  Coal is of prehistoric origin, formed from accumulation of vegetable matter, supposed to be the remains of immense forests. In past ages the deposits underwent destructive distillation from great heat and was subjected to pressure, sufficient to compress it into varying degrees of hardness. Coal is composed of carbon, hydrogen and oxygen, with small quantities of nitrogen and varying amounts of sulphur and ash. The coals from different geological formations vary in quality from the hard dry anthracites to the soft wet lignites, with the intermediate bituminous coals; all of which furnish fuels that when burned will produce amounts of heat, depending on their composition, the quantity of moisture contained and the conditions of their combustion. Carbon, of which coal is principally composed, exists in different combinations, depending on the condition of its formation. Part of the carbon is combined with hydrogen to form hydrocarbon that may be driven off when heated, and which forms the volatile portion of the coal. The remainder of the carbon appears in the form of coke—when the volatile matter is driven off—and is said to be fixed. The fixed carbon and volatile constituents together make up the combustible. Other ingredients of coal that require attention are the moisture, and the incombustible matter that forms ash. Moisture varies in quantity from as low as 0.75 per cent. in hard coal to 50 per cent. in lignite. The amounts of ash in different coals vary from 3 to 30 per cent. of the weights of the fuel. The heating value of coals differs in amount by reason of the variable quantities of fixed and combined carbons, moisture and ash. Different coals are compared in value by the number of B.t.u. per pound of dry coal that can possibly be developed when burned, and with these factors are given the percentages of moisture and ash. There are no distinct demarkations between different grades of The value of coal as a heat-producing agent is represented by the B.t.u. it is capable of turning to useful account. The price of coal should be based on the amount of heat it is capable of generating when burned. In considering the value of coal for any particular purpose, thought must be taken as to its characteristic properties, for coals that produce excellent results for one purpose may be very unsatisfactory in others. Soft coal containing a large percentage of volatile matter usually produces a great amount of smoke and unless carefully fired this will condense and form accumulations of soot that are objectionable. For reasons of this kind bituminous coals are often sold at a lower price than their rated heating value might indicate. Anthracite or hard coalpossesses bright lustrous surfaces when newly fractured, that when handled do not soil the hands. It contains a high percentage of carbon, a small amount of volatile matter and little moisture. It is greatly in demand as a domestic fuel because it burns slowly with an intense heat, practically without flame and produces no smoke. It invariably commands a higher price than soft coal, but in heating value is not superior to the better grades of soft coal. In furnaces for house heating the use of soft coal often gives better satisfaction than hard coal. The grades of hard coal found in the market will vary with the demand in any locality but those recognized by the trade are:
Hard coal of stove and chestnut sizes are those most commonly used for domestic heating, because they are well suited for Bituminous or soft coalrepresents the chief fuel of commerce. The market prices of these coals are determined largely by reason of their reputation as desirable fuel. The variations in price depend on the physical qualities, rather than on the amount of heat evolved in combustion. The compositions of coals vary markedly in different localities and often in the same locality several grades are produced. It sometimes happens that from different parts of a mine the coal will differ very much in heat value. Bituminous coals are roughly classified as coking and free-burning. The former is valuable for gas manufacture and for production of coke. The coking coals fuse on being heated, allowing the volatile portion to escape; and when the gas has been all distilled, the residue is coke. When used for gas making, the volatile portion forms the illuminating gas. When burned in a furnace, the gases from soft coal burn with a yellow flame and usually with considerable smoke. The classification of bituminous coals differ somewhat in the East from that of the West. Eastern bituminous coals are commonly graded: A. Run-of-mine coal = unscreened coal as taken from the mine. B. Lump coal = that which passes over a bar screen with 1¼-inch openings. C. Nut coal = that which passes through a bar screen with 1¼-inch openings and over one with ¾-inch openings. D. Slack = all that which passes through a ¾-inch bar screen. Western bituminous coal: E. Run-of-mine coal = the unscreened coal as taken from the mine. F. Lump coal—divides as 6-inch, 3-inch and 1¼-inch according to the diameter of the mesh through which the pieces pass the screens. G. Nut coal—varying from 1¼-inch size to ¾-inch in diameter. H. Screening = all coal which passes a 1¼-inch screen including the dust. Heat derived from coal—or any other fuel—in the process of combustion is due to oxidation. Combustion or burning is caused by rapid oxidation. When oxygen combines with Oxidation of Hydrocarbons.—In the oxidation of hydrocarbons, as that of burning coal gas, the combination of the elements forms carbon dioxide and water. The presence of the water, formed in combustion, is often shown in the formation of moisture on the bottom of a cold vessel when placed over a gas flame. The same effect is observed in a newly lighted kerosene lamp, when the film of moisture forms inside the cold lamp chimney. As soon as the surfaces become heated the moisture is evaporated. Occasionally, the accumulation of moisture in chimneys, from this cause, is sufficient during extremely cold weather to form ice in the part of the chimney exposed to the outside air. Chimneys have been known to become so stopped by accumulation of ice from this cause as to materially interfere with the draft. The fixed carbon of the coal, when oxidized, has a constant heating value of 14,000 B.t.u. per pound. The volatile hydrocarbons develop amounts of heat when burned, depending on their composition, and differ in coals from different localities. The heat obtained from the volatile part of coal depends on its chemical composition and differs very materially; it may be as high as 21,000 B.t.u. per pound, or as low as 12,000 B.t.u. per pound. A high percentage of volatile matter usually indicates a fuel that will produce a large volume of smoke, which—unless the combustion is complete in the furnace—will deposit soot as soon as it is condensed, either in the chimney or in the outside air. The ash has no heating value, and the contained moisture has a negative heating effect, because considerable heat is required to evaporate and raise it to the temperature of the gases of the furnace. In burning fuel the moisture uses up the heat of combustion in proportion as it appears in the coal. The moisture is bought as coal but requires heat to get rid of it; so the percentage of water in coal should be considered very carefully. It is customary in comparing the heating values of coals, to Semi-bituminous coalrepresents a class between the hard and soft grades. It contains less carbon and more volatile matter than hard coal. It burns with a short flame with very little smoke and is valuable as a furnace fuel. The Pocahontas coal of West Virginia is an example of this class. Semi-bituminous coal is often called smokeless coal, because in burning it produces relatively little smoke. It will be noted in the table of heat values on page 192 that coal of this variety has high heat-producing properties. It is a very friable coal and for that reason is apt to contain considerable dust. As a furnace fuel it produces—when carefully fired—very satisfactory results. Graphitic Anthracite.—This is a type of coal found in Rhode Island and Massachusetts which resembles both graphite and anthracite coal. It is gray in color, very hard and burns with extreme difficulty. Cannel Coal.—This is a variety of bituminous coal, rich in hydrocarbons. It burns with a bright flame without fusing and is often used for open fires. Lignite.—This is a type of fuel that in point of geological formation represents the condition between true coal and peat. Lignite occurs in immense deposits throughout the middle portion of the western half of the United States, where beds 20 feet in depth are not uncommon. It varies in color from black to brown and in many localities is known as brown coal. When newly mined, lignite contains a large percentage of water, sometimes as high as 50 per cent. On account of this Peat.—As a fuel, peat has been used very little in the United States on account of the abundance of the better grades of fuel, but in many parts of the country it is used locally to a considerable extent. In peat bogs from which the fuel is taken, the peat is formed from grasses and sedges which in time produce a carbonaceous mass that becomes sufficiently dense to be taken out in sections, with a long narrow spade. The peat is then built into piles where after drying it is ready to be burned. Wood.—On account of its relative scarcity and correspondingly high price, wood is no longer a commercial fuel of any consequence. The low heating value of wood as compared with coal makes it a prohibitive fuel except in forest localities. Wood is commonly sold by the cord and no attention is given by dealers to its value in heat-producing capacity. The desirability of wood as a fuel is chiefly that of reputation. It is usually considered that hickory is the ideal fire wood, dry maple a close second and that oak is next in desirability as fuel; following which are ash, elm, beech, etc., depending on the density of the wood. The price of wood per cord depends on the nearness and abundance of supply. The actual heating values of different woods as determined by Gottlieb show that per pound of dry wood there is little difference in heat value between different kinds of hard woods, and that pine gives per pound the highest value of all. The table given below was taken from “Steam” published by the Babcock-Wilcox Co.
In considering this table it must be kept in mind that the values are for dry wood per pound. As given in Kent’s “Engineer’s Pocket Book” the weights of different fuel woods per cord (thoroughly air-dried) are about as follows:
The above values in pounds of coal may be taken to represent average bituminous coals. As given by Suplee’s “Mechanical Engineers’ Reference Book,” eight samples of coals representing bituminous coals from mines east of the Mississippi River give an average heating value of 13,755 B.t.u. per pound. Charcoal.—This is made from wood by driving off the volatile constituents; the residual carbon, which forms the charcoal is a fuel that burns without smoke or flame. Charcoal is made by piling wood in a heap, which is covered with earth. In the bottom of the heap a fire generates the necessary heat for distilling off the volatile matter. Charcoal holds to wood the same relation that coke bears to coal. Coke.—This is the residue from the distillation of coal. It comprises from 60 to 70 per cent. of the original coal and contains most of the carbon and all of the ash of the coal. Coke is gray in color and has a slightly metallic luster; it is porous, brittle and in handling gives out something of a metallic ring. It is often sold for fuel as a byproduct by gas factories. In heating value gas-coke gives about 14,000 B.t.u. per pound when dry and as a consequence is rated as an excellent fuel. Clean coke burns without flame and is capable of producing an intense heat. On account of its porous nature it occupies a relatively large volume per ton. It is most successfully burned in stoves and furnaces with large fire-boxes. Gas-coke,which is the residue from the gas retorts, is somewhat inferior in heating value to coke made in ovens but it is an excellent fuel where furnaces are adapted to its use. Gas-coke is often stored, by piling it in heaps, in the open and on account Briquettes.—Briquetted coal and other fuels are produced by compressing coal dust or other powdered fuel, mixed with coal tars or other bituminous binder in sufficient quantity to cause the adhesion of the particles when pressed into form under great pressure. Owing to the relative cheapness of fuel, briquettes have been used but very little in the United States. With the advance in the price of coal of the past few years, they have found a place on the market and have become a common form of fuel. The heat value of briquettes will depend on the kind and quality of material that enters into their composition. Quite generally, they produce heat equal to the average grade of soft coal. In the Northwest briquettes made of West Virginia semi-bituminous coal sell at the same price as run-of-mine coal of the same quality. Their use has proven satisfactory as a furnace fuel and they will very likely be sold in increasing quantities. Comparative Value of Coal to Other Fuels.—Until a comparatively recent time, coal has been sold by weight and reputation alone; but conditions are rapidly approaching, which will require it to be sold according to its composition and heating value. Among manufacturers and others using large quantities of fuel, the practice of contracting for coal by specification is becoming increasingly common. The determining factors are the amounts of moisture, ash, sulphur, carbon, and volatile matter the coal contains, as well as the size of the pieces and freedom from dust. In a few of the most progressive cities, coal dealers are required to supply coal for schools and other municipal uses, which has been subject to the approval of the City Engineer. The time is not far distant when dealers will be required to submit samples of all fuel, for sale to the public, to the examination of the municipal authorities. The following table of the heating values of various fuels is taken from Benson’s “Industrial Chemistry.” British Thermal Units for One Cent from Different Fuels
Price of Coal.—The value of coal as a fuel will depend on the amount of heat it is capable of producing when burned; its price should therefore be determined by the heating value per pound of fuel as purchased. Secondary determining factors in price are those of convenience of handling and the difficulty in burning the fuel such as the size and uniformity of the pieces, the formation of clinkers, smoke and accumulation of soot. Soft coals, containing a large amount of volatile matter, usually produce much soot and smoke and as a consequence sell for a lower price than coals that produce little smoke. The selection of fuels will depend on the type of heating plant in use, whether by stoves or by furnaces. If by stoves, whether it is possible to use soft coal as a fuel. The automatically fed stove, of the base-burner type, are usually designed for the use of hard coal and in such stoves the use of soft coal would not be possible. Other stoves and furnaces are usually capable of burning soft coal with varying degrees of satisfaction, depending on the design and surrounding conditions. The following prices, from the local market, show the usual ratings of various fuels in common use. These prices vary with the locality and somewhat with the season. It is usually possible to purchase coal at some reduction in price during the summer months when the demand for coal is light.
The price of coal is determined in many localities by the distance from the sources of supply and the means of transportation. The fact that coals from all of the principal mining areas from Pennsylvania, west to Iowa, are sold at points in the Northwest for the same price, is due in greatest measure to transportation rates on the Great Lakes. The prices of Eastern coals at Duluth are such that in competition with Western coals they are sold at the same price as is shown by the table. It is usually impossible for the average householder, or even the dealer, to determine definitely the exact locality from which his fuel is mined. Even when such information is obtainable, the quality is still in doubt, unless analysis is obtainable by sample. The data given in the following tables is such as will furnish a fair knowledge of the relative heating values of coals from the principal mining areas of the United States. The data was obtained from a considerable number of authorities but chiefly from the reports of the United States Geological Survey. The different items are not intended to be exact, they merely represent reliable average conditions. The varying conditions of available heat and percentage of moisture given in the following table are such as to be of little use to those unaccustomed to problems of this kind, unless a systematic method of comparison is made of the different fuels. Approximate Composition and Calorific Value of Typical American Coals
The following table was prepared from the date of that preceding combined with the prices of various coals to be obtained in the local market. The table is intended to present a method of comparing the values of fuels from different coal areas. The consumer is interested to know the amount of heat purchased in the form of fuel. The table shows in the column headed “Heat per $1,” the number of B.t.u. purchased for $1 in coal; the number of available B.t.u. in the different kinds of coal may be taken as a relative comparison of their values as fuel. The gas-coke in the table is that sold by the local gas company. The amount of moisture in this case is relatively high because of
Semi-bituminous coal commands considerable favor as a house-heating fuel, because of the fact that it burns with much less smoke than bituminous coal. In available heat it is considerably above the Western bituminous coal and it sells at a price $1.50 higher per ton. The reason for the difference in price is not so much on account of its heating value, as because of relatively small amount of smoke produced in combustion. Other coals capable of producing more heat are sold at less price because of smoke and soot produced in burning. Hard coal at $10.50 is the most expensive coal of all. The ratio of available heat units per $1 for hard coal, as compared with the best soft coal, is as 23 is to 35. This means that at the stated prices those who burn hard coal pay the additional price, because of the physical properties it possesses. In constructing the above table, 100 pounds of coal was taken as a unit of comparison. The price per ton is that given in the table of local prices. The per cent. of moisture and the B.t.u. per pound of fuel was taken from table on page 192. In explaining the method by which the different items were obtained, it will be necessary to discuss briefly the condition of combustion and the heat losses that take place when fuel is burned. The moisture in the fuel is the undesirable part, because it requires a large amount of heat to dispose of it. It is looked upon as so much water, that must be raised in temperature from that in which it is taken from the coal bin to the temperature and condition of vapor in which it passes into the chimney. When the fuel enters the furnace the water is heated to the boiling point. In changing temperature it absorbs 1 B.t.u. for each pound of water, through each degree of change. Suppose that, as in the case of Pennsylvania bituminous coal which contains 2.44 pounds of water to each 100 pounds of coal, the coal entering the furnace was at 50°F. To raise its temperature to the boiling point (212°F.) required a change of 162°. The 2.44 pounds of water raised this amount 162 × 2.44 = 395.28 B.t.u. To change the 2.44 pounds of water, into steam at the atmospheric pressure requires 969.7 B.t.u. (heat of vaporization), practically 970 B.t.u. per pound of water. The heat required to vaporize 2.44 pounds of water is 2.44 × 970 = 2366.80 B.t.u. The vapor is now raised in temperature, to that of the furnace, which we may assume is 1200°F. The furnace being at atmospheric pressure the vapor merely expands in volume as a gas. The specific heat of steam at atmospheric pressure is 0.464; that is, 1 pound of steam requires only 0.464 B.t.u. to raise it a degree, and 2.44 pounds of water will absorb 0.464 × 2.44 × 1200 = 1356.00 B.t.u. Of this last amount of heat, approximately 50 per cent. is recovered as the gases pass through the furnace. The total loss of heat due to the evaporation of the water is
In the 100 pounds of coal under consideration, there is 100 pounds, less 2.44 pounds of water, or 97.56 of dry coal, each pound of which contains 13,732 B.t.u. as given by the table on page 193. This gives 97.56 × 12,682 = 1,339,753 = practically 1,340,000 B.t.u. From this quantity is subtracted the loss of heat, 3439. 1,340,000 - 3439 = 1,336,561 B.t.u. This represents the total available heat in 100 pounds of coal. If this quantity is now divided by the cost of 100 pounds of coal at $7.25 per ton, the result, 3,564,000 B.t.u., will be the available heat bought for $1 as given in column 7 of the table. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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