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MAYERS—FLOW PROCESSES IN U N D ERFEED STOKERS 483 beds with two different values of void volume and specific surface. The asterisks indicate the points at which instability would be reached. That the air rate at which this occurs is greatly affected by the character of the coke of which the bed is made is not of merely academic interest. In the Hell Gate tests, the air rate corresponding to an average burning rate of about 35 lb of coal per sq ft per hr was calculated to be about 1100 lb per sq ft per hr when referred to the air-admitting surface .of the stoker. However, as showr in Fig. 4, the width of the burning lane was seldom as great as the width of the tuyeres, so that the air-flow velocity in the lanes might be anywhere from IV 2 to 3 times as great as the air rate referred to the entire air-admission surface. Evidently, encroachment of the coke walls of the burning lane on the space above the tuyeres is likely to lead to conditions of instability and should be prevented by provision of ample means for breaking them down. In the neighborhood of the critical flow, when the bed is in the condition designated as “just moving,” it has some remarkable properties, which are used, for example, in advanced German designs of gas producers (20). Particles of different densities are segregated in such a bed; if the particles are lighter than those of F i g . 8 D e p e n d e n c e o f R e s i s t a n c e t o F l o w T h r o u g h F u e l B e d o n A i r - F l o w R a t e , R e f e r r e d t o A i r - A d m i s s i o n S u r f a c e (Line 1 refers to a bed of coke having the size distribution described by line 2 of Fig. 10; line 2 to th at described by line 3 of Fig. 10. Asterisks indicate upper lim its of stability.) the bed, they float out on top, while if they are heavier, they sink to the bottom. Thus, if a fuel bed could be operated stably at this point, it would automatically allow the ash to fall through the bed into the coolest regions where there would be little or no danger of slagging. Moreover, such a bed, which is most stable when operated with an interm ittent blast of air (21), as in coalcleaning tables, has the property of healing itself, that is, if channeling begins at any point in the bed, instead of the air flow being disturbed in such a way as to make conditions worse, the bed is so fluid that it immediately flows over the incipient channel and stabilizes the flow resistance. This, obviously, would be a desirable characteristic to obtain in a fuel bed. I t seems likely that apparatus designed to employ a bed in the ‘"'just moving” condition would surmount several of the disadvantages now faced by grate firing of solid fuels. If full use is made of air, flowing at velocities close to the critical by provision of a bed of adequate depth, the attainable burning rates would be much greater than are now possible. This is shown by the values of combustion rate equivalent to TABLE 1 C A LCULA TED D E P E N D N C E OF C O M B U ST IO N R ATE ON D E P T H OF F U E L B E D A N D A IR FLOW /------------------------- Com bustion rates®----------------------- ■> Referred to air-admission area, Referred to total area,b lb per sq ft per hr lb per sq ft per hr CO at top of bed, per cen t..................................... 0 15 30 0 15 30 Approximate fuel bed depth, in .c....................... < 1 4 .4 15 . . . . . . . . . Air rates, lb per sq ft per hr 150 1 3.0 1 8.0 2 3 .9 6 .2 8 .6 11.4 1000 8 6 .6 120 159 41.1 5 7.1 7 5 .6 3000 260 360 479 124 171 224 ° Rate of burning of com bustible in coke, assumed to be carbon. b Assuming ratio of tuyere stack area to total area corresponds to tuy&re width of 10 in., retort width 11 in. c For a typical high-tem perature coke burned at an air rate of 150 lb per sq ft per hr, but not in proportion to the air rate. various air-flow rates calculated in Table 1. Those in columns 2, 3, and 4 are the rates calculated for 0, 15, and 30 per cent carbon monoxide in the issuing gas, referred to the air-admission or burning-lane area; while those in columns 5, 6, and 7 are referred to the projected stoker area, on the assumption that the proportions of airadmission to projected area are approximately the same as in the Hell Gate stokers. I t is evident that material increases in rating could be obtained if we could be assured that an unstable condition, due to encroachment of the coke walls on the burning lane, could be avoided. F l o w o f C o a l a n d A s h The flow of coal in the stoker takes place almost entirely in the retorts and in the walls of the burning lanes. There is practically no motion either lengthwise or crosswise of the stoker in the burning lanes. This was shown by the fact that porcelain probes inserted into the burning lanes could remain in position almost indefinitely without being subjected to severe transverse stresses. On the other hand, when such probes were inserted into the retorts through holes in the secondary rams, they were broken off during the return stroke of the ram at every stroke. The retort performs a double function. In the first place, it has the obvious function of distributing coal to the entire length of the stoker. Coal which enters the retort near its top rises close to the front wall and is delivered to the burning lane at the head end of the stoker. Coal which enters at the bottom of the retort passes well down the stoker before being delivered to the burning lane and, in fact, in existing stokers may be forced straight out onto the overfeed section without ever having reached the burning lanes over the tuyere rows. The distribution is effected by the longitudinal flow of the coal which takes place at relatively high velocity, speeds of the order of 2.5 fph. The second function of the retort is to coke the green coal and prepare it for smokeless burning above the tuyfire stacks. This process takes place because the retort produces a transverse component of flow of coal which has risen above the level of the tuyeres, causing it to flow toward and through the coke walls which border the burning lane. . In this passage the coal is carbonized so that it is delivered to the burning lane as coke. The magnitude of this transverse component of velocity was about 0.75 fph in the stoker tested at Hell Gate, a value obtained from speeded-up motion pictures made as a part of the tests. The magnitude of this component may also be calculated from the load and the dimensions of the fuel bed, since it is obvious that coal can be burned on the tuySre stacks only as fast as it is delivered to the stacks by the retorts as a result of this transverse flow. Hence, it follows that fuel must flow across the boundary of the burning lane at the same rate as it is burned on the stack. Knowing the rate of burning from the load and the dimensions of the tuyere stacks, it is evident that the average flow across the boundary between retort and burning lane must be equal to the
484 TRANSACTIONS OF TH E A.S.M.E. AUGUST, 1941 rate of burning per foot of length of tuyere stack, divided by the depth of the fuel bed over the burning lane, i.e., the height of that boundary. An average value of the last-mentioned quantity representing the entire fuel bed cannot be estimated accurately, but Table 2 shows that the calculated values are comparable with those estimated from the motion pictures. The transverse motion of the fuel from the retort toward the center line of the tuyere stack is terminated when the fuel mass breaks off from the wall of the burning lane. Such detached masses then fall vertically downward into the lane and help to T A B L E 2 T R A N S V E R S E V E L O C IT IE S O F F L O W O F F U E L Fire condition Long pusher strokes at load of 125,000 lb per h r.............................................................. Average............................................................... Probable error.................................................... Fhort pusher strokes at load of 119,000 lb per h r.............................................................. Average........... Probable error. Observed values, fph 1.85 0.81 1.33 — 0.34 0.72 0.67 1.85 — 1.18 1.45 0.68 0.22 1.06 — 1.25 1.76 0.70 0.96 1.83 0.65 1.26 0.70 0.51 0.55 0.94 1.38 — 0.42 0.52 0.75 0.14 Calculated, fph form the continuous bed of discrete particles which exists at the bottom of the lane. On the burning lane itself the motion of the fuel is downward, just as in any other overfeed bed and, just as in an overfeed bed, the size of the particles decreases as they fall lower in the bed. At this point a mysterious situation exists. Eventually practically all of the fuel is burned out and at the bottom of the bed ash will be collected, perhaps in a dry, powdery form if the cooling effect of the air is great enough, or perhaps as small clinkers. Somehow or other a large portion of this ash gets down to the ashpit, but so far the path by which it reaches that destination has not been discovered. During the Hell Gate tests, it wras sometimes found that a layer of ash existed along the tops of the tuyeres. When this occurred, opening the guide tubes preparatory to inserting a probe was followed by a shower of ash blown down 65 through the tubes, but in an equal number of cases no ash came down and the probes read high temperatures right at the level of the tuyeres. Is the agitation of the incoming air sufficient to cause the ash to flow along the slope of the tuyfire stacks down to the ashpit? This seems hardly likely and yet the only other alternative, that all of the ash is blown up through the burning lane and then dropped out of the air stream when it slows down in the furnace, seems still more unlikely. Thus, there remains a hiatus in our knowledge of the flow of solid materials in the fuel bed. The flow of the fuel during its distribution, coking, and burning is represented in Fig. 9, which shows a perspective view of the stoker and approximate lines of travel of representative portions of the fuel. This representation is, of course, not exact or complete but represents a first estimate of how coal flows into the fire in a multiple-retort underfeed stoker. The multiple function of the retort may be responsible for some of the instability of stoker fuel beds. The rate of air flow through a bed at which instability sets in is greater the smaller the specific surface of the fuel, i.e., the more uniform in size are the particles which make up the bed, and the larger their size. Now, it has been found that, when coke is broken by impact, as in the shatter test, its size distribution may be plotted on probability coordinates, as in Fig. 10 (line 1). The slope and mean size of this distribution are characteristic of the coal and of the temperature of carbonization. This is the kind of size distribution Fia. 10 C o k e -Size D i s t r i b u t i o n s P l o t t e d t o P r o b a b i l i t y C o o r d i n a t e s (Line X, size of coke after shatter test. Line 2, size of coke discharged from a by-product coke oven. Line 3, size of coke made from same ooal as discharged from a continuous vertical retort.) APPROXIMATE DIRECTIONS OF FUEL FLOW LINES OF TRAVEL OF REPRESENTATIVE FUEL P A R T IC L E S F i g . 9 D i a g r a m m a t i c R e p r e s e n t a t i o n s o f R a t e a n d D i r e c t i o n s o f F u e l F l o w i n S t o k e r F u e l B e d s
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Page 3 and 4: Significance of Coal-A sh Fusing T
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Bulb temperature B ath ✓---------
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The Bureau of Standards paper2 give
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Discussion M. F. Behar.4 This paper
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The differential equations for the
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Filling it with mercury reduces the
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