Theoretical And Experimental Study On Mechanism Of Cellulose Pyrolysis

The global scientists are exploring the environmental-friendly and renewable resources to treat with the energy crisis and the serious environmental deteriorations. Biomass has attracted attention because it is the only renewable resources to provide fixed carbon. However, there still existed many problems during the utilization of biomass, such as rather low utilization efficiency of biomass and low energy efficiency. The fundamental investigations on the pyrolysis mechanism of cellulose should be carried out, in order to find the real solutions.In this work, the cellotriose is used as a model compound for cellulose macromolecule, the quantum chemistry computation method based on the density functional theory (DFT) is applied to investigate the all possible generant reactions during the initial cellulose pyrolysis, including the scission of glycosidic linkage, the hydroxyl elimination on pyran ring and the ring opening of pyranoside. Then the formation mechanisms of main pyrolysis products, such as anhydroglucose, 5-hydroxymethyl furfural (5-HMF), acetol, glycoaldehyde, carbon monoxide and carbon dioxide, have been studied. Using the experimental techniques of in situ FT-IR (DRIFT) and TG-MS, the pyrolysis process of cellulose has been investigated.The DFT simulation for the reactions occurred at the initial pyrolysis of cellulose is carried out, it is suggested that the scission of glysodic linkage could be more favorable based on energy criterion than the ring opening of pyran; the dehydration of pyran ring would prefer in form of pinacol rearrangement; when the formation of C=O or C=C double bonds is formed, the ring opening become more easy. As to the formation mechanism of main pyrolysis products of cellulose, it is indicated that the activation energy for 1,6-anhydroglucose formation is lower than that for 1,2-anhydroglucose formation, which is consist with the experimental data on higher 1,6-anhydroglucose content in dehydration products; when the net positive charge is carried around the pyran ring, the conversion of pyran ring to furan ring become more preferential, thus 5-HMF and furfural could be formed; the C=C, C=O bonds and the net charge on the pyran ring could promote the formation of glycolaldehyde.Based on the in situ FT-IR technique, the structural variations in pyrolysis residue of cellulose under the temperature programmed heating are investigated. The results show that the C=O bond could be formed at lower temperature than the C=C bond, which fact could also validate the results of quantum chemistry computation, i.e. the activation energy for pinacol rearrangement would be lower than that for direct combination of hydroxyl with the hydrogen on adjacent carbon.Based on the TG-MS technique, the kinetics of cellulose pyrolysis is investigated by the model-free iso-conversional technique, the apparent activation energy curve for cellulose pyrolysis with the variational conversions is obtained, and the average activation energy of pyrolysis in the conversion range between 10% and 90% could be evaluated as 164.1 kJ/mol. The sphere symmetrical phase-boundary reaction model could describe the kinetic process of cellulose pyrolysis, and the related kinetic parameters have been determined.The mechanisms of cellulose pyrolysis were made clearer than ever before via the study of simulation and experiment. The conclusions drawed out in this paper made some contributions to the theory of cellulose pyrolysis and the practical untilization of the cellulose via thermochemistry.