Effect of separated over-fire air on combustion performance of a 3 MW pilot-scale facility

•Numerically and experimentally analyzed a 3MW pilot-scale facility.•The thermal balance formula for the furnace was proposed.•The loss of the fuel combustion heat reaches to 8% of the total.•The unburnt carbon in the bottom ash leads to the loss of the fuel combustion heat.•The position that air injected into furnace affects the sensible enthalpy of air.