Stanford Synchrotron Radiation Lightsource An Office of Science User Facility

The element chromium is found in the environment in two common forms: Cr(VI), which is easily absorbed by the human body, and Cr(III), which is not. The first of these in the form of chromates can have severe adverse effects on the human body, including cancerous tumor formation and gene damage.  Normally Cr(VI) forms are not present in the approximately one billion tons of coal used annually for electricity generation in the U.S., however, a fraction of the Cr(III) in coal can become oxidized during coal combustion ending up as a Cr(VI) component in fly-ash, the major waste product from coal combustion. 

Chromium forms in coal and wood and their converted forms in fly-ash, the major product of coal and coal/wood combustion. (Image courtesy F. Huggins et al.)

A recent XAFS study conducted at SSRL Beam Line 4-3 by a team of researchers from KEMA and the University of Twente in the Netherlands and the University of Kentucky in the U.S. sought to determine how the chromium in fly-ash—which is often stored in isolated surface piles or in impoundments—behaves both during coal combustion and in ash disposal situations.  Their work determined that Cr(VI) as a percentage of the total Cr was less than 10% in fly-ash when derived from commercial pulverized combustion of bituminous coals. However, it was significantly higher (up to 30%) in ash from combustion of subbituminous and other lower-rank coals and also from coal and wood co-firing. 

The results indicate that the combustion conditions, particularly the air/fuel ratio, are important in determining the percentage of chromium as Cr(VI), and that leachable Cr(VI) contents in the ash could be minimized by better control on the excess air present during combustion and possibly by additives that promote the formation of insoluble chromate species.