Study on Catalytic Pyrolysis Mechanism of Clay Minerals to Biomass and Adsorption Characteristics of Biochar

Author:Gao Li Hui

Supervisor:wang hai feng wang yong tian


Degree Year:2019





Biomass is an abundant green energy in China,and the efficient utilization of biomass has been a hot topic in recent years.But at present,there are a series of problems in biomass pyrolysis technology,such as low gas yield,high oxygen content and high acidity of pyrolysis liquids.Catalytic pyrolysis is an effective way to improve gas yield and high value-added products in pyrolytic liquids.Conventional synthetic catalysis are suffering the problems of deactivating easily and expensive.Natural clay minerals,with the unique interlayer and surface acid sites,have attracted extensive attention of scholars at home and abroad.Therefore,this paper proposed to use natural clay minerals as catalysts to study the catalytic pyrolysis behavior and mechanism of biomass.Cellulose with single component and oak leaves with complex components were selected as raw biomass materials.Kaolinite and montmorillonite,most common and different structures of layered clay minerals,were used as catalysts compared with quartz with chemical inertness.The study mainly focused on in situ catalytic pyrolysis of clay minerals to biomass.The influence of different clay minerals on the pyrolysis process of biomass was investigated.Based on the analysis and discussion of surface active sites of clay minerals,the catalytic pyrolysis mechanism of clay minerals to biomass was further revealed.Meanwhile,the physicochemical properties of the biochars formed by pyrolysis were characterized,and the study on removal properties of methylene blue and heavy metal ions from simulated wastewater using biochars was carried out.Then the mechanism of biochar in the process of pollutants removal was revealed.Based on the above research contents,the main research work of this paper is as follows:Firstly,TGA-DSC was used to study the weightless behavior of biomass pyrolysis with minerals,and the released gas products were analyzed by in situ FTIR.The pyrolysis kinetics of biomass with clay minerals were analyzed using distributed activation energy model(DAEM).Therefore,the effects of kaolinite and montmorillonite minerals on the temperature transfer,pyrolysis behavior and pyrolysis activation energy of biomass catalytic pyrolysis were revealed.The results showed that Tmax(the temperature at which maximum weightlessness peak occurred)of oak leaves-montmorillonite and oak leaves-kaolinite was lower than that of oak leaves-quartz at the same heating rate.The maximum decomposition rate of cellulose-montmorillonite and cellulose-kaolinitewas higher than that of cellulose-quartz.The loading of clay minerals had no obvious influence on the average pyrolysis activation energy of biomass,but the loading of montmorillonite changed the pyrolysis path of cellulose and promoted theβ-elimination path at the initial time,so that the activation energy E presented a trend of slight increase and then decrease with the conversion rate X.TG-FTIR results showed that the loading of clay minerals had no effect on the types of produced products and the temperature corresponding to the maximum decomposition rate in the pyrolysis process of oak leaves and cellulose,but only had a certain effect on the yield of pyrolysis products.Secondly,the pyrolysis experiments of biomass with minerals were carried out using tube furnace pyrolysis system designed by ourselves,and yield of three kinds of products,namely gas,liquid,solid,was calculated.Light gases and heavy liquids were analyzed qualitatively and quantitatively,respectively.Combined with the surface properties analysis of clay minerals,the catalytic pyrolysis mechanism of clay minerals in the pyrolysis process of biomass was revealed.The results showed that clay minerals followed the catalytic mechanism of“crushing”,and made biomass produce more light and small molecular gases and high value-added chemical intermediates.And clay minerals could consume the bio-oil for secondary decomposition,which increased the yield of light non-condensable gas products and reduced the yield of heavy liquids.But there was no obvious effect on the solid yield.The pyrolysis test of tubular furnace showed that CO2,CH4 and H2 were the main gas products of biomass pyrolysis,and the loading of clay minerals could change the pyrolysis path of biomass,so that the output and producing rate of pyrolysis gas was improved.Meanwhile,the increment of gas increased with the total number of acid sites.The main products of oak leaves-minerals pyrolysis liquids were furfural,carboxylic acid,ketone,phenol and a small amount of hydrocarbon.And the main products of cellulose-minerals pyrolysis liquids were furans and ketones.The loading of clay minerals changed the composition of bio-oil,and reduced contents of three carboxylic acids(acetic acid,butyric acid and acetic anhydride),guaiacol and p-cresol.Furfural,a high value-added intermediate chemical,showed an increase trend with the increase of total acid number of catalysis under the synergistic catalysis of Lewis acid and Br?nsted acid on the surface and between layers of clay minerals.When the load was 10 wt%,compared with that of oak leaves-quartz,the content of furfural in the pyrolysis liquids of oak leaves-montmorillonite and oak leaves-kaolinite increased by 86%and 39%,respectively.In the bio-oil from cellulose pyrolysis,the contents of l-glucan(LG)and 1,4:3,6-didehydrated-d-pyranoglucose(DGP)showed a decrease trend,while the contents of l-gluconone(LGO),the chiral synthetic substrate,increased significantly with the increase of total acid sites in the clay minerals.Finally,the physicochemical properties of biochars were characterized,and the adsorption properties of methylene blue and heavy metals from simulated wastewater on the biochars were carried out.The adsorption behaviors of different pollutants on the biochars surface were discussed,and the mechanism of biochars in the process of pollutants removal was revealed.The results showed that biochars were carbon-rich solids.The content of C in biochar increased with the catalysis of clay minerals,while the H and O elements decreased.BET surface area of biochars produced by biomass with clay minerals was higher than that of biochars produced form biomass pyrolysis with quartz.BET surface area decreased due to the collapse of pore diameter of montmorillonite-biochar and the blockage of pore diameter by montmorillonite.The results of FTIR analysis showed that the aromatic structure of pyrolysis biochar was enhanced by the loading of clay minerals,and oxygen-containing functional groups such as dehydrocarboxyl,carbonyl and hydroxyl groups were also formed.Studies on adsorption of organics and metal ions by biochars produced from biomass pyrolysis with 5 wt%minerals showed that the adsorption of methylene blue(MB)by biochars was monomolecular chemisorption,and particle diffusion was not the only limiting step for biochar to adsorb MB.All the five metal ions were chemical adsorbed by biochar,and the total adsorption capacity was 400-600 mg/g.The properties of the adsorbed ions played a dominant role in the adsorption process.There were both monomolecular and multi-molecular adsorption for Cd2+,Co2+,Mn2+and Zn2+on biochars,while Cu2+was monomolecular adsorption on biochars from oak leaves pyrolysis and multi-molecular adsorption on biochars from cellulose pyrolysis.The CEC and BET surface area of biochars played a dominant role in the maximum adsorption capacity of biochar,while the electronegativity of heavy metal ions affected the adsorption rate.In all,the above research can provide theoretical basis for the industrialized application of biomass catalytic pyrolysis with clay minerals,and also provide application direction for the generated biochar resources,which has good practical significance.This paper contains 75 figures,21 tables and 214 pieces of references.