A Study Of Thermal Decomposition In Cellulose By Molecular Dynamics Simulation

The exploitation and utilization of the biomass energy has become the focus in the field of searching the new energy sources in order to alleviate the pressure on the environment. So, the investigation on the biomass energy technologies is one of the hottest topics all over the world.In this thesis, to probe the further mechanism of biomass energy's thermal depositopn, the cellulose was chosen as the working substance and its thermal deposition was simulated using a reactive molecular dynamics model. Cellulose, which is a kind of huge molecular polymer formed by connection betweenβ(1-4)-glycosidic bond of D-glucose, is the main component of biomass, accounting for 40%~96% of the total amount of biomass.The semi-experienced MNDO-PM3 method based on the MNDO model was adopted in the paper. Furthermore, Polak-Ribiere conjugate gradient was applied in optimization with RMS set as 0.042kJ/mol. So that series parameters of cellulose structure were obtained.Molecylar dynamic simulation was based on the different force fields which possessed different forms with its advantages and limits in each form. At first, the single chain of cellulose molecule was simulated by molecular dynamic method in different force fileds. The energy change, deposition temperature and the cracked groups of simulation process in different force fields were obtained and compared each other, of which, the results about the Amber force field matched quite well to the experimental data.The thermal decomposition process of cellulose simulated by MDS based on Amber force field was divided into four stages, heating at low temperature, starting breaking at moderate temperature, decomposition at high temperature, and complete breaking. The details and modalities of cellulose pyrolysis were obtained; besides, the production principle of cellulose pyrolysis and the corresponding distribution of productions were analyzed based on mechanism of bond breaking. It is found that the decomposition always takes place from the lowest energy. In one cellulose unite, the hydroxy (-OH) in ring breaks firstly, then hydroxy (-OH) in branched chain, finally, the ring breaks. As for the whole chain of cellulose molecule, the molecule chain decomposes from both sides to the middle gradually. The hydroxyl (-OH) of inside unite will break earlier than the ring of two-terminals.To our knowledge, there are many factors to affect the pyrolysis process of cellulose. In the paper, the heating temperature, the heating rate and the ploymerization degree of cellulose were mainly taken into consideration and the results were analyzed.The conclusions gotten from simulating results of cellulose thermal decomposition by molecular dynamic model agreed quite well with the corresponding experimental results, which turnd out that the molecular dynamics simulation could be a very powerful tool for studying cellulose thermal decomposition at the microscopic level.