Contruction And Evolution Of MILD Combustion Regime Under Different Heat Exchange Conditions For Methane

MILD（Moderate&Intensive Low Oxygen Dilution）combustion technology has a broad research prospect in improving the heating efficiency of gas-fired boilers or kilns and reducing NOx emissions owing to its uniform temperature distribution,high combustion efficiency and low NO_x emission characteristics.However,little attention has been paid to the formation and influence mechanism of MILD combustion in industrial furnaces under different heat transfer conditions.In this work,the construction method of combustion regime maps of CH₄ under adiabatic and non-adiabatic conditions were studied.And the combustion mode transformation process of CH₄ under non-adiabatic condition was discussed.Besides,effects of physical and chemical properties of the diluent on combustion regime and NO formation were analyzed.Firstly,the WSR model in CHEMKIN was used to reconstruct the CH₄ combustion regime map based on mathematical criteria provided by Cavaliere and Joannon.According to critical oxygen concentration(X_O2*),MILD combustion was divided into Unconditional MILD combustion（UMC）and Conditional MILD combustion（CMC）.Furthermore,NO emission was considered for the first time to determine the upper limit inlet temperature(T_in^up*)of MILD Combustion.MILD Combustion was further divided into High NO MILD Combustion（HNMC）and Low NO MILD Combustion（LNMC）according to T_in^up*.The NO formation mechanisms in different combustion regime whenΦ=1.0 were studied.The reason for the low NO_x in the LNMC region is that thermal NO is greatly inhibited and the no-reburning path can consume more NO.Then,the effects ofΦand diluents on classification of combustion regimes and NO formation are deeply explored.Results show that X_O2^up*and T_in^up*would be improved if we increase or decrease theΦproperly.Additionally,both diluent of H₂O and CO₂ were beneficial for decreasing the region of HTC,increasing region of LNMC,as well as improving the X_O2^up*and T_in^up*.Moreover,the effect of CO₂was more superior than H₂O for methane to realize the MILD combustion.Under MILD combustion state,N₂O route will dominate NO formation whenΦ<0.8;Prompt and NO-reburning route will contribute most to NO formation whenΦ>1.0.Under MILD combustion mode,the dilution of H₂O is most effective to reduce NO formation.WhenΦ<0.85,the reduction mainly comes from the chemical properties,and physical effect will become the mainly reason for NO formation whenΦ>0.85.For CO₂ diluent,the reduction of NO mainly comes from its physical effect on reducing temperature.Additionally,the chemical effect of CO₂will increase the reaction rates of R182 and R185,resulting the increased the NO production from N₂O route whenΦ>1.1.Finally,the detailed evolution of different combustion regimes under non-adiabatic condition has been comprehensively examined.Results show that X_O2*is found to climb as the heat loss ratio（HLR）increases.In addition,high temperature combustion（HTC）can shift to CMC or even UMC by just enlarging HLR,providing a potential solution to realize MILD combustion in practical applications.However,the combustion regime would further shift to unsteady combustion（USC）or even no reaction（NR）regions once the heat is over-extracted.Interestingly,higher T_in and lower X_O2are found able to widen the UMC region under larger HLR conditions.Besides,the shifting of combustion regime from HTC to MILD combustion by heat extraction would be more effective with CO₂ dilution than either N₂ or H₂O dilution.