Enhancement of energy and combustion properties of hydrochar via citric acid catalysed secondary char production

The present study investigates the use of hydrothermal carbonization (HTC) to upgrade agro-waste into solid biofuels and the use of citric acid (CA) as a catalyst capable of enhancing energy properties of hydrochars. HTC of pineapple waste (PA) was carried out at 180, 220, and 250 °C at a fixed 1-h residence time with and without the addition of CA. Contrarily to the current understanding with regard to the use of an acid catalyst during HTC, CA addition shows to appreciably increase hydrochar mass yields with HTC temperature, while increasing their degree of coalification and carbon retention. PA hydrochars produced with the addition of CA exhibit higher heating values (HHV) up to 29.7 MJ/kg dry basis (db), low residual ash (between 0.53 and 0.75 wt% db), and better combustion properties when compared to those of hydrochars obtained without CA addition. We show that the increase in mass yields and energy properties observed for hydrochars is due to CA catalytic effect toward back-polymerization of organics in the liquid phase to form secondary char. Secondary char formation and its role in influencing the hydrochar properties as solid biofuels are demonstrated by scanning electron microscopy, proximate, elemental analysis, and combustion reactivity. Graphical abstract