Numerical Simulation On Natural Gas MILD Oxygen-enriched Combustion Characterisitcs

MILD（Moderate&Intense Low Oxygen Dilution）oxygen-enriched combustion combines the advantages of energy saving and low NOx emission,which can cost-effectively control NOx emissions,and enrich CO₂in flue gas during natural gas combustion.However,the effects of oxygen concentration and its mixing method on MILD combustion characteristics and NO emission need to be further studied.In this paper,the MILD oxygen-enriched combustion characteristics of natural gas with premixed oxygen-enriched air were investigated numerically;then a novel non-premixed oxygen-enriched MILD burner has been proposed;and the performance of the novel burner under different diluent conditions was discussed.Firstly,MILD combustion characteristics of natural gas in the furnace built by International Flame Research Foundation（IFRF）were investigated under oxygen-enriched conditions by premixing air and oxygen,meanwhile,the establishment and maintenance of the MILD combustion state were analyzed under different oxygen concentrations and velocities.The results show that the uniformity of temperature becomes worse when oxygen is premixed with air if the burner structure remains unchanged;when the oxygen concentration is increased from 19.5%to 30%,NO production increases by 80%.Increasing the oxidant velocity can help MILD combustion to be established at higher oxygen concentrations and effectively suppress NO formation.When the inlet velocity was enhanced to 150m/s under the condition of 26.5%oxygen concentration,NO concentration in flue gas was reduced from 49ppm to 31.2ppm,which was already lower than that under the original condition.Next,a novel MILD oxygen-enriched burner is proposed,in which the oxygen is isolated by the diluent and separately injected to the furnace.The combustion behavior and NO emission characteristics of novel burner were compared with that of the original one under different oxygen concentrations.The results present that the new burner can effectively delay the mixing of fuel and oxygen,which is beneficial to the establishment of MILD combustion regime under oxygen-enhanced conditions.The designed new MILD burner has good features such as more uniform temperature distribution in comparison with original IFRF burner,moreover,NO emission is reduced by 58-88%when the oxygen concentration is 19.5%-26.5%.Finally,the combustion performance of the novel burner was further discussed when the oxygen was mixed with different diluents such as N₂,CO₂and H₂O,simultaneously,the effect of oxygen concentration on natural gas MILD combustion with CO₂as diluent was revealed.The results show that,compared with H₂O and N₂,CO₂is the best one as the diluent to achieve more homogeneous temperature distribution in MILD regime.Under O₂/CO₂atmosphere,the volume reduction of the reaction zone in the furnace is not obvious compared with that in O₂/N₂atmosphere when the oxygen concentration in the oxidants increases from 19.5%to 26.5%.Compared with oxygen-enriched air combustion,applying the novel MILD burner in oxy-fuel combustion mode（using recycled flue gas as the diluent）could obtain more uniform temperature distribution.