Hydrothermal carbonization as a pre-treatment to selectively remove ashes from bamboo

Hydrothermal carbonization (HTC) is a thermochemical treatment involving the use of subcritical water, which acts not only as solvent, but also exploits its acidity as a catalyst, promoting the carbonization reactions of biomass. In this study, HTC has been proposed as a mild pre-treatment to enhance the combustion properties of bamboo species, focusing on the reduction of fouling and particulate matter emissions, through selective solubilization of the ashes. Bamboo is one of the fastest growing plants and it can suitably grow on poor soils, such as marginal lands, making it a promising feedstock for biorefineries, even under Mediterranean climate conditions. In this context, an optimization of the HTC treatment was carried out with the aim of maximizing the ash removal, at the same time preventing the solubilization of C5 and C6 structural carbohydrates. Several HTC tests were performed using a 300 mL Parr reactor, varying the reaction temperature (160-180-200 °C) and the reaction time (1-4-8 hours), with a biomass loading of 10 wt%. The macrostructural composition of the raw bamboo and the solids recovered from the HTC tests were determined, along with the chemical composition of their ashes. Moreover, a physicochemical characterization of the corresponding mother liquors was carried out, to get useful information about their valorisation. The results indicate that the mildest HTC reaction conditions (160 °C, 1 hour) can effectively solubilize some critical inorganic elements (mainly K, Mg, and P), while preserving the macrostructural components in the pre-treated solid, thus resulting better exploitable for energy uses. The use of carboxylate-based chelating agents to improve ash solubilization is in progress, enhancing furtherly the HTC as a mild pre-treatment of bamboo species. Remarkably, this approach could be appropriate for the development of a better integrated biorefinery scheme, requiring the best fractionation/exploitation of each biomass component.