Chemical Structure Characteristics Of Char And Formation Mechanism Of Sugar During Oxidation Of Cellulose

In the process of scaling up of biomass pyrolysis and promoting its commercialization,the heat transfer bottleneck has become a major challenge due to the endothermic nature of pyrolysis,which has been motivating the research and development of oxidative pyrolysis.The current research of cellulose oxidative pyrolysis focuses on the influences of different biomass types,reactor types and scales,and pyrolysis conditions.Due to the introduction of oxygen,the oxidative pyrolysis process involves the heterogeneous oxidation of oxygen-biochar/biomass,the homogeneous gas-phase oxidation of oxygen-volatile and the common secondary reaction of biochar-volatile,which makes the research on the oxidative pyrolysis of biomass extremely complicated.In this article,cellulose with the highest content of biomass components is used as the research object to study the oxidative pyrolysis,focusing on the influences of oxygen on the chemical structure of char from cellulose pyrolysis and the mechanism of sugar formation.First,in order to explore the effect of oxygen on the pyrolysis of cellulose on a macro scale.Thermogravimetric analyzer combined with differential scanning calorimetry was used to explore the weight loss and endothermic characteristics of cellulose pyrolysis under oxidizing atmosphere,and thermogravimetric-Fourier infrared transform spectroscopy was used to study the gas release mechanism under oxidative pyrolysis.Under oxidizing atmosphere,cellulose pyrolysis presents two stages of mass loss.The weight loss process in the first stage is not simply the same as in an inert atmosphere.It will be accompanied by an exothermic reaction during the pyrolysis process.As the oxygen concentration increases,the exothermic reaction will be enhanced.While exothermic reaction of oxidation of char occurred on the second stage.The presence of oxygen promotes the degradation of cellulose and the oxidation of char and volatiles,which enhances the release of CO₂.However,the oxygen concentration has no significant effect on the activation energy of cellulose pyrolysis.As the oxygen concentration increases,the pyrolysis time decreased.Secondly,the functional groups on the char surface played an extremely important role inthe initial stage of pyrolysis.A combination of in situ diffuse reflectance infrared Fourier transform technology and two-dimensional perturbation correlation infrared spectroscopy was applied to deeply analyze the characteristics of oxidative pyrolysis of cellulose under four experimental conditions（oxygen concentration:0%,5%,10%,and 21%）.The research focuses on the effect of oxygen on the breaking and bonding mechanism in the process of cellulose cleavage,and the evolution of char structure.Intra-molecular hydrogen bonds started to break at 120℃,and the presence of oxygen promotes the destruction of the hydrogen bond network to a certain extent.The formation of saturated ketone groups in an oxidizing atmosphere was promoted,and the temperature at which unsaturated bonds appear was earlier as the oxygen concentration increased.With the increase of oxygen concentration,the relative intensities of crystalline region,glycosidic bonds,and C6 methylene and methine groups of the pyran ring declined rapidly.The temperature these groups began to drop drastically was consistent with the temperature corresponding to the formation C=O and C=C.Oxygen can promote the dehydration from the secondary reaction of the intermediate active cellulose.At the same time,oxygen promoted the increase of the double-substituents of the benzene ring of char,and the char turned to fused ring more quickly.Lastly,the pyrolysis of cellulose under different condition was carried out on a wire-mesh reactor.With limited the secondary reaction,the research mostly focuses the formation and evolution of the anhydro-sugars（levoglucosan,cellobiosan and cellotriosan）and sugar oligomers（glucose,cellobiose and cellotriose）in the intermediate active cellulose of the oxidative pyrolysis of cellulose.As the results show,the presence of oxygen advanced the decomposition of cellulose by promoting the formation of the intermediate active cellulose.The effect of oxygen was significant during the severe degradation of cellulose.With increasing oxygen concentration,the char yield decreases and the tar yield elevated.Oxygen slightly promoted the dehydration reaction of primary char.Levoglucan was still the primary product of anhydro-sugars.Yield of anhydro-sugars increased in an oxidizing atmosphere,which may be related to the consumption of the cellulose crystallization area and the promotion of the rupture of glycosidic bonds.The yield of sugar oligomers in an oxidizing atmosphere was lower than that in an inert atmosphere,especially for cellotriose.The higher the degree of anhydro-sugars oligomers,the worse the stability of anhydro-sugars.At high temperatures,it is more difficult for the presence of high degree of anhydro-sugars and sugar polymers in an oxidizing atmosphere.It is not conducive to the polymerization reaction of anhydro-sugars into high degree of polymerization sugars during oxidative pyrolysis.Oxidation reaction may it easily transfer to gaseous and liquid products.It is not conducive to the formation of char.