Chlorine and Other Halogens in Coal
AUTHOR: Robert M. Davidson
DATE: September 1996
In the Perspectives series of reports from IEA Coal Research, Chlorine and other halogens in coal reviews studies on the nature of the halogens in coal. The emphasis of the report os mainly on chlorine and, to a lesser extent, fluorine.
The report begins with a description of the techniques used to determine the concentrations of the halogens in coal. Although there are relatively few reported problems, those that exist reflect the common problems associated with sample preparation, These include the loss of volatile elements or their insufficient release from the coal.
Knowledge of the concentration of an element in coal is only part of the sum of information needed to assess its technological behavior, environmental impact, by-product potential, and geological significance. In addition, we need to know the chemical state in which the elements exist: the mode of occurrence. This is discussed for fluorine and in greater depth for chlorine.
Much of the interest in the chlorine content of coal has arisen from its reported corrosive effects in boilers. The longest chapter of the report is devoted to this topic but it has to be admitted that the evidence from the information currently in the public domain is inconclusive. The overall impression is that, although chlorine, by itself, is not responsible for most boiler corrosion, it is implicated among other factors.
The last chapter discusses the possibilities of removing fluorine and chlorine from coal. Fluorine, being associated with the mineral fraction of coal, can usually be reduced by the about the same level as the ash removal. In theory, chlorine should be removed from coal by washing but practical applications have not yet been fully proven technically. Mild heat treatment has also been proposed but again no commercial processes have been developed.