Natural Downward Smouldering of Peat: Effects of Inorganic Content and Piled Bed Height

The ash thickness formed above the combustion front of peat, which is affected by the inorganic content and the piled bed height of peat column, plays both positive and negative effects on peat smouldering fires. This work investigated the effects of the inorganic content and the piled bed height (5, 10 and 20 cm) on the natural downward smouldering mechanism of peat, both experimentally and numerically. It was observed that the char formation and char oxidation processes are stable and dominant during the downward smouldering. The spread rate of the pyrolysis front was found to decrease almost linearly with the inorganic content, while the spread rate of the char oxidation front was less dependent on the inorganic content and the piled bed height at higher inorganic content. The spread rate of the char oxidation front levelled off when the ash layer above the char layer was thick enough, which was also well-predicted by a 1-D numerical model. The model also predicted the nonlinearly increasing peak temperature and decreasing downward spread rate with depths in experiments. This work provides a better understanding of the multiple role of inorganic content in peat smouldering fires, especially for the fires in the in-depth peat layer with abundant inorganics in natural peatlands.