The relationship between the moderate or intense low oxygen dilution (MILD) regime and NO formation in the co-combustion of NH3 with bituminous coal

Abstract

NH₃/coal co-combustion technology presents an effective technical pathway to rapidly and largely reducing CO₂ emissions in coal-fired power plants. To mitigate the challenges of combustion instability and elevated NO_x emissions in NH₃/coal co-combustion, the moderate or intense low oxygen dilution (MILD) combustion technology, which has stronger stability and lower NO_x emissions, is applied to NH₃/coal co-combustion. To further clarify the internal relationship between the establishment of the MILD combustion regime and the reduced NO_x formation, the internal recirculation distribution in the furnace is reconstructed by adjusting the injection angles (α) and injection distances (l) of fuel jets. The relationship between the two is then discussed from the perspectives of combustion stability, turbulence–chemistry interaction, and particle reaction behaviors. Results show that increasing α or l reduces the temperature fluctuation in the main reaction zone by enhancing the recirculation level (K_v) upstream of the furnace, and leads to a more stable and milder combustion status. As K_v increases, the oxidation reaction rates of volatiles and char particles decrease, and their burnout time is extended. The combustion behaviors of char shift from being C_char-O₂ dominated to being C_char-O₂ and C_char-H₂O/CO₂ co-dominated, and the consumption mode of NH₃ changes from being NH₃-O₂ dominated to being NH₃-O₂ and NH₃-NO co-dominated. When α increases to +10°, the NO emission at the furnace outlet is about 860 ppm at 40 cal% NH₃ blending ratio, which is 40.2 % less than that in the initial burner arrangement with α of 0°. With the enhancement of the MILD regime, the char gasification and NH₃ reduction reactions both increase and the non-oxidizing behaviors of coal and NH₃ are greatly promoted, thus preventing the oxidation of fuel-N.