Optimization Design And Dynamic Study Of Catalytic ATR SOFC-GT Hybrid System

The hybrid system based on solid oxide fuel cell(SOFC)and gas turbine(GT)has high efficiency,low carbon emission,and fuel flexibility.The modularity of fuel cells makes it an important branch of distributed power generation technology.SOFC-GT hybrid power generation system has been widely studied and concerned since it was proposed,and has become a new research hotspot and frontier in the field of SOFC.Because there are still many problems in the SOFC-GT hybrid system that have not been fully studied,the conditions for commercial application and promotion of the hybrid system are not mature.To promote the mechanism research of hybrid systems,enrich the relevant research in the field of hybrid systems in China,and promote the implementation of this technology,this paper focuses on the performance of system efficiency,security and stability.The influence of different fuel utilization,fuel cell operating temperature and fuel pre-reforming share on the design of the catalytic auto-thermal reformer solid oxide fuel cell gas turbine(CATR-SOFC-GT)hybrid system was analyzed,and the optimization design of the hybrid system was carried out.Then the dynamic model of the hybrid system is established on the basis of optimal design,and the hybrid system’s dynamic characteristics and control strategy are studied.The specific research contents are as follows:(1)According to different research requirements,three models of CATR-SOFC-GT hybrid system are established.Firstly,the top cycle structure of the hybrid system is designed according to the coupling principle of the hybrid system.Then,a static model(Model I)for analyzing the effects of major parameters of hybrid power generation system on system performance and safety was established on Ebsilon and MATLAB/Simulink simulation platforms,a static model(Model II)for hybrid system optimization design was established,and a dynamic model(Model 3)is developed for dynamic analysis and control strategy verification of hybrid system on APROS and MATLAB/Simulink simulation platforms.(2)Based on Model I of the catalytic ATR-SOFC-GT hybrid system,the influence of fuel cell operating temperature on the power generation performance of the hybrid system under different fuel utilization(45%～90%)was analyzed.The results show that under the condition of safe operation of the system(the internal temperature gradient of fuel cell is less than 6℃/cm),60%(65%)SOFC fuel utilization and 600K(700K)external fuel reforming temperature,the hybrid power generation system efficiency is the highest,up to 73%.(3)To work out the influence of fuel cell operating temperature on the power generation efficiency of the hybrid system,this paper analyzes the influence of different fuel cell operating temperature(635-835 ℃)on the performance of the hybrid system based on the parameters of(2).The results show that the power generation efficiency of the hybrid system increases first and then decreases with the fuel cell fuel utilization.When the SOFC operating temperature drops from 835℃ to 635℃,the optimal fuel cell fuel utilization of the hybrid system decreases from 65% to 55%.When SOFC fuel utilization is 65%,the system achieves a maximum power generation efficiency of 74%at 835℃.In contrast,the lowest efficiency point(49%)was achieved at an SOFC operating temperature of 635 ° C and 90% fuel utilization.(4)On the basis of(2)and(3),fuel cell fuel utilization,water/carbon ratio at the inlet of the reformer,oxygen/carbon ratio and air temperature at the inlet of the reformer are selected as search spaces to optimize the power generation efficiency,fuel cell stack size,fuel cell inlet temperature difference and total heat transfer area of the system heat exchanger.The approximate Pareto front of optimization results is obtained by MMOPSO algorithm.Through analysis,the hybrid system can achieve more than 80%of the hybrid power generation efficiency,but the fuel cell stack is required as the main power output component of the hybrid system,which leads to a few configuration points for the hybrid system.The number of fuel cells ranges from 350 to 425,and the system can be configured with more.(5)A one-dimensional CATR model is established and verified.Then,in the hybrid power generation system(model III)constructed by this model and the one-dimensional SOFC model,the safety of the hybrid system operation is analyzed under the two conditions that the O/C ratio at the inlet of the auto-thermal reformer CATR jumps from0.25 to 0.05 and 0.45.Both experiments show that the step change of CATR operating O/C will lead to the change of SOFC’s operating state,especially when the temperature gradient in SOFC exceeds the limit of the safe temperature gradient limitation.In the O/C ratio step reduction experiment,SOFC anode fuel temperature can be controlled at700℃,but in the O/C ratio step increase experiment,due to the CATR involved in the combustion of methane generation heat excess,SOFC anode inlet temperature overtemperature,which will lead to SOFC support structure thermal stress increased,And will lead to SOFC internal temperature gradient increase,which directly threatens the safe operation of SOFC.The O/C ratio also affects the post-combustion chamber temperature and gas turbine inlet temperature of the hybrid system.Too low O/C ratio will lead to the rise of post-combustion chamber temperature,and even cause turbine inlet overtemperature,which is not conducive to system safety.(6)A complete control strategy for the CATR-SOFC-GT hybrid system was proposed.The hybrid system control system includes hybrid system power control,CATR inlet temperature control,SOFC inlet fuel and air temperature control,SOFC operating temperature control,SOFC fuel efficiency control,turbine inlet temperature control,compressor surge control,gas turbine speed control and other functions.The results of the step decrease experiment of the hybrid system power(100k W to 50 k W)show that the temperature fluctuation caused by the large range of power in the process of load change and the compressor surge caused by changes in air flow were restrained very well by the control system.The control system realizes the load tracking quickly and accurately during the experiment.