Effects Of Different Catalysts And Reaction Conditions On Biomass Pyrolysis

Biomass pyrolysis is the most efficient and potential conversion method in thermochemical conversion.However,the composition of bio-oil produced by pyrolysis is complex,and has a series of disadvantages such as high oxygen content and high water content.In addition,as another major product of pyrolysis products,biochar has the potential to be developed into activated carbon,adsorbent and supercapacitor,and its biochar structure is directly related to its performance.In order to increase the content of target compounds in bio-oils and regulate the structure of biochar,it can be controlled by changing the pyrolysis conditions of biomass to promote or inhibit certain reaction pathways.In this paper,two typical biomass materials（herbal and poplar）were selected for pyrolysis experiments.Firstly,the effect of extraction pretreatment on the pyrolysis performance of herbal raw materials was studied,and the extracted herbal residues（a kind of agricultural waste）were pyrolyzed to produce value-added products.Extracting Mao Hao（a kind of Chinese herbal medicine）with ethanol to obtain herbal waste.Herbal residues and herbal raw materials are pyrolyzed at different pyrolysis temperatures.A series of characterization studies the composition of bio-oil and the structure of biochar products.The results show that extracting herbal medicine with ethanol can affect the yield and the composition/properties of bio-oil/biochar.Herbal extraction also affects the pyrolysis of cellulose,hemicellulose or lignin in different ways.It promotes the formation of phenols with a single or condensed ring structure from lignin,while inhibiting the derivatives formed from cellulose or hemicellulose.The release of carbonyl-containing organic compounds during pyrolysis is also affected by herbal extraction.As for biochar,more carbonyl groups are retained in the biochar produced by pyrolysis of extracted herbs.However,due to structural differences caused by extraction,the thermal stability of biochar in the extracted herbal medicine is significantly lower than that in the raw material of herbal medicine.The second is to study the effect of pyrolysis temperature on biomass pyrolysis.This study paid attention on the change of functionalities in the biochar formed in pyrolysis of poplar wood in a wide range of temperature.The in situ Diffuse Reflectance Infrared Fourier transform spectroscopy characterization indicated that the formation of aldehydes and ketones functionalities initiated at 100°C,dominated at 300 to 500°C and were removed gradually at higher temperatures.The carboxyl group was less stable than the carbonyl groups.For the carbonyl groups,the aldehydes were more thermally stable than the ketone groups.Cellulose crystal in poplar decomposed slightly at 300°C and significantly at 350°C.The temperature from 250 to 350°C significantly affected biochar yields,while the drastic fusion of the ring structures in biochar occurred from 550 to 650°C,making biochar more aliphatic while less more aromatic.High pyrolysis temperature also created more defective structures in the biochar and favored the absorption of the CO₂ generated during the pyrolysis.Finally,the effect of the addition of the catalyst on the characteristics of the pyrolysis products is studied.Oxides are commonly used as support materials in catalytic reactions.This study investigated the effects of seven different oxides on biomass pyrolysis.The three biomasses of poplar,cellulose and lignin are used as raw materials for the reaction.Through experiments,different types of oxides have a certain effect on the pyrolysis reaction.Basic catalysts（K₂O,Mg O,Ca O,La₂O₃）can promote the occurrence of secondary pyrolysis in the pyrolysis process and increase the gas production.For acidic catalysts（Al₂O₃,Si O₂,ZnO）,tar yield can be effectively improved.In addition,the acidity and alkalinity of oxides have a great influence on the distribution of products in bio-oil.In the bio-oils catalyzed by acidic oxides,the yields of carboxylic acids,ketones,aldehydes,furans and phenols are higher.In basic catalysts,the yield of phenol is significantly higher.In addition,the intensity of acidity and alkalinity is closely related to the type of coke and the amount of coke.For different raw materials,it also has a great influence on the formation of coke.This is related to the unique structure between different raw materials.In addition,the catalytic behavior of transition metal oxides（Co O,Cr₂O₃,CuO,Fe₂O₃,Mn₂O₃,NiO,TiO₂,and V₂O₅）and CeO₂ for the catalytic pyrolysis of poplar wood was also studied.The metal oxides,especially Ti O₂ and Ni O could suppress further cracking of primary products,increasing the tar yield and simultaneously decreasing the gas yield.The V,Mn,Ti or Co-based catalyst promoted the formation of the heavy bio-oil,while the Ce,Cr,Cu or Fe-based catalysts were the opposite.The metal oxides（except Fe₂O₃）promoted the formation of alcohols,furan,ketones,acetic acid and phenolics in bio-oil.Fe₂O₃ catalysts suppressed formation of the derivatives from cellulose and hemicellulose except hydroxyl acetone.Hydroxyl acetone formation was promoted by almost all the oxide catalysts while hydroxyl aldehyde formation was the opposite.Remarkable coke formed over the V,Mn,Cu and Co-based catalysts.These oxides contain multiple valences and could be partially redu ced to generate oxygen vacancies,which played important roles in the polymerisation reactions.In addition,the coke species formed on the oxide catalysts were mainly polymeric with low thermal stability.In summary,through the pyrolysis of biomass under different conditions,the composition of biomass oil and the structure of biochar were studied,the pyrolysis pathway was studied,the source of carbon accumulation was explored,and some useful explorations were made for the recycling and high-value utilization of biomass.