Open Cut » Geology
Rapid evaluation of a coal resource by in-situ characterisation downhole can dramatically enhance understanding of the resource, providing new knowledge regarding coal quality - particularly grade - to inform the mining process and improve yield.
LIBS is swiftly becoming an established technology and it shows promise as a rapid analytical method for coal characterisation. LIBS hardware modified to fit smaller diameter exploration holes (<100 mm) and deliver one or several coal quality measures with an accuracy akin to laboratory testing would therefore be immensely valuable and enable coal producers to make more informed decisions in the field.
However, it has yet to be successfully adapted for use downhole due to the cost and specificity of development LIBS hardware and the unique challenges of remote tool operation in a downhole environment. A successful technology demonstration on unprepared coal core would identify the further research and engineering developments required to adapt the LIBS hardware as a downhole tool.
This proof-of-concept project was comprised of three sections:
- Task 1: Determine a device calibration protocol
- Task 2: Core specific calibration
- Task 3: Field test simulations on coal core
A sample set of 12 test coals were used to derive and optimise calibration protocols for major coal quality metrics using a handheld LIBS device.
Despite varying matrix effects between coal samples the likely major contributor to model error, viable relationships (<10% relative error) between LIBS spectra and rank, ash, fixed carbon, calorific value and ultimate analysis values could be derived. More indicative relationships (>10% relative error) for moisture, volatile matter and density were also obtained. Sulphur could not be detected during these investigations, a common finding in LIBS analysis of coal. Future work utilising spectrometers with enhanced resolution (particularly in the UV and NIR ranges) may be able to overcome sensitivity issues as demonstrated by a small number of literature publications.
The device was successfully calibrated for a 2-meter section of PQ coal core, resulting in significant enhancements in curve fitting and relative error (<2%) compared to the test coal sample set. This suggests that calibration on target coal seam and associated mineral matter, a strategy known as 'matrix-matching,' would likely be necessary to ensure accuracy for a potential future field or downhole deployable LIBS device.
LIBS coal quality results obtained from an unprepared coal core were generally encouraging, with relative errors ranging from 4-10% compared to lab testing. However, these results were inferior to the pelletised calibration results, which showed a relative error of only 1-2%. This suggests that whilst meaningful information was still obtained, the volume of spectra collected during this work and/or the laser spot size were likely insufficient to compensate for the increased roughness, heterogeneity, and larger mean grain size present in the coal core.
Testing LIBS on coal cores in a lab setting provided a controlled and reproducible experimental environment for this proof-of-concept project. The primary aim was to showcase the feasibility of obtaining meaningful results without the need for extensive sample preparation, thus evaluating LIBS as a viable technique for analysing exploration holes and warranting further investigation.
This study has demonstrated that, although not yet on par with lab analysis, viable coal quality results can be obtained directly from unprepared coal cores using a low-cost, off-the-shelf LIBS device and relatively simple chemometric calibration methods.