flash ash carbon content, therefore the primary air proportion shall account for 50% to 60%, The secondary air proportion shall account for 40 to 50%.For some coal with lower volatile, higher carbon content and higher calorific value  that is hard to combustion, the primary air proportion shall be larger. For the coal with higher volatile and  higher ash content that is easy to combustion, the primary air proportion shall be slightly smaller, and the secondary air proportion shall be larger properly. The operators shall adjust the proportioning of the primary air and the secondary air according to the different quality of coal, the proportion of primary air and the secondary air can be 1 to 1. The  secondary air can also be slightly bigger than the primary air. (hxl)