| Coal to synthetic natural gas (SNG or methane) has attracted much attention in recent years in China due to cleaner combustion of methane in comparison to other fossil fuels, such as coal and heavy oil, as well as the shortages in natural gas resource and supply. Many coal to SNG technologies have been studied in laboratory and practiced in industry,including steam-oxygen, catalytic coal gasification, and coal hydrogasification. The last one is regarded to be high in thermal efficiency, low in carbon dioxide emission and low in production cost.However, catalyst is needed to promote the rate of coal hydrogasification to make it competitive to other technologies. The catalysts studied in coal catalytic hydrogasification (CCHG) are mainly alkali-compounds and transition metals. The alkali-compounds tend to react with the inherent minerals in coal to form water-insoluble compounds, while the transition metals are prone to sintering at high temperature and poisoning by sulfur,and all of them make these catalysts difficult to recover and high in cost.In contrast, Ca-compounds are not troubled with the problem of catalyst recovery due to the abundant, low-cost and environmental benign nature.However, researches on catalysis of Ca-compounds are few and the results are somewhat contradictive. In view of this, this dissertation systematically studies the catalysis of Ca-compounds in CCHG, including the effect of reaction conditions on the catalytic activity, interaction of Ca and Fe, and reaction mechanism. The studies were carried out in a high pressure fix-bed reactor with a flow of hydrogen. The effluent gas was analyzed by gas chromatography (GC) or mass spectra (MS) while the solids were analyzed with XRD, elemental analysis, pore structure analysis and XPS. Furthermore, isotope tracer D2 is used to seek the mechanism. The main results are described in the following:(1) CaO-Fe shows remarkable catalytic activity in methane formation in char hydrogasification and the elements of Ca, O and Fe have to coexist meanwhile. The synergetic catalytic role of CaO-Fe is tentatively attributed to breaking of amorphous aromatic C-C bonds in char.(2) Only if the reaction conditions reach the threshold values, can the synergetic catalysis of CaO-Fe be activated. At temperatures higher than 750 °C and H2 partial pressures higher than 1.5 MPa, the synergetic catalysis starts remarkably. For the char containing 0.024 mmol/g (ad) Fe and 0.059 mmol/g (ad) S, the loading amount of CaO and Fe should be higher than 0.310 mmol/g and 0.170 mmol/g, respectively.(3) An induction time is necessary in CCHG before the catalysis of CaO-Fe starting, which shortens with increases in temperature and in H2 partial pressure, especially the former. The induction time may be related to diffusion of CaO into the interior of char, and due to the diffusion effect of CaO the synergetic catalysis of CaO-Fe transforms from CaO-dominated to Fe-dominated over time.(4) The effect of dispersity of metallic Fe and CaO on the synergetic catalysis of CaO-Fe is different. For metallic Fe, the Fe formed from the reduction of impregnated Fe(N03)3 is much more active in the synergetic catalysis of CaO-Fe than that from the reduction of mechanically mixed FeS2. However, CaO impregnated by Ca(CH3COO)2 and loaded by Ca(OH)2 show the similar synergetic catalysis in CaO-Fe.(5) H2-D2 exchange reaction occur remarkably at high temperature to produce HD. Hence, the isotopic tracer technique of H2/D2 mixture is difficult to be applied in the study of the dissociation of hydrogen molecule in hydrogasification.(6) In the synergetic catalysis of CaO-Fe, the role of CaO is proposed to be oxygen transfer mechanism and CO can inhibit the oxygen transfer reaction. |
