Analysing coal with x-ray diffraction

Coal remains one of the major energy sources for the world. A fast and accurate check of ash content and the mineral matter in coal enables the early recognition of coal quality and makes fast counteractions possible.  Also, some types of mineral matter can have adverse effects on the milling equipment used to grind coal, and XRD can monitor these minerals and the ratio of minerals, which affects wear of milling equipment.

X-ray diffraction (XRD)

Commercial x-ray diffractometers have been available since 1948. A diffractometer uses a monochromatic x-ray source (i.e. sealed x-ray tube), optics to condition the incident and diffracted beams of x-rays, a sample platform, a detector, and a precise goniometer to move the sample with respect to the tube and detector to produce a diffraction pattern. This pattern is unique for every crystalline phase. The pattern consists of peaks whose positions correlate to interatomic distances in a crystalline material, with a peak height due to structural factors, abundance of the phase, and absorption of x-rays in the material, and a peak shape which will correspond to instrument related effects as well as sample related effects such as crystallite size and microstrain from defects in the lattice.¹ Patterns can be compared to available databases containing thousands of reference patterns for many known minerals and materials to identify the phase(s) present in a sample.

Modern diffraction systems employ the latest advances in solid state detector technology, optics, goniometer positioning mechanics, as well as advanced software to automate the collection of data, analysis, and reporting to network or LIMs systems. These advances make it possible for personnel without any knowledge of the technique to use this equipment to produce fast and reliable quantitative analysis results with ease.

Identification of mineral phases in coal

XRD can identify the presence of mineral phases in coal. Figure 1 shows such a phase identification of a raw coal sample with high mineral content. The mineral content is the main source of the elements that make up the ash when coal is burned. All phases were matched to reference patterns in a database used by the analysis software. Besides graphitic carbon, six different mineral phases were identified in the raw coal sample:

Quartz (SiO₂).

Kaolinite (Al2Si₂O₅(OH)₄).

Calcite (CaCO₃).

Dolomite (CaMg(CO₃)₂).

Siderite (FeCO₃).

Anatase (TiO₂).

Amorphous material, mainly in the form of carbon, is represented in the scan by the hump at about 25°.

Figure 1. Phase identification results of a raw coal sample.

Quantification of mineral phases in coal

Analysis of coal using the standardless Rietveld analysis method has several advantages compared to classical quantification methods. All crystalline phases can be determined in just a few minutes. In addition, the amount of amorphous carbon content can also be calculated. Line overlaps and effects of sample preparation, such as sample height and preferred orientation, do not influence the results. Crucial process and quality parameters, such as total ash content, can be calculated directly from the phase quantification. No standards, monitors or calibration are needed for this.

Figure 2 shows the quantification of a raw coal sample using Rietveld refinement. In this example, the dry ash content was found to be 63% (quartz plus kaolinite). This compares well with a classically determined loss on ignition (LOI) of 37%, which includes the dehydration of the kaolinite in addition to the volatilisation of the organic carbon. The latter explains the difference between the LOI and the organic carbon calculated by XRD. For this example, the carbonates are negligible.

Figure 2. Quantification results of a raw coal sample using Rietveld method.

For effective combustion, coal needs to be milled to a specific particle size. The erosion and abrasion of the mills are associated with minerals, such as quartz and pyrite. Coal with higher quartz-to-kaolinite ratio needs less grinding time than coal with a lower ratio. Mineral quantification can be used to adjust the mills, resulting in energy and consumables savings.

Conclusion

XRD is a fast and reliable alternative to time-consuming and labour intensive traditional methods for analysing ash content, LOI, and minerals present in coal. Modern XRD systems are easy to use and produce valuable information to determine coal quality and potential problematic constituents.

References

M. Ermrich and D. Opper, XRD for the Analyst, PANalytical B.V., 2013.

Authors

Uwe Konig, PANalytical, Netherlands and Katherine Macchiarola, PANalytical, USA.