Process Integration And Catalyst Design Based On Calcium Looping

To tackle issues and challenges aroused by the climate and environment deterioration associated with ever-rising atmospheric CO₂ concentration since the Industrial Revolution,increasing attentions have been drawn to carbon capture,sequestration and utilization（CCSU）technologies.Among numerous CCSU technologies,calcium looping（Ca L）based on reversible conversion between CaCO₃ and Ca O is deemed as a highly prospective solution.Nowadays,Ca L based sorption-enhanced hydrogen production,integrated CO₂ capture-catalytic conversion and chemical looping oxidation processes have received extensive research and exhibit favorable practicality.Nevertheless,the key factor that impedes popularization of Ca L lies in its energy-intensive decarbonation,which,at present,is commonly powered by coal and natural gas combustion,thereby resulting in increased fossil fuel dependence and reduced CO₂ capture efficiency.Herein,we propose that the overall energy consumption of Ca L can be reduced dramatically by introducing exothermic reactions at the decarbonation stage.Calcium cobaltate（CCO）can be formed via intensively exothermic solid-state reaction between Ca O and Co in the presence of O₂,in which Ca O and Co are CO₂ sorbent and reforming pre-catalyst for sorption-enhanced steam reforming of glycerol（SESRG）.In this context,the reaction can be coupled with CaCO₃ decomposition to reduce energy consumption of the decarbonation stage.By further introducing chemical looping methane combustion（CLMC）process which proceeds prior to SESRG,H₂ consumption caused by introducing O₂ can be avoided and high-efficiency CH₄ utilization can be achieved,thereby constituting an integrated CLMC-SESRG looping process.Energy balance calculations indicate that the overall energy consumption of proposed process is 56%of that of conventional SERSG-decarbonation process.CCO is then subjected to the proposed process,with further Pt doping to enhance the capability of C-H bond activation.In a 20-cycle test,70%CH₄ conversion with over 90%CO₂ selectivity is achieved,and H₂ with 96%purity and 120%yield is produced over Pt/CCO,and the structural evolutions of Pt/CCO throughout the cyclic test is highly reversible.CH₄-TPR-MS,SEM,TEM and XPS analysis further illustrate that the formation of Pt O₂ particles on the CCO flake as well as homogeneous dispersion of remaining Pt species within bulk CCO phase is responsible for promoting CH₄ conversion,and that improved hydrogen production performance is attributed to Co-Pt solid solution formed after CH₄ reduction,which facilitates the water-gas shift reaction.It is also feasible to reduce energy consumption of Ca L by introducing CaCO₃hydrogenation reaction in the presence of H₂,by which carbonaceous fuels,such as CH₄,can be concurrently produced.Energy balance calculations suggest that energy consumption of CaCO₃ hydrogenation-carbonation process is only one third of that of conventional Ca L process.Nevertheless,to make CaCO₃ hydrogenation-carbonation process practically efficient,the essence lies in designing high-performance catalyst and clarifying the mechanisms thereof.To this end,Pt nanoparticles supported on CaCO₃ are designed and fabricated,allowing for formation of plentiful catalyst-reactant interfaces.At 360～550 ℃,100 k Pa and 100%H₂ flow,the as-synthesized Pt/CaCO₃ achieved 98%CH₄ selectivity,approaching the thermodynamic equilibrium value.In 10 cyclic hydrogenation-carbonation test,CH₄ selectivity over 95%with37 mmol g_Pt^-1 h^-1 production rate can be maintained over Pt/CaCO₃,and the structural and morphological properties of Pt/CaCO₃ are also retained.In situ XRD test reveals that Ca O is the main solid-phase product of Pt/CaCO₃ hydrogenation at 350～550 ℃,along with forming small amount of Ca（OH）₂.In situ diffuse reflectance infrared Fourier transform spectroscopy（DRIFTS）verifies that decomposition of CaCO₃ does not occur in the course of reaction,and that surface Ca（OH）₂ and adsorbed Pt-CO species are the key intermediates for Pt/CaCO₃hydrogenation,with the consumption rate of adsorbed Pt-CO species positively correlating with CH₄ production rate.Furthermore,CO-DRIFTS indicates adsorbed Pt-CO species are favorable to conform on Pt atoms located at Pt-CaCO₃ interfaces.To demonstrate the feasibility of scaled implementation of above-mentioned CaCO₃hydrogenation-carbonation process,process simulation and subsequent techno-economic analysis are conducted using coke oven gas（COG）,an abundant product from coal coking process,or wind-photoelectric to hydrogen（WPTH）produced by water electrolysis as hydrogen source,and flue gas from thermal power plant as carbon source,respectively.It is revealed that CH₄ productivities are 1.34×10⁸ and 0.35×10⁸ Nm³/y,and energy efficiencies are55.99%and 23.79%for COG and WPTH enabled CaCO₃ hydrogenation-carbonation process,respectively.In addition,the annual profit for the former scenario is 1.67×10⁸ CNY,allowing for investment recovery within 3.77 years,while the latter scenario leads to an annual loss of2.75×10⁸ CNY,and either hydrogen unit cost lowering to 0.06 CNY/Nm³ or CH₄unit price increasing to 10.13 CNY/Nm³ is required to make it profitable.At current status,COG is more suitable as hydrogen source for CaCO₃ hydrogenation-carbonation process than WPTH,and the process is preliminarily feasible for scaled implementation.