An investigation of the kinetics for the fast pyrolysis of loblolly pine woody biomass
Williams, Alexander W.
MetadataShow full item record
In the search for fossil fuel alternatives the production of bio-oil through the pyrolysis of biomass is one method which has shown evidence of scalability, meaning that the technology could be scaled up for the processing of biomass on the order of tons per day. Pyrolysis is the thermal degradation of compounds in the absence of oxygen. Of particular interest is the pyrolysis of sustainable energy crops such as Loblolly pine (Pinus taeda). The goal of this study is to develop a new method of characterizing the fast pyrolysis of biomass for the advancement of reactor design. The objectives are to determine bulk kinetic coefficients for the isothermal fast pyrolysis of biomass, evaluate the interchangeability of fast and slow pyrolysis kinetic parameters and compare generally accepted pyrolysis mechanisms derived from a common data set. A technical objective is to apply the most suitable derived kinetic parameters to model pyrolysis within a moving bed reactor. A novel fast pyrolysis micro-reactor is presented along with its design and development process. The micro-reactor allows for the control over both temperature and residence time of the reacting biomass. This system provides the experimental data for the characterization of biomass pyrolysis kinetic parameters. Thermal validation tests are presented and experimental yield results are given for raw Loblolly Pine, Avicel cellulose and Beechwood xylan for the derivation of kinetic descriptors. Cellulose and xylan results show good agreement with literature when the proper experimental conditions are met and whole wood pyrolysis results clearly demonstrate the dissimilarity between fast and slow pyrolysis apparent kinetic rates. The experimental results are then used to evaluate five different pyrolysis kinetic model configurations: single component global pyrolysis, two component global pyrolysis, product based pyrolysis, pseudo-component based pyrolysis and pseudo-component pyrolysis with an intermediate solid compound. Pseudo-component models are of particular interest because they may provide a generalized model, parameterized by the fractional composition of cellulose, hemicellulose and lignin in biomass species. Lignin pyrolysis yields are calculated to evaluate the suitability of a pseudo-component parallel non-competing superposition pyrolysis model. Lignin yields are estimated by taking the difference between whole wood pyrolysis and predicted cellulose and hemicellulose pyrolysis behaviors. The five models are then evaluated by comparison of predicted yields to the results for the pyrolysis of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies). Model evaluations show that pseudo-component superposition is not suitable as a generic pyrolysis model for the fast pyrolysis of biomass observed using the micro-reactor. Further analytical evaluations indicate that the assumption of parallel non-competing reactions between pseudo-components is not valid. Among the other models investigated the intermediate solid compound model showed the best fit to the verification experimentation results followed closely by the two component global model. Finally, the derived kinetic parameters are applied to the design of moving bed vacuum pyrolysis reactors which provide for the separation of heat and mass transfer pathways, resulting in the reduction of char entrainment and secondary reactions within collected bio-oils. Reaction kinetics and porous bed heat and mass transfer are accounted for within the bed model. Model development and predictive results are presented and sensitivity to activation energy variations investigated.