Study On The Mechanism Of K-/Ca/-fe-Compounds Catalyzed Coal Gasification

In the foreseeable future, coal is believed to be a major energy source in china. Coal gasification, as a clean coal technology can effectively improve coal utilization efficiency, reduce pollution emissions and achieve comprehensive utilization of energy. The use of catalysts in gasification, can significantly reduce reaction temperature, thereby reducing the severity of the reactor conditions and the cost of the process, moreover, achieving the target of directional gasification, making the gas product selectable. In striving to understand and unify the catalytic coal gasification process, the basic principles, that is, the mechanism of catalytic gasification need to be further investigated, which can be benefit in finding a suitable reaction conditions, technological level, and the development of new catalysts.Based on the references and conclusions of our research works, this thesis paid a special attention on the characteristic of K-/Ca-/Fe catalysts, and investigated on their catalytic activity. Then the structures of catalytic precursors and oxidation state (complex form) intermediates were analyzed, aiming at reveal the reaction path of catalytic gasification. Finally, the quantum chemistry theory which is applying to analyze the mechanism of catalytic gasification was deeply discussed. The key findings of dissertation are as follows:Firstly, catalytic activity of K-/Ca-/Fe-salts or oxides was studied by thermogravimetric analyzer (TG) coupled with gas chromatograph (GC)/Fourier transform infrared spectroscopy (FTIR) and the active sites on carbon surface was measured trough CO2chemisorption method. The results showed that during the carbon gasification process, decreasing order of catalytic activity K2CO3>Na2CO3>Fe(NO3)3>CaO was observed, and the intrinsic mineral matters in char improved the catalytic activity of KCl. CO2chemisorption analysis received satisfactory success in both interpreting catalytic effects and correlating the gasification reactivity by the parameter of strong CO2chemisorption (Cstr). In details, the gasification reactivity parameter Tmax, which represents the temperature corresponding to the maximum weight loss rate, decreased linearly with the increase of Cstr.The characteristics of catalyst (K2CO3/Ca0/Fe(N03)3) and char/graphite interaction in inert atmosphere was conducted by TG-FTIR, then the catalyst concentration and composition in the residues were detected by scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX) and X-ray diffraction (XRD) to further analyze the structures of catalytic precursors. The results showed that even in the inert atmosphere, potassium carbonate decomposed when it was heated in the presence of carbon/char at subgasification temperatures. For graphite, the interaction of carbon with potassium carbonate would firstly create a CnK cluster which prevented the evaporation of K, then above800℃, the CnK cluster would gradually thermally cleaved resulting in the evaporation of K. For demineralized coal char, K was bonded to carbon during the heating treatment, forming a complex containing O and K which showed high thermal stability at950℃. The crystal structure analysis showed that this newly formed complex couldn’t be observed in the XRD pattern. As for CaO, heating treatment would make it anchor to the carbon surface without any change, Fe(NO3)3can be reduced with the carbon substrate by redox reactions, newly forming the catalytic precursors, elementary Fe and Fe3O4.The structures of oxidation state (complex form) intermediates were studied by diffuse reflectance infrared fourier transform spectroscopy (DRIFTs) coupled with XRD analysis. The results showed that CO2chemisorbed by K-catalytic precursors at200～500℃would result in absorption bands at around1112cm-1and940cm-1, attributing to potassium phenolate and a covalently bound carbonate group (dimethyl carbonate), respectively. CO2chemisorbed by Ca-catalytic precursors would also result in absorption bands at around945cm-1, attributing to dimethyl carbonate. When treated in CO2atmosphere at600℃, Ca-catalytic precursors (CaO) almost wholly converted to CaCO3, thus, a CaO-CaCO3cycle configuration is rational for the alkaline-earth metals catalytic gasification. CO2 chemisorbed by Fe-catalytic precursors would result in absorption bands at around945cm-1and1158cm-1, attributing to dimethyl carbonate and specially chemisorbed CO2by Fe-precursors, respectively. When treated in CO2atmosphere at600℃, Fe-catalytic precursors (Fe and Fe3O4) were oxidized, mainly in the form of Fe2O3.Finally, according to the active intermediates structure analyzed above, the catalytic gasification mechanism was investigated systematically by Density Functional Theory (DFT). The results showed that:the-O-K and CaO catalytic group in the carbon substrate resulted in the energy barrier of CO desorption which is the rate-limiting step of gasification increased by a minor degree. The catalytic activity of these groups lies in the fact that the absorption of K, Ca atom could increase the atom charge of the carbon free active site, especially the ones located close to the metal atom. Consequently, more oxygen chemisorption would occur and newly form more C(O) active complex. The rate of carbon gasification should be directly proportional to the concentration of the C(O) complex.