| With the background of vigorously implementing energy conservation and emission reduction,the application of media gas-gas heater and front-located air preheater with liquid mediator that transfer heat through the intermediate mediator gradually increases in power plants.In this context,this type of heat exchanger that transfers heat through liquid mediator is collectively referred as heat exchanger with liquid mediator.S,Cl,N,etc.in boiler fuel produce acid vapor through a series of chemical reactions after burning,and dew condensation occurs on the surface of heat exchange equipment when the flue gas temperature drops to a certain value.In this paper,a new type of fluorine steel pipe plate heat exchanger was introduced.Fluoroplastic steel heat exchanger is corrosion resistant and nearly no ash deposit because of PTFE material,and has good heat transfer performance and good mechanical properties due to stainless steel.Thus,the heat exchanger greatly reduced the cost compared to pure PTFE heat exchanger.It is of great significance of studying the new fluoroplastic steel heat exchanger for energy-saving in power plants.First of all,The corrosion behavior and corrosion characteristics of fluoroplastic steel,316L stainless steel,304 stainless steel and 2205 duplex stainless steel was studied by means of mass loss measurement and potentiodynamic polarization curve.Scanning electron microscope(SEM)was used to observe the surface morphology and elemental analysis of fluorine plastic before and after corrosion.There was no obvious corrosion morphology on the surface of fluorine plastic after being corroded by H2SO4 and NH4HSO4 solution,and no new element appeared on the surface of fluorine plastic after corrosion.The mass loss of immersion corrosion test showed that the corrosion of 316L stainless steel,304 stainless steel and 2205 duplex stainless steel in the solution of H2SO4 is more intensive than in the same concentration of NH4HSO4.2205 duplex stainless steel was more easily corroded than 316L stainless steel and 304 stainless steel in the acid solution.Fluoroplastic steel was almost no corrosion in the solution of H2SO4 and NH4HSO4.According to the potentiodynamic polarization curve,corrosion current increased and corrosion rate of 2205 duplex stainless steel increased with the increasing concentration of H2SO4 and NH4HSO4.316L stainless steel is more corrosion resistant than 304 stainless steel and 2205 duplex stainless steel in different concentration of H2SO4 and NH4HSO4.Secondly,according to engineering application of the first fluoroplastic air-preheater,the corrosion resistance and ash deposit characteristics of fluoroplastic steel heat exchanger were studied.Compared to stainless steel tube,fluoride plastic pipe has a a layer of fluorine plastic.The negative influence of heat transfer performance for that there is a layer of fluorine plastic is acceptable in the theoretical analysis.The on-site tests showed that the heat transfer performance of fluoroplastic steel air-preheater is better than that of the enamel air-preheater;the thermal resistance of ash deposit is much smaller than that of the enamel air-preheater.The main composition of deposits on the surfaces of fluoroplastic steel and enamel air-preheater was analyzed by using Energy Dis-persive Spectrometry(EDS)and X-ray Diffraction(XRD).Results show that in the acid environment,the enamel tubes react with oxygen and water in flue gas to produce iron oxides and hydrated oxides,which will attach to the surface of the heat transfer tubes.But the fluorine plastic steel air-preheater is almost no corrosion in the low temperature.Finally,fluoroplastic steel pipe is proposed to be used in the heat exchanger with liquid mediator in this paper,and the heat transfer model of heat exchanger with liquid mediator is established.The model takes the total heat transfer area of heat exchanger with liquid mediator as the objective function and reduces the variables in the model through non-dimensionalization.The solution of optimal temperature of the liquid mediator was given in the end.Then the multi-function selection genetic algorithm is used to optimize the structural parameters of the heat exchanger with liquid mediator. |
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