WO1998043017A1

WO1998043017A1 - Recovery boiler for combustion of waste liquors

Info

Publication number: WO1998043017A1
Application number: PCT/FI1998/000246
Authority: WO
Inventors: Lasse Mannola; Krister Sannholm; Kari Saviharju; Liisa Simonen; Olavi Tervo; Tapani Heikkilä; Jorma Simonen
Original assignee: Ahlstrom Machinery Oy
Priority date: 1997-03-21
Filing date: 1998-03-20
Publication date: 1998-10-01

Abstract

A recovery boiler is provided that may be made larger than conventional soda recovery boilers which burn black liquor to produce energy and to recover chemicals for the kraft pulping process, by providing a particular substantially rectangular configuration of the furnace of the boiler. The ratio of the depth (defined by the side walls) and the width (defined by the front or rear wall) of the furnace is at least 1.15 to 1 (e.g. between about 1.25-1.5 to 1), and a header is provided that is substantially parallel to the side walls of the furnace and which feeds the water tubes at the bottom of the furnace. A steam drum, which is part of the water/steam circulation and which feeds the header using a plurality of downtake pipes, is preferably positioned so that the dimension of elongation of the steam drum is substantially parallel to the front wall of the furnace, and so that the steam drum is behind the rear wall. Recovery boilers may be constructed that effectively burn more than 4,000 (preferably about 5,000) tons of dry matter per day.

Description

RECOVERY BOILER FOR COMBUSTION OF WASTE LIQUORS

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a recovery boiler for burning waste liquors in the chemical pulp industry, for example a soda boiler for burning black liquor. The boiler furnace typically is defined by a bottom, front wall, rear wall, and two side walls, all formed by water-cooled tubes connected to the water/vapor circulation of the boiler. A significant part of the furnish supplied to the delignification process ends up as waste liquor containing, in addition to organic material, the inorganic chemicals required for delignification. The organic part of the waste liquor is turned into energy in a recovery boiler, the type of which depends on the pulping process used. At the same time, the cooking chemicals are turned into a reusable form. The most common pulping method is the kraft or sulphate process, in which the waste liquor is black liquor.

Black liquor is injected as small drops into the furnace of the recovery boiler. In the hot furnace, water vapor, volatile components of the dry matter, and finally components gasifiable from the dry matter, are released from the drops. The gases combust, releasing heat to the heat exchange surfaces of the boiler, and are discharged from the upper part of the boiler. The inorganic material of the black liquor accumulates in a melt bed on the bottom of the furnace, from which it is fed through different treatment stages back to the cooking process.

The furnace of a modern recovery boiler, such as a soda boiler, used for combustion of black liquor, has a structure formed by a water-cooled bottom and water-cooled side walls made of a compact membrane structure, so that a water-steam mixture under pressure flows in the tubes. The water-steam generated in this way is superheated downstream of the furnace, typically in a conventional superheater located in the shield of a " nose" above the furnace . In the superheater the heat of the flue gases generated during combustion is recovered. Typically, a boiler bank and an economizer serve as the after-heat surface in a soda boiler, and after the superheater the flue gases are directed into the boiler bank and economizer. However, it is possible to have no boiler bank at all in a boiler. Also, instead of an economizer a pre-heater of combustion air, and a direct-contact evaporator of combustion liquor, may be used as an after-heat surface. The generated, high- pressure steam is typically further used by directing it to a steam turbine, so that electricity and process steam needed at the mill are generated.

The lower section of the furnace and the bottom of the soda recovery boiler are made of water-cooled tubes, which constitute part of the pressurized section of the boiler.

Due to the structure of the bottom and the lower section of the furnace, there is natural circulation of water in the bottom tubes, i.e., the circulation is effected by a difference in density. The boiler water is thus led at a high pressure and at a temperature of, for example, about 300 degrees C to a distributing pipe (header) below the bottom of the furnace, from which the water is distributed into the bottom and side wall tubes. In the lower section of the furnace, the water flows first nearly horizontally or obliquely upwardly in the bottom tubes towards the walls, and then further upwardly through the wall tubes to the upper section of the boiler.

Since the object of the combustion of waste liquor is, in addition to the production of energy, transforming the cooking chemicals into a reusable form, it is important to choose and control the process conditions in the furnace carefully. For a soda recovery boiler this means, regardless of the supplier of the apparatus, a structure in which the bottom of the furnace is slightly inclined. On the surface of the bottom, a carbon-containing bed is formed by controlling the feed of waste liquor into the furnace and the feed of combustion air. The sulphur in the black liquor may be reduced into sulphide in the carbon bed, and the rest of the alkalis of the liquor are transformed (for the most part) into carbonates. The above- mentioned chemicals form a "melt" which flows, due to the compact structure of the bottom, through melt spouts out of the furnace into a dissolving tank, and further processing, before the chemicals can be reused in the pulping processes .

The inclination of the bottom of a soda boiler is slight, typically only a few degrees, e.g. at most about 10 degrees, which is small considering the size of the furnace; e.g. the cross-section of the furnace is typically a square up to about 14 meters x 14 meters in dimension. The inclination extends either from the front wall of the furnace to the rear wall thereof (the after-heat surfaces of the boiler, such as economizers, are located on the side of the rear wall) , from the rear wall to the front wall, or from the front and rear walls to the middle, so that the bottom is shaped as an upwardly directed V. The side walls are primarily vertical in the area of the lower part of the furnace . There are also structures in which the bottom part is wider than the upper part of the furnace, having a shape which may deviate from rectangular or quadratic, for example hexagonal; even in these cases, however, the inclination is as described above.

Good water circulation is essential for soda boiler operation, because otherwise the water-containing tubes may overheat and break. Breaking is then followed by a water leakage to the furnace, which may, in turn, result in a melt explosion causing property damage, great economic loss, and physical injury.

The reason for the conventional inclination of the bottom of the furnace is the feeding of water in the boiler, and also a desire to prevent the generation of a large melt reserve under the bed, which would, for example, make the cleaning and checking of the bottom more difficult during a shut-down. The water circulation of soda boilers. as well as other boilers burning waste liquors, typically function as a natural circulation, requiring that there be one or more steam drums in the boiler. The drum/s of the soda boiler is/are arranged crosswise relative to the boiler, and so are the headers feeding water from the drum to the front and rear walls. Consequently, the inclination of the bottom may, as a practical matter, be effected only in a manner determined by the header/s, i.e. along the longitudinal direction of the boiler. The investment costs of a chemical pulp mill may be decreased per ton of production capacity if it is possible to increase the size of the main components. The combustion boiler of the waste liquor is the most expensive individual component of a chemical pulp mill, and the capacity thereof has a significant influence on the costs. In fact, the producers of soda boilers seek to increase the size of such boilers to 3,000 - 4,000 tons of dry matter per day (the combustion capacity of black liquor in soda boilers) . The increase in capacity is limited, however, because of the need to ensure the circulation of water in the bottom of the furnace, which becomes more difficult if the dimensions of an only slightly inclined bottom are increased. Also, the control of the combustion process becomes more difficult, because a uniform feed of the liquor over a large bottom is problematic. In addition, there may be difficulties in maintaining an even temperature profile. The sootblowers of the boiler have to be very long, too, if the width of the boiler is increased.

To ensure proper water circulation in the bottom of the furnace, a separate bottom has been suggested, in which the circulation of the water is provided by a pump. Also a structure for ensuring bottom water circulation using a circulation pump is presently in use. Also, a structure is known in which the tubes of the bottom have spiral inner surfaces. The last-mentioned alternative does not improve the water circulation but does help prevent the overheating of the wall of the bottom tubes on the furnace side by improving the mixing of the generated steam and the water within the tubes.

The invention seeks to overcome the above-mentioned problems and to provide a structure for a recovery boiler that is especially suitable for boilers having a high liquor-burning capacity, i.e. above 4,000 tons of dry matter per day.

The furnace of the invention has a horizontal cross- section in the form of a rectangle, the ratio of the depth and width thereof being at least 1.15 to 1. Also, at least one header of the furnace bottom into which water is first led from the downtake pipes is parallel to the boiler, i.e. parallel to the side walls of the furnace.

The term "width of the furnace" means the horizontal length of the front wall of the furnace, and the term

"depth" means the horizontal length of a side wall of the furnace. The cross-section of the furnace is for example 14 meters (width) by 18 meters (depth) .

The capacity of a soda boiler may be increased in an economical manner to a sufficient extent if the furnace is constructed as a rectangle in accordance with the invention in such a way that the width of the furnace is increased little, but instead, the depth, i.e. the distance between the front and rear walls of the furnace, is increased significantly. The ratio of the adjacent sides of the rectangle is at least 1.20 to 1, typically about 1.25 - 1.5 to 1. The feed of the waste liquor into the furnace may be effected in such a way that the majority of the liquor and combustion air is fed from the side walls, so that the liquor and air are also evenly distributed to the middle of the furnace. The combustion in the furnace is uniform and the combustion temperature profile is also desirable.

In a boiler according to the invention, efficient water circulation in the boiler tubes is effected in such a way that at least one of the distribution headers feeding water from downtake pipes to the bottom of the furnace is parallel to the boiler, i.e. parallel to the side walls of the furnace. At least the first header located downstream of the downtake pipes (that is, water is flowing first to this header from the downflow pipes) is parallel to the side walls of the furnace. The bottom may be inclined in the direction of the shorter side of the furnace, in other words the inclination/s is/are between the side walls, so that the bottom tubes are parallel with the front and rear walls. The purpose of this inclination is to ensure the water circulation in the tubes of the bottom of the furnace, so that the steam generated in the tubes does not accumulate in the bottom tubes on the side of the furnace, which could cause overheating and breaking of the tubes. The bottom may be inclined from one side wall to the other, so that the angle of inclination is below about 15 degrees, typically between about 2 - 5 degrees . The bottom may also be a decanting bottom having a shape of a low-gradient upwardly directed V as a vertical cross-section, and the bottom-most part of which is parallel with the side walls of the furnace. The bottom may also have a horizontal flat part which is short compared with the entire dimensions, facilitating the attachment of a water feed tube, so that the bottom has the shape of a cut, low-gradient upwardly directed V as a vertical cross-section. The water feeding system preferably has a T shape. The bottom may also have another shape, for example a U shape.

The discharge of the melt from the boiler according to the invention is preferably effected from the side walls from both sides of the boiler, or from one side only, in such a way that the dissolving tank is on one side of the boiler, or on both sides thereof.

Also the drum of the water/steam circulation may be positioned parallel to the boiler, i.e. in such a way that the longitudinal axis of the drum is parallel to the side walls of the furnace (although for considerations relating only to the operation of the boiler, the drum may just as well be transverse relative to the boiler, as is conventional in recovery boilers) . In accordance with a preferred embodiment of the invention, there is at least one water-steam drum positioned behind the rear wall of the furnace, preferably approximately above the boiler bank and parallel to the front and rear wall of the boiler. The downtake pipes, through which water is led from the drum into the water tubes of the bottom of the boiler, may be positioned adjacent the sides of the boiler, parallel to the side walls. The water may first flow from the drum through the boiler bank, and from the boiler bank further into the downtake pipes disposed on the sides of the boiler. On the sides of the boiler, there are at least two downtake pipes, one on either side. If needed, water may also flow from the drum through downtake pipes disposed in one or more other manners . In the boiler arrangement according to the invention, the economizers having a vertical bank of heat transfer tubes are preferably disposed at a level which is substantially lower than the boiler bank. The drum of the water/steam circulation of the boiler may be disposed vertically above the economizers. Preferably, the drum is located above the flue channel that is between the heat exchange surfaces of the boiler bank and the heat exchange surfaces of the first economizer. This new position of the economizers facilitates the installation of the downtake pipes through which water flow from the drum into the bottom and wall pipes of the furnace. Above the economizers the downtake pipes extend directly downwardly, and in the proximity of the upper surface of the economizers they are turned to the sides of the boiler so that they are parallel to the side walls of the boiler.

According to the present invention a recovery boiler (particularly a soda boiler for burning black liquid) comprises the following: A furnace defined by a bottom, a front wall, two side walls, and a rear wall, the furnace having a width defined by the front or rear wall, and a depth defined by one of the side walls, the side walls being substantially parallel to each other, and substantially transverse to the front and rear walls. The bottom and the walls formed by water-cooled tubes. A water/steam circulation comprising at least one steam drum elongated in a first dimension, a plurality of downtake pipes, and at least one header to which the downtake pipes are connected. The downtake pipes and the at least one header being operatively connected to the water-cooled tubes so that fluid from the drum is circulated to the downtake pipes, then to the at least one header, and then to and through the water-cooled tubes. The furnace having a substantially rectangular configuration in which the ratio of the depth to the width thereof is at least 1.15 to 1 (e.g. between about 1.25-1.5 to 1) . And, wherein at least one of the headers is substantially parallel to the side walls of the furnace.

The water/steam circulation preferably further comprises heat exchange surfaces of a boiler bank and economizers, the heat transfer surfaces positioned in such a way that the economizers are at a substantially lower vertical level than the boiler bank. The economizers comprise a vertical bank of tubes. The dimension of elongation of the steam drum is preferably substantially parallel to the front and rear walls of the furnace, and the steam drum may be positioned on the opposite side of the rear wall from the front wall (that is behind the rear wall) . Preferably the steam drum may be positioned vertically above the economizers.

Preferably the soda boiler furnace is dimensioned (e.g. having dimensions of about 14 meters by 18 meters and a height of at least about 40 meters, e.g. about 50 meters) to effectively burn more than 4,000 tons of dry matter per day, e.g. about 5,000 tons/day.

The boiler preferably has a plurality of first nozzles for introducing waste liquor into the furnace, and a plurality of second nozzles for introducing combustion into the furnace. The majority (typically at least about 60%, e.g. about 75%) of the first and second nozzles are preferably disposed in the furnace side walls.

The furnace bottom may be an upwardly directed low- gradient (that is with an angle of inclination of less than 15 degrees to the horizontal) V having a bottom most portion that is substantially parallel to the side walls. The bottom most portion of the V may be an apex of the V, or the bottom-most portion may be substantially flat for ease of manufacture. Alternatively the furnace bottom may slant downwardly from one side to the other at an angle of inclination of less than 15 degrees to the horizontal. In each case the preferred angle of inclination of the water tubes to the horizontal is between about 2-5 degrees.

It is the primary object of the present invention to provide an enhanced construction of a recovery boiler that has a number of advantages, including the ability to scale up the size of a recovery boiler without encountering the problems faced in the prior art with respect to effective circulation of the water through the furnace bottom and walls. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 schematically illustrates a longitudinal cross-sectional view of a conventional soda boiler; FIGURE 2 is a schematic top plan view of the prior art boiler of FIGURE 1;

FIGURES 3a and 3b schematically illustrate an operating principle of the water circulation in the lower part of the furnace in the conventional soda recovery boiler of FIGURES 1 and 2 ;

FIGURES 4a-4c schematically illustrate an operating principle of the water circulation in the lower part of the furnace in several embodiments of recovery boilers according to the invention; FIGURE 5 schematically illustrates the structure of an exemplary bottom tube system of the furnace in a recovery boiler according to the invention;

FIGURE 6 is a schematic side elevational view of a recovery boiler according to the invention; and

FIGURE 7 is a schematic detail elevational view of a boiler according to FIGURE 6 viewed at arrows 7-7 of FIGURE 6.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGURE 1 schematically illustrates the structure of a conventional soda boiler comprising a furnace 1 defined by water tube walls comprising a front wall 2, two side walls 3, and a rear wall 4, and also comprising a bottom 5, all formed by water tubes. Combustion air is fed into the furnace 1 from several different levels, and waste liquor from nozzles 6 is also fed into the furnace 1. During combustion, a melt bed is formed on the bottom 5 of the furnace 1, from which the melt is removed through a melt spout 7 disposed in the lower part of the furnace 1.

Above the furnace 1 are heat recovery surfaces, i.e. superheaters 8, a boiler bank 9, and economizers 10, in which the heat of the flue gases generated in the furnace 1 is recovered as steam. In the superheaters 8, saturated steam is transformed into steam having a higher temperature. In the boiler banks 9, the water is boiled partly into steam, and in the economizers 10, the water is heated by the flue gases prior to leading the water into the vaporizing part 9 and the superheating parts 8 of the boiler.

The water/steam circulation of the boiler is provided by natural circulation, so that the water/steam mixture generated in the water tubes of the walls 2-4 and the bottom 5 of the furnace 1 rises through collector tubes into a steam drum 11 positioned crosswise relative to the boiler, i.e. parallel to the front wall 2. From the steam drum 11, hot water flows through downtake pipes 12 into the header 13 for the water tubes of the bottom 5. The water flow is divided by header 13 into the water tubes of the bottom 5 and further into the water tubes of the walls 2-4.

The schematic illustration of FIGURE 2 clarifies the function of the boiler and the relationship of the different parts thereof to each other. Also in this figure the furnace 1 is defined by the front wall 2, the right- hand side side wall 3a, the rear wall 4, and the left-hand side side wall 3b. In the middle of the bottom 5 of the furnace there is the header 13, from which water flows as indicated by the arrow 14 towards the front wall 2 and rear wall 4. The cross-section of the furnace 1 is rectangular, the header 13 being parallel with the front and rear walls 2, 4. The location of the steam drum 11 is in front of the front wall 2, indicating that the drum 11 is parallel with the front wall 2 of the furnace 1. On the side of the rear wall 4 of the furnace 1, downstream of the superheaters 8, there are the heat exchange surfaces, the boiler bank 9, and the economizers 10. The arrow 15 indicates the flow direction of the flue gases discharged from the economizers 10 to a conventional electrostatic precipitator and discharge stack (not shown) . Since the cross-section of the furnace 1 is rectangular, the width of the furnace 1, which is the same as the length of the front wall 2, is equal to the depth of the furnace, which is the same as the length of each of the side walls 3a, 3b.

FIGURES 3a and 3b illustrate the bottom 5 of a conventional soda boiler formed between the front and rear walls 2, 4, the tubes of the bottom 5 being parallel with the side walls 3 (not seen in figures 3a and 3b) . In FIGURE 3a, the bottom 5 is inclined from the front wall 2 and the rear wall 4 towards the middle of the bottom 5. Parallel with the front wall 2 and the steam drum 11, the header 13 is located in the middle of the bottom 5. From the main header 13 the water flows to the tubes of the bottom 5 and the water walls 2, 4, as indicated by the arrows, through the header (not shown) of each wall.

In FIGURE 3b, the bottom 5 is positioned between the front 2 and rear 4 walls, so that the bottom 5 is inclined from the front wall 2 towards the rear wall 4. Water flows into the tubes of the bottom 5 and walls 2, 4 from two headers 13' and 13'' disposed crosswise to the boiler, as shown .

The horizontal direction of elongation of the steam drum 11 relative to the boiler and headers 13, 13', 13'' is apparent from FIGURES 1, 3a and 3b. The drum 11 is disposed crosswise to the boiler, and thus parallel to the front wall 2.

As is apparent in FIGURES 3a and 3b, the inclination of the bottom 5 is determined by the header 13 or headers 13' and 13'', i.e. according to the longitudinal direction of the boiler. Should it be desirable to increase the size of the boiler, this may be effected only by increasing the width thereof, which makes water circulation in the bottom

5, and uniform feed of waste liquor introduced by nozzles

6, more difficult.

FIGURES 4a-4c illustrate the structure of a bottom 5a of a furnace according to the present invention. The bottom

5a is disposed between the side walls 3a, 3b of the furnace, so that the header 13a of the bottom 5a is parallel to the boiler. The water flows, as indicated by the arrows in FIGURE 4a, from the header 13a into the water tubes of the bottom 5a and the walls 3a, 3b, and finally into the steam drum 11a. FIGURE 4a also shows the horizontal location of the steam drum 11a relative to the boiler; that is, the steam drum 11a is substantially at a right angle to the header 13a. The drum 11a may, alternatively, be parallel to the header 13a.

The bottom 5a of the FIGURE 4a embodiment is a "decanting" bottom. That is, bottom 5a is inclined from both side walls 3a, 3b towards the middle so that a low- gradient upwardly directed V shape is provided. The V shape may have an apex (at the bottom-most portion) as illustrated in FIGURE 4a, or a horizontal part on both sides of the water feed point (as in FIGURE 4b) to facilitate ease of manufacturing. Also, the bottom may slope from just one side wall (e.g. 3a) as illustrated in FIGURE 4c. In any case the angle of inclination of the bottom 5a is below 15 degrees, preferably between about 2-5 degrees, to the horizontal.

FIGURE 5 illustrates in more detail the tube system of the bottom 5a of a recovery boiler according to the invention, wherein the bottom 5a is disposed between the side walls 3. The cross-section of the furnace as seen in FIGURE 5 is a rectangle. In accordance with the invention, the ratio of the sides of the furnace, i.e. the ratio of the length S of the side wall 3, i.e. the depth of the furnace, and the length W of the front wall 2, i.e. the width of the furnace, is at least 1.15 to 1, preferably above about 1.20 to 1, and most preferably between about 1.25 - 1.5 to 1.

Water flows out of the steam drum 11a (only a part shown in FIGURE 5, but seen in its entirely at 11a in FIGURE 4) disposed on the side of the front wall 2, through several downtake pipes 12 into the main header 13a of the bottom 5a of the furnace. The header 13a is substantially parallel to the boiler, in other words substantially parallel to the side walls 3. From the header 13a, the water flows into additional headers 16 of the bottom 5a, which are parallel to the main header 13a. From the additional headers 16, the water flows into the water tubes 20 of the bottom 5a, only a few of which tubes 20 are illustrated. The tubes 20 are substantially parallel to the front and rear walls 2, 4 of the boiler, as shown by the arrow 17. The water may also flow without an additional header 16. From the bottom tubes 20, the water further flows into the tubes 23 of the side wall 3.

Since the main header 13a is parallel with the side wall 3, which is longer than the front wall 2, the bottom tubes 20 are shorter than they would be if the main header 13a was disposed parallel to the front wall 2 (as in the prior art of FIGURES 3a and 3b) . In the FIGURE 5 embodiment, water is brought into the header 18 of the rear wall 4 and into the header 21 of the front wall 2 from the main header 13a through a plurality of supply pipes one of which, pipe 24, is illustrated in FIGURE 5. Pipes 19 forming the rear wall 4, and pipes 22 forming the front wall 2, are attached to the headers 18 and 21, respectively.

FIGURE 6 illustrates a recovery boiler utilizing the novel structural features according to the invention. The boiler of FIGURE 6 comprises a furnace 31 defined by water tube walls, i.e. front wall 32, two side walls 33, and rear wall 34, as well as the bottom 35, all of which are constructed from water tubes. The cross-section of the furnace 3 is rectangular, in which the width of the front wall 32 is, for example, about 14.5 meters, and the length of each of the side walls 33 is about 18 meters. The height of the furnace 31 is preferably above 40 meters, e.g. about 50 meters. This is significantly more than the height of conventional recovery boilers, which is about 40 meters.

Combustion air is fed into the furnace 31 on several different levels, i.e. from primary air nozzles 36, from secondary air nozzles 37, and from tertiary air nozzles 39. The boiler may also comprise other air levels. Waste liquor (e.g. black liquor) is fed into the furnace 31 by the nozzles 38. As a result of combustion, a melt bed is formed on the bottom 35 of the furnace 31 by the waste liquor. From the bed the melt is removed through melt spouts 48 disposed in the side walls 33 in the lower part of the furnace 31.

Above the furnace 31, there are heat recovery surfaces, e.g. superheaters 40, a boiler bank 41, and economizers 45 (e.g. 45a and 45b), in which the heat of the flue gases generated in the furnace 31 is recovered in the form of steam. The economizers having a vertical bank of heat transfer tubes 45 are disposed at a lower level than the boiler bank 41. The upper surface 46 of the boiler bank 41 is positioned vertically above the upper surface 47 of the economizers 45. The flue gas generated in the furnace 31 flows through the superheaters 40 to the boiler bank 41. Between the heat exchange elements of the boiler bank 41 and those of the economizer 45a, there is a flue gas channel 48. The flue gases flow through the channel 48 to heat the water flowing in the economizers 45a and 45b. The water/steam mixture generated in the water tubes of the walls 32-34 and the bottom 35 of the furnace 31 rises into the steam drum 44, which is disposed transverse to the boiler, i.e. substantially parallel to the front wall 2. From the steam drum 44, hot water flows through downtake pipes 42 into the bottom header 43, after which the water is distributed into the water tubes of the bottom 35 and further to the water tube walls. The drum 44 is disposed in a vertical direction behind the rear wall 34, directly at the flue gas channel 48 between the heat exchange elements of the boiler bank 41 and the heat exchange elements of the economizer 45a, above the upper side 47 of the economizers 45a, 45b. The downtake pipes 42 are connected to the drum 44 to lead water into the tubes of the furnace bottom 35. FIGURE 6 shows three downtake pipes 42, which are first directed straight downwardly, and then, in the proximity of the upper side 47 of the economizer 45, the pipes 42 are inclined so that they are parallel to the side wall 33 on the sides of the boiler. FIGURE 7 schematically illustrates such a location of the downtake pipes 42. The downtake pipes are positioned at substantially equal distances from each other along the whole length of the drum 44.

The downtake pipes 42 are connected to the header 43 of the furnace bottom 35, which header 43 is -- according to the invention -- parallel with the side walls 33. From the header 43, the water further flows into the bottom- forming tubes 20 and the wall tubes as seen in FIGURE 5. The present invention provides a structure for a combustion boiler of waste liquors which is especially to H¹ σ in o in

Claims

WHAT IS CLAIMED IS:

1. A recovery boiler comprising: a furnace defined by a bottom, a front wall, two side walls, and a rear wall, said furnace having a width defined by said front or rear wall, and a depth defined by one of said side walls, said side walls being substantially parallel to each other, and substantially transverse to said front and rear walls; said bottom and said walls formed by water-cooled tubes; a water/steam circulation comprising at least one steam drum elongated in a first dimension, a plurality of downtake pipes, and at least one header to which said downtake pipes are connected; said downtake pipes and at least one header operatively connected to said water-cooled tubes so that fluid from said drum is circulated to said downtake pipes, then to said at least one header, and then to and through said water-cooled tubes; said furnace having a substantially rectangular configuration in which the ratio of the depth to the width thereof is at least 1.15 to 1; and wherein at least one of said headers is substantially parallel to said side walls of said furnace.

2. A recovery boiler as recited in claim 1 wherein said furnace bottom slants downwardly from one side wall to the other at an angle of inclination of less than 15 degrees to the horizontal.

3. A recovery boiler as recited in claim 1 wherein said furnace bottom is an upwardly directed low-gradient V having a bottom-most portion that is substantially parallel to said side walls, said V having an angle of inclination of less than 15 degrees to the horizontal.

4. A recovery boiler as recited in claim 1 wherein said water/steam circulation further comprises heat transfer surfaces of a boiler bank and economizers, said heat transfer surfaces positioned in such a way that said economizers are at a substantially lower vertical level than said boiler bank.

5. A recovery boiler as recited in claim 1 wherein said water/steam circulation comprises one steam drum, and said dimension of elongation of said steam drum is substantially parallel to said front and rear walls of said furnace.

6. A recovery boiler as recited in claim 5 wherein said steam drum is disposed on the opposite side of said rear wall from said front wall.

7. A recovery boiler as recited in claim 1 wherein said recovery boiler comprises a soda boiler for burning black liquor, and wherein said furnace is dimensioned to effectively burn more than 4,000 tons of dry matter per day.

8. A recovery boiler as recited in claim 7 wherein said furnace is dimensioned to effectively burn about 5,000 tons of dry matter per day.

9. A recovery boiler as recited in claim 1 further comprising a plurality of first nozzles for introducing waste liquor into said furnace, and a plurality of second nozzles for introducing combustion air into said furnace; and wherein the majority of each of said first and second nozzles are disposed in said furnace side walls.

10 . A recovery boiler as recited in claim 9 wherein at least about 60% of each of said first and second nozzles are disposed in said furnace side walls.

11. A recovery boiler as recited in claim 4 wherein said dimension of elongation of said steam drum is substantially parallel to said front wall; and wherein said drum is positioned vertically above said economizers.

12. A recovery boiler as recited in claim 3 wherein said bottom-most portion of said V is an apex of said V.

13. A recovery boiler as recited in claim 3 wherein said bottom-most portion is substantially flat for ease of manufacture .

14. A recovery boiler as recited in claim 1 wherein said ratio is between about 1.25 - 1.5 to 1.

15. A recovery boiler as recited in claim 1 wherein said at least one header comprises a single header extending substantially parallel to said side walls.

16. A recovery boiler as recited in claim 15 wherein said recovery boiler comprises a separate header for each of said front and rear walls connected to said single header, and extending substantially parallel to said front and rear walls.

17. A recovery boiler as recited in claim 2 wherein said furnace bottom includes slanting water tubes, said tubes slanting at an angle of between about 2-5 degrees to the horizontal.