Theoretical And Experimental Research On Direct Contact Air-humidified And Nitrogen Oxides Emission Reduction

Natural gas fired boiler would be one of main heat sources in the northern China after the promotion of "coal to gas".As for the natural gas fired boiler,current flue gas waste heat recovery solution based on economizer and air preheater only recovers a part of the sensible heat of flue gas.Most of latent heat in exhaust flue gas is directly discharged into ambint atmosphere.The latent heat in exhaust flue gas accounts for ten percent of natural gas low caloricity value.And emission of nitrogen oxides from flue gas of conventional natural gas fired boilers is currently beyond that of natural emission standard.Thus quality of the atmospheric environment becomes worsey in winter.Therefore,how to recover flue gas waste heat efficiently and reduce NO_x emission greatly are two problems to be sovled for dis trict heating systems based on natural gas fired boliers in northern China.This paper has conducted experimental and theoretical studies on a new process combined flue gas waste heat recovery and nitrogen oxides emission reduction which is based on direct-contact heat and mass transfer process.Sensitive factors in heat and mass transfer process of air humidification tower were studied,and impacts of combustion air humidification on NO_x emission reduction were analyzed.In this paper,two methods of nume rical simulation and experiment were used to study the effects of liquid to gas ratio,spray water temperature and air mass flow rate on the heat and mass transfer performance of air humidification tower and the thermal efficiency of heat source firstly.Secondly,the influences of air humidification on volume fraction of N2,O2 and combustion temperature were analyzed,and it could reduce the NO_x generate rate of boiler combustion.The curve of NO_x reduction ratio is gained through experimental data conduction in this paper.And the upper limit of air humidity ratio was determined by experiment.Finally,thermodynamic parameters and structural parameters of the spray humidification system for a 2t/h boiler were optimized by using mathematical model of air humidifying tower.The design principles of the air-humidified spray tower are given.the best matching relationship between thermal parameters and structural parameters of the air humidification tower was disclosed under the given operating conditions.The new combined process could increase the thermal efficiency of heat source by about 10%,and its denitrification rate could reach 50%.Therefore,the system based on new combined process has better energy-saving effect and more economic benefits,and it would be a promising application in flue gas waste heat recovery and NO_x emission reduction of natural gas fired heating boilers.