Method and apparatus for improving chemical recovery boiler operation

Abstract

A chemical recovery boiler is described in which the primary air system is reconfigured to provide active char bed control and improved combustion in the lower furnace. A minimal number of primary air ports are used on two opposing walls to generate powerful air jets that pass across the boiler, providing physical and thermal stability to the char bed while increasing heat release and combustion stability in the lower furnace, increasing reduction efficiency and reducing carryover and emissions. Various embodiments are described, including operating strategies and multi-level black liquor injection.

Description

[Technical Field]

[0001] This description relates to chemical recovery boilers. [Background technology]

[0002] Chemical recovery boilers are well known in the pulp and paper industry as a means of recovering spent cooking chemicals and the heat generated by burning black liquor fuel within them. In the kraft pulping process, wood chips are treated (or cooked) in a digester, where they are subjected to high temperatures and pressures in the presence of caustic chemicals. During the digester digestion, the lignin that binds the wood fibers together dissolves, liberating the fibers used to make pulp. The kraft process produces relatively long fibers, which are used to make strong paper products, commonly used in packaging of all kinds. After dissolving the wood chips, the spent cooking chemicals, dissolved lignin, and unsuitable wood fibers are captured, the excess water is evaporated, and the resulting black liquor is burned in a recovery boiler. After evaporation, the moisture content of the black liquor is typically 20% to 40%, depending on the equipment and the operation of the individual mill. Black liquor solids consist of approximately 50% inorganic matter and 50% organic material (lignin and wood fiber). Black liquor is injected into a recovery boiler through one or more atomizing spray nozzles to evaporate residual moisture and burn organic material. During this combustion process, spent cooking chemicals are liberated and undergo chemical reduction. The primary spent cooking chemical in a black liquor recovery boiler is sodium sulfate (Na2SO4), which is reduced to sodium sulfide (Na2S) in the presence of heat and atomic carbon. This is an endothermic reaction that absorbs heat from the volatiles and char being burned. The reduction efficiency is the ratio of the total sulfur concentration in the smelt minus the sulfate concentration divided by the total sulfur concentration (for practical purposes, this can be expressed as Na2S / (Na2S + Na2SO4)). A well-operated recovery boiler can achieve reduction efficiencies of over 95%. While other types of recovery boilers, such as soda boilers and red liquor boilers, which use different chemical mixtures to dissolve wood chips, are also used, black liquor boilers are currently the most common type. Although we limit our discussion to black liquor boilers, the methods and apparatus described herein are also suitable for other types of chemical recovery boilers that use combustion system configurations similar to black liquor boilers.

[0003] Recovery boilers are typically constructed with a furnace (combustion) section in a square or rectangular plan shape, typically 10 to 30 feet or more on a side. The furnace floor and walls are constructed from a parallel array of steel boiler tubes, seal-welded to adjacent tubes. The floor may be flat or sloped at approximately 5°, and the walls are vertical, forming a large prismatic enclosure 30 to 100 feet high. At the top of the furnace, one of the boiler walls (typically the rear wall) is bent inward to form a bullnose section, which redirects the hot combustion gases across the convection heat transfer tubes arranged at the top of the boiler. The convection section typically consists of one or more superheaters, a steam generator (power bank or boiler bank), and one or more economizers. The tubes forming the hearth are fed by a header at one edge of the floor and turn vertically to form one of the furnace walls on the opposite side of the header. In some boilers, one or two centrally located headers feed the floor tubes, which in turn feed two opposing walls. The remaining walls are fed by their own headers, which receive boiler feedwater through one or more downcomers from a water drum at the bottom of the generating bank in a two-drum boiler, or from the drum above in a single-drum boiler. A natural circulation system cools the furnace wall tubes, in which a head of water from the cooler, denser boiler feedwater in the downcomers pushes upward against the warmer, less dense water in the furnace walls. The primary heat transfer mechanism in the furnace section of a boiler is radiation, with the majority of saturated steam being generated in the furnace walls.

[0004] Black liquor fuel is injected into the furnace at one or more locations on one or more of the furnace walls. Relatively large recovery boilers typically use three or more injection nozzles per wall, and may have a total of ten or more injection nozzles. All nozzles are typically at a single height, generally considered the working level, because injecting the liquor requires constant operator attention. The nozzle height may typically be 15 to 30 feet above the floor. As the liquor is injected into the boiler, it undergoes multiple combustion stages, including drying, devolatilization, char combustion, and smelt formation. After drying the droplets, pyrolysis gases are produced (CH4, CO2, H2), and the droplets expand like popcorn, increasing their volume and surface area as their mass decreases. This increases the buoyancy of the liquor particles during the char combustion stage, when residual carbon is consumed. After combustion is complete, the remaining inorganic chemicals form smelt droplets (a molten form of chemicals, primarily sodium sulfate). During the combustion phase, some of the liquid falls to the bottom of the boiler, where it forms a char bed. This is so named because it is composed of droplets from the char combustion phase. The char bed typically forms a mound in the center of the boiler, but the size and shape of the char bed can vary greatly from boiler to boiler and from time to time within a given boiler. Some of the liquid may land on the walls and remain there until the combustion process is complete, and some of the droplets may be entrained in the gas flow within the furnace and carried to the upper convection section of the boiler, where they may adhere to the superheater and power bank tubes and completely block the gas flow if not removed. Soot blowers are typically used to periodically blow the deposited material off the tubes, but soot blowing is not 100% effective and can consume a lot of steam, reducing the overall efficiency of the boiler. By the time the droplets reach the convection section, most of the droplets have turned into smelt along with residual carbon; this material is called carryover. In addition to this carryover, sodium vapor is produced, which combines with free sulfur and oxygen in the flue gas and condenses as sodium sulfate on the superheaters, power banks, and economizer tubes.This material, commonly called saltcake, is recovered by soot blowing, where it falls into a hopper and mixes with the incoming black liquor. The sodium fumes are beneficial in that they capture sulfur in the flue gas that would otherwise be released or require additional flue gas treatment.

[0005] The combustion air system for a black liquor recovery boiler consists of fans, air heaters, ducts, nozzles, control dampers, port cleaners, port dampers, port openings, and other instrumentation that provides and controls the flow of combustion air to the boiler. Since the first Tomlinson black liquor recovery boiler in 1929, combustion air has been blown into the boiler through a series of closely spaced openings around the bottom of the boiler, approximately 3 feet above the boiler floor. Depending on the size of the boiler, there are typically 10 to 40 or more ports arranged on each side. This was done to facilitate smelt discharge at the periphery of the bed by keeping the charbed edge lower than the center of the bed. When the charbed accumulates in the center of the boiler, the added weight compresses the bed, making smelt discharge more difficult and displacing the smelt toward the walls. The earliest recovery boilers had multiple air ports at a single level, but over the years, additional levels have been added; modern recovery boilers generally have three to six levels of air injection, with multiple ports at each level in various configurations to promote good combustion within the boiler. The lowest port level, called the primary level, consists of a series of closely spaced combustion air ports around all four walls of the boiler, spaced approximately evenly vertically about three feet above the floor. The next level up is traditionally called the secondary level, although some older boilers referred to it as the high primary level. The starting burner port is often included at this level. Modern recovery boiler air systems may have two or more levels of secondary air ports; the defining feature of a secondary port is that it is located above the primary ports and below the liquid spray ports. One to three or more levels of tertiary combustion air ports are commonly used above the liquid spray ports, with the highest level sometimes referred to as the quaternary level. Load burner ports may also be present above the liquid spray level, where gas or oil fired burners are located to increase the vaporization rate when needed.Numerous combustion air port arrangements have been used over the years, and numerous patents have been issued in recent years for improved combustion systems for recovery boilers, including U.S. Patent Nos. 5,629,599; 5,7 ...

[0006] Traditionally, the role of primary air ports has been to control the periphery of the char bed at a relatively low level to facilitate smelt evacuation around the boiler periphery. Secondary air systems are intended to control the top of the char bed to promote good combustion and are generally designed with fewer, larger combustion air ports for good passage and mixing. Secondary combustion air ports are typically located on two or all four walls. Tertiary and quaternary air ports are generally located on two opposing walls of the boiler to aid in the completion of combustion and minimize undesirable emissions.

[0007] At the bottom of the furnace, openings are made in one or two walls at bed level to allow smelt to exit the boiler through a smelt discharge port. Most smelt is generated in the char bed; some is generated on the walls; and some is generated in suspension and either falls to the char bed or is carried over as carryover and exits the top of the furnace. The recovered smelt flows into a dissolving tank, where it is diluted into green liquor and returned to the pulp mill for reconstitution into the caustic chemicals required for the cooker. To reach the smelt discharge port, the smelt must percolate through the char bed; traditionally, most of the smelt flows around the periphery of the bed below the primary port, because the char bed is shallowest there. The smelt is very hot and generates a heat flux through the bed through which it flows, which, as mentioned above, is mostly around the periphery. It has also been noted that radiation is considered the dominant heat transfer phenomenon in furnaces. Radiation heat transfer is affected by the so-called shape factor of the enclosure. For recovery boilers with a rectangular planar geometry, this means that the corners of the boiler receive significantly less radiant heat than the center of the walls. It has also been described that the tubes comprising the floor and walls form part of a natural circulation system driven by the radiant heat flux to the walls. Given the lower heat flux in the corners, there is also less water circulating through the floor tubes that extend adjacent to the boiler walls, and these floor tubes are therefore more susceptible to overheating due to the lower cooling water flow despite the higher heat flux from the smelt flow.

[0008] Another problem with traditional primary air systems is that they do not promote good combustion and overall char bed stability. It is quite common for the char bed to flatten around the bottom or lower perimeter of the primary air port, and then about 2-3 feet from the wall, the bed slopes upward into one or more mounds. The mounds may be localized, like an anthill, or more widespread, like a series of small hills. The shape of the char bed is determined by the interaction between the primary and secondary air flows and the liquid spray. In either case, a stable char bed is highly desirable to promote consistent boiler operation with high throughput and low emissions. A common problem with char beds is that the sloping-sided mounds will grow until they become unstable and collapse. Although the fuel they contain is shielded, when the mound collapses, it suddenly releases additional fuel. Nevertheless, the combustion air system has no means to respond to this (combustion air systems operate at a somewhat steady state), and therefore volatiles are not completely consumed, resulting in emission spikes. Another potential problem with unstable char beds is when mounds collapse and block primary ports, creating a blackout condition. This blocks airflow and creates combustion problems, necessitating cleanup and significant operator effort. The primary air's primary role is to control the char bed's periphery, which often means supplying more air than necessary for good combustion. The fuel-to-air ratio around the boiler's periphery at the primary level is typically highly skewed, and after entering the boiler, the air is deflected upward by the char bed, where it inhibits the secondary air jets and increases the bulk upward velocity of gases in the furnace, increasing undesirable carryover and pushing combustion higher in the furnace. Lowering combustion in the boiler is desirable to improve heat release to the walls and increase water circulation, promoting a more stable char bed, increasing sodium fume production and improving reduction efficiency.

[0009] Over the past 25 years, much research has been done to improve recovery boiler combustion, but a complete rethink and modernization of the primary air system has never been embraced. The current state of the art sanctifies recovery boilers having multiple closely spaced primary ports. The method and apparatus described below provide a final remedy to the inherent deficiencies of the current state of the art. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] U.S. Patent No. 6,742,463 [Patent Document 2] U.S. Patent No. 6,932,000 [Patent Document 3] U.S. Patent No. 7,069,866 [Patent Document 4] U.S. Patent No. 7,185,594 [Patent Document 5] U.S. Patent No. 7,207,280 [Patent Document 6] U.S. Patent No. 7,694,637 Summary of the Invention [Problem to be solved by the invention]

[0011] SUMMARY OF THE INVENTION It is an object of the present invention to provide a method and apparatus for the retrofitting of recovery boilers. [Means for solving the problem]

[0012] The chemical recovery boiler does not include a primary combustion air port in the wall opposite the ported wall, hi some embodiments, the boiler includes at least two, but not more than seven, primary ports in each of the two walls adjacent to the ported wall.

[0013] The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description that follows may be better understood. Additional features and advantages of the present invention will be described hereinafter. It will be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It will also be appreciated by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

[0014] For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a side view of a recovery boiler in which the tubes and pipes are shown as single lines. [Figure 2] FIG. 2 is a partial front view of the boiler of FIG. 1 taken along line A-A. [Figure 3] FIG. 3 is a cross-sectional view of the boiler of FIG. 1 at the level of the primary ports, the cross-section being taken along section line B-B, showing a first primary port configuration. [Figure 4] FIG. 4 is a cross-sectional view of the boiler of FIG. 1 taken along line B-B at the level of the primary ports showing different primary port configurations. DETAILED DESCRIPTION OF THE INVENTION

[0016] The principles described herein can be used to build new chemical recovery boilers and / or to retrofit existing chemical recovery boilers. The following description is directed to chemical recovery boilers and, consequently, to methods of making and operating improved chemical recovery boilers.

[0017] The following description refers to features in the accompanying drawings, in which features are identified with like numerals. Figure 1 is a side view of a recovery boiler, in which the tubes and pipes are shown as single lines. Figure 2 is a partial front view of the same boiler, identified by section line A-A. Figure 3 is a plan view of the boiler of Figure 1 at the primary port level, identified by section line B-B, showing a first primary port configuration, and Figure 4 also shows a cross section B-B, but this time showing a second primary port configuration.

[0018] Referring to Figures 1 and 2, recovery boiler 1 is an older, two-drum type using a steam drum 2 and a water drum 3. The boiler walls and floor are constructed of closely spaced tubes 33 in a vertical, parallel array that are seal-welded together to create an airtight enclosure. The tube walls are constructed with uniform tube-to-tube pitch. Boiler feedwater is supplied to steam drum 2, and a portion flows down the rear tubes 4 of power bank 5 into water drum 3, recovering heat from flue gas 6 that has passed through cross screen 7, superheaters 8-11, power bank 5, and economizer 12. Because flue gas 6 is relatively hot at the inlet to power bank 5, the water in front tube 13 absorbs more heat than the water in rear tube 4, creating natural circulation between steam drum 2 and water drum 3.

[0019] A portion of the water in water drum 3 flows down downcomer 14 toward headers 15-18. Water in header 15 flows through floor 19, up rear wall 20, through bullnose 21, and back to water drum 3. Similarly, water in header 16 flows through floor 22, up front wall 23, and through roof tubes 24 back to steam drum 2. Sidewall headers 17 and 18 feed sidewalls 25 and 26, respectively, and are released by an upper header (not shown) to return to steam drum 2. Radiant heat from the burning fuel is absorbed by walls 20, 23, 25, and 26 to produce steam, creating a natural circulation system.

[0020] Forced draft combustion air is supplied through primary port 28, primary air port 29, secondary port 38, tertiary port 39, and, if present, quaternary port 40. Most chemical recovery boilers currently in operation have a smelt discharge port on one wall of the boiler, although some older combustion engineering units discharge smelt from two opposing walls. The apparatus described herein is particularly suitable for boilers that discharge smelt from one wall, but can also be used in boilers that discharge smelt from two walls. Smelt discharge port 27 is at or slightly above the boiler floor level, while discharged wall primary port 28 and primary air port 29 are several feet higher. Around the periphery of the boiler, char bed 30 is below the primary ports but still above the level of discharge opening 31. At the discharge port 27, the flowing smelt 32 tends to maintain a relatively low char bed, but keeping the molten smelt moving freely at the discharge port can still be a problem. The molten smelt falls into the dissolving tank 34, where it becomes reusable green liquor.

[0021] Therefore, several primary ports 28 are required to operate in close proximity to the smelt outlets, preferably 1 to 10 ports, more preferably 2 to 8 ports, and most preferably 3 to 5 ports centered relative to each outlet. These ports are sized and positioned similarly to conventional primary ports on the outlet wall (in this case, the front wall 23) and may, in fact, be recycle ports if the boiler is modified according to the present disclosure. Only a few primary ports open on the outlet wall (in this case, the front wall 23) at each outlet. No conventional primary ports are required on the wall opposite the outlet (in this case, the rear wall 20) (in a single-outlet wall boiler). Preferably, the area of ​​the air ports on the wall opposite the outlet wall is zero or is less than 50%, 25%, 10%, 5%, or 1% of the area of ​​the ports on the outlet wall. When a boiler is modified according to the present disclosure, existing ports may be blocked with refractory material or some other means, or port tubes that were originally bent to form openings may be replaced with straight tubes. On the two walls adjacent (orthogonal to) the ported wall (side walls 25 and 26 in this case), conventional primary ports are absent. Instead, combustion air ports 29, which are significantly larger than conventional primary ports, are present. As with the other wall, when a boiler is modified to incorporate the present disclosure, existing primary ports are blocked, replaced with straight tubes, or replaced with new bent tubes that form new, larger port openings. In some embodiments, the primary air ports on the two walls adjacent (orthogonal to) the ported wall provide more than 80% of the primary air port area, with the air ports on the ported wall providing less than 20% of the primary air port area. In some embodiments, the primary air ports on the two walls adjacent (orthogonal) to the outlet wall provide more than 90% of the primary air port area with the air ports on the outlet wall, and the wall opposite the outlet wall provides less than 10% of the primary air port area.In some embodiments, the primary air ports on the two walls adjacent (orthogonal) to the outlet wall provide more than 95% of the primary air port area with the air ports on the outlet wall, and the wall opposite the outlet wall provides less than 5% of the primary air port area.

[0022] Conventional primary ports on recovery boilers typically measure 1 inch wide by 6 inches high to 2 inches wide by 11 inches high. The upper limit is 2.5 inches by 15 inches. The new ports as part of this disclosure will be fewer in number and much larger than the original primary ports. For example, if the primary ports on a recovery boiler are 1.5 inches wide by 8 inches high, there will be 80 such ports on the boiler, for a total primary port area of ​​1.5 x 8 x 80 = 960 square inches. Implementation of the present invention can replace 90% of the primary ports, for a total of 864 square inches. If eight new ports are installed in their designated locations, each port will have an area of ​​108 square inches. In this case, these new ports may be 6 inches wide by 18 inches high. Actual dimensions may vary from this example depending on the practical requirements of each system.

[0023] The total area of ​​the primary air ports of a recovery boiler retrofitted in accordance with the present disclosure can be approximately the same as the area of ​​the air ports of the recovery boiler before the retrofit. For example, the total area of ​​the primary air ports of a recovery boiler according to the present disclosure can be within plus or minus 40%, plus or minus 30%, plus or minus 20%, plus or minus 10%, or plus or minus 5% of the area of ​​the air ports of the recovery boiler before the retrofit.

[0024] Referring to Figure 3, the number of primary ports 29 on these two walls is dictated by the boiler size and the optimum spacing between ports. This optimum spacing may be as little as 3 feet for small boilers and as much as 7 feet for larger boilers. Because air jets expand as they flow through the boiler, larger boilers require air jets to travel farther and expand more, starting further apart to avoid interference with adjacent or opposing air jets. The minimum horizontal spacing between primary air ports is given by the formula S = 0.13 * W, adjusted to match the pitch or other characteristics of the boiler's tubes, where S is the nominal horizontal spacing between ports and W is the boiler's planar dimension parallel to the direction of the air jets, which must always be at least 3 feet. Dimension W is from the midline of one wall to the midline of the opposing wall. D is the boiler dimension perpendicular to W. The maximum number of primary ports on the opposing walls 25 and 26 adjacent to the discharge wall 23 of a square boiler is seven, unless one side of the boiler exceeds 49 feet, in which case additional ports are added to maintain S < 7 feet. For rectangular boilers, the maximum number of primary ports on the opposing walls 25 and 26 adjacent to the discharge wall 23 may exceed seven, assuming that walls 25 and 26 are longer than walls 20 and 23. Note that a square is a type of rectangle, and for our purposes, a square boiler is any boiler whose longer side is less than 3 feet longer than its shorter side. These relationships assume that the air jet direction is perpendicular to the wall from which the air jet originates. All primary air ports are evenly spaced + / - 0.25*S to allow for variations due to other features on the boiler. The spacing L1 is the distance from the ported wall 23 to the first primary air port on the adjacent walls 25 and 26, and the spacing L2 is the distance from the last primary air port on walls 25 and 26 to the wall 20 opposite the ported wall. Generally L1 = L2, but this can vary by + / - 0.25*S to accommodate boiler geometry or other features.L1 and L2 should not be less than 0.75*S because the low pressure zone created by the air jet flowing by the wall can suck the air jet against the adjacent wall.

[0025] Given that the apparatus described herein has fewer primary air ports than conventional boilers, the primary air ports must be larger. This not only provides the combustion air necessary to meet stoichiometric requirements, but also provides a higher volumetric flow rate per air jet, which improves the passage of the air jet across the char bed. A feature of some embodiments of the apparatus described herein is that fewer, but larger, primary air ports create more powerful air jets that pass across the boiler, providing the combustion air and physical agitation necessary to maintain a stable char bed. Typically, approximately 20-40% of the total combustion air enters through the primary air ports. Knowing the boiler capacity, the stoichiometric requirements for combustion air can be determined, and the required primary air port area can be determined using the following equation: A commonly used formula for determining the velocity of the air jet is given as Equation 2: V = 67.3 * ((Pd) * (T / 527))^0.5, where V is the velocity of the jet in feet per second, Pd is the differential pressure across the port in inches of water, and T is the temperature of the combustion air in degrees Rankine. The formula for determining the area of ​​each port opening is given as Equation 3: Aip = ((Q * X) / (V * Cfl * Cfo - Asp)) / 2N, where Aip is the area of ​​the individual primary air port, Q is the total amount of forced draft combustion air into the boiler, X is the fraction of forced draft combustion air injected at the primary level, V is the velocity of the air jet, Cfl is the flow coefficient, Cfo is the port opening fouling coefficient (recovery boiler port openings tend to be fouled by char and frozen smelt from black liquor), Asp is the total area of ​​the primary ports on the discharge wall, and N is the total number of primary air ports. There are other influencing variables, for example some of the ports may operate at different speeds or flow rates or may have port dampers that adjust the effective area of ​​the port openings, and the fixed area of ​​the port openings must be adjusted accordingly.

[0026] The first embodiment relates to primary ports, which are generally considered to be the lowest forced draft air ports. We further define a primary port as one whose bottom is no more than five feet above the boiler floor, with no lower forced draft air ports on the same wall, except for those that are stepped along the slope of the floor. Referring to Figures 1 and 2, the primary ports 28 on the outlet wall 23 will be located at a height suitable to protect the smelt outlet 27 from the char bed 30. On the side walls 25 and 26, the primary air ports 29 will be located at a height to provide a char bed 30 of the desired depth. While the primary ports of the apparatus described herein are generally all at the same height, this is not necessarily the case; heights may vary from port to port or wall to wall within the context of the above definition. Some boilers have sloped floors, in which case multiple ports may be the same distance above the floor but not at the same height. In this first embodiment, the primary air ports 29 are located on two opposing walls 25 and 26 adjacent to the outlet wall 23, with every port having a corresponding port opening on the opposing wall and aligned with the opposing port within plus or minus three tube pitches. The number of port openings on each wall is the same, with a minimum spacing determined by Equation 1. The minimum number of primary air ports is two on each wall 25 and 26, unless the boiler is rectangular, and the maximum is seven. For rectangular boilers in which walls 25 and 26 are three feet or longer than walls 20 and 23, the number of ports 29 can be increased by one for each additional length S. Each port opening includes a means for controlling airflow through the port by varying the port area, the port pressure, or both. For our purposes, the term "air flow" includes the volumetric flow rate, mass flow rate, and / or velocity of the air jet. In the apparatus described herein, the airflow through the ports is automatically varied based on predetermined criteria to change the airflow and combustion characteristics within the boiler, providing active control of the char bed.For example, referring to FIG. 3 , a first port on wall 25 may be set to full flow while the port directly opposite it on wall 26 is set to partial flow; a second port on wall 25 may be set to partial flow while the port directly opposite it on wall 26 is set to full flow, and so on down the wall. This creates strong jets 35 alternating with weak jets 36 across the char bed. Periodically within a range of 1 to 20 minutes, this configuration automatically reverses, so that the strong jets become weak jets and the weak jets become strong jets. The purpose of this configuration is to provide strong jets that pass across the boiler to control the top of the char bed but are opposed by weak jets to prevent the char bed from building up against the opposing wall and to prevent black liquor, smelt, and char from being blown into opposing port openings. Because the air jets have unequal strength, the point of contact is not centered on the boiler. Where the air jets meet, they tend to deflect each other upward. Staggering the contact points prevents the formation of a core of high vertical velocity within the furnace, which, as previously explained, pushes fuel, air, and combustion upward within the boiler, a detrimental effect. Equation 2 describes the square-root relationship between differential pressure and the resulting jet velocity. When jet velocity is controlled based on pressure, this means that a large difference in differential pressure is required to achieve a modest difference in velocity. For example, to double the velocity of an air jet, the differential pressure must be increased by a factor of four. When using a strong / weak jet configuration, the lower flow rate from the weak jet must be taken into account when determining the required area of ​​each port opening.

[0027] A second embodiment, referring to FIG. 4, has primary air ports staggered on opposing walls 25 and 26 adjacent to the outlet wall 23. In this case, the minimum spacing between ports follows Equation 4: Si = 0.26 * W, and the maximum optimum number of ports 29 on each wall 25 and 26 is three for a square boiler. For rectangular boilers where walls 25 and 26 are three feet or longer than walls 20 and 23, the number of ports 29 can be increased by one for each additional length Si. The nominal distance L3 from the first port on wall 25 to wall 20 must be equal to or greater than 0.37 * Si, and the nominal distance L4 from the first port on wall 26 to wall 20 must be L3 + Si / 2. ​​This allows the air jets to be evenly spaced across the boiler, with each jet staggered between two jets on opposing walls or between a vertical wall and one air jet from the opposing wall. This embodiment has the advantage of being low cost since fewer port openings are required, and collisions between the air jets are avoided, allowing the air jets to pass through the entire boiler, further reducing updrafts within the boiler. However, if there is no opposition to the air jets, the char bed can build up against the opposing wall to a height that can become problematic. As the air jets travel over the char bed, they tend to curve upward (the final direction of all the air), reducing their ability to actively control the char bed. This embodiment has the advantage of being simple in design and operation since periodic reversals of the air jets are not required.

[0028] A third embodiment takes advantage of the positive char bed control and improved combustion in the lower furnace that primary air ports provide. In most black liquor recovery boilers, the majority of the forced draft combustion air is injected below the liquor atomizing nozzles. This is for practical reasons because air is needed to burn the fuel, but the combustion air and gaseous products of combustion need to go somewhere, and in the furnace, this direction is upward. By creating a strong jet / weak jet configuration or staggering the air jets, strong localized updrafts are minimized, reducing upward pressure on the fuel droplets. These configurations have been used for many years at the secondary, tertiary, and quaternary air levels, but have not previously been used at the primary air level because it was assumed that numerous conventional primary ports would be necessary for smelt evacuation and char bed control. The primary forced draft air flow typically accounts for about 30% of the total air flow, while the secondary air flow accounts for about 40%. Using a strong / weak jet configuration or staggering secondary air jets has proven effective in improving combustion in the lower furnace, even though the primary air (roughly half of the combustion air in the lower furnace) is not involved and actually interferes with combustion. By incorporating primary air with an optimally positioned, active mixing and char bed control system using larger, more powerful air jets, combustion in the lower furnace is enhanced far beyond what is possible at the secondary level alone. This creates the opportunity to push more fuel into the char bed, and more fuel landing in the bed means less fuel burning in suspension, reducing carryover. Reduced carryover means the boiler can run longer between cleanups and / or use less steam for soot blowoff, or, important for many mills, the boiler can run at a higher load factor, increasing mill productivity. Referring to FIG. 1, the current state of the art allows one or more black liquor spray nozzles 36 to be located on one or more walls of the boiler, but they are always at the same height. For our purposes, we refer to this as the "operating level."This is because operators typically control the boiler from that level. This embodiment incorporates one or more lower black liquor spray nozzles 37 below the operating level but at least 2 feet above the primary port level, no more than 12 feet above the bottom of the boiler, and preferably below the secondary air injection point. By spraying the liquor at a relatively low height, the fuel experiences less updraft, reducing carryover and burning in the lower furnace where it resides. This is not feasible without a very powerful primary air system.

[0029] Multiple versions of these embodiments can be used. For example, any of the above-described arrangements can be implemented as a mirror image of that described or illustrated in the drawings; embodiments can be implemented in a boiler that discharges smelt from two opposing walls and has ports on the wall adjacent to the smelt discharge wall; ports can be located on the smelt discharge wall(s) and on the opposite wall; there can be an even number of primary air ports on one wall and an odd number on the opposite wall; primary air ports can be located on the shorter walls of a rectangular boiler. It can thus be seen that numerous configurations and variations can be implemented without departing from the spirit of the present invention.

[0030] Some embodiments provide a method for improving performance of a chemical recovery boiler including a smelt discharge wall, a wall opposite the smelt discharge wall, and two side walls between the smelt discharge wall and the wall opposite the smelt discharge wall, the method including the steps of blocking a plurality of primary air port openings on the smelt discharge wall such that only 2 to 8 ports near each smelt discharge port are open; blocking essentially all of the primary air port openings on the wall opposite the smelt discharge wall; and providing at least two, but not more than seven, primary side wall air ports in each of the two side walls.

[0031] In some embodiments, the method includes retrofitting an existing chemical recovery boiler, wherein the total open area of ​​the at least two but not more than seven primary ports on each of the two side walls is 80% to 120% of the primary air ports of the original chemical recovery boiler being retrofitted.

[0032] In some embodiments, the step of blocking essentially all primary air port openings on the wall opposite the smelt outlet wall includes blocking primary air port openings on the wall opposite the smelt outlet wall so that the total opening area of ​​the primary air port openings on the wall opposite the smelt outlet wall comprises less than 5% of the total area of ​​at least two but not more than seven primary side wall air ports on each of the two side walls.

[0033] In some embodiments, the volumetric flow rate and / or mass flow rate and / or velocity of at least one of the sidewall primary air ports is adjusted to be at least 25% greater or less than the volumetric flow rate and / or mass flow rate and / or velocity of at least one second primary sidewall air port, the at least one second primary sidewall air port being opposite the first sidewall primary port plus or minus three tube pitches to form a strong jet / weak jet relationship between the ports.

[0034] In some embodiments, the strong jet / weak jet relationship between the sidewall air ports is periodically and automatically reversed.

[0035] In some embodiments, the strong jet / weak jet relationship alternates sequentially from port to port along one of the two side walls of the boiler.

[0036] Some embodiments provide a method of operating a chemical recovery boiler, the method including: maintaining smelt flow at a smelt outlet by injecting air through primary smelt outlet air ports positioned near the smelt outlet, the primary smelt outlet air ports having a total primary outlet air port opening area; and injecting primary air through 2 to 8 sidewall primary air ports on each of two sidewalls intersecting the outlet wall, the 2 to 8 sidewall primary air ports having a total primary sidewall air port opening area, the total primary sidewall air port opening area comprising more than 80% of the total primary air port opening area.

[0037] In some embodiments, a total required amount of primary air injected into the furnace is determined by stoichiometry, and at least 80% of the total required amount of primary air injected into the furnace is injected through the sidewall primary air ports.

[0038] In some embodiments, less than 10% of the primary air is injected through primary air ports on the wall opposite the smelt outlet wall.

[0039] In some embodiments, primary air is not injected through an air port on the wall opposite the outlet wall.

[0040] In some embodiments, less than 10% of the primary air is injected through the smelt outlet air port.

[0041] While the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the scope of the invention, as defined by the appended claims. Moreover, the scope of this application is not intended to be limited to the particular embodiments of the processes, machines, manufacture, compositions of matter, means, methods, and steps described herein. As will be readily apparent from this disclosure to those skilled in the art, any currently existing or future-developed processes, machines, manufacture, compositions of matter, means, methods, or steps that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized in accordance with the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, and steps.

Claims

1. 1. A chemical recovery boiler having a square or rectangular plan shape, and if rectangular, having a longer side that is less than 3 feet longer than its shorter side, and whether square or rectangular, discharging smelt from one wall, having no primary combustion air ports on the wall opposite the ported wall, and having at least two but not more than seven primary ports on each of the two walls adjacent to the ported wall, wherein the spacing between each of the primary ports on the two walls adjacent to the ported wall is at least 0.13 times the plan dimension of the boiler parallel to the ported wall or 3 feet, whichever is greater.

2. 2. The chemical recovery boiler of claim 1, wherein all of the primary ports on a first wall adjacent the ported wall are within plus or minus three tube pitches of directly across from the primary ports on the wall opposite the first wall.

3. 3. The chemical recovery boiler of claim 2, wherein a jet of combustion air issues from each of the primary ports towards the interior of the boiler, the volumetric and / or mass flow and / or velocity being controlled automatically and individually for each of the ports.

4. 4. The chemical recovery boiler of claim 3, wherein the volumetric flow rate and / or mass flow rate and / or velocity of at least one first said primary port is adjusted to be at least 25% greater or less than the volumetric flow rate and / or mass flow rate and / or velocity of at least one second said primary port, and wherein said at least one second said primary port is opposed to the first said primary port by plus or minus three tube pitches, forming a strong jet / weak jet relationship between said ports.

5. 5. The chemical recovery boiler of claim 4, wherein the strong jet / weak jet relationship between the ports is periodically and automatically reversed.

6. 5. The chemical recovery boiler of claim 4, wherein the strong jet / weak jet relationship alternates sequentially from port to port along a first wall of the boiler, the first wall being adjacent a ported wall.

7. 1. A chemical recovery boiler having a rectangular planar shape, discharging smelt from one wall, and having no primary combustion air ports on the wall opposite the ported wall, wherein the spacing between individual primary ports on the two walls adjacent to the ported wall must be equal to or greater than the greater of dimension S = 0.13 times the planar dimension W of the boiler parallel to the direction of the air jets or 3 feet, and wherein at least two primary ports are provided on each of the two walls adjacent to the ported wall, and the maximum number of primary ports on each of the two walls adjacent to the ported wall is equal to or less than 7 + N, where N is (D-W) / S rounded down to an integer, and D is equal to the planar dimension of the boiler perpendicular to W.

8. 8. The chemical recovery boiler of claim 7, wherein all of the primary ports on a first wall adjacent the ported wall are within plus or minus three tube pitches of directly across from the primary ports on the wall opposite the first wall.

9. 9. The chemical recovery boiler of claim 8, wherein a jet of combustion air issues from each of the primary ports towards the interior of the boiler, the volumetric and / or mass flow and / or velocity being controlled automatically and individually for each of the ports.

10. 10. The chemical recovery boiler of claim 9, wherein the volumetric flow rate and / or mass flow rate and / or velocity of at least one first said primary port is adjusted to be at least 25% greater or less than the volumetric flow rate and / or mass flow rate and / or velocity of at least one second said primary port, said at least one second said primary port being opposed to the first said primary port by plus or minus three tube pitches to form a strong jet / weak jet relationship between said ports.

11. 11. The chemical recovery boiler of claim 10, wherein the strong jet / weak jet relationship between the ports is periodically and automatically reversed.

12. 12. The chemical recovery boiler of claim 11, wherein the strong jet / weak jet relationship alternates sequentially from port to port along a first wall of the boiler, the first wall being adjacent a ported wall.

13. 2. The chemical recovery boiler of claim 1, wherein the number of primary ports on a first one of the walls adjacent to the ported wall is an even number and the number of primary ports on a second one of the walls adjacent to the ported wall is an odd number.

14. 14. The chemical recovery boiler of claim 13, wherein the primary ports on the first wall are staggered with the primary ports on the second wall such that the primary ports on the first wall are within plus or minus three tube pitches of the center between the primary ports on the second wall.

15. 2. The chemical recovery boiler of claim 1, wherein black liquor is injected into the boiler from at least two levels, a first said level being at least 2 feet above the midline level of the primary port and no more than 12 feet above the floor of the boiler and at least 3 feet below a second said level.

16. 16. The chemical recovery boiler of claim 15, wherein the first elevation is below an elevation of at least one secondary port.

17. 8. The chemical recovery boiler of claim 7, wherein black liquor is injected into the boiler from at least two levels, a first said level being at least 2 feet above the midline level of the primary port and no more than 12 feet above the floor of the boiler and at least 3 feet below a second said level.

18. 18. The chemical recovery boiler of claim 17, wherein the first elevation is below an elevation of at least one secondary port.

19. 14. The chemical recovery boiler of claim 13, wherein black liquor is injected into the boiler from at least two levels, a first level being at least 2 feet above the midline level of the primary port and no more than 12 feet above the floor of the boiler and at least 3 feet below the second level.

20. 20. The chemical recovery boiler of claim 19, wherein the first elevation is below an elevation of at least one secondary port.

21. 15. The chemical recovery boiler of claim 14, wherein black liquor is injected into the boiler from at least two levels, a first level being at least 2 feet above the midline level of the primary port and no more than 12 feet above the floor of the boiler and at least 3 feet below the second level.

22. 22. The chemical recovery boiler of claim 21 , wherein the first elevation is below an elevation of at least one secondary port.

23. 1. A method for improving performance of a chemical recovery boiler including a smelt ported wall, a wall opposite the smelt ported wall, and two side walls between the smelt ported wall and the wall opposite the smelt ported wall, the method comprising: blocking a plurality of primary air port openings on the smelt outlet wall so that only 2 to 8 ports near each smelt outlet are open; blocking essentially all primary air port openings on the wall opposite the smelt outlet wall; and providing at least two, but not more than seven, primary sidewall air ports in each of said two sidewalls; A method comprising:

24. 24. The method of claim 23, wherein the method for improving performance of a chemical recovery boiler includes retrofitting an existing chemical recovery boiler, wherein a total open area of ​​the at least two but not more than seven primary sidewall air ports on each of the two sidewalls is 80% to 120% of the primary air ports of the original chemical recovery boiler being retrofitted.

25. 24. The method of claim 23, wherein the step of blocking essentially all primary air port openings on the wall opposite the smelt discharge wall comprises blocking primary air port openings on the wall opposite the smelt discharge wall such that a total open area of ​​the primary air port openings on the wall opposite the smelt discharge wall comprises less than 5% of a total area of ​​at least two but not more than seven primary sidewall air ports on each of the two sidewalls.

26. The method described in claim 23, wherein the volumetric flow rate and / or mass flow rate and / or velocity of at least one first of the primary sidewall air ports is adjusted to be at least 25% greater or smaller than the volumetric flow rate and / or mass flow rate and / or velocity of at least one second of the primary sidewall air ports, and the at least one second of the primary sidewall air ports is opposite the at least one first of the primary sidewall air ports at a position plus or minus three tube pitches, forming a strong jet / weak jet relationship between the ports.

27. 27. The method of claim 26, wherein the strong jet / weak jet relationship between the primary sidewall air ports is periodically and automatically reversed.

28. 27. The method of claim 26, wherein the strong jet / weak jet relationship alternates sequentially from port to port along one of the two side walls of the boiler.

29. 1. A method of operating a chemical recovery boiler, said method comprising: maintaining smelt flow at the smelt outlet by injecting air through primary smelt outlet air ports positioned near the smelt outlet, the primary smelt outlet air ports having a total primary outlet air port open area; and injecting primary air through 2 to 8 sidewall primary air ports on each of two sidewalls intersecting the smelt discharge port wall, the 2 to 8 sidewall primary air ports having a total primary sidewall air port opening area, the total primary sidewall air port opening area comprising more than 80% of the total primary air port opening area; A method comprising:

30. 30. The method of claim 29, wherein a total required amount of primary air injected into the furnace is determined by stoichiometry, and wherein at least 80% of the total required amount of primary air injected into the furnace is injected through the sidewall primary air ports.

31. 30. The method of claim 29, wherein less than 10% of the primary air is injected through primary air ports on a wall opposite the smelt outlet wall.

32. 32. The method of claim 31 , wherein primary air is not injected through a primary air port on the wall opposite the smelt outlet wall.

33. 30. The method of claim 29, wherein less than 10% of the primary air is injected through the primary smelt outlet air port.

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