Homogeneous Reaction And NO_x Ultra-low Emission Mechanisms Of MILD Oxy-combustion

Oxy-fuel combustion is considered to be one of the most promising technology for large-scale carbon capture utilization and sequestration,which has been highly concerned in recent years.Combining oxy-fuel combustion with MILD combustion?Moderate or Intense Low-oxygen Dilution?,namely,MILD oxy-combustion technology,can not only improve the stability of oxy-fuel combustion,but also produce high combustion efficiency and very low emissions of NO_x and soot.It is known as one of the most promising new combustion technologies in the 21^st century.Aiming at the current situation that the basic theory of MILD oxy-combustion is relatively scarce,the present thesis systematically studies the characteristics of homogeneous combustion mechanism and NO formation under MILD oxy-combustion by means of experiments,numerical simulation and reaction kinetics analysis.Firstly,based on the JL?Jones and Lindstedt?mechanism developed by Kim et al.,the JL global mechanism under MILD oxy-combustion is modified by comparing the peak concentration and the equilibrium concentration of CO with the detailed mechanisms.Then,seven different versions of JL global mechanism are compared and validated with the two detailed mechanisms by furnace combustion,Jet in Hot Coflow flames and Plug Flow Reactor under oxy-fuel combustion,MILD air-combustion and MILD oxy-combustion respectively.The results show that the optimized JL-MO mechanism significantly improves the prediction of peak and equilibrium concentration of main species under MILD,oxy-fuel combustion and MILD oxy-combustion conditions.Then,seven methane detailed combustion mechanisms are systematically evaluated by using error evaluation function method under atmospheric and pressurized oxy-fuel combustion conditions,and the best detailed combustion mechanism is simplified and verified.The results show that the USC-Mech II is the best comprehensive performance mechanism under oxy-fuel combustion,and it can accurately simulate the formation of CO under oxy-fuel combustion.The skeletal mechanism including 34 species and 155 reactions and the reduced mechanism with 22 species and 19-step global reactions are developed.The relative errors of the skeletal and reduced mechanisms are less than 10%compared with the detailed mechanism and experiments.Further,six methane detailed combustion mechanisms containing NO_x are systematically evaluated using the experimental data of the in-house made Perfectly Stirred Reactor Reactor.The results show that th PG2018 is the best overall prediction mechanism and can be applied to oxy-fuel and air combustion conditions.The PG2018 is systematically simplified to the skeletal mechanism with 35 species and 259 reactions.The relative error between the skeletal mechanism and the detailed mechanism is less than 20%under both oxy-fuel and air combustion conditions.Next,the classical problem of the relationship between NO emission and equivalence ratio???is systematically studied through experiments,literature analysis and reaction kinetics analysis.Two different trends of NO emission under MILD combustion are summarized,that is,NO emission can increase first and then decrease,or decrease first and then increase,and its mechanism is analyzed.The recommended reaction parameters for obtaining ultra-low NO and CO emissions?NO?5 ppm and CO?10 ppm@3%O₂?are1100 K?T_r?1500 K,Y_O2?4%,??0.85.Finally,the effects of initial conditions on NO emission of methane MILD combustion are studied by experiments and CFD?Computational Fluid Dynamics?numerical simulation in a 20 kW furnace.It is found that the optimal equivalence ratio???for minimizing NO emission is between 0.8 and 0.9,and this trend is obtained at various P and T_O conditions.In the 20 kW experiment,the near-zero emission of NO?<1 mg/m³?is achieved near the optimal?.