| Hydrogen production from biomass gasification is an important research direction for high value utilization of biomass,which has attracted extensive attention of scholars at home and abroad.Biomass pellets have the advantages of high energy density,easy storage and transportation.It is considered to be a promising biofuel.However,there is a lack of systematic research on the preparation of hydrogen-rich gases by in-situ catalytic gasification of biomass pellets.The in-situ catalytic gasification and hydrogen production mechanism of additives in biomass pellets are not clear.In this thesis,the scientific problems of hydrogen production from steam gasification of biomass pellets by in-situ catalysis of additives are discussed.Corn straw was used as gasification raw material for hydrogen production,Fe2O3,CaO and Na2CO3 were selected as additives,the in-situ catalytic action of additives in steam gasification of corn straw pellets was systematically investigated.The idea of enhancing hydrogen production by steam gasification of biomass added with blast furnace gas ash(BFGA)was put forward.The enhanced hydrogen production law and gas production characteristics in downdraft gasifier were discussed.The research contents include the effects of additives on the yields and characteristics of pyrolysis products of corn straw pellets,the in-situ pyrolysis of char on volatile matter,the reaction kinetics and hydrogen production from steam gasification of char pellets loaded with additives,and the preparation of hydrogen-rich gas from corn straw pellets strengthened by additives in downdraft gasifier.(1)Fe2O3,CaO and Na2CO3were used as additives to study the yield variation of pyrolysis products of corn straw at different final temperatures(350～750℃).It was found that most of the liquid products of corn straw pyrolysis were formed at 450℃.At this stage,CO and CO2 were the main components of the volatilized pyrolysis gas.The main products of pyrolysis were H2,CH4 and CnHm at 450～750℃C.The weight loss rate of corn straw powder with different additives was in the order of Na2CO3>Fe2O3≈no addition>CaO,while the order of weight loss rate of pellets was Na2CO3>Fe2O3>CaO>no addition.Comparing with corn straw powder before compression molding,additives can promote the release of volatile compounds in corn straw pellets,which is attributed to the better contact between the biomass components and additives in the pellets.In the pyrolysis process,melting occurred between Na2CO3 and inorganic substances in corn straw,which strengthened the contact between Na2CO3 and corn straw components,promoted the release of volatiles,and greatly improved the gas and H2 yield.Under the action of CaO,the pyrolysis liquid yield of corn straw pellets was reduced,while the pyrolysis gas yield was increased.CaO could absorb CO2 from pyrolysis gas to generate CaCO3,and promoted the generation of hydrocarbon gases.The low heating value(LHV)of pyrolysis gas was higher.At a final temperature of 750℃,it reached 13.24 MJ/Nm3.Fe2O3 inhibited the production of CH4 and hydrocarbon gases and promoted the production of H2.Fe2O3was reduced to Fe3O4 due tothe reduction of carbon and reducing gas.(2)The char of corn straw pellets was used as the steam gasification raw material,the impact of temperature and additive types were evaluated.The basic problems associated with steam gasification of biomass char were discussed to obtain hydrogen-rich gas.Apparent reaction kinetics parameters of char pellets were obtained by isothermal method.It was found that the reactivity of char pellets was significantly increased with the addition of CaO and Na2CO3.According to the conversion rate and time relationship of reaction,it was concluded that the interfacial chemical reaction mechanism function G(a)of char and Fe2O3 added char pellets was 1-(1-α)1/2.The reaction series of char pellets were changed when CaO and Na2CO3 were added,and the reaction mechanism functions were in the form of 1-(1-α)1/3and 1-(1-α)1/4,respectively.There was the kinetic compensation effect between activation energy E and pre-exponential factor A of steam gasification of char pellets.The char prepared by pyrolysis at 450℃ was a good raw material for hydrogen production by gasification,which avoided the generation of tar and retained the existence of C and H in the solid phase to the utmost extent.There was almost no biomass tar produced during the gasification process,and the H2 content varied from 64.96 to 69.44%.The ranking of gas yield and hydrogen yield of char steam gasification with three additives was Fe2O3≈no addition>CaO>Na2CO3,which was determined by the composition of char and the catalytic action of additives.When BFGA was used as additive,the hydrogen production capacity of char was improved,and its hydrogen production effect was similar to that of Fe2O3.(3)Based on the principle of Gibbs free energy minimization,the reaction limit and hydrogen production law of steam gasification of corn straw under the influence of various additives,temperature and S/B were simulated by Aspen Plus simulation platform.When the steam gasification was in the thermodynamic control zone,the calculated results of the simulation were in good agreement with the experimental results.The thermodynamic calculation results showed that Na2CO3did not participate in the reaction of organic components in biomass and had little effect on its thermodynamic equilibrium.CaO could absorb CO2 in the product gas at low temperature range(less than 750℃)and promoted the formation of H2 indirectly.Fe2O3 participated in biomass gasification as an oxygen carrier and reacted with C in biomass and CO and H2 in product gas,this promoted gasification.Based on the previous experimental analysis and thermodynamic calculation results,the relationship between additives and biomass in the process of steam gasification and the evolution path were inferred.(4)In this thesis,the idea of enhancing hydrogen production from biomass steam gasification with BFGA was put forward.A set of downdraft steam gasification experimental platform was designed and built.The process of producing hydrogen-rich gas by steam gasification of corn straw pellets was experimentally studied,and the gas production characteristics were analyzed under different reaction temperatures and steam flow rate.The experimental results showed that the downdraft fixed bed gasifier could effectively utilize the high temperature and catalytic action of the char pellets layer in the gasification zone,so that the volatile matter produced in the pyrolysis zone could be effectively catalyzed reforming.The catalytic reforming ability of the char pellets layer to the pyrolysis volatile matter could be enhanced when the char pellets were loaded with BFGA.At the steam flow rate of 0.75 kg/h and the reaction temperature range of 700～950℃,the volume fraction of H2 in hydrogen-rich product gas prepared by corn straw pellets without additives was 41.65%～56.08%,and the volume fraction of H2 in product gas was increased to 55.87%～59.20%.This indicated that the ability of corn straw pellets to produce hydrogen-rich gas in downdraft gasifier was strengthened under the action of BFGA.The aim of improving the thermal conversion efficiency of hydrogen-rich biomass products was reached,and the target of industrial waste utilization was also achieved. |
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