On the mechanism of single pellet smouldering combustion

Aiming at improving the design of wood pellet stoves and boilers, we developed original experiments to study chemical and physical processes occurring in a single pellet. We investigated medium temperature (<550 °C) and low air flowrate, where smouldering prevails. Two setups have been used, allowing to monitor the pellet weight loss, internal temperature, and the exhaust gas composition (H2, CO, CO2, O2, VOC), and modify the temperature and composition of the environment surrounding the pellet and its size. The study allowed to identify the sequence of phases involving drying, pyrolysis, partial and total oxidation of volatiles and char. The maximum rate of conversion is controlled by heat conduction, in the gas surrounding the solids and within the porous pellet, thus, it is linearly proportional to the environment temperature. At the higher temperature, the char yield decreases, and the useful heat must be extracted from volatiles combustion (in the gas phase) rather that embers (char combustion). An axial smouldering front, from the fractured edges prevails over a radial one, from the side of the pellet, due to a less permeable layer left by the extrusion process. Experiments in a nearly stagnant atmosphere clarified that both volatiles and char oxidations occur under O2 starvation; the rate is proportional to O2 available. Also, evidence of a transition from 1- to 2-films char combustion are identified, suggesting that O2 mass transfer controls the heterogeneous char oxidation first, and then the CO oxidation in the gas phase, at higher surface temperature.