Part 22

As already stated, the temperature at the beginning of combustion is the reading just before the current is turned on, or B in Fig. 25. The point C or the temperature at which combustion is presumably completed, should be taken at a point which falls well within the established final rate of radiation, and not at the maximum temperature that the thermometer indicates in the test, unless it lies on the straight line determining the final radiation. This is due to the fact that in certain instances local conditions will cause the thermometer to read higher than it should during the time that the bomb is transmitting heat to the water rapidly, and at other times the maximum temperature might be lower than that which would be indicated were readings to be taken at intervals of less than one-half minute, _i. e._, the point of maximum temperature will fall below the line determined by the final rate of radiation. With this understanding AB, Fig. 25, represents the time of initial radiation, BC the time of combustion, and CD the time of final radiation. Therefore to apply Pfaundler's correction, formula (19), to the data as represented by Fig. 25.

N = 6, R = 0, R' = .01, T = 20.29, T' = 22.83,

20.29 + 22.54 + 22.84 + 22.88 + 22.87 + 22.86 T" = --------------------------------------------- = 22.36 6

_ _ | .01 - 0 | C = 6|0 + -------------(22.36 - 20.29)| |_ 22.85 - 20.29 _|

= 6 × .008 = .048

Pfaundler's formula while simple is rather long. Mr. E. H. Peabody has devised a simpler formula with which, under proper conditions, the variation from correction as found by Pfaundler's method is negligible.

It was noted throughout an extended series of calorimeter tests that the maximum temperature was reached by the thermometer slightly over one minute after the time of firing. If this period between the time of firing and the maximum temperature reported was exactly one minute, the radiation through this period would equal the radiation per one-half minute _before firing_ plus the radiation per one-half minute _after the maximum temperature is reached_; or, the radiation through the one minute interval would be the average of the radiation per minute before firing and the radiation per minute after the maximum. A plotted chart of temperatures would take the form of a curve of three straight lines (B, C', D) in Fig. 25. Under such conditions, using the notation as in formula (19) the correction would become,

2R + 2R' C = ------- + (N - 2)R', or R + (N - 1)R' (20) 2

This formula may be generalized for conditions where the maximum temperature is reached after a period of more than one minute as follows:

Let M = the number of intervals between the time of firing and the maximum temperature. Then the radiation through this period will be an average of the radiation for M intervals before firing and for M intervals after the maximum is recorded, or

MR + MR' M M C = ------- + (N - M)R' = - R + (N - -)R' (21) 2 2 2

In the case of Mr. Peabody's deductions M was found to be approximately 2 and formula (21) becomes directly, C = R + (N - 1)R' or formula (20).

The corrections to be made, as secured by the use of this formula, are very close to those secured by Pfaundler's method, where the point of maximum temperature is not more than five intervals later than the point of firing. Where a longer period than this is indicated in the chart of plotted temperatures, the approximate formula should not be used. As the period between firing and the maximum temperature is increased, the plotted results are further and further away from the theoretical straight line curve. Where this period is not over five intervals, or two and a half minutes, an approximation of the straight line curve may be plotted by eye, and ordinarily the radiation correction to be applied may be determined very closely from such an approximated curve.

Peabody's approximate formula has been found from a number of tests to give results within .003 degrees Fahrenheit for the limits within which its application holds good as described. The value of M, which is not necessarily a whole number, should be determined for each test, though in all probability such a value is a constant for any individual calorimeter which is properly operated.

The correction for radiation as found on page 188 is in all instances to be added to the range of temperature between the firing point and the point chosen from which the final radiation is calculated. This corrected range multiplied by the water equivalent of the calorimeter gives the heat of combustion in calories of the coal burned in the calorimeter together with that evolved by the burning of the fuse wire. The heat evolved by the burning of the fuse wire is found from the determination of the actual weight of wire burned and the heat of combustion of one milligram of the wire (1.7 calories), _i. e._, multiply the weight of wire used by 1.7, the result being in gram calories or the heat required to raise one gram of water one degree centigrade.

Other small corrections to be made are those for the formation of nitric acid and for the combustion of sulphur to sulphuric acid instead of sulphur dioxide, due to the more complete combustion in the presence of oxygen than would be possible in the atmosphere.

To make these corrections the bomb of the calorimeter is carefully washed out with water after each test and the amount of acid determined from titrating this water with a standard solution of ammonia or of caustic soda, all of the acid being assumed to be nitric acid. Each cubic centimeter of the ammonia titrating solution used is equivalent to a correction of 2.65 calories.

As part of acidity is due to the formation of sulphuric acid, a further correction is necessary. In burning sulphuric acid the heat evolved per gram of sulphur is 2230 calories in excess of the heat which would be evolved if the sulphur burned to sulphur dioxide, or 22.3 calories for each per cent of sulphur in the coal. One cubic centimeter of the ammonia solution is equivalent to 0.00286 grams of sulphur as sulphuric acid, or to 0.286 × 22.3 = 6.38 calories. It is evident therefore that after multiplying the number of cubic centimeters used in titrating by the heat factor for nitric acid (2.65) a further correction of 6.38 - 2.65 = 3.73 is necessary for each cubic centimeter used in titrating sulphuric instead of nitric acid. This correction will be 3.73/0.297 = 13 units for each 0.01 gram of sulphur in the coal.

The total correction therefore for the aqueous nitric and sulphuric acid is found by multiplying the ammonia by 2.65 and adding 13 calories for each 0.01 gram of sulphur in the coal. This total correction is to be deducted from the heat value as found from the corrected range and the amount equivalent to the calorimeter.

After each test the pan in which the coal has been burned must be carefully examined to make sure that all of the sample has undergone complete combustion. The presence of black specks ordinarily indicates unburned coal, and often will be found where the coal contains bone or slate. Where such specks are found the tests should be repeated. In testing any fuel where it is found difficult to completely consume a sample, a weighed amount of naphthaline may be added, the total weight of fuel and naphthaline being approximately one gram. The naphthaline has a known heat of combustion, samples for this purpose being obtainable from the United States Bureau of Standards, and from the combined heat of combustion of the fuel and naphthaline that of the former may be readily computed.

The heat evolved in burning of a definite weight of standard naphthaline may also be used as a means of calibrating the calorimeter as a whole.

COMBUSTION OF COAL

The composition of coal varies over such a wide range, and the methods of firing have to be altered so greatly to suit the various coals and the innumerable types of furnaces in which they are burned, that any instructions given for the handling of different fuels must of necessity be of the most general character. For each kind of coal there is some method of firing which will give the best results for each individual set of conditions. General rules can be suggested, but the best results can be obtained only by following such methods as experience and practice show to be the best suited to the specific conditions.

The question of draft is an all important factor. If this be insufficient, proper combustion is impossible, as the suction in the furnace will not be great enough to draw the necessary amount of air through the fuel bed, and the gases may pass off only partially consumed. On the other hand, an excessive draft may cause losses due to the excess quantities of air drawn through holes in the fire. Where coal is burned however, there are rarely complaints from excessive draft, as this can be and should be regulated by the boiler damper to give only the draft necessary for the particular rate of combustion desired. The draft required for various kinds of fuel is treated in detail in the chapter on "Chimneys and Draft". In this chapter it will be assumed that the draft is at all times ample and that it is regulated to give the best results for each kind of coal.

TABLE 40

ANTHRACITE COAL SIZES

_________________________________________________________________ | | | | | | | Testing Segments | | | Round Mesh | Standard Square | | | | Mesh | | Trade Name |__________________|__________________| | | | | | | | | Through | Over | Through | Over | | | Inches | Inches | Inches | Inches | |___________________________|_________|________|_________|________| | | | | | | | Broken | 4-1/2 | 3-1/4 | 4 | 2-3/4 | | Egg | 3-1/4 | 2-3/8 | 2-3/4 | 2 | | Stove | 2-3/8 | 1-5/8 | 2 | 1-3/8 | | Chestnut | 1-5/8 | 7/8 | 1-3/8 | 3/4 | | Pea | 7/8 | 5/8 | 3/4 | 1/2 | | No. 1 Buckwheat | 5/8 | 3/8 | 1/2 | 1/4 | | No. 2 Buckwheat or Rice | 3/8 | 3/16 | 1/4 | 1/8 | | No. 3 Buckwheat or Barley | 3/16 | 3/32 | 1/8 | 1/16 | |___________________________|_________|________|_________|________|

Anthracite--Anthracite coal is ordinarily marketed under the names and sizes given in Table 40.

The larger sizes of anthracite are rarely used for commercial steam generating purposes as the demand for domestic use now limits the supply. In commercial plants the sizes generally found are Nos. 1, 2 and 3 buckwheat. In some plants where the finer sizes are used, a small percentage of bituminous coal, say, 10 per cent, is sometimes mixed with the anthracite and beneficial results secured both in economy and capacity.

Anthracite coal should be fired evenly, in small quantities and at frequent intervals. If this method is not followed, dead spots will appear in the fire, and if the fire gets too irregular through burning in patches, nothing can be done to remedy it until the fire is cleaned as a whole. After this grade of fuel has been fired it should be left alone, and the fire tools used as little as possible. Owing to the difficulty of igniting this fuel, care must be taken in cleaning fires. The intervals of cleaning will, of course, depend upon the nature of the coal and the rate of combustion. With the small sizes and moderately high combustion rates, fires will have to be cleaned twice on each eight-hour shift. As the fires become dirty the thickness of the fuel bed will increase, until this depth may be 12 or 14 inches just before a cleaning period. In cleaning, the following practice is usually followed: The good coal on the forward half of the grate is pushed to the rear half, and the refuse on the front portion either pulled out or dumped. The good coal is then pulled forward onto the front part of the grate and the refuse on the rear section dumped. The remaining good coal is then spread evenly over the whole grate surface and the fire built up with fresh coal.

A ratio of grate surface to heating surface of 1 to from 35 to 40 will under ordinary conditions develop the rated capacity of a boiler when burning anthracite buckwheat. Where the finer sizes are used, or where overloads are desirable, however, this ratio should preferably be 1 to 25 and a forced blast should be used. Grates 10 feet deep with a slope of 1½ inches to the foot can be handled comfortably with this class of fuel, and grates 12 feet deep with the same slope can be successfully handled. Where grates over 8 feet in depth are necessary, shaking grates or overlapping dumping grates should be used. Dumping grates may be applied either for the whole grate surface or to the rear section. Air openings in the grate bars should be made from 3/16 inch in width for No. 3 buckwheat to 5/16 inch for No. 1 buckwheat. It is important that these air openings be uniformly distributed over the whole surface to avoid blowing holes in the fire, and it is for this reason that overlapping grates are recommended.

No air should be admitted over the fire. Steam is sometimes introduced into the ashpit to soften any clinker that may form, but the quantity of steam should be limited to that required for this purpose. The steam that may be used in a steam jet blower for securing blast will in certain instances assist in softening the clinker, but a much greater quantity may be used by such an apparatus than is required for this purpose. Combustion arches sprung above the grates have proved of advantage in maintaining a high furnace temperature and in assisting in the ignition of fresh coal.

Stacks used with forced blast should be of such size as to insure a slight suction in the furnace under any conditions of operation. A blast up to 3 inches of water should be available for the finer sizes supplied by engine driven fans, automatically controlled by the boiler pressure. The blast required will increase as the depth of the fuel bed increases, and the slight suction should be maintained in the furnace by damper regulation.

The use of blast with the finer sizes causes rapid fouling of the heating surfaces of the boiler, the dust often amounting to over 10 per cent of the total fuel fired. Economical disposal of dust and ashes is of the utmost importance in burning fuel of this nature. Provision should be made in the baffling of the boiler to accommodate and dispose of this dust. Whenever conditions permit, the ashes can be economically disposed of by flushing them out with water.

Bituminous Coals--There is no classification of bituminous coal as to size that holds good in all localities. The American Society of Mechanical Engineers suggests the following grading:

_Eastern Bituminous Coals_--

(A) Run of mine coal; the unscreened coal taken from the mine.

(B) Lump coal; that which passes over a bar-screen with openings 1¼ inches wide.

(C) Nut coal; that which passes through a bar-screen with 1¼-inch openings and over one with ¾-inch openings.

(D) Slack coal; that which passes through a bar-screen with ¾-inch openings.

_Western Bituminous Coals_--

(E) Run of mine coal; the unscreened coal taken from the mine.

(F) Lump coal; divided into 6-inch, 3-inch and 1¼-inch lump, according to the diameter of the circular openings over which the respective grades pass; also 6 × 3-inch lump and 3 × 1¼-inch lump, according as the coal passes through a circular opening having the diameter of the larger figure and over that of the smaller diameter.

(G) Nut coal; divided into 3-inch steam nut, which passes through an opening 3 inches diameter and over 1¼ inches; 1¼ inch nut, which passes through a 1¼-inch diameter opening and over a ¾-inch diameter opening; ¾-inch nut, which passes through a ¾-inch diameter opening and over a 5/8-inch diameter opening.

(H) Screenings; that which passes through a 1¼-inch diameter opening.

As the variation in character of bituminous coals is much greater than in the anthracites, any rules set down for their handling must be the more general. The difficulties in burning bituminous coals with economy and with little or no smoke increases as the content of fixed carbon in the coal decreases. It is their volatile content which causes the difficulties and it is essential that the furnaces be designed to properly handle this portion of the coal. The fixed carbon will take care of itself, provided the volatile matter is properly burned.

Mr. Kent, in his "Steam Boiler Economy", described the action of bituminous coal after it is fired as follows: "The first thing that the fine fresh coal does is to choke the air spaces existing through the bed of coke, thus shutting off the air supply which is needed to burn the gases produced from the fresh coal. The next thing is a very rapid evaporation of moisture from the coal, a chilling process, which robs the furnace of heat. Next is the formation of water-gas by the chemical reaction, C + H_{2}O = CO + 2H, the steam being decomposed, its oxygen burning the carbon of the coal to carbonic oxide, and the hydrogen being liberated. This reaction takes place when steam is brought in contact with highly heated carbon. This also is a chilling process, absorbing heat from the furnaces. The two valuable fuel gases thus generated would give back all the heat absorbed in their formation if they could be burned, but there is not enough air in the furnace to burn them. Admitting extra air through the fire door at this time will be of no service, for the gases being comparatively cool cannot be burned unless the air is highly heated. After all the moisture has been driven off from the coal, the distillation of hydrocarbons begins, and a considerable portion of them escapes unburned, owing to the deficiency of hot air, and to their being chilled by the relatively cool heating surfaces of the boiler. During all this time great volumes of smoke are escaping from the chimney, together with unburned hydrogen, hydrocarbons, and carbonic oxide, all fuel gases, while at the same time soot is being deposited on the heating surface, diminishing its efficiency in transmitting heat to the water."

To burn these gases distilled from the coal, it is necessary that they be brought into contact with air sufficiently heated to cause them to ignite, that sufficient space be allowed for their mixture with the air, and that sufficient time be allowed for their complete combustion before they strike the boiler heating surfaces, since these surfaces are comparatively cool and will lower the temperature of the gases below their ignition point. The air drawn through the fire by the draft suction is heated in its passage and heat is added by radiation from the hot brick surfaces of the furnace, the air and volatile gases mixing as this increase in temperature is taking place. Thus in most instances is the first requirement fulfilled. The element of space for the proper mixture of the gases with the air, and of time in which combustion is to take place, should be taken care of by sufficiently large combustion chambers.

Certain bituminous coals, owing to their high volatile content, require that the air be heated to a higher temperature than it is possible for it to attain simply in its passage through the fire and by absorption from the side walls of the furnace. Such coals can be burned with the best results under fire brick arches. Such arches increase the temperature of the furnace and in this way maintain the heat that must be present for ignition and complete combustion of the fuels in question. These fuels too, sometimes require additional combustion space, and an extension furnace will give this in addition to the required arches.

As stated, the difficulty of burning bituminous coals successfully will increase with the increase in volatile matter. This percentage of volatile will affect directly the depth of coal bed to be carried and the intervals of firing for the most satisfactory results. The variation in the fuel over such wide ranges makes it impossible to definitely state the thickness of fires for all classes, and experiment with the class of fuel in use is the best method of determining how that

## particular fuel should be handled. The following suggestions, which are

not to be considered in any sense hard and fast rules, may be of service for general operating conditions for hand firing:

Semi-bituminous coals, such as Pocahontas, New River, Clearfield, etc., require fires from 10 to 14 inches thick; fresh coal should be fired at intervals of 10 to 20 minutes and sufficient coal charged at each firing to maintain a uniform thickness. Bituminous coals from Pittsburgh Region require fires from 4 to 6 inches thick, and should be fired often in comparatively small charges. Kentucky, Tennessee, Ohio and Illinois coals require a thickness from 4 to 6 inches. Free burning coals from Rock Springs, Wyoming, require from 6 to 8 inches, while the poorer grades of Montana, Utah and Washington bituminous coals require a depth of about 4 inches.

In general as thin fires are found necessary, the intervals of firing should be made more frequent and the quantity of coal fired at each interval smaller. As thin fires become necessary due to the character of the coal, the tendency to clinker will increase if the thickness be increased over that found to give the best results.

There are two general methods of hand firing: 1st, the spreading method; and 2nd, the coking method.

[Illustration: Babcock & Wilcox Chain Grate Stoker]

In the spreading method but little fuel is fired at one time, and is spread evenly over the fuel bed from front to rear. Where there is more than one firing door the doors should be fired alternately. The advantage of alternate firing is the whole surface of the fire is not blanketed with green coal, and steam is generated more uniformly than if all doors were fired at one time. Again, a better combustion results due to the burning of more of the volatile matter directly after firing than where all doors are fired at one time.

In the coking method, fresh coal is fired at considerable depth at the front of the grate and after it is partially coked it is pushed back into the furnace. The object of such a method is the preserving of a bed of carbon at the rear of the grate, in passing over which the volatile gases driven off from the green coal will be burned. This method is

## particularly adaptable to a grate in which the gases are made to pass

horizontally over the fire. Modern practice for hand firing leans more and more toward the spread firing method. Again the tendency is to work bituminous coal fires less than formerly. A certain amount of slicing and raking may be necessary with either method of firing, but in general, the less the fire is worked the better the results.