The improved energy efficiency of applied smouldering systems with increasing scale

Smouldering combustion has been demonstrated to be a highly energy efficient approach towards waste-to-energy. The benefits of smouldering are principally due to the matching of the energy generation and transfer time scales as well as its low quenching temperature (< 400 °C). This enables effective energy extraction of problematic wastes (e.g., because of low-volatility or high-moisture content). As the engineering applications of smouldering combustion expand, there is a growing interest in designing systems to best house a propagating smouldering reaction. Through a series of experiments, this work quantified the role of radial heat losses in the energy efficiency of different sized reactors. Thermocouples were placed throughout radial and axial coordinates and integrated to estimate the net stored energy throughout the column volume with time. The impact of heat losses was normalized as the system energy efficiency, by dividing the net stored energy by the energy added into the column for ignition and released from smouldering. The results revealed that the system energy efficiency increased from 65 ± 3% to 86 ± 5% with column radius increasing from 0.080 m to 0.300 m, respectively. As a result, scenarios that were not self-sustaining in the thin column were demonstrated to be self-sustaining in the wider column. Thus, increased system energy efficiency increased the robustness of the reaction to quenching. Altogether, this work underscores the importance of scale as a crucial design parameter enabling a smouldering system to be used as an effective waste-to-energy approach