The boiler ash and slag discharged from the fluidized bed boiler will taken away large amount of physical heat, thus worsening the operating in site, the residual S and O in ash and slag can still release SO2 and NOX outside the boiler, causing environmental pollution. For the fuel with middle and low calorific value and with ash content higher than 30 ﹪ , if the ash is not cooled, the physical heat of ash can be lost by 2 ﹪ , it can be recycled by proper heat transferring device. On the other hand, the self-handling and transportation of the hot ash is very difficult, it cannot be mechanically operated. General the temperature limit that the machinery for handling ash can bear mostly is between 150 ~ 300 ℃, the cooling for ash is a must. In order to control the materials storage capacity and proper bed height, to prevent big slag deposition and maintain good fluidization conditions, thus avoiding coking, it is necessary to control the slagging program, timing and flow. In addition, there are many fuel and desulfurizer particles in the bottom slag that have not completely reacted. In order to further improve the efficiency of combustion and desulfurization agent, it is necessary to return this part of the fine particles into the hearth. But the operation of these aspects shall be accomplished in slag-cooler.