Experimental Investigation Of Air Humidification Flue Gas Condensation Latent Heat Recovery And NO_X Purification System

The thermal efficiency of a natural gas boiler is about 90%,and the exhaust heat of the exhaust gas accounts for about 10-12%of the calorific value of the fuel.If the high-temperature flue gas is directly discharged,a large amount of energy is wasted.The NO_X content in the conventional natural gas boiler exhaust is generally higher than the Beijing Boiler Air Pollutant Emission Standard,and the pollutant discharge exceeds the standard.Therefore,the research on flue gas condensation latent heat recovery and low NO_X emission of natural gas boiler has great engineering application value.The temperature difference between the flue gas dew point temperature and the heat exchange medium is the key parameter determining the heat recovery effect of the flue gas condensation latent heat recovery device.Based on the idea of increasing the heat transfer temperature difference,this paper proposes to use the direct contact gas-water heat exchanger to preheat and humidify the combustion air,and at the same time,the flue gas is cooled and dehumidified,thereby recovering the latent heat of condensation of flue gas.The direct contact gas-water heat exchanger solves the problem that the specific heat capacity does not match when the combustion air and the flue gas are indirectly exchanged,and the low temperature combustion air becomes a low temperature cold source in addition to the heat return network in the single tower system,thereby Reduce the temperature of the heat exchange medium.The combustion air with high temperature and high moisture content flows into the combustion chamber of the natural gas boiler,and the flue gas of the natural gas boiler after combustion contains more water vapor than normal,thereby increasing the dew point temperature of the flue gas.Preheating and humidifying combustion air can increase the heat exchange temperature difference in both directions to improve the latent heat recovery of flue gas condensation.In order to study the effect of preheating humidified air on flue gas condensation waste heat recovery and reducing NOx,this paper uses a 58kW natural gas boiler as the flue gas source to build air humidification flue gas condensation waste heat recovery and NO_X purification system(single tower system).A dual cold source operation mode using hot water backwater and air is designed for the system.However,the NO_X emission reduction effect of the single tower system adopting this mode is restricted by the return water temperature of the heating network,so the single cold source operation mode using only air is optimized.The effects of factors such as liquid-gas ratio,heat network backwater temperature,air moisture content and boiler load rate on system heat recovery performance and NO_X emission reduction were studied experimentally.The experimental results show that the single tower system with double cold source operation mode can effectively recover the flue gas condensation waste heat under the conditions of 90%and 50%load rate,excess air coefficient 1.2 and three typical heat network return water conditions.The single tower system has the best heat recovery performance when the boiler has a 90%load rate and a 45°C heat network return water temperature.At this time,the total heating efficiency of the system can be increased by 12.16%,and the exhaust gas temperature is reduced to 51.52°C;in the dual cold source operation mode The single tower system can reduce the NO_X content in the exhaust from 88 mg/m~3 to 30 mg/m~3 at 90%load rate and 55°C heat network return water temperature.The single-column system with single cold source operation mode can recover flue gas condensation waste heat under the condition of 90%and 50%load rate and excess air coefficient 1.2 working condition,but the heat recovery effect is slightly worse than the double cold source mode.The heat recovery performance of the boiler is 90%at the load rate.At this time,the total heating efficiency of the system can be increased by 10.27%,and the exhaust gas temperature is reduced to 60.18°C.However,the single tower system has a 90%and 50%load rate in the single cold source operation mode.Under the working conditions,the NO_X content in the exhaust can be reduced from 88 mg/m~3 to less than 30 mg/m~3.In the single cold source operation mode,the single tower system can reduce the NO_X content in the exhaust to 22.48 mg/m~3.at 90%loading rate.The preheating and humidifying combustion air technology studied in this paper can jointly deal with the problem of flue gas condensation waste heat recovery and flue gas nitrogen oxide emission reduction.This technology has important application value for improving natural gas boiler heating efficiency and reducing NO_X emission.