The Performance Study And Optimization Of Carbon Dioxide Reduction In IGCC System

With the increasing concerns on global warming, reduction of CO2emissions has become one of the new themes for energy-saving power generation in21st century. CO2capture and storage (CCS), therefore, emerged at the right moment. In the IGCC (Integrated Gasification Combined Cycle) system, the raw gas produced in the gasification furnace, which was mainly composed of CO and H2, was burned to CO2and H2O when fed into the combustion chamber of the gas turbine after dedusting and desulfurization. The heat of the exhaust gas was recovered in the exhaust-heat boiler and the exhaust gas was discharged into environment, resulting in great greenhouse gas emission pollution. The aim of Pre-combustion CCS technology used in the IGCC system was to make the net gas go through the CO shift converter in which CO was partially converted to CO2. CO2was separated and deposited and the gas contained more H2but less CO at the entrance of the combustion chamber, which reduced the amount of CO2emission eventually and clean-burning could be realized.In this thesis, the processes of transformation of CO and CO2capture and storage were added to the basic IGCC system to design the Pre-CCS-IGCC system. MATLAB software and the compiled VB programs were used to calculate the Pre-CCS-IGCC system’s thermal efficiency and CO2emission volume. The results showed that the thermal efficiency was reduced by4.48% when the CO2ultimately emission volme was reduced to40%of the non-CCS IGCC system.Then parameters and processes of Pre-CCS-IGCC system were optimized. The results showed that with the increasing of the vapor/syngas ratio, the CO2emissions and thermal efficiency reduced simultaneously, which meant that at the expense of system efficiency, the CO2emissions could be controlled by changing the choice of the ratio of steam and gas. With the same amount of CO2emissions, increasing vapor/coal ratio, CO contained in the unpurified gas would decrease, while CO2, H2O and H2contents increased and the system thermal efficiency had a trend downward, but the variation was small; When the ratio of oxygen and coal increased, the gasification reaction temperature would increase while the effective gas compositions decrease and the system thermal efficiency would decrease with the same amount of CO2emissions. Considering the reduction of CO2emissions and limitation of gasifier’s temperature, in the Pre-CCS-IGCC system the best oxygen/coal ratio were0.86; The work of steam turbine decreased and that of gas turbine increased using the twice-transformation decarburization process and the CO2capturing energy consumption increased in the mean time, so the system thermal efficiency was reduced by1.582%than the once conversion process, consequently CO2should be removed by a direct conversion process in the physical absorption method.The system of oxy-combustion CO2circulating was computed, which was the CO2emission reduction method of sending part of the high concentration CO2gas cooled and separated from the exhaust-heat boiler to the combustion chamber of gas turbine. The CO2gas participated in the circulation and did work and the rest part was compressed and stored or liquefied and collected. The system performances with Pre-combustion CCS and O2/CO2were compared and the results showed that the thermal efficiency of system with O2/CO2was lower than that of the system with Pre-combustion CCS, but in which zero CO2emission could be achieved and the best oxygen/coal ratio was0.86. Part of the separated CO2gas was recycled back to the gasifier to participate in the gasification reaction when using the circulated CO2. Finally, the heat performance calculation and analysis of CO2cycle gasification Pre-CCS-IGCC system and O2/CO2system were conducted respectively. The results showed that the content of H2decreased and CO increased at the export of the gasifier and the reaction temperature was lower.With the same CO2emission reduction, the system thermal efficiency increased slightly compared to that without circulation.The thermal efficiency of Pre-CCS-IGCC system was42percent that kept the level of pulverized power plant when achieving the CO2emission reduction by86.55%and the industrial application of Pre-CCS technology could be achieved in future. So the research of the CO2emissions mitigation in IGCC system had high theoretical and practical significance to clean utilization of coal.