Abstract
On the central-plateau of the South African Highveld, domestic coal combustion is associated with
the release of carbonaceous aerosols often produced as dendritic carbonaceous particles and tar
balls. However, very few studies have been conducted to validate this contribution. Combustion
of coal in low-income settlements is done using self-constructed devices (predominantly braziers)
known as imbaulas. Combustion characteristics in these devices are influenced by fire ignition
methods. The most common ignition method in South African Highveld areas is the bottom-lit
updraft (BLUD) relative to the less favoured Top lit-updraft (TLUD) ignition technique. Aerosols
samples were collected using Nucleopore filters combustion phases (ignition and pyrolysis). The
JSM 5800LV SEM at the University of Pretoria and the Vega3 LM at the University of
Johannesburg were used to observe particle morphologies from coal-burning fires. Both
instruments were equipped with energy dispersive spectroscopy (EDS), which provides the
possibility to analyse particle chemical compositions. Tar balls were identified in both ignition
methods (BLUD and TLUD) and in pyrolysis (stage I and II). When sampling close to the fires (at
ignition and pyrolysis stage I), the filter material rapidly clogged and a continuous layer of liquid/
tarry substance coalesced and covered the entire filter membrane, with some pores completely
closed. At the 5 m exit point, micrographs showed distinct particles morphologies, including giant
spherical organic particles that had condensed as the exhaust stream cooled. Perfectly spherical
giant tar balls were observed in ageing smoke from smouldering combustion conditions typical of
poorly ventilated BLUD fires. The spherical tar ball particles were found as individual spherical
particles and as aggregates forming diffusion accretion chains. It is hypothesised that spherical
organic particles may have been formed by the ejection of liquid tar droplets (pyrolysis stage I
products of coal burning) from the pores of the burning coal, followed by rapid thermal
transformation upon passing through the flame or glow zone of the fire.