The Experimental And Theoretical Studies On Sorption Enhanced Ethanol Steam Reforming For Hydrogen Production With CaO-based Sorbents

Hydrogen energy,with the advantages of high energy density,clean and free pollution,has been widely considered as an ideal energy carrier.Conversion of biomass ethanol to hydrogen not only can promote the utilization of biomass resources,but also alleviate the dependence of hydrogen production from fossil fuels.As an emerging technology,the sorption enhanced reforming for hydrogen production exhibit great prospect for application,because it can produce high purity hydrogen in one step by in-situ capturing CO₂,which is stored in adsorbents for further processing.CaO-based sorbents and Ni-based catalysts have the most potential in the sorption enhanced ethanol steam reforming?SEESR?system,but they also subject the problems of loss-in-capacity and coking respectively during the long term utilization.Consequently,how to improve the cycle stability of adsorbents and the coking resistance of catalysis are the major challenges.Additionally,design of bi-functional materials with the adsorbent sites and catalyst sites mixing at the molecular level has also received increasing attentions,which can further enhance the performance of SEESR.In this work,we focus on enhancing the multiple carbonation performance of CaO based sorbent by adding metals as promoters,design and optimization of bi-functional materials,as well as exploring the effect of Ca component on the coking resistance of Ni in the bi-functional materials.The main research contents are as follows:Four kinds of CaO based absorbents modified respectively by metal promoters,such as Al,Mg,Zr and Na,were synthesized through sol-gel route.Moreover,the regulatory mechanism of metal promoters on the carbonation ability of sorbents was explored by combining experiments and DFT calculations.The results show that Al modified sorbents has the maximum initial adsorption capacity,which is attributed to its maximum specific surface area,thus providing more reactive sites for carbonation.The surface area of Na modified sorbent is the smallest,and the particle surface is smooth almost without obviously visible pores,which is not conducive to heat and mass transfer.Correspondingly,the carbonation ability of Na modified sorbent is the worst.Comparing with pure SG-CaO,although the surface area of Zr modified sorbent was reduced,it still has a better carbonation capacity,because the Zr promoter significantly improve the binding ability CaO surface with CO₂.From the view of sintering resistance,the presence of Al or Zr can significantly inhibit the movement of atoms from the nanoparticles,leading the high sintering resistance,while Mg only slightly enhance the ability to inhibit atomic migration On the contrary,the addition of Na promotes the atomic migration on the surface,leading to an aggravation of sintering.The results above indicate that metal promoters may affect the carbonation performance through three paths,such as varying the diffusion ability of CO₂ in particles,the binding energy of CO₂ with CaO surface and the migration resistance of calcium oxide grains.The final carbonation ability should depend on the superposition between the above three aspects.Comprehensively,Al should be chosen as the most suitable promoters to improve the carbonation performance of CaO.The carbonation performance of Al modified CaO-based sorbents were investigated in detail on many aspects,such as the effect of Al addition on the kinetics of carbonation and regeneration temperature,effect of Al content and calcination condition on the cyclic stability,and effect of synthesis method on the carbonation performance of Al modified sorbent.The results show that Al addition reduced the decomposition temperature of CaCO₃,and the reduction degree increases with the increasing Al content.The activation energy of Al modified sorbent was lower in both fast and slow stages of carbonation than that of pure CaO.As for the cyclic stability,with the increasing Al content the cyclic stability was also increased,but too high Al content would result in low initial CO₂ uptake.Under the harsh calcination conditions such as higher temperature and high concentration of CO₂,the cycle stability of Al modified sorbent decreased,but is still better than that of the pure CaO.In addition,the anti-sintering ability of Al modified sorbents largely depend on the distribution uniformity of Al and Ca element.The sol-gel method ensured the Al element distributed well in the sorbent,leading good sintering resistance,but dry mixing method didn't ensure that.By the molecular dynamics simulation,the sintering process of nanometer CaO particles was explored.Furthermore,the effect of Al₂O₃ addition on the sintering of CaO nanoparticles was also investigated.The results indicate that the sintering of CaO nanoparticles can be obviously divided into three stages including neck formation,rapid neck formation and slow neck formation phase.The higher the temperature is,the longer the growth duration of sintering neck will be,which will lead to the formation of sintering neck with larger width.Compared with large particles,atoms in small particles have higher migration activity in the sintering process,which may be attributed to that the small particles will melt more easily at high temperature.Regardless of particle size,the surface atoms of particles are easier to diffuse than the interior atoms.The addition of Al₂O₃ particle improves the migration and activation energy of atoms in CaO nanoparticles,inhibits the diffusion of atoms in CaO particles during the sintering process,and thus reduces the sintering degree.The Ni-CaO/Al₂O₃ bi-functional materials were synthesized by sol method.Then,their CO₂ absorption ability and performance of sorption enhanced reforming for hydrogen production were evaluated.From the perspective of CO₂ absorption performance,the Al components in the bi-functional materials play a role in inhibiting CaO sintering,but Ni components promoting the CaO sintering.When the mass fraction of Al₂O₃ was 4.5%,the material had the best carbonation performance.Further increasing Al₂O₃ content could improve the cycling stability,but also significantly reduce the initial carbonation ability.Compared with the traditional ethanol steam reforming without sorbent,the SEESR significantly increased the concentration of H₂ in the product.The bi-functional material of10Ni85.5CaO4.5Al had the best performance during the SEESR with the longest pre-breakthrough period and the highest H₂ concentration reached about 96%,which was about20%higher than that in the traditional ethanol steam reforming.In addition,the pre-breakthrough period of SEESR employing 10Ni85.5CaO4.5Al was obviously longer than using simple physical mixed material.In order to explore the potential anti-coking performance of bi-functional materials mixing Ni and Ca component at atomic level,the effect of Ca addition on the several main elementary reactions that determined the final coking degree was investigated,such as the formation,migration and agglomeration as well as elimination of monatomic C.In the case of Ca addition,the formation rate of monatomic C slight increase,which appeared that the risk of coking could boost if these monatomic C didn't gasification timely.However,the results also show that the diffusion and CO formation rate were both enhanced kinetically,especially the CO formation rate with a geometric progression,which implies that there was almost no chance to leave the excess of carbon atoms as deposit.the C diffusion and C-C bonds formation was needed to carbon deposit,and their reaction rate were also slowed down dramatically by Ca addition,thus inhibiting the coke forming.The Ca addition also weaken the interaction of C6 cluster with Ni surface,meaning the formation of encapsulating carbon could also be suppressed.Additionally,the presence of Ca on the Ni surface could significantly decrease the barrier of H₂O dissociation,thus providing sufficient oxygen intermediates to scavenge the deposited carbon.Hence,it should be believed that the presence of Ca in the bi-functional materials could well improve the coking resistance of Ni catalyst sites during the SEESR.