Influence of sequential HTC pre-treatment and pyrolysis on wet food-industry wastes: Optimisation toward nitrogen-rich hierarchical carbonaceous materials intended for use in energy storage solutions.

Due to elevated protein content, the food-industry bio-wastes are promising feedstock to produce hierarchical (micro-mesoporous) carbonaceous materials with the intended use as electrodes in the energy storage solutions. However, the high initial water content, makes their direct activation through high-temperature processes costineffective due to significant heat requirements. In this study, the influence of pretreatment with hydrothermal carbonization (HTC) on wet food-industry bio-wastes, further pyrolysed, was investigated. Selected wastes (brewer's spent grains, spent coffee grains and spent sugar beets) were pre-treated by HTC at 180 °C or 240 °C, and then pyrolysed at 500 °C or 700 °C. Obtained materials were examined using elemental analysis, gas adsorption (N2 and CO2) and FT-IR. Besides minor differences caused by the bio-composition of wastes, the general trends were similar for feedstock. The pre-treatment had a beneficial influence on the properties of all wastes. The HTC at 180 °C and pyrolysis at 700 °C for all wastes show the most promising total specific surface area 560 ± 10 m2/g and accessible specific surface area 96 m2/g. Those conditions simultaneously did not reduce the total solid yield in comparison to the one-step process. The pre-treatment at 240 °C led to elevated nitrogen incorporation in the carbonaceous structure compared to HTC at 180 °C. However, it formed a hierarchical structure that was not stable for the thermal treatment. Study proves the HTC pre-treatment at 180 °C is beneficial for the conversion of food-industry bio-wastes into hierarchical carbonaceous material for their use in the energy storage systems application.