Influence of multiple air staging and reburning on NOx emissions during co-firing of low rank brown coal with woody biomass and natural gas

In a multi-fuel case investigated, it has been demonstrated that over 50% of NOx reduction could be achieved at 0.1 cofiring at the same process temperature, and 67% if the process temperature is reduced. It has been shown that the coal blends at process temperatures above 1400 °C, with no fuel staging and OFA switched off, yield highest NOx emissions (above 1100 mg/mn3 at 6% O2 dry). On the other side, NOx emissions reduced significantly with the increase of the staging air portion and also with an increase in the distance between the burner and OFA nozzle. Furthermore, if the reburning is performed by natural gas with OFA switched on, NOx emissions reduce below 500 mg/mn3 at 6% O2 dry. If the temperature is reduced below 1100 °C, NOx emissions decreased to 400 mg/mn3 at 6% O2 dry. The results confirmed that the design and operation of multi-fuel system using coal, biomass and gas could be optimized, such benefiting further reduction of NOx emissions.