Flow and reaction characteristics at different oxygen concentrations and inlet velocities in pulverized coal MILD combustion

Abstract To better understand Moderate or intense low-oxygen dilution (MILD) combustion, numerical simulations of pulverized coal MILD combustion with different inlet velocities and oxygen concentrations are presented in this work. The flow and chemical reaction characteristics are analyzed and the MILD performance is determined by the Damkohler number, Da, and the circulation coefficient, K υ , with different oxygen concentrations and inlet velocities. Results show that the oxygen concentration has a more pronounced impact on the chemical reaction, and the inlet velocity has a more pronounced impact on the flow time scale. The K υ values are found increasing with the oxygen concentrations, which is caused by the decreased total amount of flue gas. Modified schematic diagram of combustion regimes is presented based on the analysis. The Da number increases with the increase of oxygen concentration. This is because, on the one hand, the increase in oxygen concentration leads to the decrease in the inlet velocity. On the other hand, the oxygen concentration intensifies the chemical reaction. This indicates that, with the increasing oxygen concentration, the larger K υ or the smaller Da is required to establish the MILD combustion. If the burner structure is not changed, increasing the oxygen concentration will reduce flue gas entrainment and aggravate chemical reactions, which is not conducive to the MILD coal combustion.