Materials Development And Performance Study Over Novel Calcium Looping Technology Integrated With CH₄/CO₂ Dry Reforming

The development and implementation of CO₂ capture,utilization and storage technology is a crucial way to achieve the"dual carbon"goal.Coupling calcium looping（Ca L）,chemical looping combustion（CLC）and CH₄/CO₂ dry reforming,we proposed a novel calcium looping technology integrated with CH₄/CO₂ dry reforming,aiming to realize the simultaneous CO₂capture and utilization.In such a technology,the Ca L process,based on a reversible carbonation/calcination reaction of Ca O-based sorbents,was used to capture CO₂ from the flue gas.The CLC of gaseous fuels with calcium-copper composite sorbents was utilized to provide the heat required for calcination of the Ca O-based sorbents.The CH₄/CO₂ dry reforming on Ni-based catalysts was employed for catalytic conversion of the captured CO₂.This work has designed and synthesized novel Ca O-based sorbents,calcium-copper composite sorbents and Ni-based catalysts.Meanwhile,their performances as sorbents,oxygen carriers and catalysts have been carefully studied,in order to provide theoretical and experimental support for the material development for the proposed technology.The main research contents and conclusions are as follows:In terms of the rapid deactivation of CO₂ capture performance of the Ca O-based sorbents during cyclic calcination/carbonation reactions at high temperatures,a carbonaceous microsphere template approach was developed to synthesize Ca O hollow microspheres.The CO₂ capture performance of Ca O hollow microspheres was assessed on a thermogravimetric analyzer（TGA）,and the kinetic analysis was then conducted for the carbonation reaction.The results show that the Ca O hollow microspheres demonstrated an initial carbonation conversion of 98.2%,1.3 times as high as the raw limestone.This is because the hollow microsphere structure improved the reaction rate in the kinetically controlled stage of the carbonation reaction,delayed the decline of the reaction rate,and reduced the activation energy of the carbonation reaction,thus promoting the carbonation reaction.Turning to the calcium-copper composite sorbents,due to the presence of Cu O/Cu component with low Tammann temperature,the sintering during cyclic operations was more severe than the Ca O-based sorbents,thus leading to more significant deactivation in its CO₂capture performance.To address this problem,we employed three approaches,namely the development of nanosized sorbent,hollow microsphere structure modification and stabilization modification,to improve the reactivity of the calcium-copper composite sorbents,respectively.First,a flame synthesis method was used to fabricate the nanosized calcium-copper composite sorbents,the CO₂ capture performance of which was then evaluated on a TGA.It is found that the nanosized calcium-copper composite sorbents demonstrated poor CO₂ capture performance in the initial cycles,however,its CO₂ uptake capacity increased with the increasing cycle number and became stable eventually.This is mainly due to the increase in both specific surface area and pore volume of the nanosized calcium-copper composite sorbents during cycling.Then,a one-step hydrothermal method was explored to synthesize the calcium-copper hollow microspheres.A fixed bed reactor was used to evaluate the reactivity of the calcium-copper hollow microspheres.It is found that the hollow microsphere structure improved significantly the CO₂ capture performance of the calcium-copper composite sorbents,resulting in a high CO₂ sorption rate and good cyclic stability.The calcium-copper hollow microspheres demonstrated a CO₂ uptake of 0.17 g _CO2/g_sorbentwithin a carbonation duration of the first minute,contributing to 81%of the total CO₂ uptake capacity.Moreover,the rate of decline in CO₂capture performance was only 28.6%after ten repeated cycles,which was much lower than that of the calcium-copper composite sorbents synthesized by wet mixing and co-precipitation methods.This was because the hollow microsphere structure was retained during cyclic operations,suppressing a dramatic reduction in surface area and pore volume and thus improving the cyclic stability of the CO₂ capture performance.Finally,a Pechini method was developed to synthesize the stabilized calcium-copper composite sorbents,the reactivity of which was then tested on a TGA.The results show that when the stabilized calcium-copper composite sorbent was synthesized by the Pechini method,Al₂O₃ was more suitable to be a stabilizer than Mg O,and the optimal quantity of Al₂O₃ is 15mol.%.Compared with Mg O,Al₂O₃ reacted with Ca O to form Ca₅Al₆O₁₄ during the material synthesis process,inhibiting element migrations during cycling.Thus,the elemental distribution in Al₂O₃-stabilized calcium-copper composite sorbents was more uniform than that in Mg O-stabilized sorbents,which is beneficial for the Al₂O₃ to act as a robust framework and thereby led to a much improved CO₂ capture performance.Furthermore,the improvement effect of the abovementioned three methods（i.e.,the development of nanosized sorbent,hollow microsphere structure modification and stabilization modification）on the reactivity of the calcium-copper composite sorbents was analyzed comparatively.It is found that the hollow microsphere structure modification was the best strategy,leading to the best oxidation performance and CO₂ capture performance.Turning to the Ni-based catalysts,it faced two major challenges,including the sintering of active component Ni and carbon deposition in the CH₄/CO₂ dry reforming.To solve the problem,an exsolution strategy was explored to synthesize Ni-based catalysts with high reactivity and high resistance to carbon deposition.The catalytic activity of the Ni-based catalysts was then measured on a fixed-bed reactor.The results show that when Mg O was used as the support,an extremely stable Mg O-Ni O solid solution formed during the calcination at high temperatures,which was very difficult to be reduced.Thus,very few metallic Ni nanoparticles were exsolved from the catalysts in the reduction process.Besides,the exsolved Ni nanoparticles were embedded on the surface of the supports with a narrow particle size distribution.It is noted that the addition of Zn retarded the carbon deposition on the surface of Ni-based catalysts during the reaction.Although the catalytic activity of the Ni-based catalysts was not significantly improved,the reactor blockage caused by the carbon deposition was avoided,thereby prolonging the lifetime of the Ni-based catalysts.