After the secondary air has been used in the circulating fluidized bed boiler, the  combustion area usually is divided into dense-phase zone , namely the zone under the secondary air area,  and the dilute phase zone , which is above the secondary air area. The proportion of the primary air depends of the combustion share of dense-phase area,  the combustion share of dilute phase zone is decided by the air feeding volume of secondary air and the fuel combustion rate. The combustion share percentage of the two area varies with the change of the coal quality, load, and the proportioning of primary air and secondary air. The operators can make adjustment according to operation condition.(hxl)