Chapter VIII: Part 8

(.821 .026 .016)
W = 34.56 (---- + (.0425 - ----) + ----) = 10.88 pounds.
( 3 8 8 )

If the amount of carbon which is burned and passes away as flue gas is 80 per cent, which would allow for 2.1 per cent of unburned carbon in terms of the total weight of dry fuel burned, the weight of dry gas per pound of carbon burned will be from formula (16):

11 × 10.7 + 8 × 9.0 + 7(0 + 80.3)
W = --------------------------------- = 23.42 pounds
3(10.7 + 0)

and the weight of flue gas per pound of coal burned will be .80 × 23.42 = 18.74 pounds.

The heat lost in the flue gases per pound of coal burned will be from formula (15) and the value 18.74 just determined.

Loss = .24 × 18.74 × (500 - 60) = 1979 B. t. u.

The percentage of heat lost in the flue gases will be 1979 ÷ 14500 = 13.6 per cent.

The ratio of air supplied per pound of coal to that theoretically required will be 18.74 ÷ 10.88 = 1.72 per cent.

The ratio of air supplied per pound of combustible to that required will be from formula (14):

.803 ------------------------- = 1.73 .803 - 3.782(.09 - ½ × 0)

The ratio based on combustible will be greater than the ratio based on fuel if there is unconsumed carbon in the ash.

Unreliability of CO_{2} Readings Taken Alone--It is generally assumed that high CO_{2} readings are indicative of good combustion and hence of high efficiency. This is true only in the sense that such high readings do indicate the small amount of excess air that usually accompanies good combustion, and for this reason high CO_{2} readings alone are not considered entirely reliable. Wherever an automatic CO_{2} recorder is used, it should be checked from time to time and the analysis carried further with a view to ascertaining whether there is CO present. As the percentage of CO_{2} in these gases increases, there is a tendency toward the presence of CO, which, of course, cannot be shown by a CO_{2} recorder, and which is often difficult to detect with an Orsat apparatus. The greatest care should be taken in preparing the cuprous chloride solution in making analyses and it must be known to be fresh and capable of absorbing CO. In one instance that came to our attention, in using an Orsat apparatus where the cuprous chloride solution was believed to be fresh, no CO was indicated in the flue gases but on passing the same sample into a Hempel apparatus, a considerable percentage was found. It is not safe, therefore, to assume without question from a high CO_{2} reading that the combustion is correspondingly good, and the question of excess air alone should be distinguished from that of good combustion. The effect of a small quantity of CO, say one per cent, present in the flue gases will have a negligible influence on the quantity of excess air, but the presence of such an amount would mean a loss due to the incomplete combustion of the carbon in the fuel of possibly 4.5 per cent of the total heat in the fuel burned. When this is considered, the importance of a complete flue gas analysis is apparent.

Table 34 gives the densities of various gases together with other data that will be of service in gas analysis work.

TABLE 34

DENSITY OF GASES AT 32 DEGREES FAHRENHEIT AND ATMOSPHERIC PRESSURE
ADAPTED FROM SMITHSONIAN TABLES

+----------+----------+--------+---------+----------+---------------+ | | | | | | Relative | | | | | Weight | | Density, | | | | | of | Volume | Hydrogen = 1 | | | |Specific|One Cubic| of +-------+-------+ | Gas | Chemical |Gravity | Foot |One Pound | |Approx-| | | Symbol | Air=1 | Pounds |Cubic Feet| Exact | imate | +----------+----------+--------+---------+----------+-------+-------+ |Oxygen | O | 1.053 | .08922 | 11.208 | 15.87 | 16 | |Nitrogen | N | 0.9673 | .07829 | 12.773 | 13.92 | 14 | |Hydrogen | H | 0.0696 | .005621 | 177.90 | 1.00 | 1 | |Carbon | | | | | | | | Dioxide | CO_{2} | 1.5291 | .12269 | 8.151 | 21.83 | 22 | |Carbon | | | | | | | | Monoxide | CO | 0.9672 | .07807 | 12.809 | 13.89 | 14 | |Methane | CH_{4} | 0.5576 | .04470 | 22.371 | 7.95 | 8 | |Ethane |C_{2}H_{6}| 1.075 | .08379 | 11.935 | 14.91 | 15 | |Acetylene |C_{2}H_{2}| 0.920 | .07254 | 13.785 | 12.91 | 13 | |Sulphur | | | | | | | | Dioxide | SO_{2} | 2.2639 | .17862 | 5.598 | 31.96 | 32 | |Air | ... | 1.0000 | .08071 | 12.390 | ... | ... | +----------+----------+--------+---------+----------+-------+-------+

CLASSIFICATION OF FUELS

(WITH PARTICULAR REFERENCE TO COAL)

Fuels for steam boilers may be classified as solid, liquid or gaseous. Of the solid fuels, anthracite and bituminous coals are the most common, but in this class must also be included lignite, peat, wood, bagasse and the refuse from certain industrial processes such as sawdust, shavings, tan bark and the like. Straw, corn and coffee husks are utilized in isolated cases.

The class of liquid fuels is represented chiefly by petroleum, though coal tar and water-gas tar are used to a limited extent.

Gaseous fuels are limited to natural gas, blast furnace gas and coke oven gas, the first being a natural product and the two latter by-products from industrial processes. Though waste gases from certain processes may be considered as gaseous fuels, inasmuch as the question of combustion does not enter, the methods of utilizing them differ from that for combustible gaseous fuel, and the question will be dealt with separately.

Since coal is by far the most generally used of all fuels, this chapter will be devoted entirely to the formation, composition and distribution of the various grades, from anthracite to peat. The other fuels will be discussed in succeeding chapters and their combustion dealt with in connection with their composition.

Formation of Coal--All coals are of vegetable origin and are the remains of prehistoric forests. Destructive distillation due to great pressures and temperatures, has resolved the organic matter into its invariable ultimate constituents, carbon, hydrogen, oxygen and other substances, in varying proportions. The factors of time, depth of beds, disturbance of beds and the intrusion of mineral matter resulting from such disturbances have produced the variation in the degree of evolution from vegetable fiber to hard coal. This variation is shown chiefly in the content of carbon, and Table 35 shows the steps of such variation.

TABLE 35

APPROXIMATE CHEMICAL CHANGES FROM WOOD
FIBER TO ANTHRACITE COAL

+----------------------+-------+--------+-------+ |Substance |Carbon |Hydrogen|Oxygen | +----------------------+-------+--------+-------+ |Wood Fiber | 52.65 | 5.25 | 42.10 | |Peat | 59.57 | 5.96 | 34.47 | |Lignite | 66.04 | 5.27 | 28.69 | |Earthy Brown Coal | 73.18 | 5.68 | 21.14 | |Bituminous Coal | 75.06 | 5.84 | 19.10 | |Semi-bituminous Coal | 89.29 | 5.05 | 5.66 | |Anthracite Coal | 91.58 | 3.96 | 4.46 | +----------------------+-------+--------+-------+

Composition of Coal--The uncombined carbon in coal is known as fixed carbon. Some of the carbon constituent is combined with hydrogen and this, together with other gaseous substances driven off by the application of heat, form that portion of the coal known as volatile matter. The fixed carbon and the volatile matter constitute the combustible. The oxygen and nitrogen contained in the volatile matter are not combustible, but custom has applied this term to that portion of the coal which is dry and free from ash, thus including the oxygen and nitrogen.

The other important substances entering into the composition of coal are moisture and the refractory earths which form the ash. The ash varies in different coals from 3 to 30 per cent and the moisture from 0.75 to 45 per cent of the total weight of the coal, depending upon the grade and the locality in which it is mined. A large percentage of ash is undesirable as it not only reduces the calorific value of the fuel, but chokes up the air passages in the furnace and through the fuel bed, thus preventing the rapid combustion necessary to high efficiency. If the coal contains an excessive quantity of sulphur, trouble will result from its harmful action on the metal of the boiler where moisture is present, and because it unites with the ash to form a fusible slag or clinker which will choke up the grate bars and form a solid mass in which large quantities of unconsumed carbon may be imbedded.

Moisture in coal may be more detrimental than ash in reducing the temperature of a furnace, as it is non-combustible, absorbs heat both in being evaporated and superheated to the temperature of the furnace gases. In some instances, however, a certain amount of moisture in a bituminous coal produces a mechanical action that assists in the combustion and makes it possible to develop higher capacities than with dry coal.

Classification of Coal--Custom has classified coals in accordance with the varying content of carbon and volatile matter in the combustible. Table 36 gives the approximate percentages of these constituents for the general classes of coals with the corresponding heat values per pound of combustible.

TABLE 36

APPROXIMATE COMPOSITION AND CALORIFIC VALUE
OF GENERAL GRADES OF COAL ON BASIS OF COMBUSTIBLE

+-------------------+----------------------------+--------------+ | Kind of Coal | Per Cent of Combustible | B. t. u. | | +------------+---------------+ Per Pound of | | |Fixed Carbon|Volatile Matter| Combustible | +-------------------+------------+---------------+--------------+ |Anthracite |97.0 to 92.5| 3.0 to 7.5 |14600 to 14800| |Semi-anthracite |92.5 to 87.5| 7.5 to 12.5 |14700 to 15500| |Semi-bituminous |87.5 to 75.0| 12.5 to 25.0 |15500 to 16000| |Bituminous--Eastern|75.0 to 60.0| 25.0 to 40.0 |14800 to 15300| |Bituminous--Western|65.0 to 50.0| 35.0 to 50.0 |13500 to 14800| |Lignite | Under 50 | Over 50 |11000 to 13500| +-------------------+------------+---------------+--------------+

Anthracite--The name anthracite, or hard coal, is applied to those dry coals containing from 3 to 7 per cent volatile matter and which do not swell when burned. True anthracite is hard, compact, lustrous and sometimes iridescent, and is characterized by few joints and clefts. Its specific gravity varies from 1.4 to 1.8. In burning, it kindles slowly and with difficulty, is hard to keep alight, and burns with a short, almost colorless flame, without smoke.

Semi-anthracite coal has less density, hardness and luster than true anthracite, and can be distinguished from it by the fact that when newly fractured it will soot the hands. Its specific gravity is ordinarily about 1.4. It kindles quite readily and burns more freely than the true anthracites.

Semi-bituminous coal is softer than anthracite, contains more volatile hydrocarbons, kindles more easily and burns more rapidly. It is ordinarily free burning, has a high calorific value and is of the highest order for steam generating purposes.

Bituminous coals are still softer than those described and contain still more volatile hydrocarbons. The difference between the semi-bituminous and the bituminous coals is an important one, economically. The former have an average heating value per pound of combustible about 6 per cent higher than the latter, and they burn with much less smoke in ordinary furnaces. The distinctive characteristic of the bituminous coals is the emission of yellow flame and smoke when burning. In color they range from pitch black to dark brown, having a resinous luster in the most compact specimens, and a silky luster in such specimens as show traces of vegetable fiber. The specific gravity is ordinarily about 1.3.

Bituminous coals are either of the caking or non-caking class. The former, when heated, fuse and swell in size; the latter burn freely, do not fuse, and are commonly known as free burning coals. Caking coals are rich in volatile hydrocarbons and are valuable in gas manufacture.

Bituminous coals absorb moisture from the atmosphere. The surface moisture can be removed by ordinary drying, but a portion of the water can be removed only by heating the coal to a temperature of about 250 degrees Fahrenheit.

Cannel coal is a variety of bituminous coal, rich in hydrogen and hydrocarbons, and is exceedingly valuable as a gas coal. It has a dull resinous luster and burns with a bright flame without fusing. Cannel coal is seldom used for steam coal, though it is sometimes mixed with semi-bituminous coal where an increased economy at high rates of combustion is desired. The composition of cannel coal is approximately as follows: fixed carbon, 26 to 55 per cent; volatile matter, 42 to 64 per cent; earthy matter, 2 to 14 per cent. Its specific gravity is approximately 1.24.

Lignite is organic matter in the earlier stages of its conversion into coal, and includes all varieties which are intermediate between peat and coal of the older formation. Its specific gravity is low, being 1.2 to 1.23, and when freshly mined it may contain as high as 50 per cent of moisture. Its appearance varies from a light brown, showing a distinctly woody structure, in the poorer varieties, to a black, with a pitchy luster resembling hard coal, in the best varieties. It is non-caking and burns with a bright but slightly smoky flame with moderate heat. It is easily broken, will not stand much handling in transportation, and if exposed to the weather will rapidly disintegrate, which will increase the difficulty of burning it.

Its composition varies over wide limits. The ash may run as low as one per cent and as high as 50 per cent. Its high content of moisture and the large quantity of air necessary for its combustion cause large stack losses. It is distinctly a low-grade fuel and is used almost entirely in the districts where mined, due to its cheapness.

Peat is organic matter in the first stages of its conversion into coal and is found in bogs and similar places. Its moisture content when cut is extremely high, averaging 75 or 80 per cent. It is unsuitable for fuel until dried and even then will contain as much as 30 per cent moisture. Its ash content when dry varies from 3 to 12 per cent. In this country, though large deposits of peat have been found, it has not as yet been found practicable to utilize it for steam generating purposes in competition with coal. In some European countries, however, the peat industry is common.

Distribution--The anthracite coals are, with some unimportant exceptions, confined to five small fields in Eastern Pennsylvania, as shown in the following list. These fields are given in the order of their hardness.

Lehigh or Eastern Middle Field
Green Mountain District
Black Creek District
Hazelton District
Beaver Meadow District
Panther Creek District[33]

Mahanoy or Western Field[34]
East Mahanoy District
West Mahanoy District

Wyoming or Northern Field
Carbondale District
Scranton District
Pittston District
Wilkesbarre District
Plymouth District

Schuylkill or Southern Field
East Schuylkill District
West Schuylkill District
Louberry District

Lykens Valley or Southwestern Field
Lykens Valley District
Shamokin District[35]

Anthracite is also found in Pulaski and Wythe Counties, Virginia; along the border of Little Walker Mountain, and in Gunnison County, Colorado. The areas in Virginia are limited, however, while in Colorado the quality varies greatly in neighboring beds and even in the same bed. An anthracite bed in New Mexico was described in 1870 by Dr. R. W. Raymond, formerly United States Mining Commissioner.

Semi-anthracite coals are found in a few small areas in the western part of the anthracite field. The largest of these beds is the Bernice in Sullivan County, Pennsylvania. Mr. William Kent, in his "Steam Boiler Economy", describes this as follows: "The Bernice semi-anthracite coal basin lies between Beech Creek on the north and Loyalsock Creek on the south. It is six miles long, east and west, and hardly a third of a mile across. An 8-foot vein of coal lies in a bed of 12 feet of coal and slate. The coal of this bed is the dividing line between anthracite and semi-anthracite, and is similar to the coal of the Lykens Valley District. Mine analyses give a range as follows: moisture, 0.65 to 1.97; volatile matter, 3.56 to 9.40; fixed carbon, 82.52 to 89.39; ash, 3.27 to 9.34; sulphur, 0.24 to 1.04."

Semi-bituminous coals are found on the eastern edge of the great Appalachian Field. Starting with Tioga and Bradford Counties of northern Pennsylvania, the bed runs southwest through Lycoming, Clearfield, Centre, Huntingdon, Cambria, Somerset and Fulton Counties, Pennsylvania; Allegheny County, Maryland; Buchannan, Dickinson, Lee, Russell, Scott, Tazewell and Wise Counties, Virginia; Mercer, McDowell, Fayette, Raleigh and Mineral Counties, West Virginia; and ending in northeastern Tennessee, where a small amount of semi-bituminous is mined.

The largest of the bituminous fields is the Appalachian. Beginning near the northern boundary of Pennsylvania, in the western portion of the State, it extends southwestward through West Virginia, touching Maryland and Virginia on their western borders, passing through southeastern Ohio, eastern Kentucky and central Tennessee, and ending in western Alabama, 900 miles from its northern extremity.

The next bituminous coal producing region to the west is the Northern Field, in north central Michigan. Still further to the west, and second in importance to the Appalachian Field, is the Eastern Interior Field. This covers, with the exception of the upper northern portion, nearly the entire State of Illinois, southwest Indiana and the western portion of Kentucky.

The Western Field extends through central and southern Iowa, western Missouri, southwestern Kansas, eastern Oklahoma and the west central portion of Arkansas. The Southwestern Field is confined entirely to the north central portion of Texas, in which State there are also two small isolated fields along the Rio Grande River.

The remaining bituminous fields are scattered through what may be termed the Rocky Mountain Region, extending from Montana to New Orleans. A partial list of these fields and their location follows:

Judith Basin Central Montana Bull Mountain Field Central Montana Yellowstone Region Southwestern Montana Big Horn Basin Region Southern Montana Big Horn Basin Region Northern Wyoming Black Hills Region Northeastern Wyoming Hanna Field Southern Wyoming Green River Region Southwestern Wyoming Yampa Field Northwestern Colorado North Park Field Northern Colorado Denver Region North Central Colorado Uinta Region Western Colorado Uinta Region Eastern Utah Southwestern Region Southwestern Utah Raton Mountain Region Southern Colorado Raton Mountain Region Northern New Mexico San Juan River Region Northwestern New Mexico Capitan Field Southern New Mexico

Along the Pacific Coast a few small fields are scattered in western California, southwestern Oregon, western and northwestern Washington.

Most of the coals in the above fields are on the border line between bituminous and lignite. They are really a low grade of bituminous coal and are known as sub-bituminous or black lignites.

Lignites--These resemble the brown coals of Europe and are found in the western states, Wyoming, New Mexico, Arizona, Utah, Montana, North Dakota, Nevada, California, Oregon and Washington. Many of the fields given as those containing bituminous coals in the western states also contain true lignite. Lignite is also found in the eastern part of Texas and in Oklahoma.

Alaska Coals--Coal has been found in Alaska and undoubtedly is of great value, though the extent and character of the fields have probably been exaggerated. Great quantities of lignite are known to exist, and in quality the coal ranges in character from lignite to anthracite. There are at present, however, only two fields of high-grade coals known, these being the Bering River Field, near Controllers Bay, and the Matanuska Field, at the head of Cooks Inlet. Both of these fields are known to contain both anthracite and high-grade bituminous coals, though as yet they cannot be said to have been opened up.

Weathering of Coal--The storage of coal has become within the last few years to a certain extent a necessity due to market conditions, danger of labor difficulties at the mines and in the railroads, and the crowding of transportation facilities. The first cause is probably the most important, and this is particularly true of anthracite coals where a sliding scale of prices is used according to the season of the year. While market conditions serve as one of the principal reasons for coal storage, most power plants and manufacturing plants feel compelled to protect their coal supply from the danger of strikes, car shortages and the like, and it is customary for large power plants, railroads and coal companies themselves, to store bituminous coal. Naval coaling stations are also an example of what is done along these lines.

Anthracite is the nearest approach to the ideal coal for storing. It is not subject to spontaneous ignition, and for this reason is unlimited in the amount that may be stored in one pile. With bituminous coals, however, the case is different. Most bituminous coals will ignite if placed in large enough piles and all suffer more or less from disintegration. Coal producers only store such coals as are least liable to ignite, and which will stand rehandling for shipment.

The changes which take place in stored coal are of two kinds: 1st, the oxidization of the inorganic matter such as pyrites; and 2nd, the direct oxidization of the organic matter of the actual coal.

The first change will result in an increased volume of the coal, and sometimes in an increased weight, and a marked disintegration. The changes due to direct oxidization of the coal substances usually cannot be detected by the eye, but as they involve the oxidization of the carbon and available hydrogen and the absorption of the oxygen by unsaturated hydrocarbons, they are the chief cause of the weathering losses in heat value. Numerous experiments have led to the conclusion that this is also the cause for spontaneous combustion.

Experiments to show loss in calorific heat values due to weathering indicate that such loss may be as high as 10 per cent when the coal is stored in the air, and 8.75 per cent when stored under water. It would appear that the higher the volatile content of the coal, the greater will be the loss in calorific value and the more subject to spontaneous ignition.

Some experiments made by Messrs. S. W. Parr and W. F. Wheeler, published in 1909 by the Experiment Station of the University of Illinois, indicate that coals of the nature found in Illinois and neighboring states are not affected seriously during storage from the standpoint of weight and heating value, the latter loss averaging about 3½ per cent for the first year of storage. They found that the losses due to disintegration and to spontaneous ignition were of greater importance. Their conclusions agree with those deduced from the other experiments, viz., that the storing of a larger size coal than that which is to be used, will overcome to a certain extent the objection to disintegration, and that the larger sizes, besides being advantageous in respect to disintegration, are less liable to spontaneous ignition. Storage under water will, of course, entirely prevent any fire loss and, to a great extent, will stop disintegration and reduce the calorific losses to a minimum.

To minimize the danger of spontaneous ignition in storing coal, the piles should be thoroughly ventilated.

Pulverized Fuels--Considerable experimental work has been done with pulverized coal, utilizing either coal dust or pulverizing such coal as is too small to be burned in other ways. If satisfactorily fed to the furnace, it would appear to have several advantages. The dust burned in suspension would be more completely consumed than is the case with the solid coals, the production of smoke would be minimized, and the process would admit of an adjustment of the air supply to a point very close to the amount theoretically required. This is due to the fact that in burning there is an intimate mixture of the air and fuel. The principal objections have been in the inability to introduce the pulverized fuel into the furnace uniformly, the difficulty of reducing the fuel to the same degree of fineness, liability of explosion in the furnace due to improper mixture with the air, and the decreased capacity and efficiency resulting from the difficulty of keeping tube surfaces clean.

Pressed Fuels--In this class are those composed of the dust of some suitable combustible, pressed and cemented together by a substance possessing binding and in most cases inflammable properties. Such fuels, known as briquettes, are extensively used in foreign countries and consist of carbon or soft coal, too small to be burned in the ordinary way, mixed usually with pitch or coal tar. Much experimenting has been done in this country in briquetting fuels, the government having taken an active interest in the question, but as yet this class of fuel has not come into common use as the cost and difficulty of manufacture and handling have made it impossible to place it in the market at a price to successfully compete with coal.

Coke is a porous product consisting almost entirely of carbon remaining after certain manufacturing processes have distilled off the hydrocarbon gases of the fuel used. It is produced, first, from gas coal distilled in gas retorts; second, from gas or ordinary bituminous coals burned in special furnaces called coke ovens; and third, from petroleum by carrying the distillation of the residuum to a red heat.

Coke is a smokeless fuel. It readily absorbs moisture from the atmosphere and if not kept under cover its moisture content may be as much as 20 per cent of its own weight.

Gas-house coke is generally softer and more porous than oven coke, ignites more readily, and requires less draft for its combustion.

THE DETERMINATION OF HEATING VALUES OF FUELS

The heating value of a fuel may be determined either by a calculation from a chemical analysis or by burning a sample in a calorimeter.

In the former method the calculation should be based on an ultimate analysis, which reduces the fuel to its elementary constituents of carbon, hydrogen, oxygen, nitrogen, sulphur, ash and moisture, to secure a reasonable degree of accuracy. A proximate analysis, which determines only the percentage of moisture, fixed carbon, volatile matter and ash, without determining the ultimate composition of the volatile matter, cannot be used for computing the heat of combustion with the same degree of accuracy as an ultimate analysis, but estimates may be based on the ultimate analysis that are fairly correct.

An ultimate analysis requires the services of a competent chemist, and the methods to be employed in such a determination will be found in any standard book on engineering chemistry. An ultimate analysis, while resolving the fuel into its elementary constituents, does not reveal how these may have been combined in the fuel. The manner of their combination undoubtedly has a direct effect upon their calorific value, as fuels having almost identical ultimate analyses show a difference in heating value when tested in a calorimeter. Such a difference, however, is slight, and very close approximations may be computed from the ultimate analysis.

Ultimate analyses are given on both a moist and a dry fuel basis. Inasmuch as the latter is the basis generally accepted for the comparison of data, it would appear that it is the best basis on which to report such an analysis. When an analysis is given on a moist fuel basis it may be readily converted to a dry basis by dividing the percentages of the various constituents by one minus the percentage of moisture, reporting the moisture content separately.

_Moist Fuel_ _Dry Fuel_

C 83.95 84.45 H 4.23 4.25 O 3.02 3.04 N 1.27 1.28 S .91 .91 Ash 6.03 6.07 ------ 100.00

Moisture .59 .59
------
100.00

Calculations from an Ultimate Analysis--The first formula for the calculation of heating values from the composition of a fuel as determined from an ultimate analysis is due to Dulong, and this formula, slightly modified, is the most commonly used to-day. Other formulae have been proposed, some of which are more accurate for certain specific classes of fuel, but all have their basis in Dulong's formula, the accepted modified form of which is:

Heat units in B. t. u. per pound of dry fuel =

O
14,600 C + 62,000(H - -) + 4000 S (18)
8

where C, H, O and S are the proportionate parts by weight of carbon, hydrogen, oxygen and sulphur.

Assume a coal of the composition given. Substituting in this formula (18),

Heating value per pound of dry coal

( .0304)
= 14,600 × .8445 + 62,000 (.0425 - -----) + 4000 × .0091 = 14,765 B. t. u.
( 8 )

This coal, by a calorimetric test, showed 14,843 B. t. u., and from a comparison the degree of accuracy of the formula will be noted.

The investigation of Lord and Haas in this country, Mabler in France, and Bunte in Germany, all show that Dulong's formula gives results nearly identical with those obtained from calorimetric tests and may be safely applied to all solid fuels except cannel coal, lignite, turf and wood, provided the ultimate analysis is correct. This practically limits its use to coal. The limiting features are the presence of hydrogen and carbon united in the form of hydrocarbons. Such hydrocarbons are present in coals in small quantities, but they have positive and negative heats of combination, and in coals these appear to offset each other, certainly sufficiently to apply the formula to such fuels.

High and Low Heat Value of Fuels--In any fuel containing hydrogen the calorific value as found by the calorimeter is higher than that obtainable under most working conditions in boiler practice by an amount equal to the latent heat of the volatilization of water. This heat would reappear when the vapor was condensed, though in ordinary practice the vapor passes away uncondensed. This fact gives rise to a distinction in heat values into the so-called "higher" and "lower" calorific values. The higher value, _i. e._, the one determined by the calorimeter, is the only scientific unit, is the value which should be used in boiler testing work, and is the one recommended by the American Society of Mechanical Engineers.

There is no absolute measure of the lower heat of combustion, and in view of the wide difference in opinion among physicists as to the deductions to be made from the higher or absolute unit in this determination, the lower value must be considered an artificial unit. The lower value entails the use of an ultimate analysis and involves assumptions that would make the employment of such a unit impracticable for commercial work. The use of the low value may also lead to error and is in no way to be recommended for boiler practice.

An example of its illogical use may be shown by the consideration of a boiler operated in connection with a special economizer where the vapor produced by hydrogen is partially condensed by the economizer. If the low value were used in computing the boiler efficiency, it is obvious that the total efficiency of the combined boiler and economizer must be in error through crediting the combination with the heat imparted in condensing the vapor and not charging such heat to the heat value of the coal.

Heating Value of Gaseous Fuels--The method of computing calorific values from an ultimate analysis is particularly adapted to solid fuels, with the exceptions already noted. The heating value of gaseous fuels may be calculated by Dulong's formula provided another term is added to provide for any carbon monoxide present. Such a method, however, involves the separating of the constituent gases into their elementary gases, which is oftentimes difficult and liable to simple arithmetical error. As the combustible portion of gaseous fuels is ordinarily composed of hydrogen, carbon monoxide and certain hydrocarbons, a determination of the calorific value is much more readily obtained by a separation into their constituent gases and a computation of the calorific value from a table of such values of the constituents. Table 37 gives the calorific value of the more common combustible gases, together with the theoretical amount of air required for their combustion.

TABLE 37

WEIGHT AND CALORIFIC VALUE OF VARIOUS GASES
AT 32 DEGREES FAHRENHEIT AND ATMOSPHERIC PRESSURE
WITH THEORETICAL AMOUNT OF AIR REQUIRED FOR COMBUSTION

+---------------+----------+------+-----+------+----------+-----------+ | Gas | Symbol |Cubic |B.t.u|B.t.u.|Cubic Feet|Cubic Feet | | | | Feet | per | per | of Air | of Air | | | |of Gas|Pound|Cubic | Required | Required | | | | per | | Foot |per Pound | Per Cubic | | | |Pound | | | of Gas |Foot of Gas| +---------------+----------+------+-----+------+----------+-----------+ |Hydrogen | H |177.90|62000| 349 | 428.25 | 2.41 | |Carbon Monoxide| CO | 12.81| 4450| 347 | 30.60 | 2.39 | |Methane |CH_{4} | 22.37|23550| 1053 | 214.00 | 9.57 | |Acetylene |C_{2}H_{2}| 13.79|21465| 1556 | 164.87 | 11.93 | |Olefiant Gas |C_{2}H_{4}| 12.80|21440| 1675 | 183.60 | 14.33 | |Ethane |C_{2}H_{6}| 11.94|22230| 1862 | 199.88 | 16.74 | +---------------+----------+------+-----+------+----------+-----------+

In applying this table, as gas analyses may be reported either by weight or volume, there is given in Table 33[36] a method of changing from volumetric analysis to analysis by weight.

Examples:

1st. Assume a blast furnace gas, the analysis of which in percentages by weight is, oxygen = 2.7, carbon monoxide = 19.5, carbon dioxide = 18.7, nitrogen = 59.1. Here the only combustible gas is the carbon monoxide, and the heat value will be,

0.195 × 4450 = 867.75 B. t. u. per pound.

The _net_ volume of air required to burn one pound of this gas will be,

0.195 × 30.6 = 5.967 cubic feet.

2nd. Assume a natural gas, the analysis of which in percentages by volume is oxygen = 0.40, carbon monoxide = 0.95, carbon dioxide = 0.34, olefiant gas (C_{2}H_{4}) = 0.66, ethane (C_{2}H_{6}) = 3.55, marsh gas (CH_{4}) = 72.15 and hydrogen = 21.95. All but the oxygen and the carbon dioxide are combustibles, and the heat per cubic foot will be,

From CO = 0.0095 × 347 = 3.30
C_{2}H_{4} = 0.0066 × 1675 = 11.05
C_{2}H_{6} = 0.0355 × 1862 = 66.10
CH_{4} = 0.7215 × 1050 = 757.58
H = 0.2195 × 349 = 76.61
------
B. t. u. per cubic foot 914.64

The _net_ air required for combustion of one cubic foot of the gas will be,

CO = 0.0095 × 2.39 = 0.02 C_{2}H_{4} = 0.0066 × 14.33 = 0.09 C_{2}H_{6} = 0.0355 × 16.74 = 0.59 CH_{4} = 0.7215 × 9.57 = 6.90 H = 0.2195 × 2.41 = 0.53 ---- Total net air per cubic foot 8.13

Proximate Analysis--The proximate analysis of a fuel gives its proportions by weight of fixed carbon, volatile combustible matter, moisture and ash. A method of making such an analysis which has been found to give eminently satisfactory results is described below.

From the coal sample obtained on the boiler trial, an average sample of approximately 40 grams is broken up and weighed. A good means of reducing such a sample is passing it through an ordinary coffee mill. This sample should be placed in a double-walled air bath, which should be kept at an approximately constant temperature of 105 degrees centigrade, the sample being weighed at intervals until a minimum is reached. The percentage of moisture can be calculated from the loss in such a drying.

For the determination of the remainder of the analysis, and the heating value of the fuel, a portion of this dried sample should be thoroughly pulverized, and if it is to be kept, should be placed in an air-tight receptacle. One gram of the pulverized sample should be weighed into a porcelain crucible equipped with a well fitting lid. This crucible should be supported on a platinum triangle and heated for seven minutes over the full flame of a Bunsen burner. At the end of such time the sample should be placed in a desiccator containing calcium chloride, and when cooled should be weighed. From the loss the percentage of volatile combustible matter may be readily calculated.

The same sample from which the volatile matter has been driven should be used in the determination of the percentage of ash. This percentage is obtained by burning the fixed carbon over a Bunsen burner or in a muffle furnace. The burning should be kept up until a constant weight is secured, and it may be assisted by stirring with a platinum rod. The weight of the residue determines the percentage of ash, and the percentage of fixed carbon is easily calculated from the loss during the determination of ash after the volatile matter has been driven off.

Proximate analyses may be made and reported on a moist or dry basis. The dry basis is that ordinarily accepted, and this is the basis adopted throughout this book. The method of converting from a moist to a dry basis is the same as described in the case of an ultimate analysis. A proximate analysis is easily made, gives information as to the general characteristics of a fuel and of its _relative_ heating value.

Table 38 gives the proximate analysis and calorific value of a number of representative coals found in the United States.

TABLE 38

APPROXIMATE COMPOSITION AND CALORIFIC VALUE OF CERTAIN TYPICAL AMERICAN COALS

____________________________________________________________________________ | | | | | | | | | | | | No. | State | County | Field, Bed | Mine | Size | | | | or Vein | | | | | | | | | | | | | | | ____|_______|________________|________________|_______________|_____________| | | | | | | ANTHRACITES | | ____|_______|_________________________________________________|_____________| | | | | | | 1 | Pa. | Carbon | Lehigh | Beaver Meadow | | 2 | Pa. | Dauphin | Schuylkill | | Buckwheat | 3 | Pa. | Lackawanna | Wyoming | Belleview | No. 2 Buck. | 4 | Pa. | Lackawanna | Wyoming | Johnson | Culm. | 5 | Pa. | Luzerne | Wyoming | Pittston | No. 2 Buck. | 6 | Pa. | Luzerne | Wyoming | Mammoth | Large | 7 | Pa. | Luzerne | Wyoming | Exeter | Rice | 8 | Pa. | Northumberland | Schuylkill | Treverton | | 9 | Pa. | Schuylkill | Schuylkill | Buck Mountain | | 10 | Pa. | Schuylkill | | York Farm | Buckwheat | 11 | Pa. | | | Victoria | Buckwheat | 12 | Pa. | Carbon | Lehigh | Lehigh & | Buck. & Pea | | | | | Wilkes C. Co. | | 13 | Pa. | Carbon | Lehigh | | Buckwheat | 14 | Pa. | Lackawanna | |Del. & Hud. Co.| No. 1 Buck. | ____|_______|________________|________________|_______________|_____________| | | | | | | SEMI-ANTHRACITES | | ____|_______|_________________________________________________|_____________| | | | | | | 15 | Pa. | Lycoming | Loyalsock | | | 16 | Pa. | Sullivan | | Lopez | | 17 | Pa. | Sullivan | Bernice | | | ____|_______|________________|________________|_______________|_____________| | | | | | | SEMI-BITUMINOUS | | ____|_______|_________________________________________________|_____________| | | | | | | 18 | Md. | Alleghany | Big Vein, | | | | | | George's Crk. | | | 19 | Md. | Alleghany | George's Creek | | | 20 | Md. | Alleghany | George's Creek | | | 21 | Md. | Alleghany | George's Creek | Ocean No. 7 | Mine run | 22 | Md. | Alleghany | Cumberland | | | 23 | Md. | Garrett | | Washington | Mine run | | | | | No. 3 | | 24 | Pa. | Bradford | | Long Valley | | 25 | Pa. | Tioga | | Antrim | | 26 | Pa. | Cambria | "B" or Miller | Soriman Shaft | | | | | | C. Co. | | 27 | Pa. | Cambria | "B" or Miller | Henrietta | | 28 | Pa. | Cambria | "B" or Miller | Penker | | 29 | Pa. | Cambria | "B" or Miller | Lancashire | | 30 | Pa. | Cambria | Lower | Penn. C. & C. | Mine run | | | | Kittanning | Co. No. 3 | | 31 | Pa. | Cambria | Upper | Valley | Mine run | | | | Kittanning | | | 32 | Pa. | Clearfield | Lower | Eureka | Mine run | | | | Kittanning | | | 33 | Pa. | Clearfield | | Ghem | Mine run | 34 | Pa. | Clearfield | | Osceola | | 35 | Pa. | Clearfield | Reynoldsville | | | 36 | Pa. | Clearfield | Atlantic- | | Mine run | | | | Clearfield | | | 37 | Pa. | Huntington | Barnet & Fulton| Carbon | Mine run | 38 | Pa. | Huntington | | Rock Hill | Mine run | 39 | Pa. | Somerset | Lower | Kimmelton | Mine run | | | | Kittanning | | | 40 | Pa. | Somerset | "C" Prime Vein | Jenner | Mine run | ____|_______|________________|________________|_______________|_____________|

_____________________________________________________________________ | | | | | Proximate Analysis (Dry Coal) |B. t. u.| | No. |________________________________________| Per | | | | | | | Pound | Authority | | Moisture | Volatile | Fixed | Ash | Dry | | | | Matter | Carbon | | Coal | | ____|__________|__________|________|_________|________|______________| | | | | | | | | | | | | | | ____|__________|__________|________|_________|________|______________| | | | | | | | 1 | 1.50 | 2.41 | 90.30 | 7.29 | | Gale | 2 | 2.15 | 12.88 | 78.23 | 8.89 | 13137 | Whitham | 3 | 8.29 | 7.81 | 77.19 | 15.00 | 12341 | Sadtler | 4 | 13.90 | 11.16 | 65.96 | 22.88 | 10591 | B. & W. Co. | 5 | 3.66 | 4.40 | 78.96 | 16.64 | 12865 | B. & W. Co. | 6 | 4.00 | 3.44 | 90.59 | 5.97 | 13720 | Carpenter | 7 | 0.25 | 8.18 | 79.61 | 12.21 | 12400 | B. & W. Co. | 8 | 0.84 | 6.73 | 86.39 | 6.88 | | Isherwood | 9 | | 3.17 | 92.41 | 4.42 | 14220 | Carpenter | 10 | 0.81 | 5.51 | 75.90 | 18.59 | 11430 | | 11 | 4.30 | 0.55 | 86.73 | 12.72 | 12642 | B. & W. Co. | 12 | 1.57 | 6.27 | 66.53 | 27.20 | 12848 | B. & W. Co. | | | | | | | | 13 | | 5.00 | 81.00 | 14.00 | 11800 | Carpenter | 14 | 6.20 | | | 11.60 | 12100 | Denton | ____|__________|__________|________|_________|________|______________| | | | | | | | | | | | | | | ____|__________|__________|________|_________|________|______________| | | | | | | | 15 | 1.30 | 8.72 | 84.44 | 6.84 | | | 16 | 5.48 | 7.53 | 81.00 | 11.47 | 13547 | B. & W. Co. | 17 | 1.29 | 8.21 | 84.43 | 7.36 | | | ____|__________|__________|________|_________|________|______________| | | | | | | | | | | | | | | ____|__________|__________|________|_________|________|______________| | | | | | | | 18 | 3.50 | 21.33 | 72.47 | 6.20 | 14682 | B. & W. Co. | | | | | | | | 19 | 3.63 | 16.27 | 76.93 | 6.80 | 14695 | B. & W. Co. | 20 | 2.28 | 19.43 | 77.44 | 6.13 | 14793 | B. & W. Co. | 21 | 1.13 | | | | 14451 | B. & W. Co. | 22 | 1.50 | 17.26 | 76.65 | 6.09 | 14700 | | 23 | 2.33 | 14.38 | 74.93 | 10.49 | 14033 | U. S. Geo. S.| | | | | | | [37] | 24 | 1.55 | 20.33 | 68.38 | 11.29 | 12965 | | 25 | 2.19 | 18.43 | 71.87 | 9.70 | 13500 | | 26 | 3.40 | 20.70 | 71.84 | 7.46 | 14484 | N. Y. Ed. Co.| | | | | | | | 27 | 1.23 | 18.37 | 75.28 | 6.45 | 14770 | So. Eng. Co. | 28 | 3.64 | 21.34 | 70.48 | 8.18 | 14401 | B. & W. Co. | 29 | 4.38 | 21.20 | 70.27 | 8.53 | 14453 | B. & W. Co. | 30 | 3.51 | 17.43 | 75.69 | 6.88 | 14279 | U. S. Geo. S.| | | | | | | | 31 | 3.40 | 14.89 | 75.03 | 10.08 | 14152 | B. & W. Co. | | | | | | | | 32 | 5.90 | 16.71 | 77.22 | 6.07 | 14843 | U. S. Geo. S.| | | | | | | | 33 | 3.43 | 17.53 | 69.67 | 12.80 | 13744 | B. & W. Co. | 34 | 1.24 | 25.43 | 68.56 | 6.01 | 13589 | B. & W. Co. | 35 | 2.91 | 21.55 | 69.03 | 9.42 | 14685 | B. & W. Co. | 36 | 1.55 | 23.36 | 71.15 | 5.94 | 13963 | Whitham | | | | | | | | 37 | 4.50 | 18.34 | 73.06 | 8.60 | 13770 | B. & W. Co. | 38 | 5.91 | 17.58 | 73.44 | 8.99 | 14105 | B. & W. Co. | 39 | 3.09 | 17.84 | 70.47 | 11.69 | 13424 | U. S. Geo. S.| | | | | | | | 40 | 9.37 | 16.47 | 75.76 | 7.77 | 14507 | P. R. R. | ____|__________|__________|________|_________|________|______________|

APPROXIMATE COMPOSITION AND CALORIFIC VALUE OF CERTAIN TYPICAL AMERICAN COALS--Continued

____________________________________________________________________________ | | | | | | | | | | | | No. | State | County | Field, Bed | Mine | Size | | | | or Vein | | | | | | | | | | | | | | | ____|_______|________________|________________|_______________|_____________| | | | | | | 41 | W. Va.| Fayette | New River | Rush Run | Mine run | 42 | W. Va.| Fayette | New River | Loup Creek | | 43 | W. Va.| Fayette | New River | | Slack | 44 | W. Va.| Fayette | New River | | Mine run | 45 | W. Va.| Fayette | New River | Rush Run | Mine run | 46 | W. Va.| McDowell | Pocahontas | Zenith | Mine run | | | | No. 3 | | | 47 | W. Va.| McDowell | Tug River | Big Sandy | Mine run | 48 | W. Va.| Mercer | Pocahontas | Mora | Lump | 49 | W. Va.| Mineral | Elk Garden | | | 50 | W. Va.| McDowell | Pocahontas | Flat Top | Mine run | 51 | W. Va.| McDowell | Pocahontas | Flat Top | Slack | 52 | W. Va.| McDowell | Pocahontas | Flat Top | Lump | ____|_______|________________|________________|_______________|_____________| | | | | | | BITUMINOUS | | ____|_______|_________________________________________________|_____________| | | | | | | 53 | Ala. | Bibb | Cahaba | Hill Creek | Mine run | 54 | Ala. | Jefferson | Pratt | Pratt No. 13 | | 55 | Ala. | Jefferson | Pratt | Warner | Mine run | 56 | Ala. | Jefferson | | Coalburg | Mine run | 57 | Ala. | Walker | Horse Creek | Ivy C. & I. | Nut | | | | | Co. No. 8 | | 58 | Ala. | Walker | Jagger | Galloway C. | Mine run | | | | | Co. No. 5 | | 59 | Ark. | Franklin | Denning | Western No. 4 | Nut | 60 | Ark. | Sebastian | Jenny Lind | Mine No. 12 | Lump | 61 | Ark. | Sebastian | Huntington | Cherokee | Mine run | 62 | Col. | Boulder | South Platte | Lafayette | Mine run | 63 | Col. | Boulder | Laramie | Simson | Mine run | 64 | Col. | Fremont | Canon City | Chandler | Nut and | | | | | | Slack | 65 | Col. | Las Animas | Trinidad | Hastings | Nut | 66 | Col. | Las Animas | Trinidad | Moreley | Slack | 67 | Col. | Routt | Yampa | Oak Creek | | 68 | Ill. | Christian | Pana | Penwell Col. | Lump | 69 | Ill. | Franklin | No. 6 | Benton | Egg | 70 | Ill. | Franklin | Big Muddy | Zeigler | ¾ inch | 71 | Ill. | Jackson | Big Muddy | | | 72 | Ill. | La Salle | Streator | | | 73 | Ill. | La Salle | Streator | Marseilles | Mine run | 74 | Ill. | Macoupin | Nilwood | Mine No. 2 | Screenings | 75 | Ill. | Macoupin | Mt. Olive | Mine No. 2 | Mine run | 76 | Ill. | Madison | Belleville | Donk Bros. | Lump | 77 | Ill. | Madison | Glen Carbon | | Mine run | 78 | Ill. | Marion | | Odin | Lump | 79 | Ill. | Mercer | Gilchrist | | Screenings | 80 | Ill. | Montgomery | Pana or No. 5 | Coffeen | Mine run | 81 | Ill. | Peoria | No. 5 | Empire | | 82 | Ill. | Perry | Du Quoin | Number 1 | Screenings | ____|_______|________________|________________|_______________|_____________|

APPROXIMATE COMPOSITION AND CALORIFIC VALUE OF CERTAIN TYPICAL AMERICAN COALS--Continued