Numerical Simulation Of Natural Gas Combustion And NO_x Emission Characteristics By Cold Wall

Under Chinese“Double Carbon”target driven policy,the energy structure has been gradually improved,the proportion of coal consumption has decreased year by year,and the proportion of natural gas consumption has increased year by year.From 2009 to the end of2020,the proportion of natural gas consumption has increased from 0.9%to 8.2%.Although natural gas is relatively clean fuel,it produces NO_x pollutants due to high temperature in the combustion process.The emission of NO_x pollutants can no longer meet the increasingly stringent requirements for the emission limits of air pollutants.Therefore,it is of great engineering significance to study the combustion process of natural gas and the generation law of NO_x.In this thesis,the numerical calculation method is used to study the combustion reaction mechanism and dynamic characteristics,explore the interaction between chemical reaction and velocity field,temperature field and concentration field,and analyze the interaction mechanism between cold wall and flame,so as to obtain the near wall combustion theory of natural gas and the generation law of reducing NO_x.The specific research results are as follows:(1)A full mixed flow reactor model was used to describe the combustion process of methane and air combustion system.The 325 step elementary reactions of methane combustion were screened by sensitivity analysis method,and the elementary reactions that had the most significant impact on temperature and main component concentration were obtained.The comparative analysis showed that the obtained simplified reaction mechanism met the requirements of calculation accuracy,and could be used to simulate the combustion characteristics and NO_x generation characteristics.(2)By adjusting the parameters of methane and air combustion system,the results of chemical reaction kinetics calculation show that when the equivalence ratio increases from 0.5to 1.5,the generated NO_x mole fraction first increases and then decreases.When the equivalence ratio is 0.95,the NO_x mole fraction reaches the peak value,which is 0.21%;Under the same equivalence ratio,when 10%water vapor and 10%nitrogen are added respectively,the mole fraction of NO_x generation can be reduced by 0.16%and 0.12%at most,and the greater the amount,the greater the effect of inhibiting NO_x generation;Under lean burn conditions with equivalence ratio less than 1,increasing the amount of oxygen can reduce the amount of NO_x generated.When increasing 10%oxygen,the mole fraction of NO_x generated will decrease by 0.055%;Under the same equivalence ratio,the combustion temperature of hydrogen is higher than that of methane,and the mole fraction of total NO_x is0.18%higher at most.(3)The kinetic model of combustion process is established based on the simplified reaction mechanism.The calculation results show that under the same inlet jet velocity of 8m/s and the equivalence ratio of 0.8,1.0 and 1.2 respectively,the NO_x emission concentration obtained is consistent with the chemical kinetic calculation results.When the equivalence ratio is 1 and there is no cold wall,the NO_x concentration at the outlet of the combustion space is the highest,with a value of 60.36mg/m~3.After the cold wall is added at the flame front,the maximum combustion temperature is reduced by 200K,and the output of NO_x at the outlet is reduced to 48.22mg/m~3.(4)When the equivalence ratio is kept at 1 and the inlet jet velocity is changed to 6m/s,8m/s and 10m/s respectively,the temperature in the combustion space gradually increases with the flow velocity,and the amount of NO_x generation increases accordingly.When there is no cold wall with a flow rate of 10m/s,the maximum NO_x generation is 67.64mg/m~3.After adding cold wall,the NO_x concentration is reduced to 53.59mg/m~3.(5)When the equivalence ratio is 1.0 and the jet velocity is 8m/s,the positions of the cold wall on the axis of the combustion space are 160mm,190mm and 220mm respectively.The calculation results show that the cold wall is located at 160mm,and the NO_x emission concentration is 38.65mg/m~3.The cold wall is located at 190mm(near the flame front),and the NO_x emission concentration is 48.22mg/m~3.The cold wall is located at 220mm,and the NO_x emission concentration is 57.23mg/m~3.The comprehensive analysis shows that the location of the cold wall near the flame front not only ensures high combustion efficiency,but also significantly reduces the emission concentration of NO_x.(6)Under the condition of keeping the flow constant,the calculation results show that the combustion temperature of the two jets is higher,which has obvious superposition effect and produces more NO_x.By adjusting the spacing between the two nozzles to 0.5d,1.0d and 1.5d respectively,the superposition effect of the temperature field generated by the two jets is reduced,and the emission of NO_x is reduced from 121.65mg/m~3to 95.28mg/m~3.When the cold wall is set and the nozzle spacing is 1.5d,the NO_x generation is reduced from95.28mg/m~3to 75.07mg/m~3.