Chemistry of Trace Inorganic Elements in Coal Combustion Systems: A Century of Discovery

Coal fueled the Industrial Revolution and the global expansion of electrification in the 20th century. In the 21st century, coal use has declined in North America and Europe but continues to increase in Asia. Coal contains many of the elements of the Periodic Table, in percent levels or trace amounts (parts per million or parts per billion). The impact of many of these elements on the environment via air emissions and water discharges from coal-fired plants has been studied, with decades of research on their chemical transformations within combustion systems and their fates upon re-introduction into the environment. The transformations of trace elements present in coal burned during combustion can be categorized as thermal volatilizations from the coal in the furnace, thermal decomposition of trace element compounds inside the coal, encapsulation inside ash structures through high-temperature vitrification, oxidation of trace elements with the myriad of species contained in flue gas through gas phase (homogeneous) reactions or catalytic (gas-solid) reactions, adsorption and/or reactions with active sites on entrained fly ash particulates contained in the flue gas, and absorption into solutions. These transformations can, in many cases, impact the fraction of these trace elements that are removed by various pollution control devices compared to the fraction released into the environment. The sampling and measurement of trace elements, in the inlet coal, outlet flue gas, aqueous scrubber solutions, and ash matrices, represent a significant challenge. This review focuses on the behavior of trace elements in industrial coal combustion systems with an emphasis on what has been learned over the past century uniquely related to the use of coal in boilers for electricity and heat production. Key accomplishments in measurement, modeling, and control of trace element emissions in coal-fired systems are highlighted.