Experimental And Numerical Simulation Study On Flue Gas Recirculation Combustion Technology Of Gas-fired Boiler

Gas-fired boilers play an irreplaceable role in the development of heavy oil in oilfields.Aiming at the problem of high nitrogen oxide emissions from gas-fired steam injection boilers,this paper uses experimental study,numerical simulation and thermal calculation methods to research the relevant measures to reduce boiler nitrogen oxide emissions,and analyzes the impact of low-nitrogen combustion technology on boiler thermal efficiency.Firstly,the effects of air coefficient,boiler heat load and flue gas recirculation on NOx emission were studied in an indoor combustion experimental device using propane as fuel.The results show that:（1）With the increase of air coefficientα,the temperature and NOx concentration in combustion chamber increase first and then decrease.Whenα=1.1,the maximum temperature and NOx concentration reach 1557 K and 75 mg/m³;the CO concentration decreases first and then increases with the increase ofα.Whenα=1.1,the heat loss due to incomplete combustion of fuel is the lowest,which is 14 ppm.（2）With the increase of boiler heat load,the temperature and flame length in the combustion chamber increase,and the temperature and NOx concentration in the combustion chamber gradually increase with the increase of boiler heat load.（3）When the flue gas recirculation rate is 20%,the NOx emission can be reduced by 34.7%.But at this time,the CO concentration rises rapidly and the incomplete combustion loss of fuel increases,so it is appropriate to control the flue gas recirculation rate within 15%.Then,based on the computational fluid dynamics software FLUENT,the combustion and pollutant control model of the indoor combustion experimental device is established.Among them,the turbulent flow uses the Realizable k-εmodel,the combustion uses the finite rate/vortex dissipation combustion model,and the NOx generation rate uses the Zeldovich mechanism to predict.The combustion characteristics of NOx were simulated under different combustion conditions.The simulation results are in good agreement with the experimental results,which shows that the model can predict the combustion and pollutant generation law of gas-fired boiler.At the same time,the abundant information provided by the numerical simulation results further reveals the mechanism of NOx generation in combustion process controlled by relevant factors,and points out the direction for reducing NOx emission of gas-fired boilers.Then,a LPG fired steam injection boiler is simulated by using the gas fuel combustion and pollutant control model.The simulation results show that:（1）With the increase of air coefficientα,the maximum temperature in the combustion chamber first increases and then decreases,reaching the maximum value of 2279 K whenα=1.1;the concentration of NOx also increases and then decreases with the change of air coefficient,and the maximum NOx concentration after conversion is 139 mg/m³.（2）With the decrease of boiler heat load,the maximum temperature of fuel combustion,flame length and NOx concentration also show a monotonous downward trend.The maximum temperature of combustion decreases from 2279K to 2249 K,and the NOx concentration also decreases from 152 mg/m³ to 60 mg/m³.（3）Keeping the air coefficient and heat load unchanged,when the flue gas recirculation rate increases from 0%to 20%,the maximum temperature of combustion decreases from 2279 K to1920 K,and the NOx emission concentration also decreases from 152 mg/m³ to 57 mg/m³,but the CO emission concentration at the outlet of combustion chamber increases from 61 mg/m³to 133 mg/m³.Finally,based on the numerical simulation results at the exit of combustion chamber of YZF-11-21-P boiler,the thermal calculation method is used to study the heat transfer characteristics and boiler thermal efficiency of the high-temperature flue gas produced by combustion after entering the convection section of the steam injection boiler.Calculations proves that:（1）With the increase of air coefficientα,the radiant heat transfer in the combustion chamber decreases and the heat transfer in the convection section increases;the q₂ of boiler exhaust gas increases with the increase ofα,while q₃ of boiler gas increases first and then decreases with the increase of air coefficientα;Considering the changes of heat loss,incomplete combustion loss and exhaust heat loss,the thermal efficiency of steam injection boiler first increases and then decreases with the increase of air coefficientα,and the boiler efficiency is higher nearα=1.1.（2）With the increase of steam injection boiler load,radiation heat transfer,convection heat transfer,NOx concentration and exhaust heat loss will increase;As the increase of heat loss of exhaust gas is less than the sum of heat loss and incomplete combustion loss of gas fuel,the thermal efficiency of boiler increases gradually with the increase of load;therefore,on the premise of meeting the NOx emission standard,the thermal load of boiler should be increased as much as possible.（3）With the increase of flue gas recirculation rate,the radiation heat transfer of flue gas in the combustion chamber gradually decreases,and the heat transfer in the convection section gradually increases;the heat loss q₂and incomplete combustion heat loss q₃ of the boiler gradually increase with the increase of flue gas recirculation rate,resulting in the gradual decrease of the thermal efficiency when the flue gas recirculation rate increase.Therefore,when flue gas recirculation is used to reduce NOx emission concentration,it is necessary to comprehensively consider its adverse impact on boiler economy.