This review embraces approximately 100 publications on chlorine (Cl) in coal. Topics reviewed in this paper include: (a) historical introduction; (b) some peculiarities of Cl environmental geochemistry; (c) estimation of the Cl coal Clarke value; (d) occurrence of high-Cl coals; (e) mode of Cl occurrence in coal; (f) factors influencing the distribution of Cl in coal; and (g) origins of Cl in coal.
- The world average Cl contents in coals (coal Clarke of Cl) for hard and brown coals are, respectively, 340 Β± 40 and 120 Β± 20 ppm. The average Cl content on an ash basis is 1435 ppm which is much more than the Clarke value for sedimentary rocks (150 ppm); hence, chlorine is, on average, a highly coalphile element. Just this alone shows that the authigenic sorption fraction, Cl sorb , must be dominant in coals.
Aside from "normal" coals with average Cl contents, there are "saline" coals, strongly enriched in Cl, up to 1 wt.% and more. Although there are different genetic concepts, it seems that such coals were mainly formed in epigenesiswhen coal beds were influenced by basinal chloride brines (often by hot ore-forming brines, containing also some ore elements such as As, Pb, Zn, etc.).
The modes of Cl occurrence in coal are surprisingly varied. Among them are found inorganic salt-like Na and other chlorides, as well as the Cl-bearing silicates, carbonates, sulfides, as well as dissolved chlorides in pore moisture. Organic-associated Cl seems to predominate in coal. It may consist of two different types. One (minor) site ("true" Cl org ) may be as covalent-bonded Cl in coal organic macromolecules. The major organic Cl is represented by "semi-organic" Cl, as anion Cl -, sorbed on the coal organic surface in pores and being surrounded by pore moisture. These are HCl-complexes bonded with bases, such as quaternary nitrogen.
If Cl in coal is mainly of syngenetic origin, a "true" Cl org form may be of value, and Cl is concentrated in low-ash gelified coal macerals. If coals were submerged by epigenetic impact from basinal brines, Cl min (NaCl and other salts) and "semi-organic" Cl sorb forms have to be dominant. In such instances, Cl may be enriched in fusain and in high-ash coals.
There is a complicated relationship between Cl-contents in coal and coal rank. On the one hand, the coals subjected to epigenetic brine influence are, as a rule, bituminous coals that have been submerged to considerable depths where basinal brines exist. On the other hand, if coals have a rank higher than low volatile bituminous A (C daf more than 86%), the Cl content decreases due to decreasing sorption capacity of the high carbonized coal organic matter.