Coal analyses using CCSEM/QEMScan

Application of CCSEM to Coal Ash Related Problems

The transformation of the mineral matter in coal to ash residues is a fundamental issue when trying to assess the deposition and erosion potential for a coal upon utilisation. It is well known that deposition of ash in boiler systems, and erosion of boiler surfaces is a particle specific phenomenon.

Traditional ash analysis techniques, such as x-ray fluorescence (XRF), do not provide adequate information for the assessment of these aspects of coal quality. An advanced analytical technique, the computer controlled scanning electron microscope, has been used at the CCSD to provide a measurement of coal mineralogy. The CCSEM technique has the advantage over the traditional ash analysis technique in that it is a particle to particle measurement on the coal. This measurement, when combined with predictive models developed at the centre, allows the common problems of ash deposition and erosion of surfaces to be predicted.

Minerals in Coal

A large variety of minerals can be found in black coals. Three main aspects can be defined for the minerals present in a coal:

1. Mode of Occurrence: affects the conditions (temperature and gaseous environment) experienced by the mineral particle within the combustion environment.
2. Chemical Form of the Minerals: affects the character of the resultant ash residue(s) e.g. sticky or abrasive.
3. Size of Mineral Grains: affects the transport of mineral residues to surfaces and the time-temperature history for residues.

All of these factors are important when trying to assess deposition and erosion potential.

The CCSEM Technique

For CCSEM analysis approximately 2000 mineral grains in a coal sample are analysed for:

• Mode of occurrence
• Chemical form
• Size and shape

An example of the mineral distribution obtained, in respect to chemical form, is shown in Figure 1.

Predicting Ash Deposition Using CCSEM

The use of models developed by the centre allows the deposition potential for the coal to be determined from the size, type and associations for the minerals in the coal. This is shown for a range of coals in Figure 2. The type of deposition encountered e.g. slagging or fouling can also be determined.

Figure 2: Example of the CRC Ash Viscosity Deposition Index ranking of coals using information derived from CCSEM analysis

Predicting Erosion Using CCSEM

The conventional erosion index for PC boilers is directly related to the percent of free silica present in the ash. It assumes that:

• the silica equivalent of 1.5 times the alumina content is bound in clays.
• the remaining silica is present as free silica.
• all free silica is quartz.

Work at the centre has indicated that such simplistic assumptions can unfairly disadvantage some Australian coals. The CCSEM technique, being a particle specific technique, is able to measure free silica more accurately than the standard XRF technique. This provides a more robust index of coal quality in respect to erosion (Figure 3).

Figure 3: The conventional erosion index for PC boilers based on free quartz in ash, and the advanced index specifically for superheater tube erosion based on CCSEM analysis.

For more information contact:

Prof Terry Wall
Program Leader
University of Newcastle

Dr Raj Gupta
Senior Lecturer
University of Newcastle