Organic-acid-assisted hydrothermal carbonization of spruce–pine–fir biomass: Enhancing carbon content, heating value, and hydrochar functionality

Hydrothermal carbonization (HTC) converts biomass into carbon-rich material, but improving surface chemistry, fuel quality and combustion properties at reduced severity remains challenging. This study used response surface methodology to determine the extent to which citric acid (CA) catalysis replaced HTC severity in spruce-pine-fir sawdust. Quadratic models for solid yield (SY), higher heating value (HHV), carbon content (CC) and fuel ratio (FR) were statistically significant (R 2 > 0.85; p < 0.0001). Numerical CC optimization identified 280 °C, 129 min and 13.9 wt% CA as the optimum condition, where predicted CC, HHV and SY (74.20 wt%, 31.60 MJ kg −1 , 52.37 wt%) matched the experimental values (71.02 wt%, 30.24 MJ kg −1 , 51.00 wt%) within 2.69-4.50% error. At the milder condition of 230 °C, 135 min and 17.5 wt% CA, hydrochar exceeded the CC and fixed carbon of its non-catalytic counterpart (68.84 vs 63.12 wt%, and 44.04 vs 32.20 wt%, p < 0.001). Relative to non-catalytic HTC at 280 °C, the catalytic 230 °C hydrochar showed 3.5-fold higher BET surface area (87.77 vs 25.08 m 2 g −1 ), higher surface acidity (2.30 vs 1.68 mmol g −1 ), lower pH, and pHPZC (3.13 vs 3.66; 3.87 vs 4.12). This acidity advantage survived washing, indicating retained structural rather than leachable functionality relevant to contaminant binding. Despite a 50 °C drop in HTC temperature, TGA/DTG showed that CA extended burnout temperature by 86.8 °C (872.9 vs 786.1 °C), while lowering ignition temperature by 79 °C and combustibility index by 11%. CA catalysis thus reduces thermal severity while advancing energy density, surface functionality, combustion profile, and thermal resistance for combined energy-environmental application.