Investigation of the CN and C₂ emission characteristics and microstructural evolution of coal to char using laser-induced breakdown spectroscopy and Raman spectroscopy

Laser-induced breakdown spectroscopy (LIBS) was employed to study the spectral characteristics of the C₂ and CN emissions of three types of coal and their pyrolysis char samples. By analysing the atomic and molecular emissions, the electron temperature, electron density and rotational temperature were calculated to evaluate the characteristics of the laser-induced plasma. The correlation between the molecular emissions and coal combustion characteristics was also investigated, which was explained from the aspect of microstructural evolution obtained by Raman spectroscopy. The molecular emissions were gradually inhibited with increasing pyrolysis temperatures, however the molecules tended to distribute at a higher equilibrium temperature. The rotational temperatures increase by 1000–3300 K relative to the raw coal for chars prepared at pyrolysis temperatures of 293–1173 K. The reduction of C₂ and CN emissions, which are thought to be mainly caused by small aromatic rings and amorphous carbon structures, is consistent with the Raman spectroscopy results in that the microstructures evolve towards a more stable large aromatic-ring skeleton after pyrolysis. When the carbon conversion ratio is X_c = 50% for three kinds of coal, the R² values of the exponential fitting between C₂ emission and carbon conversion rate are all higher than 0.99.