What are spheroidal carbonaceous particles?

Fossil-fuels, such as coal and oil, are burned at high temperatures to produce heat and power for electricity generation and other industries. At temperatures of up to 1750°C and at a rate of heating of approaching 104°C^-1 the droplets, or pulverised grains of fuel, are efficiently burned even though they only remain in the furnace for a matter of seconds.

SCPs and IASs should not be confused with 'fume' or 'soot'. Fume particles are sub-micron spheres (or clusters of spheres) derived from the condensation of volatilised non-combustible material. Soot is formed during the combustion process (as opposed to SCPs which result from the incomplete combustion of the fuel) and although carbonaceous, is usually sub-micron in size and bears no morphological resemblance to SCPs.

Some definitions

Although SCPs and IASs are produced by the alteration of original particles passing through the furnace, and soot is produced during combustion processes, there has been some confusion in the literature. However, it is important to realise that these studies are referring to the same particle types. For example, SCPs have been variously referred to as soot, soot balls, soot particles, soot spheres, soot spherules, carbon particles, elemental carbon particles, carbonaceous particles, charcoal particles, coarse carbonaceous spheres and spheroidal fly-ash particles (SFP), although this latter may also have included black IASs. Recently, however, the term spheroidal carbonaceous particle or spheroidal carbonaceous fly-ash particle (both abbreviated to SCP) has become the usual term and is now widely used.

The nature of their formation and their porosity means that both oil and coal SCPs are never spherical. However, they do have some degree of sphericity to their morphology and for this reason they are termed spheroidal. IASs are spherical and come in a variety of colours from colourless through yellow, red, brown and black, depending on their elemental, and in particular iron, content. Therefore, the counting of black 'spheres' and terming them SCPs may lead to further confusion as to the exact nature of the particle types included in the study.

More background information on SCPs and IASs can be found in: Rose, N.L. (2001). Fly-ash particles. In: Last, W.M. and Smol, J.P “Tracking Environmental Change Using Lake Sediments: Volume 2. Physical and Chemical Techniques”. Kluwer Academic Publishers, Dordrecht, The Netherlands. pp. 319 - 349.

Sizes

Fly-ash particles can vary in size from sub-micron to greater than 200 µm. However, these largest sizes are rarely seen in environmental samples as they are more efficiently trapped by particle arrestor technologies and also do not travel very far from their sources.

SCPs identified in lake sediments are most usually within the range 5 - 50 µm (like the one shown on the left), depending on the distance from source, whilst IASs are generally smaller. In remote areas the size distribution of identifiable particles is usually curtailed to less than 10 µm. Most SCP identification is usually undertaken by light microscopy and this also limits the lower size range of identifiable SCPs to greater than 2-4 µm (depending on the magnification).