Research Of Low NO_x Combustion Based On The Flue Gas Recirculation For Direct Vented Domestic Gas Water Heater

With the high attention to air quality in China,more and more attention has been attracted on nitrogen oxide emission,as the commonly used household gas appliances,domestic gas instantaneous water heater is also one of the pollution sources.To solve this problem,the method of recirculation flow of exhaust gas has been selected to reduce NO_x emission in this paper.With the experiments,the emission characteristics of flue gas with and without recirculation are studied,and the best scheme of recirculation flow of exhaust gas is proposed for the selected gas instantaneous water heater,which verifies the feasibility of recirculation flow of exhaust gas in reducing emission of NO_x of domestic gas instantaneous water heater.With the analysis of the formation mechanism of nitrogen oxide and carbon monoxide in the combustion products of domestic gas instantaneous water heater,it is concluded that the emission of nitrogen oxide and carbon monoxide is related to combustion temperature,combustion temperature,excess air coefficient,oxygen content,etc.As the formation mechanism of the nitrog en oxide and carbon monoxide are coupled,the emission of carbon monoxide may be changed with the reduction of the emission of nitrogen oxide.Comparing with the analysis of the commonly used measures to reduce the formation of NO _x,it is confirmed that the recirculation flow of exhaust gas is an effective way to reduce NO _x in domestic gas instantaneous water heater.Through the elaboration of the operation principle of the domestic gas instantaneous water heater,the experimental prototype is selected,the experimental system is established,and the experimental process,the experimental conditions and experimental methods are determined.The location of the flue gas recirculation,the type of the flue gas recirculation fan,the control method of the return flow and the inner diameter of the circulation pipe that affect the experiment are determined.With the experiment of the domestic gas instantaneous water heater,under the same condition of recirculation flow,the flue gas emission has been tested with the change of the heat input,and the results show that:1)the content of oxygen in the flue gas all decreased under different heat input in the recirculation model;2)with the burner all combusted,the concentration of nitrogen oxide in the flue gas is decreased and the concentration of carbon monoxide increased slightly with the recirculation flow of exhaust gas;3)with the half of burner combusted,the concentration of nitrogen oxide in the flue gas does not decreased and the concentration of carbon monoxide increased rapidly with the recirculation flow of exhaust gas.Then,the most suitable volume of recirculation flow of exhaust gas is determined with the change of volume of recirculation flow under different heat input,and the experiment results show that:1)with the burner all combusted,the concentration of nitrogen oxide in the flue gas is decreased with exponential function and the concentration of carbon monoxide increased with power function with the recirculation flow of exhaust gas,and it is considered that the best proportion of recirculation flow of exhaust gas is 35%according to the emission standard of carbon monoxide;2)with the half of burner combusted,the concentration of nitrogen oxide in the flue gas decreased gradually when the proportion of recirculation flow is less than 15%,and the fluctuation trend is displayed when the proportion is greater than 15%,however,the concentration of carbon monoxide increased rapidly when the proportion of recirculation flow is greater than 15%,so the most suitable recirculation proportion of 15%is determined.With the analysis of the low nitrogen combustion technology of balanced domestic gas instantaneous water heater based on flue gas recirculation,the feasibility of the method of flue gas recirculation to realize low nitrogen of gas water heater is verified,which provides a certain reference value for the low nitrogen optimization of domestic gas instantaneous water heater.