Study On Hydrogasification Of Coal Char Catalyzed By Iron-Based Catalyst

Natural gas from coal can make up for the energy structure characteristics of"rich coal,poor oil and little gas"in China.The direct hydrogasification of coal to methane has been extensively studied because of its high thermal efficiency,simple process and economy.The use of catalysts makes the harsh conditions required for coal hydrogasification more mild.In this paper,the effect of iron-based catalysts on catalytic hydrogasification performance was studied.The effects of Fe-Co-Ca,the dispersion of Fe and the evolution of Fe-Ca on the catalytic hydrogasification of coal char were investigated.The changes of coal char and catalyst were analyzed by XRD,Raman,FT-IR,TEM and BET.The main conclusions were as follows:?1?Cobalt has been proved to be very active in coal/char catalytic hydrogasification?CCHG?,it is expensive.Partial replacement of cobalt by iron was considered in this work in order to reduce the cost of catalyst.Co-Fe-Ca ternary catalysts are studied for hydrogasification of demineralized coal char in a fixed-bed reactor.The CH₄ yield and CH₄ formation rate was studied on catalytic hydrogasification.The optimum loading of 1.0Co-0.5Fe-1.5Ca?wt%? was studied.The properties of catalyst and char were studied by XRD,TEM and Raman.The results showed that partial substitution of Co with Fe was feasible.In the presence of 1.0Co-0.5Fe-1.5Ca?wt%? catalyst,the methane yield reached 71.96%,even higher than 71.63% of 1.5Co-1.5Ca?wt%? catalyst.The formation of carbides was not found after hydrogasification of Co-Fe-Ca ternary catalyst,but only in the form of metal reduction.This may be due to the inhibition of the formation of Fe₃C by Co or the rapid conversion of Fe₃C by Co.The interior of coal char loaded with Fe-Ca is very smooth.When Co-Fe-Ca coexists,many holes are formed in coal char and the structure is rough.The addition of catalyst inhibits the graphitization process of coal char,and increases the reaction active sites and reactivity of coal char.?2?In this paper,the effect of dispersibility of Fe on coal chars with different specific surface areas on catalytic hydrogasification performance was studied in a pressurized fixed bed reactor.The chars and catalysts were characterized by XRD,BET,H₂-TPR,FTIR,TEM and Raman spectroscopy.The results show that the active site and graphitization degree of coal char are not the only factors affecting the catalytic gasification reaction,but the dispersion of catalyst has a greater impact on the reaction.The larger the specific surface area of coal char is,the more uniform the Fe catalyst disperses on the surface of coal char and the smaller the average grain size of the active component of catalyst,which can promote the formation of the catalytically hydrogenated mesophase product Fe₃C and methane yield.For 900-Char with higher specific surface area,the methane yield can reach 53% when the hydrogen pressure is 2 MPa,the temperature is 750? and the Fe loading is 5wt%.The effect of Fe catalyst loading on catalytic hydrogasification was investigated at 900-char.The methane yield increased first and then decreased,and the Fe loading had a saturation point.?3?The evolution of Fe and Fe-Ca catalysts during char catalytic hydrogasification was investigated in a pressurized fixed-bed reactor.The effects of Fe and Fe-Ca catalysts on CH₄ yield and CH₄ formation rate were studied.The change of the catalysts and char was also characterized by means of the transmission electron microscopy?TEM?,X-ray diffraction?XRD?,Temperature-programmed reduction?TPR?,etc.The relationship between the CH₄ formation rate and the ratio of Fe₃C to Fe was discussed to identify the rate determining step,as well as possible reaction mechanism.The results show that the role of calcium on char catalytic hydrogasification is not only dispersion of iron-group metals and desulphurization but also promotes the reduction of metal iron as well as carburization reaction.As for Fe-Ca catalyzed case,a negative correlation between V_CH4 and ratio of Fe₃C to Fe was found during hydrogasification.The larger the proportion of Fe₃C is,the slower the methane formation rate becomes.These results indicate that iron carbide is a stable intermediate product of catalytic hydrogasification.The hydrogenation of iron carbide to form methane is the rate determining step for Fe catalyzed hydrogasification.