In order to reduce the NOxemission during the low-quality blended coal combustion

a novel technique combining an additional separated over fire air（ SOFA） and boiler combustion system retrofit was carried out in a 300 MW tangential-fired boiler. Through three low-quality blended coals firing experiments

the impacts of SOFA

the tertiary-air and shrinked-middle wind on NOxemission and boiler efficiency were obtained. The results indicated that with the increase of air damper opening degree of SOFA

the NOxemission firstly decreased and then increased

while the boiler efficiency decreased. Meanwhile

by comparison with using the tertiary-air

the NOxemission reduced rapidly

but the unburned carbon slowly increased without the tertiary-air. Furthermore

the proper shrinked-middle wind could not only improve the coal combustion efficiency

but also reduce the NOxemission. Hence

air staging combustion adjustment should obtain a proper point for both the low NOxemission and the optimal boiler efficiency.