Research On Reaction Characteristics And Mechanism Of Lignite Semicoke Hydrogasification

The increasing demand for limited natural gas resource amount arouse thegrowing natural gas gap, thus coal-based methanation technology has causedpeople’s concern again. Effects of pyrolysis conditions, operational conditionsand catalyst on the hydrogasification of Inner Mongolian lignite semi-coke wasinvestigated by using a self-developed high-temperature and high-pressurefixed-bed reactor under conditions of1000℃and12MPa; changes of thechemical and physical properties of semi-coke during hydrogasification processwere studied by means of XRD, FT-IR and BET analysis technology; and thereaction mechanism of semi-coke hydrogasification was also investigated in thispaper. The main conclusions were list as follows:1. Raising pyrolysis temperature will change the active constituent contentof coke, promoting the polycondensation of coke, and leading the decrease ofcoke’s hydrogasification reactivity, final carbon conversion and methaneemission. The optional pyrolysis temperature of Inner Mongolia lignite was600℃, the optional time of conatant temperature was15min, the optionalheating rate was10℃/min. In the pyrolysis condition of600℃and normalpressure, the pyrolysis atmosphere(hydrogen, nitrogen and pyrolysis air) has little effect on the hydrogenation methanation reaction characteristics for semicoke.2. The carbon conversion and the yield of methane will be increased withthe decrease of semi-coke particle size (in the range of0.15-0.70mm), and theincrease of reaction temperature(in the range of700~900℃), hydrogen flowrate(in the range of18.75~93.75mL·(min·g)-1), reaction pressure (in the rangeof1~5MPa)and heating rate(in the range of10~30℃/min). Within the range ofstudy, the optimal semi-coke grain size, hydrogen flow rate, reactiontemperature, reaction pressure and heating rate were0.25-0.35mm,1200mL/min,800℃,3.0-4.0MPa and30℃/min separately.3. In the hydrogasification process, content of C element and ash increasedgradually, content of H, O, N, S elements and volatile matter decreasedgradually in the semi coke. Three diffraction peaks, KFeO2,(002) and SiO2,were included in the semicoke XRD spectrum. The vibration of-CH3,-CH2,-OH,-NH, C=O and other active groups decreased and even disappeared in charwhen the methanation reaction carried on for80min. In the process ofmethanation reaction, the average pore size of semicoke decreases first and thenincreases, the total pore volume and mesopore volume increased gradually,micropore volume and specific surface area increases first and then decreases.4. When the particle size of catalyze changes in the range of0.18-0.83mm.Compared with the reaction without catalyzer, the product yield of CH4increased by0.72~15.37%, the product yield of C2H6increased by2.18~10.37%, the product yield of CO increased by57.81~162.09%, the product yield of CO2decreased by59.79~84.55%, the product yield of C2H4decreased by16.22~80.82%. With the increase of temperature, the final carbon conversion ofsemicoke hydrogasification and production of CH4and CO will be increased,while the production of C2H6、C2H4and CO2changes less. In the range ofinvestigation, the optimal hydrogen flow rate, semi coke particle size, particlesize of catalyst, potassium carbonate added amount and reaction temperature ofthe semi-coke catalytic hydrogasification reaction were:1200mL/min,0.25-0.38mm,0.38-0.83mm,30%(wt%),800℃separately.5. The semi-coke catalytic hydrogasification reaction could be divided intothree stages: hydropyrolysis stages, rapid hydrogasification reaction and lowspeed hydrogasification reaction. In the hydropyrolysis stages, the main reactionwere thermal cracking reaction of active group like the alkyl side chain,carboxyl, hydroxyl, ether and ketone, whose gas products includes CO, CO2,CH4, C2H6and C2H4. The main reaction of rapid hydrogasification reaction wasthe hydrogenation reaction of active group inside of semi coke. The hydrogenmolecules will firstly adsorbed in the active gasification reactivity site of carbonstructure inside of semi-coke, then transfer the C of active carbon structure toCH4. The main compents of gas product in this period is CH4. The main reactionof low speed hydrogasification reaction was the hydrogenation reaction of lesshydro-active fused ring frame carbon structure inside the semi coke, and CH4isthe only compents of gas product.