Trace Elements and Their Mobility in Coal Ash/Fly Ash from Indian Power Plants in View of Its Disposal and Bulk Use in Agriculture

Abstract

The dynamic growth of coal consumption in India in order to meet the demand in power, along with high ash content in coal (-30-40% wt.) will result in generation of about 140 Mt/yr of fly ash (FA) by 2020. This creates a problem of its environmentally safe utilization and disposal. The use of FA in agriculture as soil amendment seems to be particularly attractive to India as high-volume low-technology application, and potential sink for almost unlimited amounts of coal combustion waste. In the chapter, the macro-and trace element mobility in FA has been analyzed and exemplified in field studies: (i) on impact of FA slurry pond on ground water quality; (ii) on effect ofF A-amendment on a crop yield along with trace element contents and uptake from FA-amended soil. With respect to major chemical and phase characteristics, FA from Indian power plants does not differ from that generated in other countries of the world. Trace element content in FA is of about an order of magnitude higher than in coal and markedly exceeds the average concentrations in soil. FA can be classified as alkaline aluminum silicate with predominantly low CaO content, and low buffering potential ratio BPR ranging from 0.64 to 4.25, mean <2. This suggests the susceptibility ofF A to acidification in time due to weathering transformations and developing the extensive trace constituents release from the disposed FA in the Delayed Release (ill) stage of leaching. The analysis of 1996 survey showed deep adverse alteration of ground water quality in the vicinity of FA slurry pond under operation that reflected the Washout (I) stage of leaching. Release of soluble macro-constituents was a major process, while the most of trace elements was within their stability fields in solid phase. Due to the weathering transformations, the change of leaching pattern in time according to Dissolution (II) and Delayed release (ill) stages is anticipated. The field trials with use as soil amendment of different doses of FA (from 25 to 500 t/ha) in acidic red and alkaline alluvial soils conducted by CFRI (CSIR) India in 1994-2000, showed increase of different crop yield up to 45-75% at FA dose of 200 t/ha, and from 16 to 33% at FA dose of 500 t/ha. The simultaneous increase of total and DTPA-extractable trace element concentrations in both types of soil was also Chemistry of Trace Elements in Fly Ash, edited by Sajwan et a/. Kluwer Academic/Plenum Publishers, 2003 25 noticed. Crops displayed diverse susceptibility to metal uptake: soybean > linseed > jowar > wheat. In the light of presented data, FA disposal in unprotected surface ponds is not environmentally safe. Application in agriculture seems to be a prospective sink for FA, though its large-area uncontrolled agricultural use may cause an irreversible soil or water contamination in the long-range period. The caution and pollution prevention principles suggest avoidance of FA application in acidic soils, and use it entirely in the well-buffered alkaline/neutral soils, with careful selection ofF A doses and cultivated crops.