Numerical study on oxy-biomass co-firing in a cement rotary kiln

Cement manufacturing is among the industries with the highest energy consumption and pollution emissions. Combining oxy-fuel combustion with the technology of co-firing biomass with coal is a promising way to reduce pollutant and carbon emissions. Based on a 6000 t per day cement rotary kiln, the performance of oxy-biomass co-firing technology is investigated by CFD modeling. Cases under different biomass ratios (0%-30%) and O2 concentrations are simulated. Combustion characteristics including temperature field, wall heat flux distribution, NOx emissions, etc. are widely assessed. It is found that biomass co-firing can significantly reduce ignition delay caused by high CO2 concentration during oxy-fuel combustion. A flame distribution similar to the conventional air-fired condition is obtained under conditions of 33% O2 concentration and 10% biomass co-firing ratio. The wall heat transfer is enhanced in oxy-fuel cases. With the increase of biomass co-firing ratio, the wall heat flux tends to be more uniform. Oxy-fuel combustion can effectively reduce NOx emissions and the fuel-N conversion ratio. Biomass co-firing under oxy-fuel conditions can reduce the fuel-N conversion ratio from 10.9% to 8%, but it will lead to a slight increase in NOx emissions from 848 ppm to 899 ppm. It is necessary to control the co-firing amount of biomass to achieve effective combustion and pollutant emission control.