Mathematical modeling of fast biomass pyrolysis and bio-oil formation. Note I: Kinetic mechanism of biomass pyrolysis

This paper discusses the research activities done at Politecnico di Milano in the field of the detailed kinetic modeling of pyrolysis and combustion of biomass and bio-oil formation. Different critical steps are involved in this multicomponent, multiphase and multiscale problem. The first complexity relies on biomass characterization with the selection of reference species: cellulose, hemicellulose, lignins, and extractives. Fast pyrolysis involves kinetic mechanisms, first in the solid phase for biomass pyrolysis, then in gas-phase for secondary reactions of released products. These mechanisms involve large number of species and reactions, which make computations expensive. They need to be simplified, while still maintaining their description capability. Lumping procedures are extensively applied to allow the development of the overall model. Multistep pyrolysis mechanisms of reference species are discussed in this Note, with several comparisons with experimental data. A peculiarity of the model is its ability to provide detailed compositions of pyrolysis products and solid residue. Catalytic effect of ash on pyrolysis products is also discussed. A companion paper will discuss the successive or secondary gas phase reactions of pyrolysis products, together with the heterogeneous reactions of residual char. Finally, the modeling of bio-oil formation requires a comprehensive description of the coupling of kinetic and transport processes, both at the particle and the reactor scale

Third generation Biomass. Classification and characterization of Algae fuels

Third generation biomasses are still largely unexplored. This paper first collects and classifies available information on the nature and main features of macro- and micro-algae, then develops a novel model of algae fuel characterization, based on a limited number of reference components: protein, carbohydrates and lipids. Based on elemental analysis and simple atomic mass balances, the biochemical algae composition is predicted. Despite the limited number of species and the rough assumptions to reduce the complexity of the overall problem, this model is already able to satisfactory predict algae composition

Yield, composition and active surface area of char from biomass pyrolysis

Biomass materials represent promising carriers for both heat, energy and chemicals production. Nevertheless, several aspects must be intensively investigated and understood, leading to a better design and optimization of industrial combustors, gasifiers and pyrolyzers. The first objective of this work is to update the POLIMI multistep kinetic mechanism of biomass pyrolysis, focusing on the prediction of both yield and composition of the solid residue (biochar). To this end, a large set of literature experimental data was collected and organized into a database, which was further used to finely tune and validate the proposed kinetic mechanism. Then a method to estimate the biochar active surface area is introduced, deriving from the biochar composition predictions. Keeping the previous agreements with the rate of biomass pyrolysis, formation and distribution of gas and tar products, the novelty of this work is the additional potential to predict the evolution of biochar yield and composition in a wide range of operative conditions, predicting also some important surface features. The model describes the solid residue as a mixture of pure carbon together with lumped metaplastic compounds, which represent the whole range of oxygenated and hydrogenated groups bonded to the carbonaceous matrix. These metaplastic species are released to the gas phase with their own kinetics and describe both mass loss and elemental composition change of the biochar. These are relevant topics because a comprehensive evaluation of biochar composition and its structural characteristics is crucial for an accurate description of the successive oxidation and gasification processes

A predictive model of biochar formation and characterization

Biomass is increasingly being recognized as a promising carrier for both heat, energy and chemicals production. However, several aspects still require intense research activity towards a better design and optimization of industrial combustors, gasifiers and pyrolyzer. The objective of this work is to update the CRECK kinetic mechanism of biomass pyrolysis, allowing a better prediction of both yield and composition of the solid residue (biochar). Moreover, further model modifications allow to better describe the variability of hemicellulose in different biomass. To this end, a large set of literature experimental data is collected and organized into a database, which is used to further tune and validate the proposed kinetic mechanism. Although the kinetic model maintains the previous agreement in respect of the rate of biomass pyrolysis, formation and distribution of gas and tar products, the novelty of this work is the greater attention to the predictions of biochar yield and composition, in a wide range of operative conditions. The model describes the solid residue as a mixture of pure carbon together with lumped metaplastic compounds, which represent the whole range of oxygenated and hydrogenated groups bonded to the carbonaceous matrix. These metaplastic species are released to the gas phase with their own kinetics and describe the change of both mass loss and elemental composition of the biochar. These comprehensive predictions of biochar composition are crucial for an accurate description of the successive oxidation and gasification processes