December 27, 1918. ________________________________ THE COLLIERY GUARDIAN. _____________________________________________________________ 1355 THE POSSIBILITIES OF POWDERED COAL.* By W. G. Wilcox. (Continued f rom page 1198.J Principle of Mixing. A study of the methods for making such a mixture immediately shows that the methods commonly used in making uniform mixture of two miscible liquids or a uniform solution of a solid in a liquid, or the methods used in mixing finely ground solids are not only useless in this case, but will actually separate the coal dust from the air. Ordinary mixing is done by agitation ; this agitation is usually accomplished by baffling, stirring, shaking, or similar devices. When, however, such methods are applied to a mix- ture of gas and finely divided solid, the solid tends to separate out, due to its much higher specific gravity. This, in fact, is the principle of the well- known cyclone dust collector. Any mixing device which results in such agitation of the dust and air as to give a centrifugal effect will tend to separate out instead of mixing the air and the dust. Any mixing device along these lines must necessarily fail to give an intimate, perfect mixture. The importance of intimately mixing the coal dust and air cannot be exaggerated. The rapidity of com- bustion is a direct measure of the intimacy of mix- ture. This is well illustrated by comparing the ordinary gas flame with the flame obtained in the Bone combustion system. The Bone system consists in forcing the proper proportions of air and gas through a diaphragm having numerous small inter- stices, which results in a mixture that is nearly perfect. When this mixture is ignited on the other side of the diaphragm, we have only a film of flame. Importance of Thorough Mixing. The poorer the mixing, the longer the flame. The flame simply outlines the area in which combustion is taking place, and the length of the flame is a measure of the time element necessary to accomplish combustion. This time element — other conditions being equal—is absolutely a function of the intimacy of mixture. This was noted by Breckenridge some ten years ago (Bulletin No. 325 of the United States Geological Survey), who stated: “The conclusion is 'reached that the velocity of combustion decreases enormously from the surface of the fire to the rear of the combustion chamber, where it is relatively very small. The practical application is that little is to be gained by adding further length of smooth com- bustion chamber, which would be commercially as poor an investment of capital as to add to the length of a Corliss engine cylinder and stroke. We must resort to thorough mixing.” And further: “Mere length of combustion chamber counts for little; that mixing is what counts.” These two excerpts from Breckenridge’s study of 400 steaming tests have since been amply confirmed by the work of Kreisinger, Augustine and Ovitz (Bulletin No. 135 of the Bureau of Mines). The work of these investigators confirmed and emphasised the previous observations of Breckenridge, and they also stated: “Evidently the length of the flame depends not only on the nature of the combustible, the excess of air and the rate of firing, but also to a large degree on the rate of mixing of the combustible gases with the oxygen of the air. It has been shown that the tendency of the gases is to flow in parallel streams even when the air was introduced into the furnace in many small streams through the tuyeres of the furnace.” So far we have considered this type of fuel from the point of view of the possibilities which it affords. We have also studied the characteristics of the mix- ture of coal dust and air in order to ascertain what methods should be employed to ensure delivery «to the furnace of an intimate controlled mixture of fuel with the requisite amount of air. A fuel having the great possibilities offered by a finely divided com- bustible is of extreme importance, especially under present conditions. Because of these possibilities, all of which are capable of being realised, and which are satisfied by commercial equipment now on the market, there is less excuse for permitting inefficient operation with this fuel than with stoker or hand-fired practice. Apparatus properly designed and fundamentally correct in principle will give proper efficiency. Such apparatus, by giving an intimate and controlled mixture of fuel and air to the furnace, will - permit the highest furnace efficiency. Characteristics of Powdered Coal and Fuel Oil. Just as in the past there has been a remarkable failure to realise the necessity for intimately mixing air and coal dust, neither has there been sufficient con- sideration of the characteristics of this fuel when burning. Pow’dered coal has the characteristics of a rich fuel of somewhat higher kindling temperature than producer gas, natural gas or fuel oil. To illus- trate the fact that it is a rich fuel, we can compare the available B.Th.U. in a cubic foot of a correctly proportioned mixture of powdered coal and air and the available B.Th.U. in a cubic foot of a correctly proportioned mixture of pure methane and air. Taking a Pittsburg vein coal (heating value 14,157 B.Th.U.) of the following analysis—volatile, 35-4 per cent. ; fixed carbon, 58*5 per cent. ; ash, 6-1 per cent., —we find that a cubic foot of a correctly proportioned mixture of coal dust and air has available 107 B.Th.U., w while a mixture of pure methane and the proper amount of air has available per cubic foot 62-3 B.Th.U. A further illustration is shown by the theoretical maximum flame temperature of several fuels; — * Paper read before the Western New York Section of the American Chemical Society. Theoretical Maximum Flame Temperature Using Cold Air. Degs. Cent. Hydrogen _____________' _______ 2.010 Carbon monoxide __________________2,050 Natural gas ________________________1,806 Pure methane ________________________1,958 Pittsburg coal (analysis given above) ... 3,470 Factors Controlling Rapidity and Completeness of Combustion. The rapidity of combustion and the completeness of combustion of a mixture of coal dust and air depend upon a number of factors: The velocity and pressure at which it is passed into the combustion chamber.—If the velocity of the in- coming stream of powdered coal and air is above the velocity of flame propagation, combustion will not take place until the mixture has slowed down to a point where it does not exceed the velocity of flame propa- gation. When powdered coal is fired at high pressure and high velocity, combustion frequently does not begin until a point 4 to 6 ft. from the mouth of the burner. A similar example is found in the plumber’s blow torch when too much air is used, or : in the Bunsen burner when the gas pressure is too ; high. High pressure high velocity firing not only slows down combustion, thus increasing the size of combustion chamber necessary, but has a destructive action on the furnace. It has been well established that high velocities in the combustion chamber or a blow torch effect due to firing at high pressure (whether oil or gas be used as a fuel) are always very destructive to the brickwork. This action is increased in high pressure firing of powdered coal, since, in addition to the erosional effect of gases at high tem- perature travelling at high velocity, there is a fluxing action by the melted ash. Furthermore, with such high velocity combustion the slagged ash will be carried along mechanically, leading to further furnace troubles. In one case this resulted in a serious deposit of slag on the mud drum of a vertical waste heat boiler at the end of a long reverberatory furnace. Slowing down the velocity not only hastens combustion, but makes it possible to eliminate much of the slag. When the velocity is low the coalesced particles of slagged ash are either larger than will be carried by the velocity of the gas, or this condition is so nearly approached that a slight change in direction of the flame will result in dropping out the slag. Thus, in addition to being correct combustion and necessary in order to avoid excessive furnace maintenance costs, low pressure, low velocity combustion permits by slight change in flame direction dropping out a very large percentage of the slagged ash in the early part of combustion, where it can be removed and will not interfere seriously with efficient metallurgical opera- tions. The velocity of combustion is not only dependent upon the fineness of the particles of coal, intimacy of mixture and the velocity of the stream of com- bustible and air, but is affected by the temperature of the combustion chamber. The kindling tempera- ture of a mixture of powdered coal and air is higher than either that of oil or gas; consequently, for successful and complete combustion it is necessary that the combustion chamber be maintained above a certain minimum temperature, and that combustion is practi- cally complete before the products of combustion pass . over the heat-absorbing surfaces. Just as you can extinguish a gas flame by passing over it a piece of wire gauze, so the effect of a chilling surface will be even more marked with this combustible material than burning gas, since the particle of coal is infinitely larger than a molecule of gas, and the kindling temperature is also higher. This has a direct appli- cation in the successful firing of locomotive type boilers, water tube boilers and return tubular boilers. If the combustion of powdered coal is not sufficiently developed before the flame enters the tubes of the locomotive type boiler, combustion will be checked and coked coal settle out in the tubes. If, on the other hand, combustion is sufficiently developed before the flame is brought in contact with the heat-absorbing surface, a complete combustion and high efficiency are obtained. In any furnace operation and in furnace design, this must be borne in mind if success is to be expected. Rapidity of Combustion of Low Pressure Mixture. A study of the flame developed by a low pressure intimate mixture of coal dust and air shows that com- bustion is extremely rapid. In a copper reverberatory furnace in Florence, Colorado, where this type of com- bustion is used, coal burned at the rate of approxi- mately one ton per hour develops a flame which vanishes within 6 ft. of the burner, combustion being complete at that point. In order to bring out exactly what this means, let us translate it into terms of natural gas, in which case the fuel comsumption would be approximately 26,000 cu. ft. per hour, or 433 cu. ft. per minute. Assume this quantity of gas to be burned at low pressure and developing a flame only 6 ft. long. Gas samples taken in the flame show a content of CO2 as high as 16 per cent, only 5 ft from the mouth of the burner. This will give an example of the rapidity with which combustion can be obtained, and the extremes which are possible in shortening the flame. It is equally possible with proper equipment to lengthen the flame until it will spindle out a distance as great as 100 or 120 ft. However, with an intimate controlled mixture this must be done by supplying insufficient air. Under such conditions combustion is incomplete and the flame spindles out because com- bustion continues to develop throughout the length of the furnace as air leakage supplies additional oxygen. This is an additional proof of the statement previously made, that the length of flame is an actual measure of efficiency of mixing and the adjustment of the fuel air ratio. Thus. it is seen that we have changed entirely the characteristics of coal as commonly known. Powdered coal is a fuel of extreme flexibility in that the amount burned can be varied within wide limits. It is a fuel which develops a flame the length of which can be adjusted. The character of the flame can be altered to suit the metallurgical operation. In short, the basic fuel, coal, has acquired the characteristics of oil or gas, but with better and closer control than in the case of oil or gas. Furthermore, the possibilities of this fuel are not only capable of realisation, but are actually being utilised in commercial practice to-day. To the flame characteristics of a rich fuel, developing a flame like oil or gas, is added a degree of control not yet obtainable in burning either oil or gas. This statement is made advisedly. The possibilities of such combustion for the improvement of processes, for fuel economy, for increasing output, through its ease of control and elimination of heavy labour, are to-day realised by few. Due to the psychological attitude of labour and the scarcity of skilled operatives, it is far more difficult than ever before to secure high effi- ciency and good operation in hand firing, stoker firing, or in producers; in short, wherever such efficiency depends upon constant watchfulness and hot, heavy disagreeable work. For these conditions powdered coal substitutes an ease of control such that the equip- ment can be handled by an old man or a boy, while it is so simple that a man of ordinary intelligence can soon be taught all that is necessary for good efficiency in operation. The possibilities of such control in the place of present day combustion methods, which permit high efficiency only by the most strenuous effort, through substituting for these a type of com- bustion whereby high efficiency is easily obtained, is certainly of great importance to us at the present time. Control of Heating Conditions in Chemical Processes. To those who are in touch with many diversified chemical industries, the possibility of maintaining within close limits heating conditions which are oxidising, reducing or neutral at will, conditions which once adjusted will remain constant—barring mechani- cal or electrical interruptions — will undoubtedly suggest many cases where such combustion will offer new economies and betterment in their processes. A good example of this is shown in a powdered coal installation in a malleable iron foundry in Buffalo. The positive and simple control offered by their equip- ment has not only resulted in the highest fuel ratio yet reached in malleable iron practice, namely, be- tween 5 and 6 lb. of castings per pound of coal, but also through maintaining a slightly reducing con- dition in the annealing oven the oxidation of the cast iron pots has been so reduced that the cost of pots per ton of castings has been cut from 2-25 dols. to 83 cents. These two results are concrete proof of the possibilities of this type of combustion. The high fuel ratio shows efficient combustion; the decrease in pot cost shows control of combustion; and the high fuel ratio in conjunction with the maintenance of a reducing condition shows exactness of control. A still more striking example of the possibilities of controlled combustion may be cited of a powdered coal installation now being made. In this particular process a cast iron container is maintained con- tinuously at a temperature of 1,000 degs. Cent. This is not only close to the melting point of the material, but is a temperature so high that the oxidation caused by the unavoidable excess of air met in ordinary methods of combustion results in enormous main- tenance costs due to quick failure of the container. In this installation it is confidently believed that the combustion can be so controlled as to cut oxidation to a minimum so that the life of the container will be greatly increased. Burning Low Grade Fuels in Suspension. In burning low grade fuels in suspension, the only loss in efficiency is that due to lower flame tempera- ture caused by the increasing amount of ash and the heat lost in the ash. Coals running as high as 30 to 40 per cent, ash can be successfully burned; combus- tion will be complete and maximum flame temperature for that fuel will be obtained. The only difference between burning a fuel of this type and a high grade fuel is the difference in flame temperature and the heat loss in the slagged ash. An ash whose slagging character would lead to enormous difficulties in pro- ducer or stoker operation will simply melt the easier when the coal is burned in powdered form and can therefore be dropped out earlier in the furnace, which in many cases constitutes an actual advantage. The lignite fuels of the Middle West and the Far West can be burned successfully and are being burned successfully in powdered form. An enormous field is opened up through ability to burn with efficiency these lower grade fuels. It will eliminate long hauls of fuel by our railroads, release motive power and cars, and conserve our fuel reserves. An unusually good illustration is found in two coals previously mentioned, one a Virginia coal more than 14,000 B.Th.U. and less than 6 per cent, ash, and the other a Colorado coal. Both of these are practically unavailable as fuels when using stokers, grates or producers, because of the chemical nature of the ash and its mechanical form. When, however, these coals are pulverised, the distribution of the ash in the lump coal has no effect, while the low melting point of the ash in many cases offers a decided advantage. These coals both afford splendid fuels of high heating value when burned in powdered form. The Colorado fuel is being burned successfully this way. The River Smelting and Refining Company,, which is burning this Colorado fuel, has cut the fuel requirements per ton of ore smelted in a reverberatory to one-eighth that necessary when this coal was burned on grates. ________________________________ Mr. C. H. Wordingham, who since 1903 has been head of the Electrical Department of the Admiralty, has re- signed his position as Director of Electrical Engineering in order to return to consulting practice. His address after January 1 will be 7, Victoria-street, Westminster. He has been retained by the Admiralty to act in an advisory capacity.