Simulation and flowsheeting of agro-industrial residues torrefaction: the case of tomato peels waste

Torrefaction is a thermal pretreatment for biomass feedstocks of various origin, which is usually carried out in an inert atmosphere, at ambient pressure and in a temperature range of 200-300 °C. It has the ability to reduce the main logistic and application limitations of biomass, arising from its heterogeneity, low bulk density, low energy density, hygroscopic behavior and fibrous nature. During torrefaction a combustible gas ('torgas') consisting of different organic compounds 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. This paper focuses on the valorization of biomass made available from low-value, wet agro-industrial residues. The aim of this work is to provide the conceptual design and technical analysis of a torrefaction process for recovery and upgrade of wet tomato peels, which are a typical industrial waste in the Campania region (IT). The Aspen PlusTM software was used to depict the flowsheeting of the investigated torrefaction process, to develop and solve material and energy balances of the whole process, to carry out the internal heat integration steps. A novel aspect is the modeling of the torrefaction reactor, which was carried out by taking advantage of experimental correlations available in the literature from authors' previous work. Drying of the wet biomass feedstock results a very energy-demanding operation. The main output of this study is the calculation of the process energy demand from external sources. Therefore, the paper discusses how far the torrefaction process of high-moisture tomato peel residues is from autothermal operation, provided the best available process design options and internal heat integration steps.