Environmental Aspects of Trace Elements in Coal

There is an urgency to increase our knowledge of trace elements in coal and of their role during mining, treatment and usage. Currently the most important aspects are those related to the environment, often because of fears of untoward effects. It is expected that this compelling interest will continue for at least a decade, and indeed information on new coals will be needed, in some cases to satisfy guidelines or more stringent requirements. Hence, this book has been planned to give a comprehensive view of environmental aspects of trace elements in coal. It is our aim to provide a reasonably balanced account. Although every effort was made to be objective, there is an inevitable influence of our experience. The choice of a small volume, rather than an encyclopedic work, means that some people will perceive the absence of topics which they would have chosen.

The contents of trace elements in coals are clearly relevant to any environmental aspect during mining, beneficiation and usage. There are published data for 66 trace elements in coals, surely not a surprising fact ‘for a naturally occurring material formed under varying conditions over a long period’ (Swaine, 1985). There is a comprehensive account of trace elements in coals, including tabulated data for the 24 elements of environmental interest (Swaine, 1990). Results should be stated on a coal basis and in order to avoid the error of inferring that ash is an inherent part of coal, reference should be made to mineral matter in coal and to ash yield, a product of the heating of coal under stated conditions at a specific temperature.

Coal utilisation, especially coal combustion, causes the release of many inorganic elements into the environment. Various pollution control procedures and devices are used by the coal industry to minimise the release of these elements. Selective mining and coal cleaning procedures reduce the amount of the inorganic constituents in coal prior to combustion. Electrostatic precipitators, chemical additives, baghouses, and flue-gas scrubbers reduce particulate and element emissions after combustion.

Coal is known to concentrate many elements, for example Ge, U, and Se (Goldschmidt, 1954; Swaine, 1990). This is partly due to the interactions of organic matter in a reducing environment with solutions entering the swamp, precipitation of minerals, breakdown of other minerals, along with the concentrating effect of some plants and the potential for elements to bind chemically to coal’s organic components. Geology plays an important role in the nature, concentration and distribution of elements in coal. The geochemical nature of a coal seam is strongly influenced by geological parameters (Zubovic, 1976; Bouska, 1981; Swaine, 1975, 1990).

Geochemistry of coal seams is variably affected by weathering and natural heating. Weathering of coal implies natural oxidation taking place at low temperature (< 30°C) for several years. In contrast, natural heating is a high temperature event and occurs due to implacement of plutonic or invasion of volcanic rocks into coal seams often for short durations or self-burning of coal seams due to forest fire or lightning.

Concern over potential environmental and health effects of trace elements has grown in recent years. In the United States, the Clean Air Act Amendments of 1990 identify 189 hazardous air pollutants (HAPs), of which 12 are elements and their compounds found in coal, usually in trace amounts. These elements are Sb, As, Be, Cd, Cl, Cr, Co, Pb, Mn, Hg, Ni and Se. In addition to these specific elements, radionuclides are also listed as HAPs; these too are known to occur naturally in coal. Finally, F, in the form of hydrofluoric acid, is listed.

In the Netherlands about 40% of the electricity is generated by coal nowadays. Only imported bituminous coal is fired. Typical values for this coal are a calorific value of 27 MJ/kg, an ash content of about 11% and a sulfur content of 0.7%. Pulverised coal-fired dry bottom boilers are installed exclusively in the Netherlands. The flue gases are cleaned by high-efficiency cold-side electrostatic precipitators (ESPs) and nowadays they are also cleaned by flue-gas desulfurisation (FGD) installations of the lime(stone)/gypsum process in all large coal-fired power plants.

Coal utilisation is an important source of trace-element emissions to the atmosphere (Clarke and Sloss, 1992). In a typical pulverised coal-fired power station combustion takes place in a furnace operating at temperatures above 1400°C. Coal is injected into the furnace and ignited while in suspension. As the particles are heated, volatile matter is vaporised and combustion occurs. Minerals undergo thermal decomposition, fusion, disintegration, and agglomeration. The final solid products of combustion are usually spherical ash particles, which may subsequently undergo further processes such as coalescence with other particles or expansion due to internal gas release. The formation of molten ash droplets marks the highest temperatures, and a significant fraction of the volatile forms of elements will exist in the gas phase. Some larger mineral particles may be only partly melted, and refractory minerals with high melting points may escape melting regardless of size. The main formation mechanism of coarse ash particles (> 2 µm) is carry-over of a proportion of the mineral matter in the feed coal. A portion of the non-combustible material is retained in the furnace as either slag or bottom ash. The rest of the inorganic material exits in the flue gases as flyash and vapour.

Increasing industrialisation, worldwide, has led in the past 50 years to a major increase in the global production of energy. Most of this energy is produced from the combustion of the fossil fuels coal, oil and natural gas. Emissions into the atmosphere of particular concern to plume transport and dispersion, from tall stacks, are the gaseous products SO₂, NO and NO₂. In addition significant quantities of fine particles may be also emitted into the atmosphere.

Trace elements in the atmosphere are mostly associated with particulates, although there may be some in the gaseous phase. There are diverse sources, natural and anthropogenic. Amongst natural sources are the weathering of rocks and soils, volcanism, erosion of metal-rich surface deposits, thermal spring activity, reactions at water surfaces, forest fires, biological methylation and plant growth (Swaine, 1984). Anthropogenic sources include mining operations, smelting and other industrial activities, combustion of wood, oil and coal, waste incineration, agricultural operations, tyre- and engine-wear and cremation (Swaine et al., 1989). Enhanced inputs may occur near sources, but these are diluted by wind action which disperses trace elements long distances, thereby adding to background concentrations.

Much has been written about flyash probably mainly because it is the most important by-product of the combustion of coal. During and after combustion, trace elements in the coal are redistributed into the vapour phase, bottom ash and flyash (Figure 10.1), the latter being the ultimate prime site for most trace elements. Some of the inorganic products from the decomposition of the mineral matter remain in the boiler where they flow down and are removed as bottom ash. Most of the particulate matter (flyash) is swept along with the combustion gases and ultimately removed very efficiently by particle-attenuation devices (electrostatic precipitators; fabric filters — Chapter 8). However, most of the Cl, Hg and F and small proportions of B and Se tend to be emitted with the stack gases (Chapter 7) together with some fine flyash particles which carry trace elements to the atmosphere (Chapter 10). As expected, trace elements in coal (Chapter 2) are found enhanced in flyash by a factor related to the ash yield of the coal. For dry bottom furnaces, 100 t coal yields an amount of flyash equal to about 0.8 times the % mineral matter which is close to 0.8 times the % ash yield at about 800°C. The composition of flyash depends on several factors including the composition of the coal, the furnace configuration, conditions during combustion, the efficiency of particle attenuation and the point of collection of the sample. These variables give rise to wide ranges of concentration of trace elements.

Most power stations currently operate wet ash disposal systems. However, this method of ash disposal is being subjected to increasing scrutiny as there is a potential for contamination of surface and groundwaters by trace elements leached from the ash (Carlson and Adriano, 1993). Very high liquid to solid ratios of 10:1 to 20:1 are typically used in ash sluicing systems (Chu et al., 1978). Consequently, large volumes of water containing elements dissolved from the ash are produced. The composition of the water in an ash pond will be determined by the ash itself, by the quality of water used for sluicing, and by the relationship of the ash pond to other components of the power station water circuit (Figure 12.1).

We examine the general topic of the biogeochemical cycling of trace elements and trace-element environmental concerns in surface coal-mine land reclamation¹. Emphasis is placed on research trends and results from the arid and semi-arid regions of North America. We discuss the importance of conducting pre- and post-mining trace element inventories that both anticipate potential reclamation problems and help define possible long-term ‘ecosystem’ stability. Particular emphasis is placed on understanding the source, release and transport mechanisms, and fate of trace elements under various reclamation environments and how this relates to sustainable plant establishment and animal risk factors.

Although the full impact of a Chapter can only be gained by studying it, some people may find it useful to have an overall summary of the main conclusions from each Chapter. Hence, this matter is being addressed together with indications of deficiencies in data and other information.