Structural transformations and hydrocarbon generation of low-rank coal (vitrinite) during slow heating pyrolysis

Abstract To explore the liquefaction behaviors of low-rank coal vitrinite, the transformations accompanying vitrinite pyrolysis were examined at 10 °C per hour between 350 and 520 °C. Structural characterization was every 10 °C for FTIR and every 20–30 °C for 13 C NMR and chemical analysis. The gaseous yield of C 1 –C 5 was evaluated by gas chromatography (GC). From the transformations, identified by these various techniques, the structural evolution at this heating rate generally occurred in three distinct stages: 350–390, 410–470, and > 490 °C. There was a linear increase of 1 f a (FTIR-determined aromaticity) between 410 and 470 °C. The hydroxyl content decreased while the aro-OH signal increased at temperatures 1 f a during the 490–520 °C. The ratio of aromatic bridge carbon to aromatic peripheral carbon ( x BP ) is an indicator for the size of the basic structural size (BSU). The x BP of vitrinite (0.21) was similar to naphthalene ( x BP  = 0.20) and smaller than the value for anthracene ( x BP  = 0.28). At 440 and 510 °C, the x BP is 0.28 and 0.32 respectively, corresponding to anthracene or phenanthrene ( x BP  = 0.31), but x BP  = 0.37). The increasing x BP demonstrates the aromatization as the slow pyrolysis process continues. There was a significant reduction in the oxygen content between 350 and 390 °C, then the content slowly increases between 390 and 430 °C (due to preferential loss of hydrocarbons), and finally with a slow reduction > 430 °C. When > 470 °C, FTIR data indicated that much of the oxygen was in ether form. The highest contribution of OH (FTIR), the highest C 1 –C 5 hydrocarbon generation rates, and a transition in the aliphatic carbon occurs at 380, 470, and 510 °C. The temperature programmed pyrolysis work here is of interest in coal utilization processes where there is a slow-heating rate.