Modified Ni/γ-Al₂O₃ Catalytic Reforming Of Methane And Carbon Dioxide And Its Thermochemical Recovery Of Waste Heat

CO₂ emission from fossil fuel combustion is a major factor to promote global warming.The CO₂reforming of CH₄（dry reforming）reaction can convert greenhouse gases（CO₂ and CH₄）into synthesis gas（CO+H₂）with H₂/CO ratio close to 1,which is essential for the formation of longer chain hydrocarbons through the carbonylation,dimethyl ether synthesis and Fischer Tropsch reaction.Dry reforming process provides a solution to these key problems facing today’s society,through its ability to utilize both CH₄ and CO₂,for synthesis gas（syngas）production which has dual significance of environmental protection and resource utilization.Although Ni based catalyst has good economy,it is sintered and deposited carbon easily in the high reaction temperature which leads to the decrease of catalytic activity and even deactivation.This disadvantage is a bottleneck problem of Ni based catalyst industrial application for dry reforming.In order to solve the problems of carbon deposition and endothermic reaction process of dry reforming over Ni/γ-Al₂O₃ catalyst,Ca O and CO were used to modify Ni/γ-Al₂O₃ catalyst to improve the stability and anti-carbon deposition ability of the catalyst,and the catalytic activity of the modified catalyst in dry reforming was studied.The coupling mechanism of thermochemical recuperation system by dry reforming was studied from waste heat of flue gas.This technology realizes the dual effection of carbon recovery and efficient utilization of waste heat.The reaction mechanism and thermodynamic analysis of CH₄-CO₂ reforming reaction were studied systematically.In order to optimize the reaction conditions,the effects of reaction temperature（500～1200℃）,pressure（0.5～2.0atm）,CH₄/CO₂ molar ratio（0.5～2）and trace O₂ and H₂O on the conversion,selectivity and carbon deposition were analyzed in dry reforming reaction.The conversions of CH₄ and CO₂ can reach98.4%and 97.0%respectively at low pressure（1atm）and CH₄/CO₂ ratio=1.The selectivities of H₂ and CO can reach 94.7%and 96.9%respectively,and the coke deposition rate is only 5.7×10^-23%in mass.Ni/γ-Al₂O₃ catalysts with different Ca O loading and Ni Ca O/γ-Al₂O₃catalysts with different Co loading were synthesized by impregnation method for dry reforming reaction.The effects of loading Ca O and Co on the catalytic performance of Ni10/γ-Al₂O₃ catalysts for CH₄-CO₂ reforming reaction was investigated.The prepared samples were characterized by X-ray diffraction（XRD）,N₂ adsorption（BET）,temperature programmed reduction（H₂-TPR）and scanning electron microscopy（SEM）techniques.The results showed that adding Ca O can improve the performance of Ni based catalyst,and adding Co can improve the coke deposition resistance and stability of Ni10Ca O3/γ-Al₂O₃ catalyst.Ca O can improve the dispersion of active metals on the support,inhibit the formation of nickel-aluminum spinel,and promote the catalytic activity of the catalyst.Co affects the interaction between the metal and the support.The formation of Ni-Co intermetallic compound.The coke deposition resistance and stability of the catalyst were enhanced by loading Co metal.With the loading content increasing of Ca O,the conversions of CH₄ and CO₂reached 92.0%and95.0%,the selectivities of H₂ and CO reached 87.9%and 94.2%,respectively.Ni8Co2Ca O3/γ-Al₂O₃ has the most excellent catalytic activity,stability and coke deposition resistance.The conversions of CH₄ and CO₂ reached 92.6%and 96.7%respectively,the selectivities of H₂ and CO reached 82.8%and 98.4%respectively.In addition,Ni8Co2Ca O3/γ-Al₂O₃ possessed high catalytic stability during 100 h time on stream.The coke deposition mainly comes from the pyrolysis of methane at high temperature in the catalytic reforming of methane and carbon dioxide.It is main reason for the deactivation of the catalyst that the carbon fiber destroys the link between the active metal and the support,and a dense encapsulated carbon was formed which is difficult to eliminate.The results showed that high temperature is beneficial to the main reforming reaction.When the temperature is higher than 900℃,coke would be elimited by the carbon gasification reaction.However,the coke deposition rate from methane cracking is higher than that of carbon gasification reaction,and the energy consumption of the reaction would increase too.Coke deposition leads to catalyst deactivation due to coke fiber breaks the link between metal particles and the suport,and the dense packing of polymerized carbon is eliminated difficultly and implies a drastic decrease of the catalyst activity.Although the content of coke deposited is determined by the employed operating conditions,Nickel-based catalysts have carbon deposits inevitably during the CH₄-CO₂ catalytic reforming process.High temperature and low pressure could reduce the amount of carbon deposition on the catalyst.But the effect of temperature on the amount of carbon deposition on the catalyst is more sensitive than the effect of pressure.The performances of anti-sintering and anti-coke deposition can improve with Ca O/Co modified Ni/γ-Al2O3 catalysts significantly.Ni8Co2Ca O3/γ-Al₂O₃catalyst possessed high catalytic stability during 100 h on stream at 800℃and atmospheric pressure.the coke deposition rate arbon conversion is only 1.5%under the optimized condition of operating conditions.The technical schemes of termochemical waste-heat recuperation systems were pointed out based on dry reforming process or triple reforming of methane process.In this paper,energy analysis of thermochemical recuperation system by methane reforming was studied.The efficiency of dry reforming thermochemical waste heat recuperation process depended on the H₂/CO ratio in the synthesis gas after reforming.The conversions of methane and carbon dioxide effected the heat recovery efficiency.The coefficient of heat transformation constantly increases with increasing of temperature and addition carbon dioxine to feed stock.The reforming reaction heat was same too.The thermochemical waste heat recuperation of methane dry reforming can save methane supply and reduce carbon emission.The technology effectively can achieve a carbon emission reduction of 14.4%.The thermochemical waste heat storage recuperation process of methane tri-reforming which can effectively improve the heat utilization efficiency and reduce fuel consumption is a simplifies the process.a recovered heat increases with increasing exhaust gas temperature.The results showed that the ideal temperature of the methane tri-reforming reaction is higher than 800℃.The increasing reaction temperature would improve the conversion rate of CH₄ and CO₂,and increase the yield of syngas.The H₂/CO ratio of synthesis gas can be controlled at 1.5～2.0 generally.The conversions of CH₄ and CO₂ can reached 99%and88%respectively.When a small amount of oxygen is added into the feed,the heat recovery and thermochemical conversion of the thri-reforming could reach 144.9k J·mol-CH₄^-1 and 27.4%respectively.