Trace elements in coal: Associations with coal and minerals and their behavior during coal utilization – A review
Introduction
Coal is responsible for about 40% of electricity generation in world. The large abundance of coal makes it a reliable, long-term fuel source for both domestic and export purposes in countries like Australia, Canada and South Africa. One recent study puts global coal reserves at about one trillion tons, enough to last about 250 years at the current rates of consumption. Another study estimated the life of coal reserves at 1500 years. These studies suggest that crude oil may only last for another 40–60 years and natural gas may be there for 70–120 years. By the year 2020, coal could overtake crude oil as the world’s most important source of energy. In fact, on a global basis, coal is the world’s most abundant fossil fuel reserve.
Trace elements (TEs) such as mercury, arsenic and selenium present in coal are known to be of concern for public health. These TEs even when present in parts per million levels in coal can result in emission of several tons of these pollutants in environment. Table 1 shows the classification of those TEs of particular environmental concern related to coal utilization.
These elements mostly evaporate during combustion and condense either homogeneously as sub-micron ash or heterogeneously onto already existing fine ash, the former one being more difficult to be captured. Clean coal technology development is, therefore, a priority area for research and needs continuous improvements in increased efficiency and decreased pollutant emission.
The inorganic matter in coal including TEs is not uniformly distributed. The coal–mineral and mineral–mineral associations play an important role in the formation of fine particles and in subsequent condensation of TEs in various phases. Any retention of these elements in fly ash particles is strongly influenced by their association with other minerals in individual coal and mineral grains and their relative volatility. Usually, there is a higher concentration of minerals in the high reflectivity component of coal (inertinite). This component burns comparatively slowly. This can cause retention of some of these elements in selected ash particles, particularly those with low volatility. Thus the mode of emission of these elements is strongly linked with nature of coal and formation of fine particles.
The emission and distribution of TEs in different streams depends on several factors, such as, modes of occurrence of TEs in coal, mineral content and distribution in coal, physical processes such as turbulence, pollution control devices, temperature profile, and so on.
The principal objective of this paper is to look over closely the current status of the knowledge concerning the TEs deportment during coal utilization processes.
Section snippets
Trace elements in coal
Nearly most of the elements in the periodic table can be detected in measurable amounts in coal. However, little is known about the mode of occurrence or distribution of these elements. Also, accurate measurement of some elements, such as Au, Ag, Bi, In, Re, Tm, and the platinum group elements (PGE) has not yet been carried out, due to the limitations in the range of detection of modern analytical techniques [2]. Elements in coal can be classified into three broad groups, based on their
Modes of occurrence and association of TEs in coal
The amount of TE emission from coal combustion is strongly dependent on the modes of occurrence in coal. The mode of occurrence includes information about the specific mineral an element forms, dispersion within a particular host mineral or maceral, the fraction of the coal the element is associated, the oxidation state that element occurs, and so on. Those elements which are associated mostly with the coal organic and sulfide fractions tend to vaporize and then adsorb on fine particles when
Behavior of trace elements in combustion and gasification systems
The contribution of anthropogenic sources of fine particles resulted from coal utilization to global budgets of the various TEs is normally estimated by emission factors. Pacyna [25], [26], [27], [28] has reviewed the emission factors for a range of TEs from coal combustion in different types of boilers and for different countries. The data in this review date back to 1979 which in turn, makes these emission factors inapplicable for several reasons. The types of coals used in each country may
Particulate control strategies
Different levels of control strategies have been employed in coal utilization processes. The foremost strategy is pre-utilization removal of TEs. A significant portion of minerals in coal is removed by a number of methods which is closely associated with association of TEs in coal. Those elements associated with minerals are easy to be removed in coal cleaning. Minami et al. [60] using air dense medium fluidized beds (ADMFB) were able to reject a substantial fraction of mercury in original coal
Conclusion
In this study, a brief account of the environmental hazards of coal energy is addressed mainly focused on TE emission and the current status of legislation concerning TEs. This review shows the need for reducing pollutant emission from coal utilization due to the great environmental and health impacts and the current and future regulation. The behavior of TEs during coal combustion is discussed, with a number of information about modes of occurrence of TEs in coal; the partitioning of TEs
Acknowledgements
The authors are thankful for the financial support from AERI and from NSERC under Discovery Grant 341140-07.
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