Experimental study of moisture content effects on the transient gas and particle emissions from peat fires

Peat fires are a global-scale source of carbon emissions and a leading cause of regional air quality deterioration, especially in Southeast Asia. The ignition and spread of peat fires are strongly affected by moisture, which acts as an energy sink. However, moisture effects on peat fire emissions are poorly understood in the literature. Here we present the first experimental work to investigate transient gas and particle emissions for a wide range of peat moisture contents (MCs). We include drying, ignition, smouldering spread, and even flaming stages. Peat samples conditioned to different MCs were burnt in the laboratory where a suite of diagnostics simultaneously measured mass loss rate, temperature profiles, real-time concentration of 20 gas species, and size-fractioned particle mass. It was found that MC affects emissions, in addition to peat burning dynamics. An increase in MC below a smouldering threshold of 160% in dry basis leads to a decrease in NH3 and greenhouse gas emissions, including CO2 and CH4. The burning of wet peat emits more coarse particles (between 1 and 10 µm) than dry peat, especially during the ignition stage. In contrast, flaming stage emits mostly soot particles less than 1 µm, and releases 100% more fully oxidised gas species including CO2, NO2 and SO2 than smouldering. The examination of the resulting modified combustion efficiency (MCE) reveals that it fails to recongnise smouldering combustion with sufficient accuracy, especially for wet peat with MC > 120%. MCE confuses drying and flaming, and has significant variations during the ignition stage. As a result, MCE is not valid as a universal fire mode indicator used in the field. This work fills the knowledge gap between moisture and emissions, and provides a better understanding which can help mitigate peat fires.