Study On Recovery Of Waste Heat From Low Temperature Flue Gas By Spraying Falling Film

With the upgrading of energy structure and the intensification of prevention and control of atmospheric pollutants,natural gas consumption is increasing year by year in China.Boiler is an important consuming equipment of natural gas,and its thermal efficiency is about 90%.The flue gas of gas fired boilers is rich in water vapor and its latent heat accounts for more than 10% of the low calorific value of natural gas,which constitutes the main heat loss of gas fired boilers that has a large space for waste heat recovery.Falling film flow has the advantages of simple structure,high heat transfer coefficient and low power consumption.At present,there is no publicly reported of low temperature flue gas waste heat recovery based on falling film spraying technology.Therefore,this paper proposes a scheme of waste heat recovery that the effect of waste heat recovery of different falling film spraying schemes and the main influencing factors were studied,which combined with vertical falling film condensation technology on the basis of existing low temperature flue gas waste heat recovery technology.Firstly,the principle and research methods of gas-water direct contact heat transfer in the process of waste heat recovery from spray falling film flue gas are analyzed in detail.Then in order to study the effect of falling film spraying technology on waste heat recovery of flue gas,a gas boiler flue gas waste heat recovery system was designed and a falling film heat transfer experimental bench was set up in order to carry out the experimental research.The heat transfer effect of different falling film devices(conical baffle and vertical plate)in the spray zone was investigated.The effects of water.gas ratio and air inlet temperature on waste heat recovery of these two types of falling film devices are discussed emphatically.Take the conical baffle as an example,the main factors that affected the heat transfer efficiency of the spray tower were investigated,which include the size of the hole,the distance between the holes,the initial spray angle,the direction of the spray,the spray density,row number and row spacing.The heat transfer characteristics of smooth plate and baffle,the different baffle lengths and the different number of different falling plate structures were also investigated.The results show that the heat transfer efficiency of conical baffle spraying is higher than that of plate falling film spraying under the same conditions(water.gas ratio and air inlet temperature)and the heat transfer performance of falling film spraying with baffles is better than that of smooth flat falling film spraying.For all spraying schemes,the heat transfer efficiency increases gradually with the increase of water.air ratio or the decrease of air inlet temperature within the scope of investigation.The heat transfer efficiency and the rise of spray water temperature four rows of baffles are better than that of two rows of baffles.The paper obtaines the main factors affecting waste heat recovery by designing different types of falling film devices,changing the structure,size and number of falling film devices and other parameters.It can provide an experimental basis for the application and optimization of waste heat recovery of gas flue technology based on the falling film spraying.