Analysis of spectral properties for coal with different volatile contents by laser-induced breakdown spectroscopy

In view of the inevitable influence of variable coal quality for laser-induced breakdown spectroscopy (LIBS) detection, the effects of changes in volatile content on the properties of the laser-induced plasma and spectral signals were investigated using five coal samples, which cover a wide range of volatiles and their coal chars. Results showed that the intensities of most elements in coal were lower than those in coal chars including the major elements (C, H, O and N) and mineral elements (Si, Al, Ca, etc), along with the vaporization and ablation processes of volatile. Volatiles were easily affected by the thermal mechanism due to the high plasma temperature and more volatiles were vaporized in coal samples with higher volatile content. These increase the ionization of volatiles, resulting in the significant decrease of line intensities of mineral elements (Si, Al, Ca existing in ash). There is no significant relationship between the line intensities of major elements (C, H, O and N) and volatile content. Furthermore, the elemental concentration of sample surface areas was measured to reveal the difference between original sample surface, heat-affected area and internal surface of ablated crater to characterize the influence of laser pulse thermal mechanism. The concentration of C in the internal surface of crater and heat-affected area of coal sample was higher than that in the original surface, due to the evaporation of volatiles and other elements caused by thermal mechanism. On the other hand, the concentrations of most mineral elements of the internal surface were lower than that in the original surface. This may be attributed to thermal mechanism, caused by the different rates of volatilization and atomization processes of elements.