Influence of Lignin on Smoldering Propagation

Smoldering combustion during wildfires contributes significantly to emissions of pollutants, can burn for days or months, may damage roots and soil, and can transition to flaming combustion. Mitigating these hazards requires an understanding of how physical parameters control smoldering combustion, such as the chemical composition of the fuel. The main organic constituents of biomass are cellulose, hemicellulose, and lignin. Understanding how these constituents influence smoldering is an important step in further developing physics-based models and developing understanding that applies across multiple fuel sources. Previous experimental studies have investigated how varying the amount of cellulose and hemicellulose in fuel influences smoldering behavior, but have not considered the impacts of varying the lignin content. The objective of this study was to identify the influence of lignin on smoldering behavior. This objective was achieved by experimentally and numerically studying the smoldering behavior of various concentrations of lignin in mixtures of cellulose and hemicellulose. These were tested at densities of 200 and 300kg/m3. An infrared camera and thermocouples were used to determine the propagation of the smoldering front in the horizontal and vertical directions, respectively. A one-dimensional reactive porous media model with global chemistry was used to simulate downward smoldering propagation. The horizontal and downward smoldering propagation velocities decrease when more lignin is present due to the slower pyrolysis rates and higher activation energy of lignin. Additionally, results from simulations match this trend for downward propagation. At higher lignin contents, the effect of the mass percentage of cellulose and hemicellulose on downward and horizontal smoldering decreases, indicating that lignin content has the largest impact on smoldering velocities of the three constituents. Increasing the density decreases both the horizontal and vertical propagation velocities due to lower oxygen diffusion and the additional mass being consumed.