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inner thermometer. On plotting the readings for the two thermometers it will be indicated at the crossing of the two lines. This point is of such great significance from the standpoint of spontaneous combustion of coal that it is here designated as the critical oxidation temperature for the particular
Vol. 17,No. 2
Deterioration and Spontaneous Combustion of Coal in Storage’ By S. W. Parr U N I V s R S I T Y OF ILLINOIS, URBANA,
ILL.
T
TNne /n Mhufes Figure 6
coals studied. Determinations of this point were made upon a number of freshly prepared samples and also upon parallel samples of the same coals which had been subjected to oxidation or what might be called complete “weathering” or saturation with oxygen at a temperature below the critical point. Discussion of Results
A few typical charts giving the results on freshly prepared samples are first presented as Figures 3,4, and 5. The critical oxidation temperatures for the samples of Illinois coal are within a range of 138’ to 143’ C., and the corresponding temperature for the Pocahontas coal is 202’ C. It seems fair to conclude that, in the case of these coals in storage, assuming free accessibility of oxygen, a temperature within the mass approximating the range indicated by the charts would denote a speedy progression to the stage of ac-
HE first organized attempt along scientific lines to bring about improved conditions in the operation of the coal industry in the United States reached a definite status when the administrative board of the American Engineering Council, in 1923, officially authorized the appointment of a committee on storage of coal. This committee has made a comprehensive study of the various problems pertaining t o the topic,2 such as production, distribution, transportation, handling, and storage, but it is evident that the factor of fundamental importance, and the one that is directly associated with every phase of the problem, is the matter of storage, with the attending questions of deterioration and spontaneous combustion. It is entirely appropriate, therefore, that in this symposium we call attention t o this report, not only because of the related information it contains, but because of the profound importance of the subject, not only to the coal business, but in some form or other t o almost every phase of our industrial, economic, and social life. Let it be emphasized, also, that the problems connected with the storage of coal depend upon chemical phenomena to a far greater extent than any other, and their solution must be worked out by the chemist. It is the purpose of this paper, therefore, to review briefly those chemical activities which are recognized as peculiarly characteristic of coal and to indicate the results of recent studies along this line. Indeed, it is believed that the accompanying papers will convey more conclusive evidence as to the underlying factors connected with the storage of coal than has heretofore been available. Coal and Free Oxygen
The ability of coal to absorb oxygen from the air is well recognized. A summarized statement of the conclusions growing out of avidity of coal for oxygen is as follows: 1-Coal,
by reason of its peculiar composition and structure,
is capable of absorbing very considerable quantities of oxygen.
Figure 7
tual ignition. Although these charts are based on the use of pure oxygen the conditions under contact with the oxygen of the air would show a lag as to time, but substantially in agreement as to effect, provided other things, such as radiation, are the same. Figure 6 repeats the conditions on the Montgomery County coal, but after it had been saturated with oxygen-that is, “weathered”-at an oven temperature of 100” C. A consistent increase in the critical range of oxidation is evident. Figure 7 represents the temperature required to produce autogenous oxidation for the several stages of “weathering” with coal from Saline County.
2-In general, the coals with higher free or textural moisture have the greatest avidity and the largest capacity for oxygen. 3-The avidity of a coal for oxygen is augmented by a rise in temperature. &By the process of adsorbing or condensing oxygen upon the surfaces, including the interior or textural structure of the coal, the oxygen is in the best possible form for promoting the oxidation of the material with which it is associated. This readiness to pass over from the form of surface combination is enhanced by ( a ) heat, ( b ) the catalytic properties inherent in the material, and (c) the unsaturated character of many of the constituents of the coal, which thereby accentuate the readiness with which chemical combinations are entered into. 5-Under suitable conditions the sulfur of iron pyrites reacts chemically with oxygen, resulting in the generation of heat. Deterioration
OXYGENhsomTIoN-The first result of oxygen absorption on the part of coal at ordinary atmospheric temperatures is physical rather than chemical. It would be expected, therefore, that at normal temperatures and for a reasonable length of time the actual deterioration of coal as indicated by loss of heat units is apparent rather than real. The increase of weight due to the absorption of oxygen is therefore in the Received August 5, 1824. “Industrial Coal: Purchase, Delivery, and Storage.” American Engineering Council, The Ronald Press Co., 1914. 1
3
Report of the
INDUSTRIAL A N D ENGINRERING CHEMISTRY
February, 1925
main sufficient to account for the apparent decrease in heat units. Evidence of this statement is given in Figure 5 of the paper by Parr and Milner in this symposium, where the curve for the formation of carbon dioxide at normal temperatures shows no appreciable increase, and again in Figure 6 of the same paper the loss in heat units at normal temperatures and for a similar period of time, 48 days, is very small. Compare also in this connection the persistent increase of weight as shown by all samples under all conditions until a limit of from 7 to 9 per cent of the original weight of the coal has been attained owing t o the absorption of oxygen. It will be well a t this point to add graphs by Cheng3 (Figure 1) in which air is used l0
carbon dioxide as well as the positive drop in calorific values which accompanies this range of temperatures. DISINTEGRATION--AI1 increase of fines implies a variable or uncertain control of air supply in the process of combustion and hence a lowering of the over-all efficiency in the generation of steam. Although this difficulty may be one calling for more intelligent handling of the fuel on the grates, the extent to which disintegration is likely to occur upon storage is an important factor to be considered. Here again the coals differ very widely in their disintegrating tendency. I n general, the more dense coals and those carrying the lower percentages of free or textural moisture break down the least. This general statement, with the amount of increase in fine material for type coals, is summarized in Table IIa6
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Table 11-Summary
of Boiler Tests Average overall efficiencies Percent
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instead of oxygen. The same characteristic is seen as to the increase of weight at normal temperatures; the only difference observable is a lag in the matter of time, Note that in Figure 2, with the temperatures maintained at approximately 25" C., the unit of time is in weeks, even though the atmosphere used was oxygen. Still another evidence as t o a general agreement between an increase in weight and the loss of heat units is afforded by Table I, summarized from the results of storage in car lots over a period of six years.4 From this accumulation of data the statement seems warranted that for normal temperatures there is substantially no loss in actual heat units, the apparent loss being accounted for by a corresponding increase in weight of the coal due to oxygen absorption. Table I-Results
of Storage of Coal for Six Years -UNIT COAL,B.T. u.-
Difference by per cent
PER CENT ASE, DRYBASIS DifDitAfter ferAfter ferFresh 6 years ence COAL SIZE Fresh 6 years ence Vermil- Nut 13.76 11.45 2.31 14632 13934 698 4.8 ion Screen- 15.18 14.11 1.07 14791 13302 1489 10.1 County ings SangaNut 16.86 12.96 3.90 14613 13790 823 6.6 mon Screen- 17.13 14.95 2.18 14524 13604 920 6.3 County ings William- Nut 14.41 13.41 1.00 14838 14265 573 3.86 son Screen- 14.72 13.03 1.69 14716 14170 546 3.70 County ings Note that these values indicate only general conditions. Part of the loss in ash should be accredited to oxidation of sulfur and leaching out of the soluble material.
OXYGENCOMBINATION-The foregoing discussion refers to temperatures in storage which have not exceeded 50" C. If temperatures are maintained for a time between 75"and 125' C., then actual loss of heat value occurs, presumably because the absorbed oxygen passes over to the chemically combined state. Under these conditions coal in storage would show a decided drop in heat units. An actual loss of values of from 8 to 15 per cent may occur. These conclusions are justified by noting in the graphs already referred tos the loss of water and
* University of Illinois, thesis for master's degree, 6
63.5
Heating of Coal in Storage
\
4
64.2 63.8
1924.
Parr, University of Illinois, Eng. Dept. Sta., Bull. 97 (1917). See Figures 3 to 6,paper by Parr and Milner, p. 115.
Reference has already been made to the behavior of oxygen, in that it is absorbed, presumably, first in the physical stage, from which it passes over, slowly at atmospheric temperatures and rapidly at elevated temperatures, into chemical combinations with unsaturated compounds or in the ultimate forms of water and carbon dioxide. The heating of coal in storage may therefore be summarized as follows: 1-A high percentage of free or textural moisture in a coal indicates a high capacity for oxygen absorption. Such coals, immediately upon breaking out from the seam, begin to lose moisture until an equilibrium with the moisture of the atmosphere is established, thus making way for the accession of oxygen. 2-The l:-trgerthe lumps of coal the slower the interchange as between the free or inherent moisture and oxygen. Conversely, the more finely divided the material the more rapid the interchange and also the greater the superficial area of the coal particles. S
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Figure 2
3-Any departure from normal temperature upward, unless checked or dissipated, is a vital factor, whether such increase of temperature arises from external sources or is the result of chemical combinations within the coal itself. 4-Accessibility of air within the coal mass, thereby augmenting the supply of oxygen beyond the amount taken up by absorption, is essential for carrying the oxidation forward, thus bringing the temperature of the mass up to the point of ignition. 5-Accessibility of air, involving ready circulation to an extent which will carry away the initial increments of heat as rapidly as formed, will prevent a rise of temperature. 6-Complete prevention of an additional oxygen supply beyond that absorbed by the coal before entering the storage pile will insure against any appreciable or dangerous rise of temperature. 7-The oxidation of pyritic sulfur requires the presence of free moisture as well as oxygen. The augmenting of the heat in 8
Parr, loc. tit., adapted from Table 17.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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any manner greatly accelerates the oxidation processes for both the hydrocarbon and the sulfur components of the coal.
The behavior of coal toward oxygen as thus summarized is shown in Figure 3, based on the absorption capacity of finely divided material at varying temperatures. Although the curve is diagrammatic in character, it is in fact the assembly of data from many different experiments, and accurately expresses the relative behavior of bituminous coals a t the various temperature ranges indicated.' Oxduhon of Dry Cod
Figure 3
From a study of the various charts it would appear that a t normal temperatures-that is, below 50' C.-the oxygen is taken on as a physical rather than a chemical accession, at first with extreme slowness, then more rapidly. As the temperature rises there is given off in determinable quantities both water and carbon dioxide. These products begin to appear a t about 65" to 85' C., depending on the length of time the coal has been subjected to oxygen saturation. After 100" C.has been passed there is a constantly accelerated rise until about 140" or 150" C.,when the temperature very rapidly proceeds to the ignition point. With reference to the initial or first stage of oxygen absorption at normal temperatures, coals differ in the ease with which they pass over from the merely physical to a chemical combination. It has already been pointed out that high-moisture coals have a higher capacity for the absorption of oxygen; and, therefore, are able to acquire a larger supply of potential danger. The more dense and harder coals show less abrasion and accumulation of fines in the process of handIing and shipment. Note should be made here of the fact that as a general rule density and hardness accompany low free moisture, while lightness and friability are accompanied by the higher percentages of moisture. It is also true, as above noted, that coals differ in the character of their carbon compounds with reference to the ease or readiness with which they combine with oxygen. Here is a critical phase of these initial reactions to which we must look, in the absence of extraneous sources, for the starting point in the building up of the heat increments. But concerning these compounds and how to identify or label them, we do not know. Actual or authenticated illustrations of types or mines producing coal with an excessive ease toward this type of oxidation are very difficult to obtain. Up to the present time the only source of information is that of experience or well-proved behavior, and even a reputation for poor storing properties could be reversed where certainty of control as to the fundamental conditions is maintained. 7 The curves and data contributing to this composite graph have been for the most part developed by R. T. Milner, C. C. Coons, and T. Y.Cheng.
Vol. 17, No. 2
Although this feature of greater initial activity as a property of certain kinds or types of coal is yet a problem for study, even if a solution were devised and a distinction made possible, it would only mean that certain coals of definite designation could be stored carelessly with the probability of getting by without loss, while other coals, also of definite designation, could only be stored safely with careful observance of the fundamental conditions as herein set forth. The fact remains that all bituminous coals have numerous hydrocarbon compounds of the cyclic or aromatic type, unsaturated and hence readily oxidizable, but varying as t o the readiness with which they oxidize. I n some coals the sulfur, either because of structure or size of particles, is more readily oxidized than in other cases. While we are a t present more inclined to charge these initial heat reactions to carbonaceous material than to sulfur, it is by no means wise to ignore the presence of pyritic sulfur as a possible source of heat. It may also be noted that the dull bands and the mineral charcoal are the portions that contribute most largely to the ash and quite uniformly also to the sulfur content of the commercial material as received. In this finely divided material, therefore, are possible factors which may accentuate the danger due to the forms in which sulfur occurs. Moreover, these conditions as to sulfur increase are not constant, especially in the layers of mineral charcoal. Indeed, this material may vary as to its relative amount in the output of the same mine, and differences in behavior may frequently be ascribed to sulfur forms and sizes rather than to the coal proper. Summary
It is now possible t o predict the behavior of coal under the various conditions met with in storage practice, and to prescribe the procedure to be followed. These conditions are exceedingly simple and may be enumerated as follows: 1-Anthracite has a minimum capacity for oxygen absorption and can be stored substantially in any quantity without deterioration or danger of loss from spontaneous combustion. 2-In storing bituminous coal i t is of fundamental importance that the coal be free from dust or duff-that is to say, material in any considerable quantity that would pass a '/e-inch screen. In the case of lump coal, an addition of fine material in masses too large to admit of carrying away the initial increments of heat by transpiration of air currents will invite local heating in the areas where the fine coal is located. Conversely, slack or screenings with occasional zones of nut or lump sizes where air may enter will heat first at the margins of the areas where the coarse coal occurs. Illustrations of these conditions will readily suggest themsqlves. A storage mass of lump coal adjacent to a pile of slack will heat at the border line. The piling of screenings in a way to create channels for the free passage of air, as by coning, with the consequent rolling down the side of nut sizes, will insure heating at the margins of the chimney thus formed. Posts, girders, or braces may afford a ready access of air and promote heating at such points. Indeed, i t may be noted in this connection that screenings can be successfully stored if conditions are maintained which prevent the circulation of air. The opening up of channels for a limited admission of air within the interior of such material must be avoided. In brief, there must be free circulation or no circulation of air. 3-Every possible accession of extraneous heat must be avoided. The reason has already been made obvious. A few of the possible sources of heat need only to be enumerated-hot walls or bulkheads, steam pipes, conduits carrying hot pipes or hot air, the sun's rays, etc., etc. Any interior temperature approximating 75" C. should be considered as having reached the danger line. 4-The height of piles is limited only by the increased tendency toward segregation of sizes with increase of height. Theoretically, with actual observance of the conditions of free air or n o air, there is no limit to the height to which storage piles might be carried, but practically, with the added chance of violating those conditions, relatively shallow piles are safest. The height should, of course, be governed by the conditions as above emphasized. In case of doubt use moderate heights, say from 10 to 15 feet. There is the added advantage of dissipating the heat
February, 1925
INDUSTRIAL AND ENGINEERING CHEMISTRY
by conduction and radiation, as also the possibility of quick removal of hot spots for use under the boilers. 5-The question of coal types has little significance compared with the importance of maximum voids as secured by screened and sized lump, accessibility of air currents, and absence of extraneous heat. Any bituminous coal with normally good storing qualities may be stocked in a manner to insure freedom from heating. As a matter of fact, the conflicting statements often met with concerning the storage qualities of the same coal, even of coal from the same mine, are evidence of the truth of these propositions. Indeed, the same statements may be extended to include coal mixtures, and emphasis is here intended to be put primarily upon the fundamental requirement of absence of fine material. 6-Coal in storage which has acquired a temperature of, say, 75' C., will proceed to the point of ignition unless the conditions are altered. A continuance of that temperature for any considerable length of time, a few days a t most, will result in a reduction of heating value, and in case of a coal to be used for carbonization, a weakened structure in the resulting coke. Rating of Coals for Storage Purposes
It is difficult to summarize the information thus presented in terms of possible rating as to storage adaptability of specific coals. The fundamental factors involved, however, would seem to suggest the propriety of assigning storage qualities on the basis of size rather than type. For example, the high-moisture coals of the lignitic type would be set down as the most difficult or impossible to store without heating. The next in order of betterment would be the bituminous coals having from 12 to 15 pet cent of moisture. The paramount factor in all bituminous coals, however, is fineness of division. This would include, of course, tendency to breakage and the production of fines in the process of shipping and handling. Those coals which disintegrate least on weathering are less ready to start active chemical reactions in the pile. Lump sizes free from duff may be safely stored. I n coals of the low volatile or Pocahontas type we have a low water content and a greatly reduced avidity for oxygen, but these qualities, which should contribute directly to the storing properties of these coals, are frequently offset by structural peculiarities that contribute to the ease with which they disintegrate and develop fine material. However, it should be noted that, because of the greatly reduced adsorption capacity of such coal for oxygen, the fines do not have anywhere near the danger that attaches to fine material from the highmoisture coals. Although initial reactions resulting in the development of heat may have their origin in the carbonaceous substance, independent of the presence or absence of sulfur, still it must appear that this latter element constitutes an added feature requiring consideration. However, since the danger from sulfur largely centers in the duff, and since the avoidance of this material is essential also because of the ease of oxidation of the carbonaceous portion, an added argument appears for eliminating the fine material from the mass.
Copper Production Breaks Peace-Time Records The production of copper in 1924 broke all previous records except those for the years during the World War, according to the Department of the Interior. The output from domestic sources during the last year was 1,628,000,000 pounds, compared with 1,435,000,000 pounds in 1923, an increase of more than 13 per cent over that year, which itself was the highest on record with the exception of the war years. The smelter production of copper in December, as estimated by the producing companies, was 137,000,000 pounds, a little higher than the average monthly production for the year, or at the rate of about 1,644,000,000 pounds a year. The production of new refined copper from domestic sources, determined similarly, was about 1,764,000,000 pounds, compared with 1,464,000,000 pounds in 1923. I n 1924 the production of new refined copper from domestic and foreign sources amounted to about 2,293,000,000 pounds, compared with 1,980,000,000 pounds in 1923.
123
Deteriorations of Midwest Coals under Different Storage Conditions-Effect on Burning Qualities' By N. R. Beagle ILLINOIS POWER & LIGHT CORP.,PEORIA, ILL.
T
HE Illinois Power & Light Corporation is a public utility corporation operating an Electric Railway System of over five hundred miles, as well as street railways and gas, electric, steam heat, and municipal water supply properties in six of the midwest states. The larger properties, however, are in Illinois, Missouri, Kansas, and Iowa, and therefore the greater percentage of coal used comes from the Illinois field. The remainder of the supply comes from Kentucky, Kansas, Missouri, and Iowa. Necessity for Coal Storage
Unfortunately, it is essential that a certain reserve coal supply be always kept on hand a t a utility plant, for service must be maintained and railroad deliveries are not always prompt. Then, too, there is the necessity of building up a big reserve about once each year, for strikes and rumors of strikes are always with us. In recent years the biggest storage was in 1922, when a reserve of about 220,000tons was built up on account of the coal strike that year. I n the spring of 1924 the storage reserve was only about 125,000 tons, the usual reserve of other years being between these two limits. During 1923 the total coal used was near 1,OOO,OOO tons of screenings of various sizes between ll/qand 2 inches. Most of this supply goes to about thirty boiler plants, which are usually equipped with chain-grate, stoker-fired boilers of different capacities ranging from 400 to 600 horsepower each, using natural draft. Because of tonnage burned the storage reserves have to be large, and they must be screenings of the sizes noted above. There is a wide difference in storability between midwest coals and those mined in the eastern coal fields, and it should be kept in mind that only the former are under discussion here. Since lack of sufficient personnel at the laboratory has prevented extensive experimental testing, a large part of this paper covers observations made during the last ten years, in which period considerably more than 1,000,000tons of screenings from the various districts have been stored at several pointn and later burned in the many power plants. Storability of Different Coals
The general characteristics of the screenings as they are received from various coal districts are given in Table I. Careful observations of their behavior while in storage, together with a study of results obtained when the stored coals are burned, indicate that they may be ranked for storage 'in the order given, the first named being the most suitable. Rules Governing Open Storage
Where screenings are to be placed in open storage, the following rules, if closely followed, will aid materially in reduoing deterioration loss: 1-Do not make piles more than 10 feet deep. 2-Do not put two sizes of screenings in the same storage pile, and use great care that the coarse and fines do not become segregated in strata. 1
Received October 3, 1924.
