| Boiler exhaust is a rich low-temperature waste heat resource.In order to avoid low-temperature corrosion of the flue and equipment,the design exhaust temperature of coal-fired boilers in China is mostly around 130 ℃,and some high-sulfur coal boilers can reach160 ℃ or even higher.On the other hand,the latent heat of vaporization is abundant in the low temperature and wet saturated flue gas formed by limestone-gypsum desulfurization process.If a flue gas deep treatment system can be designed and use low-pressure economizer(GWH)and desalinated water condenser(CDH)made of low-temperature corrosion-resistant materials to connect this system in series,it can deeply condense the low-temperature and wet-saturated flue gas while recovering the waste heat well.The application of the system can play the dual role of energy saving and emission reduction with better promotion value and competitiveness.Firstly,this paper describes a detailed study of the characteristics of a new material of fluoroplastic steel resistant to low temperature corrosion.Fluoroplastic steel refers to a composite material processed by hot melting and molding of soluble Polytetrafluoroethylene(PFA)or other fluoroplastic materials directly on the surface of a steel pipe and combining each other seamlessly.The fluoroplastic steel low-pressure economizer should be installed behind the dust collector to reduce the wear of the flue gas dust on the surface fluoroplastics.Fluoroplastic steel has good one-sided corrosion resistance and can resist the corrosion of strong acids such as aqua regia,strong alkalis,strong oxidants and most organic solvents.The relative thermal expansion coefficient of the fluoroplastic steel in the axial direction is 4.38×10-5 cm/(cm·℃),and the O-ring flexible connection structure is used in practical applications.Fluoroplastic steel has good dust resistance,but also note the flue gas control,wall temperature control and reasonable setting of ash blowing.The use temperature of fluoroplastic steel should be ≤260 ℃,avoid the influence of welding high temperature on the material and scratches on the outer surface during manufacturing and installation.Long-term stable operation of fluoroplastic steel can be guaranteed in low temperature flue gas environment under reasonable setting and correct use.Secondly,this paper discusses the whole design process of flue gas heat exchanger of fluoroplastic steel.Fluoroplastic steel flue gas heat exchanger is a tubular heat exchanger.Heat exchange and structural design should be firstly designed,and then the stress and vibration check.While ensuring that the heat exchange and resistance performance of the equipment meet the requirements,stress and vibration should also be controlled to ensure the safe operation of the equipment.Thirdly,this paper studies the process design of the low-temperature flue gas deep waste heat recovery system of the new fluoroplastic steel material.The system is composed of fluoroplastic steel GWH and CDH and the auxiliary systems,which can realize deep recovery and utilization of flue gas waste heat,water saving,further reduction of pollutant emissions such as dust /NOx/SOx,and reduction of the white smoke plume.The total added smoke resistance of the system is no more than 400 Pa,and the fan allowance of the original system can generally meet the requirements of the system.The system can improve the efficiency of desulfurization,reduce the water consumption of desulphurization tower and contribute to the operation of desulphurization tower,but attention should be paid to the influence of the system on the water balance of desulphurization tower.What’s more,the system can also improve the removal efficiency of the unit’s fine particles,remove part of moisture from the flue gas and discharge a large amount of salt substances.The total benefit of the system is 2.499 million yuan,the cost can be recovered within one year.Finally,this paper verifies and analyzes the actual application in the project.Under rated conditions,the average heat exchange of GWH is 1518.1 k W;and CDH is 1721.5 k W;the average total heat exchange coefficient of GWH is 51.0 W/(m2·K);the total average heat exchange coefficient of CDH is 220.7 W/(m2·K);The average value of the collected flue gas condensate is 2.16 t/h.In the automatic state,the operating state of the system is mainly affected by the main steam load of the unit.After the system has been in operation for half a year,the flue gas resistance parameters of the system were normal,and the furnace shutdown inspection found that there is no obvious dust accumulation on the surface of the heat exchange tube,no traces of corrosion or stress damage,and the relative displacement of the tube film is within the expected range.It shows that the system has shown strong reliability and can operate safely and reliably for a long time in a low-temperature flue gas environment. |
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