Torrefaction process for the improvement of solid lignocellulo-sic biofuel

Biomass for energy production has been extensively studied in the recent years. In order to overcome some constrains imposed by the chemico-physical properties of the biomass, several pre-treatments have been proposed. Torrefaction is one of the most interesting as torrefied biomass holds a wide range of advantages over raw biomass: the devolatilization of water and some oxygenated compounds determine the increase of the calorific value, both on mass and volumetric basis. The increase of the density approximately up to 750-850 kg/m3, reduces the transportation costs. The reduction of the moisture content increases the resistance of the biomass to the biological degradation, thus facilitating its storage for long periods [1]. The process raises some concerns when waste biomass is used as feedstock as toxic pollutants such as heavy metals can contaminate it [2-4]. This is the case of plants used for soil restoration through phytoremediation technique. The work aims at studying the torrefaction as eco-sustainable process for the combined production of a solid biofuel with improved characteristics with respect to the starting material and a vapor fraction, embedded in the gas carrier flow, to be directly burned in a MILD combustion burner (hence in highly diluted and pre-heated conditions) for energy recovery. The study wants to address the need of optimize the torrefaction stage with respect to energy sustainability and environmental impact. To this aim the energetic content of the torrefaction products as a function of the process temperature has been evaluated. At the same time the fate of the heavy metals in the raw biomass (Cd, Pb) at the different torrefaction temperature has been studied and their stability in the solid torrefied biomass has been investigated and compared to the stability in the in the raw biomass.