I N D U S T R I A L A N D ENGINEERING CHEMISTRY
808
Vol. 21, No. 9
SYMPOSIUM O N BOILER ROOM CHEMISTRY Presented before the joint meeting of the Divisions of Industrial and Engineering Chemistry Gas and Fuel Chemistry and Water Sewage, and Sanitation Chemistry a t the 77th Meeting of the American Chemical Society, Columbus, Ohio, April 29 to May 3, iQ29. The paper by Edmund Taylor and H. F. Johnstone, entitled “Determination of the Sulfur Content of Ga-es from Boiler Furnaces,” which was presented as a part of this Symposium, will appear in the October 15 issue of the Analytical Edition.
Rate of Burning of Individual Particles of Solid Fuel’ H. K. Griffin, J . R . Adams,*and David F. Smith PITTSBURGH
EXPERIMENT STATION,u.
An apparatus and method have been described for determining the rate of burning of individual particles of solid fuels under controllable conditions of furnace temperature, particle size, and oxygen concentration. Typical data for three sizes of coal and two sizes of semi-coke and active charcoal have been presented and discussed as follows: ( a ) Under the experimental conditions, fuels containing high percentages of volatile matter show a pronounced increase in burning time with increasing furnace temperature; ( b )fuels containing practically no volatile matter show a much smaller temperature coefficient over the temperature range covered in the experiments; and (c) active charcoal requires a considerably longer period to burn than coal or semi-coke of the same size (weight per particle).
P
ULVERIZED fuels, which in the United States usually
mean bituminous coals, are finding a steadily increasing use in power plants. The boiler furnace burning pulverized fuels may also serve as an outlet for the semi-coke produced in increasing quantities by low-temperature carbonization processes; this condition would be of importance in conservation and smoke abatement. T o attain it, however, the greater cost of the coke, arising from the expense of manufacture and from its lower heating value, will have to be offset either by the sale of by-products or by a greater utility of the coke in the boiler room, or by both. Fundamental data on the burning characteristics of powdered fuels are meager, although they are obviously essential to the rational design of furnaces intended for their use. Further, although powdered coal is successfully and advantageously used in many installations, the burning characteristics of powdered semi-coke are largely known. As a first approximation, the rate of liberation of heat energy in a given boiler installation, other things being equal, will depend on two main factors: (1) the rate of propagation of the flame through the cloud of powdered fuel and air, because this determines the number of fuel particles ignited per unit time; and (2) the time of burning of the individual particles of the fuel, because this determines the rate of liberation of energy from the particle, once it is ignited. Both factors probably are functions of other variables, some of the latter undoubtedly being common to both factors. A study of the fundamental phenomena occurring in the combustion of powdered fuels has been conducted during the past two years by the Pittsburgh Experiment Station of the U. S. Bureau of Mines jointly with the Carnegie Institute of Technology and the Mining Advisory Board. Although the data obtained during the first year did not permit evalu1
Received April 2, 1929.
Published by permission of the Director,
U.S. Bureau of Mines, Carnegie Institute of Technology and the Mining Advisory Board. (Not subject to copyright) 1 Research fellow, 1928-29, Carnegie Institute of Technology.
s. BUREAUO F MINES, PITTSBURGH, P A . ating the absolute rate of flame propagation, apparently the flame propagates more rapidly through a suspension of powdered coal in air than through a similar “cloud” of powdered . semi-coke of the same fuel concentration and particle size.3 As a further step in the fundamental study of this problem, during the college year 1928-29 the rate of burning of the individual particles of certain solid fuels under known conditions of furnace temperature, particle size, and oxygen concentration was determined. Apparatus and Method
Figures 1 and 2 illustrate the experimental method of determining the rate of burning of the individual particles of solid fuels. They show, respectively, the furnace in which the particles were burned and the revolving-drum camera for determining the period during which each particle was in process of combustion. The essential parts of the furnace are the six nichrome ribbons that serve both as the heating element and as the walls of the furnace proper; the silica glass windows; and the feeder by which the particles, which have been previously sized, are dropped into the furnace. The nichrome ribbons are each 34 inches (86.4 cm.) long and 0.01 inch (0.25 mm.) (KO.30 B. & S. gage) thick. Two strips are each 1 inch (2.5 cm.) wide, and the other four strips are each 3/4 inch (1.9 cm.) wide. The six ribbons are disposed (Figure 1, A A ) about the periphery of a hexagon, 1 inch (2.5 cm.) on a side, held in such arrangement primarily by attachment at the ends to hexagonal studs on the furnace terminals. The two 1-inch (2.5-cm.) ribbons are placed opposite each other, and the other four are disposed as indicated in Figure 1. This arrangement leaves two opposite vertices of the heating element open to form the windows. The upper furnace terminal carries the weight of the heating element and the lower terminal. The latter is free to move through the hole in the transite board furnace end, as the heating element expands with temperature. Springs a t the bottom of the furnace put a slight tension on the heating element and aid in keeping the element in alignment. Slots are cut in the furnace shell or exterior (made of 6-inch (15-cm.) Shelby seamless steel tubing), which align with the windows in the heating element. Narrow ties, on 6-inch (15-cm.) centers, are left when milling such slots; otherwise the tube would not retain its cylindrical shape. The sectional slots in the furnace shell make necessary fused and polished silica-glass windows, which are placed directly against the “edge” of the heating element in 6-inch (15-cm.) sections. Likewise, the pieces of transite board that provide a clear passage for the light from the interior to the exterior of the
*
Thesis submitted b y D. I,. Reed as part requirement for an M.S. degree, Carnegie Institute of Technology, 1927-2s.
IA-DCSTRIAL A S D ENGINEERIXG CHEMISTRY
September, 1929
Dus
I
DUST FEEDER
Water
5
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Mica window
I
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809
It was found necessary t o introduce the smaller particles farther down in the furnace, because when introduced near the top the relatively violent convection current? in the neighborhood of the water-cooled terminal made it difficult to take photographs. The feeder for this purpose was identical in principle with the top one. Differences in construction details included passing the stem of the funnel through one of the furnace window spaces a t an angle of 45 degrees with the vertical and producing the vibration by means of an eccentric, of variable amplitude, driven by a variable speed motor. The camera used (Figure 2 ) is an ordinary 8 by 10 inch view camera modified as required for the purpose. The regular camera back is replaced by a n aluminum plate, containing an opening, with rabbeted edges, about 4 by 10 inches (10 by 25 cm.). A removable drum box, with slide and cover as indicated in the figure, fits into the rabbets, where it is held by special thumbnuts. The box with drum is thus readily removed and transported to the dark room. The drum proper is a piece of 33/~inch (9.5-em.) brass tubing, which is just the correct size t o take a 10 by 12 inch (25.4 by 30 em.) photographic film or paper with a slight lap. The geometrical arrangement of the drum assembly is such as to bring the surface of the drum in the plane normally occupied by the ground glass of the camera. It is thus possible t o focus sharply on the burning particles by placing the regular camera back in position. The lens used is an anastigmat of speed F 3.5, fitted with a front shutter. Peripheral speed of the drum is obtained by means of a spark gap, focused on the drum surface and controlled by a 50cycle tuning fork. I n making an experiment the drum is started and allowed to run a n instant to attain constant speed. The drum-box slide is then removed and the spark plug allowed to function for about one revolution of the drum. The vibrator of the feeder is then started. and a t about the same instant the shutter of the camera is opened. After enough particlesusually about 50-have been fed into the furnace and burned the camera shutter and drum-box slide are closed. The drum box is then removed and carried to the dark room for development of the film or paper.
SEmoNA-A
-Terminal
Stoek view camera ..___.___..
Figure 1-Furnace for Determining Time of Burning of Individual Particles of Powdered Fuels
furnace are made in sections 6 inches (15 em.) long. Both silica windows and transite board pieces are supported from special brackets attached by screws t o the furnace shell. The slots on the outside of the furnace shell are covered with transparent mica. Space between heating element and shell is filled with short-fiber asbestos. Slow and uniform feeding of the fuel particler, as well as keeping them cool until they enter the furnace at the end of the funnel stem, is essential. The former is accoinplished by sifting them, as previously sized, through a screen placed in a long-stemmed metal funnel. The funnel is vibrated, with some control over period and amplitude, by means of a large electric buzzei. The particles are kept cool until they enter the furnace by sheathing the steam of the funnel with a water jacket formed of concentric tubes, as shown in the enlarged section of the feeder in Figure 1.
I
Figure 2-Longitudinal Vertical Section of Revolving-Drum Camera for Photographing Burning Fuel Particles
Description of Fuels and Method of Preparation
The fuels used were as follows: (1) a powdered coal from the Cabin Creek district of West Virginia, described a s Kanawha gas coal; (2) a semi-coke produced from this coal by the McEwen-Runge process of low-temperature carbonization; (3) a beehive coke; and (4) a n activated charcoal. Table I gives the proximate chemical analyses of the first three of these fuels.
x I0
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I t u r i m a l r A n a l y b i s of Fuels 'TerfeB
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12,780
18.260
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71.6 12.6
2.1 13.8
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IFigurr 3 -Minus 45 t 50 Mesh Coal.
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Vol. 21, s o . 9
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As the cod and semi-coke were received in pulverized form, preparation of the siimples consisted in screening the fuels t h r o u g h screens of the desired mesh. The samples were screened twice bo aid in eliniinating fines which have ~1 tcndrncy to adhere to tlie larger part,iclea and might hee o m e se par a t ed when the fuel is introdoced into the furnace. Otherwise, these would introduce 811 error in the t i m e of hurning for the g i v e n particle size. Aftor
Figures 7 to 14 arc typical photogrxphs of burning pwtides, as ohtained with the revolving-druni camera, OS the varimii fuels of different sizes and at different temperatures. In rnplaiiation of the pliotogra,plis (see Figure 7), rotation of the camera drum moves the filmfrom left to r i Q i i t . The timing dots hy which the drum speed is deter&ned are visible at the u n w r edae of the rictnre. The friel pariicles mow d o n k a r d , at least initially, so that if the particles eontimied to fntl at a uniform rate the tracks rvould be straight lines sloping downward oii the right. Actually, while the particles fall initially, they start 1.0 rise soou after ignition, thus producing tlic curved traces sliown in the photographs. Ohviondy, then, so long as the particles mose in R vertical line (otherwise the particles \vould pass from view hecanso of the narrow Iurnaee windnu), the time of huming is ohtsinahle from the projection of the trnee on the time axis, lreing aet,nally the leiigth Of such ~ (linear) speed OS tlie film. Deprojcetion dil-ided I Jt.he pnrture f m i n inntion i n a vertical line, while int,roducing i n i error i n tlie time t.trus cnniputed, was small. Moreover, the readtarit. error iibvifJns1y tends to be eliniinat,ed over a consideralile nimiher of pnrticles, although if the particle describes an intriciitc s p s c e cnrve some error may result. Error from sucli s~~urccs, liou~ever, was negligihlc, as e+ douced Iiy the rt~pr~,docibilityof tlie enperimental values, ivliich were snrpiisingly good for work of t h i s type. All the pliotopaplis slnnvn Were made on regnlar ~ J ~ I I diromntic film, which is scnsitive, not nrily to t,hc usual pliotoy;aplric \wive lengths, hut til tlie red rays as well. A 11 paper, known as Eastmnri So. I recording pitper, was iisrd in general for ohtainiiig the plidographs, because hy so doing inore &ita rould lie iiht,ained in the time available. :'atwe when rovering the field rather
wlmt, Iowr temperatlm:s,
r,)r
I l i w r e 4 ~ M l n u s 4 5+5OMeshSeml-Cuke. X 16
ortiow, enrli of difierciri character; the earlier portion that the rolatile mattcr is driven d f wnd buI'lled din p1iri011. After such iLpp:x
