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 carbonization reactions in biomass. In this study, HTC is proposed as a mild pre-treatment for enhancing the combustion properties of bamboo species, focusing on reductions in 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, optimization of the HTC treatment was carried out with the aim of maximizing the ash removal and 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 was determined, along with the chemical composition of their ashes. Moreover, physicochemical characterization of the corresponding mother liquors was carried out to obtain useful information about their valorization. The results indicate that the mildest HTC reaction conditions (160 °C, 1 hour) could effectively solubilize certain critical inorganic elements (mainly K, Mg, and P) while preserving the macrostructural components of the pre-treated solid, thus resulting in them being more easily exploitable for energy uses. The use of carboxylate-based chelating agents to improve ash solubilization is in progress, enhancing HTC further as a mild pre-treatment for 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.