Smoldering fire in peatland is different from flaming wildland fire. The spread of smoldering peat fire is slow, low-temperature and more persistent, releasing a large amount of carbon and smoke into the atmosphere. In this work, we investigated the downward and upward smoldering spread of a peat column up to 30 cm height under variable moisture content (MC) and packing density. In the experiment, the downward fire spread rate was found to decrease with the increasing depth. As MC increased from 10% to 70%, the spread rate increased, which was the opposite to what was previously observed in the horizontal spread of smoldering. For the case of in-depth ignition, upward smoldering spread was not observed if the igniter was more than 15 cm below the free surface. This is thought to be because of the limited oxygen diffusion. If ignited, the smoldering fire first spread upward to the free surface, and then a secondary-stage downward spread was activated because of the multi-step oxidation process. These results suggested that (1) the oxygen diffusion controls the downward spread of peat fire, and (2) the incomplete peat oxidation sustains the upward fire spread. This study provides the novel physical understanding of the role of oxygen and moisture in the downward and upward spread of smoldering peat fires and may provide guidelines to prevent and suppress fires in peatlands.
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