Torrefaction of woody waste for use as biofuel

Biomass for energy production has been extensively studied in the recent years. To overcome some constraints imposed by the chemical-physical properties of the biomass, several pretreatments have been proposed. Torrefaction is one of the most interesting pretreatments because torrefied biomass holds a wide range of advantages over raw biomass. The devolatilization of water and some oxygenated compounds influences the increase in the calorific value on both a mass and volumetric basis. The increase in the density reduces the transportation costs. Moreover, the decreased moisture content increases the resistance of biomass to biological degradation, thus facilitating its storage for long periods. Under torrefaction conditions, approximately 10-40 wt% of the initial biomass is converted into volatile matter including liquid and non-condensable combustible gases.1,2 The energy efficiency of the process could greatly benefit the exploitation of the energy content of these products. Recent studies and technological solutions have demonstrated the possibility to realize polygeneration systems that integrate torrefaction/pyrolysis to a combustion process with the aim of obtaining torrefied material/biochar and/or energy from biomass. Some examples include Pyreg, Pyreg-Aactor GT3, TorPlant, and Top Process.4 The identification of the main volatiles produced under torrefaction regime is useful for the optimization of the operating conditions of the integrated system. The integrated process raises some concerns when biomass from phytoremediation and wood from demolition and construction activities are used as feedstock because they could contain potential toxic elements (PTEs). During the torrefaction treatment, the fate of PTEs should be controlled in order to avoid their release in the gas phase and to evaluate the extent of their concentration in the torrefied biomass. The present work aims at studying torrefaction as an eco-sustainable process for the combined production of a solid biofuel with improved characteristics with respect to the starting material and a combustible vapor phase, embedded in the gas carrier flow, to be directly burned for energy recovery. Herein, torrefaction tests on Populus nigra L. branches from phytoremediation, and demolition wood were conducted at three temperatures, 250, 270 and 300 °C, at a holding time of 15 min. The energetic content of torrefied materials was determined. At the same time, the fate of the heavy metals (Cd, Pb, and Zn) in the raw biomass at different torrefaction temperatures was studied, and their mobility in the torrefied biomass was investigated and compared to the mobility in the raw biomass.