Coal (What Your Parents Never Taught You…)

By Buecker, Brad

Coal is compressed plant matter that over millions of years transformed into a high-carbon material. Age, type of initial vegetation and location of deposit formation are all significant factors in the quality of a coal deposit. Scientific research indicates that the first plants to grow on land appeared over 400 million years ago1. This early vegetation consisted mostly of leafless shoots. Within a relatively short period of time, cosmologically speaking, after the first plants appeared extensive forests covered much of the world. The period from 350 to 275 million years ago is known as the Carboniferous period and during this time much coal formation began. This was a period of globally warm temperatures, which were fertile for plant growth.

At the end of the Carboniferous period and for about 135 million years thereafter, coal formation in the Northern Hemisphere greatly diminished. But with the onset of the Cretaceous period around 135 million years ago, plant growth and coal formation resumed, although by this time more complex vegetation, including plants and trees with protected seeds, had begun to dominate the landscape.

Thus, the coal that we use today developed over hundreds of millions of years from a wide variety of vegetation.

To understand the general properties of coal, it is first necessary to understand the basic chemical composition of plant life. The main building block of vegetation is cellulose, whose general chemical structure is shown in Figure I2. Cellulose belongs to a class of compounds known as carbohydrates, whose name naturally comes from the fact that the compounds are composed of carbon, oxygen and hydrogen. Other materials that comprise plant structures include lignin and hemicellulose, which are also carbon-based compounds.

The prerequisites for coal formation were the vast swamps that covered much of the earth in prehistoric times. Vegetation that dies on dry land is acted upon by aerobic microorganisms. This means that much of the decomposition product will enter the soil as humus (small organic clumps) or will completely break down to carbon dioxide. But when vegetation dies in a swamp where oxygen-deficient conditions exist, a much different process occurs. The first step in formation of the coal we use now was anaerobic bacterial attack of the dead vegetation. Microorganisms consumed hydrogen and oxygen, which increased the carbon content. This mechanism, known as the biochemical phase of coalification, was self-limiting, as the bacterial action produced compounds that were lethal to the organisms themselves. However, over time the partially decomposed matter was overlaid by other material including more vegetation and soil.

TABLE 1 PROPERTIES OF U.S. COALS INCLUDING ASH ANALYSES

TABLE 2 CLASSIFICATION OF COALS BY RANK

This process had two principal effects. First, it placed the material under increasing pressure. second, it moved the deposits deeper underground where temperatures were, and still are, warmer.

The combination of pressure and heat caused additional loss of oxygen and hydrogen. This phase is known as the geochemical phase of coalification. The sequence of materials as coal matures is wood, peat, lignite, subbituminous, butuminous, anthricite. Carbon content increases from about 44 percent in wood to over 90 percent in anthracite. Theoretically, a completely mature coal would have the chemical composition of graphite, which is complete carbon.

The complex carbohydrates within vegetation build up from sugars and starches that the plant produces through photosynthesis. During the coalification process, some of these compounds metamorphosed to low-weight organic molecules that are not bound to the main coal structure. The smaller organic compounds are known as volatiles because they vaporize with increasing temperature. Volatiles were driven off during the coalification process. As a result, increasingly mature coals contain less volatile content.

One might be tempted to think that age is a primary factor in coal’s maturity. While this is true in some cases, the two most important factors were pressure and temperature. Coals that were buried deep and located in high temperature zones, say underneath a region of volcanic activity, matured much more quickly than older coals subjected to less heat and pressure.

Examples of the different types of coal can be found all over the world. In the United States, the Appalachian area around western Pennsylvania, West Virginia, Ohio, Kentucky, and even stretching into Alabama contains enormous deposits of bituminous coal. Illinois also sits on top of an extensive bituminous deposit. Also well known is a large subbituminous deposit beneath Wyoming and Montana. Because much of this latter coal is mined in an area near the Powder River, it is known as Powder River Basin (PRB) coal. Significant lignite deposits are in North Dakota and to a lesser extent in Texas.

Figure 1 BASIC STRUCTURE OF CELLULOSE

Tables 1 and 2 outline the common properties of several prominent coals found within the United States. A description of each type- including peat, the starting material-is outlined below.

Peat

A visual examination of peat provides a clear example of the intermediate stage between plant life and coal deposits. Peat may range from a light-colored substance with recognizable pieces of plant matter to a black material that looks like coal. Although peat is continually being compacted by overlying material, it is still subject to microbiological decomposition in the biochemical phase of coalification. A general rule of thumb is that it takes 100 years for a two- to three-inch layer of peat to form.1 Some modern swamps have peat layers up to 30 feet deep, which means they have been undisturbed for thousands of years. Not uncommon are layered coal deposits, where each seam is separated by soil and minerals. This suggests that some ancient swamps produced a layer of peat, died out, then redeveloped to start the process again.

Lignite

By continguing to compress and heat peat, lignite and its more immature precursor, brown coal, are produced. While peat is not considered to be coal, lignite is, although plant material is often still clearly evident in lignite deposits. Carbon content in lignite is around 70 percent, while oxygen content is around 25 percent. This is the first fuel listed in the ASTM (American Society for Testing and Materials) coal classification table. As is clearly evident, the heating value (quantity of energy available from combustion) is the lowest of all coals, with a range of 6,300 to 8,300 British thermal units (Btu) per pound (14,653 – 19,305 kilo- Joules-kJ-per kilogram) on a moisture- and ash-free basis. Lignite- fired boilers are typically much larger than other boilers because long residence times are required to extract the energy from the fuel. Lignite is not a common fuel of choice and is mostly used at minemouth power plants, in which the fuel is conveyed directly from the mine to the plant. Lignite contains much volatile matter and is the easiest coal to ignite. It is the coal most prone to spontaneous combustion in coal piles, bunkers and so on. Although lignite has much less moisture than peat (30 percent as compared to 70 percent) the percentage is still quite high. This must be taken into account when designing fuel handling and drying systems for lignite.

Subbituminous

The next step in coal’s evolution is subbituminous. This coal has a 75 percent carbon content with only 20 percent oxygen. Like lignite, the ASTM ranks this coal on energy content. Subbituminous coal has a high volatile content and is subject to spontaneous combustion when stored improperly or for too long in storage piles or coal bunkers. The high volatile content gives subbituminous coal good ignition properties within a boiler. This same property can cause explosions in coal-grinding mills. This aspect requires careful mill operation and safety devices, such as “sweeping” mills upon shutdown and installation of well-designed inerting systems.

Bituminous

Bituminous coal offers many advantages. It has a high heating content and enough volatiles to ignite quickly in a boiler, but, in general, is less prone to spontaneous combustion than subbituminous coal or lignite. Bituminous coal also has just the right properties for coke production, which is vital to the steel industry. Other factors aside, bituminous coal would be the preferred fuel for many coal-fired boilers. The presence of significant sulfur concentrations in many bituminous deposits has proven to be a negative factor in an increasingly stringent air pollution control regulatory climate.

Anthracite

Anthracite represents the most mature coal. Volatile matter is very minor making it a difficult coal to ignite. The only significant deposit of anthracite in the United States is in Pennsylvania, so other than for home heating purposes in the 19th and part of the 20th Century, anthracite has not been heavily exploited as an energy source in this country.

Coal Impurities

Some impurities accumulated during original plant growth, but most came from external sources that are strongly dependent upon where or in what conditions the coal formed. Two natural eleme\nts in coal are sulfur and nitrogen, which were contained in amino acids and metabolites produced by the original vegetation. Fuel-bound sulfur generally accounts for only a small portion of the sulfur in a coal deposit, as most comes from minerals transported with water that filters through coal seams. Peat, of course, has a very high moisture content, but even mature coals, including bituminous, contain many cracks and crevices that allow the passage of water.

A common impurity is iron sulfide, FeS^sub 2^. Iron sulfide is believed to have come from swamps originally flooded with brackish water containing sulfates. Anaerobic bacterial breakdown of * the sulfates produced sulfides, which combined with iron. FeS^sub 2^ is the most troublesome impurity of many eastern bituminous coals.

Soil and many natural minerals consist of complex metallic silicates, so virtually all coals contain silicon and aluminum in significant quantities. These materials constitute the major percentage of ash. Sodium and potassium are important impurities, as they can cause a great deal of trouble in the backpass of a boiler. Sodium, especially if it is in the form of a salt such as sodium chloride, volatilizes during combustion. As temperatures drop in the backpass, the sodium condenses and acts as a binding agent that causes ash particles to accumulate on superheater and reheater tubes. Potassium is a primary ingredient of corrosive ash deposition products that can afflict superheater and reheater tubes.

An interesting aspect that may be observed in Table 2 is that PRB coals typically contain less ash than eastern coals. This seeming advantage is offset by the fact that the calcium content of the ash is higher. Calcium increases the strength of ash deposits, and thus is a factor in backpass fouling.

Table 2 also illustrates the ash fusion temperatures for a variety of coals. The temperatures represent the ranges over which coal ash begins to soften to the point where it reaches maximum fluidity. The melting point, or more precisely the ash fusion range, is crucial in boiler design. Modern, pulverized coal boilers are designed such that any ash that gravitates to the furnace walls is relatively solid by the time it reaches this periphery. Otherwise, semi-molten ash would build up on the tubes and greatly restrict heat transfer. The cyclone boilers of the 1960s and 1970s were designed to produce a molten ash in the cyclones that flowed out through cyclone “taps” to the bottom of the boiler, where the molten liquid drained to the water-filled slag tank for quenching. Newer fluidized bed boilers operate at temperatures of 1,500 F to 1,700 F, which is well below the ash fusion temperatures of any of the coals shown.

Coal’s melting point is crucial in boiler design

References

1. Kitto, J.B. and S. Stultz, Eds. STEAM, 41st Edition, Babcock & Wilcox, a McDermott Co., Barberton, Ohio, 2005.

2.B. Buecker. “Hitting the Gas: The development of biorenewable energy from crops and other vegetation is gaining momentum,” Environmental Protection, pp 32-37, Vol. 17 No. 6, July/August 2006.

3.Sinder, J.G., Ed. Combustion: Fossil Power, published by Alstom, formerly Combustion Engineering, Windsor, Conn., 1991.

By Brad Buecker, Contributing Editor

Author

Brad Buecker is an air quality control specialist at a large, Midwestern power plant. He has previous experience as a chemical cleaning services engineer, a water and wastewater system supervisor, and a consulting chemist for an engineering firm. He also served as a results engineer, flue gas desulfurization (FGD) engineer, and analytical chemist for City Water, Light & Power, Springfield, Illinois, USA. Buecker has written more than 70 articles on steam generation, water treatment, and FGD chemistry, and he is the author of three boohs on steam generation topics published by PennWell Publishing, Tulsa, Oklahoma, USA. Buecker has an AA in pre-engineering from Springfield College in Illinois and a BS in chemistry from Iowa State University. He is a member of the ACS, AIChE, ASME, and NACE.

Copyright PennWell Publishing Company Nov 2006

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