| The wet flue gas desulfurization（WFGD）process is widely employed for the control of SO2 concentration in coal-fired power plants to achieve ultra-low emissions.Generally,the flue gas after desulfurization will become saturated with a temperature of 50℃.Statistics show that more than 100t/h water vapor is contained in the flue gas emitted by a 300MW unit.The direct discharge of wet flue gas will waste a large amount of water and heat resources.In addition,it can also cause chimney corrosion,"white smoke","chimney rain" and many other problems.Consequently,the recovery of water and heat from wet flue gas can not only reduce water and coal consumption from power plants,but also weaken the influence of a series of environmental problems caused by wet flue gas emissions.Therefore,this technology is of great significance and has high value in engineering application.In this study,the low-temperature saturated flue gas from coal-fired power plants after WFGD is studied.In this study,a series of researches were explored including theoretical analysis of heat and mass transfer process between flue gas and absorption solution,change law of heat and mass transfer performance,pilot test of water and heat recovery from flue gas,and optimization of solution absorption system for consumption reduction,laying the theoretical foundation for the engineering application.The physical parameters of low-temperature saturated flue gas and commonly used absorption solution were analyzed in this study.Based on the phase equilibrium theory,the concept of equivalent moisture content of solution was proposed to characterize the solution vapor pressure.In addition,the energy and mass conservation equations for the reaction between flue gas and solution were established,and the analytical solutions of heat and mass transfer process were obtained.The influence of number of mass transfer units（NTU）and the liquid-gas mass ratio（R）on the flue gas outlet state was also analyzed.Based on the decoupling analysis of the heat and mass coupling reaction between the flue gas and absorption solution,the independent variable enthalpy difference Δh and the relative humidity difference Δφ ere proposed to characterize the driving force of the heat and mass transfer process between the flue gas and absorption solution.Moreover,the influence of inlet state parameters of flue gas and absorption solution on heat and mass transfer performance was analyzed on temperaturehumidity diagram.The regional characteristics of flue gas outlet state were also analyzed in this study.The above studies could provide guidance for the subsequent experiments.To further explore the mechanism of heat and mass transfer between the low-temperature wet flue gas and absorption solution,a countercurrent adiabatic falling-film test platform was built.CaCl2 solution was selected as the absorption solution.The influence of flue gas and CaCl2 solution parameters on the heat and mass transfer performance was analyzed.Results showed that the water and heat recovery efficiencies decreased with the solution temperature and increased with the solution concentration.When the solution temperature was 35℃ and the concentration was 45%,the highest water and heat recovery efficiencies of 50℃ wet flue gas were 67.5%and 58.1%,respectively.The increase of flue gas inlet temperature had positive effect on water and heat recovery efficiencies,which was more obvious in high-temperature flue gas.The water and heat recovery efficiencies of the low-temperature wet flue gas decreased with the Re of the flue gas,and increased with the Re of the CaCl2 solution.In addition,the test results showed that almost all heat released by the reaction of flue gas and CaCl2 solution was transferred to the solution side.The mass transfer coefficient of the gas side was much higher than that of the liquid side,indicating that the mass transfer resistance was mainly concentrated on the solution side.As the flue gas Re increased from 3700 to 6350,the gas-side mass transfer coefficient increased by 50.7%while as the CaCl2 solution Re increased from 120 to 380,the liquid-side mass transfer coefficient increased by 250.4%.The heat transfer coefficient was positively correlated with solution temperature,flue gas temperature and solution Re,and negatively correlated with solution concentration.The heat transfer coefficient was between 0.51 and 2.09kW/（m2·K）in the tests.In this study,the synergistic reaction between CaCl2 solution and SO2 was carried out,and the solubility of the experimental precipitation product was tested because there was still a small amount of SO2 existing in low-temperature flue gas after wet desulfurization.The results showed that the composition of the precipitation product in the reaction was mostly CaSO4·2H2O,only a small amount of CaSO4 existed in the precipitation.The solubility of CaSO4·2H2O in CaCl2 solution first increased and then decreased with the solution temperature.When the solution temperature was 65℃,the maximum solubility under different solution concentrations was obtained.The pH value of CaCl2 solution increased with the solution temperature and concentration,and the CaCl2 solution with higher pH value had greater SO2 removal performance.O2 was necessary for CaCl2 solution to absorb SO2.When the O2 content was between 5%and 15%,the promotion effect on SO2 removal was most obvious;when O2 content was higher than 10%,the reaction precipitation products almost all changed from CaSO3 to CaSO4.The addition of Ca（OH）2 could increase the pH value of the CaCl2 solution and enhance the removal efficiency of SO2.In addition,Ca（OH）2 could also reduce the solubility of CaSO4·2H2O in the CaCl2 solution and promote the formation of precipitation.As the Ca（OH）2 concentration increased from 0 to 0.05mol/L,the removal efficiency of SO2 increased by more than 25%;and the solubility of CaSO4·2H2O in 40%CaCl2 solution with a temperature of 40℃ decreased from 0.133g/L to 0.077g/L.The regeneration performance of CaCl2 solution was studied,and a low-pressure flashing regeneration test system was built to explore the effects of the solution temperature,concentration and regeneration pressure on the evaporation performance.The results showed that CaCl2 solution with a higher temperature and a lower concentration had a higher evaporation rate.With the regeneration pressure increased from 8kPa to 18kPa,the evaporation rate of CaCl2 solution dropped from 3.27g/L to 0.83g/L,and the evaporation rate dropped from 8.93%to 2.27%.To further reduce the regeneration energy consumption of CaCl2 solution,a novel non-evaporation solution extraction regeneration technology based on the difference in water solubility of the extractants at different temperatures was proposed.DIPA,EBA and DMCHA were selected as the solvents.The water solubility characteristics of different solvents at different temperatures were measured,and the influence of the CaCl2 solution concentration on the extraction efficiency and Cl-removal performance were also explored in this study.Results showed that DIPA had highest extraction efficiency,the extraction efficiency decreased with the CaCl2 solution concentration.A highest extraction efficiency of 4.1%was achieved when the CaCl2 solution concentration was 20%.The Cl-removal efficiency increased with the CaCl2 solution concentration.DMCHA had the highest Cl-removal efficiency of>99%,and the Cl-concentration in the product water was approximately 2000mg/L.Based on theoretical analysis and mechanism test results,a pilot test system was designed and built.Low-temperature wet flue gas with a volume of 1000Nm3/h was introduced in the system for water and heat recovery with the method of solution absorption.In the pilot test,CaCl2 solution with a temperature of 50℃ and a concentration of 40%was selected as the absorption solution.In addition,the liquid-gas ratio was 2L/m3.The water recovery efficiency of 44.0%and the heat recovery efficiency of 35.5%were achieved when the system was in stable operation.The average temperature difference of the cooling water reached 19.3℃,and the maximum temperature difference was 20.5℃.As a result,the coefficient of performance（COP）value of this system was 1.21.The pollutants concentration of the inlet and outlet flue gas was tested,and the results showed that the SO2 removal efficiency in the test was between 40%and 55%while the average NOx removal efficiency was 9.6%.The phenomenon of white smoke existing in the outlet flue gas was completely eliminated.In addition,the quality analysis on the condensation water was conducted,and the results showed that the recovered water was in great quality,with a pH of approximately 4.3.According to the results of the pilot test,the mothod of net present value（NPV）was applied to evaluate the economics of the solution absorption system when applied to a 300MW unit.The results showed that a recovery period of 4.95 and 4.70 years were achieved when condensed water was used for industrial and domestic,respectively.In order to reduce energy consumption during solution regeneration,then increase the COP values,the solution absorption system was optimized in this section.Based on low-pressure flashing,an open absorption heat pump system（OAHP）applied for water and heat recovery from low-temperature wet flue gas was proposed.In addition,two optimized systems,twostage regeneration open absorption heat pump system（TROAHP）and part-regeneration open absorption heat pump system（PROAHP）were also proposed.Thermodynamic models were established for the above systems,and the COP values of the systems under different parameters were calculated.The simulation results showed that the COP of the heat pump system decreased with the temperature and concentration of the CaCl2 solution,and increased with the flue gas temperature.With the pressure of solution regeneration increased,the COP of the OAHP system first increased and then decreased.When the regeneration pressure was 70～90kPa,the COP of the OAHP system increased by 0.5%-2.2%.TROAHP system achieved the highest COP value when the regeneration pressure was 70kPa,which was 4.2%higher than the heating regeneration system.When the regeneration ratio RR was 0.4 to 0.9,the COP of the PROAHP system increased by 2.1%to 17.3%,and when the RR was 0.6,the highest COP value of 1.7 was obtained.The exergy analysis of different systems showed that when the regeneration pressure exceeded 80kPa,the exergy efficiency of the TROAHP system was higher than that of the OAHP system;and when the regeneration ratio RR exceeded 0.6,the exergy efficiency of the PROAHP system was higher than that of the OAHP system.The exergy loss of the heat pump system was mainly concentrated in the regenerator,accounting for more than 54.9%.In order to further reduce the energy consumption,a novel heat pump system based on nonevaporation regeneration method was proposed.The thermodynamic calculation results showed that compared with heating and flash regeneration,the energy consumption of extraction regeneration was reduced by 22.7%～51.6%.The novel open absorption heat pump system associated with extraction regeneration could achieve a COP of 1.71 and an exergy efficiency of 8.53%.The application of solution absorption technology in the water and heat recovery from low-temperature wet flue gas has advantages in high water and heat recovery efficiencies and obvious synergistic removal of pollutants.Through the theoretical analysis of the heat and mass transfer between the low-temperature wet flue gas and the CaCl2 solution,the pilot test performance research and the optimization of the solution absorption system for reducing energy consumption,this study can provide theoretical support and practical guidance for the application of the solution absorption technology in the low-temperature wet flue gas water and heat recovery process. |
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