Microstructural evolution of coal to char after pyrolysis using laser-induced breakdown spectroscopy and Raman spectroscopy

Laser-induced breakdown spectroscopy (LIBS) and Raman spectroscopy were used to analyze the microstructural characteristics after coal pyrolysis. Two coals and their demineralized coals were pyrolyzed at a fixed-bed furnace. LIBS molecular spectra of demineralized coal should be first reported. Experimental results show that the decrease of Raman baseline drift revealed by Raman spectroscopy correlates with the dissipation of substituent, aliphatic, and carbonyl groups with increasing pyrolysis temperatures. Minerals in coal slightly suppress the baseline drift before pyrolysis, but it accounts for less than 10% variation of the α. The LIBS and Raman spectra data show that coal undergoes dry degassing, substituent and aliphatic decomposition, and aromatic-ring condensation as the pyrolysis temperature increases. In the combustion test, the demineralized char has an analogous combustion rate at α ≈ 0 since the char has a similar structure. The minerals in raw coal can improve the combustion rate by at least 39% relative to the demineralized coal. The catalytic effect caused by minerals is more pronounced than thermal inactivation caused by stable structural dissipation from 673 to 1273 K. Raman spectroscopy and LIBS offer fresh insight into the coal-maceral transformation, which provide excellent tools to diagnose structural change from quasi-coal to quasi-char rapidly.