Experimental and numerical study on establishment, fuel-N conversion, and heat transfer of swirl, non-premixed MILD, and premixed MILD combustion

Moderate or intense low-oxygen dilution (MILD) combustion has become much more attractive due to its advantage of low NO_x emissions. This paper undertakes a combined experimental and numerical study to examine the behaviors of establishment, fuel-N conversion, and heat transfer of swirl, non-premixed MILD, and premixed MILD combustion with non-preheated air in a 20-kW laboratory-scale furnace. In particular, the threshold wall temperature for achieving non-preheated CH₄/air MILD combustion in the non-premixed and premixed fuel/air jet modes is experimentally obtained. In swirl, non-premixed MILD, and premixed MILD combustion, fuel-NO emission is measured over a wide range of NH₃ mole fractions in the fuel ($X_{{\text{NH}}_{\text{3}}}$) from 0 to 3 % and fuel–air equivalence ratios (φ) from 0.8 to 1.2 when burning NH₃-doped CH₄; meanwhile, their difference in the fuel-N conversion mechanism is numerically revealed, especially from fuel-lean to fuel-rich conditions. The characteristics of energy balance and heat transfer in swirl, non-premixed MILD, and premixed MILD combustion are further comparatively analyzed. Results show that, the threshold wall temperature for forming non-preheated MILD combustion is reduced from ∼ 915 to ∼ 825 K when the non-premixed mode is changed to the premixed mode. Compared to swirl combustion, MILD combustion can reduce fuel-NO emissions by over 25 %. Interestingly, in MILD combustion, the non-premixed mode shows lower NO emissions under fuel-lean conditions due to less NO formation via ${\mathrm{NH}}_3\stackrel{+\mathrm{OH},H,O}{\to }{\mathrm{NH}}_2\stackrel{+O}{\to }\mathrm{HNO}\stackrel{+{H,\mathrm{OH},O,O}_2}{\to }\mathrm{NO}$, whereas NO emissions are lower in the premixed mode under reducing conditions as a result of less NO formation via HNO + H/OH → NO and more NO reduction via conversion of NO to NO₂. Moreover, fuel-NO reduction, which proceeds mainly by reburning and selective non-catalytic reduction (SNCR) via $\mathrm{NO}\stackrel{+{\mathrm{CH}}_{i=0-3}\mathrm{or}\mathrm{HCCO}}{\to }(\mathrm{CN}/\mathrm{HCNO}\to )\mathrm{HCN}\to \cdots \to N_2$, $\mathrm{NO}\stackrel{+{\mathrm{NH}}_2,\mathrm{NH},N}{\to }{N}_2$, and $\mathrm{NO}\stackrel{+\mathrm{NH}}{\to }{N_{2}O\stackrel{+H}{\to }N}_2$, becomes more important in MILD combustion than in swirl combustion. Compared to swirl combustion, MILD combustion is characterized by lower heat flux transferred to the furnace walls (e.g., 47.3 %) and more exhaust energy loss through the chimney (e.g., 52.7 %) under non-preheated conditions.