Boreal peatlands, which make up nearly 70% of global peatlands, are increasingly recognised as important to smouldering fires that can release large amounts of carbon and pose safety and environmental hazards. Most previous studies have concentrated on tropical peat and homogeneous samples, leaving a gap in knowledge about the combustion behaviour of boreal peat and the effect of size. This study experimentally examines the effect of particle size on the smouldering combustion behaviour of boreal peat from Thurso, Scotland, UK, focusing on key parameters related to fire dynamics, such as ignition probability, spread rate, and mass loss. In addition, this work introduces an experimental definition of the critical moisture content (cMC) for smouldering, providing a quantitative threshold for sustained combustion under varying conditions. Controlled laboratory experiments in a 20 ×20 cm reactor with open surface were conducted using two particle size ranges (diameter ≤ 2 cm and ≤ 5 cm) at moisture contents ranging from 120% to 180% (dry basis). Results show that for particles ≤ 2 cm, the spread rate decreases linearly as moisture increases, with a cMC of 178%. Conversely, particles ≤ 5 cm display a faster decline in spread rate and a lower cMC of 146%, due to reduced surface area-to-volume ratios that limit heat transfer. Indicating sustained smouldering in smaller peat at higher moisture levels, while larger particles fail to ignite beyond a moisture content of 178%. These results emphasise the combined role of particle size and moisture in affecting smouldering ignition and spread, offering insights for evaluating smouldering fire hazards in boreal regions and enhancing loss prevention measures in peatland ecosystems. • Natural boreal peat from Thurso, UK, was studied in laboratory conditions. • New experimental definition of critical moisture content for smouldering ignition. • Small particles, or agglomerates, can ignite at higher moisture contents than larger particles. • Particle size and moisture content jointly affect smouldering ignition and spread.
Mulyasih et al. (Sun,) studied this question.