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Online LIBS analysis of ash in coal – Part 2

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.

Online analysis of the ash content in coal
Product coal has to fulfill different demands from customers. One important aspect that has an influence on the quality of the coal is the ash content. The ash itself consists of different elements and their compounds, whereas the composition differs in general from deposit to deposit and layer to layer in the deposit.

In contrast to other analysis methods, LIBS is not limited by low elemental concentrations. In contrast to prompt gamma neutron activation analysis (PGNAA), it can be used to analyse concentrations below 3 m.-%, as well as for light elements. This makes it possible to analyse washed coal that has reduced ash content in this concentration range and below.

To perform an online analysis of the ash content in coal, the analyser is mounted above the conveyor belt (Figure 2).

Figure 2: Schematic view of the analyser mounted above the conveyor belt.

The conveyor belt transports an amount of 80 tph at a speed of 2.5 m/sec. The analyser can be moved across the whole width of the conveyor belt to perform the analysis at different locations. For the application in the current example, a rather slow data acquisition rate is sufficient, in this case v = 10 Hz. Every 100 nsec a measurement is performed when all elements of interest are analysed. The analyser system is fully automated for 24/7 operation and performs automated recalibration measurements at defined time intervals.

To give an overview of the degree of refinement that is generated by a continuous analysis of the washed coal, the data that is achieved during a period of a month is shown in Figure 3.

Figure 3: One month overview of the on-line data. The measured concentration for Mg is plotted as a function of the time. Each single measurement is represented by a grey dot. The moving average is plotted as solid curve to show the general trend.

The element magnesium (Mg) is shown as an example for all other elements that are detected simultaneously. Every single measure within this period of one month is represented by a grey dot. To get a better overview of the trend, the moving average is plotted as a solid curve. The measured Mg concentration varies between CMg = 0.032 and 0.043 m.-%. A detailed view of the 48 hours time period marked by an arrow in Figure 3 is shown in Figure 4. The Mg concentration variation from 0.040 m.-% down to 0.033 m.-% is clearly seen.

Figure 4. Detailed view of the measured magnesium (Mg) concentration.

The one-month concentration variation trend is shown in Figure 5 for Si.

Figure 5. One-month overview of the online data. The measured concentration for Si is plotted as a function of the time. Each single measurement is represented by a grey dot. The moving average is plotted as solid curve to show the general trend.

The measured Si concentration varies between CSi = 0.345 and 0.425 m.-%. Compared to the measured Mg concentration (Figure 3), the trend for the Si concentration is similar but not identical. Figure 6 shows the detailed view of the same time period as in Figure 4. The Si concentration sensitively increases on 27 July, while the Mg-concentration stays at its lower level. This indicates a variation of the elemental composition of the ash.

Figure 6. Detailed view of the measured Si concentration.

The ash content can be calculated from the main components of the ash that are analysed simultaneously in each measurement. Figure 7 shows its variations over a whole month. Like in the previous figures, grey dots represent the data points for each single LIBS measurement, obtained on-line by the analyser.

Figure 7: One-month overview of the ash content variation measured by LIBS, compared to the routine laboratory results. The single LIBS measurements are represented by grey dots, the average values by solid circles. The lines connecting the average values are drawn to guide the eye. The laboratory values, obtained hourly, are shown as open triangles, connected by dashed lines for easier comparison with the online results.

The solid circles represent the hourly averages of the LIBS measurements and are connected by a solid line to guide the eye. In addition, the laboratory results of the hourly routine samples are plotted in the diagram as open triangles and connected by a dashed line to guide the eye.

The trend and the absolute values of the LIBS and the laboratory analyses are in good agreement, especially considering the overall low absolute values of the ash content. Again, Figure 8 shows a detailed view of the two days period considered previously (Figures 4 and 6).

Figure 8. Detailed view of the ash content in coal for a two day period.

The results of the laboratory vary from 1.8 m.-% to 2.5 m.-%, while the average values of the online analyser range from 1.5 m.-% to 2.4 m.-%. Both analyses show a similar trend and comparable absolute values, except for the time period indicated by an arrow in Figure 8. The laboratory results show a sudden jump of the ash content from one analysis to the next, which is not visible in the average results of the online analysis. Then the decreasing trend of the ash content is similar for laboratory and online results, although at different absolute values.

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.

The third part of the article, discussing the deviation between the LIBS and routine laboratory analysis that resulted in the above case study, can be reached here.

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

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