581 / 2019-01-18 20:55:17

Experimental study on effective thermal conductivity of ash particles in coal-fired flue gas

effective thermal conductivity; ash deposition; ammonium bisulfate; Waste Heat Recovery System (WHRS); denitrification

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The ash deposition is the major concerns when evaluating the performance and reliability of tail heat exchanger of coal-fired boiler. Wherein, the effective thermal conductivity of ash particles is one of the most important parameters determining the heat transfer through deposit. Base on it, experiments and analyses have been carried out to investigate the influence of denitrification and flue gas Waste Heat Recovery System (WHRS) on effective thermal conductivity of ash particles by parallel hot-wire method in this paper. The results show that with use of SCR/SNCR, ammonium bisulfate (NH4HSO4) produced contribute to the increase of effective thermal conductivity of ash particles, which can be categorized into two zones according to the action mechanism of NH4HSO4. In both zones, the growth rate of effective thermal conductivity increases first and then decreases gradually. Furthermore, the rise of filling density and water content enhance the effective thermal conductivity of ash due to the porous and irregular structure of ash particles, but a small increase (60 ℃-170 ℃) in temperature has little effect on it. With use of WHRS, it is found that effective thermal conductivity of ash particles goes up with the increase of SO3 content when the flue gas temperature is lower than the Acid Dew Temperature(ADT), and the higher the content of acid condensation, the stronger effect on effective thermal conductivity. In addition, the formation of liquid sulfuric acid and ammonium bisulfate promote the accumulation between particles and increase the particle size, which in turn enhance the effective thermal conductivity. Thus, it is necessary to consider the influence of flue gas composition or products (NH4HSO4 and H2SO4) on the effective thermal conductivity for the thermal calculation of heat exchanger and the improvement of heat transfer efficiency in the tail of coal-fired boilers.