Optimal Modification Of Supercritical CO₂ Power Cycle Integrating Carbon Capture And Gasification Of Coal

The control of greenhouse gas CO₂ emission is a hot issue related to curbing the trend of global warming for sustainable development of mankind.Coal is recognized as the main foundation of the Chinese energy sources and the proportion of coal-fired electric power accounted for more than 50%in China's power structure.Huge amounts of coal combustion will lead to large amounts of pollutants and greenhouse gas CO₂emission.According to China's commitment at the Paris climate conference,it is imperative to implement CO₂ capture in a coal-fired power plant otherwise it will be winnowed out.However,the traditional scheme of capturing CO₂ in flue gas consumes a large amount of energy,which usually results in about 10%penalty for thermal efficiency of coal-fired power plants.Therefore,it is necessary to optimize the scheme of CO₂ capture.Supercritical carbon dioxide power cycle has high thermal efficiency under the condition of medium and high temperature heat source,which is endowed with tremendous advantages and potential for power generation under various heat sources.Oxygen fired fuel power generation system using liquefied natural gas(LNG)or supercritical water coal gasification syngas as fuel is studied in this paper.The semi closed s-CO₂ power cycle is used to realize the integration and optimization of energy conversion and CO₂ capture process,so as to solve the problem that the thermal efficiency of power plant will be decreased in the process of CO₂ capture by traditional methods.The main research contents and achievements are as follows:(1)Aiming at the problems related to the high turbine inlet temperature(1150?),high material requirements and complex process flow of Allam cycle,an improved cycle(Allam-Z cycle)with relatively simple system suitable for turbine inlet temperature of about 900?and the turbine back pressure directly set near the critical point(7.21MPa/30?)was proposed and studied.Therefore,the dilemma of using low turbine backpressure due to limitation of turbine exhaust temperature for suitable material for heat exchangers and using the compressors to boost the pressure to near the critical point after the regeneration process to dissipate heat to the environment in Allam cycle can be avoided.When the condensation temperature is 30?,the turbine inlet parameters are 30 MPa/700?and 30 MPa/900?,respectively,and the turbine outlet pressure of the Allam-Z cycle is 7.21 MPa and that of the Allam cycle is 4 MPa respectively,the power output efficiency of Allam-Z cycle is 50.87%and 43.64%,and the equivalent net efficiency of the Allam-Z cycle is 48.05%and 40.83%,which are2.15%and 2.96%higher than that of the Allam cycle under the same conditions.(2)A novel power generation system(Allam ZC cycle system)integrating supercritical water coal gasification technology and oxygen fired fuel s-CO₂ power cycle is constructed.Under higher s-CO₂ turbine back pressure,the high-temperature exhaust gas from turbine(720??760?)is used to provide heat required for the reaction of supercritical water gasification of coal.Thermodynamic method for parameter optimization with black-box model is applied to find the proper operation range of turbine parameters.The range of turbine inlet temperature is recommended at a fixed CO₂ condensing temperature.Since the turbine exhaust gas is used firstly for heating supercritical water gasification of coal process,the maximum temperature of exhaust gas entering the regenerator is lower than 700?(the value of Allam cycle entering the regenerator is 760?),so the problem of high temperature in regenerator is also avoided.Under the parameters of turbine inlet parameters of 25MPa/1000?,condensation temperature of 25?and ASU specific power consumption of 0.245k Wh·kg^-1(O₂),the net efficiency of the cycle achieves 47.3%with full CO₂ capture.(3)The modified scheme(Allam-ZC2)of increasing the pressure of syngas by adding a fuel compressor between the gasifier and the combustors is proposed to further increase the inlet pressure of turbine,which eliminates the limitation of gasification reaction pressure on the operating parameters of s-CO₂ turbine.The simulation results show that this scheme reduces the mass flow rate of high-pressure and low-temperature CO₂ fluid in the regenerator,and solves the problem of poor matching of heat transfer temperature difference due to the lower specific heat capacity of the exhaust gas at the high-temperature side of the regenerator.At the same time,the coupled influence trend of cooling water temperature and turbine back pressure on cycle performance is revealed.The optimal matching curves of turbine back pressure in subcritical and supercritical range under different cooling water temperature are given,which can not only meet the requirement of the pump inlet fluid density for using pump to raise the pressure,but also maintains high cycle efficiency.Under conditions of the turbine inlet parameters of 30 MPa/1000?and turbine backpressure of 6.4/7.5 MPa,cooling water temperature of 15/25?and ASU specific power consumption of 0.245k Wh·kg^-1(O₂),the net efficiency of the Allam-ZC system achieves 48.1%/44.9%respectively with full CO₂ capture.(4)Helical baffle heat exchangers have the advantages of high efficiency and low flow resistance,and also suitable for high temperature and high pressure conditions.A general scenario of the heat transfer system in a 300MW power generation unit with sextant or trisection helical baffle heat exchangers of the Allam-ZC cycle for reference in practical applications is designed.The model of mass center equivalent rectangle(MCER)was adopted to calculate the cross-sectional area of the helical flow channel of helical baffle heat exchangers,which solves the problems of large deviation from the actual situation with the existing minimum cross-sectional area model and impossible to predict the Nusselt number or friction coefficient of shell side with the general Reynolds number correlation formula.Taking the heat exchanger HX2 as an example which is for heat transfer between turbine exhaust gas and circulation CO₂,the influence of main structural parameters on performance indexes is discussed.The results show that the scheme of small diameter sextant helical baffle heat exchanger has obvious advantages.Finally,the geometrical parameters of the main heat exchangers of the heat transfer system in the Allam-ZC cycle were designed and presented.The calculation results provide reference data for the optimal design of heat exchangers in the Allam ZC cycle system.In this paper,the innovative research was carried out on the integration and simplification of oxygen fired fuel system and semi-closed s-CO₂ power cycle using liquefied natural gas or supercritical water gasification of coal syngas as fuel.The proposed cycle process is not only simplified and optimized,but also more suitable for the technical level allowed by the existing turbine materials.The integration of energy consumption of the cycle system and the heating process of supercritical water gasification of coal is more reasonable.The results of this dissertation provide a new feasible technical route for new coal-fired power plants to achieve high efficiency for power generation with zero CO₂ emission.