SOFC System Performance Based On CO₂ Gasification Of Char

The syngas being rich in CO could be used as fuel for SOFCs. Effective power generation process can be realized after electrochemical reaction and then the high temperature and concentration CO₂ flue gas can be gained, captured and effectively utilized. An efficient method for the flue gas utilization is that it can be used to gasify coal（char）. Syngas produced by the gasification is rich in CO and it can be directly used by SOFCs. As a result, a circulation system of CO₂ gasifying of char coupled with SOFCs power generation is proposed. Firstly, the sensible heat in high temperature CO₂ can be utilized to improve power generation efficiency of the system.Secondly, the high temperature CO₂ is employed as carbon source, which can increase CO volume fraction in the syngas under certain condition and then increase the generating capacity of SOFCs. Thirdly, the increasing high concentration CO₂ can be partly captured to maintain the material balance of the system. As a result, efficient power generation and high concentration CO₂ capture can be realized starting from coal utilization.To simplify the study char instead of coal is used in the thesis. The research content in the thesis includes the following parts. First, the（catalytic） reactivity of char with CO₂ and O₂/CO₂ is studied. Second, the syngas catalytic preparation using fluidized bed gasifier is studied. Third, effects of syngas on the electrochemical performance of the SOFCs are studied. Fourth, based on the above results the system power generation efficiency, material balance and energy balance are calculated and effects of gasification conditions and high temperature CO₂ on the material balance and system power generation efficiency are studied.In chapter 1, research progress on CO₂ gasifying of char and SOFC power generation system is reviewed respectively. The coal or carbon gasification coupled with SOFC power generation is introduced. In chapter 2, the designing and operating of the fluidized bed gasifier and the thermogravimetric analyzer in the thesis are introduced respectively. Also other related apparatus, materials and research method in the study are presented.In chapter 3, the reactivity of char-CO₂ and char-O₂/CO₂ reaction is studied. The results showed that Bayer process red mud has significant catalytic ability on char-CO₂ and char-O₂/CO₂ reaction. The catalytic char-CO₂ reactivity with 8wt% Bayer process red mud is comparative with 10% adding content of K2CO3 when the CO₂ flow rate is0.35L/min and the reaction temperature is 1373.15 K. The carbon both can becompletely converted using the two catalysts when the gasification time reaches to 55 minute. The maximum comprehensive reaction characteristic index（S） value of9.67E-11 is attained when the char-O₂/CO₂ reaction is catalyzed by 5wt% Bayer process red mud adding content. Fe2O3, Ca O and Na OH catalyst are the main excuse for the improved char reactivity.The un-reacted core shrinking mode, hybrid model and modified volume model are used to study the catalytic char-CO₂ reaction dynamic. The results show that the modified volume model is preferable to interpreting the catalytic dynamic and activation energy is 26.109KJ/mol. Coats-Redfern and Doyle model are used to interpret the catalytic char-O₂/CO₂ reaction. Doyle model has a better explanation for the reaction and the activation energy is 102KJ/mol.In chapter 4, based on the optimized catalytic char gasification conditions by the TGA, the fluidized bed gasifier was utilized to study the effect of fluizied bed char-CO₂（catalytic）gasification conditions on the syngas preparation. The results show that the maximum CO volume fraction is 86.53% when the gasification temperature being of1100 ℃, MCO₂/Mchar being of 0.4 and adding content of Bayer process red mud being of8%.Effect of reaction conditions of char-O₂/CO₂ gasification on the syngas composition is also investigated. The maximum CO volume fraction value of 90.24% is achieved when the MO₂/Mchar being of 0.29, MCO₂/Mchar being of 0.4 and reaction temperature being of 1100℃.In chapter 5, based on the syngas prepared by char fluidized bed（catalytic）gasification, effects of syngas composition and gasification conditions on the performance of Ni O-YSZ//YSZ//YSZ-LSCF cell are investigated. The results show that the maximum power density of the cell is 336 m W/cm2 under the optimum gasification conditions. The voltage of the cell decreases by 30% after 11 h stability test and carbon deposition in anode is the main reason for performance degradation.To overcome the cell performance degradation because of carbon deposition, study of performance of LSCFN//LSGM//LSCFN cell under the same fuel atmosphere is proposed. The maximum power density under the optimum gasification conditions is431 m W/cm2. Its voltage decreases by 4% after 54 h stability test and its electrochemical performance is improved compared with the performance of Ni O-YSZ//YSZ//YSZ-LSCF cell. The results illustrate that LSCFN//LSGM//LSCFN cell is preferable to using the syngas.In chapter 6, based on the research above, 100 KW circulation system of high temperature and high concentration CO₂ gasifying of char coupled with SOFC power generation is set up and effects of different gasification simulation process parameters on the material balance, system power generation efficiency are carried out. The system power generation efficiency increases by 4.72% when the O₂ flow rates increases from8.2kg/h to 9.4kg/h, CO₂ capture content and the ratio of CO₂ capture content increase by5.1kg/h and 2.61wt% respectively. The system power generation efficiency and CO₂ capture content both increases to maximum values of 57.68% and 37.44kg/h separately when the CO₂ flow rates increase to 11kg/h, but the ratio of CO₂ capture content decreases gradually with the increasing of CO₂ flow rates. The system power generation efficiency, CO₂ capture content and ratio of CO₂ capture content increase by 1.31%,0.62kg/h and 0.29wt% respectively when the CO₂ preheating temperature increase from30℃ to 800℃. As a result, the high temperature and high concentration CO₂ flue gas has important significance to improve the system power generation efficiency.In conclusion, the thesis obeys the thinking of unit to system. The catalytic char-CO₂（O₂/CO₂） reactivity by Bayer process red mud are studied and optimized. The results show that Bayer process red mud has perfect catalytic performance for char gasification. Based on the optimization it can be seen that Bayer process red mud can improve the syngas composition in fluidized bed char-CO₂（O₂/CO₂） gasification.Syngas prepared by the fluidized bed gasification is used for the cells. Effects of syngas composition and char（catalytic） gasification process conditions on the cells’ performance are achieved. LSCFN//LSGM//LSCFN cell has better electrochemical performance when the cells are fueled by the syngas prepared. Based on the above research, 100 KW circulation system of high temperature and high concentration CO₂ gasifying of char coupled with SOFC power generation is set up. It can be attained that the power generation efficiency of the circulation system can be improved when the high temperature and high concentration CO₂ flue gas produced by SOFCs is used as gasifying agent in the fluidized bed gasification. The material balance of the circulation system can be maintained by the capture of the increasing CO₂ content in the operation of the circulation system.In summary, the research has established a foundation for the circulation system of the high temperature and high concentration CO₂ exhaust gas gasifying of char coupled with SOFCs power generation.