The black smoke is always emitted from the chimney in the chain-grate stoker-fired boiler at the time of ignition at ambient temperature and restart of the combustion after temporary flameout in China. The purpose of the work is to reduce the black smoke emission. A laboratory fixed-grate model has been used to simulate the combustion of coal in chain-grate stoker-fired boiler. The CO and O-2 concentration in the flue gas have been measured with a flue gas analyser, and the black smoke emitted from the chimney has been screened with Charge Couple Device (CCD) video camera. Power 2(#) coal, sized at 5-25 mm, has been fired in the fixed-grate model. The secondary air has been used to enhance the turbulence in the furnace after the numerical simulation. The results of experiments show that the emission of the black smoke at the time of ignition of the coal at ambient temperature is more serious than that of restart of the combustion after the temporary flameout for the case of the temperature; the secondary air is helpful for reducing the black smoke emission for enhancing the intensity of the turbulence in the furnace; selection of coal particle size is necessary, the smaller the size of the coal is, the more serious the black smoke emission is, and the effect of reducing the black smoke emission with the secondary air is more evident with smaller-size coal. The industrial test has been employed to study the effect of the air demand. It indicates that rational stoichiometric air/fuel ratio is helpful for reducing the black smoke emission in the restart of the combustion after the temporary flameout. Copyright (c) 2006 John Wiley & Sons, Ltd.