Natural convection smouldering of peat: minimum dimensions of reactor and quenching limit

This study investigates the propagation of smouldering combustion in peat under natural convection within open reactors. Emphasis is placed on evaluating the effects of reactor scale, particle size distribution, and organic matter content on the overall burn rate, as well as on two critical near-wall phenomena: the unreacted fuel margin and the local temperature gradients at the reactor boundary . Several experiments were conducted using high-bulk-density (HBD) peat formed in Brazil and Northern Hemisphere Sphagnum peat in box reactors with internal dimensions of 20 × 20 × 20, 20 × 20 × 15, and 20 × 20 × 10 cm. The results show that smouldering propagation is governed by the combined effects of fuel properties and the thermal confinement imposed by reactor geometry. Coarser particle classes, lower organic matter contents, and smaller reactor sizes systematically weakened the combustion front, leading to lower burn rates, a thicker unreacted fuel margin, and steeper near-wall temperature gradients. Additional thermocouple-based diagnostics revealed that smaller reactors exhibited lower peak temperatures and steeper near-wall temperature gradients. This provides direct evidence that lateral heat losses intensify as reactor size decreases, ultimately controlling the expansion of the unreacted fuel region near-walls. The present study offers two major contributions. First, it identifies the minimum critical reactor size for natural convection smouldering, revealing that a 20 × 20 × 15 cm configuration acts as a practical threshold for sustained propagation, while a 20 × 20 × 20 cm geometry ensures robust burning, even for fuel beds with lower organic matter or coarser particles. Second, it experimentally verifies that decreasing the reactor scale steepens near-wall temperature gradients, confirming that the quenching mechanism is governed by elevated boundary heat losses. These findings provide objective guidelines for reactor design and improve the physical understanding of lateral heat-loss effects in open smouldering systems. Novelty and significance statement This study advances the fundamental understanding of peat smouldering under natural convection by establishing rigorous minimum-scale criteria for open reactors. Crucially, this work demonstrates that decreasing reactor size systematically steepens near-wall temperature gradients, consequently increasing the heat loss flux and expanding the near-wall unreacted fuel region. This provides direct, quantitative evidence that localized extinction is driven by intensified lateral heat losses. Ultimately, these findings deliver essential physical insights and practical design thresholds for laboratory-scale smouldering research.