Biofuel production via dry and wet torrefaction of agro-industrial waste: a comparative study

Hydrothermal carbonization (HTC), also known as wet torrefaction, is a thermochemical process where wet organic substrates, in an oxygen-free environment, are directly converted into a solid enriched in carbon that is referred to as hydrochar. HTC is carried out in a liquid water environment, in a temperature range of 180–250 °C, in autogenous vapor pressure of water up to 4.0 MPa, and reaction times ranging from few minutes to several hours. In the present work, HTC was used to achieve fuel upgrade of two types of agro-industrial wastes: olive tree trimmings (OT) and olive pulp (OP). Hydrochar yield (MY), elemental and proximate composition, thermal stability, higher heating value (HHV), and energy yield at different reaction temperatures (180, 200, 220, 235 and 250 °C) were assessed for fixed reaction time (30 minutes) and solid load (dry biomass to water ratios - B/W - equal to 0.25) [1]. HTC results were compared with results previously obtained on the same substrates treated through low temperature pyrolysis (LTP), also known as as dry torrefaction (temperature: 200, 250 and 300 °C; reaction time: 30 min) [2]. Table 1 reports results relevant to both thermo-chemical processes. Data related to temperatures of 200 and 250 °C is particularly interesting for a comparison between treatments. Hydrochars showed lower MY and sensibly higher HHVs than the analogous LTP chars. Both OT and OP hydrochars showed higher degree of carbonization (lower H/C and O/C ratios) when compared to the corresponding LTP chars. The HTC process proved to be more severe than the LTP process when operating at the same temperature and reaction time. According to the data of OT and OP reported in Table 1, LTP experiments have to be carried out at temperatures 50 °C higher than those used in HTC in order to obtain bio-chars having the same HHVs: HTC requires milder conditions than LPT to induce coalification and enhance energy properties of chars. Interestingly, SEM analysis demonstrated that HTC chars presented at their surface coke particles with a sphere-like geometry. Such spheres are due to polymerization, in the aqueous phase, of molecules previously released in the same phase by OT and OP during HTC. Such coke particles were not present in the LTP chars. As a whole, HTC demonstrated to be a very effective technology to upgrade high-moisture residual biomass such as OT and OP to hydrochars resembling the biochars obtainable through dry torrefaction. [1] Volpe M., Fiori L., Journal of Analytical and applied Pyrolysis 124 (2017) 63-72. doi:10.1016/j.jaap.2017.02.022 [2] Volpe R., Messineo A., Millan M., Volpe M., Kandiyoti R., Energy 82 (2015) 119–127. doi:10.1016/j.energy.2015.01.011