Research On The System Performance Of The Hybrid SOFC-GT System With Low Fuel Utilization And External Reforming

Fuel cell technologies possess the capability of converting chemical energy to electric energy efficiently and cleanly.Solid oxide fuel cell（SOFC）-gas turbine（GT）hybrids have some potentials to reach electrical efficiency higher than 70%and prolong the SOFC lifetimes,while maintaining relatively lower emissions than other fossil power plants.Due to the intrinsic modularity of fuel cells,SOFC-GT hybrid systems are also attractive for distributed applications.For those reasons,this hybrid system has been attrackting attentions all over the world.SOFC fuel utilization（U_f）and reforming process are the two critical factors affecting the system performance in the SOFC-GT hybrid system.SOFC U_f is usually defined as the ratio of fuel consumed in SOFC to the total fuel entering the stack.SOFC U_f affects not only the cell efficiency,cell voltage,power density and degradation rate of the SOFC,but also influence the thermal management and system performance of the entire system.Reforming process has a big impact on anode inlet fuel composition,which also affects the system configuration due to the strong cooling effects of fuel reforming.Work in the literature have demonstrated that high efficiency could be achieved with low U_f in the hybrid SOFC-GT system.But,the research on the system performance of this hybrid system under low U_f are still very limited.Meanwhile,most research focus around the internal reforming configuration in the SOFC-GT hybrid due to its expected high efficiency and low capital cost.Both SOFC U_f and reforming process have the ability to manage the energy distribution in the hybrid system.However,there is few work could be found in the literature which explores the coupling effects of SOFC U_f and reforming process on the system performance of the SOFC-GT hybrid system.This study focus on the system design of the hybrid SOFC-GT system under different U_f and fuel pre-reforming ratios.Moreover,this work is the first ever investigation to explore the feasibility of achieving high efficiencies with low SOFC U_f and external reforming in a pressurized natural gas hybrid SOFC-GT system.The main work in this dissertation are shown as follows:1.Based on auto-thermal reforming configuration（reforming process was decoupled with the rest of the system）,the system performance of the hybrid SOFC-GT system was analyzed over different pre-reforming ratios（reforming temperature from 600–1000 K）and SOFC U_f（from 45-90%）.Results showed that the system efficiency of external reforming was generally lower than internal reforming.In the low U_f regime,the system efficiency of internal reforming was about 10 percentage points higher than external reforming.However,this advantage decreased to 3 percentage points at 90%U_f.The highest system efficiency of 74%was achieved at 60%U_f and 600 K reforming temperature,but this design condition required the system operating at extreme conditions（SOFC cathode inlet air temperature was higher than 1150 K and turbine inlet temperature was above 1400 K）.2.After considering the results from 1,a system configuration with locating reformer at turbine inlet was studied to explore the feasibility of achieving high efficiency with low U_f and external reforming.With this system configuration,the system efficiencies of the most design conditions were higher than 70%.The difference between the highest and the lowest system efficiency was only 6 percentage points,demonstrating that the system efficiency was not sensitive to U_f and reforming process.Based on the simulation results,a relatively high system efficiency（70%）could be maintained with 50%U_f and 1000 K reforming temperature.Based on the analysis of system operating parameters,the safe operation of the hybrid system under low U_f and external reforming was preliminary verified.3.The system performance of the system configuration studied in 2 was investigated under part load operation condition（80%,60%and 40%design load）.Results showed that the system efficiency of the hybrid system was relatively high under part load operation（70%under 80%design load,65%under 60%design load,56%under40%design load）,showing the great potential of load following.According to the results,the low U_f and low cathode air flowrate operating condition has some potentials to maintain a relatively high system efficiency and prolong the lifetime of SOFC under part load operation.4.A dynamic one-dimensional natural gas reformer model was established to investigate the impacts of external reforming process on the dynamic response of the hybrid SOFC-GT system based on auto-thermal reforming system configuration.Specifically,after the O₂ molar fraction at reformer inlet was increased to 5%,the dynamic response of the hybrid system was studied based on two different control strategies:open-loop control and constant turbine speed control.Results showed that the increase of O₂ concentration at reformer inlet could lead to the increase of SOFC U_f and SOFC operating temperature,which resulted in the decrease of turbine speed and finally caused the fast decrease of SOFC cathode inlet air flowrate and the increase of SOFC temperature.With implementing the constant turbine speed control,the hybrid system could run smoothly,but it is still necessary to pay attention to the increase of SOFC temperature and the decrease in cell voltage and power density during this dynamic operation.5.Finally,considering the application of fuel cell technologies in distributed generation systems,an optimization design method of a hybrid power generation system which consists of wind,solar,fuel cell systems was proposed in this dissertation.With taking the unstable power output of the renewable energy into consideration,a multi-objective optimization framework based on energy supply reliability,efficiency,and capital cost of the hybrid system was established.Then Hamersley sequence sampling and sequential quadratic program method were employed to get the Pareto solution of the multi-objective optimization design problem of the hybrid system.