Numerical Simulation Of MILD Combustion Of MSW Pyrolysis Gas

In view of the increasing of garbage quantity in China,the medium and low calorific value gas（referred to as waste pyrolysis gas）obtained by pyrolysis treatment has been developed rapidly.The technology of mixed combustion with Moderate and Intense Low Oxygen Dilution（MILD）reduces local high temperature zone and reduces the discharge of pollutants by increasing momentum and diluting reactants.The combination of the two makes the MILD combustion of garbage pyrolysis gas have a significant advantage.In this paper,the typical pyrolysis gas of waste is taken as the research object,and the combustion characteristics and rules of mild are studied by numerical simulation.Firstly,the chemical dynamics research based on the perfectly stirred reactor（PSR）shows that the maximum temperature rise and NO_x concentration increase first and then decrease with the increase of excess air coefficient;the increase of flue gas circulation rate makes the maximum temperature rise of each excess air coefficient decrease obviously,and the effect on NO_x concentration and CO concentration decreases significant.When the flue gas circulation rate is 1 and 2,the excess air coefficient of the corresponding NO_x concentration value is100pmvd15%O₂ is about 1.36 and 0.76.By comparing the relationship between excess air coefficient and temperature rise,NO_x and CO concentration under different flue gas cycle rates of gri-mech3.0 mechanism and C2 mechanism,as well as the absolute rate of production CH₄and H₂ of GRI-Mech3.0 mechanism and C2 mechanism,it can be seen that GRI-Mech3.0mechanism is enough to describe the characteristics of MILD combustion.Secondly,the closed homogeneous batch reactor（CHBR）combustion model was used for transient analysis.The results show that during the process of hot desorption gas ignition,CH₄is converted into H₂ and CO before the ignition reaction occurs for 0.0043s;when the ignition reaction occurs in 0.0043s,CH₄、H₂ and CO react to generate H₂O and CO₂ at different rates.The residence time of the mixture in the high temperature region after ignition is closely related to the formation of NO.The critical excess air coefficient without flue gas circulation is 0.5 and2.33 when dividing the combustion area of MILD.With the increase of flue gas circulation rate,the MILD combustion region increases in both horizontal and vertical dimensions.In the horizontal dimension,the high temperature combustion（HTC）region and the conditional MILD combustion（CMC）region decrease synchronously,while the unconditional MILD combustion（UMC）region increases;in the vertical dimension,the HTC region decreases gradually and the CMC region increases relatively.Compared with HTC condition,the differences of UMC condition are as follows:the yield of thermal NO and non-thermal NO is lower when combustion reaction occurs;the yield of NO approaches zero after ignition;the reaction routes of three combustion stages are different,and the yield of NO in UMC condition is lower than that in HTC condition;the final concentration of NO_x is 143.8 ppmvd15%O₂ in HTC condition and 7.6 ppmvd15%O₂ in UMC condition.Thirdly,the PSRN model was used to study the mild combustion of pyrolytic gas.It was found that the increase of flue gas entrainment rate had more significant influence on combustion results when the entrainment rate was low;when the entrainment rate reached 4.6,the CO emission was lower than 50ppmv;when the entrainment rate was 3,NOx emission was the highest,and its concentration was 11ppmv.The increase of excess air coefficient will increase the flue gas quantity simultaneously,and the high value will make the fuel dilution too high and not conducive to combustion;when the excess air coefficient is 1.45,the CO concentration is about 50ppmv;when the excess air coefficient changes within the range of 1-3,the emission of NO_x is under 5ppmv.Compared with the fuel inlet temperature,the effect of the dilution air inlet temperature on the reactor temperature is significant,but the high air inlet temperature will make the temperature increase significantly affect the pollutant emission;when the temperature of the imported dilution air is below 1030K,the CO concentration is lower than 50ppmv;when the dilution air temperature is 800-1200K,and the NO_x emission concentration is below 7ppmv.The results show that the increase of hydrogen content in the pyrolytic gas component has little influence on NO_x emission,but it can significantly reduce the emission concentration of CO;when the volume fraction of hydrogen is increased from 0to 90%,the emission of CO is reduced from 74ppmv to 6ppmv;the NO_x emission of different hydrogen content is lower than 2.2ppmv.Finally,the three-dimensional simulation of MILD combustion in a burner is carried out based on FLUENT software.The results show that the increase of the number of air nozzles makes the air better package and mixing effect on fuel,the peak temperature of combustion increases,the average temperature and NO emission concentration first drop and then rise.In this study,the number of air nozzles n=4 is appropriate.The increase of fuel nozzle diameter is not conducive to the formation of flue gas reflux and low oxygen concentration area,and the peak temperature of combustion increases,but the proportion of high temperature area is small and the NO emission fluctuates slightly.In this study,D_f=8 mm is appropriate.With the increases of air nozzle diameter,the jet mixing is advanced,the recirculation zone decreases,the combustion reaction is violent,and the temperature level and NO emission increase,even beyond the range of clean combustion.In this study,D_a=8 mm is appropriate.With the increase of nozzle spacing,the combustion area is wider,the dilution of flue gas is enhanced,and the reaction is more uniform,which is beneficial to mild combustion.However,if the nozzle spacing is too large,the combustion is not sufficient.The optimal value must consider the jet velocity and the overall equipment size.In this study,the nozzle spacing S=15 is appropriate.When the calorific value fluctuates within±20%,the MILD combustion results change.