Numerical investigation of biomass co-combustion with methane for NOx reduction

This paper aims at exploring a cleaner way of waste wood biomass grate-firing with a focus on NOX emission. The objective is to evaluate the potential of biomass co-combustion with methane for NOX abatement. NOX model involving three major gaseous N-species: NO, NH3 and HCN is incorporated into the dynamic fuel bed model, in which the dependency of species release on the local oxygen is accounted for. Coupling with freeboard simulation, the NOX emission from a real scale grate boiler is predicted with good accuracy and details. Waste wood and methane co-combustion with fuel-staging is investigated. The methane co-combustion ratio is on an energy basis, and three values are chosen, 10%, 20%, and 30%. In the fuel-staging setup, it is found that a higher co-combustion fraction produces a better NOX reduction effect. This reduction is a result of less nitrogen input and NO reburning from hydrocarbon and other N-species. Moreover, while cutting down the total excess air, the reduction effect is enhanced from lower stoichiometry but accompanied by a diminished dilution for the NOX emission at stack. The results show that the reduction efficiency can achieve up to 45.3%.