Thermodynamic Analysis Of Combined System Based Solid Oxide Fuel Cells With Zero CO₂ Emission

Solid oxide fuel cell(SOFC)is one of the most promising technologies in the future because of its high fuel utilization,flexible fuel selection,low relative cost and environmental friendliness.Porous electrodes and connecting materials of fuel cell separate the reactants of cathode and anode,and the reactants and products of cathode are not directly contacted with that of anode.When methane is used as the fuel,if the anode exhaust is fully mixed and burned with pure oxygen,the combustion products are only consisted of H2O and CO2,and the concentration of CO2 is high,which makes it possible to achieve the goal of CO2 zero emission in the SOFC combined system.This paper presents a SOFC/GT hybrid system based on SOFC and Gas Turbine(GT),which can make CO2 zero emission come true.Because the working temperature of SOFC/GT system is high,the waste heat from the exhaust still has recyclable value.A combined cooling heating and power system(SOFC/GT/ORC/ARC)based on the SOFC/GT system,the CO2 capture system,the Organic Rankine cycle and the absorption refrigeration cycle is established,in which the Organic Rankine cycle and the absorption refrigeration cycle are used as bottom cycle to recover the SOFC/GT system waste heat.A power system(SOFC/GT/KC)based the SOFC/GT system,the CO2 capture system and the Kalina cycle is combined,in which the Kalina cycle is used as bottom cycle to recover SOFC/GT system waste heat.In this paper,firstly,the electrochemical model of SOFC is established,and the model validation and performance analysis are carried out.The thermodynamic model of the main equipment of SOFC/GT system is established.Thermodynamic parameters of each state point in the system and system performances are calculated and simulated under design conditions,and the changes of performances are studied under variable conditions.The results show that the electrical efficiency and the exergy efficiency of SOFC/GT system are 50.99%and 54.93%,respectively,under the design conditions When the current density is 0.2 A/cm2,the net-work of the SOFC/GT system is the most and the electrical efficiency is the largest.The maximum of electrical efficiency is 50.99%at the SOFC inlet temperature of 700?,and the maximum of exergy efficiency is 55.84%at the SOFC inlet temperature of 600?.Then,thermodynamic models of bottom cycle for SOFC/GT/ORC/ARC system and SOFC/GT/KC system are established,respectively.Thermodynamic parameters of each state point in the combined system and the performances parameters are calculated under the design conditions,and the influence of the key parameters on the performances of the combined system are analyzed.The results show that,under the given conditions,the trigeneration efficiency,the electrical efficiency and the exergy of SOFC/GT/ORC/ARC system are 72.23%?52.83%and 59.96%,respectively,and the electrical efficiency and the exergy of SOFC/GT/KC system are 54.77%and 58.4%,respectively.Compared with no CO2 capture system,the efficiency of combined system decreased by 2.05%.In SOFC/GT/ORC/ARC system,the maximum exergy efficiency is 60.1%at the SOFC inlet temperature of 625?,and the maximum of the trigeneration efficiency and the electrical efficiency are 73.19%and 52.95%,respectively,at the SOFC inlet temperature of 700?.In SOFC/GT/KC system,the largest exergy efficiency is 58.77%at the SOFC inlet temperature of 625?,and the largest electrical efficiency is 54.77%,obtained at the SOFC inlet temperature of 700°C.Through the analysis of the variable conditions,it can be known that the effects of the main parameters on the combined system performances are different.When conditions allowable,choosing appropriate parameters is good for the improvement of system performances.