Numerical study of co-firing lignite and agricultural biomass in utility boiler under variable operation conditions

•Model to predict biomass/coal co-combustion with user-oriented interface presented.•Suitability of direct co-firing agricultural residues with lignite addressed.•Partial boiler load resulted in reduction of NOx and SOx up to 34% and 9.5%.•Lower thermal loads gave increased unburnt C in bottom/fly ash of biomass fuels.•Burner arrangement strongly affected aerodynamics, flame and pollution abatement. The co-combustion of biomass and coal in a utility boiler could provide cleaner power production and ensure sustainable utilization of the solid fuels. This paper aims to numerically investigate complex processes in the tangentially-fired 900 MWth boiler furnace during direct co-firing of lignite and biomass with 10% thermal share of agricultural residues (wheat straw, corn straw and soybean straw) under variable boiler loads (100%, 85% and 70%). Simulations are conducted by means of in-house developed computer code, supported by the specially designed user-friendly graphical interface. Co-firing of agricultural residues provides lower pollutant emissions, somewhat higher furnace exit gas temperature and increase in unburnt carbon in bottom/fly ash, compared to the lignite combustion without biomass. Soybean is found to be the most suitable for co-firing regarding its ash melting point, however due to its abundance and availability the wheat straw is selected for this study. Co-combustion at partial boiler loads results in reduction of NOx and SOx up to 34% and 9.5%, respectively. Burners arrangement and furnace aerodynamics affect the abatement of pollutants. This study may help the global efforts in fighting the climate change, efficiently and cost-effectively, thus offering considerable economic and social benefits. [Display omitted]