Effect of ambiance on the coal characterization using laser-induced breakdown spectroscopy (LIBS)

The influence of surrounding gases, such as He, N-2, atmospheric air, and Ar, and gas flow rate on the laser-induced breakdown spectroscopy (LIBS) characterization of coals in free space is studied. The atomic and molecular carbon (C-2 and CN) emission intensities are observed to be higher in Ar and N-2 ambiance. Quantitative analysis of carbon and ash content in different coal samples is carried out using the carbon bound atomic and molecular emission signals and the ash forming elements (Si, Fe, Mg, Al, Ca, Na, and K) signals. The sum of the LIBS emission of the all and major ash forming elements increased linearly with an increase in the ash content. Similarly, the ratio between the carbon signals (C I, CN, and C-2) and the sum of major ash forming elements (Si, Al, Fe, and Ca) also showed a linear increase with the increase in carbon content in coal samples. The linear coefficient of regression, R-2, was estimated to be 0.67, 0.58, and 0.85, and the root mean square of calibration samples was estimated to be 5.71, 5.82, and 5.57 wt% using the partial least square regression (PLSR) method for air (no flow), N-2, and Ar atmosphere, respectively. The precision and accuracy of the carbon measurement in coal samples by the LIBS technique using the PLSR method were higher in the presence of Ar than air or N-2 atmosphere due to the plasma shielding effect.