Translated Abstract
Coal has been the main energy in China and will still dominate during the next few decades. The flue gas produced from coal combustion contains a large amount of solid particles and SO3, which will cause severe low-temperature corrosion and particles deposition in the low temperature heat exchanger. At present, a lot of research work has been done on the characteristics of flow and heat transfer, fly ash deposition and low temperature corrosion in heat exchangers. However, the fundamental problems have not been solved. Many factors affect the flow, heat transfer, corrosion and fouling of heat exchangers, and these factors influence each other. So it is still quite required to further explore the mechanisms of the flow and heat transfer, low-temperature corrosion and fly ash deposition in heat exchangers.
(1) A novel type of finned oval tube is employed in this dissertation. The fin with bleeding dimple and rectangular vortex is developed based on the H-type finned oval tube. The flow and heat transfer characteristics of the novel finned oval tube are simulated and compared with the ordinary H-type finned oval tubes. It is found that the flow and heat transfer characteristics are excellent in the new finned oval tube. Then the characteristics of acid corrosion and fly ash deposition are also studied. The acid and water vapor condensation rate and the acid dew point temperature all decrease in the new finned oval tube. So the new finned tube can reduce the low temperature corrosion effectively. When the particle is large, the fly ash deposition will be decreased on the wall surface, but it will be increased on the fin surface.
(2) It is necessary to solve the acid corrosion and fly ash deposition in the low-temperature heat exchanger, after obtaining good characteristics of flow and heat transfer. The acid dew point temperature is the critical temperature of sulfuric acid vapor condensation, it is also the basis for studying low temperature corrosion. A new prediction model for acid dew point temperature is proposed based on the theory of gas-liquid equilibrium and the principle of component diffusion. The various factors affecting the acid dew point temperature are discussed, including the nature of coal type, excess air factor, combustion temperature, wall temperature, flue gas velocity and fly ash particles in the flue gas. Different from the traditional estimation formula for acid dew point temperature of flue gas, the method can simulate the local acid dew point temperature of flue gas at different positions on the wall surface. It makes the calculation more accurate about the acid dew point temperature of flue gas. The prediction method has a wide range of applications and can operate easily. It is of guiding significance for local prevention of low temperature corrosion in heat exchangers.
(3) The acid and water vapor condensation rate has important effect on the corrosion rate on the heat transfer surface. The turbulence influence is added in the acid and water condensation laminar flow model, and the improved condensation model is added to the FLUENT software through Uuser Defined Functions (UDF). The acid and water condensation rates and wall acid concentration in a complex heat exchanger are studied and the effects of acid vapor and water vapor concentration in the flue gas, the flue gas temperature and fin structure on the condensation rates of acid and water vapor are discussed. It is found that the condensation rates of acid and water vapor increase with the partial pressure of acid and water vapor in the flue gas, but decrease with the increase of flue gas temperature. The condensation rates of acid and water vapor are largest on the first row tube in heat exchanger.
(4) A numerical study is conducted on the characteristics of fly ash deposition in heat exchanger. The critical adhesion angle, the critical adhesion velocity and the critical removed velocity of particles are calculated by the particle transport model, particle deposition model and particle removed model. They are the conditions for determining the state of particles deposition on the heat transfer surface. The deposition number and quality of particles are simulated on the heat transfer surface. It is analyzed that the effects of several key factors (particle size, flue gas velocity, fin structure, and tube arrangement) on the deposition of particles on the heat transfer surface. The particles are easily deposited on the first row tube in in-line tube bank. However, the particles are more easily deposited on the second and third tube rows than the first tube row in the staggered tube bank.
(5) A coupling model of acid condensation and fly ash deposition is proposed to numerically solve the distributions of the low temperature bonding ash and loose ash on the heat transfer surface. The low temperature bonding ash easily deposits on the windward side of the fins and tube, but the loose ash easily deposits on the tails of the fins and the tube. The loose ash increases with the concentration of sulfuric acid vapor in the flue gas, while the low temperature blooding ash decreases with the concentration of sulfuric acid vapor. The number of low-temperature bonding ash is largest in the first row in in-line tube bank, while the number of loose ash is larger on the rear row tubes in the staggered tube bank.
Translated Keyword
[Acid dew point temperature of flue gas, Acid vapor condensation, Fin structure, Flow and heat transfer, Particles deposition]
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