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Rapid and accurate measurement of coal quality has great significance for efficient use of coal at thermal power plants. Laser-induced breakdown spectroscopy (LIBS) combined with chemometric methods has many unique advantages in coal analysis. In this study, four calibration models, based on partial least squares regression (PLSR), support vector regression (SVR), artificial neural network (ANN), and principal component regression (PCR), were applied assisted by the LIBS technique for the quantitative analysis of coal quality. In order to find the optimal calibration method with LIBS for coal analysis, the spectral data of 40 standard coal samples with pressed-pellet pretreatment were acquired through a LIBS experimental setup, and the modeling efficiency and prediction accuracy of the four chemometric methods were compared based on these spectral data. As a result, the modeling efficiency of PLSR was found to be the highest, that of SVR was the lowest, and that of ANN ranked third. In terms of prediction performance, ANN was found to work better than the other three chemometric methods, and the average absolute error (AAE) of prediction of ash content, volatile matter content and calorific value were 0.69%, 0.87%, and 0.56 MJ kg-1, respectively. ANN can seek the best compromise of modeling efficiency and prediction accuracy and is demonstrated to be an optimal multivariate calibration method with LIBS for online measurement of coal quality at thermal power plants.

Sulfur accurate determination plays a vital role in the evaluation of coal quality. In this work, double-pulse laser-induced breakdown (DP-LIBS) in a helium atmosphere was presented and applied in the sulfur concentration quantification of coal. The ablation plasma generated from the sample surface by the first laser (Nd:YAG laser at 1064 nm, 50 mJ per pulse) was re-excited orthogonally by the second laser pulse (Nd:YAG laser at 1064 nm, 120 mJ per pulse). The intensity of sulfur spectral lines has been enhanced with DP-LIBS and the interference from O I 926.6 has been weakened in the helium atmosphere. The calibration curves of S I 921.3 nm, S I 922.8 nm and S I 923.8 nm have been established for 10 coal samples with S concentration ranging from 0.28 wt% to 4.01 wt%. The best result was achieved with R2 = 0.992, LoD = 0.038 wt%, and RMSECV = 0.143 wt%. The overall results adequately demonstrate that DP-LIBS combined with a helium atmosphere has excellent analytical performance for sulfur determination in coal and enormous potential for on-line detection in the coal industry.