Speciation of chromium in Australian coals and combustion products

Abstract

Trace elements in coal combustion ash products are potentially hazardous to the environment and some of them have high carcinogenic potency. They also have the potential to bioaccumulate providing the major limiting factor for safe disposal or beneficiation of the coal combustion ash products. The various forms and oxidation states of the trace elements may determine the ultimate environmental fate and health impacts. For example, in case of chromium, the trivalent Cr(III) is an essential nutrient while Cr(VI) is strongly carcinogenic. Moreover, Cr(VI) originates mostly from anthropogenic industrial sources and is classified as a Group A inhalation carcinogen by the US environmental protection agency (EPA), and is also classified as one of the ‘priority pollutants’ in the national pollutant inventory (NPI) in Australia. Knowledge of the chromium species in coals and ash by-products becomes important for conducting comprehensive risk assessment of their emissions from coal-based power plants. This study focuses on chromium speciation analysis of coals and ash by-products. US EPA methods 3060A and 7199 were used for Cr(VI) determination in samples. To quantify Cr(VI) and total chromium in the samples, Ion Chromatography with Post Column Derivatisation & Colorimetric Detection and Inductively Coupled Plasma Mass Spectrometry (ICP–MS) were used, respectively. The more toxic hexavalent chromium was detected in most of the fly ash samples with concentrations up to 1.38ppm. Enrichment of concentration of total Cr and Cr(VI) was found with respect to decreasing particle size range of fly ash particles. The bottom ash samples contained primarily the trivalent form of chromium. Laboratory ash and char were also produced by carrying out combustion and pyrolysis experiments on two different types of coal in a laboratory-based drop tube furnace. Chromium in laboratory-produced char and ash was mainly in trivalent form. Possible gas phase chromium reactions likely to occur in the combustion zone are also discussed.