Catalytic hydrothermal carbonization of corn and rice straws with citric acid: Implications for charcoal production and combustion performance

Hydrothermal carbonization represents a promising environmental approach for converting biomass into high-quality biochar fuel. Despite extensive discourse, limited research has focused on fuel properties and combustion performances of biochar as well as reaction mechanisms under catalysts. This study scrutinized properties of biochar from citric acid-catalyzed carbonization. Straw (corn/rice) was hydrothermally carbonized in 5 % citric acid solution with a solid-liquid ratio of 1:10 g/mL at 190–250 °C for 3 h. Results revealed an increase in C and a decrease in O with elevated temperature and citric acid concentration. The calorific values for corn and rice straw biochar ranged from 23.53 MJ/kg to 29.81 MJ/kg and from 23.14 MJ/kg to 26.98 MJ/kg, respectively. This study highlighted diverse impacts of hydrothermal reaction temperature and citric acid concentration on biochar attributes, calorific value, energy density, mass yield, energy yield, and thermal stability. It elucidated the influence of citric acid concentration on biochar burnout temperature and the role of hydrothermal reaction temperature and heating rate in biochar combustion characteristics. Citric acid effectively reduced biochar activation energy in low-temperature areas and prolonged the biochar combustion reaction time, achieving highly favorable comprehensive combustion characteristics at 47.56×10−7. The findings present the potential for biochar production with enhanced performance for integrating and co-firing with conventional fuels.