Chromium In Ash Research Articles

Direct speciation of chromium in coal combustion by-products by X-ray absorption fine-structure spectroscopy

Chromium X-ray absorption fine-structure (XAFS) spectroscopy has been used to determine the relative amounts of Cr(VI) and Cr(III) in coal combustion by-products, principally fly-ash and bottom ash. The method, which is based on the relative heights of the pre-edge peaks for the different Cr oxidation states in X-ray absorption near-edge structure (XANES) spectra, can be used to speciate as little as 50 ppm of chromium in ash. The results indicate that the fraction of Cr(VI) oxidation state present in combustion ash samples from most commercial combustion plants is typically below or close to the detection limit (approximately 3–5% of the total chromium). There are exceptions, however, that have been found to reach as high as 20%. Such findings provide justification for a reappraisal of whether or not chromium should be considered a significant hazardous air pollutant in coal combustion.

96/02228 Determination of chromium oxidation states in coal combustion products by XAFS spectroscopy

Chromium XAFS spectroscopy has been used to determine the relative amounts of Cr(VI) and Cr(III) in ash samples obtained from coal combustion. The method, which is based on the relative heights of the pre-edge peaks for the different Cr oxidation states in XANES spectra, can be used to speciate as little as 50 ppm of chromium in ash. The results indicate that the fraction of Cr(VI) oxidation state present in combustion ash from commercial combustion plants is typically at or close to the detection limit (approx. 3% of the total chromium). Such findings provide justification for a reappraisal of whether or not chromium should be considered a significant HAP in coal combustion.

Speciation of arsenic and chromium in coal and combustion ash by XAFS spectroscopy

While there are a variety of methods to determine the concentration of trace elements in coal and ash, there have been few attempts to determine the speciation of these elements. In this paper, it is demonstrated that X-ray absorption fine structure (XAFS) spectroscopy is capable of providing speciation information at realistic concentration levels of 10–100 ppm, provided a solid-state multielement germanium detector is used. The initial studies have concentrated on arsenic and chromium. For arsenic, two principal forms of occurrence are observed in coal: As contained in pyrite and As(V) in arsenate (AsO 4 −3). In one coal (Pittsburgh #8, DECS-12), the As is present as arsenopyrite. The As in pyrite is readily oxidized under ambient conditions to the arsenate form. In combustion ashes, all arsenic is in the form of arsenate, with at least two forms of arsenate present; As in aluminosilicate slag and calcium orthoarsenate are possibilities. Chromium in coal and in ash is observed to be present predominantly (> 95%) in the Cr +3 state. Chromium oxyhydroxide is the standard compound whose XAFS spectrum most closely resembles that of the chromium in coal, while the chromium in ash may be incorporated into the aluminosilicate slag phase.