The mechanism of fuel-type NOx formation in combustion process of coal and its elimination have been analysed.Using the feature of excess air coefficient

which is very sensitive along the height of furnace in the event of volatile N being converted into NO

a rich-fuel zone of a1 has been tried to establish in the combustion process of coal

making the fuel N in said zone to be converted into volatile N as completely as possible

at the same time

promoting the volatile N to be converted into N2 under condition of reductive atmosphere

thereby

to reach the purpose of greatly decreasing NOx.A test study on combustion with staged distribution of the secondary air has been carried out on a 220 t/h boiler in one power plant

results of study show that the emission of NOx is the lowest under condition of adopting air-distributing mode for three-level's secondary air burners according to succesively staged air-feeding mode from bottom to top

and the emission of NOx is the highest under condition of adopting the waist-drum type air-distributing mode.The formation regularity of carbon content in fly ash is just inverse with that of NOx.