Mechanism And Critical Technology Of Ultra-low No_x Emission In Flameless Combustion Of Nitrogenous Metallurgical Gas

The existence of fuel-nitrogen components in metallurgical gas not only increases the complexity of the NO_xgeneration path,but also emphasizes the importance of the control of fuel-NO_xformation in metallurgical furnaces.This study systematically investigates the reaction mechanism of nitrogenous gas.The detailed and simplified reaction mechanisms for nitrogenous gas combustion are developed.The nitrogen conversion pathway of the fuel-nitrogen in metallurgical gas combustion is also obtained.Moreover,this work develops the key technology of ultra-low NO_xemissions for the flameless combustion of nitrogenous metallurgical gas.Firstly,based on the jet-stirred reactor experiment and chemical kinetics simulation,the best detailed mechanism PG2018 is optimized.A new detailed reaction mechanism（PG2018mod）,including 151-species and 1408-step,is obtained,which improves the prediction accuracy of metallurgical gas combustion.Then,the new detailed mechanism PG2018mod is systematically simplified by methods of Directed Relation Graph with Error Propagation（DRGEP）and DRGEP-aided sensitivity analysis（DRGEPASA）.A nitrogen-containing skeletal mechanism（PG2018mod-S）including 38-species and 316-step is obtained.The validation results of the skeletal mechanism in predicting the ignition delay time,combustion temperature,nitrogen concentration,flame propagation,and extinction agree well with the detailed mechanism and experiments.The relative error is less than10%.Moreover,the skeletal mechanism can achieve the calculation acceleration of about15.8 while maintaining the accuracy of the original detailed mechanism.Thus,the skeletal mechanism is suitable for computational fluid dynamics（CFD）simulation.Secondly,based on the jet-stirred reactor,the detailed reaction mechanism PG2018mod is used to perform the chemical kinetic analysis.The nitrogen conversion pathways of three kinds of metallurgical gases are systematically examined according to sensitivity and reaction path analyses.The results show that,under fuel-lean and stoichiometric conditions,the fuel-NO is produced when the temperature exceeds about1000 K.And the fuel-NO is increased with the increase of temperature,while it is decreased with the increase of equivalence ratio,residence time（>0.1 s）,and the CO₂concentration.When there is initial NO,the lowest NO concentration of the coke oven gas combustion occurs at the equivalence ratio of approximately 1.5,and the maximum NO-reburning reduction is achieved in the temperature range of 950-1200 K.Compared with the blast furnace and converter gas,the significant amount of CH₄in coke oven gas can significantly enhance the NO-reburning reduction.Thirdly,based on a 20 k W laboratory-scale furnace,the experiment and CFD simulation are carried out to investigate the flameless combustion for suppressing the fuel-NO formation.The CFD simulation is performed with the finite-rate combustion modeling coupled with the developed 38-component nitrogen skeletal mechanism PG2018mod-S.The experimental validation shows that the CFD method has high accuracy for the combustion of nitrogen-containing gas and the formation of fuel-nitrogen.It is found that there is a critical equivalence ratio for establishing flameless combustion,and the critical equivalence ratio is decreased with the increase of air preheating temperature.In the equivalence ratio range of 0.67-0.87,flameless combustion can reduce the homogeneous fuel-NO emission by 60%-85%,compared with the swirl flame combustion.Finally,through experiments and numerical simulation in a 3 MW pilot-scale furnace of an iron and steel plant,a pilot-scale flameless burner of metallurgical gas,which can realize flameless combustion without highly preheating air,is successfully developed.The field test shows that the NO_xemission of the flameless furnace is reduced by about 66%compared with the conventional furnace.The developed flameless combustion of metallurgical gas can directly meet the new environmental protection requirements of ultra-low NO_xemission in the iron and steel industry.