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.