Technical Market Support » Thermal Coal
Coal-fired power stations produce large quantities of waste ash and fly ash represents around 80-90% of the total produced. Fly ash contains many trace elements of environmental concern that may become mobile in the ash dam water resulting in the potential to impact on the surrounding environment. The leaching of trace elements must therefore be monitored to assess this potential impact.
Power stations use laboratory-based standard leach tests (USEPA, ASTM) on the fly ash to assess the potential release of trace elements, the main objective being to determine whether the ash meets landfill quality guidelines. These tests however do not simulate ash dam environments and cannot be used to gain knowledge of the potential environmental impact of the ash dam on the surrounding ecology. Most of the leach data obtained are proprietary to the individual power stations and not generally available in the public domain. There is no general information on fly ash leaching which can be drawn on to assess the leaching characteristics of fly ash produced from Australian thermal coals.
With the increasing requirement for coal suppliers to provide data of leachability of fly ash in their contracts, this knowledge gap is creating a degree of uncertainty on the suitability of Australian coals to meet environmental regulations imposed on power station operations. There is considerable variability in the leaching characteristics of the fly ashes produced in Australian power stations; this is dependent on the different combustion conditions, the chemistry and the prevailing disposal conditions.
A database on the leaching of trace elements from fly ash was therefore considered essential to fill the knowledge gap on this important environmental issue. A major feature of the database should be that it provides reliable comparative data on the long term leaching characteristics of trace elements under ash storage site conditions.
The objectives of this project were the following:
- To establish a database on the leaching of environmentally sensitive trace elements from fly ashes produced by Australian power stations burning bituminous coal.
- To generate long term leaching characteristics under ash dam conditions using column leaching
- To compare standard leach test data with column leach test data.
- To determine the effect of combustion conditions on the leachability of trace elements.
- To assess the ecotoxicology of the leachates.
In order to establish a comprehensive database, fly ash samples derived from bituminous coals from 9 power stations in Queensland, New South Wales and Western Australia were used. Fly ash samples obtained from the Australian Combustion Technology Centre (ACTC) Pilot Scale Combustor were used to test the effect of combustion conditions. The column leach tests were carried out over periods of up to 24 months.
Conclusions
Leach rate data for 32 elements from the column experiments showed large variations between power station fly ashes. ANZECC water quality guidelines were used as reference values for elements of environmental concern. For some elements (boron, selenium, molybdenum and vanadium), some fly ashes required elution volumes up to 40 times larger than others to leach to compliance levels. Results showed that leachates from all power station fly ashes could pose environmental compliance problems with these elements. Some of the acidic ashes posed problems with cadmium, arsenic, copper, nickel and zinc.
Relationships were found between the amount of selenium leached and the amount of iron and calcium in the fly ash. The amount of selenium that was leached was inversely related to the amount of iron and calcium in the fly ash.
Chalcophile elements including copper, zinc, lead and arsenic were prominent among those with the highest percentage leached values. This is probably because of their volatility at combustion temperatures and consequent enrichment on the surface of the ash particles.
Relationships were also established between the pH of the fly ash leachates and the alkaline and acidic components of the ashes. An increase, by a factor of about 15, in the ratio of alkaline to acidic oxides in the ash correlates with a pH change from 4 to 12 in the leachate.
This allowed predictions to be made of the expected leaching behaviour of the ash.
The column leaching studies recorded delays in the appearance of some elements, particularly arsenic, barium, boron, molybdenum, selenium and vanadium which for some fly ashes had maximum leachate concentrations after several liquid:solid volumes had passed through the columns. Many of these elements may be present as compounds which are essentially insoluble in the initial leachate pH caused by the presence of other species; as the leaching progresses, this condition can change and result in a leachate matrix in which they become soluble. This emphasises the importance of column leaching in assessing the long-term leachability of some elements.
Total amounts of trace elements leached using the standard batch methods and the column leach showed that the TCLP test, which is commonly used by power stations, gave results that were more than three times higher than the column results for alkaline ashes. For acidic ashes the TCLP data agreed with the column data.
Column leach tests on the fly ashes produced by the ACTC pilot scale combustor at two different temperatures showed differences in the leaching behaviour between ash pairs. Generally it appeared that the higher temperature ashes leached less elements of environmental concern than the lower temperature ashes.
Bioavailability studies on column leachates showed that the toxicity changed as the leaching progressed. Although aluminium and selenium were present at levels in excess of the ANZECC guidelines, there was no inhibition of algal growth until arsenic levels rose to about 5 times the guideline. This underscores the need to utilise such tests to properly evaluate trace element toxicity of ash dam effluents.