Experimental Study And System Simulation On Hydrogen Production From Staged Catalytic Gasification Of Biomass

As a renewable and plentiful carbon-neutral energy source,biomass can be efficiently utilized to replace fossil energy to produce"green hydrogen,"a zero-carbon clean energy,which is expected to address energy shortages and environmental pollution.Biomass gasification is an efficient and economical thermochemical hydrogen production technology with a high reaction rate and gas yield that has good application prospects in large-scale industry.To solve the problems of high gasification temperatures,high energy consumption for heating,complex product gas components,low H₂conversion and H₂concentration in hydrogen production from biomass gasification technology,this paper proposes a two-stage system combining calcium looping gasification and catalytic reforming to produce hydrogen from biomass,which respectively facilitate the decrease of gasification temperature to avoid the corrosion of equipment due to the release of alkali metals at high temperatures,and the increase of catalytic temperature to promote tar removal and syngas conversion,while reducing catalyst deactivation and improving its recyclability.Ca（OH）₂was selected as a calcium adsorbent for alkaline degradation of biomass,tar cracking,in-situ CO₂adsorption to enhance steam reforming and supply heat.Fe/Ce bimetallic modified Ni-Ca adsorption catalytic bifunctional catalysts（Fe/Ce-Ni-Ca for short）were prepared for catalytic reforming to directionally control the quality of syngas for improving the H₂selectivity,aiming to stabilize the production of high concentration H₂at a relatively low temperature and hence the reduction of energy consumption for heating and hydrogen separation in actual industry.The research work in this paper is summarized as follows.Firstly,an experimental study on the mechanism of hydrogen production from staged catalytic gasification of biomass was carried out in a two-stage fixed-bed reactor.The coupling mechanisms of Ca（OH）₂adsorbent and gasification temperature on the gasification characteristics of three typical biomass（rice husk,corn straw and sawdust）and three major components of biomass（cellulose,hemicellulose and lignin）were investigated to verify the promotion effects of Ca（OH）₂for in-situ CO₂adsorption at lower temperatures on the reactions of water gas reforming（WGS）,steam methane reforming（SMR）and C₂-C₃hydrocarbons reforming,and the opposite promotion effect of gasification temperature on the reaction rate and exothermic Ca（OH）₂carbonation and WGS reaction was indicated that the optimum gasification temperature（T_g）of 500°C.The effects of Fe/Ce-Ni-Ca catalysts with different preparation methods and metal doping ratios on hydrogen production from staged catalytic gasification of biomass were studied.The effects of operating parameters such as catalytic temperature（T_c）,steam/biomass carbon molar ratio（H₂O/C）,and catalyst/biomass mass ratio（C/B）on the concentration and yield of product gas components were studied to obtain the optimum condition of T_g=T_c=500°C,H₂O/C=5,and C/B=2.5.Among all the prepared catalysts,sol-gel Ce_0.7Ni₁Ca₅exhibited the best activity and stability in hydrogen production,CO₂adsorption,and resistance to carbon deposition,obtaining the maximum H₂concentration and H₂yield(85.81 vol.%and 35.82 mmol g_biomass^-1,respectively),which were42%higher than the H₂concentration and more than 4.5 times higher than the H₂yield without the catalyst,and remaining stable during 10 cycles.Various characterization analyses of the catalysts revealed the catalytic mechanism of strong Ni-O-Ce interaction inducing the creation of more oxygen vacancies that facilitate the fracture of O-H bonds in water to form H₂,and the enhancement mechanism of Ce O₂on the reactivity and cycling stability of Ni-Ca catalysts:Ce O₂as an inert component inhibited the sintering of Ni O and Ni particles,improving the metal dispersion uniformity and hence the increase of active sites,while delaying the agglomeration of Ca O,stabilizing its carbonation activity and CO₂adsorption.The high oxygen transport capacity and strong redox behavior of Ce O₂can not only form a rich mesoporous structure,increasing the pore volume and specific surface area of the catalyst and promoting WGS and SMR reactions,but also promote the oxidation of amorphous carbon containing low-molecular aromatic or aliphatic compounds with a lower degree of graphitization,consequently lowering carbon deposition.Then,a scale-up experiment was carried out in a laboratory-scale bubbling fluidized bed to verify the feasibility of the two-stage system combining calcium looping gasification and catalytic reforming.A measure for improving the cycling performance of Ca（OH）₂ carbonation-calcination was proposed,i.e.,the supplementation of fresh Ca（OH）₂at a Ca/C（Ca（OH）₂to biomass carbon molar ratio）of 0.5 in each calcination helps to activate the regenerated Ca O via hydration reaction in time,maintaining its carbonation activity and CO₂adsorption,and stabilizing CO₂and H₂concentrations.Under a condition of Ca/C=1.5,H₂O/C=1.5,T_g=500°C,T_c=800°C and using the Ce-Ni/γAl₂O₃catalyst prepared by impregnation method,the obtained H₂concentration was nearly 20%higher than that without the catalyst,and the H₂yield exceeded more than 3.5 times,and remained stable during 5continuous reaction tests.Finally,the two-stage system combining calcium looping gasification and catalytic reforming was simulated and verified by Aspen Plus.A system model more in line with the practical industrial application was built through using fuel heating method,waste heat utilization,CO₂capture,and cryogenic separation and purification of hydrogen to obtain high-purity H₂.The optimum condition was determined by sensitivity analyses to be Ca/C=0.5,H₂O/C=1,T_g=400°C and T_c=600°C.The optimal model scheme was determined by thermal balance analyses and economic evaluations to obtain the maximum H₂concentration of 97.67 vol.%,the highest hydrogen production efficiency of 57.99%,and the lowest hydrogen production cost of（?）9.47/kg.This paper presents an experimental study and system simulation on the process of hydrogen production from staged catalytic gasification of biomass.The experimental results can provide a theoretical basis and guidance for the biomass gasification process improvement,operation parameter optimization,catalyst preparation and its modification method.The simulation results are also of reference value for the future technology of hydrogen production from biomass via a two-stage system combining calcium looping gasification and catalytic reforming in terms of improving hydrogen concentration,hydrogen production efficiency,and economics,while achieving negative CO₂ emissions.