1. To ensure the normal coal particle size. 

2. The slag-feeding cannot be too much and the slag-feeding speed cannot be too fast. 

3. Ensure proper thickness of material layer inside each bunker of slag-cooler. 

4. Ensure low carbon content in the slag discharged. 

5. Ensure sufficient fluidization of boiler bed material, complete combustion and bed temperature higher than 850 ℃ as much as possible.