Characterisation of wood combustion and emission under varying moisture contents using multiple imaging techniques

Abstract

In this study, the complex combustion process of natural wood, with different moisture contents, has been characterised using a pioneering approach that integrated Mid-Wavelength Infrared (MWIR) Hyperspectral Imaging (HSI), Schlieren imaging, Long-Wavelength Infrared (LWIR) thermal imaging and visual imaging. The experiment involved the combustion of oak wood samples with moisture contents varying from dry (defined as 0%) to 30%. This setup enabled a detailed investigation into the combustion process, examining aspects such as weight loss rates, thermal behaviours, spectral radiance of gas emissions and flow structures. Higher wood moisture levels were associated with decreased fuel consumption and sustainability of the combustion. The radiance ratios of CH4/CO2 and CO/CO2 were higher in samples with increased moisture during the initial combustion stages, indicating reduced efficiency in gas-phase combustion. The different combustion stages: flaming and smouldering, were characterised by the thermal profiles. Results demonstrated the moisture level significantly affected the initiation of both flaming and smouldering combustion, as moisture can suppress the thermal pyrolysis of flammable gases during ignition and the evaporation of the residual water can cool the surface locally after ignition. Additionally, the samples with high moisture content transitioned more quickly and directly to smouldering combustion. This transition is further evidenced by lower emissions of flammable gases and a significant decrease in the intensity gradient of flow structures in high moisture conditions. This comprehensive study highlights the potential of multi-wavelength techniques in combustion research. The findings have significant implications for optimising biomass combustion processes in various applications, contributing to the broader field of energy and combustion science.