Linking Thermoplastic Development and Swelling with Molecular Weight Changes of a Coking Coal and Its Pyrolysis Products

摘要

The thermoplastic development of an Australian coking coal was investigated by linking thermal swelling with changes in molecular weight of its pyrolysis products. Coal thermal swelling was investigated together with volatiles evolution and characterization of generated volatiles (including volatile tar and light gases). The molecular weight distributions of coal and its solvent extracts were measured by using laser desorption/ionization time of flight mass spectroscopy (LDI-TOF-MS). Solvent extraction (by acetone and tetrahydrofuran (THF)) was initially used on the raw coal to aid interpretation of thermoswelling by volumetric expansion measurements. The removal of 2% solvent-soluble matter from the raw coal (the mobile phase) reduced its swelling extent during heating by up to 22% (from 86% down to 68% and 64% for acetone- and THF-residues, respectively). Volatile release after solvent treatment remained unaffected. This suggested that the majority of the coal's swelling behavior could be attributed to the formation of heat-generated liquid matter (the Metaplast) during pyrolysis. Broad molecular weight changes were found in the solvent extractable component (metaplastic material extracted by acetone and THF) of the semicoke. Prior to softening (350 degrees C), the extractable components were composed of molecules mainly <500 Da. The upper limit in molecular weight distribution of solvent extracts increased significantly to 1800 Da at the onset of swelling (400-450 degrees C) and decreased back to similar to 500 Da at the end of swelling (500 degrees C). The spectra showed that the volatile tar and acetone extract (the light solvent extract) consisted of similar repeating structures separated 12-14 Da apart. As the treatment temperature increased, the molecular weight distribution of volatile tar increased in molecular mass, approaching that of the acetone extract distribution (similar to 600 Da). The THF extract molecular weight distribution was a mixture of 12-14 and 24 Da repeating units which only became apparent at molecular weight above 600 Da. The LDI-TOF-MS analysis of the solid coal showed that it contained a distribution of molecular structures centered at 2000 Da and spanning between 500 and 7000 Da. This raw coal spectrum also contained multiple repeating mass lines every 24 Da apart. Overall, these results suggested that the coal consisted of complicated structures which subsequently degraded into smaller fragments capable of forming a complex intermediate liquid phase and a distribution of lighter volatile tar species.