US2617394A - Boiler - Google Patents

Description

Nov. 11, 1952 W. S. PATTERSON BOILER Filed Dec. 30, 1949 Word 8. Patterson BY ATTOR EY Patented Nov. 11, 1952 UNITED STATES PATENT OFFICE Combustion Engineering-superheater,

Inc.,

New York, N. Y., a corporation of Delaware Application December 30, 1949, Serial No. 137,600

2 Claims.

This invention relates in general to an improved method of burning fuels containing ash and to an improved construction and operation of fuel burning apparatus especially designed and particularly adapted for carrying out this method. More specifically the present invention relates to a furnace arrangement of a forced circulation vapor generating unit.

Modern high-temperature high-capacity vapor generators are generally divided into two classes depending on the method of ash removal. In one class ash is removed from the furnace in a dry state by Way of ash hoppers. In the other class of vapor generating unit the ash is removed from the bottom of the furnace in a fiuid state and it is this latter class to which the present invention applies. Such so-called slagging furnaces are usually designed for burning bituminous or semi-bituminous coals in a finely divided condition. In the operation of such furnaces the gas temperature in part of the furnace is normally maintained above the ash fusion temperature of the fuel so that a large percentage of the ash content of the fuel can be removed from the furnace chamber in a molten state.

The general object of the present invention is the provision of an improved method and apparatus for burning an ash-containing solid fuel which method and apparatus are characterized by a high rate of heat release per cubic foot of furnace volume in one or more primary chambers, a high fuel burning efiiciency, and. separation of and removal of substantially all of the recoverable ash content of the fuel in a molten condition before the combustion gas leaves the furnace.

Another object is the provision of an improved fuel burning apparatus in which in spite of high furnace heat release positive cooling of furnace wall surfaces is accomplished so as to permit operation at high heat absorption rates and high steam pressure without resorting to refractory protection of the furnace wall tubes.

More specifically the invention is concerned with an improved construction of a multi-chamber ash-slagging furnace in conjunction with forced-circulation steam generating apparatus, one object being a more compact and space-saving furnace design.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the inventions operating advantages and other specific objects obtained by its use,

reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described one preferred embodiment of my invention.

Of the drawings:

Fig. 1 is a sectional elevation of a forced cirv Illustrative embodiment of the invention In the drawings character A denotes as a whole the lower portion of a furnace chamber bounded by four vertical walls, namely two side walls l0 and I2, front wall It and rear wall I6; and also by a floor l8. Partition walls 20 and 22 within said lower furnace portion A extend from side to side and from the floor IS upwardly dividing furnace portion A into primary compartments a, b and secondary compartment c, the outer primary compartments 0, and b respectively communicating with secondary central compartment 0 by way of opening 24 adjacent wall l2 and opening 26 adjacent wall l0. Roof 28 forms the top boundary of compartment 0. and roof 36 forms the top boundary of compartment 1). Openings 24 and 26 extend between floor It and roofs 28 and 30 respectively, or some portion thereof.

Compartment 0 is left open at the top to facilitate discharge of combustion gases from lower portion A of the furnace chamber through opening I9. Outer side walls l0 and I2 and partition walls 20 and 22 are extended in an upwardly direction to form together with roof 3| the boundary of the upper portion B of said furnace chamber, this upper portion B being an extension of secondary compartment 0 and containing 0]: communicating by way of gas discharge opening 31 with heat absorbing surfaces which in the illustrative installation represented include hightemperature superheater section 0, reheater section D, low-temperature superheater section E, and upper and lower economizer section F and G respectively.

The furnace side of outside walls I0, [2, l4 and I6 and also partition walls 20 and 22 are equipped with vertical wall tubes 25 (see Fig. 2 particularly) which are organized in communication with the forced circulation system of the steam generating unit in the following manner:

The forced circulation pump I receives water from drum J by way of conduit K and discharges under pressure through conduit L, strainer M and tubes N into lower side wall headers O and through tubes V to front wall header P. There are two headers 0, one on each side of the unit feeding water wall tubes 25 of furnace side wall In and [2 respectively. These water wall tubes extend from headers O upwardly to upper water wall headers Q of which there are also two, one on each side of the unit. Some of the water flowing through these water wall tubes is evaporated into steam and a mixture of water and steam is discharged from headers Q into drum J by way of connecting tubes R thereby completing identical forced circulation circuits for these two side walls In and I2.

A portion of the water entering lower water wall header P flows upwardly through the water wall tubes 25 of front furnace wall l4 thence horizontally over roof 28 downwardly through tubes of partition wall 20 and upwardly through tubes on the other side of this wall continuing along front wall 2| of upper portion B of furnace chamber into drum J thereby completing the front wall circuit of the forced circulation system. Those of the tubes of roof 28 located in line with opening 24 will of course turn up at the upper border 33 of opening 24 and continue to form front wall 2 l,

The remaining portion of the water entering header P proceeds along tubes forming floor l8, up through wall I6, horizontally over roof 30 thence down partition wall 22 and up on the other side and continuing upwardly to form rear wall 23 of upper furnace portion B, thence horizontally along roof 3| of upper chamber B back to drum J thereby completing rear wall circuit of the illustrative installations forced circulation system. Here again those tubes of roof 30 which lie opposite to opening 26 will continue downwardly in the plane of partition wall 22, turn to form the upper edge 35 defining opening 26, hence flow upwardly to join the remaining tubes of partition 22 in forming rear wall 23.

While water passes through front wall circuit and through rear wall circuit some of it will be evaporated and a mixture of steam and water from these circuits is collected in drum J together with steam and water mixture from aforementioned side wall circuits. Steam separation, washing and drying in a conventional manner takes place in drum J, whereupon the steam passes from drum J via drum T (if desired) and conduits 29 into and through superheaters E and C and thence to outlet header U.

primary air is delivered from an outside source (not shown) to burners 32 arranged in a vertical row in the front portion of side wall I!) of compartment a and to burners 34 arranged in a vertical row in the rear portion of the side wall [2 of compartment b and discharged at high velocity into these furnace compartment to be burned therein. Secondary combustion air may be admitted into the furnace by way of rows of air nozzles 36 and 38 vertically arranged in the furnace compartment corners on each side of the vertical burner row 32 and 34 respectively or in the usual manner through the burners 32. In the illustrated installation fuel is burned at a high combustion rate in both compartments a and b. Burners 32 and 34 impart a rotating turbulent motion to the fuel and air mixture while discharging same into the furnace chambers a and 1). Secondary air emitted from air nozzles 36 and 38 impinges on said rotating fuel streams creating additional turbulence and thereby aids in the combustion process.

In the operation of the installation illustrated the gas temperature is normally maintained in compartments a, b and 0 above the fuel ash fusion temperature and combustion is expected to be completed by the time the gas leaves compartment 0. Due to the impact of furnace gas against the walls II and I2, 20 and 22 at the turns 24 and 26 ash in a fluid state collects on these walls and flows to the bottom I8 of the furnace forming a slag p001. Two circular openings 40 are provided inthe floor H! of compartment 0 to facilitate drainage of molten ash into slag pit H from which the fluid ash can be removed by any known conventional method.

The air supplied to the furnace both as primary and secondary combustion air is preferably preheated a substantial amount to speed up the ignition of the fuel. The proportion and size of the fuel burning compartments a and b in conjunction with the high combustion rate is such as to impart a considerable velocity to the combustion gas approaching and passing through openings 24 and 26 into intermediate furnace compartment 0. A change of direction of flow of 180 deg. forced upon the gas stream by partition walls 20 and 22 facilitates the separation of suspended slag droplets from the outgoing gas and also retards the passing of combustible from furnace chambers a and b. Since one object of my A characteristic feature of a forced circulation design is the use of orifices installed in the var ious circuits, preferably at the inlet to the water Wall surfaces, for the purpose of adjusting the flows of water through these circuits to variations in heat absorption and circuit flow resistance. Accordingly in the preferred embodiment illustrated in Figs. 1 and 2 orifice plates (not shown) are installed in the tube ends of side water walls In and I2 at their junction with headers O and in the tube ends of front wall l4 and floor [8 at their junction with header P. This provision insures a controlled supply of cooling water to all circuits in proportion to length and heat absorption and greatly contributes to elimination of furnace tube failures at high combustion rate operation by permitting the use of small diameter thin wall tubes. 7

Solid fuel in relatively finely divided form (crushed or pulverized) and mixed with carrier or invention is the separation of liquid slag from the furnace gas primarily by impinging of the gas upon the furnace walls, a high velocity of the furnace gas passing through the furnace compartments a, b and c is essential, so that the combined action of impact and of change in direction of flow causes effective separation of the liquid slag droplets and solid particles from the gas stream.

During operation the Vertical furnace walls l4, I6, 20 and 22 will be covered with a thin layer of liquid slag due to the turbulence created by rotative discharge of fuel and the secondary air streams issuing from the corners adjacent the burners and the high temperature existing in compartments a and b. The portion of wall 12 and wall 22 enclosing in part compartment a and 0 respectively and the portion of wall [0 and wall 20 enclosing in part compartments 1) and 0 respectively are as set forth above, primarily instrumental in collecting ash by impingement of the gas stream upon their surfaces. This accumulation of ash is enhanced by centrifugal action due to change in direction of the gas flow.

While the gas streams are entering compartment 0 an upwardly change in direction of flow is imparted causing the stream to enter through gas discharge opening IS th upper portion B of the furnace chamber where the exposed water walls effectively cool the gas before it proceeds to flow through ga discharge opening 31 and over heating surfaces C, D, E, F and G giving up heat to these heat absorbing surfaces and leaving the steam generating unit by way of gas duct X. By adjusting dampers S the gas flow over superheater E is controlled, thereby serving as a means for regulating superheated steam temperature.

With the described method of fuel burning combustion of the fuel particles will proceed at a high rate. Portions of the furnace walls l0, l2, I4, I6, 20 and 22 covered with heavy deposit of molten slag will also collect heavier unburned coal particles imbedded therein by impingement and centrifugal force. A stream of oxygen-rich furnace gas is continuously scrubbing these wall portions at high velocity and it has been demonstrated that under such conditions coal deposited on the molten slag surface will become completely burned. Since the lower portion of the furnace chamber will be operated at temperatures above the fusion temperature of the ash and collect most of the ash, the gas will be quite free of ash and sufficiently cooled before leaving the upper portion B of said furnace chamber through gas discharge opening 37, so that any ash still remaining in the furnace gas will be in a dry state. Dry ash which falls or is removed from the walls of the upper furnace chamber and accumulates at the bottom of compartments 0 will melt and also flow to the slag openings 40.

The Problem which invention solves Summarizing the problem, the high superheated steam temperature demanded by modern steam power generating practice requires in most cases a high furnace outlet gas temperature which may well equal or exceed the ash fusion temperature of many coals mined in the United States and other countries. In the operation of conventional pulverized coal fired furnaces where the gas leaving temperature exceeds the ash fusion point of the coal, a considerable percentage of the ash in the fuel is carried in suspension in the gas stream leaving the furnace in the form of tiny liquid ash droplets. Upon contact of these gases with convection heat absorbing surfaces, the aforesaid suspended slag droplets will solidify and deposit on said heat absorbing surfaces causing reduction in the transfer of heat from gases to slag covered surfaces and eventual costly shutdown of the steam generating unit for cleaning purposes.

This problem of eliminating slagging difficulties in convection heating surfaces is being approached from several directions. One line of attack is directed at the lowering of the furnace outlet gas temperature to a value safely below the ash fusion temperature of the coal. Other efforts are directed towards eliminating the greater portion if not all of the ash in the combustion chamber before the gases reach any convection heating surface, in other words disposing of the ash through an opening in the furnace bottom while the ash is still in a fluid state.

As previously indicated my invention is concerned with this latter approach.

While in the prior art, in working along this line, attempts have been made to arrive at a solution by various designs of slagging furnaces, these attempts have been only partially successful. Some conventional designs of slagging bottom furnaces of high capacity (in the neighborhood of 1,000,000 lb. steam per hour) have a relatively low overall heat release per cubic foot of furnace volume. The ratio of furnac heating surface to furnace volume being low in large units, the total furnace volume therefore is far out of proportion and inconsistent with economical furnace design practice. Gas velocity in the furnace is very low and the contact of combustion gas with furnace heating surface very limited.

If a higher heat liberation is attempted in such a conventional furnace the gas temperature leaving the furnace chamber increases beyond safe limits, unless furnace heat absorbing surfaces are added by further increasing the volume of the furnace or by adding more heating surface such as partition walls without increasing the volume of the furnace. Attempts in the prior art had been made to follow the latter course, however difficulties were encountered in sufliciently cooling these additional heating surfaces by water circulation. These surfaces therefore had to be covered with a refractory layer to prevent their destruction. My invention has overcome this difficulty by adapting a well known mode of forced circulation to a uniquely designed multiple chamber furnace characterized by an exceedingly high combustion rate in the primary portion of the furnace.

The proposed design allows the use of relatively small diameter thin Walled tubes in contrast to relatively large diameter heavy walled tubes which must be used in natural circulation design in order to obtain adequate fluid flow.

Some research was also directed at separating the ash suspended in the products of combustion by the action of centrifugal force. M invention not only uses centrifugal force but in addition it employs a separation method entirely new and novel in the art of burning slag forming fuels. This novel method consists in impingement of a high velocity gas stream, carryin slag droplets, on slag covered partition walls whereby said slag droplets having a low Viscosity by virtue of their high temperature splatter against the water cooled wall surface, adhere thereto due to the wall cooling effect lowering their viscosity, and form a viscous layer of running slag which continuously retains other slag droplets being hurled against them.

The aforesaid effect is obtained in my preferred embodiment of the invention by confining the high velocity gas stream between two walls and forcing this stream to abruptly change direction by substantially deg. entering another confining channel formed by furnace walls, thereby causing the heavier-than-gas slag droplets to separate and splatter against the wall surfaces circumscribing the turn.

It can be appreciated by anyone skilled in the art that these surf-aces take a considerable punishment and failure of these furnace walls Without protective refractory coating would occur in a relatively short time. However by employin the earlier-disclosed forced circulation means with such high combustion rate operation I have succeeded in cooling these trouble spots continually and effectively.

Advantages obtained by invention Among the advantages obtainable by the herein-disclosed unique organization of my invention, mention may be made of the following:

First advantaiga fhe principal characteristics of my invention such as the high velocity of the combustion gases (flowing from outer chambers a and 1) into central chamber and thence upwardly) together with the accompanying abrupt changes in direction, and consequent impingement of those gases on viscous and absorbing slag coatings (within the lower furnace portion A), results in an extremely high slag elimination ef ficiency thereby minimizing slagging accumulations on subsequent convection heating surfaces (within the upper furnace portion B and thereafter) Second advantage-The forced circulation water cooled furnace partition walls (including and 22) uniquely arranged to compress the gases into high velocity multiple streams (into and through central chamber 0) and also to form high combustion rate multiple firin chambers w-b makes it possible to increase the surfaceover-voluine ratio of the furnace to such a marked degree that considerable savings in first cost can be realized without incurring a risk of excessive maintenance cost on water walls when in operation.

Third advantage-Em unique combination of my invention in using forced circulation circuits in a high combustion rate, high velocity, ash slagging multiple furnace allows the designer complete freedom in arranging furnace heating surfaces such as partition walls and firing chambers for maximum ash separation efficiency without at the same time having to be concerned with and being limited by Water circulation problems which are ever present in the conventional natural circulation boiler.

Fourth advantage.Combining forced circulation circuits with slagging bottom, multiple chamber furnace affords the advantage, residing in the free use of furnace wall tubes of relatively small diameter permitting the employment of a proportionally thinner tube wall for a given steam pressure. In large diameter tubes having a relatively thick wall, such as required in natural circulation design, excessive thermal stresses resulting from a high temperature difference between outside and inside surface of the tubes, contribute greatly to failure of furnace tubes. A thinner tube wall results in a lower temperature difference reducing thermal stresses considerably and thereby minimizing tube failures. Small diameter tubes are permissible in forced circulation boilers because a positive and sufficient flow of water through the tubes is assured at all times regardless of size, shape and location.

Fifth advantage.ln addition forced circulation design in conjunction with my uniquely arranged partitioned multiple chamber furnace affords operation of the steam generating unit over a wide load range without undue difficulties and with high ash collection efliciency even at very low loads. Thus low-load operation is accomplished by firing fuel into one chamber only maintainin in the gas passages a high impinging velocity essential for efiicient ash removal and at the same time providing an adequate flow of fluid through all water wall heating surfaces including those in the unfired chambers thereby preventing possible damage to those water cooled tubes.

My design also affords a higher superheated steam temperature at low load, than can be obtained with conventional design, by reducin the effectiveness of some of the furnace heating surface when operating with one firing chamber only.

Sixth adcantage.-The herein-disclosed novel method of burning ash-containing fuel in crushed or pulverized form at high combustion rates in the presence of high velocity air streams and confining the resultant products of combustion between slag covered partition walls spaced and arranged so as to cause a high velocity flow and abrupt change of direction of the gas stream, not only results in the separation of ash droplets from said stream but also causes heavier unconsumed coal particles to be ejected and thrown against the sticky ash coating of the walls at the turn and then burned therein by the scrubbing action of the oxygen-rich gas stream. This effectively contributes to reduce combustible loss in fly ash thereby increasing the operating efficiency of the steam generator.

Seventh adt-antage.--Conventional slagging bottom furnaces cannot be operated efficiently with coarsely ground coal because of excessive combustible loss. My design however permits the burning of comparatively coarsely ground or crushed coal since my method of slag elimination by impinging on slag covered furnace and partition walls at the same time causes rapid consumption of coal particles hurled against and imbedded in said. slag coating and by a continuous consequent scrubbing with oxygen-rich gas.

Thus, from the foregoing, it can be appreciated by those skilled in the art how a long felt unfulfilled need in burning ash-containing finelydivided fuel in an efiicient and economical manner has been satisfied by the above described invention herein disclosed.

While I have illustrated and described a furnace with burners 32 and 34 firing in directions perpendicular to walls 10 and [2 these burners also can be arranged to fire in other ways; i. e., they may be arranged for corner firing as indicated by nozzles 36 and 38 of Fig. 2. Also it is apparent that other modifications may be made in the construction of the inventive apparatus shown such as changes in size, proportion and number of furnace compartments and partition walls.

Although the above described preferred embcdiment of my invention is presented as being applied to a forced circulation boiler, my uniquely arranged multi-chamber furnace, employing partition walls for the express purpose of increasing the heating surface as Well as forcing the combustion gas into flow paths to eliminate within the lower furnace most of the ashes carried by said gas, can also be applied to a steam generating unit designed for natural circulation.

Therefore it will be understood that while I have illustrated and described herein only one form of apparatus embodying the invention, those skilled in the art will appreciate that changes and modifications such as those aforementioned may be made in said disclosed apparatus without de+ parting from the spirit and scope of the invention as characterized by the claims.

What I claim is:

v 1. In apparatus for burning slag forming fuel, the combination of a furnace chamber of generally rectangular cross section defined by first and second side walls, front wall and rear wall, floor and roof; first and second spaced and generally vertical partitions extending crosswise of said chamber respectively from said first side wall and from said second side wall and dividing the chamber into three parallel compartments including a central compartment sandwiched between a front outer compartment and a rear outer compartment; means forming a gas discharge opening in the roof of said central compartment substantially coextensive with the width and length of that compartment; means adjacent the second-Wall side of said first partition forming a first unobstructed gas communication openin from said front outer compartment into said central compartment; means adjacent the first-wall side of said second partition forming a second unobstructed gas communication opening from said rear outer compartment into said central compartment which second opening is located on the opposite furnace chamber side from said first gas communication opening; means for introducing a high velocity stream of primary air and slag forming fuel in suspension into the portion of said front outer compartment that is remote from said first gas communication opening leading to the central compartment; means for introducing a high velocity stream of primary air and slag forming fuel in suspension into the portion of said rear outer compartment that is remote from said second gas communication opening leading to the central compartment; means for also introducing secondary air into the aforesaid fuel-entering portion of said front outer compartment and into the aforesaid fuel-entering portion of said rear outer compartment to assist in combustion of the said slag forming fuel burned in those two outer compartments, whereby there is created in said front outer compartment a high velocity stream of combustion products flowing towards said first gas communication opening and in said rear outer compartment a high velocity stream of combustion products flowing towards said second gas communication opening thus forcing those gaseous products of combustion to make turns of substantially 180 degrees as same pass into said central compartment; and inner exposed metallic surface lining the walls, roof, floor and partitions of the aforesaid three compartments, said central compartment containing only heat absorbing surface composed of said inner metallic wall surface, thereby affording an unobstructed passageway for the flow of gases therethrough.

2. In a steam generator, the combination of a first outer compartment and a second outer compartment, said compartments being spacedly positioned side by side; an inner compartment sandwiched between said first outer compartment and said second outer compartment, said inner compartment having a first wall in common with said first outer compartment, a second opposite wall in common with said second outer compartment, two opposing end walls and a roof; means in said first common wall and adjacent one of said two end walls for providing a lateral passage from said first outer compartment into said inner compartment; means in said second common wall and adjacent the other of said two end walls for providing a lateral passage from said second outer compartment into said inner compartment; nozzle means for discharging fuel and air streams for combustion into that portion of each of said outer compartments which is remote from said lateral passage thereof; inner exposed metallic surface lining the walls of the aforesaid inner compartment, said metallic wall surface constituting the only heating surface contained within said inner compartment, thereby affording an unobstructed passageway for the flow of gases therethrough; means forming a gas uptake passage through the roof of said inner compartment; and heat absorbing surfaces located in said gas up-take passage, whereby the stream of combustion gases being discharged into said up-take passage from said inner compartment is substantially free of temperature stratification while passing over said heat absorbing surfaces.

WARD S. PATTERSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,731,428 Lundgren Oct. 15, 1929 1,738,636 Caracristi Dec. 10, 1929 1,966,054 Wheeler, Jr July 10, 1934 2,204,350 Frisch June 11, 1940 2,258,235 Barnes Oct. 7, 1941 2,285,442 Kerr June 9, 1942 2,357,303 Kerr et al Sept. 5, 1944 FOREIGN PATENTS Number Country Date 71,857 Sweden Feb. 13, 1928 833,423 France July 18, 1938

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