Energy Analyses And Process Integration Of Coal-fired Power Plant With Sodium-based Sorbents

Climate change has become an urgent problem and the increased greenhouse gases emission is a significant cause.CO₂ capture and storage（CCS）technology is considered to be an efficient solution to reduce CO₂ emission and slow down the global warming.Among all CO₂ capture and storage technologies,sodium-based CCS technology is thought to be a promising method due to the advantages of low cost,easy accessibility and good compatibility with existing coal-fired power plants（CFPPs）.It is necessary to evaluate the efficiency penalty of coal-fired power plants（CFPPs）using sodium-based solid sorbents prior to industrial applications.In this study,energy analyses and process integration of a CFPP with CO₂ capture system were conducted.The specific research was introduced in the following aspects:The energy analysis for a typical 300MW coal-fired power plant coupled with a sodium-based CO₂ capture system was conducted in Aspen Plus^?with achievable Na₂CO₃ loading and conversion ratio.Without heat recovery,the energy consumption for the CO₂ capture process was 7.23 GJ/tCO₂,higher than that of typical Monoethanolamine（MEA）-based CCS technologies,which is in the range of 3.8 to 4.2 GJ/tCO₂.Exergy analyses showed that most of export heat exergy loss arose in the sorption reactor and cooling bed,accounting for almost 40%of the total exergy.Heat from cooling bed and heat from released gas of desorption reactor were at a higher energy level,while the energy grade of heat from sorption reactor was too low to recover.The huge energy consumption was the biggest problem that needed to be solved for commercial application of the sodium-based post combustion CCS technology.Therefore,several heat recovery approaches were introduced to reduce the huge energy consumption,recovering sensible heat from cooling bed to preheat sorbents before desorption and recovering the heat of the regenerated gas to replace the low-pressure heaters.Simulation results indicated that the energy consumption could be decreased to 4.04 GJ/tCO₂ and net efficiency penalty was 15.87%.Sensitivity analyses showed that energy consumption showed independence on sorption/desorption temperature but great dependence on the Na₂CO₃ loading.Combined with existing experimental researches,30wt%-40wt%of Na₂CO₃loading was considered to be optimum for not only high CO₂ capture efficiency but also acceptable energy consumption.Absorption heat pump was introduced for further optimization of this sodium-based CO₂ capture system.The successful application of absorption heat-exchange unit provided the possibility to recover sorption reaction heat by low-temperature return water.Based on this,an economical system（CCPP-CUⅠsystem）connected with heat network was proposed,including the coal-fired power plant with CO₂ capture（CCPP）and a cogeneration unit（CU）with absorption heat pump.Three cases were discussed for comparasion.Thermal analyses showed that energy consumption of CO₂capture system in case C would be reduced to 1.08 GJ/tCO₂.Meanwhile,the fuel utilization coefficient of this whole system increased to 67.62%,showing great economic benefits.Different from CCPP-CUⅠsystem,CCPP-CUⅡsystem was disconnected from absorption heat pump.Energy analyses showed that energy consumption of this CCPP-CUⅡsystem was 1.14GJ/tCO₂,close to the thermal performance of CCPP-CUⅠsystem in case B and C.When the amount of extracted steam was constant,sensitivity analyses showed that the temperature of sorption reactor and low-temperature return water had greater influence on the energy consumption than other parameters.If the amount of extracted steam changed along with the flow of heat-network water,the exergy efficiency of this CCPP-CUⅡsystem decreased with the increase of heat capacity because of the exergy loss of heat-exchange process.