| Under the pressure of environmental pollution and energy shortages, technologies that can realize clean, effective and rational utilization of energy are being explored all around the world. In recent years, the researchers pay much attention to coal chemical looping process, because it is characterized by high efficiency and inherent CO2separation. Coal chemical looping gasification (CLG) is a promising industrial application technology, which couples chemical looping process with catalytic coal gasification. CLG process uses oxygen carriers to transfer lattice oxygen and heat to coal for gasification, which can obtain objective product by adjusting the ratio of oxygen carrier, gasifying agent and fuel. In this thesis, coal chemical looping gasification process with calcium-based oxygen carrier is innovatively explored, the main results and conclusions are summarized as follows:Firstly, the effects of operating conditions on reactivity of Ca-based oxygen carrier with coal during chemical-looping gasification were investigated in a fluidized bed reactor, employing the steam as gasification-fluidization medium. The influence of operating conditions on the gasification performance was obvious, such as temperature, stream/coal ratio, coal types and supporter. The average carbon conversion rate and cold gas efficiency of bituminous coal, lignite and anthracite increased with increasing temperature from750℃to950℃. Effect of temperature on bituminous coal was more evident than the other coals. At950℃, the average carbon conversion rate and cold gas efficiency of bituminous coal was4.52%/min and87.95%, respectively. The residence time of these coals was shortened with the increase of steam flow rate. Three Ca-based oxygen carriers were prepared by loading CaSO4on γ-Al2O3, SiO2and bentonite respectively. The carbon conversion rate and cold gas efficiency of bituminous coal is90.24%and80.32%, reacting with CaBen at950℃. The wear rate of CaBen particles is only0.87%after a200-min wear test. The mechanical performance of CaSO4/bentonite (CaBen) oxygen carrier is well.Secondly, Three Ca-based compound oxygen carriers were prepared by mechanical mixing-impregnationmethod. The reactivity performances of the modified Ca-based oxygen carriers were improved. At900℃, the reactivity of these oxygen carriers with bituminous coal was ordered as CaKBen>CaCaBen>CaFeBen>CaBen. The cold gas efficiency of bituminous coal maintains at about85%during ten redox reactions, which indicates that the oxygen carriers have an excellent recycling stability. The X-ray diffraction (XRD) images show that the oxygen carriers exist mainly in the form of CaSO4and K2CO3after ten circle redox reactions. The SEM images indicate that the surface of the oxygen carriers become porous after ten circle redox reactions, which promotes the chemical looping gasification. The kinetics of coal chemical looping gasification is well explained by active site extending model, and the synergy of CaSO4and K2CO3is verified during the coal chemical looping gasification process.Finally, an investigation into the sulfur release of Ca-based oxygen carrier during the chemical-looping gasification was presented in this thesis. The influences of temperature and stream flow rate on the sulfur release were obvious. An increase of steam flow rate and of temperature both accelerate the rate of adverse reactions. The time needed for H2S content to reach equilibrium value was shortened and the peak value of COS content appeared earlier. The investigation into the sulfur release patterns of these compound oxygen carriers showed that when CaCaBen was used in chemical-looping gasification the peak content of COS reduced by2.76times and the time needed for H2S content to reache an equilibrium value extended by nearly5times. This indicated that CaO can inhibit the side reaction of CaSO4and CaS. |
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