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

To explore the liquefaction behaviors of low-rank coal vitrinite, the transformations accompanying vitrinite pyrolysis were examined at 10 degrees C per hour between 350 and 520 degrees C. Structural characterization was every 10 degrees C for FTIR and every 20-30 degrees C for C-13 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 degrees C. There was a linear increase of (1)f(a) (FTIR-determined aromaticity) between 410 and 470 degrees C. The hydroxyl content decreased while the aro-OH signal increased at temperatures < 440 degrees C. There was a slow increase of (1)f(a) during the 490-520 degrees 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 degrees C, the X-Bp is 0.28 and 0.32 respectively, corresponding to anthracene or phenanthrene (x(Bp) = 0.31), but < pyrene (x(Bp) = 0.37). The increasing xBp demonstrates the aromatization as the slow pyrolysis process continues. There was a significant reduction in the oxygen content between 350 and 390 degrees C, then the content slowly increases between 390 and 430 degrees C (due to preferential loss of hydrocarbons), and finally with a slow reduction > 430 degrees C. When > 470 degrees C, FTIR data indicated that much of the oxygen was in ether form. The highest contribution of OH (FTIR), the highest Cr-CB hydrocarbon generation rates, and a transition in the aliphatic carbon occurs at 380, 470, and 510 degrees C. The temperature programmed pyrolysis work here is of interest in coal utilization processes where there is a slow heating rate.