Mechanism Study In The Pyrolysis Of Cellulose With Different Crystallinity Index And Its Model Compounds Under The Catalytic Effects Of KCl And CaCl₂

Protruding from several kinds of renewable energy resource,biomass plays a prominent role in the development of clean energy,and its industrialization is an important part of the reform process of Chinese efficient,clean and safe energy.In the common lignocellulosic biomass,cellulose occupies 35?50% of the dry weight,whose pyrolysis mechanism has been attracting the attentions of scholars of various countries for several decates.However,there are still several problems remaining to be solved.First,the function mechanism of the effects of cellulose's fine structure?i.e.the degree of polymerization and crystallinity index?and inherent alkali and alkaline earth metals?AAEM?on the evolution of hydrogen bonds network during pyrolysis is still not clear.Second,numerous secondary reactions in cellulose pyrolysis can disturbe the revealing of the generation characteristics of intermediate active cellulose and the primary catalytic effects of AAEM.Third,all the available models for cellulose pyrolysis ignored the influences of the diversity of cellulose on the pyrolysis,which restricts the universality of the models for various kinds of biomass.Facing the above problems,this work conducted the researches on the mechanism of cellulos pyrolysis and AAEM catalysis in different temperature intervals,which focused on the generation and evolution of intermediate active cellulose.Fistly,a combination of in situ diffuse reflectance infrared Fourier transform technology?in situ DRIFT?and two-dimensional perturbation correlation infrared spectroscopy?2D-PCIS?was applied to on-line monitor the evolution of hydrogen bonds network and other functional groups during cellulose pyrolysis with the effects of crystallinity index,KCl and CaCl₂.The results show that more compact hydrogen bonds network in crystalline region than that in amorphous region protected the hydroxyls and pyran rings in cellulose chain from dehydration reactions and abscission,resulting in the enhanced emergence temperature of the saturated carbonyls with 270? from 240? in amorphous cellulose.In regard with KCl and CaCl₂,both of them increased the cleavage of intra-molecular hydrogen bonds,especially intra-molecular O?2?H···O?6?,while Ca²⁺,being the stronger Lewis acid,behaved more efficient and strongly promoted dehydration reactions and the rupture of glycosidic bonds and pyran rings.Secondly,following the collapse of hydrogen bonds network is the rapid depolymerization of cellulose leading to intermediate active cellulose.This work used the wire-mesh reactor to limit the secondary reactions and explore the primary catalysis of AAEM on the formation of the intermediate active cellulose.The research mostly focuses on the formation and evolution of the anhydro-sugars?levoglucosan,cellobiosan,maltosan and cellotriosan?in the intermediate active cellulose through the high performance liquid chromatography analysis method?HPLC?after a pre-column benzoyl chloride derivatization.As the results show,levoglucosan,cellobiosan and cellotriosan in the intermediate active cellulose mostly come from the direct depolymerization of cellulose chain,while matosan is more likely from the secondary transglycosylation reactions of anhydro-sugars like levoglucosan.Besides,low?<350??and high?>400??temperatures go against to the anhydro-sugars yield in volatiles.The former is bad for the formation and evaporation of anhydro-sugars,and the later is benefitial to the non-sugar products.In addition,Ca²⁺mainly influenced the thermal stability of cellulose and increased the formation of intermediate active cellulose at low temperatures,and K⁺increased the formation of char and water-insoluble gas products at the expense of anhydro-sugars in high temperature intervals.Thirdly,aiming at the evolution of intermediate active cellulose and the effects of AAEM at high temperatures,this work selected levoglucosan,maltosan,maltose,cellobiose and?-cyclodextrin as the model componends.The pyrolysis experiments were conducted on TG-FTIR-MS and Py-GC/MS to explore their pyrolysis characteristics and products distribution.The results show that the reducing end from hydrolysis reactions pefer to the formation of furans such as 5-hydroxymethylfurfural,while the levoglucosan end from transglycosylation reactions pefer to the formation of pyrans such as levoglucosenone.In addition,Ca²⁺showed a stronge promotion effect on the secondary reaction of intermediate active cellulose,and K⁺can interact with the reducing ends.Furthermore,after the iso-conversion analysis?Kissinger-Akahira-Sunose method?of celluloses with different crystallinity index?CI?,the initial active energy of 146 kJ/mol was obtained for amphous cellullse,which is close to the published active energy of the hydrolysis reaction?141 kJ/mol?.Then,based on Bio-Polimi mechanism,a modified model was proposed for cellulose pyrolysis to consider the effects of CI.The modified model divides cellulose into a crystalline region and an amorphous region with an assumption that the crystalline region and the amorphous region can depolymerize through transglycosylation reaction,while the amorphous region can also depolymerize through hydrolysis reaction.The fitting of the undetermined kinetic parameters was conducted through MATLAB based on Cantera reaction kinetics library.The fitting program consists of two steps,i.e.genetic algorithm and pattern search,which can avoid the effects of different intial input values on the fitting results.Compared with the original Bio-Polimi mode,the modified one successfully predicted the effects of CI on the weight loss of cellulose and the yields evolution of char,levoglucosan and intermediate active cellulose.