Study On Carbonation Reactivity Of CaO-based Sorbents And Sorption-enhanced Steam Methane Reforming For Continuous Hydrogen Production

Hydrogen energy is regarded as the most promising clean energy carrier in the 21st century.Sorption-enhanced steam methane reforming（SESMR）can produce high-purity hydrogen in one step while reducing carbon emissions by removing CO₂ in situ through high-temperature solid sorbents,which has great prospects for development.CaO-based sorbents are considered to be the most potential materials in the SESMR system due to its wide range of sources,low price,and large theoretical absorption capacity.How to improve the cyclic reaction activity and adsorption stability is still a key issue.There is still a lack of intrinsic understanding of the mechanism of CaO carbonation reaction with CO₂ and Al doping modification mechanism.Besides,the influence of water vapor and catalytic component Ni on the CO₂ capture process of CaO-based sorbents has not been studied in detail.In addition,the coupling method of sorbent and catalyst is also important to the hydrogen production of SESMR.Based on the above analysis,a systematic study was performed on the carbonation reactivity of CaO-based sorbents and sorption-enhanced steam methane reforming for continuous hydrogen production.The main contents are as follows:Firstly,the carbonation reaction rate of micron and nano-scale CaO were tested using a thermogravimetric analyzer under different temperatures.Meanwhile,the reaction molecular dynamics（Reax FF-MD）simulations were performed to detect the time-dependent properties and behaviors.Also,the dynamic evolution of product and reactant structure during chemical reaction process were obtained,revealing the actual mechanism of temperature on the carbonation reaction of CaO with CO₂.Results show that the reaction rate can be reconvered by increasing temperature even after adsorption saturation,and the greater temperature difference,the faster heating rate,the more obvious improvement in the carbonation rate.At the beginning of heating,the thickness of the product layer remains unchanged,but the density increases.During the secondary kinetic reaction,the formation of carbonate groups on the surface is the key step for the adsorption rate,which is obviously different from the initial reaction.By tracking species,it is found that CO₂ molecules on the CO₂-Ca CO₃interface can react with carbonate anion(CO₃^2-)through an oxo-Grotthuss mechanism via a pyrocarbonate anion,C₂O₅^2-.In particular,the recovery of rapid carbonation rate is mainly attributed to the instantaneous formation of a large number of polycarbonate ions.To address the poor cyclic stability casued by sintering of calcium oxide-based sorbents,Al₂O₃-stabilized CaO-based sorbents with different mass ratios of component were prepared by using dry/wet mixing,co-precipitation and sol-gel combustion methods.The effects of synthetic method and aluminum addition on the CO₂ capture properties and microstructure of CaO-based sorbents were analyzed from different perspectives,based on adsorption performance and structure characterizations.Results show that the sorbent with 25wt.%Al₂O₃ synthesed by sol-gel combustion displays the surperior performance in term of both capture capacity and durability.Al₂O₃ can react with CaO to generate inert skeleton material,which can effectively avoid particle agglomeration and improve its stability.After 30 cycles,its CO₂ capture capacity can still be maintained at 0.28 g/g,which is much higher than that of commercial CaO（0.11 g/g）.Solid-state nuclear magnetic resonance（NMR）shows that the order of the material structure decreases with the increase of Al content,and some Al ions enter into CaO lattice,causing internal structure distortion.There is a phenomenon of Al O₄to Al O₆ conversion during cycles,which forces Al ions to penetrate into the network and become a modifier,resulting in a decrease in the sintering resistance of the Al-O support.Water vapor as one of the reactant exists during the reforming process,so it is of great significance to study the effect of steam on the high-temperature CO₂ capture of CaO-based sorbents.The experiments combined with the Reax FF-MD calculation methods were used to explore the co-adsorption behavior of CO₂/H₂O on the CaO nanoparticle,and the promoting mechanism of steam on carbonation reaction was clarified by tracking the product and structure evolution.The experimental results show that the addition of steam has almost no effect on the initial rapid stage of CO₂ adsorption,but promotes the slow carbonization stage,and the promoting effect is more apparent at low temperatures.H₂O could significantly improve the outward diffusion of solid ions including Ca²⁺and O_ca^2-and the inward diffusion of CO₃^2-group under low temperatures.The OH groups from H₂O dissociation always stay on the surface of particles in the form of free radicals and can not directly react with CO₂.Protons could quickly combine with lattice oxygen to generate the hydroxyl and continuously diffuse into the particles through oxygen atom exchange.The strengthening effect of steam on the carbonation reaction is mainly achieved by promoting ion diffusion.Subsequently,the influence of the catalytic component Ni on the adsorption performance and anti-sintering properties of CaO during the process of sorption-enhanced hydrogen production was evaluated through CO₂ adsorption test,morphology and structure characterizations combined with density functional theory（DFT）calculations,and the synergistic mechanism of metal loading and inert oxides on the reaction characteristics of CaO-based sorbents were discussed essentially.The results show that Ni has a weak promotion effect on the initial kinetic adsorption of CO₂ with CaO,but it would also block pores and weaken the adsorption of Ca₄O₄ clusters,leading to more severe sintering.The inert component Al₂O₃ can effectively improve the sintering resistance of CaO by enhancing the surface adsorption of Ca₄O₄ clusters.On the Ni-Al₂O₃ surface,the adsorption energy of Ca₄O₄ clusters is still much higher than that of on the pure CaO（100）surface,meaning that the material can still maintain good sintering resistance.The Ni-Ca interaction is not conducive to the performance of CaO sorbent.The strong Al-Ca interaction is beneficial to the improvement of stability,and the strong Ni-Al interaction could weaken the anti-sintering effect of the inert carrier Al₂O₃.Finally,the Al-modified CaO-based sorbent was pelletized by the carbon template method,and its sorption enhancement effect in the methane steam reforming process of methane was tested.The influence of reaction conditions（temperature,water-gas ratio and space velocity）and the combination of catalyst with adsorbent（powder/particle mechanical mixing,layered arrangement and bifunctional materials at a molecular level）on the hydrogen production during the pre-breakthrough period was investigated.The results show that the sorbent pellets have a fluffy porous structure after granulation,and still show good adsorption performance under low CO₂ partial pressure,as well as good mechanical properties.The optimal reaction temperature range of SESMR is 550-600℃.Increasing the water-gas ratio and reducing the flow rate can effectively increase the CH₄ conversion rate and H₂ purity.Under uniform mixture of the catalyst particles and sorbent particles,the volume fraction of H₂ in the outlet products reaches almost 100%,and there is no attenuation during 15 cycles.When the switching time of reaction material is 60 minutes,the doual reactors can continuously produce high-purity（>96%）hydrogen.