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

Abstract

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 °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 °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 °C; however, the elements in deposition tended to form eutectics, e.g., NaSiO 3 , Na−Si−Al−O, and so on, at higher temperature (932−1117 °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.

Release and Transformation Behaviors of Sodium, Calcium, and Iron during Oxy-fuel Combustion of Zhundong Coals

“…The other minor chemical forms of Ca, water-soluble and insoluble forms, present negligible variation with O 2 content. The release and transformation of AAEMs are largely related to their occurrence modes with coal . As illustrated in Figure d, the Fe of the insoluble form first increases apparently with the increasing O 2 content in oxy-fuel combustion, and then becomes asymptotically constant, whereas the Fe of the HCl-soluble form shows an opposite evolution compared with the insoluble Fe.…”

“…Only a small quantity of water-soluble calcium is released or transformed into other chemical forms as the combustion temperature is beyond 1000 °C. However, Lin et al 54 found out that the release of Ca was substantially enhanced with the pyrolysis temperature up to 1000 °C and then kept constant hereafter, while magnesium (Mg) presented a different varying tendency. The transformation behaviors of AAEMs were strongly influenced by the corresponding occurrence modes and thermal conversion conditions.…”

“…The release and transformation of AAEMs are largely related to their occurrence modes with coal. 54 As illustrated in Figure 11d, the Fe of the insoluble form first increases apparently with the increasing O 2 content in oxy-fuel combustion, and then becomes asymptotically constant, whereas the Fe of the HCl-soluble form shows an opposite evolution compared with the insoluble Fe. With the increase of O 2 content, one part of HCl-soluble Fe is further discharged into gaseous phase, while some HCl-soluble Fe left is converted into the insoluble form.…”

Catalytic effect of alkali and alkaline earth metals in different occurrence modes in Zhundong coals

“…For potassium, insoluble potassium is the most abundant form, and the total content of potassium is relatively lower than the total content of sodium. Nevertheless, the results for calcium and magnesium are different from those of previous research . This difference occurred because NH 4 Cl was used as an alternative buffer solution instead of NH 4 OAc solution to determine the contents of ion‐exchangeable Ca and Mg species in the two coals.…”

Effects of inherent alkali and alkaline earth metals on nitrogen transformation during steam gasification of Shengli lignite

“…It is obvious that alkali and alkaline earth metals are removed by acid-washing. There are four types of alkali and alkaline earth metals found in coal: water-soluble, ion-exchangeable, hydrochloric soluble and stable forms (Lin et al 2016;Bai et al 2017). Among these four types,hydrochloric-soluble alkali metals and alkaline earth metals is organic salt presenting as coordination form in coal structure.…”