Analysing for Trace Elements

Where are the trace elements in the coal and where do they go?

The release of trace elements to the environment as a result of coal fired power generation is an area of increasing community concern. Recent moves to include coal-fired power stations in the National Pollutant Inventory will act to further focus attention on the environmental credentials of Australian coal.

Environmental impact assessments of coal utilisation processes usually require expensive mass balance studies on operating plants and experimental combustors. Knowledge of the mechanisms by which trace elements are distributed between the flue gas, fly ash and bottom ash could provide essential data for predictive modelling of the environmental impacts, which minimises the need to carry out expensive plant tests.

A basic consideration for developing deportment mechanisms is to determine the modes of occurrence for trace elements in coal. Most trace elements are associated with the coal mineral matter and some are present in the coal organic matter. The mineralogical association for the particular trace element will determine its behaviour during combustion.

Work at the CCSD has determined the association of trace elements with mineral groups and organic matter. Eight Australian bituminous coals were analysed using sequential selective chemical leaching to determine the association of trace elements with the following mineral phases:

• Oxides, Carbonates and monosulphides
• Pyrite
• Silicates
• Organic Matter

Figure 1: Trace element associations

An example of these associations for selected trace elements is shown in Figure 1.

This information assists in understanding the fate of trace elements during and post combustion. For example, elements associated with either pyrite or the organic matter in the coal tend to volatilise and exit with the flue gas, often as a condensate on the surface of fine ash particles.

Figure 2 shows how trace elements from a particular feed coal have partitioned during combustion.

Figure 2: Example of the partitioning of trace elements during combustion

Key Findings

• Most trace elements have similar principal residences in Australian coals.
• Associations with mineral groups are variable.
• Hg, As, Se, Ni, Pb, Cu and Zn are predominantly associated with sulphide minerals and organic matter.
• The association of trace elements with the coal minerals or organic matter can significantly influence the extent of vaporisation and hence the proportion exiting the plant in the flue gas.
• Trace elements detected in the flue gas were those predominantly associated with sulphide minerals.

Future work will enable the development of predictive models linking the trace element distribution in the feed coal to the power station wastes produced.

For more information contact:

Mr Ken Riley
Research Scientist
CSIRO DET