Study on Recovery and Utilization Way of Latent Heat in Coal-fired Flue Gas

Author:Zuo Min

Supervisor:dong yong

Database:Doctor

Degree Year:2019

Download:11

Pages:162

Size:16279K

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As a major consumer of coal and water resources,coal-fired power plants undertake the task of energy saving and emission reduction while achieving the goal of electricity production.The saturated humid flue gas after wet desulfurization(about 50-55℃)contains a large amount of vapor and latent heat.The vapor and latent heat come from two parts:One is vapor generated by water evaporation and hydrogen combustion in coal.The other is vapor absorbed from desulfurization slurry in wet desulfurization tower.The latent heat carried in flue gas has not been recovered,leading to a great reduction of boiler efficiency based on high calorific value.The vapor absorbed by flue gas from desulfurization slurry accounts for about 80%of the water consumption in wet desulfurization tower,becoming the biggest water consumption of wet desulfurization.And a small amount of particulate matter such as desulfurizer and desulfurization gypsum carried in saturated wet flue gas will have adverse effects on the environment.So,if the vapor and latent heat in wet flue gas could be recovered simultaneously by effeective technical means and the latent heat in flue gas could be rationally utilized,it would be of vital practical significance for improving the thermal efficiency,promoting the popularization of wet desulfurization technology in water-deficient areas,eliminating the“white smoke,and so on.In this paper,the saturated humid flue gas after wet desulfurization of coal-fired power plants is taken as the research object,and the recovery and efficient utilization of latent heat from saturated humid flue gas is taken as the research objectives.The detailed thermodynamic theory analysis of latent heat recovery is carried out.A method of recovering latent heat from flue gas based on flash evaporation and condensation-heat pump is proposed.The different utilization ways of latent heat after being upgraded by heat pump are studied.The humidity-temperature chart of flue gas(d-t chart)was drawn.Based on the flue gas d-t chart,the changes of flue gas state during processes of adiabatic humidification and cooling condensation were explained.The sources of latent heat and vapor in flue gas were identified,and the recovery potential of latent heat and water from flue gas was calculated.On the basis of boiler efficiency based on low calorific value and high calorific value respectively,the benefit of flue gas regeneration with latent heat was studied.Four typical types of coal were selected and the effects of coal composition(mainly moisture content)on adiabatic saturation temperature,dew point temperature,recovery potential of latent heat and benefit of flue gas regeneration with latent heat were analyzed in detail.The results showed that:the higher the moisture content of coal,the greater the recovery potential of latent heat,and the latent heat recovery of flue gas has the greatest impact on the benefit of flue gas regeneration.The flue gas from lignite combustion has the greatest latent heat and water recovery potential.For 300 MW coal-fired power plant,when the flue gas temperature decreases from adiabatic saturation tenperature to 30℃,the latent heat recovery reaches 77.09 MW and water recovery reaches 115.6 t/h.At this time,the thermal efficiency of boilers based on high calorific value reaches 95.2%.In view of the similar structure characteristics between wet desulfurization tower and direct contact condenser,a new method of latent heat and water recovery based on flash evaporation and condensation-heat pump was proposed,including single-stage flash evaporation and condensation-heat pump system and two-stage flash evaporation and condensation-heat pump system,which provided a new direction for flue gas latent heat recovery.A pilot-scale system of single-stage flash evaporation and condensation was built.The variation of flash evaporation degree with superheat was studied experimentally and the composition of flash evaporation vapor was tested.Then,the feasibility of producing low-temperature slurry and recovering clean water by flash evaporation and condensation was verified.In order to improve the thermodynamic performance of flash condensation-heat pump as much as possible,the connection mode between the heat output side of flash condensation-heat pump and the external cooling water side was studied.Taking single-effect absorption flash condensation-heat pump system as an example,the thermodynamic performance calculation models of single-stage flash condensation-heat pump system,two-stage series system and two-stage parallel system were established and the calculation programs were programmed.The calculation results show that:when the low-temperature slurry of 35℃ is prepared by flash evaporation,the coefficient of performance(COP)of two-stage series system is the highest,up to 1.65,followed by two-stage parallel system,and the smallest single-stage flash condensation-heat pump system with 1.59 and 1.55,respectively.Finally,the economic benefits of district heating using latent heat recovered by two-stage series system were analyzed.The annual net income could come to 20.659 million CNY with latent heat recovered from flue gas of lignite combustion,while that was only 5.712 nillion CNY with latent heat recovered from flue gas of anthracite combustion,but the static payback period(about 39 months)is the same.Other utilization ways of latent heat were studied from the inside and outside of the thermal system respectively:Based on the principle of cascade utilization,the heat pump regeneration system of latent heat by preheating air was proposed on the basis of conventional low-temperature economizer system and optimized low-temperature economizer system,which realized the replacement of latent heat and high-level flue gas waste heat at the tail of boiler.Taking a 1000MW coal-fired power plant as an example,the detailed energy analysis,exergy analysis and techno-economic analysis were carried out.By using method of equivalent enthalpy drop and EBSILON software,it was found that:Compared with the reference unit,the proposed system could increase the net efficiency by 0.59 percentage points and save the standard coal consumption by 3.36 g/(kW·h),which were 0.16 percentage point and 0.98 g/(kW·h)more than those of the optimized low-temperature economizer system.The exergy utilization diagrams analysis showed that the exergy loss of the proposed system was 3.03 MW less than that of the optimized low-temperature economizer system and had better thermal performance.The techno-economic analysis showed that the annual net income is 11.7 million CNY,which is 2.867 million CNY higher than that of the optimized low-temperature economizer system.Combined with air preheater bypass,the proposed system was further optimized.The net efficiency could reach 44.39%,which is 0.89 percentage points higher than that of the reference unit,the standard coal consumption could be reduced by 5.24 g/(kW h),the exergy loss is 17.9 MW,10.42 MW lower than that of the optimized low-temperature economizer system,and the annual net income is 24.02 million CNY.Combining the process of chemical energy converting into heat energy,the heat pump regeneration system of latent heat by drying low-rank coal was proposed,which realized the replacement of low-level flue gas latent heat and high-level heat in furnace.Taking a 600 MW lignite-fired coal-fired power plant as an example,the detailed energy analysis,exergy analysis and techno-economic analysis were carried out.The results showed that the net efficiency could be increased from 42.25%to 43.61%,which is 0.3 percentage points more than that of the unit with conventional steam drying system,the exergy loss decreased from 47.1%to 44.4%,and was further reduced to 44.08%after recovering flue gas latent heat,and the annual net income is 29.854 million CNY,which is 5.972 million CNY more than that of the power plant with conventional steam rotary drying system.In addition to the two utilization ways mentioned above,the thermal efficiency could be improved by reducing the steam extraction of the environmental protection system.Aiming at the problem of high energy consumption of conventional organic amine carbon capture,the heat pump upgrading of latent heat and organic amine carbon capture were systematically integrated.A first-stage flue gas cooling and condensation integrated system and a two-stage flue gas cooling and condensation integrated system were proposed,in order to reduce the energy consumption of carbon capture simultaneously from processes of absorption and regeneration.The conventional carbon capture system and proposed system were simulated by Aspen plus software based on the rate model.It was found that the integrated systems of first-stage flue gas cooling and condensation and two-stage flue gas cooling and condensation reduced the energy consumption of carbon capture by 17.4%and 20.9%respectively.The related parameters affecting the energy consumption of two-stage integrated system were optimized,and the optimum parameters were obtained:the temperature of second stage flue gas is 25℃,the inlet position of second stage flue gas is 15-20,the split ratio of rich liquid is 0.4,and the feed position of second rich liquid is 11.Under the above optimum parameters,the two-stage flue gas cooling and condensation integrated system can reduce the energy consumption of carbon capture by 22.5%.