Geochemistry of leachates from coal ash

Abstract

The combustion of coal around the world for power generation produces huge volumes of fly ash. In Europe alone this amounted to about 40 Mt in 2000 of which less than 50% was utilized. The waste ends in lagoons, ash mounds, and landfill sites. Coal ashes have high concentrations of many trace elements, some of which are of environmental concern. Although the origin of elements in coals is not considered in this chapter, other aspects of the geochemistry are, and in particular the location of elements within the coal as this influences the behaviour of elements during combustion. During combustion many elements are volatile and are concentrated on the surfaces of the ash particles. Analyses of input coal and combustion residues from Eggborough power station (UK) demonstrate retention of the majority of elements in the solid combustion products, and analyses of size-fractionated fly ash have enabled the percentage surface association to be calculated, which for elements such as As and Mo is considerable. A consideration of the general leaching behaviour leads to the conclusion that it is the surface-associated elements that are most susceptible to leaching in the aqueous environment. The pH is an important control on trace element mobility in water, and in leachates from fly ash ranges from 3.3 to 12.3. Highsulphur coals generate acidic leachates, but not exclusively as the laboratory and field data demonstrate in case studies on UK coals. Batch and column leaching tests on fly ashes are reviewed and data presented for fresh fly ash and weathered fly ashes from two UK mounds dating back 17 and 40 years. The weathered ashes do have lower leachate concentrations than the fresh ash, but in spite of their ages they would not be considered to be inert. The batch leaching tests are of value in simulating high liquid-to-solid ratios encountered in ash lagoons, whereas the column leaching tests relate more closely to ash mounds. Finally, the results of field studies are reviewed and data presented for samples from boreholes in the two ash mounds. Analyses of the ash and extracted porewaters demonstrate depth-related changes due to reaction of the ash with the infiltrating water and whether or not equilibrium was established. Calculations demonstrate that the porewaters achieve saturation with respect to gypsum at depth in the boreholes. Infiltration over the years has led to detectable changes in the concentrations of some of the major elements in the bulk ash, such as Ca, and this enables realistic infiltration rates to be calculated. However, there are not comparable changes in the concentrations of the trace elements in the ash because the rate at which elements are being removed in solution is not sufficiently great.

Boron and Strontium Isotopic Characterization of Coal Combustion Residuals: Validation of New Environmental Tracers

“…In fly ash, a minor portion of strontium is potentially leachable due to surface adsorption, but the bulk is associated with the silicate phases and therefore the leaching is relatively slower. 24,26 Another consideration would be the addition of lime (CaO) or CaCO 3 with a different Sr isotopic ratio in SO 2 scrubbers as part of the Flue Gas Desulfurization (FGD) process. Such an addition could have an impact on the Sr isotopic fingerprints of leachates originated from these CCRs (i.e., CaCO 3 typically contains 500−1500 ppm of Sr).…”

Geochemical Controls of Coal Fly Ash Leachate pH

“…When CCPs are mixed with water, the initial pH of the mixture can range from about 4 to greater than 12. Spears and Lee (2004) generalized that about 70% of all fly ashes produce a neutral or alkaline leachate. Coal fly ash derived from U.S. bituminous coal can yield an alkaline or acidic reaction when initially mixed with water.…”

“…Alkaline leachate from fly ash was the result of the hydrolysis of Ca and Mg oxides that form during the combustion of coal. Spears and Lee (2004) generalized that acidic fly ash leachate can be attributed to the sorption of SO 2 or the condensation of sulfuric acid from the gas stream onto ash particle surfaces. Another possible source of acidity could stem from the hydrolysis of Al 3+ derived from aluminum sulfates.…”

Partitioning of major and trace inorganic contaminants in fly ash acid mine drainage derived solid residues

“…In China, thermal power plants produce nearly 160 million tons and over 50 % of the fly ash is used in different applications such as bricks manufacture, construction of dams and in the cement industry (Okafor and Opuene, 2007). FA tends to accumulate toxic elements such as heavy metals at the high temperatures involved during its generation Spears and Lee, 2004) and is considered an environmental hazard in South Africa and other parts of the world. Anions (Cl -, SO 4 2-), oxy-anions of Se, As, Mo, B and Cr and cations (Al, Fe, Na, K, Ca, Sr, Ba, Zn, Cu, Cd and Mg) are leached from the ash heaps by the wastewater derived from the ash slurry or by subsequent infiltration by rain upon disposal (Adriano et al, 1980;Eary et al, 1990;Mattigod et al, 1990;Reardon et al, 1995;Nameni et al, 2008).…”

“…Anions (Cl -, SO 4 2-), oxy-anions of Se, As, Mo, B and Cr and cations (Al, Fe, Na, K, Ca, Sr, Ba, Zn, Cu, Cd and Mg) are leached from the ash heaps by the wastewater derived from the ash slurry or by subsequent infiltration by rain upon disposal (Adriano et al, 1980;Eary et al, 1990;Mattigod et al, 1990;Reardon et al, 1995;Nameni et al, 2008). Of main environmental concern are the toxic trace elements, Pb, Cr, B, Mo, As and Se (Adriano et al, 1980;Carlson and Adriano, 1993;Abbott et al, 2001;Spears and Lee, 2004) that are likely to leach upon disposal. The pH of a fly ash suspension for example in water can vary depending on the S content of the parent coal (Plank and Martens, 1974).…”