Heat Condensate Recovery Research And Application Of Gas-fired Boiler Flue Gas

As China adjusts energy structure, natural gas boilers will replace coal boilers in cities used widely in the heating supplying more and more, and how to use natural gas efficiently is becoming an important topic in the field of Energy and Environmental Engineering. The flue gas of natural gas boilers contain as much as15to20percent of water vapor commencing volume fraction, while the temperature of natural gas boiler flue gas is always above150℃. This part of the vapor is superheated, so its latent heat of vaporization can not be used and exhausted with the flue gas, causing huge heat loss. This paper focuses on the condensating heat recovery from natural gas boiler flue gas, systematically discussing the study theoretical and practical use.First, learning from Nusselt heat transfer coefficient of laminar flow of pure water vapor condensation, drive the expression of the heat transfer coefficient when the moisture flue gas condensing on the vertical wall and horizontal tube. As the non-condensable gases occupy the majority share of the flue gas, there exists a non-condensable gas boundary layer making the heat transfer coefficient of flue gas much lower than that of pure water vapor.Second, this paper researches the flue gas of natural gas used in Jinan, and analysis sensible heat and latent heat relations about the number and its trends. This paper raises that recycle flue gas latent heat of vaporization is the key factor to guarantee the condensing heat exchanger’s recovery efficiency. The paper also researches the exergy balance analysis of the natural gas boiler before and after the installation of the condensing heat exchanger, trying to find out the link of the exergy losses。Third, when studying the balance between losing heat of flue gas and gaining heat of boiler feed water, it is found that only boiler feed water will not gain enough heat to make the flue gas temperature as low as condensating enough water vapor in flue gas. Without enough water vapor condenating, the condensing heat exchanger’s efficiency is limited. To solve this problem, the paper gives a new idea to use combined heat exchanger which will use two or more low temperature heat source at the same time to gain heat from the flue gas and can make sure that the temperature of the exhaust flue gas will be low enough to ensure the latent heat of water vapor in the flue gas can be recovered as much as possible. To solve specific issue, this paper designs a combined heat exchanger, using boiler feed water and bathwater as low temperature heat source to recover the heat. And taking the deduced heat transfer coefficient as a reference, the author designs a finned-tube condensing heat exchanger which uses heating backwater to recover the heat.After installing the condensing heat exchangers, the author analysis the condensing heat exchangers’operating parameters, and find out that they work well as expected. The temperature reduced to below40℃, which will ensure the latent heat of vaporization can be recovered adequately. The condensing heat exchangers used on heating boilers can recover more heat when the heating backwater’s temperature is lower.