Hydrotreatment of Eucalyptus sawdust: The influence of process temperature and H2SO4 catalyst on hydrochar quality, combustion behavior and related emissions

Abstract

Hydrothermal carbonization (HTC) presents a promising avenue for waste-to-energy conversion, owing to its cost-effectiveness, low emissions, and adaptability. This study employed HTC to convert Eucalyptus grandis sawdust residues into hydrochars (HC). HTC was conducted for 60 min in a high-pressure steel reactor within glycerin thermal bath equipment under different temperatures (150, 170, 190, and 220 °C) and reaction mediums (water and H2SO4 concentrations of 0.1 and 0.2 mol L–1). The resulting HC underwent thorough analysis, encompassing proximate, ultimate, and morphological assessments and investigations into combustion behavior and potential emissions. Moreover, a new indicator, the hydrotreatment severity index (HSI), is proposed to provide a standardized measure of hydrotreatment severity and property predictions. Higher temperatures and increased H2SO4 concentrations were found to reduce solid yield (ranging from 43.21 % to 90.72 %) and induce morphological changes, including an abundance of carbon microspheres, particularly at higher acid concentrations. These modifications substantially improved higher heating values (HHV), up to 34 % (HHV of 25.72 MJ kg−1 achieved at 220 °C with 0.2 mol L–1 H2SO4) compared to raw feedstock. Interestingly, the intermediate temperature of 190 °C contributed significantly, enhancing HHV by 33 %. All HTC processes greatly enhanced the combustion performance and ignitability of hydrochars. The most favorable reactivity (S = 7.61E7 %2 min−2 °C−3) was observed at 220 °C with 0.2 mol L–1 H2SO4, along with an estimated CO2 emissions of 2,058.46 kg per ton of hydrochar. These results underscore the efficacy of the simplified methodology and reactor system, emphasizing the critical roles of H2SO4 as catalyst and temperature in shaping the characteristics and performance of hydrochars from a representative Brazilian residue, Eucalyptus grandis.