Study of the competing chemical reactions in the initiation and spread of smouldering combustion in peat

Smouldering combustion of natural fuel layers such as peatlands leads to the largest fires on Earth and posses a possible positive feedback mechanism to climate change. In this paper, we use an experimental methodology to study the smouldering combustion of samples of peat under a wide range burning conditions. Vertical samples (30 mm deep and 125 mm in diameter) are ignited by radiation on the top free surface and the smouldering front propagates downward against a forced flow of oxidizer. By varying the oxygen concentration ([O2]) and the ignition conditions we investigate the competing pyrolysis and oxidation reactions. A reaction framework with two regimes is consistently observed. The measurements show that a char species is formed by the competing pyrolysis and oxidation reactions in the first regime resulting in net char production and in the second regime char oxidation results in conversion of the char to ash. Lower mass loss rates and the larger residual mass at lower [O2] suggest that a wider smouldering front is required to sustain combustion as the [O2] is decreased. These results improve our understanding of smouldering phenomena and the role of the competing chemical reactions.