Self-Heating of Biochar during Postproduction Storage by O₂ Chemisorption at Low Temperatures

Abstract

<p>Biochar is attracting attention as an alternative carbon/fuel source to coal in the process industry and energy sector. However, it is prone to self-heating and often leads to spontaneous ignition and thermal runaway during storage, resulting in production loss and health risks. This study investigates biochar self-heating upon its contact with O<sub>2</sub> at low temperatures, i.e., 50–300<sup>◦</sup> C. First, kinetic parameters of O<sub>2</sub> adsorption and CO<sub>2</sub> release were measured in a thermogravimetric analyzer using biochar produced from a pilot-scale pyrolysis process. Then, specific heat capacity and heat of reactions were measured in a differential scanning calorimeter. Finally, a one-dimensional transient model was developed to simulate self-heating in containers and gain insight into the influences of major parameters. The model showed a good agreement with experimental measurement in a closed metal container. It was observed that char temperature slowly increased from the initial temperature due to heat released during O<sub>2</sub> adsorption. Thermal runaway, i.e., self-ignition, was observed in some cases even at the initial biochar temperature of ca. 200<sup>◦</sup> C. However, if O<sub>2</sub> is not permeable through the container materials, the temperature starts decreasing after the consumption of O<sub>2</sub> in the container. The simulation model was also applied to examine important factors related to self-heating. The results suggested that self-heating can be somewhat mitigated by decreasing the void fraction, reducing storage volume, and lowering the initial char temperature. This study demonstrated a robust way to estimate the cooling demands required in the biochar production process.</p>