US2594267A

US2594267A - Chemical furnace

Info

Publication number: US2594267A
Application number: US729538A
Authority: US
Inventors: Leslie S Wilcoxson
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1947-02-19
Filing date: 1947-02-19
Publication date: 1952-04-22
Anticipated expiration: 1969-04-22

Description

April 22, 1952 L. '5, W L QX ON 2,594,267

CHEMICAL FURNACE Filed Feb. 19, 1947 2 SHEETS-SHEET 1 INVENTOR LEslieJZUi/coxson 011W ATTORNEY April 22, 1952 L. s. WILCOXSON 2,594,267

CHEMICAL FURNACE Filed Feb. 19. 1947 2 SHEETS--SHEET 2 O O O O O O O O O O O O O O O O O O OOOOOOOOOOOOOOOOOO OOOOOOOOQOOOOOOOOOO ATTO R N EY Patented Apr. 22, 1952 CHEMICAL FURNACE Leslie S. Wilcoxson, Ridgewood, N. .L, assignor to The Babcock & Wilcox Company, New York, N. Y., a corporation of New Jersey Application February 19, 1947, Serial No. 729,538

Claims.

The present invention relates to improvements in the construction and arrangement of the heat absorbing surface of fluid heating units fired by a slag forming fuel, and more particularly, to the construction and arrangement of vapor superheating surface of vapor generating units designed for firing with a slag-forming fuel at combustion zone temperatures above the fuel ash fusion temperature.

In certain types of fluid heating units heretofore proposed incorporating multi-looped pendant non-draining type vapor superheating surface in advance of the main convection heated vapor generating tube banks to secure the desired high superheat temperatures, the vapor superheating surface has been subjected to a high temperature high velocity heating gas flow vertically or diagonally over the most vulnerable portions of the superheating surface, 1. e. the lower loops of the superheater tubes, wherein solids and liquids carried over from the liquid and vapor drum tend to collect. Protection of this tube area is particularly important during starting-up periods since such heating of the tube loops tend to cause slugs of collected liquid to be violently ejected against the hot upper tube metal sections of the superheater and cause warping of the superheater tubes. The likelihood of trouble in this tube area is enhanced where the lower tube loops are wholly or partly exposed to furnace radiation. Where incombustible ash constituents of the fuel are entrained in the heating gases, it is likely that a fluid or semi-fluid ash condition will exist at the superheater bottom loops in such installations. The effect of furnace radiation and heating gas temperature with such an arrangement is reduced on ascending the superheater elements. The temperature drop causes the molten ash to undergo transition to a plastic sticky state, causing it to adhere to and build up on the superheater tubes. This results in a nonuniform gas distribution over the superheater elements, and tends to cause overheating of'the tube metal and warpage and possibly failure of the superheater tubes. Slagging of the super: heater tubes increases the operating labor costs for cleaning.

The main object of my invention is the provision of an improved construction and arrangement of heat absorbing surface of the character described which insures substantially uniform and thermally efficient convection heat transfer conditions over substantially all of the pendant type vapor superheating surface while providing a relative low temperature low velocity heating gas flow over the lower tube loops of the superheater and effectively shielding the same from furnace radiation. A further and more specific object is the provision of a heating gas flow in a fluid heater of the character described which minimizes ash or slag deposition on the pendant vapor superheating surface and facilitates the gravity return to the combustion zone of ash or slag dropping from the superheater and depositing on a special gas deflecting baffle shielding the superheating surface from furnace radiation. A further specific object is the provision of a special nose bafile constructed and arranged below the vapor superheater to shield the superheater tubes from direct furnace radiation and to deflect the normal vertical flow of heating gases to a circuitous path upward and through an open fluid cooled furnace pass, to secure a uniform distribution of heating gases transversely across the superheater and consequently safe tube metal temperatures in and a longer superheater life. A further specific object is the provision of a relative arrangement of nose baffle and furnace walls insuring a high heating gas velocity past and parallel to the lower portion of the superheater tubes and a heating gas flow requiring a substantially degree turn before entering the superheater.

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 invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described a preferred embodiment of my invention.

Of the drawings:

Fig. l is a sectional elevation of a pulp liquor recovery unit constructed in accordance with my invention;

Fig. 2 is a horizontal section taken on the line -2-2 of Fig. 1;

Fig. 3 is a section taken on the line 3-3 of Fig. 1; and

Fig.- 4 is an enlarged sectional view of the baflle shown in Fig. 2.

While in its broader aspects, my invention is adapted for use in any fluid heating unit incorporating vapor superheating surface in the upper portion of the furnace chamber, it is particularly designed and especially useful in the construction of heat and chemical recovery units intended for the incineration of residual digester 3 liquor, known as black liquor, from soda and kraft pulp mills.

In the black liquor recovery unit illustrated, a vertically elongated furnace chamber of rectangular horizontal and vertical cross-section is defined by vertical front and rear walls I I, I2, vertical side walls [3, an inclined roof l4, and an inclined hearth [5. A smelt spout H5 at the lower end of the hearth discharges molten chemical ash into a dissolving tank ll. The front wall H is cooled by a row of water tubes [8 extending from a transverse bottom header I9 throughout the front wall height and thence along the roof I4 to a transverse steam and water drum arranged rearwardly of but adjacent to the plane of the rear furnace wall I2. A row of water tubes 2! extends from the header [9 along the hearth i5 and thence upwardly along the rear wall l2. Rows of water tubes 22 extend from longitudinal bottom headers 23 along the side walls I3 to corresponding top headers 24. The tubes I 3, 2| and 22 have their lowermost sections covered on their furnace sides by studs and initially plastic refractory in a well known manner to define a high temperature combustion zone and their remaining upper sections provided with fiat plate studs, of the type shown in Fig. 4, to form steam generating surface of greater heat absorbing capacity than that in the combustion zone section. All of the pressure parts are top supported from the recovery unit steelwork, as indicated in Fig. 1. A spray nozzle 26 is oscillatably mounted in the front wall II and in operation discharges a horizontal sheet of relatively coarse particles of black liquor against the furnace side and rear walls, drying thereon to a reactive char, which falls in lumps to the hearth, where it is burned by the addition of preheated combustion air supplied from a forced draft fan 21 through an air heater 28 and divided between primary air ports 29, secondary air ports 30, and tertiary air ports 3| at different levels in the furnace chamber side and rear walls.

With the apparatus described the black liquor is introduced at concentrations of 50-70% solids, the solids consisting of combustible organic compounds and incombustible sodium-sulphur compounds. The organic constituents are burned as a char in the combustion zone at temperatures in excess of the fusion temperature of the chemical ash residue. The inorganic chemicals are reduced by reaction with the char and discharged in a molten condition through the smelt spout. The remaining chemical ash passes upwardly in the furnace chamber as a chemical fume and molten particles entrained in the high temperature heating gases leaving the combustion zone. The entrained particles of molten chemical ash undergo transition through a plastic state to a final condition of dry chemical particles as the temperature of the carrier heating gases is progressively lowered by the heat absorbing surface in the gas flow path.

The steam and water drum 20 supports a plurality of depending

vertical banks

33 and 34 of closely spaced steam generating tubes arranged rearwardly of but adjacent to the plane of the rear wall 12 and connected at their lower ends to a transverse water drum 35. A depending water tube cooled baifie 36 between the

tube banks

33 and 34 separates serially connected downflow and upflow gas passes 31 and 38 respectively. The gas pass 31 receives heating gases from the furnace chamber through a gas outlet 39 in the plane of the rear wall l2 and subjacent to the steam and water drum 20, with the tube bank 33 extending across the entire gas outlet.

A second vertical baffle 40 extends upwardly along the rear side of the tube bank 34, defining the rear side of the pass 38 and the front side of a serially connected economizer pass 4|. A vertical bank of economizer tubes 42 connected to upper and lower

transverse headers

43 and 44 respectively is located in the pass 4|. Feed water is supplied to the header 44 and discharged from the upper header 43 through tubes 45 to the drum 20. The lower end of the economizer pass opens into a flue 46 containing the air heater 28 and leading to an induced draft fan 41, from which the gases are delivered to the usual gas treating apparatus (not shown) for the recovery of heat and chemicals.

To secure the desired superheat temperatures, a steam superheater of the pendant non-draining type and consisting of a bank of multi-looped tubes 50 having their forward and rear ends connected to external transverse steam inlet and outlet headers 5| and 52 respectively, is positioned in the uppermost portion of the furnace chamber In immediately in front of and extending across the heating gas outlet 39. The superheater headers and upper tube loops are externally supported, while the lower end portions of the superheater tubes are interconnected by spacer loops, as indicated in Fig. 1. The superheater tubes have a front to rear and side to side spacing greater than the spacing of the tubes in the

boiler banks

33 and 34.

In accordance with my invention, the superheater 50 and its associated parts are proportioned and arranged to insure a heating gas flow at a substantially uniform temperature and gas velocity transversely of all portions of the superheater tubes except across the lower loop portions of the tubes, across which the gas flow is maintained as low as possible and at a low temperature. For this purpose the lower superheater tube loops are all arranged at a level or levels below the lower end of the heatin gas outlet 39 and a nose bafile 55 arranged to pro- J'ect forwardly from the plane of the rear wall l2 below the superheater. The nose bafile is defined by a row of bent tubes 56 having their lower ends connected to a transverse header 51 arranged rearwardly adjacent the rear wall i2 at a level slightly below the water drum 35 and connected thereto. As shown in Fig. 4, the tubes 56 have fiat plate studs 58 diametrically welded thereto and short cylindrical studs 59 positioned along the rear side of the tubes. The

plates

58 and 59 cooperate to hold in position a layer of initially plastic refractory material 60. The bafile so defined extends from the upper end of the rear wall [2 to form a relatively steep portion 550,, and is then forwardly inclined at a lesser angle to define the lower surface 55b of the nose baflle. The bafile section 55b extends forwardly va substantial portion of the furnace depth as hereinafter described, and preferably to a point at least in vertical alignment with the foremost tube length of the superheater 50 to shield the lower tube loops from furnace radiation. The bafiie has a rounded nose portion 550 at its forward end and is then rearwardly inclined at an angle greater than the natural angle of repose of dry chemical ash depositing thereon. The upper bafile surface 5501 thereby formed extends rearwardly to the tube bank 33 and thence upwardly for a short distance parallel to the superheater tubes, terminating at a level above the uppermost lower superheater loop to define the lower end of the gas outlet. The tubes 56 continue therebeyond without studs or stud plates to the steam and water drum and form in effect the first row of the boiler tube bank 33. In the embodiment illustrated the lower bafile surface 552) is arranged at an angle of degrees, while the upper baiile surface 55d is arranged at an angle of 50 degrees.

The heating gases generated by the char burnin on the hearth and volatiles burning in suspension in the lower part of the furnace chamber leave the lower part of the furnace chamber as a substantially vertical gas stream occupying the entire chamber cross-sectional area. The nose bailie cooperates with the furnace front and side walls to form in effect an orifice 6| in the gas flow path substantially reducing the available gas flow area and thus correspondingly increase the heating gas velocity at a level slightly below the level and forwardly of the lower superheater tube loops. In accordance with my invention the nose baffle should extend forwardly at least one-third of the furnace depth and in the embodiment illustrated the nose bafiie extends horizontally approximately 40% of the furnace depth to secure the desired gas velocity condition and direction. With these proportions the orifice 6! will have approximately the same flow area as the heating gas outlet 39 and be substantially less than the gas fiow area at the entrance to the superheater 50, i. e. the distance between the nose of the bafiie 55 and the roof I4 will be considerably greater than the distance between the nose and front wall H.

In conjunction with the described nose baiile arrangement the front wall H is extended up-- wardly in a vertical plane to a level approximating the level of the upper end of the gas outlet 39 so that the roof I4 will have only a slight inclination toward the steam and water drum 20. This wall formation thus provides a vertically elongated water cooled open pass 82 throughout the height of the superheater tubes and forwardly thereof, providing a gas flow space from which the heating gases make an approximately 90 degree turn to enter the superheater section. The substantial amount of heat absorbing area present in the open pass reduces the temperature of the heating gases and entrained chemical ash particles.

To further cool the gases and entrained chemical particles prior to the gases entering the superheater section and more closely spaced tubes of the boiler banks, a slag screen 65 of substantial depth is arranged along the lower side of the nose baflle and forwardly of the superheater. The sla screen is formed by staggered rows of steam generating tubes with the tubes in each row having twice the side-by-side Spacing of the superheater tubes. Six rows of screen tubes are illustrated, with the two foremost rows 65a formed by bending alternate rear wall tubes 2i out of alignment in planes parallel to the nose baflie lower surface 5% and thence upwardly parallel to the superheater positing on the convection surface.

around each element of the superheater is obtained at all times because the upper superheater tube loops are nested into the sections of the screen tubes extending below the roof and a refractory baffle 10 is arranged to enclose the horizontally inclined roof portions of the screen tubes and associated superheater tube loops. This arrangement fixes the lateral spacing of the superheater tubes so that the gas flow spaces between them will be maintained uniform.

In apparatus of the character described, the relatively low fusion temperature of the included incombustible fuel constituents and the necessity of maintaining a combustion zone temperature above this fusion temperature makes the problem of maintaining clean convection heat transfer surfaces of prime importance, not only from the standpoint of heat transfer rates, but also with respect to the operating load on the induced draft fan. The entrained molten chemical ash particles pass through a plastic state to a final condition of dry chemical particles in the gas streamas the gas temperature is progressively lowered. The chemical ash from black liquor even when in a dry condition has a peculiar tendency to adhere and cake on the convection heat transfer surfaces, insulating those surfaces against the transfer of heat and restricting the gas flow area. The present apparatus is designed and operated to minimize the chemical ash carryover and to facilitate the removal of any entrained chemical particles de- The introduction of the liquid fuel in a relatively coarse stream and deposition on the walls coupled with a control of the combustion air to limit the volume of gases generated results in a low carryover of both chemical fumes and entrained ash particles. The desired reduction in the temperature of the heating gases and entrained ash particles is secured by the water cooled furnace walls and the slag screen tubes before the gases contact with the more closely spaced superheater and boiler tubes. The gas deflecting action of the lower surface of the nose baffle causes much of the gases to first'flow over the lower inclined section of the screen tube bank prior to entering the orifice 6i and open pass 62 and to be directed towards the front wall of the furnace. The velocity increase given to the furnace gase by the nose bafile restriction causes the gases to flow upwardly throughout the open pass 62 and to make a substantially degree turn before flowing transversely across the upper vertical section of the screen tubes. The gas deflecting action of the nose bafile causes the entrained chemical particles to be deflected away from the slag screen and toward the water cooled front wall from which deposited ash particles return by gravity to the lower part of the furnace chamber.

The described construction of the upper part of the furnace chamber results in a gas flow across the superheater tube lengths which is substantially uniform in temperature and velocity, except adjacent the lower superheater tube loops. In an operating installation of the character described, the velocity, direction and temperature of the gases were measured at different levels in front of the superheater under normal operating conditions, as indicated in Fig. 1 by the arrows at the points a, b, c, d and e, the direction of the arrows indicating the gas direction at each point, and the length of the arrows indicating the relative gas velocities. It was found that a stagnant Point Dog. F

The substantially uniform temperature and velocity of the gases over the superheater except its lower tube loops coupled with a steam flow through the superheater parallel to the gas flow insures that the superheater tube metal temperature will be kept at a minimum for the designed superheat temperatures, lightening the duty on the uperheater and insuring safety of operation and a long superheater tube life. The vertical arrangement of the superheater tubes minimizes ash deposits on the superheater and facilitates ash dislodgement during the periodic cleaning operations. The chemical ash drops onto the upper surface of the nose baffle and the slope thereof causes the deposited material to flow or slide downwardly by gravity towards the center of the furnace. The low gas temperature and practically zero gas velocity of the gases along the upper surface of the baffle facilitates the gravity discharge of the deposited chemical particles in a dry condition.

With the described contruction the heating gase leaving the gas outlet 39 will be relatively clean and flow downwardly along the, tube bank 33. The bottom of the gas pass 31 is provided with a large gas turning chamber 80 formed by the rear side of the nose bafile. The gases makr a low velocity turn prior to entering the gas pass 38. Any ash particles still entrained in the gasetend to be eliminated in the chamber 80 and pre-- cipitate into the dust hopper 8| from which the can be readily returned to the recovery systen. A baffle 82 depending from the drum 35 and tel-- minating short of the hopper bottom prevents gas flow around the lower side of the drum 3:: while permitting ash separated in the gas passes 31 and 38 to drop into the hopper 8|. The gases flow upwardly through the pass 38 and downwardly through the economizer pass 4| and air heater tubes to the induced draft fan 41.

While in accordance with the provisions of the statutes I have illustrated and described herein the best form of the invention now known to me, those skilled in the art will understand that changes may be made in the formof the apparatus disclosed without departing from the spirit of the invention covered by my claims, and that certain features of my invention may sometimes be used to advantage without a corresponding use of other features.

I claim:

1. Apparatus for the recovery of chemicals and heat from waste liquor containing inorganic chemicals and combustible organic matter comprising walls defining a vertically elongated furnace chamber of substantially rectangular crosssection and having a heating gas outlet in the rear upper portion thereof, means for spraying said waste liquor into said furnace chamber, a hearth at the bottom of said furnace chamber on which combustion of the organic matter is completed and the inorganic chemicals are smelted, a smelt outlet adjacent said hearth, the furnace chamber front wall extending to substantially the same height as the upper end of said gas outlet, a bank of closely spaced vapor generating tubes at the rear side of said gas outlet, a group of looped pendant vapor superheating tubes in the uppermost portion of said furnace chamber in front of said gas outlet and separated from said front wall by an open pass extending substantially the full length of said superheater, an inwardly tapering nose bafile projecting forwardly from the rear wall of said furnace chamber to a point above said hearth substantially in vertical alignment with the foremost superheater tube loop and with its rearwardly inclined upper surface arranged subjacent to the lower loops of said vapor superheating tubes and shielding the same from radiation from the lower part of the furnace chamber, and said nose bafile and furnace chamber front wall collectively defining a restricted gas flow passage at one side of and subjacent to the forward tube lengths of said superheater.

2. Apparatus for the recovery of chemicals and heat from waste liquor containing inorganic chemicals and combustible organic matter comprising walls defining a vertically elongated furnace chamber of substantially rectangular crosssection and having a heating gas outlet in the rear upper portion thereof, means for spraying said waste liquor into said furnace chamber, a hearth at the bottom of said furnace chamber on which combustion of the organic matter is completed and the inorganic chemicals are smelted, a smelt outlet adjacent said hearth, the furnace chamber front wall extending to substantially the same height as the upper end of said gas outlet, a vertical bank of vapor generating tubes at the rear side of said gas outlet, a group of multilooped pendant vapor superheating tubes in the uppermost portion of said furnace chamber in front of said gas outlet and separated from said front wall by an open pass of substantially uniform gas flow area extending substantially the full length of said superheater, an inwardly tapering nose baflle projecting forwardly from the rear wall of said furnace chamber with its rearwardly inclined upper surface arranged subjacent to the lower loops of said vapor superheating tubes and shielding the same from radiation from the lower part of the furnace chamher, and said nose bafile projecting over said hearth at least one-third the depth of said furnace chamber and cooperating with said furnace chamber front wall to define a restricted gas flow passage at one side of and subjacent to the forward tube lengths of said superheater.

3. Apparatus for the recovery of chemicals and heat from waste liquor containing inorganic chemicals and combustible organic matter comprising walls defining a vertically elongated furnace chamber of substantially rectangular crosssection and having a heating gas outlet in the rear upper portion thereof, vapor generating tubes lining the front and side walls of said furnace chamber substantially throughout their vertical extent, means for spraying said waste liquor into said furnace chamber, a hearth at the bottom of said furnace chamber on which combustion of the organic matter is completed and the inorganic chemicals are smelted, a smelt outlet adjacent said hearth, said furnace chamber front wall extending to substantially the same height as the upper end of said gas oulet, a vertical bank of vapor generating tubes extending across the rear side of said gas outlet, a group of multi-looped pendant vapor superheating tubes in the uppermost portion of said furnace chamber in front of said gas outlet and separated from said front Wall by an open pass of substantially uniform gas flow area extending substantially the full length of said superheater, an inwardly tapering nose baffle projecting forwardly from the rear wall of said furnace chamber to a point above said hearth substantially in vertical alignment with the foremost tube loop and with its rearwardly inclined upper surface arranged subjacent to the lower loops of said vapor superheating tubes and shielding the same from radiation from the lower part of the furnace chamber, said nose bafiie cooperating with said front wall to define a restricted gas flow passage at one side of and subjacent to the forward tube lengths of said superheater and having a gas flow area substantially less than the gas flow area at the entrance to said superheater.

4. Apparatus for the recovery of chemicals and heat from waste liquor containing inorganic chemicals and combustible organic matter comprising walls defining a vertically elongated furnace chamber of substantially rectangular crosssection and having a heating gas outlet in the rear upper portion thereof, means for spraying said waste liquor into said furnace chamber, a hearth at the bottom of said furnace chamber on which combustion of the organic matter is completed, and the inorganic chemicals are smelted, a smelt outlet adjacent said hearth, the furnace chamber front Wall extending to substantially the same height as the upper end of said gas outlet, a vertical bank of vapor generating tubes extending across the rear side of said gas outlet, a group of looped pendant vapor superheating tubes in the uppermost portion of said furnace chamber in front of said gas outlet and separated from said front wall by an open pass extending substantially the full length of said superheater, an inwardly tapering nose baiilc projecting forwardly from the rear wall of said furnace chamber with its rearwardly inclined upper surface arranged subjacent to the lower loops of said vapor superheating tubes and shielding the same from radiation from the lower part of the furnace chamber, and a screen bank of vapor generating tubes extending along the forwardly inclined lower surface of said nose baffie and upwardly along the front side of said vapor superheating tubes, said nose bafile projecting over said hearth at least one-third the depth of said furnace chamber and cooperating with said front wall to define a restricted gas flow passage at one side of and subjacent to the forward tube lengths of said superheater.

5. Apparatus for the recovery of chemicals and heat from waste liquor containing inorganic chemicals and combustible organic matter comprising walls defining a vertically elongated furnace chamber of substantially rectangular crosssection and having a heating gas outlet in the rear upper portion thereof, vapor generating tubes lining the front and side walls of said furnace chamber substantially throughout their vertical extent, means for spraying said waste liquor into said furnace chamber, a hearth at the bottom of said furnace chamber on which combustion of the organic matter is completed and the inorganic chemicals are smelted, a smelt outlet adjacent said hearth, said furnace chamber front wall extending to substantially the same height as the upper end of said gas outlet, a vertical bank of vapor generating tubes extending across the rear side of said gas outlet, a group of rearwardly stepped multi-looped pendant vapor superheating tubes in the uppermost portion of said furnace chamber in front of said gas outlet and separated from said front wall by an open pass of substantially uniform gas flow area extending substantially the full length of said superheater, an inwardly tapering nose bailie projecting forwardly from the rear wall of said furnace chamber to a point over said hearth substantially in vertical alignment with the foremost superheater tube loop and with its rearwardly inclined upper surface arranged subjacent to the lower loops of said vapor superheating tubes and shielding the same from radiation from the lower part of the furnace chamber, and a screen bank of vapor generating tubes extending along the forwardly inclined lower surface of said nose bafiie and upwardly along the front side of said vapor superheating tubes, said nose bafile and screen collectively projecting across a major portion of the depth of said furnace chamber over said hearth and cooperating with said front wall to define a restricted gas flow passage at one side of and subjacent to the forward tube lengths of said superheater.

LESLIE S. W'ILCOXSON.

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

UNITED STATES PATENTS Number Name Date 1,983,364 Gordon Dec. 4, 1934 2,023,031 Rohrer Dec. 3, 1935 2,252,071 Gay Aug. 12, 1941 2,258,467 Owens Oct. 7, 1941 2,284,601 Shellenberger et a1. May 26, 1942 2,293,833 Kuhner et al Aug. 25, 1942 2,313,251 Marshall Mar. 9, 1943 2,416,053 Grossman Feb. 18, 1947 2,416,462 Wilcoxson Feb. 25, 1947