Environmental Assessment of Thermal Coals

The environmental impact of trace elements during coal combustion relates to their emissions to air and water. Operators of power plants are required to conform to environmental standards through legislated statutory limits and operating agreements. The assessment of potential environmental impact is based on the measurement of atmospheric emissions and the leaching of trace elements in ash dams, when wet disposal is used, or at land fill sites. The assessment can only be carried out during combustion trials in a power station test. This is very expensive and difficult to organise particularly for new product coals. The overseas thermal coal market is very competitive and the environmental impact of trace elements is under increasing scrutiny particularly by Japanese utilities where air emissions are of major concern. This is due to the capture of volatiles in flue gas desulphurisation (FGD) units and their subsequent presence in the wastewater drainage. Prior knowledge of the potential environmental impact could therefore assist marketing.

In the absence of data from power station tests it is not possible to assess the potential environmental impact. There is however comprehensive data available from trace element mass balance studies on power stations and studies of the leachability of trace elements from power station fly ashes. These data could be used to relate air emissions and the leaching of trace elements directly to the trace element profile of the coal. This would overcome the need for power station tests and provide a means for assessing the environmental impact. 

OBJECTIVES

The aim of this project was to provide the means for rapidly estimating the potential environmental impact of trace elements of major concern from the trace element profile of the coal. This would be achieved by establishing factors which relate the trace element levels in the coal directly to their air emissions and the leaching from fly ash. An easy-to-use computer software program would be written to facilitate the rapid processing of the results.

CONCLUSIONS

From a series of trace element mass balance tests on power stations air emission factors, based on the mass percentage distribution of the elements in the flue gas, were established which enabled the air emission rates of trace elements of major environmental concern to be estimated using only the concentrations of the elements in the coal. The elements were antimony, arsenic, boron, beryllium, cadmium, cobalt, chromium, copper, fluorine, mercury, manganese, molybdenum, nickel, lead, selenium, vanadium and zinc.

Good recoveries in the mass balance tests were obtained for most of the elements. However the recoveries for boron, fluorine and selenium were low. These elements are volatile and it was evident from the mass balance data that they were not efficiently collected in the gas sampling train used to measure flue gas emissions. Air emission factors for these elements were therefore obtained from the difference between the mass flows of the coal input and the fly ash output.

A suite of test coals from a number of power stations and their corresponding fly ashes was used to validate the air emissions. However because measurements of flue gas emissions was not undertaken on these coals the validation could only be made for boron, fluorine, mercury and selenium by comparing the calculated emissions using the factors and the emissions calculated from the trace element mass difference between the coal and fly ash for the individual coals.

Good correlations (better than 0.9) were found for boron, fluorine and mercury. For selenium the correlation was only 0.61. Boron, fluorine, mercury and selenium are volatile elements and their presence in the flue gas is of major environmental concern.  Although the emission rates for the other elements could not be validated they were very low and consistent with the levels found in power station flue gas measurements.

A most significant finding was that the standard method for measuring trace elements in the flue gas was unsuitable for determining the emission of boron, fluorine and selenium. These elements are not efficiently trapped by the acidic solutions in the gas sampling train because they are present as acidic species in the flue gas. Because of this, the use of mass balance data was found to be a more reliable measurement of their emissions.

Factors for calculating the leaching of trace elements were established using a suite of power station coals and their corresponding fly ashes. Data for the Toxicity Characteristic Leaching Procedure and the ASTM standard method were obtained. The agreement between calculated and measured concentrations of trace elements in the leachates was generally to within a factor of three. Since the concentrations of most elements in the leachates were significantly below water quality guidelines the agreement was considered satisfactory.

A computer software package was developed to facilitate the rapid calculation of the trace element emissions to air and the levels in leach solutions from trace element concentrations in the coal.