Abstract:
A multiple level combustion air system for a chemical recovery boiler incorporates a single level of secondary air ports, with the air ports having an aspect ratio greater than 12. A second embodiment incorporates one or more tertiary levels of combustion air ports with the ports having an aspect ratio greater than 12. Further embodiments incorporate a combination high aspect ration combustion air port and auxiliary fuel burner opening; damper and cleaner means; and the injection of gases through one or more of the combustion air ports.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     In the pulp and paper industry, recovery boilers are used extensively to reclaim spent cooking chemicals from pulp digesters. A variety of dilute chemicals are used in the process to dissolve the lignin in wood chips to liberate the cellulose fibers. At the end of the cooking cycle the fibers are separated from the cooking chemicals, the fibers going off to make paper pulp and the chemicals to be recycled. The separation of the chemicals from the fibers is imperfect, however, resulting in a substantial amount of fibers carried away with the chemicals. In order to be recycled, the excess dilution water in the chemical bath is evaporated resulting in a solution of 20%-30% water, the solid portion being organic material from the wood and the spent inorganic cooking chemicals. When the excess water is substantially removed, the solution (called “black liquor”) becomes the fuel for the recovery boiler. The black liquor is sprayed into the boiler in an atomizing fashion where it goes through several processes during combustion. The black liquor mostly falls to the bottom of the boiler where a “char bed” is formed. The cooking chemicals, carried in an oxidized state in the black liquor must be chemically reduced to be reused. This is an endothermic reaction requiring a driving force produced by the heat of combustion. The oxygen consumed during combustion creates the required reducing environment in and around the char bed, thereby creating the required conditions to reclaim the spent cooking chemicals. The reclaimed chemicals flow out the bottom of the boiler as a molten smelt. The excess heat generated by the combustion of the black liquor is used by the boiler to generate steam which in turn is used to generate electricity and/or as a heat source for a variety of mill processes, including evaporating the black liquor and drying paper. Of course the actual chemical and physical processes are much more complicated than described here. Many details of recovery boiler design, operation and chemistry have been described in many patents granted over the years.  
         [0002]     Recovery boilers are required to be part of both the chemical loop and the steam loop in the modern pulp mill. Because of this dual role, the recovery boiler is an essential piece of equipment in the pulp mill and represents a very significant capital investment. Improving recovery boiler operation is critical to improving the productivity of the mill. Areas of improvement include run time, throughput, thermal efficiency, reduction efficiency, operability, maintenance and emissions. A common thread between these factors is that they are all affected by the combustion in the boiler. Improving the combustion can have significant benefits in all of these areas and the most significant influence on the combustion in the boiler is the design and operation of the combustion air system. The combustion air system consists of the fans, ducting, port openings (in the boiler tube walls), dampers, port cleaners, and other control means used to force combustion air into the boiler.  
         [0003]     Combustion air is by far the largest mass flow entering the boiler with air to fuel ratios of around 5:1 (mass basis). The manner in which the combustion air is introduced is vital to the improved operation of the recovery boiler. Historically, combustion air has been introduced at a variety of levels around the perimeter of the boiler, each level defined by a plurality of air ports arranged in horizontal rows. The ports are formed by bending one or more of the boiler tubes to reveal an opening in the otherwise airtight boiler wall. Because the boiler tubes run vertically, the air ports typically take on a more or less rectangular profile. The number of air levels varies widely, with current trends indicating a single primary level (the lowest level); one, two or three secondary levels (levels between the primary level and the liquor spray); and one or more levels of tertiary air (levels above the liquor spray). The liquor is generally sprayed into the boiler through one or more openings in the boiler walls, usually at the same elevation, at a height sufficient for the liquor to dry and at least begin combusting before reaching the char bed. For the sake of clarity, the upper secondary level may be at the liquor spray level if it is fed by the secondary fan or by heated air. Similarly, the lowest tertiary level may be at the liquor spray level if it is fed by the tertiary fan or unheated air. Furthermore, levels above the lowest tertiary level are sometime called quaternary air. For the sake of this discussion, we will refer to only primary, secondary, and tertiary levels, as defined above. Additionally, recovery boilers are sometimes used to incinerate non condensable gasses that would otherwise be vented to atmosphere. It is also possible to re-circulate flue gas from the boiler back to the boiler to improve combustion and/or control gas temperature. The subject invention is applicable to all of these cases as well as corresponding arrangements on power boilers (boilers used primarily to produce steam).  
         [0004]     The current state of the art in recovery boiler combustion air system design incorporates two or three secondary air levels and two or more tertiary air levels, with the combustion air ports at each level being more or less evenly interlaced with corresponding ports opposite, all levels having a corresponding arrangement, and the ports on each level more or less aligned vertically with the ports above (or below). For example, each level may have three ports on a first wall interlaced with two ports on the opposite wall at the same level. In this case there may be five ports at each level in a “three interlaced with two” arrangement, with the “three interlaced with two” repeated at each level. Many other combinations are possible, but generally all of the levels are arranged corresponding to each other. Several patents have been granted on these designs with more pending and are referred to in the industry as “vertical” or “stacked” air systems.  
         [0005]     While these new combustion air system designs have been shown to work well in theory and in practice, they can be expensive to implement. It is generally accepted that the best geometry for a combustion air port opening is round. The air jet produced by a round opening has the least surface area for its mass flow therefore should be better able to penetrate the depth or width of the boiler to improve the mixing and combustion in the boiler. The adage “mixed is burned” applies. It is not usually practical to make round openings, however, as this would require bending more tubes to get the required open area and would limit the efficacy of port dampers. Rectangular ports have been a common compromise, with the ports made as short and wide as possible while bending as few tubes as possible, to achieve a low aspect ratio (height/width) port shape. It has been recently found, however, that round (and low aspect ratio) ports are not the optimum shape for combustion air ports in recovery (and other) boilers.  
       SUMMARY OF THE INVENTION  
       [0006]     In accordance with the invention, a system provides an elongate port which can be adjusted to provide variable port opening size.  
         [0007]     Accordingly, it is an object of the present invention to provide an improved port and port damper system.  
         [0008]     It is a further object of the present invention to provide an improved recovery boiler damper configuration.  
         [0009]     It is yet another object of the present invention to provide an improved boiler air adjustment system.  
         [0010]     The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a perspective view of the slot port damper system with an automatic port cleaner as installed in a recovery boiler;  
         [0012]      FIG. 2  is a perspective sectional view taken along line  1 - 1  of  FIG. 1 ;  
         [0013]      FIG. 3  is an end view of the system of  FIG. 1 ;  
         [0014]      FIG. 4  is a side view of the system of  FIG. 1 ;  
         [0015]      FIG. 5  is a side sectional view taken along line  5 - 5  of  FIG. 4 ;  
         [0016]      FIG. 6  is a view from the interior aspect of the furnace of an alternative slot port system;  
         [0017]      FIG. 7  is a perspective view of the system of FIG.  6 ;  
         [0018]      FIG. 8  is a side view of the system of  FIG. 6 ;  
         [0019]      FIG. 9  is a perspective view of the system of  FIG. 6  taken from outside of the furnace;  
         [0020]      FIG. 10  is a top view of the system of  FIG. 6 ;  
         [0021]      FIG. 11  is an end view of the system of  FIG. 6  from outside the furnace; and  
         [0022]      FIG. 12  is an end view of the configuration of the  FIG. 11  system illustrating closure of lower damper doors. 
     
    
     DETAILED DESCRIPTION  
       [0023]     The advent of computational fluid dynamic (CFD) modeling of recovery boiler combustion has shown that it is much easier to penetrate the full horizontal width or depth of a recovery boiler with a non-round air jet than was previously thought. Furthermore, combustion air systems with multiple ports arranged immediately above each other can be significantly improved by replacing the multiple ports with a single ultra high aspect ratio port (hereafter referred to as a “slot port”). The slot port has at least three performance advantages: First, the increased surface area to mass flow creates an air jet with more surface area to interact with the fuel in the boiler. The turbulence over this much larger surface area improves mixing and combustion in the boiler. Second, the vertical elongation of the air jet resists deflection by the upward flowing combustion gases since only the narrow width of the jet is exposed to the flowing gas. Third, the air jets from slot ports are narrower for a given mass flow therefore “interlace” better with air jets from the opposite wall. That is, there is more space between the jets allowing the jets to pass in opposite directions with less interference. The “shearing action” between two opposite jets may create good turbulence but frequently the interference between jets causes them to skew from their intended path. The second two characteristics above allow the slot port jets to overcome the drag of the added surface area and more effectively penetrate the boiler. The static pressure in the ducts feeding the combustion air to the boiler is the driving potential for the air jets, and also has to overcome the frictional losses as the air passes through the port opening. The slot port, with much larger surface area, has greater frictional losses. Normally this would require higher static pressure but the slot port air jets penetrate the boiler more easily therefore requires less static pressure to drive the jets. This offsets the increased frictional pressure drop.  
         [0024]     Economically the slot port is advantageous because it reduces the work required either in new boiler construction or the modification of an existing boiler. For example, a “stacked air” system may require the replacement of 48 tube sections requiring 96 tube welds, but a corresponding slot port system requires the replacement of only 18 tubes with 36 welds. This results in a significant savings in material, labor, risk, and boiler outage time.  
         [0025]     The subject invention comprises a combustion air port, in any quantity, in any arrangement on the boiler, with an aspect ratio greater than 12. The aspect ratio is defined as the height of the port opening squared divided by the open area of the port, not including any castings, inserts or dampers. In other words just the tube opening itself, including any attached membrane, in a clean and cold condition. If H=port height, A=port area, and AR=aspect ratio, than AR=H2/A. Port openings with aspect ratios up to 12 are common on recovery boilers but higher aspect ratios have been avoided because of the bias to low aspect ratio ports. High aspect ratio dampers have been used but only in conjunction with low aspect ratio ports thus limiting their overall height to that of the port opening. These ports do not have adequate flow capability for the modern air system designs. The invention further includes parameters on port opening sizes and damper and cleaner means that work with the slot port concept.  
         [0026]     Typical boiler wall construction consists of heavy steel tubes, set vertically in a planar array, closely spaced and seal welded directly together, or more commonly with up to one inch or more separation distance between the tubes, and the gap filled with the same width steel bar (the membrane) and seal welded to the tubes. The port openings are created by bending one or more of the straight boiler tubes to the side to create the opening. If the desired opening width is wider than can be achieved by bending the tubes in the plane of the tube wall, or to keep the number of bent tubes to a minimum, the tubes may be bent out of the plane of the tube wall. It is also possible to create the openings by simply eliminating the membrane over a short vertical distance between the tubes. Older boilers may use refractory to fill the gaps between the tubes, in which case some of the refractory is eliminated in way of the port openings. Generally when tubes are bent to form the openings, some of the membrane is also removed. The invention uses corresponding construction techniques but since the ports are very narrow, typically no more than one tube requires bending.  
         [0027]      FIGS. 1-5  show a first embodiment, wherein (in the illustrated embodiment), two sets of adjustable dampers  12 ,  14  are positioned to divide the elongate port into upper, middle and lower port opening segments. Each damper is individually adjustable, to expand or contract to cover a wider or narrower segment of the port, so as to adjust the air flow through a particular upper, middle or lower portion of the air port. One manner to accomplish this is by extending an adjustment rod  16 ,  16 ′ which causes the center hinge position  18 ,  18 ′ of damper leaves  20  to move toward the port and the interaction of the leaves with the port causes the leaves to open and narrow the air port opening in the region of the damper leaves, and by retracting the adjustment rod, thereby pulling the hinge point  18  of the leaves away from the port which allows the leaves to move toward one another and provide a greater opening size for the port. The illustrations of  FIGS. 1-5  employ two sets of dampers to define  3  port segments  22 ,  24 , and  26 . An automatic port cleaner  29  is also illustrated installed in  FIGS. 1-5 .  
         [0028]      FIGS. 6-12  illustrate an alternative embodiment, wherein the system is adapted for use in a case where a starting burner, for example, would be installed at the port. The wide portion  30  at the bottom of the port is to accommodate the starting burner, and includes a box  32  with a horizontally sliding damper portion  34  having an elongate rod type handle  36 , to allow adjustment and control of the air flow through the box portion. When the starting burner is in use, the horizontal damper at the top is typically more open, and left and right damper-members  38  within the box are retracted to be parallel to the sides of the box and allow the maximum opening at the lower portion of the air port. When the horizontal damper is closed somewhat, as in the case when the starting burner is not being used (in which case the burner apparatus (not shown) may be retracted, the left and right damper members close the wider lower port portion down so as to have substantially the same width as the upper slot shape port portions have (visible in  FIG. 12 , where the left and right doors are closed, as opposed to  FIG. 11 , where the doors are open). The adjustable port damper system comprises plural individual port damper members  40  (six such members shown in the illustrations) above the box portion, whereby the dampers are adapted to swing away from the port opening or to swing so as to block a portion of the port opening. In the illustrations, two lowermost dampers  40 ′ are swung to the blocked position, so that the lowermost portion of the port is blocked off, while the  4  upper dampers  40  are open. The dampers include attachment points at the rear thereof (relative to the furnace interior) and are hingedly secured at one edge so that by extension or retraction of an actuation member in attachment with the attachment points, the dampers can be individually opened or closed, enabling any combination of open and closed damper settings to provide control of the overall port area as desired.  
         [0029]     The subject invention is applicable to the improvement of combustion air systems for existing boilers, but it is useful to improve the design of new boilers as well. It is equally useful for power boilers as well as recovery boilers.  
         [0030]     While preferred embodiments of the present invention have been shown and described, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the invention in its broader aspects. The appended claims are therefore intended to cover all such changes and modifications as fall within the true spirit and scope of the invention.