Performance Investigation Of Gasification Chemical-Looping Combustion Combined Cycle

Observation of the increased concentration of carbon dioxide in the atmosphere and the thereto connected global warming effect has made prevention of carbon dioxide emission from power plants an important field of research. Most fuels used in thermal power plants are fossil fuels, which release CO₂ to the atmosphere on combustion. To decrease the emission of CO₂ from fossil fueled power plants, an increase in the power conversion efficiency with less emission or zero emission is necessary. New combined cycle systems based on gasification and chemical-looping combustion (CLC) is proposed in this paper. The system has better efficiency than that of conventional PC power plant and offers a potential method for CO₂ capture. After pressurized, coal slurry is lead to a pipe-typed gasifier, which is immersed in the CLC air reactor and takes in the heat released from the air reactor. After removal of particulates, the raw syngas is used as the fuel of CLC fuel reactor. The technique involves the use of a metal oxide as an oxygen carrier that transfers oxygen from the combustion air to the fuel, and the direct contact between fuel and combustion air is avoided. The products of combustion, i.e. carbon dioxide and water, are kept separate from the rest of the flue gas. After condensation of the water almost pure CO₂ is obtained, without any energy lost for the separation. Combined with the technology of gas turbine cycle and steam cycle, a new combined cycle system is realized and the total efficiency will get improved. The new zero emission system is called coal gasification CLC combined cycle (GCLC-CC). Two other systems based on CLC are included for comparation with GCLC-CC in this paper. One is called CLC intergrated gasification combined cycle (CLC-IGCC) where gas turbine combustion system is replaced by CLC reactors in an integrated gasification combined cycle and the other is coal gasification CLC rankine cycle (GCLC-RC) in which syngas is used as the fuel of CLC reactors which are designed as a circulating fluidized bed boiler and a fixed fuidized bed boiler to generate steam as conventional PC boiler.Performance of those three systems is investigated. Operating parameters are studied and optimized such as the gasifier operation temperature and pressure, mass fraction of water and coal, the mole ratio of calcium and sulfer, the outlet temperature of air reactor and supplementary firing reactor, pressure ratio of compressor, the coolant fraction of turbine, circulating mass flow rate of the oxygen carrier, etc. Most of parameters related to the CLC reactors are discussed to examine the influence on system efficiency. Energy consumption needed to capture CO₂ has properly been considered. Commercial software ASPEN PLUS is used as a tool to simulate the three-system performance.The results show that an efficiency of 44.3% can be achieved under certain conditions with GCLC-CC, and 90.1% of CO₂ is captured, i.e. CO₂ emission level is 72.37 gCO₂/kWh. For CLC-IGCC, the efficency achieved can be 41.55% with CO₂ emission 125.77 gCO₂/kWh. The efficiency of GCLC-RC is only 32.69% at zero emissions level. The results show that CLC system has high potential for power generation with inherent CO₂ capture.