Design And Analysis Of The Oxy-CFB Power Plant Integrated With Energy Storage For Peak Shaving

The rapidly increasing renewable energy generation has put forward higher requirements for the peak-shaving capacity of coal-fired power plants.Oxy-fuel combustion technology is one of the most promising carbon capture technologies for coal-fired power plants.At present,there are few systematic studies on the combination of oxy-CFB(circulating fludized bed)and energy storage.In this work,a 330 MWe oxy-CFB power plant was selected as the reference power plant.Based on the reference power plant,oxy-CFB power plant with liquid O2 storage system was proposed.An optimization design is highly necessary for the recovery of the energy during the transformation from liquid O2 into gaseous one.This work also proposed an oxy-CFB power plant with liquid O2 storage and cold energy recovery systems.The thermodynamic performance and technical economy of different power plants were compared and analyzed.The specific research contents and results are as follows:The reference power plant was constructed and simulated with Aspen Plus,and its thermodynamic performance in rated-load(BRL)and part-load(50% THA)was analyzed.The results showed that the total auxiliary electricity consumption significantly increased due to the usage of air separation unit(ASU)and CO2 compression and purification unit(CPU).As a result,the net efficiency in rated-load and part-load were 24.83% and 23.83% lower,respectively.Exergy analysis showed that the combustion and flue gas heat exchange subsystem of the reference power plant had the largest exergy losses in rated-load and part-load,but they were 18.98 MW and 9.76 MW lower than that of the air combustion unit,respectively.It shows that the combustion and flue gas heat exchange subsystem had a higher energy utilization level under the oxy-fuel combustion condition than that under the air combustion condition.Due to the usage of ASU and CPU in the reference power plant,a large exergy loss occurs.The gross exergy efficiency of the reference power plant in rated-load and part-load is 8.81% and 8.64% lower than that of the air combustion unit,respectively.Based on the reference power plant,oxy-CFB power plant with liquid O2 storage system was proposed and simulated with Aspen Plus.Its thermodynamic performance in peak and off-peak periods was analyzed.The results show that the net power efficiency of the oxy-CFB power plant with liquid O2 storage system in the peak period reached 32.13%,which was7.30% higher than that of the reference power plant.In off-peak period,the excess power output was used to prepare liquid O2,and the power consumption of liquid O2 preparation was0.369 k Wh/kg.This higher power consumption resulted in the net power efficiency of only12.11%.The reduction in net power efficiency was not due to energy loss but was stored as cold energy in the liquid tank.In peak period,liquid O2 was directly released as gaseous O2 at normal temperature and pressure,and the gain exergy of ASU subsystem was 0.Therefore,energy saving transformation of the liquid O2 release process should be emphasized to reduce exergy loss.An oxy-CFB power plant with liquid O2 storage and cold energy recovery systems was proposed and simulated with Aspen Plus.Initial investment was considered,one-stage,two-stage and three-stage cold energy recovery systems are designed and analyzed.Based on sensitivity analysis and comprehensively considering the equipment investment and net power output,a two-stage cold energy recovery system and 5.0 MPa pump discharge pressure were selected,and the corresponding maximum net power output was 12.97 MWe.The net power efficiency of the oxy-CFB power plant with liquid O2 storage and cold energy recovery systems was 1.48% higher than that of the oxy-CFB power plant with liquid O2 storage system in peak period due to the coupled cold energy recovery system.In peak period,exergy analysis results can be summized as follows: 1)liquid O2 was directly released into gaseous O2 at normal temperature and pressure,and the exergy efficiency of ASU was 0%;2)the cold energy recovery system recovered the cold energy,and the exergy efficiency reached 28.32%.Several parameters were compared and analyzed to determine the best system scheme,such as the thermodynamic performance and economic feasibility of the reference power plant,the oxy-CFB power plant with liquid O2 storage system,and oxy-CFB power plant with liquid O2 storage and cold energy recovery systems.Compared with the other two power plants,the oxy-CFB power plant with liquid O2 storage and cold energy recovery systems showed the best performance in peak period,which were net power output of 294.22 MWe and net power efficiency of 33.39%.The peak-shaving coefficient was almost 2.77,which has the best peak-shaving capacity.Exergy analysis shows that the gross exergy efficiency of the two power plants with energy storage system was significantly higher than that of the reference power plant in peak period.Economic analysis showed that the operation and maintenance costs of the three power plants were greatly reduced through the sales of liquid O2,liquid CO2 and carbon trading.The levelized cost of energy of the oxy-CFB power plant with liquid O2 storage and cold energy recovery systems was 30.90 $/MWh,which was the lowest value.Sensitivity analysis showed that coal price and liquid CO2 price had a greater impact on the levelized cost of energy of the three power plants,while carbon trading price and liquid O2 price had a relatively small impact on the levelized cost of energy.The levelized cost of energy can be significantly reduced through selling captured liquid CO2,which was conducive to the wide application of oxy-fuel power plant.Considering the thermodynamic performance and economic feasibility of the three power plants,the oxy-CFB power plant with liquid O2 storage and cold energy recovery systems had the best performance,which could provide a solution for the improvement of the oxy-CFB power plant in the future.