chem I ~upplement
edited by: MICHAELR. SLABAUGH HELENJ. JAMES Weber State College Ogden. Utah 84408
The Geochemistry of Coal II. The Components of Coal Harokl H. Schoberl Fuel Science Program, Pennsylvania State University, University Park, PA 16602
From Peat to Coal The formation of peat represents the end of the hiochemiis no commercially minable brown coal in the United States, but i t is a very important fuel resource in central Europe and cal phase of coalification. The subsequent events are rein Australia. Brown coal and iignite contain easily recognizferred to as the geochemical phase, or metamorphosis. Actuable plant remains, including coalified tree stumps or logs. ally there is not a sharp transition between the two phases, At this stage the geochemical processes reduce the moisture since some bacterial aition may continue in coal, and some geological processes, such as compression and increase in content and increase carhon. On an as-mined basis, peat may temoerature..mav alreadv have hemn durine the accumulabe 90% moisture, brown coal 60%, and lignite 3040%. On a tion of peat. moisture-and-ash-free basis, peat and lignite contain about Peat accumulation is halted bv a chanee in the environ60 and 70% carbon, respectively. Bituminous coal forms as temperature and pressure conment. In a low-lying coastal swamp, for example, a rise in the tinue to exert an influence on the buried deposit over hunsea level could result in the sea advancina into the swam^. dreds of millions of years. Continued codification of lignite drowning the plants. The sea might bringwith i t inorgan& sediments that would build up on top of the peat. These through subbituminous to hituminous coal results in a progressive increase in carhon content. A typical lignite might sediments provide a shield aga&t further decay; if instead contain 72% carhon, 5%hydrogen, 21% oxygen, and 1%each the peat hecame exposed to air, aerobic decav could destroy of nitrogen and sulfur. By comparison, a high volatile A it. Cbntinued accumulation of inorganic materials compresihituminous coal might contain 85% carhon, 5.5% hydrogen, es the peat further and results in the peat becoming huried 7% oxygen, and 1-2% each of nitrogen and sulfur. more &d more deeply. As depth of burial increasesiso does The loss of oxygen reflects continuing decomposition of the temperature. The combined effect of the geochemical the orieinal olant comDonents. which were com~arativelv aeents of temperature and comnression is to transform the rich inoxygen. In term; of thecomposition of the peat to coal. coal, coalification increases the carhon content and, initially, As in more familiar reactions carried out in the laboratorv. decreases oxygen. The hydrogen content is relatively cononce the reactantsand their concentrations have been estahlished. the ~hvsicalconditions that affect the reochemical stant through the bituminous coals, but decreases during the reactions inthe formation of coal are time, temperature, and formation of anthracite. A significant reduction in oxken, pressure. Time and temperature are the most important. and some drop in hydrogen, in the transition from wood to peat is consistent w i t h t h e anaerobic decay reaction disThe temperatures are established, in most cases, hy the cussed previously. A similarity of composition of peat and natural thermal gradients in the earth. The thermal gradilignin reflects the resistance of lignin to decay; lignin is the ents likely vary from place to place of the earth, but have principal survivor of decay a t the peat stage. As coalification been estimated to he of the order of 1 0 4 0 'Clkm ( I ) to 3050 OC/km ( I ) . If the surface temperature is ahout 30 OC in a nroeresses. the oxveen content continues to drop. alone with G n o r d&rease"in hydrogen, these changes bding tke retropical environment, a peat deposit buried about 2 k m sults of dehvdration and decarboxvlation reactions. As coalcould experience temneratures of the order of 100 OC. At through the bituminous coals toward these temperatures condensation of phenols, dehydration, ification anthracite, reductions in hvdrogen become more proand decarboxvlation could occur. ( E x a m ~ l e of s these reacnounced relative to the loss of oxken. The reactions ;hat tions that occur with simple organic molecules would he, contribute to the decreasing hydrogen are dealkylation of respectively, the formation of di(hydroxypheny1)methane arenes, aromatization of cycloalkanes or hydroaromatics, from reaction of phenol and p-hydroxyhenzyl alcohol, styand condensation of isolated rings into polycyclic systems. rene from 1-~henvlethanol, and benzene from henzoic acid.) (Some examples of these reactions applied to simple organic These r e a c t i k cause a loss of CO? and H1O. Peat may have molecules are, respectively, the conversion of ethylben~ene a compositionof,say 5 5 4 C,R'H, and 3O0bO (neglecting the to benzene and ethene (2); cyclodecane to naphthalene (3); relatively small amounts of nitrogen and sulfur). After being and cis-stilbene t o phenanthrene ( 4 ) . ) cooked for about 30 million years, the product may contain Anthracite is the highest rank of coal and, in most cases, is about 638 C. 5.58 H. and 23% 0. Because reaction rimesare also the oldest coal. The formation of anthracite involved measured in tens of millions of years, even reactions that are extremelv slow in the l a h o r a t o ~ mav" occur to an a-. ~ ~ r e c i a h l e verv- hieh temperatures and pressures exerted on the coal seams as the s;rrounding rocks were folded during a mounextent over the eons of geological time. tain huilding event. Most of the anthracite in the United This first product of metamorphosis is brown coal, which and was formed at States is in northeasterm~enns~lvania would be calied lignite B in the ASTM classification: There
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the time the Annalachian Mountains were raised. The composition of anthracite is about 95%C, 3% H, and 2% 0. With most of the hydrogen and oxygen gone, virtually all traces of the original plant material have disappeared; even under the microscope there are no visible remnants of plant structures. To reach high carhon and very low hydrogen and oxygen contents requires extensive aromatization and condensation of rings into polycyclic systems. The aromatic systems in anthracite may contain in excess of 30 condensed rings (5). With a carhon content annroachine 100%and the formation of very large sheets of arbmatic rings, increased coalification among the very high rank coals develops structures that become more and more like graphite. As lone as the nrincinal source of the temperature exnerienced h i t h e co&fying material is the thermal gradient in the earth, there will be a relationship between the ape and rank of acoal: the older a coal, the higher its rank. This is hecause the older the coal is, the longer it will have been exposed to elevated temperatures. Among coals in the United States, lignites are 60-70 million years old, bituminous coals ahout 300. and anthracites nerhans 350. Furthermore. the deeper a c o i is buried, the loiger it bill he exposed to th; high temperatures arising from the thermal gradients in the earth and the higher will he those temperatures to which it is exnosed. aeain resultine in the formation of hieher rank. ~ h u the s temperature a n d the time a t temperature play major roles in determining rank. Provided that the strata have not been disturbed, the deeper a coal, the older it is and the higher the rank it will have. This relationship was first expressed by C. Hilt over 100 years ago ( 6 ) ,based on a study of the coals of Wales, Germany, and France. In the lahoratorv. reactions can be accelerated hv increasing the temperat& or retarded by keeping the temperature cooler than normal. This same effect is observed in the geochemical processes in the formation of coal. If material undergoing coalification is heated to temperatures much greater than provided by normal thermal gradients, as by a sudden intrusion of magma (an example of a process known as contact metamorphism (7)),the rank may become much higher than predicted by Hilt's rule. Anthracite deposits in the Rocky Mountains are less than 100 million years old, the coal attaining this rank in a geologically short time because of the unusually high temperatures provided by intrusions of magma probahly associated with the mountain building. Coals of the same geological age on the plains may still be of subbituminous rank. In the Soviet Union a large deposit of brown coal outside of Moscow isover 300 million vears old. It has evidently never experienced deep burial andthe associated elevated temneratures that would orornote coalification to higher ranks. The Organic Components ol Coal Examining a rock-a piece of granite, for exampleshows that it is cornnosed of discrete mains of minerals. The minerals may be obvious to the unaided eye and can certainly be seen with a hand lensor microsco~e.An examination of coal shows that it too has a structural order with discrete components analogous to the minerals in a rock. In hituminous coal it is often possible to distinguish distinct bands of material. Four types of these bands, which are called lithotypes, are distinguished on the basis of appearance and fracture hehavior. Vitrain is glossy black with a conchoidal fracture; clarain, glossy with horizontal fractures; durain, hard material with a dull, granular appearance; and fusain, material that looks much like charcoal. The different lithotypes are a result of different conditions that existed during deposition of the coal. For example, fusain looks like charcoal because in fact it was-fusain may have formed in fires in the coal swamp that partially carbonized or "charcoalized" the trees. The charcoallike structure persisted into the coal. Examination of lithotypes with a microscope shows that
Composltlon Dlflerences among Maceral Groups ( 1 )
~xinne Vitrinite inerlinite
85.5 83.5 86.8
7.3 5.1 3.9
0.5 0.8 0.0
0.9 0.9 0.7
5.8 9.7 8.0
they have components on a finer scale. These components are roughly analogous to the minerals in a rock and are called macerals. The analogy of minerals and macerals is not exact, because, on a molecular level, a maceral may contain a variety of compounds rather than heing a pure chemical suhstance. However, the fact that a piece of coal consists of a variety of macerals in the same way that a rock may consist of a variety of minerals leads us to call coal an organic rock. The microscopic examination of the components of coal is called coal petrology; a useful introductory hook ( 8 ) and a standard reference (9) are available. Minerals are conveniently grouped into various categories: the carbonates, silicates, sulfides, and so on. Similarly, macerals are categorized in maceral groups: vitrinites, exinites, and inertinites. The maceral groups isolated from a specific coal show distinct differences in composition. An example for a high volatile hituminous coal is shown in the table, condensed from (lo), where the compositions are shown on a dry, ash-free basis. The lithotypes are composed of different aggregations of the maceral groups. Vitrain, for -example, is predominantly vitrinite, with smaller proportions of exinite and inertinite. The macerals arise from different kinds of codified nlant material. The decomposition of lignin produces humus, a homoeenous black material. Coalification of humus nroduce~maceralsin the vitrinite group. Since lignin is relatively rich in oxygen, vitrinites tends to have the highest oxygen content among the macerals of a given coal. The exinite eroun of macerals form from specialized components of the planis. Spores become the maEerd sporinite,iesins become resinite, and leaf cuticles become cutinite. The plant materials which eventually become exinites are generally aliphatic and hydrogen-rich; consequently, the exinites have the highest hvdroien content of-the maceral mouns. Carbonized wood-heches the maceral fusinite in &e inertinite group. The carbonization nrocess involves loss of alinhatic fraements by thermal cracking, so inertinites have'the highest carbon and lowest hvdroeen contents of the maceral erouns. These relationshipsare $lustrated by the data in thetahie. The various comuonents of plants have different molecular structures. or kxample, the waxes in a leaf cuticle are esters of long-chain aliphatic alcohols and carboxylic acids, whereas resins are hased on cyclic diterpene structures with an unesterified carhoxyl groups. (Terpenes are isoprenoid compounds having structures which are hased on the CsH8 structural unit of isoprene, 2-methyl-1,3-hutadiene (ll).) Because of their different molecula~structures.these compounds will responddifferently to heat,solvents, and chemical reaeents. Althoueh the orieinal structures will likelv be modifizd during coa~fication.~he resulting cutinite anci resinite macerals will nevertheless have different volatile matter and fixed carhon contents, different heating values, and different behavior in solvent extraction or chemical reactions. Therefore, if we determine the kinds of macerals and the amount of each in a coal sample, we should he able to infer something about the thermal and chemical hehavior of the coal. This determination is called petrographic analysis. I t isespecially important in predicting coal behavior for coke production. The macerals and lithotypes form the links between a plant and a piece of coal. A cypress tree standing in a coal swamn contained classes of materials for maintainine its structure, providing food and energy, and for performing specialized services. Each class of plant component contains
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specific molecular species: cellulose and lignin as structural materials, carbohydrates and fats as energy sources, and waxes and resins for special needs. At a molecular level, each of these classes of components codifies to specific, microscopically identifiahle macerals that have composition linked to the original plant materials. Individual macerals form maceral groups, which in turn comprise the macroscopic lithotypes. The lithotypes make up the "whole coal".
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The lnorganlc Components of coal
The formation of ash indicates that coal contains inorganic suhstances in addition t o the carhonaceous, plant-derived material. I t is often possible to see specks of minerals in coal with a magnifying lens or microscope, or sometimes with the unaided eye. Several processes led to the incorporation of these inorganic materials into the coal. One source of inorganics is the plants themselves. As part of their life processes some plants may accumulate significant amounts of inorganic matter. A modern example is the scouring rush, a tiny descendant of the gigantic treelike rushes that flourished in the coal swamps of the Carhoniferous era. The scouring rush accumulates silica in its epidermis: its name comes from the fact that it was useful t o -pioneers and native Americans for scouring cooking ware. As such plants died and decaved. their inoraanic components would accumulate along with the coalifyilg plant material. The water in the coal-forming environment serves as an excellent transport medium to bring in dissolved ions and smallmineralmains. Some of the oxygenin peat is present in carhoxyl groups that are able to f&ctionas ion~exchange sites, capturing dissolved ions from solution. The specific inorganic constituents accumulated by the coal a t this stage will depend upon the nature of the nearby rocks from which the water-borne inorganics were leached or eroded, the patterns of water flow in and through the peat deposit, and on the ion-exchanee canacitv of the peat. As codification continues, the Eoal becomes less porous or permeable to water. However, brown coals and lignites still provide pathways for groundwater (in fact, some seams of these coals are eood aauifers), and they still contain carhoxyl sites for ion exchange: The alkali andalkaline earth cations are the principal species participating in the ion exchange. The continu$ flow of water through the hrown coal and lignite seams may provide an opportunity for exchange of ions between the coal and water. Thus the concentration of exchangeable ions, such as sodium and calcium, may change with time. Another consequence of the ion-exchange ability is that these low-rank coals may show great variability in their inoreanic com~osition.whether measured verticallv or laterally through t6e seam.'Bituminous coal does not contain carboxvl erouos and does not disdav . . - the ion-exchange behavior ofth;? lower rank coals. Even after the coal has been buried and compacted, water can still percolate through the cracks or joints in the seam. Minerals can he deposited in these cracks by precipitation. Examples are calcite, gypsum, kaolinite, and pyrite. Pyrite is the source of much of the sulfur in hituminous coals. Often pyrite accounts for no more than about 5% of the total amount of inorganic matter in coal, hut its presence in coal is a matter of concern far out of proportion to its concentration, because of its role in forming sulfur oxides during comhustion.
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Sulfur in Coal Although the total sulfur content of most coals is less than 5%, and in many cases close to 1%,sulfur is the most notorious of the components of coal. This notoriety is a result of the conversion of the sulfur to sulfur dioxide and trioxide when the coal is burned. If emitted to the air, these oxides can cause serious air pollution problems; alternatively, expensive equipment is required & remove the oxides from the flue gases of furnaces and boilers. The ways in which sulfur is 292
Journal of Chemical Education
incorporated in the coal and methods for removing it hefore the coal is burned are topics of great - interest in coal chemistry (12). Sulfur occurs in coal in three forms: organic sulfur, which is part of the molecular structures in the coal itself; pyritic sulfur, which consists of pyrite and its dimorph marcasite; and so-called sulfate sulfur. which is orimarilv m s u m and iron sulfates. In low-rank cbals half t i e total sulfur content mav he oreanic. hut in most hituminous coals ." nvritic sulfur is predomin&t. The organic sulfur is principally a remnant of sulfur-containina com~ounds.such as proteins. in the orieinal plant tissue.~ow&er, m k h of the sulfur in coal arises from bacterial attack on aqueous sulfate ions. In anaerobic conditions bacteria can convert sulfate to sulfur:
.-..
4Ht
+ 2S04-z = 25 + 3 0 +~ 2H20
or sulfate can react with organic matter: ~ S O I+ - ~CeH120e = 6HC03-
+ 3H2S
The hydrogen sulfide can react with iron compounds to precipitate iron sulfides. The iron sulfides are madually con. ker& to pyrite. The sulfate concentration of fresh water is usually very low, hut in marine waters sulfate has the second highest concentration amona the anions. This difference in water differences for the sulfur content of chemistry has coals in the United States (13).Coals in the Rocky Mountain region were deposited in swamps along rivers or in river deltas, and were protected from the sea by coastal ridges. These coals are usuallv verv low in sulfur. The coals in Illinois and surrounding states were deposited from marine or hrackish water a t a time when a sea effectively split the North American continent in half. These coals can he very hieh in sulfur. up to 8%in some cases. Coals were deposited in'the ~ ~ ~ a l a c h iunder a n s conditions that changed from the fresh water of an inland sea to hrackish or saline when i t became an emhayment of what is now the Gulf of Mexico. As a result, the coals in Appalachia have variable sulfur contents, in the range of 0.5% to 5%. Coal is a material of highly variahle composition. Even the carbonaceous portion mav contain. for example, 65 to 95% carbon and 2 about 3d%oxygen: The cardoniceous portion of the coal is physically heterogeneous, being composed of varying amounts of different macerals, each of which has characteristic chemical and phvsical properties. Associated with the carhonaceous materialis a vaiiable amount of moisture and variable amounts of inorganic suhstances. These facts make coal one of the most complex and complicated substances chemists can study. Ilespitc thc complexity of coal ax a substance, despite the complexity of the formation of coal, and despite the millennia ieouiied for the processes to occur. it is important to remember that the pknciples of chemistry have-not somehow gotten lost along the way. Coal has the chemical composition and physical properties that we observe because it is the nroduct of a seauence of specific chemical reactions occ&ing with an assemblage of specific chemical compounds. Although a large number of compounds compose the "starting material" for the formation of coal, their structures are, for the most part, known. The reactions that they undergo are ones that have been fairly well studied with simpler compounds in the laboratory: hydrolysis, dehydrathermal cracking, cyclization, and tion, de~arbbx~lation, aromatization. T o be sure, the kinetics, mechanisms, and structures of the products of those reactions are not all well understood. Working out the details of the kinetics and mechanisms, and of the structural relationships in coals remains a major challenge for organic geochemists. The formation of coal was introduced as a detour in the global carbon cycle. Instead of plant matter decomposing
entirely to carbon dioxide and water, intermption of the decay process has produced the world's enormous coal deposits. Now as we extract the coal from the earth and burn it, we humans are completing the find step in the cycle by returning the carbon locked in the coal to the atmosphere. ~h~ returnof massive amounts of carbon dioxide may have profound long-term climatic consequences, the "greenhouse effect" (14,15): Literature Cited 1. Van Kreuelen, D. W. Cool: Elsevier: Amsterdam, 1981. 2. Gate.B.C.:Katzer. J. R:Sehuit, G.C.A.Chsmistryof Catolytie Rocrssaa;M=G~aw-
Hill: New York, 1979; Chapter 1. 3. Fieser, L. F.; Fieaer, M. Aduancrd Organic Chamisfry; Reinhold: New York, 1961: Chanter R. ~ ....-...l..
5. Gemteim, B. C.: Muwhy, P. D.; Ryan. L. M. In Cool Sfruclure: Meyeri, R. A.. Ed.: Academic: New York, 1982: Chapter 4.
s hilt.^.^. V ~ ~ . D O Y ~ S1813.17.194. ~~.I~~. 7. stutzer,0. Geology olCool; Univ. of Chicago- Chicago, 1940: Chapter WII.
8. Busti". R. M.; Camem". A. R.;Grieve. D. A.; Kalkreuth, W. 0.Cool Petrology: Itr Principles, Methods, and Applieofions; Geological A%soeistion of Canada: St. ~ohn's,NF, 1983. 9. Stech,E.;etd.Stoch's Textbook o f C d Petmlogy: Gebruder Borntraeger: Stuttgart, 1982. 10. M ~ C L O M ~ Y M. . T. A C IS^, ~ ~77.111. 11. Roberts. J. D.;Cknerio. M. C. Basic Binciples ~(OrgonicChemistry: Benjamin: New York. 1984: Chapter 30. 12. Tsai, S. C.Fundommfok ofCoolB~nefieiofionand Utilization: Elsevier: Amsterdam, 1982; Chapter 5. 13. Given.P. H. In CaolScisnce;Gorbatv.M. L.:Lersen. J. W.; Wender, L.,Eds.;Acedemic: New York, 1984: Vol. 3. 14. Fowler, John M. EnergyondthcEnuironmenG MeGraucHill: New York. 1984: Chapfer 9. 15. Rase. David J. Lmrninaobout Enarm: Plenum: New York. 1986 Chaotter 3.
4. Mallory, C. W.; Mdlory. F. B. Org. React. 1984,30,1
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