Simulation Studies On The Process Of Partially Premixed Combustion

Since the implementation of the"coal to gas"policy,natural gas combustion has become the main source of nitrogen oxides,therefore,low-NO_X combustion technology has important significance.The experimental result shows that the oxygen diluted partially premixed/oxygen enriched supplemental combustion(ODPP/OESC)based on the physical process control can balance the chemical reaction rate and effectively reduce NO_X by adjusting the space distribution of fuel/oxidant.In order to further explore the internal flow field of ODPP/OESC and to deepen understanding of its combustion process and NO_X generation mechanism,based on the experiment,the detailed chemical reaction mechanism of methane combustion—GRI-Mech3.0 was applied to FLUENT flamelet model to simulate the partially premixed one-dimensional counterflow flame and three-dimensional swirl flame.The simulation of ODPP/OESC counterflow flame shows that the recirculation zone formed on the two sides of the outlet of the partially premixed gas nozzle is the main cause of the bulge of the counterflow flame center at the lower partially premixed equivalenec ratio,while the two sides of the recirculation zone disappear,the counterflow flame center bulge disappears,the flame tends to straight at the increase of the partially premixed equivalence ratio.On the basis of one-dimensional counterflow flame,the partially premixed swirl flame is studied deeply.The simulation results show that the same flame structure with the experiment.Furthermore,it is found in the research of its combustion flow field structure that oxygen concentration of the partially premixed gas decreases,the corner recirculation zone decreases,the central recirculation zone increases,the velocity gradient of the shear boundary layer of the two zones increases.The dynamic combustion zone and the central recirculation zone are formed in two zones of high temperature.Finally,the influence of the changes of operating parameters on the partially premixed swirl combustion process and NO_X formation mechanism was studied.The dynamic combustion zone temperature of partially premixed combustion(PPC)is higher than ODPP/OESC.The temperature of the dynamic flame is greatly reduced and the diffusion flame temperature is kept constant by reducing the dilute oxygen concentration,so the ODPP/OESC can balance the rate of the chemical reaction between the dynamic combustion zone and the diffusion combustion zone.The NO generated in the dynamic combustion zone is mainly thermal NO,while in the diffusion combustion zone,prompt NO is increased and the proportion is increased.ODPP/OESC reduces the flame temperature by reducing the dilute oxygen concentration,thus inhibiting the formation of thermal NO in dynamic combustion zone.While the rich oxygen concentration of the diffusion combustion zone is increased,the nitrogen concentration is decreased and prompt NO is reduced rapidly.The NO emission concentration at the chimney exit can be significantly reduced compared to PPC.With the increase of the partially premixed equivalence ratio,the temperature of the dynamic combustion zone and the diffusion combustion zone decrease,and the NO emission concentration tends to decrease.The temperature distribution in the dynamic combustion zone and the diffusion combustion zone is more uniform and the local high temperature zone is reduced by improving the swirl intensity,which can effectively inhibit the generation of thermal NO in the dynamic combustion zone.The NO generation mechanism in the diffusion combustion zone changes in the high swirl number,so NO of the thermal type increases slightly,while NO of the prompt type decreases slightly.