Experimental investigations and modeling of devolatilization based on superimposed kinetics of biomass

A non-isothermal one-dimensional model has been developed to describe biomass pyrolysis at fast heating rate (600-10⁴ Ks^-1), high temperatures (up to 1500C) and is valid for different biomass particle sizes (< 10 mm). The model was developedto estimate the yields of volatile gas and char. The model relies on the concept applied in fast pyrolysis of cellulose throughthe formation of an intermediate liquid (so called metaplast) which reacts further to char and gas. The kinetics of the fastpyrolysis was described by the Broido-Shafizadeh scheme.The influence of particle size and shape was included in the model. Cylindrical representation of a biomass particle shapewas chosen to be the most suitable in the pyrolysis model. The evolution of devolatilization time required for the completepyrolysis showed that the particles with a mean diameter < 0.45mm may be considered as thermally thin at high heatingrates. The predicted results by one-dimensional model are in agreement with the experimental work, and emphasize a keyrole of intra-particle heat conduction in biomass particles > 0.45 mm.The potassium influence on the char yield was implemented in the model based on the experimental results in the wire meshand drop tube reactors with respect to the stronger catalytic effect of potassium on the char yield at low and intermediateheating rates compared to pyrolysis at high heating rates. The heating rate and potassium content affected significantly thechar yield as evi denced from the experimental data obtained in the wire mesh and drop tube reactors. Thus, the modelincluding these two parameters provides an acceptable fit of char yield to the experimental data. The present results showedthat the proposed kinetic model for the fast biomass pyrolysis is relatively simple and predicts reasonably accurately the charyield of woody and herbaceous biomass particles < 10mm using one fixed set of kinetic parameters valid for woody andherbaceous biomass.