Structural evolution of solid-fuel combustion using laser-induced breakdown spectroscopy and Raman spectroscopy

One of the major challenges regarding measuring chemical structures with laser-induced breakdown spectroscopy (LIBS) is that, it is difficult to track the source of organic components from atomic spectra since these structures have a similar elemental composition. However, LIBS molecular emissions (C₂, CN, etc.) provide a possibility to investigate these similar compositions. In the present study, an experimental investigation was conducted to diagnose the structural evolution during coal combustion. The structural evolution of char particles was also evaluated as a reference using offline Raman spectroscopy. The significance of minerals on plasma emission was investigated based on plasmas of raw and demineralized coals. As combustion progresses, coal particles show the overall dissipation of active structures such as G_r, V_L, and V_r bands, and the aromatization of char structures. The minerals in coal may increase the free–free bremsstrahlung and free–bound recombination events, resulting in a significant continuous baseline. Presence of minerals also reduces the breakdown threshold and enhances the LIBS's emissions. The char's structural transformation is regarded to be correlated with the rotational temperature (T_rot) of CN molecules of LIBS's emissions. The mathematical correlations of linear fitting (R²: 0.94) and exponential fitting (R²: 0.99) exist between the drift coefficient and the T_rot. This structural transformation might change the LIP's CN formation mechanism and lead to the T_rot reduction.