Coal-Fired Power Generation Handbook. James G. Speight
may be concentrated in a specific coal bed, which may make that bed a valuable resource for those elements (such as silver, zinc, or germanium). Some elements, however, have the potential to be hazardous (for example, cadmium or selenium), particularly if they are concentrated in more than trace amounts. Although as many as 120 different minerals have been identified in coal, only approximately 33 of these minerals commonly are found in coal, and, of these, only approximately eight minerals are sufficiently abundant to be considered major constituents.
When coal is combusted, as in a coal-fired power plant to generate electricity, most of the mineral matter and trace elements generally form ash. However, some minerals break down into gaseous compounds, which go out through the furnace flue. Pyrite, for example, breaks down into the individual elements iron and sulfur and each element combines with oxygen to become, respectively, iron oxide and an oxide of sulfur – the sulfur oxides are emitted in the flue gases while the iron oxide become part of ash.
In some highly oxidative conditions, ferric oxide (Fe2O3) may be formed.
Some trace elements also dissociate from their organic or mineral hosts when coal is burned – most become part of the ash, but a few of the more volatile elements, such as mercury and selenium, may be emitted in the flue gas.
The term coal quality is used to distinguish the range of different commercial steam coals that are produced directly by mining or are produced by coal cleaning (Speight, 2013). The factors considered in judging the quality of a coal are based on, but not limited to, (i) heat value, (ii) moisture content, (iii) mineral matter content, reflect as mineral ash after combustion, (iv) fixed carbon, (v) sulfur content, (vi) the content of major, minor, and trace elements, (vii) the coking properties, (viii) the petrologic properties, and (ix) the organic constituents considered both individually and in groups. The individual importance of these factors varies according to the intended use of the coal. These properties are determined in the United States according to standards established by the standard test methods developed and published by the ASTM International (formerly the American Society for Testing and Materials, ASTM) and are usually denominated in English units (e.g., Btu/lb for heating value on a mass basis) (ASTM, 2020).
As a side note at this point, in addition to the difference in heating value (i.e., Btu/lb), electricity generating units fueled with subbituminous and lignite coals tend to operate at lower efficiency (higher heat rate) than units fueled with bituminous coal. This can lead to differences in generating capacity when using different coals.
Generally, coal quality for steam coals (i.e., coal used for electricity generation) refers to differences in heating value (Btu/lb) and sulfur content (% w/w) although other characteristics such as grindability or ash fusion characteristics are also specified in coal sale agreements. While not as obvious as the impact of sulfur content on environmental emissions, differences in the moisture content and heating values among different coal types affect the emissions of carbon dioxide upon combustion, with higher-rank bituminous coals producing 7 to 14% lower emissions than subbituminous coals on a net calorific value basis (NRC, 2007).
Coal quality is now generally recognized as having an impact, often significant, on coal combustion, especially on many areas of power plant operation (Leonard, 1991). The parameters of rank, mineral matter content (ash content), sulfur content, and moisture content are regarded as determining factors in combustibility as it relates to both heating value and ease of reaction. In addition, although not always recognized as a form of cleaning or beneficiation, the size of the coal can also make a difference to its behavior in combustion (power plant) operations. Hence, the need for one, or more, forms of cleaning (pretreatment) prior to use.
Thus, run-of-mine coal (i.e., coal taken straight from the mine) is dirty and contains impurities that are not a part of the organic coal matrix. Part of this problem arises from the composition of coal while another part arises as a result of the inclusion of rock into the coal during mining operations.
Furthermore, uncertainty in the availability and transportation of fuel necessitates storage and subsequent handling of coal. Maintaining an available supply of coal at the mine site or, more appropriately at the power plant, has the advantage of availability when supply disruption occurs and tends to overcome the perceived disadvantages of build-up of inventory – space constraints, deterioration in quality, and potential fire hazards. Other minor losses associated with the storage of coal include oxidation (leading to spontaneous ignition and property changes), wind and ground loss. It is also worthy of note that a 1% oxidation of coal has the same effect as 1% mineral matter in coal and wind losses may account for nearly 0.5 to 1.0% of the total loss.
The main goal of good coal storage is to minimize ground loss and the loss (with the associated danger) due to spontaneous combustion. Formation of a soft carpet, comprising coal dust and soil causes ground loss (also referred to as carpet loss). On the other hand, gradual temperature builds up in a coal heap, on account of oxidation and may lead to spontaneous combustion of coal in storage. The measures that would help in reducing the carpet losses are (i) preparing a hard ground for coal to be stacked upon, and (ii) preparing standard storage bays out of concrete and brick. In process industry, modes of coal handling range from manual to conveyor systems and are designed according to the differences in the properties of the coal (Narasiah and Satyanarayana, 1984). It is often advisable to minimize the handling of coal so that further generation of fines and segregation effects (due to coal size) are reduced.
But before the issues regarding stockpiling of coal are presented (below) it is necessary to consider the means of recovery of coal (aka, coal mining).
3.2 Coal Recovery
Recovery of coal (coal mining, coal recovery) is the act of removing coal from the ground and has been practiced throughout history in various parts of the world and continues to be an important economic activity (NRC, 2007; Speight, 2013, 2020).
A modern coal mine is a highly mechanized industrial plant that has to meet strict standards of engineering design and operation. The size, power, strength, monitoring and control features, and automation of mining equipment dwarf those of even a few decades ago. The overall coal mining process consists of several sequential stages: (i) exploration of a potentially economic coal seam to assess minable reserves, environmental issues, marketable reserves, potential markets, and permitting risks, (ii) analysis and selection of a mining plan, (iii) securing the markets, (iv) developing the mine, (v) extracting the coal, (vi) processing the coal if necessary, and (vii) decommissioning the mine and releasing the property for post-mining use.
The uncertainties concerning resource and reserve estimates also apply to the grade or quality of the coal that will be mined in the future. At present, it is difficult to project spatial variations of many important coal quality parameters beyond the immediate areas of sampling (mostly drill samples). Almost certainly, coals mined in the future will be lower quality because current mining practices result in higher-quality coal being mined first, leaving behind lower-quality material (such as coal with a higher mineral content leading to higher yields of combustion ash yield, higher sulfur emissions, and/or higher concentrations of potentially harmful elements. The consequences of relying on poorer-quality coal for the future include (i) higher mining costs, such as the need for increased tonnage to generate an equivalent amount of energy, greater abrasion of mining equipment, (ii) transportation challenges, such as the need to transport increased tonnage for an equivalent amount of energy, (iii) beneficiation challenges, such as the need to reduce ash yield to acceptable levels, the creation of more waste, (iv) pollution control challenges, such as capturing higher concentrations of particulates, sulfur, and trace elements; dealing with increased waste disposal, and (v) environmental and health challenges. Improving the ability to forecast coal quality will assist with mitigating the economic, technological, environmental, and health impacts that may result from the lower quality of the coal that is anticipated to be mined in the future (NRC, 2007).
Furthermore, coal seams are located in a variety of geologic