Energy Analysis And Optimization Of Mea-absorption CO₂ Capture Sysytems For Coal-fired Power Plants

CO2 capture in coal-fired power plant is of great importance for reducing CO2 emission. Monoethanolamine-based chemical absorption is a comparatively mature technology for large-scale CO2 mitigation in power plants. However, this technology causes large energy consumption and reduces the efficiency of power generation unit. Therefore, energy utilization analysis and evaluation for the CO2 capture system, analysis and optimization for the energy-saving method are carried out. The main contents are summarized as follows:The mathematical models for the carbon capture system are established and energy consumptions are calculated. Then the energy analysis and evaluation for the CO2 capture system are carried out. The results show that the specific heat consumption is 4.854GJ/kg CO2 and the specific exergy consumption is 2.003GJ/kg CO2. The energy consumption is very large, mainly because of the large heat consumption in the reboiler. In addition, the exergy efficiency and the waste heat recovery efficiency are low.Pinch analysis for the heat exchanger network in CO2 capture system is used to examine the possibility for reducing the energy penalty. Then the heat engine, heat pumps and refrigerators are integrated into the capture system to further recover the waste heat. Pinch analysis shows that the pinch temperature is 109℃, and the maximum energy saving is 19%. Heat integration analysis shows that placement of the absorption refrigerator below the pinch to recover the waste heat to liquidize the CO2 gas is feasible.Effect of the different extraction point on the capture system and the power plant are analyzed. In order to utilize the fifth extraction steam’s energy, optimization mathematical models for the back-pressure turbine and the steam ejector are established and solved. Analysis and optimization results show that the sixth stage extracted steam from the steam turbine unit is the best option. Using the back-pressure turbine and steam turbine to utilize the fifth extracted steam energy are effective and feasible. In addition, a new evaluation index, which is named the specific exergy comsumption of the extracted steam, is proposed to evaluate the degree of perfection of steam energy utilization process of the CO2 capture system.Rational methods including the heat exchanger and the absorption heat pump are proposed to recover the heat of the lean solution. And the optimization mathematical models for the heat exchanger and the absorption heat pump are established and solved. For the heat exchanger to recover the heat of the lean solution, optimization mathematical models with the total net profit as object function are established and solved. And based on the thermodynamic calculation results for the absorption heat pump, the effect of the outlet temperature of the lean solution on the coefficient of performance, the required mass flow rate of the lean solution and the hightest temperature of the solution in the generator are analyzed. The results show that the heat exchanger to recover the heat of the lean solution is the best option.Rational methods including the heat exchanger, the absorption heat transformer, the steam ejector and the absorption refrigerator are proposed to recover the heat of the condensate. And the optimization mathematical models are established and solved. First of all, for the heat exchanger to recover the heat of the condensate, optimization mathematical models with the total net profit as object function are established and solved. And then, based on the thermodynamic calculation results for the absorption heat transformer, effect of the outlet temperature of the condensate on the coefficient of performance, the absorption temperature and the required mass flow rate of the cooling water are analyzed. Finally, the optimum designs for the steam ejector and the absorption refrigerator are carried out. Comparison of the different methods shows that the heat exchanger to recover the heat of the condensate is the best option.Rational methods including the absorption heat transformer, the steam ejector and the absorption refrigerator are proposed to recover the heat of the CO2+H2O stream. And the optimization mathematical models are established and solved. Firstly, effect of the outlet temperature of the driven water on the coefficient of performance, the absorption temperature and the required mass flow rate of the cooling water are analyzed based on the thermodynamic calculation results for the absorption heat transformer. Then, the optimum designs for the steam ejector and the absorption refrigerator to recover the heat of the CO2+H2O stream are carried out. Comparison of the different methods shows that the absorption refrigerator to recover the heat of the CO2+H2O stream is the best option.The integrated energy-saving superstructure which includes all possible energy-saving methods is established. And the optimization mathematical models for the integrated energy-saving superstructure are established and solved. In addition, sensitivity analysis for the optimal integrated energy-saving method is performed. Based on the pinch analysis results, the optimization results of the utilization the fifth extraction steam, the analysis and optimization results of the heat recovery of the lean solution, the condensate and the CO2+H2O stream, possible energy-saving method are proposed which can be considered in the integrated energy-saving superstructure. Optimization results show that the integrated energy-saving method which uses the back-pressure turbine is the best. For the optimal integrated energy-saving method, the specific exergy consumption is 1.574GJ/kg CO2. If the back-pressure turbine is not to be considered, the integrated energy-saving method which the fifth extracted steam is introduced into the reboiler is the best, and the the specific exergy consumption is1.690GJ/kg CO2.