Effect of rare earth oxides on the formation of semi-char from low-rank coal pyrolysis: a comparative study based on X-ray diffraction and Raman analysis

Abstract

ABSTRACT Rare earth elements that exist in coal have the potential to influence the pyrolysis of coal that needs to be understood. This study provides understandings on the effect of rare earth oxides, cerium oxide (CeO2) and lanthanum oxide (La2O3), to the formation of semi-char during low-rank coal pyrolysis. Various parameters including amorphous carbon fraction (C amo), aromaticity (f a), and crystalline indexes (average crystalline diameter L a, average crystalline height L c, inter-layer spacing d 002) were examined via quantitative X-ray diffraction (XRD) analysis and Raman analysis. The use of CeO2 enhanced the f a (e.g., from 62% to 68% at 400°C) but decreased the C amo (54–52% at 400°C), while La2O3 had contrary effect, indicating that CeO2 promoted the produce of crystallite carbon and the aromatization of semi-char but La2O3 suppressed that. Moreover, the presence of La2O3 caused decrease of L a (from 1.738 to 1.315 nm at 550°C) and L c (from 1.036 to 0.847 nm at 550°C) combined with the increase of d 002 (0.348 to 0.358 nm); while CeO2 caused increase of L a and d 002 but decrease of L c. The correlations between XRD and Raman analysis were investigated to further confirm the catalytic performances of CeO2 and La2O3. The different effect of the two catalysts was likely because of the different catalytic performances to the aliphatic side-chains grafted on the edges of aromatic layers. The study suggests that the structure of semi-char from low-rank coal pyrolysis can be turned by the addition of rare earth oxides.