Ash is the residual or leftover material from combustion processes, including coal burning. Two types of ash are produced from industrial coal combustion processes – bottom ash and fly ash. Figure 4.7 summarises the coal combustion process in coal-fired power stations.


Bottom ash is the coarser residual or leftover matter found in the grate or combustion chamber once combustion has finished (for example, similar to the ash seen in a wood-fired heater in a home).

Fly ash is finer material that can be captured in gas when gas is emitted from the coal combustion process. Fly ash is captured when the exhaust gas stream passes through the pollution control system found in a combustion chamber. Depending on the efficiency of the pollution control system, a small amount of the fly ash can be released into the atmosphere.54

Figure 4.7 Coal combustion process in a coal-fired power station55



Fly ash is made up of non-flammable mineral elements that are found in raw coal. The final composition and particle size found in fly ash varies depending on a number of factors. These factors include the type of coal, the temperature of combustion, availability of oxygen, the type of combustion process and whether the coal is pre-treated prior to combustion (eg pulverised coal).56


An issue for the Board to consider is whether the smoke and particulate matter from the Hazelwood mine fire can be described or characterised as fly ash. Based on the data available to independent expert Ms Claire Richardson, Managing Director and Principal Consultant, Air Noise Environment Pty Ltd, the smoke and particulate matter and ash from the Hazelwood mine fire was different from particulate matter and ash caused in coal combustion during normal mine operations. This was because:

  • additional sources of smoke and particulate matter from biomass burning (bushfires) were occurring at the same time as the Hazelwood mine fire
  • coal combustion happened in the open air as opposed to a controlled environment such as a combustion chamber (as shown in Figure 4.7 above)
  • the temperature of combustion and the variation in temperature of combustion spatially and temporally as the fire progressed was different
  • the use of water and firefighting foam to quench the fire would have changed the combustion characteristics and provided an additional source of chemical constituents (eg from the additives used in the firefighting foam).57

Further, fly ash produced in a power station has a larger fraction of PM2.5 and a significantly lower proportion of carbonaceous material (carbon) than the ash samples collected from areas affected by the Hazelwood mine fire.58 Put simply, the ash produced by the Hazelwood mine fire had less carbon and PM2.5 than fly ash.

Despite not being classified as fly ash, the ash from the Hazelwood mine fire undoubtedly caused significant distress for the community. The Department of Health advised the community that the ash was generally too large to be breathed into the lungs, but could cause irritation to the skin, eyes, nose and throat.59

A range of metals and organic compounds were present in the ash that was produced from the Hazelwood mine fire. If breathed in, and where the levels exceed health guidelines, there is potential for adverse health effects.60 The potential for adverse health impacts is proportionate to the overall concentration of particulate matter inhaled, the particle size distribution and the chemical composition of the particulate matter.61 Fortunately, the ash from the Hazelwood mine fire produced significantly less fine particles than found in fly ash (6 per cent compared to 27 per cent).62