Coal Analysis, Characterization and Petrography
L. Petrakis and D. W. Grandy Gulf Research & Development Company pl~sburgh,PA
In recent years coal has become the focal point of the work of many chemists who have no previous formal training in the subject. This situation is likely to continue, given the recent emphasis by the Federal government on coal as the solution to some of our energy problems. The government programs include the solvent refined coal (SRC), Exxon donor solvent (EDS), and H-coal processes for making liquids, various coal gasification projects, and magneto-hydrodynatics (MHD) for using the energy released in the combustion of coal to make electricity directly. Since coal has been historically the province of a small group of geologists and engineers, the terminology and measured properties in the literature have reflected the interests of these groups. There is agreat amount of work in the literature ( I ) , often with divergent and seemingly unrelated terminologies. The purpose of this paper is to present a brief review of some of the important properties of coal and the systems of nomenclature in a way that will be useful to chemists and those who may be incorporating coal in their classroom presentations. Coal is a very complex, heterogeneous mixture of organic compounds and minerals. A very simple but useful analogy is to think of coal as being like a fruitcake (2). The various comoonents in the olant material. such as wood, hark, sap, ~eavks,etc., whit h make up coal are analogous to the fruits; nuts, and hatter which are used to make a fruitcake. Considerable mixing before "baking" occurs in hoth cases. How well "done" a cake is has a parallel in coal science called the rank o i a coal, or the extent of melornorpl~isrn.The concentration and distrihution of particular components in coal originating from different parts of the plant determine the coal's pelrography. Geoloeicallv. ,. .. coal is a sedimentarv. ..oreanic rock. rather than a mineral, although it isoitpn thought ofasoneolour..mineral resources." I t does not meet the soecitications of'an inoraanic substance of definite composition, nor is it a cryst&ne solid.
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Coal Analysis In order to classify a coal, certain properties must be ascertained. These parameters, along with the general classification, are used by the prospective user of the coal to determine its suitability for a particular application. We review here, briefly, the most common types of analyses employed and their relative merits and uses. Proximate Analysis Proximate analysis includes the determination of moisture, ash cmtent, fixed carbon, volatile matter, and heat content. Volatile matter and heat content are used in the ASTM classification of all coals (3). Proximate analysis is used commonly in gross industrial applications such as the purchase of coal for generation of heat for various processes. A ourchaser would want to know how much inert material. moisiure, and ash he ii buying, as well as how much coal he wili deed based on its heat content for a particular application. Volatile matter and fixed carbon percentages are of particular interest to makers of metallureical coke. Proximate analyses are performed according to standard methods proposed by the American Society for Testing and Materials (ASTM); however, other methods
which differ somewhat have been used (4). . . Moisture content is determined by weight loss after drying a coal sample in an oven at 100-1 1O0C swept by dry air r.51. Volatile matter is determined hy heating a coal sample tu 95O0C in a platinwn crucible for 7 min. Volatile matter is the weight loss percentage minus moisture percentage 16). Ash content is determined hg hurning a coal sample tu constant weight (7).The percentage of fixed carhon is calculated hv suhtractine the ash. moisture. and volatile matter percentages from 100. Ultimate Analysis The ultimate, or chemical analyses, are used for the more thorough scientific investigation of coals, such as classification of coals and determination of a coal's suitability for various applications. Elements commonly determined in coal are hydrogen, carbon, nitrogen, and sulfur. Oxygen content is usually calculated hy difference and, therefore, may he affected seriously by the accumulated errors in the other analyses and trace elements, such as chlorine, not usually determined. More recently, neutron activation analysis for oxygen is being employed. An empirical relationship known as the Mott-Spooner formula is used to calculate heat content of coals from chemical analyses (8).When the heat content of a coal is known, this relationship can be used to check the analysis and may serve to point out possible errors or warn the analyst that he is dealing with an unusual coal. The utility of any analysis depends upon which basis it is reported. Four common bases are used: as-received, dry, dry ash free (daf), and dry mineral matter free (dmmf). The asreceived basis is probably of the most use to the common consumer. The dry basis would appear to have no real ad\,antage, except to compare ash contents, and it is not commonly used. The dry ash free (daf) and dry mineral matter free (dmmf) bases are commonlv used to classifv coals and to compare them chemically. he dmmf basis is preferred, since corrections are made for the effect of loss of hydrated water from clays, sulfur loss from pyrite, and COz from carbonate minerals in the ashing process. There are several formulas for correcting the ash content to the mineral matter content, the Parr formula or some modification of it being the most common (9).Although these correction formulas have been known for more than 50 years, it appears that some authors are unaware of their significance (10). In addition to the more common analyses used in coal classification, some other parameters describing a coal's physical nature or its reaction to heat are sometimes useful. Among the many parameters measured are grindability ( l l ) , plasticity (121, and free swelling index (13). Rank The rank of a coal may he defined as the extent to which the organic material has matured during geologic time in going from ueat to anthracite. The maturation orocess is known variously as coalification, metamorphism, or carbonification. The change in rank of acoal is determined by depth of burial, pressure, and temperature; these are interrelated through geothermal gradients. Assuming no magmatic heating is present within a given vertical sequence of coal seams, the rank of the coals is found to increase with deuth. This relationship is known as Hilt's Law (14). Volume 57, Number 10, October 1980 / 689
Table 1. Class Anthracite
Bituminous
Subbitumi-
ASTM Classlflcatlonof Coals by Rank
Group Meta-anthracite Anthracite Semi Anthracite Low volatile Medium volatile High "01. A High "01. B High vol. C
Caiorlfic Value (Bt~Ilb)~
Subbit A
>I4000 13000-14000 1 1 500-13000 10500-11 500. 10500-11 500
Subbit B Subbit C Lignite A Lignite B
9500-10500 8 300- 9 500 6 300- 8 300 98 92-98 86-92 78-86 69-78
