edited bv:
chem I ~upplement
MICHAEL R. S L A B A U ~ H
HELENJ. JAMES Weber State College Ogden. Utah 84408
The Geochemistry of Coal I. The Classification and Origin of Coal Harold H. Schoberi Fuel Science Program, Pennsylvania State University, University Park, PA 16802 Since coal is extracted from the earth, and since its formation involved chemical nrocesses. a studv of the oriein and composition of coal is a part of thd broad subject of ge&hemistry. The predominant elements in coal are carbon, hydrogen, and oxygen, with lesser amounts of sulfur and nitrogen. There is no doubt that coal was formed from plants; remains of plant components, such as pollen, can befound in many coals, and organic compounds similar in structure t o compounds known to occur in plants can be isolated from coals. For these reasons the study of coal can be classified more narrowly as a branch of organic geochemistry. The carbon content of coal may vary from 65 to 95%; its age, from about 30 million years to over 300 million. The plants that contributed to the formation of coal have included long-extinct giant rushes and tree ferns through species still flourishing today. Certainly thegeochemistry of coal is asubject of great com~lexitv.but nevertheless the conversion of plant material to coal-can he explained and understood b y the same principles that apply toreactions carried out in the laboratory. In the laboratory we most often will vary the temperature of a reaction t o affect its rate or t o favor the formation of certain products. We will see that one of the major factors affecting coal formation is temperature, and, in the latter staees. that pressure also has an effect. G o i t of us regard coal as fuel to be burned as a source of energy; that is, our focus on coal is mainly on its end use. The use of coal is certainly not without problems. Among the most notorious of the problems are the emissions of sulfur oxides from combustion of sulfur in coal and the collection and safe disposal of the ash, the noncomhustible inorganic componentsbf coal. The reactions that coal undergoes when i t is used involve, just as any other chemical, the breaking of bonds, forming of new bonds, and the conversion of molecular structures into new molecules. The molecular structures and chemical bonds that exist in the coal. and therefore govern its behavior when we use it, are not random accidents of nature, but in fact are the logical outcome of a sequence of geochemical transformations applied to the organic matter from which the coal was formed. The geochemistry involved in coal formation establishes the composition and properties, and those in turn establish the behavior during coal utilization. The Concept of Rank If we observe samples of coals collected from around the world. we can see at once the materials we call "coal" ranee from a moist, loosely consolidated brown substance to avery hard, lustrous, "black diamond." Because the properties of coal vary so widely, some method of classification and noamonr- kinds of coal menclature is needed to distinauish having similar properties. There is a continuous gradation of composition and prop-
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242
Journal of Chemical Education
erties from the original unaltered plant material through coals of increasing carbon content t o nearly pure graphitic carbon. The positFon of a particular coal along this sequence from plant material to graphite represents the extent of its geochemical maturation. The extent of maturation of a coal determines its rank. Since there is a continuous increase in carbon content with increasine maturation. and since areater degrees of maturation u ~ u &require ~ longer times, the rank of a coal nrovides a aualitative indication of its ape and carhon content. Coal claBsification is made on the basis of rank. The rank classification system used in the United States derives from the needs for classifying coal for its principal commercial uses: combustion for electric nower eeneration and product of coke for the metallurgical kdustr;. If coal is heated sliehtlv .. .above 100 "C. water is driven off. The weieht lossaccompanying this heating defines the moisture content of the coal. Further heatinr of the samule in the absence of air, to 950 "C, produces a &iety of materials including the oxides of carbon and manv oraanic com~ounds.These compounds are not determined Ldividualiy. Rather, they are lumped together as"volatile matter".'l'he weight lossduring this experiment is said to hea measure ofthe volatile content of the coal.'rhe volatile matter dues not pre-exist in theroal. waitine there to hedrivenout at 950 OC. hut rather is formed by theimally breaking apart some of thd organic components of the coal. The material remaining after volatile matter bas been driven off is a char rich in carbon. Since the carhon in the char is not volatile, it is, t o use another very old chemical term, "fixed". Heating the char in air burns the fixed carhon, leaving an ash. The ash is weighed, and the fixed carbon content is calculated by subtracting the sum of moisture, volatile matter, and ash from 100%. The determination of moisture, volatile matter, ash, and fixed carbon is known as the proximate analysis of coal. The term "proximate" has a special meaning in analytical chemistry, and refers to the practice of reporting a group of components together as a single generic material, without determinina each of the comnonents individuallv. In proximate analysisof coal thespeciiiccompounds in the vulntilemntter are never determined, but rather all of the materials evolved at 950 OC are collectively referred to as volatile matter. (Another example is the reportina of water hardness as "calcium cerhonati"regardless of the-species actually cnntrihutina to the hardness.) The name unfortunately sounds like a contraction of "approximate", but in fact a proximate analysis is based on carefully defined and rigorously followed procedures. A proximate analysis is not approximate analysis. The organic portion of the coal is really contained entirely in the volatile matter and fixed carbon. In a sense, moisture ~~
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and ash are extraneous impurities. I t is only the carbonaceous portion of the coal that chanees durine maturation. andit is only the carbonaceous portion that burns to liberatd energy. Thus, to compare one coal with another i t is often desirable to compare bnly the carbonaceous or organic portion, that is, to compare the volatile matter and fixed carbon contents on a moisture-free or moisture-and-ash-free basis. Correcting the as-analyzed results to either of these bases is astraiehtforward exercise in aleebra. Strictlv sneakine. ".however, there is no ash in coal. i s h is the &oiuct of phase chanees and reactions occurrine to the inoreanic comDon e n c (usually called mineral matter) originzly in the coal during the combustion of the fixed carbon. Because the transformkion of mineral matter to ash results in a change of weight, by dehydration of clays and the oxidation of pyrite to iron oxides for example, the weight of ash is not the weight of the original mineral matter. Since it is often much easier to measure ash than mineral matter.. eouations have been developed to calculate mineral matter content from the ash determination. In the United States the most commonly used is the Parr formula ( I ) , which is
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Mineral matter = l.OB(Ash) + 0.55Sulfur Once the mineral matter has been calculated it is,also possihle to express coal analvses on a mineral matter free basis. The ciassification system developed by the American Society for Testing and Materials (ASTM) is based on the dry, mineral-matter-free volatile matter and fixed carbon content and the moist, mineral-matter-free heating value. The heating value is the amount of energy liberated when the coal is burned and in the United States is reported in English units as BtuAb. For coal classification, the heating value is reportedona moist, but mineral-matter-free hasis.Theclassification svstem is shown in the table.'l'he AS'I'M classification estabcsbes four classes of coal, each of which is subdivided into two or more groups. In common usage the names of the ranks are taken from the class names-anthracite, bituminous, subbituminous, and lienite. Two additional terms are sometimes used in connection with the classification of coal. Coal type refers t o the amount of materials that can be assigned to the different classes of coal organic components (the macerals, discussed helow) and to the kinds and amounts of mineral matter (3).Coal type derives from the kinds of plant material that contributed to the formation of the coal and from the kinds of reactions prevailing during the decay of the plant material and its conversion to coal. There is no generally accepted system for classifying coal by type analogous to the rank classification, so rank is the much more commonly used term. Coal ASTM Coal Classlflcatlon bv Rank 121
Class and gmup I. Anthracitic 1. Metaanthraclte 2. Anthracite 3. Semianthracite 11. Bituminous 1. LOWvolatile 2. Medium volatile 3. High volatlle A 4. High volstlle B 5. High volatile C Ill. S~bbitumlno~s 1. Subbit~minowA 2. Subbituminous B 3. Subbituminous C IV. Llgnitic 1. Lignite A 2. Lignite B
Fixed carbon, %
Volatile maner. %
Heatlng value. Btullb
