Abstract
In biomass-to-energy systems, torrefaction represent an interesting pre-treatment, as it increases the feedstock energy content and its suitability for the subsequent thermal conversion process. The aim of this work is to develop and validate a numerical model for the torrefaction of common reed (Phragmites australis). In order to predict the rate of the biomass decomposition (i.e., yield of the reaction products) and characterise the thermofluidynamics of the reactor, a heat, mass and momentum transfer model based on a finite volumes method (f.v.m.) representation has been developed and supplemented with a torrefaction kinetic model, consisting of a two-step scheme previously proposed. The kinetic model has been calibrated utilizing the results of thermogravimetric analysis applied to common reeds. Experimental tests have been also performed on reeds in a bench-scale batch apparatus, varying the main process parameters. The validation of the proposed kinetic scheme has been carried out following two different approaches. Through the first (simplified) approach, the finite elements model has been used to estimate the temperature distribution inside the reactor during the experimental tests. An average (experimental) torrefaction temperature has been then calculated and used as input of the kinetic model. In the second approach, the proposed kinetic scheme has been implemented directly in the f.v.m. model, solving numerically the mass and energy conservation equation. The results obtained by means of the two modelling approaches have been then compared with the experimental data. The proposed thermochemical and fluid dynamic model seems to be a suitable tool for the simulation of a torrefaction reactor and useful for the characterisation and optimization of such kind of systems.