style="font-size:15px;"> Hazardous emissions from bituminous coal combustion include particulate matter (PM), sulfur oxides (SOx), nitrogen oxides (NOx), trace metals such as lead (Pb) and mercury (Hg), vapor-phase hydrocarbon derivatives (such as methane, alkane derivatives, alkene derivatives, and benzene derivatives) and polychlorinated dibenzo-p-dioxin derivatives and polychlorinated dibenzofuran derivatives. When burned, bituminous coal can also release hazardous gases such as hydrogen chloride (HCl), hydrogen fluoride (HF), and polycyclic aromatic hydrocarbon derivatives (typically represented as PAHs or PNAs). Incomplete combustion leads to higher levels of and polycyclic aromatic hydrocarbon derivatives (which are carcinogenic) but burning bituminous coal at higher temperatures reduces its carbon monoxide emissions. Therefore, large combustion units and well-maintained ones generally have lower pollution output. Bituminous coal has slagging and agglomerating characteristics.
Bituminous coal combustion releases more pollution into the air than subbituminous coal combustion, but due to its greater heat content, less of the fuel is required to produce a given output of electricity. Therefore, bituminous, and subbituminous coals produce approximately the same amount of pollution per kilowatt of electricity generated.
Bituminous coal is the most common coal – bituminous coal and subbituminous coal represent (cumulatively) more than 90% of all the coal consumed in the United States. When burned, bituminous coal produces a high, white flame. Bituminous coal includes two subtypes: thermal and metallurgical.
Thermal coal is sometimes called steam coal because it is used to fire power plants that produce steam for electricity and industrial uses. Locomotive trains that run on steam may also be fueled with bit coal (bituminous coal). However, steam coal, which is not a specific rank of coal, is a grade of coal that falls between bituminous coal and anthracite, once widely used as a fuel for steam locomotives. In this specialized use it is sometimes known as sea coal in the United States. Small steam coal (dry small steam nuts, DSSN) was used as a fuel for domestic water heating. In addition, the material known as jet is the gem variety of coal. Jet is generally derived from anthracite and lacks a crystalline structure so it is considered to be a mineraloid. Mineraloids are often mistaken for minerals and are sometimes classified as minerals, but lack the necessary crystalline structure to be truly classified as a mineral. Jet is one of the products of an organic process but remains removed from full mineral status.
Coking coal (also known as metallurgical coal) is able to withstand high heat and is used in the process of creating coke necessary for iron and steel-making. Coking coal is able to withstand high heat. Coking coal is fed into ovens and subjected to oxygen-free thermal decomposition (pyrolysis), a process in which the coal is heated to approximately 1100 C (2010°F). The high temperature melts the coal and drives off any volatile compounds and impurities to leave pure carbon. The purified, hot, liquefied carbon solidifies into coke (a porous, hard black rock of concentrated carbon) that can be fed into a blast furnace along with iron ore and limestone to produce steel.
Bituminous coal contains moisture up to approximately 17% w/w and has a fixed carbon content on the order of 85% w/w with a mineral matter content up to 12% w/w. Bituminous coal can be categorized further by the level of volatile matter it contains; high-volatile A, B, and C, medium-volatile, and low-volatile. Approximately 0.5 to 2% w/w of bituminous coal is nitrogen.
More than half of all available coal resources are bituminous and, in the United States occur in Illinois, Kentucky, West Virginia, Arkansas (Johnson, Sebastian, Logan, Franklin, Pope, and Scott counties), and locations east of the Mississippi river.
Particles of waste bituminous coal that are left over after preparation of commercial-grade coal (coal fines), which are light, dusty, and difficult to handle, traditionally were stored with water in slurry impoundments to keep them from blowing away.
New technologies have been developed to reclaim fines that were formerly considered waste. One approach is to use a centrifuge to separate the coal particles from slurry water. Other approaches have been developed to bind the fines together into briquettes that have low moisture content, making them suitable for fuel use.
1.4.4 Anthracite
Anthracite (also known as hard coal) is the highest rank of coal (ASTM D388) and is the oldest coal from geological perspective – it is actually considered to be metamorphic. It is a hard coal composed mainly of carbon with little volatile content and practically no moisture.
Anthracite is deep black and often appears to be of a metallic nature because of the glossy surface. Compared to other coal types, anthracite is much harder, brittle, and has a glassy luster, and is denser and blacker with few impurities. When burned, anthracite produces a hot blue flame and, as a result, is primarily used for space heating by residences and businesses in and around the northeastern region of Pennsylvania, where much of it is mined.
Anthracite is considered the cleanest burning of all coal types and produces more heat and less smoke than other coals, and is widely used in furnaces. It is largely used for heating domestically as it burns with little smoke. Some residential home heating stove systems still use anthracite, which burns longer than wood.
Anthracite burns at the highest temperature of any coal and typically produces up to 13,000 to 15,000 Btu per pound. Waste coal discarded during anthracite mining (called culm) and has a heat content has less than half the heat value of mined anthracite and a higher ash and moisture content. It is used most often in fluidized bed combustion (FBC) boilers.
Anthracite has a high fixed carbon value (80 to 95%) (Chapters 2, 5) and a low sulfur as well as a low nitrogen (less than 1% each). Volatile matter is low at approximately 5% w/w, with 10 to 20% w/w of mineral ash produced by combustion. The moisture content is approximately 5 to 15% w/w and the coal is slow-burning and difficult to ignite because of the high density – consequently few pulverized coal-fired plants use anthracite as the fuel.
Anthracite is considered non-clinkering and free burning because (when ignited) it does not coke or expand and fuse together. It is most often burned in underfeed stoker boilers or single-retort side-dump stoker boilers with stationary grates. Dry-bottom furnaces are used because of the high ash fusion temperature of anthracite. Lower boiler loads tend to keep heat lower, which in turn reduces nitrogen oxide emissions.
Particulate matter, or fine soot, from burning anthracite can be reduced with proper furnace configurations and appropriate boiler load, under-fire air practices, and fly ash reinjection. Fabric filters, electrostatic precipitators (ESP), and scrubbers can be used to reduce particulate matter pollution from anthracite-fired boilers. Anthracite that is pulverized before burning creates more particulate matter.
Furthermore, it is worthy of note that even in the terminology of anthracite there are several variations which, although somewhat descriptive, do not give any detailed indications of the character of the coal. For example, some of the terms which refer to anthracite are: black coal, hard coal, stone coal, which should not to be confused with the German steinkohle or the Dutch steenbok, which are terms that include all varieties of coal with a stone-like hardness and appearance, blind coal, Kilkenny coal, crow coal (from its shiny black appearance), and black diamond. However, as the importance of the coal trade increased, it was realized that some more definite means of classifying coals according to their composition and heating value was desired because the lines of distinction between the varieties used in the past were not sufficiently definite for practical purposes (Thorpe et al., 1978; Freese, 2003).
Anthracite is scarce and only a small percentage of all remaining coal resources are anthracite. Pennsylvania anthracite was mined heavily
