Research On Water-cooled Low-NO_x Burner For Gas-fired Heating And Hot Water Combi-boilers

Since the 21^st century,people have paid more attention to environmental issues,the large amount of nitrogen oxides produced by the combustion of gas appliances has became one of the society focuses.At the same time,more and more low-NO_x combustion technologies and low-NO_x burners have been used in gas water heaters which share more parts of markets.The water-cooled low-NO_x burner of domestic gas instantaneous water heater for heating and hot water have low-NO_x emission characteristic,but the reason of inhibiting the NO_x formation lack of the systematic research.In this paper,a comprehensive study on the performance of the water-cooled low-NO_x burner and the gas-fired heating and hot water combi-boilers with the burner is carried out.The main contents and conclusions are as follows.?1?The theoretical research method is used to analyze and compare the structural characteristics of water-cooled low-NO_x burner with conventional burner.Referring to the existing water-cooled low-NO_x burner structure,the key dimensions of the burner nozzle,ejector and fire hole were designed and calculated,and a copper burner sample was processed.?2?Through a combination of experimental research and theoretical analysis,this copper water-cooled low-NO_x burner was compared with a conventional burner,and it was found that water-cooled low-NO_x burner is approximately 10 ppm at rated load,which is much lower than conventional burner.The reasons for reducing the emission of NO_x include:the primary air is larger than 1 in the working state;the flame height is short;the water-cooled pipeline can reduce the temperature of the premixed gas.The gas flow model of the burner was established and studied by numerical simulation.The results show that the mixing condition and distribution of the premixed gas in the burner cavity under the rated load is good,which is conducive to the uniform distribution of the flame and reducing the local high temperature zone of the flame.?3?The flue gas emissions and combustion of copper water-cooled low-NO_x burner and aluminum water-cooled low-NO_x burner in conventional boiler and condensing boiler were compared by experiments.The performance of different structural boilers and different burners was analyzed,and the applicability,advantages and disadvantages of the burners were analyzed.The conclusion is that copper water-cooled low-NO_x burner has short and stable flame under rated working conditions,high thermal efficiency,extremely low-NO_x concentration,and the surface temperature of the burner is below 200?.But its CO emission concentration is higher than conventional burner.When installed in a condensing boiler,the CO emission concentration increases slightly,the NO_x emission concentration decreases,and the burner surface temperature is lower than 90?under rated conditions.The aluminum water-cooled low-NO_x burner has poor performance,insufficient air replenishment,flame non-uniformity and instability,and high CO and NO_x emission concentrations.?4?Aiming at the shortcomings of the two water-cooled low-NO_x burners,a number of improved methods were proposed.The experimental method was used to study nozzle improvement,burner gap improvement and burner cavity shape improvement for the two water-cooled low-NO_x burners,and effectively improve the combustion performance of existing water-cooled low-NO_x burners.The main conclusions include:24 kW copper water-cooled low-NO_x burner has the best pollutant emission control level at a distance of 5 mm from a nozzle of 0.89 mm and a distance of5 mm from a nozzle of 0.90 mm.When a gap is added between the copper water-cooled low-NO_x burner,the introduced secondary air can reduce the CO emission concentration in the flue gas,and the NO_x emission concentration increases.After modifying the cavity shape of the aluminum water-cooled low-NO_x burner,the NO_x emission concentration is significantly reduced,but the CO emission concentration is obviously increased and there is a problem of insufficient load.