Effects of the fuel-lean coal/air flow damper opening on combustion, energy conversion and emissions in a supercritical down-fired boiler

•Effect of fuel-lean flow damper opening (FLD) in a down-fired boiler is investigated.•Relation of water-wall and steam temperature in a supercritical boiler is discussed.•On decreasing the FLD, heating of the fuel-rich coal/air flows gradually accelerates.•Specific coal consumption is lowered and energy conversion efficiency is improved.•The optimum NOx emissions and carbon in fly ash are as low as 672 mg/m3 and 4.51%.