| The flue gas dew point is the key indicator of low temperature corrosion, which restricts the deep utilization of waste heat at the rear of the boiler. Traditional thermodynamic acid dew point calculation and measure mainly consider the influence of sulfuric acid vapor in flue gas, water vapor and oxygen, but ignore the influence of fly ash to the dew point. Dew point change has a great relationship with fly ash in actual application. In ash deposits existing condition, the corrosion situation of heat exchangers which operating under the thermodynamic acid dew point is not as serious as expected. Therefore, it is not appropriate to use thermodynamic acid dew point as the judgment standard of low temperature corrosion, which leads to higher exhaust temperature and lower boiler efficiency; The new design standard should fully consider the effect of ash to dew point and the environmental factors in the process of measurement, but the corresponding research is less. At the same time, with the implementation of energy conservation and emissions reduction and the development of low temperature waste heat recovery technology, there have been many heat exchange equipments operating below the dew point temperature, but the research of heat transfer characteristics and flow characteristics below the acid dew point has not yet started.As a consequence, beginning with several aspects such as thermal state model experiment, micro component analysis, soot particle characteristic composition analysis, the paper studies the formation mechanism of the dew point in the condition of ash, designs a new type of dew point test device, and research the rule of different heat transfer characteristics of heat transfer components and flow characteristics under the dew formation operation condition based on accurate measurement of dew point. The main work is as follows:1. As a result of acid condensation on the surface characteristics of ash particle impact, the ash is more easily to deposit on the surface of heat exchanger after absorbing acid, then the ash layer become more thicker, the thermal resistance of ash layer rises, the heat transfer coefficient goes down. The experiment concluded the change rule of heat transfer characteristics varied with outer wall temperature through direct measurement, studied the effect of steam condensation and ash deposition on heat transfer characteristics; and found the change rule of ash deposition and acid corrosion on heat transfer pipe according to ash deposition and corrosion situation on the outer wall surface; then the ash samples under different situations were analysis to get the micro surface state and elemental composition, then the more deeper influence theory of ash on dew point was found, the engineering acid dew temperature which is more suitable for actual flue gas environment was proposed and it’s measurement theory was proposed.2. An instrument used to measure the engineering acid dew temperature was proposed and measured the acid dew temperature in actual flue gas environment, then the dew temperature in experiment situation which can direct the design and reform of heat exchanger was measured. The measured acid dew temperature was compared with traditional dew point, these comparison can provide theoretical direction for heat recovery system design in power plant. A new evaluation standard for flue gas low-temperature corrosion was established.3. Based on the formation mechanism of the acid dew point in engineering applications, the test simulation of heat exchange about thermal performance in a single tube under the environment of ash and acid was established, and then researched its characteristic variation under the coupling of steam condenses and fouling. With different diameters of pipe and test conditions, the acid dew point in the pipe heat exchanger was validated in engineering applications. Through the analysis of a single tube influence of the dew point, the foundation about the selection of key equipments in measuring devices was established for the acid dew point. Research about the variation of the characteristics of the heat transfer in different diameters, flow rates of cooling water and temperatures on the outer wall, especially the effects on the heat transfer coefficient under different tube Re, made a detail analysis, and fitting them got the experimental correlation under the conditions of fouling, acid dew point and water dew point.4. Based on the principle of acid dew point test in the engineering applications, a test of simulation about hot spot under the in-line and staggered arrangement of tubes was built, and representative elliptical finned tubes,round H-fin tube bundles and oval H-fin tube bundles were selected to make site thermal tests. The heat transfer and flow characteristics of three types of tubes under different amounts of steam condensation were compared comprehensively, research about characteristic variation of heat exchanger in different types of tubes, layouts, flow rates and temperatures of tube wall, and detail discussions about the influence mechanism that how each factor works for the heat transfer characteristics and resistance characteristics, and summaries about overall EER values made a guidance for the selection methods of heat exchangers in different states.5. This paper simulated the main components of the flue gas, set up the experimental model that can reveal the formation mechanism of the acid dew point for the engineering application. The ash sample influenced by the sulfuric acid vapor condensation was made controlled by different flue gas temperature, acid content, water content and the soot concentration. The influence rule which condensate liquid of the acid brought to the performance of the soot formation was analyzed by experiment of the particle diameter, the adhesion and the sample heat conductivity coefficient.The following conclusions was obtained through the research above.(1) A brand new concept called acid dew point was put forward, which can replace the original concept the thermodynamics acid dew point as the new criterion for the engineering application, the formation mechanism was analyzed theoretically, we also measured the acid dew point in engineering practice in the test condition, the electron microscope energy spectrum and XRD were applied in the experiment.(2) The experimental equipment for the acid dew point was manufactured and used in the experiment. The result was obtained that the acid dew point was lesser35℃below the thermodynamics acid dew point. This will lay the foundation for the waste heat utilization in the low temperature.(3) The rule of heat transfer coefficient outside the tubes of double-pipe heat exchangers was obtained. As the average temperature of the outer wall and thermodynamic acid dew point super-cooling degree both go down, the overall heat transfer coefficient decrease, the heat exchange process step changes, and the heat transfer coefficient greatly reduced three times. Through comparative analysis, four stable heat transfer area in the experimental area were found, which were clean area, stable soot formation area, soot formation and acid condensation coupling area, soot formation and water condensation coupling area. (4) Heat transfer and resistance characteristics of typical heat exchange tube bundles in six working condition of different arrangement were obtained, and fitting experimental correlations under different working conditions were given. Inflection points of acid dew point and water dew point in engineering application were obtained.(5) Heat transfer and flow characteristics of three types of pipes under the condition of different amount of steam condensation were synthetically compared, and theoretical guidance for heat exchanger selection under different conditions was given. The oval "H" type finned tubes arranged in-line are superior to other forms under the acid dew point, which have better heat transfer and resistance characteristics.(6) The effects of the soot particle characteristic changes in the undercooling by smoke, acid content, water content and the concentration of soot particles was obtained, and the variation curve with the growth of a single factor was obtained. The formation mechanism of the acid dew point engineering applications was analyzed, and the best working conditions of the device in the different working positions was obtained.The concept of engineering applications acid dew point in this paper set a new standard to determine the low temperature flue gas corrosion, and laid the theoretical foundation for ultra-low temperature waste heat recycling technology research. The heat exchanger experimental model established in the condition of acid and ash created conditions for the development of the utilization of the waste heat. The variation law of heat transfer and pressure drop characteristics in the condensing environments of a single tube and different types of fin tube bundles provided new ideas and methods for heat exchanger design and selection working under the acid dew point. The conclusions had some theoretical research value and engineering applications value. |
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