Catalytic Methane Decomposition Over Selective Gasification Residue Of Coal Char

Hydrogen appears to be one of the excellent energy carriers,and it has broad application prospects in fuel cells and internal combustion engines.At present,among the various hydrogen production strategies,hydrogen production by steam reforming of natural gas is the most mature technology that contributes to about 48%of the whole world’s hydrogen consumption.However,this process produces large amounts of CO_x,and thus it is usually accompanied by complicated water-gas shift,separation and purification steps.Moreover,the by-product is carbon dioxide.Catalytic methane decomposition(CMD,CH₄（g）→C（s）+2H₂（g）,ΔH_298K=75 kJ/mol)can pave the route for co-production of CO_x free hydrogen and valuable carbon materials（such as carbon nanotubes and nanofibers）,which is a potential pathway for developing low-carbon and eco-friendly hydrogen economy.However,there are still some problems and challenges for utilization of conventional metal and/or carbon catalysts.Therefore,it is necessary to further investigate and develop suitable catalysts to promote the commercialization process of catalytic methane decomposition for hydrogen production.Coal gasification is the main method for large-scale industrial production of syngas（or hydrogen）in China.Based on the above background,this paper designs and develops a two-stage process for selective gasification of coal char and hydrogen production by methane decomposition.By regulating properties of the gasification residue from coal char,the catalytic activity and stability of the gasification residue can be enhanced and improved in the methane decomposition reaction.Firstly,an appropriate amount of nickel and/or cerium component is introduced in the gasification process of coal char,and Ni⁰ crystallites with higher dispersion can be formed in-situ by controlling the operating conditions of the gasification process and utilizing the carbothermal reduction during the gasification process.The gasification reaction rate is promoted and thus achieving co-production of hydrogen-rich gas and gasification residues（expressed as Ni/carbon hybrid）with large surface areas（217-265 m²/g）.When the Ni/carbon hybrid is used as a catalyst for catalyzing methane decomposition,high activity（methane conversion up to about 90%at 850℃）and high stability（no decrease in catalyst activity during600 min reaction time）can be obtained.The superior result also contributes to formation and growth of the fibrous carbon deposits.On this basis,this paper designs and develops a high-efficiency nickel-based catalyst for hydrogen production from catalytic methane decomposition by introducing some additional nickel and K₂CO₃ into partial steam gasification of coal char.The high-reduction nickel microcrystals(Ni⁰/(Ni⁰+Ni²⁺)ratio is as high as 76%-81%)are synthesized in-situ by the carbothermal reduction reaction and the activation of K₂CO₃ in the gasification process.The gasification process is competent for co-generation of hydrogen-rich gas and nickel/carbon hybrids with large surface areas（around 86-149 m²/g after washing off the residual potassium salts）.The nickel/carbon hybrid as the gasification residue could serve as the catalyst for hydrogen production from catalytic methane decomposition,exhibiting a high and stable methane conversion（up to 80%-87%）at 850℃.It is observed that K₂CO₃ has a positive effect on formation and activity improvement of the nickel/carbon catalyst,and the beneficial effects of co-production of hydrogen and filamentous carbon can be realized in the 600-min process of hydrogen production by catalytic methane decomposition.The implementation of the integrated process of selective gasification of coal char and hydrogen production by catalytic methane decomposition could be a potential strategy towards process intensification,providing a feasible sample for the development of hydrogen production technology with high economic value,low risk and environmentally sustainable,especially for small to medium-scale facilities adapted to most parts of the worldwide regions.