## 5 In-Line Application of LIBS in Industrial Production
### Coal and Coal Ash  
Coal is still one of the major resources for the global production of electricity. Around 39% of the total electricity production worldwide was based on coal in 2015 [450]. Moreover, coal is an important raw material in various industries. The combustion efficiency of coal depends on various material properties such as chemical composition (major elements C, H, O, N, and S), heat value, moisture content, volatile matter, fixed carbon, and ash content [92, 451]. In order to optimize the efficiency of power generation and to reduce the environmental pollution (e.g., by emission of SO\({}_{2}\)) a technology for rapid in-line or at-line chemical analysis is needed. Coal is usually analyzed by standardized ASTM laboratory methods (American Society for Testing and Materials), which requires several days for the results to be obtained. Prompt gamma neutron activation analysis (PGNAA) and X-ray fluorescence (XRF) are methods enabling in-line analysis of coal. However, radiation safety issues are of relevance for both methods. Furthermore, PGNAA requires the use of an isotope source and XRF does detect only the heavier elements (Z \(\geq\) 11). LIBS is a promising candidate for coal quality detection due to its advantages of real-time, in-situ, and multi-element measurement capability. A schematic of a LIBS measurement system installed in a coal-fired power plant is shown in Figure 5a [452, 453]. The LIBS system was installed above a conveyor belt to analyze coal material that was transported from the crushing station to the coal bunkers (PPL Generation's Montour Power Station). For calibration, an artificial neural network (ANN) model was developed and trained with calibration samples (ASTM reference analysis). Figure 5b shows results from validation tests of the in-line LIBS system using approx. 120 coal samples that were grabbed and then analyzed by LIBS and a laboratory method. The LIBS results for the Sulphur concentration (red and black symbols, Figure 5b) were in good agreement with the lab reference analyses (black lines, Figure 5b). The measurement of S was required for optimal operation of the SO\({}_{2}\) reduction system of the plant. Besides S, the LIBS analyzer was detecting also Al, C, Ca, K, Mg, Na, Fe, Si, and Ti in real-time. The ash content in coal transported on a conveyor belt can be monitored online by PGNAA. However, the neutron radiation in PGNAA represents a potential health hazard requiring strict regulatory demands. LIBS does not require radioactive materials for operation and LIBS systems can be designed more compact and of less weight compared to PGNAA systems. Laser Detect Systems has developed one of the first mineral analysis systems using LIBS and has pioneered the in-line analysis of coal ash [110]. The system was installed at a conveyor belt of a coal mine in South Africa for a four-month field trial (Figure 6a). The analytical performance of the LIBS system was compared to a PGNAA system installed in the same line. LIBS analyzed the surface of the coal material on the belt while the PGNAA signal depended on the volume of the irradiated material. The field trial demonstrated successful online coal ash content monitoring by the LIBS analyzer. The key elements in coal and coal ash, C, Mg, Al, Si, Ca, Fe, and Ti, were measured and the in-line ash quantification by LIBS and PGNAA had the same accuracy with a mean absolute error of \(\pm 0.5\%\) (Figure 6b). The concentration of unburned carbon in fly ash is an important criterion for evaluating the combustion efficiency of coal-fired power plants and the commercial value of fly ash  
Figure 5: LIBS in-line analyzer of coal. (**a**) Schematic of measurement system for coal on conveyor belt in coal-fired power plant. (**b**) Measured concentration of Sulphur in coal for approx. 120 samples (validation tests). Adapted from [452,453]. Figure 6: LIBS in-line analyzer of coal ash. (**a**) Measurement system installed at a conveyor belt (photograph: LDS Laser Detect Systems).