A sequential reaction pathway for self-sustained carbonization of wet biomass enabled by unsaturated fatty acids

Self-sustained carbonization is a controlled self-heating process inspired by spontaneous heating phenomena in organic materials, in which internally generated biological and chemical heat is used to dry and carbonize wet biomass with minimal external heat input. However, initiating this process from ambient temperature remains challenging because biological heat generation becomes limited above approximately 70 °C, leaving a transitional range before thermochemical self-heating becomes effective at higher temperatures. This study investigated whether oils rich in unsaturated fatty acids could provide additional heat within this transitional range (70–110 °C) and thereby enable self-sustained carbonization of wet dairy manure under pressurized conditions. Four oils with different degrees of unsaturation, including waste cooking oil, were added to dairy manure, and temperature profiles, gas evolution, oxygen consumption, operational performance, and product properties were evaluated. Oil addition enabled a continuous temperature increase from ambient temperature to approximately 300 °C, whereas untreated manure exhibited unstable behavior and stagnation near 70 °C. Gas analysis and the sterilized-manure experiment supported a sequential heat-generation pathway: microbial respiration was primarily responsible for heat generation below approximately 70 °C, lipid oxidation likely provided additional heat over the transitional range of 70–110 °C, and thermochemical oxidation became increasingly important above approximately 100 °C. The resulting carbonized products exhibited coal-like elemental characteristics. These findings demonstrate that oils rich in unsaturated fatty acids can provide additional heat through lipid oxidation, facilitating the transition from microbial respiration to thermochemical self-heating.