Energy Dissipation Characteristic Analysis And System Integration Optimization Of Large-scale CO₂ Capture For Fossil Power Plants

The global climate change caused by greenhouse gas emissions is the most serious environmental issue facing humanity in this century.CO₂ is the main type of greenhouse gas,whose annual emission accounts for 80%of the total greenhouse gases produced by human activity.China is one of the world’s largest emitters of CO₂ and fossil-fired power plants are responsible for more than half of the total CO₂ emission.Therefore,CO₂ capture and storage technology in fossil-fired power plants is essential to achieve CO₂ mitigation in China and all over the world.In this thesis,fossil-fired power plant is selected as the research object and the hugh energy consumption for CO₂ capture in fossil-fired power plants is focused on by author.Firstly,under the unit level,the energy consumption characteristic of the CO₂ capture unit is analyzed and the integration principle is summarised.Secondly,under the system-wide level,the overall integration optimization is carried out based on the above integration principle in term of the coal-fired power plant and natural gas combined cycle（NGCC）power plant respectively.Lastly,the comparasion of monoethanolamine-based（MEA-based）and Cal-looping CO₂ capture is presented to analyze the effect of different carbon mitigation approaches on the thermal and techno-economic property of the power generation system.In this way,the overall performance of the de-carbonizaiton fossil-fired power plants can be evaluated.Specifically,Energy analysis of CO₂ capture unit is carried out.As the results shown,the high energy consumption of CO₂ capture is due to the following reasons:the significant heat supplied by extracted steam for sorbent regeneration;the dismatch of energy level between extracted steam and reboiler requirement;the discharge of the surplus energy during CO₂ capture process.Therefore,several integration principle is generalized for the integration between the CO₂ capture process and energy utilization system,namely the reduction of extracted steam flowrate,reasonable optimization of the extracted steam energy grade and the surplus energy recovery of the CO₂ capture unit.For the coal-fired power generation unit,a new decarbonization integration system is proposed based on the energy input optimization of CO₂ capture,which using steam ejector to develop a new treatment process of extracted steam and reducing the energy wastage caused by the parameter dismatch of extracted steam and reboiler requirements.The results show that through energy input optimzation the exergy losses in the extracted steam surplus pressure pretreatment,the extracted steam temperature pretreatment and heat dissipation of CO₂ capture unit decreased obviously,which leads to an improvement of exergy efficiency from 25.6%to 30.8%,namely 5.3 percentage points reduction of CO₂ capture efficiency penalty.The cost of electricity（COE）and the cost of CO₂ avoided（COA）of the proposed power plant decreased by 20.9%and40.0%,respectively.It means significantly improvements of overall performance.In term of the characteristic of NGCC power plants,including lower CO₂concentration of flue gas,higer temperature of flue gas entering CO₂ capture unit and lack of regenerative heaters in the steam cycle,a new integration system is proposed based on waste heat cascade utilization of CO₂ capture process.In the proposed system,exhausted gas recycle（EGR）is adopted to increase the CO₂ concentration of flue gas and reduce the specific regeneration heat;CO₂ supercriticle cycle is utilized to recover the sensible heat in flue gas and increase the power output;prepressurion of CO₂ is used to recover partly the latent heat for the sorbent regeneration.The results show that the net power output in new system increases from 445.92 MW to 474.77 MW,which means the CO₂ capture efficiency penalty is 2.9 percentage points lower than that in conventional de-carbonization power plants.The COE is 9.9%lower than that in the de-carbonization NGCC power plant without integration.For NGCC power plants,a new integrated system of power generation,CO₂capture,and heat supply is developed.As the de-carbonization NGCC power plant integrates with the heat supply subsystem,the low-grade and large-flow heat from the CO₂ capture process can be mostly recovered,which is sorbent cooling heat,CO2cooling heat and multi-stages compressor intercooler heat.The results show that the net power output of the new system is higher than the de-carbonization NGCC power plant without system integration by 2.0 percentage points.In heating period,247.59MW heat is recovered for the heat supply unit,the overall energy utilization efficiency is improved by 27.0 percentage points.The COE and COA of the new system decreased to 82.61$/MWh and 76.67$/t CO₂,respectively,which are 9.8%and 27.3%lower than that of the decarburization NGCC system without integration.To comprehensive evaluate the performance of MEA-based CO₂ capture,the comparision of MEA and Cal-looping CO₂ capture is carried out.As the results shown,when NGCC power plant integrates with MEA-based CO₂ capture uint,the net efficiency increases from 46.0%to 48.6%and it can be improved by 0.3 percentage points by EGR.For the Cal-looping CO₂ capture,with the same capture ratio,the efficiency penalty is 0.8 percentage points higher than that in MEA-based case.After adopting EGR,the net efficiency of Cal-looping NGCC power plant is slightly lower than that of MEA-based decarbonization NGCC power plant with EGR.Consequently,the COE of Cal-looping decarbonization NGCC power plant is 78.70$/MWh,which is 3.44$/MWh lower than that of MEA-based decarbonization NGCC power plant.