Mechanism Study Of Cellulose Pyrolysis

The exploitation and utilization of biomass energy will relieve the pressures caused by fossil fuel shortage and environmental pollution, which is beneficial for sustainable development in China. Biomass pyrolysis technology, which can convert low-quality biomass to high-quality liquid fuel or gas, also some valuable chemical materials, has been paid great attentions during these years. As the most important constituent in biomass, a deep research on cellulose pyrolysis will be helpful to understand the pyrolysis behavior of biomass, and therefore improve the exploitation and utilization of biomass energy. A summarization of the representative work on cellulose pyrolysis was given first in this thesis.Experiments on pyrolysis kinetics of pure cellulose and cellulose added with little metal salt were done on a thermogravimetric balance. A consecutive-competitive reaction mode was obtained to describe the cellulose pyrolysis process, on the basis of stage division of thermal degradation of cellulose. Combined with the Brodio-Shafizadeh kinetics model, the reaction mode was thought to correspond with the formation of active cellulose (AC), followed by two competitive reactions to produce char and volatile. Processes of three main reactions were largely influenced by the catalysis of metal salt, which would result in the difference of apparent activation energy.A series of experiments were done on a heat-radiation reactor to investigate the production of bio-oil, char and gas with the reaction parameters, such as the temperature of thermal radiation resource (TRS), gas residence time, thickness of cellulose material. A maximum aqueous bio-oil yield of 86.29 wt % was obtainedat the TRS temperature of 610℃,While, effects of free water content, metal salt and acid-pretreatment werei also been studied, to be advantageous to the mechanism research on cellulose pyrolysis.GC-MS analysis of bio-oil showed that the major components of bio-oil have a tendency of competition in formation, and high temperature could enhance the production rate except saccharide. With the increase of gas residence time, levoglucosan underwent an intensive secondary-decomposition to produce the bio-oil, which was almost same in the composition with that obtained from cellulose except the levoglucosan itself. The result confirmed that the competitive-consecutive reaction mode was also valid in volatile formation.The catalysis of metal salt mainly occurred in the solid phase of cellulose and catalyzed some reactions selectively in the form of ion. K+ accelerated the cracking and splitting process of molecules, while Ca2+ mainly influenced the reforming reaction to yield more heterocyclic compounds. Acid-pretreatment catalyzed the decomposition and dehydration reactions intensively, which varied with the type of acid adopted in pretreatment.A reasonable explanation on the formation of levoglucosan and hydroxyacetaldehyde was given to construct different reaction pathway, according to the TG analysis with mechanism experimental results. On the basis of the analysis of primary reaction, secondary reaction and main products formation, the mechanism model of cellulose pyrolysis was proposed.At the end of this thesis, an integrated model of cellulose pyrolysis was developed to simulate the pyrolysis behavior of cellulose, including the formation of major compounds and the development or evolutionof major pyrolysis products. The model reflected the effect of heat and mass transfer inner and outer the cellulose, the difference of products' characteristics, and the secondary reaction in gaseous phase. The result indicated the development and distribution of gas, bio-oil, char and intermediate products during pyrolysis process, which had a good agreement with the experimental results and would be beneficial for the optimization of reaction parameters and reactor design.