Part 15 (1/2)
_Effect of the Rate of Combustion on the Extent of Combustion s.p.a.ce Required._--With the same coal, the volume of the volatile combustible distilled from the fuel bed per unit of time varies as the rate of combustion. Thus, when this rate is double that of the standard, the volume of gases and tar vapors driven from the fuel is about doubled. To this increased volume of volatile combustible, about double the volume of air must be added, and, if the mixture is to be kept the same length of time within the combustion s.p.a.ce, the latter should be about twice as large as for the standard rate of combustion. Thus the combustion s.p.a.ce required for complete combustion varies, not only with the nature of the coal, but also with the rate of firing the fuel, which, of course, is self-evident.
_Effect of Air Supply on the Extent of Combustion s.p.a.ce Required._--Another factor which influences the extent of the combustion s.p.a.ce is the quant.i.ty of air mixed with the volatile combustible.
Perhaps, within certain limits, the combustion s.p.a.ce may be decreased when the supply of air is increased. However, any statement at present is only speculation; the facts must be determined experimentally. One fact is known, namely, that, in order to obtain higher temperatures of the products of combustion, the air supply must be decreased.
_Effect of Rate of Heating of Coal on the Extent of Combustion s.p.a.ce Required._--There is still another factor, a very important one, which, with a given coal and any given air supply, will influence the extent of the combustion s.p.a.ce. This factor is the rate of heating of the coal when feeding it into the furnace. The so-called ”proximate” a.n.a.lysis of coal is indeed only very approximate. When the a.n.a.lysis shows, say, 40% of volatile matter and 45% of fixed carbon, it does not mean that the coal is actually composed of so much volatile matter and so much fixed carbon; it simply means that, under a certain rate of heating attained by certain standard laboratory conditions, 40% of the coal has been driven off as ”volatile matter.” If the rate or method of heating were different, the amount of volatile matter driven off would also be different. Chemists state that it is difficult to obtain accurate checks on ”proximate” a.n.a.lysis. To ill.u.s.trate this factor, further reference may be made to the operation of the up-draft bituminous gas producers.
In the generator of such producers the tar vapors leave the freshly fired fuel, pa.s.s through the wet scrubber, and are finally separated by the tar extractor as a black, pasty substance in a semi-liquid state. If this tar is subjected to the standard proximate a.n.a.lysis, it will be shown that from 40 to 50% of it is fixed carbon, although it left the gas generator as volatile matter. It is desired to emphasize the fact that different rates of heating of high volatile coals will not only drive off different percentages of volatile matter, but that the latter itself varies greatly in chemical composition and physical properties as regards inflammability and rapidity of combustion. Thus it may be said that the extent of the combustion s.p.a.ce required for the complete oxidation of the volatile combustible depends on the method of charging the fuel, that is, on how rapidly the fresh fuel is heated. If this factor is given proper consideration, it may be possible to reduce very materially the necessary s.p.a.ce required for complete combustion.
_The Effect of the Rate of Mixing the Volatile Combustible and Air on the Extent of the Combustion s.p.a.ce._--When studying the effects discussed in the preceding paragraphs, the rate of mixing the volatile combustible with the supply of air must be as constant as practicable.
At first, tests will be made with no special mixing devices, the mixing will be accomplished entirely by the streams of air entering the furnace at the stoker, and by natural diffusion. Although there appears to be violent stirring of the gases above the fuel bed, the mixture of the gases does not become h.o.m.ogeneous until they are about 10 or 15 ft. from the stoker. The mixing caused by the air currents forced into the furnace at the stoker is very distinct, and can be readily observed through the peep-hole in the side wall of the Heine boiler, opposite the long combustion chamber. This mixing is shown in Fig. 20. _A_ is a current of air forced from the ash-pit directly upward through the fuel bed; _B_ and _B_ are streams of air forced above the fuel bed through numerous small openings at the furnace side of each hopper. Those currents cause the gases to flow out of the furnace in two spirals, as shown in Fig. 20. The velocity of rotation on the outside of the two spirals appears to be about 10 ft. per sec., when the rate of combustion is about 750 lb. of coal per hour. It is reasonable to expect that when the rate of mixing is increased by building piers and other mixing structures immediately back of the grate, the completeness of the combustion will be effected in less time, and a smaller combustion s.p.a.ce will be required. Thus, the mixing structures may be an important factor in the extent of the required combustion s.p.a.ce.
To sum up, it can be said that the extent of the s.p.a.ce required to obtain a combustion which can be considered complete for all practical purposes, depends on the following factors:
(_a_).--Nature of coal,
(_b_).--Rate of combustion,
(_c_).--Supply of air,
(_d_).--Rate of heating fuel,
(_e_).--Rate of mixing volatile combustible and air.
Just how much the extent of the combustion s.p.a.ce required will be influenced by these factors is the object of the experiments under discussion.
_The Scope of the Experiments._--With this object in view, as explained in the preceding paragraphs, the following series of experiments are planned:
[Ill.u.s.tration: Fig. 20.
SECTION THROUGH STOKER SHOWING MIXING OF GASES CAUSED BY CURRENTS OF AIR]
Six or eight typical coals are to be selected, each representing a certain group of nearly the same chemical composition. Each series will consist of several sets of tests, each set being run with all the conditions constant except the one, the effect of which on the size of the combustion s.p.a.ce is to be investigated. Thus a set of four or five tests will be made, varying in rate of combustion from 20 to 80 lb. of coal per square foot of grate per hour, keeping the supply of air per pound of combustible and the rate of heating constant. This set will show the effect of the rate of combustion of the coal on the extent of s.p.a.ce required to obtain combustion which is practically complete. Other variables, such as composition of coal, supply of air, and rate of heating, remain constant.
Another set of four or five tests will be made with the same coal and at the same rate of combustion, but the air supply will be different for each test. This set of tests will be repeated for two or three different rates of combustion. Thus each of these sets will give the effect of the air supply on the extent of combustion s.p.a.ce when the coal and rate of combustion remain constant.
Still another set of tests should be made in which the time of heating the coal when feeding it into the furnace will vary from 3 to 30 min. In each of the tests of this set, the rate of combustion and the air supply will be kept constant, and the set will be repeated for two or three rates of combustion and two or three supplies of air. Each of these sets of tests will give the effect of the rate of heating of fresh fuel on the extent of combustion s.p.a.ce required to burn the distilled volatile combustible. These sets of experiments will require a modification in the stoker mechanism, and, on that account, may be put off until all the other tests on the other selected typical coals are completed. As the investigation proceeds, enough may be learned so that the number of tests in each series may be gradually reduced. After all the desirable tests are made with the furnace as it stands, several kinds of mixing structures will be built successively back of the stoker and tried, one kind at a time, with a set of representative tests. Thus the effectiveness of such mixing structures will be determined.
_Determining the Completeness of Combustion._--The completeness of combustion in the successive cross-sections of the stream of gases is determined mainly by the chemical a.n.a.lysis of samples of gases collected through the openings at these respective cross-sections. The first of these cross-sections at which gas samples are collected, pa.s.ses through the middle of the bridge wall; the others are placed at intervals of 5 ft. through the entire length of the furnace. Measurements of the temperature of the gases, and direct observations of the length and color of the flames and of any visible smoke will be also made through the side peep-holes. These direct observations, together with the gas a.n.a.lysis, will furnish enough data to determine the length of travel of the combustible mixture to reach practically complete combustion.
In other words, these observations will determine the extent of the combustion s.p.a.ce for various kinds of coal when burned under certain given conditions. Direct observations and the a.n.a.lysis of gases at sections nearer the stoker than that at which the combustion is practically complete, will show how the process of combustion approaches its completion. This information will be of extreme value in determining the effect of shortening the combustion s.p.a.ce on the loss of heat due to incomplete combustion.
_Method of Collecting Gas Samples._--The collection of gas samples is a difficult problem in itself, when one considers that the temperature of the gases, as they are in the furnace, ranges from 2,400 to 3,200 Fahr.; consequently, the samples must be collected with water-cooled tubes. Thus far, about 25 preliminary tests have been made. These tests show that the composition of the gases at the cross-sections near the stoker is not uniform, and that more than one sample must be taken from each cross-section. It was decided to take 9 samples from the cross-section immediately back of the stoker, and reduce the number in the sections following, according to the uniformity of the gas composition. Thus, about 35 simultaneous gas samples must be taken for each test. The samples will be subjected, not only to the usual determination of CO_{2}, O_{2} and CO, but to a complete a.n.a.lysis. It is also realized that some of the carbon-hydrogen compounds which, at the furnace temperature, exist as heavy gases, are condensed to liquids and solids when cooled in the sampling tubes, where they settle and tend to clog it. To neglect the presence of this form of the combustible would introduce considerable error in the determination of the completeness of combustion at any of the cross-sections. Therefore, special water-cooled sampling tubes are constructed and equipped with filters which separate the liquid and solid combustible from the gases. The contents of these filters are then also subjected to complete a.n.a.lysis. To obtain quant.i.tative data, a measured quant.i.ty of gases must be drawn through these filtering sampling tubes.
_The Measuring of Temperatures._--At present the only possible known method of measuring the temperature of the furnace gases is by optical and radiation pyrometers. Platinum thermo-couples are soon destroyed by the corrosive action of the hot gases. The pyrometers used at present are the Wanner optical pyrometer and the Fery radiation pyrometer.
_The Flow of Heat Through Furnace Walls._--An interesting side investigation has developed, in the study of the loss of heat through the furnace walls. In the description of this experimental furnace it has been said that the side walls contained a 2-in. air s.p.a.ce, which, in the roof, was replaced with a 1-in. layer of asbestos. To determine the relative resistance to heat flow of the air s.p.a.ce and the asbestos layer, 20 thermo-couples were embedded, in groups of four, to different depths at three places in the side wall and at two places in the roof.
In the side wall, one of the thermo-couples of each group was placed in the inner wall near the furnace surface; the second thermo-couple was placed in the same wall, but near the surface facing the air s.p.a.ce; the third thermo-couple was placed in the outer wall near the inner surface; and the fourth was placed near the outer surface in the outer wall. In the roof the second and third thermo-couples were placed in the brick near the surface on each side of the asbestos layer. These thermo-couples have shown that the temperature drop across the 2-in. air s.p.a.ce was much less than that across the 1-in. layer of asbestos; in fact, that it was considerably less than the temperature drop through the same thickness of the brick wall.
The results obtained prove that, as far as heat insulation is concerned, air s.p.a.ces in furnace walls are undesirable. The heat is not conducted through the air, but leaps across the s.p.a.ce by radiation. In furnace construction a solid wall is a better heat insulator than one of the same total thickness containing an air s.p.a.ce. If it is necessary to build a furnace wall in two parts on account of unequal expansion, the s.p.a.ce between the two walls should be filled with some solid, cheap, non-conducting materials, such as ash, sand, or crushed brick. A more detailed account of these experiments may be found in a Bulletin of the U.S. Geological Survey ent.i.tled ”The Flow of Heat Through Furnace Walls.”