The Influence of Cellulose Stage Gasification on Hydrogen Production Based on Modified Fe/Ni Catalyst

Author:Zou Jun

Supervisor:chen han ping yang hai ping


Degree Year:2019





Hydrogen is not only an essential industrial chemical raw material,but also an ideal energy carrier for its renewable and clean characteristics.But 92%of hydrogen are produced from fossil fuels.While biomass are abundent,environmental friendly and renewable,which makes them ideal for producing hydrogen.Biomass gasification have many advantages like wide adaptability of materials,high speed conversion and simple process requirements,all these encourage biomass gaisifcation to be the most promising method to produce hydrogen.However,biomass gasification is a quite complicated process,in which the AAEM will be released out,conversion degree is low,the content of tar is high,the catalysts might get deactivation and the concentration of hydrogen is low.Those disadvantages restrict its application in energy and chemical industry and need to be solved.So this study try to solve these above problems and the main results are as the followings.This paper focus on two stage gasification which separates the whole process into the pyrolysis of biomass and the steam reforming of volatiles.When the conventional nickel-iron based catalysts are used in biomass gasification,it is found that the nickel-iron oxides play the role of oxygen carrier as well.Furthermore,they form a loop by volatile reducing and steam oxidizing.The chemical inner-looping gasification(CILG)reduces the NiO/Fe2O3-Al2O3 by using hydrocarbons such as char and methane as reducing agents,and then oxidizes by steam to produce hydrogen,which increases the hydrogen yield and content significantly.Ni has better catalytic effect than Fe on volatile refoming,but Fe has higher oxygen capacity which shows better performance in CILG and resulting in higher hydrogen yield and content.When the ratio of Ni:Fe is 2:3,the hydrogen yield and content get the maximum value as 17.26 mmol g-1 cellulose and 42.05%,respectively.When the water feeding rate was 0.1 g min-1 at 850°C,the total gas yield reached a maximum of 42.28 mmol g-1 cellulose.The total gas yield increased first and then decreased slightly with increasing water feeding rate.It could be owing to that when excessive steam is introduced into the system,which absorbs a large amount of heat,resulting in a decrease in the residence time and local temperature of the gas.Thich is not conducive to the reaction of the volatile steam reforming,and thus the total gas production is decreased.Compared with temperature,the water feeding rate has a greater impact on hydrogen yield,which indicates that water feeding rate shows higher effect on CILG.Furthermore,the concentration of hydrogen increases with the increase of water feeding rate,and the temperature corresponding to the maximum value decreases with the increase of water feeding rate.This is mainly due to that when the water feeding rate is excessive,the water gas shift reaction take a large proportion in the whole reaction system promoted by CILG.And the reaction is an exothermic reaction,the rising temperature causes the water gas shift reaction to proceed in the reverse direction which results in a decrease in the hydrogen concentration and the H2/CO ratio.Realized the importance of oxygen capacity,the photocatalytic and electrocatalytic active component CeFeO3 has drawn attentin for its advantages of high capacity and mobility of oxygen,as well as fast reaction rate.It is applied to biomass steam gasification system for the first time in this research.The results show that CeFeO3 could be generated at800 oC or higher temperature after steam gasification of biomass without forming CeO2/Fe2O3 clathrate.And CeFeO3 has excellent high-temperature catalytic performance.The optimal ratio of CeO2/Fe2O3 catalyst for hydrogen production is Ce:Fe=3:7.And hydrogen production increases 20%with the catalyst prepared by impregnation method than that prepared by coprecipitation method.However,the catalyst prepared by the coprecipitation method was more stable in catalytic performance.Due to high capacity and mobility of oxygen of CeO2,its introduction not only improves the oxidation performance of iron-based catalysts,but also promotes the oxidation of possible carbon deposits on the catalyst surface and enhances the inner-looping chemical gasification and improves the stability of the CeO2/Fe2O3 catalyst.Then,considering the catalytic activity of iron and the CO2 absorption by CaO,this paper explores the synergistic effect of Fe and Ca composite catalysts in the series of reactions such as volatile catalytic reforming,CO2 absorption and water gas shift in biomass steam gasification,and also the effect on the inner-looping chemical gasification.When Ni and Ce are respectively introduced to modify the iron-calcium metal oxide catalyst to improve the hydrogen production and the stability of the catalyst during the steam gasification of biomass,the results show that Fe and Ca are mainly exist in the form of stable structure of Ca2Fe2O5 for both CeFeCa or NiFeCa catalysts,which resulted in a higher reduction temperature.Ce and Ni mainly existed in the form of CeO2 and NiO respectively.The CeFeCa catalyst exhibits micro-strain,and some Ca atoms undergo isomorphous substitution in the CeO2 lattice,which improves the ability of CeFeCa catalyst to adsorb CO2at high temperature.Therefore,the water-gas shift reaction shifts to the right at high temperature,which promotes more hydrogen production.In the life cycle experiments,the gas production with CeFeCa catalyst is more stable.While the production of hydrogen and carbon monoxide increased in the first cycle reaction with NiFeCa catalyst,and then decreased significantly with the increase of the number of cycles.The former is owing to its perfect surface activity of CeO2,which alleviates the thermal sintering of Ca2Fe2O5 in CeFeCa catalyst.The latter was due to the severe thermal sintering of active components Ni and Ca2Fe2O5,thus the activity of catalysts decreased.Since there are some parts of AAEM released out which effect the volatile steam reforming or deposite on the surface of catalyst and influence the activity of catalyst.Thereby,this paper uses cellulose whose ash content is as low as 0.02 wt%and AAEM concentration can be neglected as biomass sample.Since the potassium occupies high content of AAEM in biomass and is highly volatile in the form of chlorine,the cellulose is impregnated with KCl to simulate the biomass pyrolysis and the potassium enters the steam reforming stage with the volatile.Combined with these two steps,the influence of volatile potassium on the Ni/Al2O3 is studied.The results show that low water feeding rate could cause coke deposite on the catalyst surface while the water feeding rate higher than 0.1 ml min-1 could promote the active phase Ni oxidized into NiO.The deposition amount of potassium on catalyst has no relationship with the amount of K loaded in the cellulose sample and kept as 0.5 wt%of the total weight of the reacted catalyst.In the life cycle test,the amount of K deposited on the catalyst surface are increased with the life time,while almost linear negative relationship is observed between the increase of the amount of potassium deposited on the reacted catalyst and the reduce of hydrogen production.The introduce of KCl on cellulose promotes the production of filamentous carbon in multi-time test.Finally,the hydrodynamic behavior of the two stage gasification reactor was numercally inspected through caculating the residence time distribution.A computational fluid dynamic model was developed to solve the momentum and component balance equations of the reactor system.Pulse experiments were carried out to characterize the mixing and flow within the reactor by introducing tracer impusle with concentration of 100mol/m3 for 1s approaching the Dirac delta function and then recorded the concentration at the outer boundary.Since comparartment modelling has much smaller computational need than CFD,a compartment model was also developed to model the hydrodynamic structure and compared with the results of CFD model.