Mass and energy balances for a stand-alone tomato peels torrefaction plant

Torrefaction is an emerging thermal pretreatment of biomass, which produces a solid biofuel having superior handling, milling, storage and co-firing properties compared to raw biomass. During the process a combustible gas (‘torgas’) consisting of different organic compound is also produced in addition to the torrefied solid product. In a properly designed and operated torrefaction system the torgas may be combusted to generate heat for the drying and torrefaction steps, thus increasing the overall process efficiency. In this paper, a simple process simulation of a stand-alone torrefaction plant with internal heat integration was performed to assess whether autothermal operation is conceivable for high moisture tomato peel residues (TPs). Results show that for typical torrefaction conditions where about 20-30% of the dry mass is removed in the form of volatile gases (i.e., 285 °C and 30 min for TPs), the process cannot be autothermal and, consequently, an additional utility fuel is required. Under these conditions, in fact, the total thermal energy potentially available in the torgas was approximately 72% lower than the overall energy required for torrefying raw tomato peels, which have 80.5% initial moisture content. The net thermal efficiency of the whole conversion process was estimated to be approximately 70%, whereas the energy yield of the torrefaction unit was 85%. This suggests that for high moisture content agro-industrial residues the integration of torrefaction unit with another plant providing waste heat may be a better option compared to stand-alone plant with internal heat integration in order to save the overall energy efficiency.