INDUSTRIAL A-VD ENGINEERIhrG CHEMISTRY
March, 1923
249
Combustion of Powdered Coal'.' By Henry Kreisinger and John Blizard COMBUSTION ENGINEERING CORPORATION, AND U. S. BUREAU OB MINES,PITTSBURGH, PA.
HE OBVIOUS essenvary, that the large partiThe combustion of powdered coal in suspension in air is not a tial in burning powcles will have the greater novelty, but f e w trustworthy experiments have been carried out to dered coal is to bring vertical velocity relative show what losses occur and what eflciency is attained in powderedthe' solid coal itself into to the furnace gases, and coal plants. contaot with the oxygen that this velocity will inThis paper contains a review of some tests carefully conducted of the air in order that the crease with the temperaby the Bureau of Mines and the Combustion Engineering Corporafixed carbon and the gases ture of the gases and the tion on boilers fired with powdered coal, which show that coal when given off by the heating of density of the particles. powdered may be burned with greater thermal eficiency for steam the coal may-be completely This velocity of the raising than when burned by any other methods. burned. The coal is ground particles relative to the so that the greater portion air and furnace gases is will pass through a 200-mesh sieve, and consists of parti- of vital importance in burning powdered coal, since withcles of various sizes, as shown by the following measurements3 out it the coal would have to rely solely on diffusion of coal which had been passed through a 200-mesh sieve: for its supply of oxygen. Coal requires about 10,000 times its own volume of air a t atmospheric temperature to Diameter, microns 60 50 40 30 20 10 5 2 form about 60,000 times its own volume of gas at furnace Diameter, temperature, so that if a sphere of coal of 10 microns radius inches 0.0025 0.002 0.0016 0.0012 0.0008 0.0004 0.0002 0.0001 enters a furnace in a sphere of air a t ordinary temperature Relative frequency of just sufficient to burn it, this sphere of air will be over 210 occur87 100 156 660 1750 6200 25600 155000 rence microns in radius, and unless the mixture remained in the Per cent of furnace for a -considerable time or the coal were moved total volume 22.5 14 9 11.9 21.3 16.7 7.4 3.8 1.5 relatively to the air as described before, it would be impossible to burn it in the air, the bulk of which must come from The last line of this table shows very roughly that the the outer part of the sphere. bulk of the coal passing through a 200-mesh sieve has a mean It is unnecessary to tell the chemist that particles of powdiameter varying from 60 to 20 microns, and that the total dered coal are of immensely greater size than gas molecules, weight is fairly evenly divided among particles of these sizes. though unfortunately the engineer in the past has confused the combustion of powdered coal with the combustion of gas. EFFECTOF AIR AXD FURXACE GASESON VELOCITY OF Further, gas requires only from one to ten times its own volPARTICLES ume of air to burn it. When such a heterogeneous mass of powdered coal is permitted to fall in air, its acceleration soon becomes zero, owing to its resistance to the air, and it then falls with a constant velocity. These equilibrium velocities of a powder in air have been determined for coke of density 1.9, anthracite of density 1.5, and coal of density 1.3, by M. E. Audibert,' who gives the following formulas showing the relationin c. g. s. units between the equivalent radius r , and the equilibrium velocity u of the coal in air:
T
+ + +
For coke (density 1.9): r = 2 X 10-4u 100u2 X 10-8 For anthracite (density 1 . 5 ) : r = 2.22 X 10-4u 123u2 X 10- 8 For coal (density 1.3): r = 2.35 X 1 0 - 4 2 ~ 1 3 8 2 ~X~ 10-8
Thus, for coal of a mean diameter of 20 microns the velocity is about 8 cm. per sec. and of 60 microns about 23 em. per sec., or nearly three times as great. Equilibrium velocities were €ound also by this investigator in atmospheres of hydrogen (density 0.069) and coke-oven gas (density 0.47) to be greater than that of air, and he finally gives the following useful approximate formula for the velocity of particles of density d, in air a t a temperature t o C., in c. g. s. units: zd = 1.231 r ( l
- 25r)(d
+ 2.1)(1 1-0.75t X
The foregoing shows that when coal enters a furnace the vertical velocities of the widely different sizes of particles mill 1 Presented before the Section of Gas and Fuel Chemistry at the 64th Meeting of the American Chemical Society, Pittsburgh, Pa., September 4 to 8, 1922. 2 Published by permission of the Director, U. S. Bureau of Mines. 8 Private communication by G. St. George Perrott and S. P. Kinney, of the Bureau of Mines. 4 A n n . mines, Tome I, third publication of 1922.
FIG. 1-POWDERED-COAL FURNACE INSTALLED A T THE LAKESIDE POWER STATION OF THE MILWAUKEE ELECTRIC RAXLWAY A N D LIGHTCOMPANY
250
INDUSTRIAL AND ENGINEERING CHElMISTRY
METHODAT THE MILWAUKEE ELECTRIC RAILWAYAND LIGHTCOMPANY'S STATION One method of burning coal is shown in Fig. 1, which illustrates a furnace installed a t the Lakeside Power Station of The Milwaukee Electric Railway and Light Company. The coal to be burned enters the furnace through six burners situated in the top of the furnace. The coal is brought to the burners through six pipes, mixed with about one-tenth of the total air supply needed for combustion, and with a velocity of about 100 to 150 ft. per see., varying with the rate of combustion. This velocity is greater than the equilibrium velocity of the particles in still air, which equilibrium velocity will therefore first be established by the smaller particles near the top of the furnace and last by the larger particles near the bottom of the furnace, where owing to combustion they are reduced in size. The remainder of the air comes down vertically through openings around the burners and horizontally through ports in the front wall of the furnace. The horizontal velocity of the air tends to sweep the particles toward the back of the furnace and causes the smaller particles to pass to the back with a greater velocity than the larger particles. Finally, the flame is carried vertically upward by the draft. Thus, the flame is U-shaped, and the larger particles which require longer time to burn are situated on its under and outer portion, dwell longer in the furnace, and have a higher velocity relative to the gases in the furnace. As the coal particles burn, they gradually become smaller in size and their velocity relative to the furnace gases becomes less. Thus, the longer path of the larger particles of coal makes them more nearly equal in size to the smaller particles having shorter path by the time all reach the upper parts of the rising arm of the U-shaped gas travel. The size of the furnace must be so proportioned to the maximum rate of combustion that practically all the coal particles are reduced to the ash residue by the time they reach the boiler. Any unburned particles of coal that reach the boiler tubes are carried through the boiler and their heat content is not liberated for generating steam. Coal particles that are too large never attain their equilibrium velocity, and therefore fall to the bottom of the furnace after they have passed through the downward arm of the U-path. If the percentage of these very large particles is small, the process of combustion continues after they have reached the bottom of the furnace, and they are completely burned. However, if their percentage is high, they accumulate on the bottom of the furnace, and soon form a thick layer so that only the particles a t the surface of the layer come in contact with the air. Under such conditions these very large particles become mixed with the refuse and are lost. With coking bituminous coals there is still another factor that affects the equilibrium velocity of the coal particles and their process of combustion, and that is the swe'lling of the particles when they become heated upon their entrance into the furnace. When the coal particles have attained the temperature a t which the coal becomes plastic, the gases that are driven off from the inner part of the particles expand, or blow the particles out into hollow spheres. These spheres, then, have much larger diameter and smaller density than the original particles of coal before heating, and their equilibrium velocity changes. Some of these hollow spheres burst after they have passed through their plastic state and the fragments of the shells float or sail through the gases and their equilibrium velocity changes according to whether they present the edge or the face to the stream of gases. Again, while the coal particles are in the plastic state and are being blown into spheres, they may collide and stick together, thus forming larger pieces or clusters of hollow
Vol. 15, No. 3
spheres. These clusters have different equilibrium velocity than the original particles of coal. The process of combustion of powdered coal is rather complicated and does not lend itself easily to formulation according to known physical and chemical laws. Some of the hollow spheres pass through the furnace and boiler and are found in the flue dust, and can be examined readily with a low-power microscope.
FIG. %-RESULTS
OF THE
BOILERT E S T S CARRIED POWER STATION
O U T AT
LAKESIDS
The method of feeding the coal and air into the furnace and the path of their travel is illustrated in Fig. 1. The principal results of the tests are shown graphically in Fig. 2. The results are plotted on the rating developed by the boiler, and each point represents one test 24 hrs. long. The four lower curves are the most interesting from the standpoint of combustion. The lowest curve gives the losses due to incomplete combustion, which includes carbon monoxide in the flue gases as well as the unburned solid carbon carried by the gases out of the furnace. On most of the tests this loss was about 1 per cent of the heat in coal fired. At the lowest and also a t the highest ratings the losses are nearly 2 per cent. The larger losses with the low ratings are probably due to low excesq of air shown by the third curve from the bottom. At the high ratings the larger losses are probably due to the fact that the particles of coal passed faster through the furnace and did not have sufficient time to be completely burned.
March, 1923
INDUSTRIAL AiVD ENGINEERING CHEMISTRY
The second curve shows that with the particular furnace the highest economical rate of heat evolution is about 23,000 B. t. u. per cu. ft. of effective combustion space. If the rate of heat evolution were carried beyond this point, more particles of only partly burned coal would be carried out of the furnace with the gases. The third curve from the bottom shows that with a fur-
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nace of the proper size powdered coal can be burned nearly completely with an excess of air of 10 to 25 per cent. The two highest curves give the thermal efficiency of the furnace and boiler. The efficiency of boiler and superheater is well above 80 per cent, and that including the economizer reaches 90 per cent, which is about the highest efficiency so far attained on a large power-plant, steam-generating unit.
Thermal Operation of Modern Regenerator Coke Ovens' By D.W. Wilson,2 H.0.Forrest,B and C. H.Herty, Jr.8 BUFFALO STATION, SCHOOL OF CHEMICAL ENGINEERINGPRACTICE, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, LACKAWANNA, N. Y .
Several careful thermal e.@ciency tests have been made on coke owns by students in the School of Chemical Engineering Practice of the Massachusetts Institute of Technology, and the figures thus obtained form the basis of this paper. The equipment studied was one sixtyoven battery of regenerator ovens fired by a part of the gas produced from the coal. The coal used consisted of coal of 33 per cent volatile matter and coal of 18 per cent volatile matter so mixed as to produce about 27 to 28 per cent volatile in the mixture. The coke produced was for use in blast furnaces. The coke plant was completely equipped with apparatus for recovery of the usual by-products. The excess colpoven gas was used for raising sfeam or in steel-reheating furnaces. The immediate objects of these tests were-first, to obtain a comprehensive heat balance on the ovens: second, to determine from this the thermal eficiency of the battery; and third, to obtain any information possible regarding the heat eflecl of coking this particular coal mixture.
ITH conditions of the past few gears the question of fuel economy has been rapidly becoming vital to all producers and users of fuels. Probably, more efficient use of coal would be the greatest factor tending to reduce necessary fuel consumption, and consequently effect a considerable fuel economy. With this in mind, in its field work, the School of Chemical Engineering Practice of the Massachusetts Institute of Technology has taken advantage of all opportunities to study the use of coal in its various forms, particularly as coal, as coke, as producer gas, as coke-oven gas, and as blast-furnace gas. A f i s t consideration in investigating the thermal operation of any machine is a knowledge of the distribution of the heat energy supplied to the unit. This best takes the form of a comprehensive heat balance. This method has the following distinct advantages: First, the reliability of the experimental methods and results is known by the agreement obtained between total heat input and total heat output; and, second, from a thermal standpoint the operation of the particular unit is covered completely, and, consequently, unforeseen future uses for the results are usually provided for. Of most importance, however, a heat balance makes possible a comparison between heat distribution on similar types of furnaces which will often indicate the directions in which efforts can profitably be expended in striving for improvement. 1 Presented before the Section of Gas and Fuel Chemistry at the 64th Meeting of the American Chemical Society, Pittsburgh, Pa , September 4 t o 8, 1922. 2 Director. 3 Assistant Director.
DATANEEDED Obviously, in lnaking the tests it was first necessary to define the system under consideration. This was done as follows: The system studied was the entire battery of sixty ovens, starting with the coal as charged to the ovens, the fuel gas and air as delivered to the battery, and ending with the hot coke as pushed out of the ovens, the crude coke-oven gas as it left the ovens, and the waste gases as they entered the stack. This, then, involved a knowledge of the following information : HEATINPUT:
HEATOUTPUT.
Coal 1. Temperature 2 . Heating value 3. Analysis 4 . Quantity Air Humidity 1. Temperature 2. 3. Quantity Fuel Gas (purified or debenzolized coke-oven gas), 1. Temperature 2 . Analysis Quantityvalue 3. Heating 4. (A) Coke 1. Temperature 2 . Specific heat 3 . Analysis 4. Heating value 5 . Quantity Crude coke-oven gas (foul gas) 1. T a r Temperature b) Heating value c) Quanti'ty 2 . Ammonia ( a ) Heating value ( b ) Quantity 3. Light oils Heatinz value ( b ) QuantiFy 4 . Water condensed (both free and combined water in coal) ( a ) Quantity 5 . Purified gas ( a ) Temperature ( b ) Analysis ( c ) Heating value Id) Ouantitv %Taste (chimney) gages 1. Temperature 2 . Analysis 3. Humidity 4. Quantity (D) Radiation and convection 1. Temperature of unit areas 2 . Wind velocities 3. Surface exposed
I")
The quantity unit used throughout was one net ton d r y coal charged. The length of test was necessarily some multiple of the coking time of 18 hrs.-in this case 54 hrs., o r three consecutive cycles.
EXPERIMENTAL METHODS Representative samples of coal were obtained as t h e charging car was filled from the bins a t the ovens. Each
