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Technical Market Support

QEMSCAN for Characterisation of Coal and Utilisation By-Products

Technical Market Support » Thermal Coal

Published: May 08Project Number: C14073

Get ReportAuthor: David French, Colin Ward, Alan Butcher | CSIRO Energy Technology, University of New South Wales, Intellection

QEMSCAN is an automated electron beam image analysis technique, originally developed by CSIRO, Division of Minerals and now by a CSIRO spin-off company Intellection Pty. Ltd., which has been widely applied in the minerals industry. However, its use in the coal and related industries has been limited. The technique has the potential to provide quantitative mineralogical and textural data for assessing coal behaviour during preparation, in power generation (pf and FBC combustion), coking and steel production, gasification and the management and utilisation of coal by-products and wastes.    QEMSCAN analysis can provide unique phase-specific information not obtainable from other analytical techniques which is of relevance to the coal producing and coal utilisation industries. The methods of data acquisition offer advantages over other SEM based techniques in that measurements are made at the individual pixel level rather than averaging over an area and greater use is made of the chemical composition for phase identification rather than relying upon back scattered electron intensity.  

The objective of this project was to develop new QEMSCAN analytical techniques for coal and coal by-product characterisation that will provide unique phase-specific information that can be used by the coal industry to address technical and marketing issues, to optimise existing utilisation technologies and assist in the development of new technologies.

Sample preparation techniques were reviewed and techniques for the preparation of coke and char samples were investigated. Coal samples are prepared using carnauba wax which offers sufficient contrast to allow the measurement of both coal and inorganic phases, thus enabling the relationships of the organic matter with the inorganic phases to be determined. Samples of fly ash and similar materials can be prepared using standard epoxy resins. Sample preparation techniques for the preparation of coke and char were investigated as carnauba wax and common epoxy resins are unsuitable for the mounting of coke and char samples. Four specialist resins were tested, one of which shows some promise and its potential should be further investigated.

The use of chemistry and/or back-scattered electron intensity to identify individual macerals was investigated. Although chemical differences do exist between the various maceral groups these are insufficient to use as suitable discriminant due to the short counting times used in a QEMSCAN analysis. Similarly, there is considerable overlap in the BSE intensity values so they also cannot be used as a reliable discriminant. The use of optical images and coregistration with the corresponding QEMSCAN images should be investigated as this is considered to be the most promising approach to maceral identification and determination of relationships between the individual macerals and the inorganic phases.

Organically associated inorganic elements such as sulphur, calcium and iron are of interest with respect to utilisation and possible catalytic role of the latter two elements in gasification and coke utilisation. Good sensitivity is obtained for sulphur at the levels typically found in coal and its use as a possible discriminant should be further investigated. The other inorganic elements occur at very low levels and cannot be reliably detected using the current counting strategy. Although the counting time could be extended to increase the sensitivity, to do so would increase the total acquisition time to impractical levels at which the analysis would cease to be cost-effective. If such elements are of interest then the use of other techniques such as electron microprobe analysis should be considered.

High-quality reference energy-dispersive X-ray spectra were collected of typical coal and ash minerals, coal macerals, and ash glass compositions in boiler deposits and gasifier slags and these were used for development, testing and refinement of the SIPs used for coal characterisation. Further work on SIP development is required in specific areas such as reliable identification of the clay minerals and better discrimination of glass compositions in utilisation by-products.

Case studies illustrating how QEMSCAN analysis could be employed were carried out in four areas of coal production and utilisation viz.

  • Coal preparation
  • Pf combustion - Boiler deposits-  Fly and bottom ash
  • Fluidized bed combustion
  • Gasification

It was also intended to investigate potential applications of QEMSCAN analysis to coke and char studies but lack of a suitable mounting medium prevented this.

These case studies have demonstrated that QEMSCAN analysis can provide unique information not readily obtainable by other means. In particular, data can be obtained on particle size and shape, phase identification and abundance, mode of occurrence and association of the identified phases. Unlike other techniques such as X-ray diffraction, QEMSCAN analysis can supply information on variations in chemistry of the amorphous phase which is relevant to issues such as ash deposition in pf boilers and ash behaviour in fluidised bed systems.  

However, it is not an analytical panacea and should always be used in conjunction with other techniques as chemical analysis, quantitative X-ray diffraction analysis, and electron microprobe investigation. QEMSCAN is a beam analysis technique and, as such, has an effective resolution of 1-2μm. Consequently, fine-grained minerals such as dispersed quartz in coal and mullite in fly ash may not be identified and this must be borne in mind when comparing QEMSCAN data with for example, quantitative XRD mineralogy. Also, polymorphs such as those of silica (quartz, tridymite and cristobalite) cannot be identified and phases of similar composition such as hematite and magnetite cannot be reliably distinguished due to the short counting times employed in a typical QEMSCAN analysis.

At this stage of development, QEMSCAN should be used as an investigative tool rather than for routine analysis. To maximise the usefulness of the information obtained from a QEMSCAN analysis, discussion with the client prior to analysis is required so that the nature of the problem to be addressed is clearly understood and the most appropriate data collection strategy can be selected.

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