Lightning-induced smoldering ignition of peat: Simulation experiments by an electric arc with long continuing current

Abstract

Lightning is one of the essential causes of wildfires. However, the ignition criteria for lightning-caused wildfires have significant limitations due to a lack of understanding of the ignition mechanism. This work aims to reveal the mechanism of lightning-induced smoldering ignition of wildland fuels and explore a more reasonable ignition criterion by experimental means. The solidified peat cake samples were ignited by the electric arcs with different discharge currents (from 100 to 220 A with a step of 30 A) and durations (from 40 to 480 ms with an interval of 40 ms). The observations show that the ignition process follows a unique flaming-to-smoldering mode consisting of three stages: discharge heating (Stage I), thermal feedback (Stage II), and sustaining smoldering (Stage III) stages. The flame column that appears in Stage I is attributed to intense gasification and burnup of the peat, and seems to have little contribution to the ignition outcome. The carbonized region formed in Stage I, not the sample gasified in the discharge channel, is the main energy deposition to drive the successful smoldering ignition. It is found that the discharge duration required for smoldering ignition decreases with increasing discharge current (Iarc) when Iarc<160 A, but weakly depends on the discharge current when Iarc≥160 A. Critical discharge energy is also required when Iarc<160 A. A segmented ignition criterion is proposed based on the critical discharge duration and energy. For the peat cake sample, a minimum size, initial temperature, and absorbed energy of the carbonized zone are required for a sustaining smoldering ignition.