Initial Deposition Feature during High-Temperature Pressurized Pyrolysis of a Typical Zhundong Coal

This work clarifies the diffusion and initial deposition behavior of volatilizable elements during pressurized pyrolysis of a typical Zhundong high-basic (i.e., high proportion of alkali and alkaline earth metallic species, AAEMs) coal. The concentrations of AAEMs in raw coal and its derived char as a function of pyrolysis temperature and pressure were duly measured; besides, the features of innermost deposit in specific areas of pyrolysis reactor were particularly investigated. The release of AAEMs was strongly related with their occurrences in coal. Specifically, Na, mostly in a water-soluble form in raw coal, still existed as solid and/or liquid phase below 1000 degrees C and thus could be partially adsorbed and retained in the porous structure of char, which could be enhanced by elevated pyrolysis pressure with the average weight loss of Na in the range of 16.0-29.1 wt %. Nevertheless, the diffusions of K (6.8-17.3 wt %), Ca (16.4-29.7 wt %), and Mg (10.7-21.2 wt %) were substantially dependent on the decomposition of minerals and the diffusive force of volatiles. The species (mainly Na, K, Ca, Mg, S, and Cl) in the pyrolysis gas were responsible for the generation of corrosive aerosol during the pyrolysis of high-basic coal. Elements with various forms in the aerosol could mutually combine and form deposition precursor (mainly as NaCl, Ca-S-O, MgO, Na-S-O, etc.). The deposits were shaped at lower temperature (< 423 degrees C) largely by fine carbonaceous substances, inherent minerals, and trace amounts of Ca-bearing matter. Abundant crystals, such as NaCl in the aerosol, were deposited on the probe at 514-932 degrees C;however, the elements, in deposition tended to form eutectics, e.g., NaSiO3, Na-Si-Al-O, and so on, at higher temperature (932-1117 degrees C). Subsequently, the diffusion of AAEMs in different forms resulted in the initial deposition on the surface of the probe after experiencing a series of physical and chemical processes and gave an appreciable ash-fouling propensity during pressurized pyrolysis. Moreover, the remaining minerals in char with high proportion of basic compounds were apparently dissociated with char; such minerals demonstrated sintering and agglomerated morphologies, posing a latent threaten to the subsequent utilization of char.