| With the increasing attention to energy conservation and environmental protection in China,the proportion of natural gas in the energy composition of heating in North China is increasing.Recovery of the waste heat of flue gas is the main way to improve the efficiency of natural gas utilization,relieve the pressure of gas consumption and reduce the economic burden of the govemment and enterprises.It is an important technical route to realize the efficient recovery of flue gas waste heat by using the direct contact heat exchange of gas-water.At present,the main research and application is single stage gas-water heat exchange,in which a lot of entranspy dissipation will be produced.In this paper,the model of gas-water contact heat exchange is established,and then the heat exchange process is reasonably divided.The water is divided into several sections by volume and spraying into flue gas at different positions.The theoretical model is verified by experiments,and then the influence of subsection heat exchange on system configuration is studied.In this paper,different segmented methods are used to reduce the entranspy dissipation caused by gas-water heat exchange,so that the energy efficiency and economy of flue gas heat recovery system are improved.In order to accurately simulate the process of gas-water heat exchange,spraying tower model of gas-water contact heat exchange was established by numerical method.The heat and mass transfer process of gas-water in the spraying tower was studied and the influence of various factors on the heat exchange effect was analyzed.The atomization characteristics of typical vortex and outer spiral nozzles were tested and analyzed.The normal distribution of water droplet size and the conclusion that the mean diameter of sotell can be used for comprehensive heat transfer calculation have been obtained.From the point of view of reducing entranspy dissipation,for the saturated gas-water heat exchange,the technical ways of selecting the reasonable segmented points in different ways were put forward.The numerical solution of segmented points has been obtained through geometric analysis.The analytical solution of partial segmented points can be obtained by using Taylor expansion to simulate the flue gas heat exchange,and the numerical solutions were verified.According to the characteristics of superheated flue gas,a method to solve the segmented point is proposed.According to the segmented points,the entranspy dissipation and the corresponding equipment parameters can be calculated quantitatively.When the exhaust gas temperature is 10?,the four stage spraying can reduce entranspy dissipation by more than 95%compared with the first stage spraying.With the more segmented points,the height of the spray tower and the spray water flow will increase.The experimental platform and demonstration project have been set up to verify the theoretical results.The vertical reverse-flow spraying tower for gas-water heat exchange and the flue gas waste heat recovery system of typical gas boiler were built respectively.Through the test,the error between the simulated and measured value of flue gas and water were generally within 2?,which verifies the model of spray tower.At the same time,using twostage spray compared with one-stage spray can reduce the unmatched entranspy dissipation by over 62%.This paper studies the influence of segmented heat exchange on the configuration of flue gas heat recovery system,models for equipments in the flue gas heat recovery system has been established,evaluation indexes in terms of system energy efficiency and economy have been proposed.The subsection process of flue gas waste heat recovery system has been constructed,and the application of subsection spray heat exchange in typical flue gas waste heat recovery system has been studied.Compared with single-stage heat exchange,the segmented heat exchange has certain advantages in system energy efficiency and economy.When the exhaust gas temperature is 10?,the four-stage spray can reduce the energy consumption by over 36%and the total cost by more than 28%compared with the one-stage spraying. |
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