Transactions

More documents

Recommendations

Info

MAYERS—FLOW PROCESSES IN U N D ERFEED STOKERS 487 F i g . 11 A r r a n g e m e n t o f S t o k e r f o r O b s e r v i n g V a r i o u s C o m b i n a t i o n s o f C o a l - D i s t r i b u t i n g E l e m e n t s the total air and, in most instances, this limit is much lower. We have tried a number of times to produce large quantities of combustible gases to be burned later with secondary air and have had no success. It is impossible to make a gas producer from the multiple-retort underfeed stoker. We agree that secondary air to be most effective should be introduced through high-velocity jets. From the author’s analysis, it appears that the secondary air would also be more effective if introduced at right angles to the retorts. We will have an installation operating in the near future with secondary air introduced transversely of the retorts and also using highvelocity jets. For turbulence, we have also used the combustion gases taken from one of the rear passes of the boiler. These gases are introduced through high-velocity jets. Since the oxygen content of these gases is low, larger quantities can be used with no increase in the total weight of gas discharged. Secondary air is used primarily to control smoke emission from the stack. In addition to the elimination of smoke, there is also more complete combustion of solid carbon in the furnace. Slagging of tube surfaces is also decreased. We have been studying the flow of fuel through the retort. These studies have been made on actual stokers, with windows placed so that the movement of the coal in the retort could be observed. The object has been to find some method by which more coal could be fed through the retort without increasing the surface movement of the fuel; in other words, reduce the agitation of the burning fuel over the tuyeres. Various combinations of coal-distributing elements have been observed by arranging a stoker of the type illustrated in Fig. 11 of this discussion, for inspection of the moving coal. The velocity and direction of flow in the various sections were marked on the glass windows and later charted to scale for further analysis. The construction eventually decided upon was installed in a stoker which was operating on low-volatile bituminous coal. The customer desired additional capacity from the unit and the change permitted him to operate reliably at a 27 per cent increase in output. Additional applications of the principle demonstrated have indicated improvements with many other •types of coals. D. J. M o s s h a r t . 4 In the main, the concepts and hypotheses constructed by the author parallel and confirm principles which 4 Assistant Chief Engineer, Stoker Department, Westinghouse Electric & Manufacturing Company, Philadelphia, Pa. for many years have been recognized and employed in the design and operation of stokers. These principles, evolved under operating conditions and based on uncounted observations made from above and below the fuel bed, have been proved in the trial, success, and adoption of designs and methods employing them. The writer feels that the author’s excellent analysis is worthy of objective study by all combustion engineers, and that it is appropriate to offer certain comment supplementary to it. As a preliminary remark, any hypothesis or analysis of a combustion process may be more thoroughly understood (or safely rejected) if it is constructed in two ways, (a) by considering the coal as fuel and the air as supporting combustion thereof, (6) by considering the air as fuel and the coal as supporting combustion. The phenomena discussed in the early part of the paper offer an excellent illustration. Ignition will be lost if the flow of coal is too rapid through a stream of air constant in quantity. The illustrated typical structure of the fuel bed is fundamental. I t is obtained with all sorts of solid fuel; with coke breeze and with subbituminous coals, which do not coke or agglutinate. The formation of coke walls along each side of the burning lane is helpfully incidental; it provides stability and inhibits blowing away of the fuel. It is suggested that the finding of free oxygen just above the fuel in the retort was probably due to some specific circumstance associated with the particular setup used. Ordinarily there is a short, low'-velocity but hot flame, or rather assortment of flames, flowing hither and yon over the coal in the retort. This flame is aspirated into the jet over the burning lane, the slowness of its aspiration being due to the fact that it lies between two aspirators of substantially equal power and wavers between them. The hypothesis of a stagnant layer of cool, unconsumed air is questioned. How can it long exist in the presence of hot coke and combustible gases? The practical application of the principles here discussed is quite simple. The fuel burns almost entirely in the burning lane. It burns partly as raw coal (finer particles sifting through the coke walls), partly as coke, and partly as gases and vapors, evolved in the formation of the coke walls and aspirated into the flame or jet of partly consumed air which issues from the lane. The average combustion result (C02 or excess air) obtained is a function of the depth of the lane and the size and disposition of the fuel particles therein. Under given conditions of fuel and load, this establishes the thickness of fuel bed required and this thickness is controlled to maintain the desired result, i.e., the feed of coal is controlled and the distribution of coal is adjusted to give a fairly uniform combustion condition over the entire stoker. At any given spot, the quality of combustion probably varies over a wide range of excess air with such great rapidity that the 50- or 100-cc sample taken by the Orsat is a composite of several values. However, with an infinite number of spots, the integrated condition obtained some distance above the fuel bed is one of fairly uniform quality. This burning in lanes is by no means exclusively characteristic of the underfeed section of the stoker. Link-grate stokers, with link-grate overfeed sections of length equal to that of the retorts, burn coal in precisely the same way. The link grates have two components of motion, propelling and breaking, separately regulated. The laned fuel bed received by them from the underfeed section is carried along in this characteristic form until it nears the end of the grate, the burning lanes gradually widening, and the fuel lanes finally disappearing. Quality of combustion is controlled by controlling the rapidity with which the laning is obliterated, i.e., by altering the relation between the propelling and the breaking components of motion.
488 TRANSACTIONS OF T H E A.S.M.E. AUGUST, 1941 This naturally leads to consideration of the author’s statement of mystification over the manner in which ash gets to the ashpit. This question has so far been answered only by observation of the progress of the ash and concurrent rationalization. It is believed that the following conclusions are not only tenable but fairly close to the complete story: 1 Since nearly all of the ash must remain as a residue in the coke which bums over the tuyeres, it is exposed to temperatures generally higher than the fusing temperature of any ash. Most of it fuses and melts. 2 The fused ash is heavier than the burning coke with which it is associated, so it drips or drops down through the burning lane to be chilled to solid form and comes to rest on the tuyeres and among the coolest particles of coke above them. Thus, there is deposited on the tuyeres a layer of ash of a thickness which is minimum at the upper end of the stoker and maximum at the lower. 3 The stream or column of coal in the retort with its confining walls of coke moves as already described. The ash is slowly but surely dragged along by the moving column. Sometimes it agglomerates to form a clinker on the tuyeres which stands still and grows until positively seized by adjacent columns and moved integrally with them. (Probably, the loose ash flowed around the author’s probes and the clinkers, if any, obligingly withheld movement while the probes were in place. High temperatures right at the tuyeres usually mean that a clinker has just passed by and swept the ash away.) 4 Thus, the ash is moved by the passing coal and, thus, it must finally be ejected from the underfeed section of the stoker, i.e., by ejecting enough coal from the retorts to carry the ash away. Overfeed sections are provided for the express purpose of burning the coal used for this function. Herein is the principal reason for the application of the linkgrate overfeed section. It provides for burning the coal ejected from the retorts in order to dispose of the ash. Additionally, it comprises a large portion of the total stoker area where the concentration of ash is highest and, being a positive conveyer, eliminates in this area dependence upon movement of the coal to move the ash. In undertaking to burn any and all varieties of coal on standardized apparatus there naturally occur instances where it is impractical or impossible adequately to control combustion solely by regulating distribution of the coal. Then it becomes necessary to use secondary air. The method of injecting the air involves more than velocity. Basically it involves the energy required to obtain penetration of the jet to the area needing the air, i.e., the area of the stoker above which the flame is dense and smoky. This energy is the product of mass and velocity and, if the mass usable is great enough, the velocity can be low. Again, method is important. Penetration rather than flame deflection is to be sought. Turbulence will more readily be obtained with a few large jets boring into the flame than with many small but powerful jets trying to push the entire mass of flame hither or yon. For instance, we find that jets of 20 to 40 sq in. area spaced 3 to 5 ft apart and using air pressures of 8 in., and often less, give greatest penetration and produce greatest turbulence. J. E. T o b e y .6 This paper represents the culmination of many years of special study, both theoretical and practical, of combustion on multiple-retort underfeed stokers. The writer has followed the author’s work with a great deal of interest and anticipation. In fact on two occasions, he visited Hell Gate Sta- 6 Vice-President in charge of Engineering, Appalachian Coals, Inc., Cincinnati, Ohio. Mem. A.S.M.E. tion, while the tests were being conducted, and crawled into the doghouse underneath the stoker with the author, from which location the probes were introduced into the fuel bed. The author has been an earnest seeker after truth and has worked tirelessly to secure the data presented in this paper. From personal observations, the writer agrees that there is need for a speed up in the coking process on underfeed stokers and that many present burning troubles would be eliminated if the fuel could pass more quickly and completely from the raw-coal state to coke. It is believed that the author’s idea of a pulsating air blast has merit from the standpoint of classifying ashy material and coke in the tuyere lane and increasing the mobility of the former. In addition, the writer has been of the opinion for a long time that a pulsating air blast might create a scavenging effect on burning coke, exposing fresh surfaces to oxygen attack, tending to remove insulating films of ash and inert gas. It is hoped that stoker engineers will collaborate with the author in developing a higher-duty stoker. R. S. J tjlsstjd.6 In his description of the structure of the fuel bed, the author speaks of a “thin skin of coke” covering the top of the green coal in the retort. The writer’s observation of numerous stoker fuel beds, both with furnace glasses and the naked eye, has never revealed such a “coke skin.” On the contrary his observations have indicated that, as the heated coal emerges from the retort, the pieces become soft, their edges round off, and they become plastic. The degree of plasticity attained varies with the fuel, some coals becoming so plastic as to appear a “frothy” mass floating over the retort, whereas, others soften but slightly and can be distinguished throughout their evolution from green coal-to coke formation. It is this plasticity, which if extreme, produces “matting” or “islands,” sometimes extending into the burning lanes, which appear black when viewed with furnace glasses. Optical-pyrometer readings of these plastic masses reveal temperatures from 1600 to 2000 F, depending upon the fuel sizing, degree of plasticity of the fuel, combustion rate, and other factors. The presence of a stagnant layer of unconsumed combustible over the retort of both single- and multiple-retort stokers has been observed by the writer. This is particularly noticeable with single-retort stokers of the “on-off” type after lengthy off periods. A steam or heat demand starting up the feed and forced-draft fan creates dense clouds of slowly rising gases over the retort which ignite as they are drawn into the high-velocity air and gas streams issuing from the coke fissures in the burning zone. Not only is high-velocity primary air desirable for entraining and burning unconsumed combustible gases above the retort, but it also assists in the rapid formation of coke as the fuel passes from the retort to the burning lane, provided of course that such velocity does not “lift” or “blow” considerable fuel from the bed. Calculation of the transverse velocity of fuel over the grates of single-retort stokers at various combustion rates have shown speeds of from 0.3 to 1 fph, which would appear in line with those determined by the author at the Hell Gate tests at probably higher combustion rates. Again referring to “on-off” type single-retort stokers, the writer has observed the tendency of the fuel bed to “heal” and correct unstable fuel-bed conditions such as “blowholes,” large coke masses, and “islands” of plastic fuel during off periods. This is probably due to the plastic nature of the fuel bed permitting it to flow into cavities and the additional creation of coke fissures due to contraction of the coke caused by fuel-bed temperature changes. The author’s four suggestions for the design of a heavy-duty 6Address, Harmon-on-Hudson, N. Y. Assoc. Mem. A.S.M.E.
Page 1 and 2: Transactions of the Significance of
Page 3 and 4: Significance of Coal-A sh Fusing T
Page 5 and 6: BAILEY, ELY—COAL-ASH FUSING TEM P
Page 7 and 8: BAILEY, ELY—COALrASH FUSING TEMPE
Page 9 and 10: BAILEY, ELY— COAL-ASH FUSING TEM
Page 15 and 16: BAILEY, ELY—COAL-ASH FUSING TEMPE
Page 17 and 18: 480 TRANSACTIONS OF TH E A.S.M.E. A
Page 23: 486 TRANSACTIONS OF TH E A.S.M.E. A
Page 27 and 28: L ubrication of G eneral E lectric
Page 29 and 30: DANTSIZEN—LUBRICATION OF GENERAL
Page 35 and 36: Stress and Deflection in R eciproca
Page 37 and 38: BOYER, KUTVINEN—STRESS AND D EFLE
Page 39 and 40: BOYER, KUIVINEN—STRESS AND DEFLEC
Page 41 and 42: Some P articu lars of Design and O
Page 43 and 44: BLIZARD, FOSTER—SOME PARTICULARS
Page 49 and 50: C ondenser T ubes and T h eir C orr
Page 51 and 52: CLARKE, WHITE, UPTHEGROVE—CONDENS
Page 53 and 54: CLARKE, W HITE, UPTHEGROVE—CONDEN
Page 55 and 56: CLARKE, W HITE, UPTHEGROVE— CONDE
Page 61 and 62: CLARKE, W H ITE, UPTHEGROVE—CONDE
Page 67 and 68: Therm om etric Time Lag T h e p urp
Page 69 and 70: ate of change will be n = T J L . I
Page 71 and 72: Bulb temperature B ath ✓---------
Page 73 and 74: The Bureau of Standards paper2 give
Page 75 and 76:
Discussion M. F. Behar.4 This paper
Page 77 and 78:
The differential equations for the
Page 79 and 80:
Filling it with mercury reduces the
Page 81 and 82:
546 TRANSACTIONS OF TH E A.S.M.E. A
Page 83 and 84:
Page 85 and 86:
Page 87 and 88:
Page 89 and 90:
Page 91 and 92:
Page 93 and 94:
558 TRANSACTIONS OF T H E A.S.M.E.
Page 95:
560 TRANSACTIONS OP TH E A.S.M.E. A
show all

Transactions

Create successful ePaper yourself

Delete template?

Save as template?