International Correspondence Schools SCRANTON, PA. Instruction Paper WITH EXAMINATION QUESTIONS FIRST EDITION PRINCIPLES OF COKING 858 INTERNATIONAL TEXTBOOK COMPANY SCRANTON. PA. ADVICE TO THE STUDENT Study a few pages at a time—do not skip from one section of the Paper to another. If examples are given in the text, compare the solutions carefully with the rules, formulas, or other text matter relating to them. If there are Examples for Practice, some or all may be worked, also; but this work need not be sent to the Schools for correction. If you meet with any difficulty, write us for help—using the “Information Blank.” If there are any statements you do not understand, let us know, and we will explain them in detail. Pay particu¬ lar attention to the definitions; a correct understanding of them is essential. Review the entire subject; then write out your answers to the Examination Questions at the end of this Paper, and send your work to us for examination and correction. Your answers should occur in the same order as the questions are printed. Do not write out the questions—merely write their numbers. If you are unable to answer some question, write lis, on an Information Blank, for assistance. If you need help in your studies, ask for it. If necessary, we will assign to you a Special Instructor, who will give you personal attention and assist you in mastering the subject. It is our aim to come into as close touch with our students as possible, and we therefore request you to keep us informed at all times regarding your progress and any difficulties you may meet with in your studies. International Correspondence Schools Copyright. 1906, by International Textbook Company. Entered at Stationers’ Hall, London. All rights reserved. Printed in the United States 2-fO.A PRINCIPLES OF COKING THE MANUFACTURE OF COKE DEFINITIONS AND GENERAL PRINCIPLES 1. When certain bituminous coals are heated in an enclosed space from which air is more or less completely excluded, the volatile matter of the coal is first driven off as a dense smoke, while the main mass of the coal fuses and runs together, at the same time expanding in volume. The passage of the escaping gases through the plastic mass causes it to be drawn out into elongated cells, giving it a sponge-like structure. When no more gases are evolved, there remains a hard, cellular, dark-gray residue, consisting essentially of the fixed carbon and the ash of the coal, together with small amounts of sulphur and phosphorus, and usually a little moisture and traces of unexpelled, volatile, combustible matter. This residue is called coke, and the coal is said to be coked. Coke is better adapted for certain metallurgical purposes than the coal from which it is made. 2. Products of Coking. —The products of the coking process are solid and gaseous. The solid products are coke and ashes. The gaseous products are the moisture expelled from the coal, and the volatile combustible portions of the coal; from this gaseous product, fuel gas, illuminating gas, ammonia, and tar may be separated. The products obtained from the gases are called by-products, because in the ordinary process of coking the gases escape into the air and are wasted, or at most are used only for fuel purposes. Copyrighted by International Textbook Company. Entered at Stationers' Hall , London 2 PRINCIPLES OF COKING §68 It is possible to save these by-products, and a coking plant at which they are saved is called a by-product plant. Numerous compounds may also be extracted from the tar, such as oils, medicinal compounds, and the so-called coal- tar colors; but as the extraction of these is carried on in a chemical manufactory entirely apart from the coke plant, the term by-products, as ordinarily used in connection with coke making, refers simply to the gas, tar, and ammonia water recovered at the coke plant. 3. Uses of Coke.—Probably 95 per cent, or more of the coke produced in the world is used in blast furnaces or foundry cupolas, but it is also used in the manufacture of water gas, producer gas, and as a domestic and locomotive fuel, and, in general, for any purpose where a quick, smoke¬ less fuel is required. Powdered coke is used, as a substitute for graphite, in the manufacture of foundry facings used on the inside of molds in making castings; for surface-hard¬ ening steel; and for making malleable-iron castings and arc-light carbons. PROCESSES OF MANUFACTURE 4. Coke is made in open pits or mounds, in beehive or some similar form of oven, in retort ovens, or in gas retorts. The first method is seldom used now, except to test samples of coke for their coking qualities. The second method, commonly known as the beehive method , was, until recently, the only method used to any extent in the United States; and while it is still the prevailing method, the third method has been quite extensively introduced. The third method, known as retort , or by-product, coking , is the one prevailing in England and on the continent of Europe. 5. Open-Pit Coking. —The open-pit method of making coke is illustrated in Fig. 1, which shows a perspective sec¬ tional view of an open-pit plant. The mounds of coal to be coked are described indiscriminately as banks, pits, and ricks, and the coke made as bank coke, pit coke, and rick coke. For the purpose of making pit coke, the ground is leveled for a 68 PRINCIPLES OF COKING 3 width of 14 feet and then surfaced with coal dirt or coke breeze, preferably the latter if it can be obtained. On this is spread a layer of coal 18 inches thick and as long as the rick is to be. Cross-flues a 6 inches wide and 10 inches deep are then made, as shown, by piling up lumps of coal or, Fig. 1 better, coke; the central flue b is made 12 inches wide and 10 inches deep in the same manner as the side flues. At the junction of the center and side flues, a central flue c, which acts as a chimney, is constructed with coarse pieces of coke or with stones. Dry wood is placed in the flues, after which 4 PRINCIPLES OF COKING §68 they are covered over with billets of wood; coal is then piled up until the mound is completed, as shown. The coking of the mound is started by setting fire to the kindling wood at the base of the flue c. The first gases given off are very black and at first do not burn, but subse¬ quently ignite and burn freely. The success of the process depends on keeping the fire evenly distributed throughout the mass, a matter of some difficulty in loosely constructed mounds and particularly on windy days. The coke burner should entirely or partially close the flues on the most freely burning side. The smoke changes from black to yellow and then to light blue, and when the blue flames (due to the burn¬ ing of carbon monoxide to carbon dioxide) appear the process is completed. This requires from 5 to 6 or more days. The pile is gradually covered, as the coking proceeds, with sod or clay from the bottom upwards, and all the openings stopped with wet coke ashes. After cooling for 4 or 5 days, or, on an average, on the tenth day after the initial firing, the cover is removed in places and the coke cooled by water before drawing. If more haste is necessary, water may be applied through a hose down the flues. This water, being converted into steam, penetrates the mass of the mound and soon extinguishes any fire. The yield of coke in such pits is small; but with care its quality is excellent. 6. Beehive coke is produced in a hemispherical brick chamber called a beehive oven from its resemblance to the old form of beehive. The initial heat for each charge, after the first, is supplied by that remaining in the walls of the oven from the preceding charge. Before an oven is first charged, the walls are heated up for several days by means of a wood or coal fire. In coking, only enough air is admitted into the oven to furnish oxygen to burn the combustible volatile matter drawn off from the coal by the heat, and the combustion of this volatile matter supplies the heat for carrying on the coking process. The method is wasteful, as some of the fixed carbon is always consumed and no attempt is usually made to recover any of the by-products. Owing §68 PRINCIPLES OF COKING 5 to the excellent quality of the product made in beehive ovens from good coking coals, and to the comparative cheapness of the plant, this process, though wasteful of the products, is widely used. 7 . Retort, or by-product, coke is made in long, narrow, upright ovens of firebrick. The heat is supplied, from start to finish, by the combustion of a portion of the volatile matter of the coal, not in the coking chamber itself, as in the beehive oven, but in flues in the walls of the oven, or in a special combustion chamber from which the intensely hot gases are conveyed through the passages in the walls of the oven proper. The product is properly called retort-oven coke , but is very frequently known as by-product coke , owing to the fact that when such ovens are used the by-products are generally saved. The coke resulting from the manufacture of illu¬ minating gas is also a retort coke. Any coal that will coke in the beehive oven will give good results in the retort oven; and many coals that will not coke in the beehive oven give very satisfactory products in retorts. The retort-oven process has in its favor, aside from making good coke from coals giving poor or indifferent results in the beehive oven, the possible and usual recovery of products otherwise wasted and which in some instances have a pecuniary value fully equal to that of the coke. The details of the manufacture of coke in beehive or by-product ovens are fully given in Coking in Beehive Ove?is, Parts 1 and 2, and By-Product Coking, Parts 1 and 2. COKING COATS 8. The term coking coal is usually understood in America to refer to coal that will make a good metallurgical coke in the ordinary beehive oven. The general meaning of the term is, however, any coal from which a good metallur¬ gical coke can be obtained in any practicable form of coke oven. Although many attempts have been made to determine 6 PRINCIPLES OF COKING §68 what is essential to a coking coal, it is not known why cer¬ tain coals will coke and others will not. 9 . The term cement, or binder, is often applied to the substance or substances in coal on which the coking property seems to depend. The composition and nature of this binder have never been determined, and indeed it is not definitely known that it is a distinct substance, as the property of coking may depend on certain physical proper¬ ties. Since, however, certain coals coke and others do not, and certain coals coke in the beehive oven and others do not, but can be coked in a retort oven, there is a difference between coals; and for want of a better term to explain this difference, the substance in the coal, or the property of the coal on which the difference depends, is known as the binder, or cement. Although it is not possible to determine exactly on what the coking of a coal depends, certain conclusions based on observation have been reached, which are useful in deter¬ mining the probability of a coal being a coking coal. Sub¬ sequent and more extended observations may prove many of these conclusions to be incorrect. CHEMICAL COMPOSITION OF COKING COALS 10. Various attempts have been made to explain the coking and non-coking of various coals from the chemical compositions of the coals—as, for instance, the relation between the fixed carbon and the volatile matter, or between hydrogen and oxygen, etc.—but such attempts have failed, for one coal may coke well while another of about the same chemical composition may not coke at all. Table I gives analyses of some typical coking coals. 11. Volatile Matter. —Attempts to explain the coking or non-coking of coals by the amount of volatile matter they contain or by the ratio between the amount of volatile matter and the other ingredients in the coal, have failed, since coals yielding good coke range all the way from 13 to 40 per cent, in volatile matter. §68 PRINCIPLES OF COKING 7 Although coals extremely high in volatile matter, such, for example, as cannel, can.rarely be coked in their natural state, at least in present-day ovens, it is possible that some TABLE I Locality Chemical Composition of Coking Coals Remarks Moisture 212° F. Per Cent. Volatile Matter Per Cent. Fixed Carbon Per Cent. Ash Per Cent. Sulphur Per Cent. Phosphorus Per Cent. Pennsylvania: Connellsville . . 1.86 30.12 59.61 8.41 .78 .024 Best coking Broad Top . . . 1.28 18.40 71.12 7-50 1.70 Trace Good coking Bennington . . . 1.20 23.68 68.77 5-73 .62 .017 Good coking Johnstown . . . .72 16.49 73-84 7-97 I.97 Dry coking Greensburg . . . 1.02 33-50 61.34 3.28 .86 Good coking Armstrong Co. . .96 38.20 52.03 5 -i 4 3-66 Pitchy coking West Virginia: 7 Pocahontas . . . 1.01 18.81 72.71 5*191 OO Best coking Fairmont .... 1.50 36.70 54-80 7.00 2.10 Alabama: Birmingham . . 2.10 25-77 68.35 3-70 .07 Brookwood . . . 1-75 24-15 65.55 8-55 1.40 Gamble. 2.78 24.67 61.96 10^59 •43 Tennessee: Jellico. 4.40 3 I -56 61.87 1.86 .31 Briceville .... •57 30.41 63.04 3.62 .23 Illinois: Mt. Carbon . . . 2.08 38.20 53-47 8.02 .63 .027 Pitchy coking Colorado: El Moro .... •95 29.82 56.41 12.82 .41 Good coking Crested Butte . . .72 23-44 71.91 3-93 •36 Good coking Mexico: Coahuila Coal Co. 1.60 15.00 '67.64 12.01 .86 preliminary treatment might render them adaptable, though of course, owing to their small content of fixed carbon, the yield of coke would necessarily be small. 8 PRINCIPLES OF COKING §68 12. An unsatisfactory classification of coals based on the volatile contents is sometimes given as follows: A rich coking coal contains from 35 to 40 per cent, of volatile matter and produces a spongy open and soft coke. These coals require a moderate degree of heat to coke them, and seem to contain an excess of cement or binding material and to require its partial expulsion by moderate heat before the actual coking begins. Normal coking coals contain from 25 to 35 per cent, of volatile matter; in Connellsville coal a content of about 32 per cent, is generally accepted as a standard. These coals give equally good results in any kind of oven. Dry coking coals are those containing 20 to 25 per cent, of volatile matter. Many coals with this amount of volatile matter will not give a good hard coke in the beehive oven, but will coke satisfactorily in the retort oven; on the other hand, some of the best coking coals, such as the Poca¬ hontas, contain only about 20 per cent, of volatile matter. 13. Fixed Carbon.— Some coals low in fixed carbon and normally non-coking may be made to coke by mixing them with pitch or tar, which seems to supply the binder otherwise lacking. 14. Moisture.— Coals containing a small percentage of water when freshly mined are generally coking, while those that contain a high percentage of water when freshly mined are seldom or never coking coals. For example, five coals ranging from 7.77 to 9.99 per cent, in moisture, with an average of 7.93 per cent., were non-coking; while coals con¬ taining from 1.72 to 1.98 per cent, were coking. The Cre¬ taceous and Tertiary coals of the West, running as high as 15 or more per cent, moisture, do not usually coke; but where for any reason the moisture has been reduced to a small amount, by metamorphic action for example, the coal will frequently coke. Local or regional metamorphism may, and probably does, affect other constituents than the moisture, but the fact remains that coals very high in moisture do not usually coke in a beehive oven. §68 PRINCIPLES OF COKING 9 15. Ash. —Within very wide limits, the amount of ash has no effect on the coking qualities of coal. Coals ran¬ ging from 3 per cent, to 20 per cent, in ash will coke, yield¬ ing a product varying approximately from 4.5 per cent, to 30 per cent, in ash. Naturally, every unit of ash displaces one of fixed carbon, thus lessening the heating power of the coke. It is claimed by many that a certain amount of ash is essential to a good coke, giving the desired strength to the cell walls; and the statement is made that a coke carrying from 10 to 11 per cent, of ash is harder and stronger than one containing from 6 to 7 per cent. 16. Sulphur. —The amount of sulphur that a coking coal can contain depends on the amount of sulphur that the result¬ ing coke can contain and still be salable. No amount of sul¬ phur in a coal up to this point affects its coking properties. Sulphur is to some extent volatilized during coking; but since it takes approximately li tons of coal to make a ton of coke, the coke usually contains about the same percentage as the coal from which it is made; that is, if the coal contains 1 per cent, of sulphur, the coke will contain 1 per cent. This is an approximate rule only, and some cokes contain less sulphur than the coal from which they were made; others contain more. Sulphur exists in coal in several forms: first, as iron pyrites or sulphide of iron, FeS 2 ; second, as gypsum or calcium sulphate; third, as organic sulphur combined with carbon, oxygen, and hydrogen. Some authorities also give a fourth form as free sulphur. If sulphur is present as iron pyrites, a considerable amount (about one-half) is driven off in coking; but if present as gypsum, none is removed. In fourteen coals examined by one chemist, the average percentage of sulphur was 1.591, of which 1.152 was in combination with iron and .439 existed “free.” The sulphur contained in the resulting cokes amounted, on an average, to .952 per cent, of the sulphur of the coal, showing an expulsion of 40.16 per cent, of the total sulphur during coking, since A-591 .952 \ iqq _ jg per cen j-. \ 1.591 / 10 PRINCIPLES OF COKING §68 These results seem to show that all the “free” sulphur does not pass off with the volatile matter in the process of coking, as is often supposed. In twenty-five coals examined by the same person, the percentage of sulphur expelled by coking varied from 57.92 to 14.75 per cent., the average being 38.50 per cent. Various unsuccessful experiments have been made to reduce the sulphur in coke by mixing the coal with salt, lime, or other chemicals before charging it into the oven. If present as pyrites, the sulphur can be greatly lessened by passing the fine coal through some one of the various wash¬ ing machines. The attempts to remove sulphur from coke before its use, by heating in air or oxygen at and above atmospheric pressure, have not proved successful; in every case, a portion of the fixed carbon was consumed, with a corresponding increase in the percentage of ash. Where, therefore, a careful handling and subsequent washing of the coal will not remove the excess of sulphur, it is scarcely to be hoped that this can be accomplished in the coke ovens. 17. Phosphorus is not removed in the process of coking and is concentrated in the coke. It cannot usually be reduced in amount by a preliminary washing. The greater part of the phosphorus sometimes occurs in a certain section of a coal seam, in which case the percentage of phosphorus in the coke may be kept down by coking only a portion of the seam. For instance, in the Pittsburg seam from which the celebrated Connellsville coke is made, the amount of phos¬ phorus increases gradually from bottom to top of the seam; hence, the best coke is made from coal taken from the bottom part of this seam. The coking properties of the coal do not seem to depend in any way on the amount of phosphorus contained. 18. Foreign Substances. —Foreign substances, such as lime and lime feldspar, which sometimes occur in the cleats of a coal bed, seem to render a coal non-coking; while the same coal without the feldspar may coke. The Northumber- land-Durham field in England illustrates this point. On the §68 PRINCIPLES OF COKING 11 northern, or Northumberland, side of the fault dividing the coal basin, the majority of the seams have their cleats filled with plates of lime feldspar from tV inch to } inch thick, while on the southern, or Durham, side this feldspar is almost entirely wanting. Both fields supply coking coals, the larger number and better quality from the Durham, but in each instance it is only the coals that do not have the feldspar that coke. If the fine coal is washed and separated from the feldspar and other impurities, a fair coke may be made from the cleaned slack that results, indicating that the feldspar prevents the coking of the coal. GEOLOGICAL POSITION OF COKING COALS 19. The geological position of a coal seems to. have little or no effect on its coking properties. Any coal, whether belonging to the Upper or Lower Carboniferous, Triassic, Jurassic, Cretaceous, or even Tertiary Age, may coke if properly treated, or it may not; but certain forma¬ tions, such as the Carboniferous, are more apt to yield good coking coals than others. Again, certain portions of a given formation may contain better coking coals than another part, as in the Clyde basin in England, where all the seams in the upper coal measures are non-coking, and all in the limestone series, or lower coal measures, are coking. In a more limited sense, a certain coal of a certain formation may coke wher¬ ever found, or it may coke in one portion of the field and not in another. The coal from one bed may coke, while that from another bed only a few feet above or below it may not. All that can be gathered from geological position is that certain seams of certain formations will probably yield better coke than other seams in the same or other measures. PHYSICAL PROPERTIES OF COKING COALS 20. Texture. —The effect of texture on the coking qualities of a coal may be considered in a twofold light: first, as the coal occurs in place in the mine; and second, as it is charged into the ovens. 12 PRINCIPLES OF COKING §68 The structure of the coal in place in the mine seems to have little or no effect on its coking- quality, and a soft mushy coal and a hard columnar or blocky coal may make equally good coke when properly treated. 21. The condition of the coal when charged into the oven may and usually does have a marked effect on the coke pro¬ duced. Although good coking coal will usually make good coke when charged as run of mine, it will usually make better coke when charged in the form of slack. Certain coals that will not coke in lump form make excel¬ lent coke when ground to slack. All coals, even of the high¬ est coking qualities, give better results when charged evenly sized. The reason for this seems to be that the fine condi¬ tion of the coal permits the more rapid evolution of gas from the increasing number of surfaces exposed to the action of heat; the fusing or melting of the pitch-like constituents of the coal is more complete; and the gas more readily and easily forces its way through the fused mass, producing the open cellular structure essential to good coke. The impor¬ tance of a preliminary crushing is more marked with coals low in volatile matter than with others. In these, the slack fuses and cokes first and often only the surface,of the lumps is coked, the coal in the center being merely charred. As a general rule, the lower in volatile matter, the finer should be the coal to give the best coke. On the other hand, the coking qualities of certain light coals may be increased by subjecting the slack to a prelimi¬ nary compression and charging the coal in artificial lumps. This is done in Germany in preparing otherwise non-coking coals for use in retort ovens. • „ 22. Effect of Weathering;. —When coal is exposed for a length of time to ordinary atmospheric moisture and heat, its coking qualities are generally wholly or partially destroyed. The fact is of importance in showing that freshly mined coal should be coked at once, and has a bearing in sampling a coal field for its coking properties. Ordinarily, samples are taken where the coal has been more or less weathered, 68 PRINCIPLES OF COKING 13 and it must be borne in mind that experimental lots of coke made therefrom will not be equal to that made when the plant is constructed and mining under normal conditions. 23. Effect of Process and Temperature. —Coking depends on the temperature; if this is low and slowly applied, the volatile matters are expelled without fusion and no coke results; on the other hand, if the heat is high and rapidly applied, the coal, if of the coking class, will coke. The effect of process and temperature on the quality of the coke should be considered together, since the main difference between the two chief processes, beehive and retort, con¬ sists in the way of applying the heat in the two processes. In the beehive, the temperature is at first quite low and slowly increases until, toward the end of the process, the maximum is reached. The heat also comes mainly from one side of the charge. In the retort, the temperature is more nearly the same from beginning to end, and is applied to the charge from all sides. The first is a slow process; the latter, rapid. Other things being equal, within certain limits, the higher the temperature of the oven, the greater will be the yield. This is shown by the fact that, if an oven is charged at once after drawing and before it has time to cool, t*he yield is much greater than if it is allowed to stand empty for several hours with the door open. The higher the temperature of the oven and the longer the coal is exposed to the heat of the oven, the harder and more dense is the coke. YIELD OF COKE - 24. The theoretical yield of coke from any coal is obtained by adding together the percentages of the solid parts of the coal, the fixed carbon, and the ash, as given by ^proximate analysis. The theoretical yield is not generally reached in beehive-oven practice, as some of the fixed carbon is burned during the process of coking; it may, however, be exceeded in retort-oven practice. 14 PRINCIPLES OF COKING §68 Assuming a coking coal to contain: Per Cent. Moisture. 1.20 Volatile combustible matter 31.50 Fixed carbon. 59.801 67.3 per Ash. 7.50J cent, coke Total.100.00 Sulphur. .80 Phosphorus. .006 the theoretical yield would be 67.3 per cent, of the coal charged into the oven. Coking coals with sufficient volatile matter to supply the heat required in the process of coking will approximate more closely to the theoretical yield than coals containing a smaller amount of volatile matter and where the deficiency has to be made up by the burning of a portion of the fixed car¬ bon. This loss of carbon is sometimes made up by the decomposition, at a high temperature, of some of the volatile hydrocarbons and the deposition of some of the carbon on the coke. In the Connellsville region, the yield of coke is nearly equal to the theoretical yield, which, according to the analysis given in Table I. is about 66f per cent, of the weight of the coal. It therefore requires if tons of Connellsville coal to make 1 ton of coke ( 100 -- 66f = li). In the Pocahontas field, however, although the theoretical yield of coke, as given by Table I, is greater than in the Connellsville region (being about 77 per cent.), the actual yield of coke is only from 58 to 61 per cent, of the weight of the coal coked. This difference in yield is due to the smaller amount of volatile matter in Pocahontas coal, requiring that a larger amount of the fixed carbon be burned in the oven to furnish the heat to coke the coal. According to the figures here given, it requires If tons of Pocahontas coal (100 -f- 60 = if) to pro¬ duce 1 ton of coke. 25. When coal is coked in a retort, the yield of coke generally exceeds the theoretical yield calculated by the §68 PRINCIPLES OF COKING 15 method given in Art. 24. The amount of this increase varies, but is usually from 5 to 10 per cent. This increase is due to the decomposition at a high temperature of the hydro¬ carbon gases contained in the volatile combustible matter of the coal and the deposition of the carbon on the coke, and also to the fact that none of the carbon of the coal is burned in the retort to furnish heat for the coking process, as is the case in the beehive oven. The yield of coke in a by-product oven is frequently 75 per cent, of the weight of the coal coked when the theoretical amount of coke in the coal is only about 66 per cent. If the yield is 75 per cent., it will require li tons (100 -r- 75 = li) of coal to produce 1 ton of coke. 26. Approximate Composition of Coke. —If the proximate analysis of a coking coal and the number of tons required to make a ton of coke are known, the approximate analysis of the coke may be determined as follows: Rule I. —Multiply the percentage of ash in the coal by the number of tons of coal required to make a ton of coke; the product will be the amount of ash in the coke. Rule II. —The percentage of fixed carbon in the coke is then obtained by subtracting the amount of ash from 100 per cent. This approximation neglects the amount of sulphur and phosphorus in the ash of the coal, and also any small amounts of moisture and volatile matter in the coke, but is close enough to give a general idea of the composition of the coke. The percentage of sulphur in the coke is assumed to be the same as in the coal. (See Art. 16.) Rule III. —The percentage of phosphorus in the coke is obtained by multiply big the percentage of phosphorus in the coal by the number of tons of coal required to make 1 ton of coke. The application of these rules is shown by the following example: . Example. —Calculate the approximate composition of cokes made in the beehive and by-product ovens from the coal of which an analysis is given in Art. 24. 16 PRINCIPLES OF COKING §68 Solution.— Beehive Coke Retort Coke Ash. 7.5 X 1.5 = 11.25 7.5 X H = 10.00 Fixed carbon (by difference) 88.75 90.00 100.00 100.00 Sulphur. .80 .80 Phosphorus . . . .006 X 1.5 = .009 .006 X l£ = .008 27. A slightly more accurate method is sometimes used, in which, in the analysis of the coal, the percentages of sul¬ phur and phosphorus are given separately from the per¬ centage of ash. In calculating the theoretical yield of coke, the fixed carbon, ash, one-half the sulphur, and all the phos¬ phorus are added. Thus, if the approximate analysis of a coal is: Per Cent. Volatile matter. 34.79 Fixed carbon . 57.86 Ash (without sulphur and phosphorus) . . 6.19 Sulphur. 1.144 Phosphorus. .016 Total. 100.000 the theoretical yield of coke will be 57.86 + 6.19 + .572 (i of 1.144) + .016 = 64.638. It therefore requires 100 -r- 64.638 — 1.54 tons of coal to make 1 ton of coke. 28. To Calculate the Gain or Loss in Fixed Carbon. If the analyses of a coal and of the resulting coke are known, the loss or gain in fixed carbon over the theoretical amount determined as above may be calculated as shown below, the following analyses having been given: Per Cent. Per Cent. Volatile matter. Fixed carbon. Ash (without sulphur and phos¬ phorus) . Sulphur. Phosphorus . in Coal in Coke • 34.79 0.00 57.86 89.20 6.19 9.50 1.144 1.276 .016 .024 100.000 100.000 Total . §68 PRINCIPLES OF COKING 17 As was calculated in Art. 27, it will take 1.54 tons of this coal to make 1 ton of coke. Then, the theoretical fixed carbon in 1 ton of coke should be 57.86 X 1.54 = 89.10 per cent., but as the analysis of the coal shows 89.20 per cent., there is evidently a slight gain in carbon. Similarly, for the sulphur X 1-54 = .88 per cent, sulphur, while the analysis of the coke shows 1.276 per cent. This shows that the assumption made in Art. 27 that one-half of the sulphur goes into the coke is not as accurate as the assumption made that the amount of sulphur in the coke is about the same as in the coal. (See Art. 26.) For the ash, 6.19 X 1.54 = 9.53 percent., which is very close to the percentage given by analysis. _ VARIETIES OF COKE 29. According to the length of time that the charge of coal remains in the oven, the resulting coke is called 24-, 48-, or 72-hour coke. Different varieties of coke are also named, from the uses to which they are put, as follows: Furnace coke and foundry coke are, as their names imply, used respectively in producing pig iron in the blast furnace and melting the same in the foundry cupola. Gas-house coke is the residue remaining in the retorts or chambers of an illuminating-gas plant after the distillation of the gas. This really is a by-product of gas manufacture; it is soft and porous and is of little use except for domestic heating, the manufacture of producer or water gas, and where a cheap smokeless fuel is required. Domestic, or crushed, coke is coke that is crushed and separated into sizes—nut, stove, egg, etc.—and used for domestic fuel. Stock coke is coke that is allowed to remain on the yard for some time, that is, is stocked owing to scarcity of orders or cars. It discolors and is thought by some to deteriorate in quality, and sometimes commands a lower price, though such coke is kept stocked a much shorter time in the oven yard than it is in the stock pile of a blast furnace. 18 PRINCIPLES OF COKING §68 Soft coke is a light, spongy, large-pored coke, produced when heating the oven prior to making good coke, or when ovens are cold, or when coal is not thoroughly coked, or when too much air is admitted to the oven, or in retorts with a low fire. Black ends are due to imperfect coking, to pulling- coke too soon, or to a cold oven floor. Black ends may occur in beehive or by-product ovens, particularly in beehive since the coal is admitted through a tunnel head in the center and comes from the center toward the side. Thus, the larger pieces of coal and slate roll down the sides to the floor of the oven, and if there is any appreciable quan¬ tity of slate, black ends will occur, or if the oven is not hot, the larger pieces of coal will not be properly coked through. Black coke is coke that lacks the silvery luster of the ordinary beehive product that has been watered inside the oven and is dark in appearance. Red coke is coke that has a reddish cast in places. This is produced where the ash contains much iron, or where the charge remains too long in the oven, so that a larger portion than usual of the fixed carbon is burned. It is also made when the water used in quenching the coke is contaminated with sulphate of iron. Run-of-oven or run-of-yard are terms analogous to run-of-mine coal, and refer to the coke taken as it occurs at the ovens. Hand-picked, or selected, coke is coke in large lumps selected for their good appearance and quality and loaded by hand to suit the requirements of a particular customer or for purposes of exhibition. Breeze, screenings, or forkings are the small pieces breaking from the larger lumps in drawing and handling the coke, and which fall between the tines of the coke forks when handling from the yard into cars. These are gathered from time to time, and sometimes screened to separate them from the fine ashes and brick dust and shipped to market or else made into briquets. §68 PRINCIPLES OF COKING 19 Short coke is coke occurring in short pieces; it is some¬ times made purposely by coking shallow charges of coal, or by coking coal in a beehive oven for 24 hours, or less. CHEMICAL AND PHYSICAL PROPERTIES OF COKE 30. Furnace coke is usually 48-hour beehive coke, or 24- or 36-hour retort-oven coke; it must fulfil certain chemi¬ cal and physical requirements. The essential element in coke is the carbon, as that is what produces the heat when the coke is burned; all other elements contained in it may be considered impurities. Chemically, it must not exceed a certain maximum in impurities, such as ash, sulphur, and phosphorus, the amount of the latter two elements allowable depending largely on the use to which the iron made from the coke is to be put. In smelting iron ore for Bessemer pig iron, the coke should not exceed 10 per cent, ash, 1 per cent, sulphur, and .02 per cent, phosphorus. The composition of a coke should be uniform to insure regularity in the working of the furnace and uniformity in the amount and quality of the iron produced; hence, coke with an excess of black ends should not be used if it can be avoided, as such ends are an evidence that the coke has been poorly made. The effects of the several impurities in coke are briefly as follows. 31. The ash has no fuel value, and as the percentage of ash increases there is a corresponding decrease in fixed carbon, necessitating the use of more fuel and limestone to flux the ash. A coke averaging 11 per cent, of ash and varying only from 10i to lli per cent, is a better blast¬ furnace fuel than one averaging 10 per cent, but varying from 7 to 13 per cent. The same is true, in but slightly less degree, as to sulphur and phosphorus. 32. Sulphur renders iron red short, that is, brittle when hot, and even though a part of the sulphur in the coal is removed in the coking process, the coke may contain a greater percentage of sulphur than the coal from which it was made, since it takes usually about li tons of coal to 20 PRINCIPLES OF COKING 68 make a ton of coke. A considerable part of the sulphur in the coke passes into the iron in the blast furnace; hence, the amount of sulphur in the coke should be made as low as possible by washing the coal before it is coked, if the coke made from the given coal would contain more than the per¬ centage of sulphur allowable in a furnace coke. 33. Phosphorus renders iron cold short, that is, brittle when cold. Very little, if any, of the phosphorus is removed in the coke oven or in the blast furnace or cupola, and prac¬ tically all of the phosphorus in the coke goes into the iron. 34. In general, the analysis of a good furnace coke should be about as follows for Bessemer pig iron: Per Cent. Fixed carbon . 89.55 Ash (including sulphur and phosphorus) . 9.10 Volatile matter . .50 Moisture . .85 Total.100.00 Sulphur. .80 Phosphorus. .015 35. The physical requirements of blast-furnace coke are hardness, great crushing strength, and as cellular or porous a structure as is consistent with these qualities. It is still largely held that the bright, silvery, semimetallic luster of beehive coke is essential to a good blast-furnace fuel, this gloss preventing the taking up of carbon by carbon dioxide (C0 2 + C — 2 CO) in the upper part of the furnace. This opinion is by no means universally held, but reports of blast-furnace work show that more retort coke (lacking the luster) than beehive is frequently required to produce a ton of pig iron. 36. Foundry coke is supposed to be 72-hour beehive coke or 36-hour retort coke; but much of the so-called foundry coke is ordinary 48-hour furnace coke from which the soft pieces and black ends have been thrown out, though often even this is not done. At some coke plants, the only §68 PRINCIPLES OF COKING 21 distinction made between furnace and foundry coke is that the former is loaded into open cars, and the latter into box cars. Foundry coke properly comes in larger, longer, and harder pieces than furnace coke, and is selected with more care to prevent the loading of soft pieces and black ends. A higher price is paid for this increased work and time in manufacture and handling. The qualities that make a coke desirable for blast-furnace fuel likewise render it suitable for foundry work. The ash should not be excessive, as it occupies the space of fixed car¬ bon. Sulphur is injurious, as it makes the iron hard; a portion of it is taken up by the limestone flux, if a flux is used, but a large amount enters the iron. The amount of phosphorus in a coke is seldom sufficient to interfere with the use of the coke in the cupola, even when making malleable-iron castings. 37. General Uses for Coke.—Coke, in comparatively small amounts, is used for a number of other purposes than those mentioned, such as domestic use, fuel for locomotives, wherever a clean, smokeless fuel is required, as in bakeries and breweries, and in the manufacture of producer and water gas. As a domestic fuel, the use of coke is increasing rapidly, particularly since the introduction into the United States of the retort oven. Ordinarily, beehive coke or retort-oven coke is used for domestic purposes; but any coke will answer. Coke for domestic use is broken in rolls similar to those used for breaking coal and then screened to sizes known as egg, stove, and nut, corresponding to the similarly named sizes of anthracite. The coke crusher is located close to the ovens, and the large coke is generally loaded into small cars running on the coke yard, and hauled directly to an elevator, which hoists it up to the crushing rolls. The final screened product is col¬ lected in bins from which it is drawn into railroad cars. As ordinarily fired, coke is an expensive fuel for domestic use. It burns freely and produces an intense heat, so that 22 PRINCIPLES OF COKING §68 small quantities at a time should be added at frequent inter¬ vals in a firebox adapted to its consumption, and not large quantities at a time, as with the slower-burning anthracite. Properly handled, however, it is a very satisfactory and clean fuel. Bakers, brewers, and others requiring a clean, smokeless fuel usually buy stock or soft coke, because of its cheapness. Gas-house coke is also largely consumed for this purpose. For the manufacture of producer and water gas any coke will answer, but generally the smaller sizes, such as breeze, or forkings, soft coke, and stock coke, are used. In such coke, the sulphur is of importance, as it cannot be removed from the producer gas and is consequently injurious when the gas is burned in direct contact with iron. From water gas it can be removed by passing the gas through scrubbers, which absorb the sulphurous substances, but of course at an increased cost for plant and maintenance. LABORATORY TESTS OF COKE 38. Aside from the usual chemical analysis to determine the percentage of impurities in coke, various tests may be made on the physical properties, the chief of which are: (1) crushing strength, (2) hardness, (3) the proportion of cells to solid matter, (4) capacity of coke to dissolve in hot carbon dioxide, CO 2 . 39. Crushing Strength.—The crushing strength of coke is usually determined on an inch cube by some one of the various crushing machines made for compression tests. Good coke has an ultimate crushing strength of, from 1,200 to 2,200 pounds per square inch, depending on the coal from which it is made and the process by which it is coked. 40. Hardness. —The hardness of a coke is that of the materials forming its cell walls, and for good furnace fuels is about 2.5. It is determined by the usual methods of miner¬ alogy or by placing a cube of coke at a fixed pressure against an emery wheel revolving at a known rate of speed. The §68 PRINCIPLES OF COKING 23 loss in weight of the sample in a given time serves as a basis for comparison with other cokes. 41 . Percentage of Cell Space. —The percentage of cell space in good cokes varies from 44 to 56, and the deter¬ mination of this percentage requires care. A cube of con¬ venient size, say 1 cubic inch, is prepared representing a fair section of the coke, carefully brushed from all adhering particles, heated to expel moisture, cooled, and weighed in air. The same cube is then soaked in water under the receiver of an air pump until the pores of the coke are thoroughly filled with water, and then weighed. From the specific gravity of the solid portion of the coke, not including the cell space, and the weights of a cubic inch of the coke in its natural form, when dry, and when saturated with water, it is possible to calculate the percentage of cell space in the coke. For example, if 1 cubic inch of coke when dry weighs 15 grains, and when saturated with water weighs 23 grains, the coke has absorbed 8 grains of water; that is, in the coke there is sufficient space to hold 8 grains of water. If the specific gravity of the solid portion of the coke is 1.75, a volume of coke equal to this space will weigh 8 X 1.75 = 14 grains. Therefore, a piece of solid coke would weigh 15 + 14 = 29 grains. It follows, therefore, that the actual weight of coke multiplied by 100 and divided by the weight of the coke, if it were solid, gives the percentage of solid coke in the mass; and the weight of coke lost by the cellular structure multiplied by 100 and divided by the same factor gives the percentage of cell space. In the present case, Coke or body . Cells 42 . The specific gravity of the coke may be deter¬ mined approximately as follows, or more accurately by any of the well-known methods of determining the specific gravity of a substance. 24 PRINCIPLES OF COKING 68 Let a = weight of dry coke; b = weight of water it can absorb; c = loss in weight in water of coke saturated with water; x — specific gravity of solid part of coke. Then, (c — b) : a = 1 : x Example. —A piece of dry coke weighs 20 grains, but when satu¬ rated with water it weighs 30 grains when weighed in the air, and only 8 grains when weighed in water, (a) What is the specific gravity of the coke? (£) What is the percentage of cell space? ( c) What is the percentage of solid coke? Solution. — (a) By applying the formula for the specific gravity, a 20 20 i a X c - b 22 - 10 “ 12 “ 1 ' 66 ' AnS ‘ (b) The weight of coke equivalent to the cellular space is 1.66 X 10 = 16.6 gr.; if solid, the coke would weigh 16.6 + 20 = 36.6 gr. The amount of cell space is 16.6 X 100 36.6 (c) The amount of solid coke is 20 X 100 36.6 = 45.36 per cent. Ans. = 54.64 per cent. Ans. 43. Capacity of Coke to Dissolve in Hot Carbon Dioxide. —A weighed quantity of coke is tested in a tube in a current of hot carbon dioxide, and the issuing gas is analyzed for its percentage of carbon monoxide; or, the coke remaining in the tube, is weighed after the test. In the first instance, the percentage of carbon monoxide in the issuing gas, and in the second the loss in weight of the coke, indi¬ cates the solvent effect of carbon dioxide on the coke in the charge. Good furnace cokes, when subjected to the first test, give a gas showing a little more than 5 per cent, of carbon monoxide; and when submitted to the second test, they show but little loss in weight. 44. Field Tests of Coking Coals. —The only certain test of the coking qualities of a coal is to try it in a coke oven, and, whenever possible, an amount of coal sufficient to §68 PRINCIPLES OF COKING 25 give one or more complete tests should be shipped to a coking plant and there tried. If the coke fails to coke in the beehive oven, it should then be tested in the retort oven. PREPARATION OF COAL FOR COKING 45 . Necessity for Preparation. —Good coking coals, unless high in sulphur or extremely slaty, require no especial preparation for coking, except that they should be broken up into reasonably small fragments. Although the best coke can be obtained by first sizing the coal, this is not always necessary or practicable. In soft or friable coals, like those of the Connellsville region of Pennsylvania, the coal is broken and sized sufficiently in the mining and by the subse¬ quent loading into the mine cars, dumping into the coal bins, drawing from the bin into the larries, and the final charging and leveling in the coke oven. In mining this coal, a pick is used and the cutting is distributed evenly over the entire face of the working place, or room, that is being excavated. The larger pieces of coal occasionally produced are broken by hand before being loaded into the mine car. When coals are hard, particularly where they are not rich in volatile matter (containing less than 32 per cent.), the lumps must be crushed in order to be successfully coked. If the lumps are too large, the heat of the oven will not penetrate to their centers and the outside of a lump will be coked while the center will be found to contain raw coal. Large lumps also consume too much time in coking and tend to retard the proc¬ ess. When large and small pieces occur in the same charge, the coking of the lumps will necessarily consume much more time than is required by the fine portion of the charge. Uniformly sized material cokes or burns downwards at a regular rate, until the bottom of the oven is reached, when the process should be complete. Coke produced from crushed coal is more uniform in texture and can be drawn from the ovens in larger pieces than coke made from uncrushed coal. The larger pieces of slate may be removed by screening and washing. Coke made from crushed coal 26 PRINCIPLES OF COKING §68 presents a better appearance than that made from uncrushed coal, as its structure shows no large pieces of slate, as is sure to be the case when the coal is not crushed. The practice of crushing coal for the manufacture of coke is gradually * being adopted in many parts of the United States and Canada. The machines used for crushing the coal prior to coking are the same as are used in crushing it prior to washing; but, as a general rule, when the coal is crushed but not washed the crushing is carried to a much finer degree. 46. The practice of washing coal that is to be coked to reduce the amount of sulphur and ash in the resulting coke is steadily increasing. Experiments have shown conclusively that not only can the value of the coke be increased by first washing the coal, but that certain coals that do not coke with¬ out washing can be coked after the excess of slate has been removed by washing. The methods of crushing and wash¬ ing coal are fully explained in Coal Washhig. PRINCIPLES OF COKING EXAMINATION questions (1) What is coke? (2) What is meant by the by-products of the coking process? (3) For what is coke used? (4) (a) By what processes is coke made? (b) For what is the open-pit method of making coke mainly used? (5) What is meant by the term coking coal? (6) What is meant by the cement or binder of a coking coal? (7) How can some so-called non-coking coals be made to coke? (8) What effect has the amount of moisture in a coal on its coking properties? (9) Does the amount of ash in a coal affect its coking qualities? (10) Has the amount of sulphur and phosphorus in a coal any effect on its coking properties? (11) What becomes of the sulphur in a coal when the coal is coked? (12) What effect have lime and feldspar when mixed with a coking coal? (13) Has the geological position of a coal bed any effect on the coking properties of the coal in the bed? §68 2 PRINCIPLES OF COKING §68 (14) ' Why should coals be crushed before they are coked? (15) What is meant by the term theoretical yield of coke? (16) Calculate the theoretical yield of coke from a coal of the following analysis, coked in a beehive oven: Per Cent. Moisture. 2 Volatile combustible matter. 30 Fixed carbon. 60 Ash. 8 Total. 100 Ans. 68 per cent. (17) Why is it possible to obtain more than the theo¬ retical yield of coke when coal is coked in a retort oven? (18) If the theoretical yield of coke is 50 per cent, of the weight of the coal, how many tons of coal will be required to produce a ton of coke? Ans. 2 T. (19) (a) How may the approximate percentage of ash and phosphorus in a coke made from a given coal be calcu¬ lated? ( b) How is the percentage of fixed carbon in the coke obtained? (20) What are the principal varieties of coke? (21) What are the requirements of a good furnace coke? (22) For what are the principal laboratory tests made on the physical properties of coke? (23) How are coals prepared for being charged into the oven? SUPPLIES FOR STUDENTS In order to do good work, it is very necessary for our students to secure the best materials, instruments, etc. used in their Courses. 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