Computational-fluid-dynamics simulation of nitrogen oxide emission in a vertical-type waste incinerator under oxyfuel- and air-combustion conditions.

Oxyfuel combustion can significantly enhance carbon capture; however, its effects on NOX emissions are not yet fully understood in municipal solid waste (MSW) incineration. In this study, NOX generation in a vertical-type waste incinerator under air- and oxyfuel-combustion conditions was investigated using a combined experimental and computational-fluid-dynamics (CFD) modeling approach. Pyrolysis and combustion experiments were conducted using refuse-derived fuel (RDF) to determine NOX precursors. Thermogravimetric analysis (TGA) was employed to quantify the thermal degradation behavior and determine the kinetic parameters of RDF and char. Gas chromatography analysis identified NH3 and HCN as the dominant NOX precursors, which were subsequently used in modeling NOX formation. The Kilpinen-97 reaction mechanism was implemented in COMSOL Multiphysics to simulate NOX formation in a zero-dimensional (0D) waste bed model and a two-dimensional (2D) combustion and recombustion chamber model. The results revealed that oxyfuel combustion significantly reduced NOX emissions by 12% compared to air combustion. This reduction was primarily attributed to the higher CO concentration under oxyfuel conditions, which enhanced NOX reduction through radical-driven reactions. Moreover, the 2D CFD model predicted lower NOX emissions than the 0D model, highlighting the impact of carbon dioxide's physical properties on the flue gas mixing quality. Overall, these findings emphasize the potential of oxyfuel combustion to reduce NOX emissions and promote carbon capture in waste-to-energy plants, thus presenting a promising pathway toward more sustainable waste management.