Study on pyrolysis characteristics of pretreated high‑sodium (Na) Zhundong coal by skimmer-type interfaced TG-DTA-EI/PI-MS system

Various pretreatment methods has been used to upgrade the high-Na Zhundong coal to solve the inherent problem with its thermal conversion. To provide a better understanding of pyrolysis characteristics of the pretreated high-Na Zhundong coal, a system of thermogravimetry-differential thermal analysis coupled with mass spectrometry equipped with skimmer-type interface and with electron ionization and photoionization capability (TG-DTA-EI/PI-MS system), was employed to analyze qualitatively the pyrolysis characteristics of the pretreated high-Na Zhundong coals. Three types of coal samples including the raw coal, H2O-washed coal, and HCl-washed coal were pyrolyzed in this study. The total pyrolysis yield, evolved volatile gases composition and individual gas evolving behavior were evaluated. It was shown that the pretreatments affected insignificantly the total yield but did significantly the volatile gases composition, the individual gas yield and production rate. The larger aromatic compounds of C7H8O (anisole), C9H12 (propylbenzene), and C10H14 (butylbenzene) disappeared gradually after the coals were pretreated. Especially, the HCl-washing pretreatment reduced greatly the yield and production rate of inorganic gases of H2, CO, CO2, and aromatic compounds of C6H6O (phenol), C8H10 (ethylbenzene), whereas improved greatly those of the smaller aliphatic compound of C3H6 (propene). It was believed that the absence of the NH4Ac-soluble Na species and HCl-soluble Na species was a main factor to affect the volatile gases composition, the individual gas yield and production rate, through altering the reaction routine of bond breaking and cross-linking during pyrolysis.