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Online LIBS analysis of ash in coal – Part 3

World Coal,

C.D. Gehlen and J. Makowe, Laser Analytical Systems and Automation GmbH, Germany, discuss the use of laser induced breakdown spectroscopy (LIBS) for online analysis of coal.

Discussion of deviations between LIBS and routine laboratory analysis
The reasons for the observed deviations for the ash content measured by LIBS and by routine laboratory analysis are most likely of different natures.

First of all, the materials analysed by LIBS and by the routine laboratory procedure are in no way identical. The routine sampling is performed manually, four times per hour. Each time, a few kilograms of material are collected in few seconds from the material stream. Mixing these four samples gives the routine sample that represents the average of a one hour production. This sample is then analysed in the plant facility under production conditions, in order to keep the time delay between sampling and the availability of the analysis result as short as possible (2- 4 hours in this case).

In contrast, the LIBS measurements are regularly distributed across the whole material that passes underneath the analyser within the hour. In total about 36,000 LIBS measurements are obtained during one hour and averaged to get the average ash content. The comparison of the LIBS results with the routine analyses is therefore the comparison of (i) the average of four prepared samples, taken at four particular moments in time and (ii) the average of several thousand measurements distributed across the whole material that passes the analyser during that time (80 t in this case). The results could be expected to be identical only if the effectively analysed material was perfectly homogenous.

Second of all, the LIBS results indicate a strong short-term variation of the material composition of the order of minutes, as shown by the sharp increases and decreases of the ash content. The involved time intervals are shorter than the time in between the routine sampling. Therefore, these variations cannot be detected by the routine sampling procedure.

Furthermore, one should keep in mind that the given time intervals for the manual routine sampling are indicative. Strictly identical ash values measured by the plant laboratory over several hours (see Figure 8 for example) are highly unlikely for a natural material like coal.

Finally, different people perform the sampling and the analysis, while all LIBS analyses are fully automated and performed by the same analyser without sample preparation. According to the experience of the plant facility, the observed differences of the laboratory and LIBS results are in the range of differences observed by comparing the results obtained by different operating personnel.

As a conclusion, most of the observed deviations between the routine laboratory analysis and the automated online analysis originate from the non-identical effectively probed coal samples, from the different sampling procedures and from the operator effect.

The use of LIBS as an analytical method is highly motivated by its online capability and the high degree of automation of LIBS systems for reliable 24/7 operation. In contrast to conventional techniques:

  • The analytical results are available without time delay, as sample preparation and transportation are not required.
  • Variations of the material conditions, such as the change of humidity, the change in material grain size or the change in material density, can be recognised during the analysis and accounted for.
  • Recalibration procedures, essential for long-term operation of any measuring system, can be automated and do not require an operator.
  • Due to the contactless measuring principle, the required maintenance is minimal.
  • LIBS systems can easily be automated for online applications, e. g. for the characterisation of material streams or sensor based sorting.
  • The achievable accuracy and detection limits depend on the chemical elements, the bulk material of the sample, the time that is available for the analysis and the layout of the LIBS system.

This article was first presented at Coal Prep International 2013 and is presented here by permission of Penton Media. Coal Prep International 2014 will take place in Lexington, Kentucky between 18 April and 1 May 2014.

Written by C.D. Gehlen and J. Makowe, Laser Analytical Systems and Automation GmbH.

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