| In recent years, the syngas and hydrogen generation from solid fuels has been paid much attention. Chemical-looping gasification (CLG) of coal is a clean and effective technology for syngas generation. Sharing similar basic principles with chemical-looping combustion (CLC), CLG also uses oxygen carriers to transfer lattice oxygen to the fuel. The major advantage of this process is that the oxygen carrier can supply the heat needed for gasification without costly oxygen production unit, served as catalysis for the gasification, and prevent the SOX and NOx emission. Iron-based oxygen carrier, with high reactivity, low price and environmental benignancy, is considered as a promising oxygen carrier for the chemical looping process. In this thesis, coal based chemical looping gasification process is systematically explored. All the experiments were carried out in a fluidized bed reactor, used steam as the gasification-fluidization medium. The main results and conclusions were summarized as follows:Firstly, feasibility analysis of main reactions in the system of Fe-based CLG was investigated by thermodynamic analysis. The influences of operating conditions (such as temperature, stream/coal ratio, oxygen carrier/coal ratio and coal types) on the gasification performance were studied from a single gasifier level. The generating efficiency of H2and CO, the carbon conversion efficiency and the gasification rate increased with increasing temperature from840℃to960℃. The concentration of syngas reached a maximum,67.28%, when the steam fed into the reactor at flow rate of2.5g/min. Two kind bituminous coal, Shenmu coal and Beisu coal, were employed. It can be found that the reactivity of Shenmu coal was higher than Beisu coal, which can be explained by the obvious effect of the ash and the pore structure of coal on the chemical looping gasification process. Carbon conversion efficiency increases from55.74%to81%with increasing Fe2O3-to-coal mole ratio from0to2, furthermore. In particular, syngas generation showed an increase and then decrease.Secondly, Fe-based oxygen carrier was modified by three metallic oxides (K2CO3. CaO and NiO) using the wet impregnation method. The reaction rate of the modified Fe-based oxygen carrier was improved. The reactivity and regeneration of three kinds of Fe2O3composite oxygen carriers in6cycle redox experiments were demonstrated. At920℃, the reactivity order was presented as Fe4A16K1>Fe4A16Cal>Fe4A16> F34A16Ni1. Moreover, the X-ray diffraction (XRD) images showed that the Fe2O3composite oxygen carrier was completely regenerated after6cycle rcdox experiments. The Fe2O3composite oxygen carrier particles have been demonstrated excellent oxygen carriers for chemical-looping gasification of coal.Finally, an investigation into the mechanism and kinetics model of the gasification process was presented in this paper. The peak fitting technique was used to analyze the gasification reaction rate curve on the reaction mechanism of coal and Fe2O3composite oxygen carrier. It indicated that complex reactions of CLG included three stages, the reaction between Fe2O3, coal char and steam, the gasification of coal char, and the reaction of Fe3O4to FeO occurred. It illustrates that the presence of Fe2O3oxygen carrier evidently favored the chemical reaction rate and enhanced the generation of syngas, which would have a positive effect on the coal conversion. The reaction rate of CLG was improved by adding the modified metallic oxides. By the experimental data, among non-catalytic reaction models, the modified random pore model is the best to fit the experiment data among five kinetic models... |
PDF Full Text Request