Abstract:
A ground supported single drum power boiler is described combining a refractory lined and insulated stepped floor; refractory lined and insulated combustion chamber; integrated fuel chutes configured to pre-dry wet solid fuel; internal chamber walls; configurable combustion air systems; a back pass with after-burner ports and cross flow superheaters; and a rear wall that acts as the downcomers feeding the other walls. A second embodiment is adaptable as a gasifier.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The invention relates to solid fuel boilers. 
       BACKGROUND OF THE INVENTION 
       [0002]    Power boilers have long been used by industries and utilities to produce steam for power production and process requirements. These boilers come in many types and sizes but the present invention is an improvement on bed-fired water tube boilers ranging in steam production from 25 tons per hour to 300 tons per hour or more. The fuel may consist of bark, sawdust, wood chips, biomass trimmings, wood or other biomass pellets, urban waste, tire derived fuel (TDF), crushed coal, pet coke, sludge, fiber line rejects or other solid fuel, or a combination of fuels, and may have moisture content as high as 60%. These boilers are typically constructed of heavy wall steel tubes welded side by side into wall panels that form the front, rear and side walls of the boiler. The lower portion of this box forms the combustion chamber of the boiler and is sometimes called the furnace. The tubes are typically 2½″ to 3″ in diameter and spaced apart 3″ to 4″ center to center. The gaps between the tubes are filled with steel strips about ¼″ thick by the width of the gap. The entire panel is seal welded air tight. The lower ends of the wall tubes are welded into larger diameter horizontal header pipes that feed water to the walls. The tops of the wall tubes are also connected to larger diameter horizontal collector pipes that carry the water away from the walls to the steam drum, located at the top of the boiler. The front wall tubes are typically bent over to form the roof of the boiler and those tubes can terminate in a collector pipe or directly to the steam drum. Similarly the rear wall tubes are typically bent to create a “bullnose” or “nose arch” to direct combustion gasses across the convective section of the boiler and then terminate in a water drum, steam drum, or collector pipe at the top of the boiler. The top of the bullnose is usually at the elevation of the water drum. Downcomer pipes connect the steam drum or water drum at the top of the boiler to the header pipes at the bottom of the tube walls and feed water from the drum to the walls. The bottom of the boiler can be a travelling or vibrating grate, tilting grate, sloping grate, step grate, fluidized bed, or a Stepped Floor as described in U.S. patent application Ser. No. 12/557,085. Fuel enters the boiler through a chute or chutes penetrating one or more walls of the boiler and may be broadcast into the boiler by a fuel distributor, for example, as described in U.S. patent application Ser. No. 12/406,035. The fuel falls to the floor or grate where it is mixed with air and burns. The heat released by the burning fuel is absorbed by the wall tubes and heats the water in the walls, where the water expands thermally and starts to boil. The heated and boiling water is less dense than the water in the downcomer pipes therefore a natural circulation is created with hotter water rising in the tube walls and cooler water descending in the downcomer pipes. The natural circulation is an inherent safety feature of these boilers as the circulation rate increases as more fuel is burned and more heat released in the combustion chamber. 
         [0003]    As the water circulates from the steam drum, down through the downcomers, up through the walls, and back to the steam drum, some of the water may boil but most of the boiling occurs in the steam generating bank, sometimes called the boiler bank. In older two drum boilers, the generating bank is a set of tubes connecting the bottom of the steam drum to the top of a water drum, sometimes called a mud drum, located up to thirty feet or so directly below the steam drum. There are generally hundreds of tubes connecting the two drums. The generating bank is arranged so that hot gasses from the furnace flow across the tubes and heat the water circulating inside. About half of the tubes in the generating bank of a two drum boiler are up flow tubes and the remainders are down flow tubes. The gas cools as it passes through the generating bank, therefore the first tubes the gas contacts (the front tubes as the gas flow through a boiler is generally front to back) are hotter and more boiling occurs in those tubes. The boiling water is less dense therefore the water circulates from the steam drum down through the rear tubes to the water drum then up through the front tubes back to the steam drum. The steam drum is generally about half full of water with saturated steam being released at the surface. The steam goes through a set of moisture separators and then to the superheaters. In newer single drum boilers there is no water drum, instead, the generating bank is fed by external (non-heated) downcomers from the steam drum and the water circulates down the downcomers and back up through all of the generating bank tubes to the steam drum. Single drum boilers are less expensive to build because the drums, especially with hundreds of tube penetrations, are the most expensive components. They also have other advantages including more flexible arrangements for locating the steam drum. 
         [0004]    Some boilers also have sets of tubes located just at the furnace exit and arranged to cross the boiler at the top of the combustion chamber. These are called screen tubes or screens, and are often arrayed as platens in which several tubes are in close parallel arrangement, one on top of another, extending from the front or rear wall of the boiler through the opposite wall. These platens are generally separated 12″-15″ apart side to side and slope upward slightly to the other side of the boiler, or they may bend part way across the boiler and rise up vertically through the roof. The screen tubes are fed by external (non-heated) downcomers from the steam drum at their lower end and relieved back to the steam drum at their upper end. Water circulates from the steam drum or water drum through the screens and back up to the steam drum. The screens are located where the gasses are very hot and absorb heat predominantly by radiation. 
         [0005]    After the steam leaves the steam drum it goes to the superheaters. These are sets of tubes typically located at the top of the boiler, above the screen tubes and in front of the generating bank. The superheaters increase the temperature of the steam from the saturation temperature in the steam drum to the final temperature required by the process or the power plant. The superheater tubes are typically arranged as vertical platens with up to a dozen tubes or more in close parallel arrangement front to back in each platen. There are many platens located across the width of the boiler with a spacing of 6″-15″ between platens. There are frequently three or more superheater sections with connecting pipes and/or desuperheaters between the sections. Desuperheaters or attemporators control the final steam temperature by spraying water into the steam, or other means. The superheater tubes start at the top of the boiler and drop vertically to just above the bullnose then run up and down a number of times before exiting back through the roof. The steam passes through the superheaters just once therefore the superheaters are not part of the boiler circulation circuits. 
         [0006]    After the boiler flue gasses exit the generating bank they typically flow through an economizer or an air heater. Economizers are tube bundles either in cross flow or parallel flow to the gas stream through which the feedwater passes once and is heated and then goes to the steam drum. The feedwater is controlled to maintain the water level in the steam drum. Feedwater makes up for the steam that is produced and exits the boiler. Upon entry into the drum, feedwater is baffled and mixes with some of the water already within the steam drum to flow to the downcommer pipes or downcommer tubes. This feedwater mixed zone has higher density, which provides the driving head for the natural circulation in the boiler. The economizer may be located immediately after the generating bank integral with the boiler, or it may be located downstream from a tubular air heater or a dust collector. 
         [0007]    Some of these boilers are supported from underneath (ground supported) but most, especially larger boilers, are hung from the top and expand downward as they heat up. A “hung” boiler requires a very strong and expensive structure to support the boiler. 
         [0008]    Boilers as described above have been in use for many years and the technology is very mature, but they are very expensive and have significant operational limitations. Grate fired boilers and fluidized bed boilers are limited in the temperatures they can tolerate in the lower furnace otherwise they will over heat the grate or sand bed. They also do a poor job of mixing the combustion air and fuel because the air flow arrangement is dictated by the requirements to cool the grate or fluidize the sand bed. This leaves little setup flexibility to improve combustion in the boiler. Mechanical grates suffer from poor reliability and fluidized bed boilers suffer from excessive sand erosion and sand agglomeration. These deficiencies are addressed with the introduction of stepped floor and fuel drying chute technologies as described in U.S. patent application Ser. Nos. 12/557,085 and 12/471,081 respectively, and provisional application 61/522,939, all three of which are hereby incorporated by reference. Those technologies may be incorporated into some embodiments of the present invention to improve the combustion of difficult to burn fuels. 
       SUMMARY OF THE INVENTION 
       [0009]    An object of the invention is to provide an improved system for firing of biomass and other solid fuels for steam production and gasification. 
         [0010]    Various embodiments of the present invention include several novel features that can significantly reduce the capital cost of the boiler while further improving the ability to burn difficult fuels. These features also make the boiler adaptable as a gasifier that can be used in a Fischer-Tropsch (F-T) process to produce liquid fuels. For the purposes described herein, a gasifier is a device that heats a biomass or other fuel to produce pyrolysis gases with little or no combustion of those gases in the gasifier. The pyrolysis gases or “syn-gas” is taken off, cleaned, and burned separately or may be sent to an F-T process. Gasifiers used in F-T processes may incorporate a separate chamber where a heat transfer media, typically sand, is heated, the sand is then transferred to the gasifier chamber where it heats and gasifies the biomass. The advantage of this “sand based” system is it can be operated under pressure, the gasification temperature can be tightly controlled, and the liberated pyrolysis gasses are not diluted. Sand-based systems are complex, may be difficult to scale up, and may suffer from sand erosion. A second embodiment of the invention is described below that is much simpler in design and operation while maintaining the advantages and minimizing the disadvantages of typical gasifiers. 
         [0011]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should 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 should also be realized 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. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0012]    For a more thorough understanding of the present invention, and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0013]      FIG. 1  is a sectional side view of the boiler. 
           [0014]      FIG. 2  is a sectional front view. 
           [0015]      FIG. 3  is a sectional plan view cut through combustion chamber  1  and viewed from above with the front of the boiler to the left. 
           [0016]      FIG. 4  is a sectional plan view cut through front pass  5  and viewed from above with the front of the boiler to the left. 
       
    
    
     DESCRIPTION 
       [0017]    The accompanying drawings are referred to in the following description.  FIG. 1  is a sectional side view of the boiler and  FIG. 2  is a sectional front view.  FIG. 3  is a sectional plan view cut through combustion chamber  1  and viewed from above with the front of the boiler to the left.  FIG. 4  is a sectional plan view cut through front pass  5  and viewed from above with the front of the boiler to the left. The numbers in this text refer to like numbers illustrated in the drawings. Referring now to  FIG. 1 , the combustion chamber  1  is formed by a reinforced concrete foundation  2  that rises 20 feet or more above the ground. Foundation walls  2  support front tube wall  3 , left tube wall  25 , right tube wall  26 , and rear tube wall  27  of the boiler and intermediate foundation wall  7  supports intermediate tube wall  4  that separates the front pass  5  (the furnace) from back pass  6 . Therefore in plan view the foundation is a rectangle with an intermediate wall  7  running side to side separating the foundation into a front chamber  8  and a rear chamber  9 . The concrete has openings formed as necessary for personnel access, ducts, conveyors, combustion air ports, fuel injection, etc. The lower headers  10  sit on top of the concrete foundation and in turn support the tube walls  3 ,  4 ,  25 ,  26 ,  27  and the rest of the steel boiler structure above. A stepped floor  11  and ash grate  12  are installed in the foundation front chamber  8  and suitably supported by features built into the foundation. A steel plenum duct  13  lines the four walls of the inside of the foundation front chamber  8  from the top of the stepped floor  11  to the top of the foundation walls. Plenum duct  13  is comprised of inner and outer walls parallel to the foundation walls and spaced about 1½″ apart with the outer walls in contact with the inside of the concrete foundation walls that form combustion chamber  1 . 
         [0018]    Some or all of the combustion air and/or recirculated flue gas fed to the boiler flows through plenum duct  13 , therefore plenum duct  13  acts as a thermal barrier between combustion chamber  1  and foundation  2  and intermediate foundation wall  7 , and also as an air heater for the air and/or recirculated flue gas flowing to the boiler. Immediately to the inside of plenum duct  13  is a layer of thermal insulation  14 , parallel to and in contact with the inner wall of plenum duct  13 . The insulating layer  14  extends around the inside of the four walls of plenum duct  13  and extends from the top of stepped floor  11  to above the top of concrete foundation  2  and intermediate foundation wall  7 . Insulating layer  14  may be comprised of insulating type refractory, ceramic paper, ceramic bats, poured refractory, precast refractory tiles, or some combination of these or other suitable materials. Insulating layer  14  may be from 4 inches to 6 inches thick. Immediately to the inside of insulating layer  14  is a layer of working refractory  15 , parallel to and in contact with the inner surface of insulating layer  14 . Working refractory  15  extends around the inside of the four walls of insulating layer  14  and extends from the top of stepped floor  11  to above the top of concrete foundation  2  and intermediate foundation wall  7 . Working layer  15  may be up to 6 inches thick and the top edge of the four walls of working layer  15  are shaped to form a protective curb  16  reposed against the boiler tube walls. Working layer  15  is comprised of a high strength abrasion resistant refractory material with an operating temperature limit of up to 3000 degrees Fahrenheit. The refractory of working layer  15  also has relatively high density and thermal conductivity. 
         [0019]    The construction of stepped floor  11  is similar to the construction of the walls of the combustion chamber  1  in that it has a plenum duct  17 , an insulating layer  18 , and a working layer  19 . Some or all of the air and/or gas flowing through plenum duct  13  may then flow through the stepped floor plenum duct  17  before flowing out through slots  44 , located at the base of each step riser, and into combustion chamber  1 . Stepped floor plenum duct  17  is designed to support stepped floor  11  while the combustion air and/or gas flowing there-through provides cooling for stepped floor plenum duct  17 . Imbedded in stepped floor  11  are stepped floor cleaners  49  located at the base of each step riser and oriented to blow one or more steam jets out through slots  44 . The steam jets, one or more per step, are programmed to blow periodically and oscillate along the length of the step to clean ash, sand, rocks, etc. from the steps and push that material down to ash grate  12 . Unburned fuel will also be cleaned from the steps, much of which is ready to burn and will ignite rapidly. Some of the unburned fuel will burn out on ash grate  12 . The stepped floor described above is more specifically detailed in U.S. patent application Ser. No. 12/557,085. Stepped floor cleaners  49  can also be arranged, when not blowing steam and cleaning the steps, to blow pressurized air onto the fuel piles to continually agitate the fuel and inject combustion air into the fuel. Some of the combustion air and/or recirculated flue gas flowing through plenum duct  17 , or from other ducts, is routed through side-sweep ports  45  and injected into combustion chamber  1  from two opposing walls. Side-sweep ports  45  are generally arranged with a lower level of ports 10 to 18 inches above the steps and an upper level 24 to 48 inches above the lower level. An upper level port is located more or less directly above each lower level port and although the two levels are created by being either an upper or lower port, any two or more ports at either level may or may not be at the same vertical elevation. The arrangement of side sweep ports  45  can induce the circulation of gasses in combustion chamber  1  depending on which ports are turned on and off. For example, referring to  FIG. 3 , if side sweep ports  45  on left wall  25  are, from front to back, off, on, off, on, and the ports  45  on the right wall are on, off, on, off, then, when viewed from above, clockwise circulation  36  will develop at the front of combustion chamber  1 , counterclockwise circulation  37  will develop at the middle, and clockwise circulation  38  will develop at the rear of combustion chamber  1 . Other circulation patterns can be developed depending on the number of side sweep air ports and which ports are turned on and off. Side sweep ports  45  are fitted with automated dampers that can be programmed to turn on and off in unison and at prescribed intervals. This will “flip-flop” the damper settings and reverse the circulation patterns to prevent a long term bias in the fuel formation or other aspects of the boiler operation. The purpose of side sweep ports  45  is to distribute fuel more uniformly and control the fuel bed formation; provide good mixing of air and pyrolysis gasses for improved combustion and heat release; establish a circulation pattern that works with the geometry of the boiler and other air ports; and provides a back-up source of combustion air should there be a blockage or other problem with combustion air slots  44  in the floor steps. 
         [0020]    A first purpose of the described construction around the perimeter of combustion chamber  1  is to retain as much of the heat of combustion as possible in the combustion chamber, without transmitting that heat to adjacent boiler walls, as in conventional boilers, or to the adjacent supporting structures of some embodiments of the present invention. This will increase the temperature in the combustion chamber and allow the firing of wetter fuel. Insulating layer  14  is required to minimize the heat transfer out of combustion chamber  1  and working layer  15  is required to provide protection to insulating layer  14 . While insulating layer  14  has low thermal conductivity some heat will still be transmitted through it therefore plenum duct  13  is required to prevent overheating concrete foundation  2  and intermediate foundation wall  7 . A second purpose of the construction is to act as a heat sink around the combustion chamber. Working layer  15  has a relatively high heat capacity; therefore it will retain a lot of heat. If the temperature drops in combustion chamber  1 , due for example to a batch of overly wet fuel, the heat contained in working layer  1  will be radiated to combustion chamber  1  where it will help to dry and ignite the wet fuel and stabilize the combustion process. In other words, working layer  15  acts as a “thermal flywheel”. The construction of the stepped floor with plenum duct  17 , insulating layer  18 , and working layer  19  works in the same way as the construction of the walls of combustion chamber  1  described above. 
         [0021]    Fuel is fed to the combustion chamber of the boiler through upper fuel chutes  20 , lower fuel chutes  21 , and fuel distributors  22 . Each of upper fuel chutes  20  are comprised of boiler tubes making a three sided chute, integral with the front wall of the boiler, with the interior of the chutes open to front pass  5  (the interior of boiler). Each of lower fuel chutes  21  are comprised of reinforced concrete and form a three sided chute with the interior of the chutes open to the interior of combustion chamber  1 . Fuel distributors reside at the bottom of fuel chutes  21  and inject the fuel into the boiler. Refractory tiles  24  are stacked one above another across the opening between upper fuel chute  20  and the interior of the boiler  5 , and across the opening between lower fuel chutes  21  and the interior of combustion chamber  1 . Refractory tiles  23  serve to retain the fuel as it is falling through the chute and also to radiate heat absorbed from the front pass  5  and combustion chamber  1  to the falling fuel. The stack of refractory tiles  23  ends below the top of upper fuel chute  20  and above the bottom of fuel chute  21  leaving a passageway for combustion gasses to flow into the top of upper fuel chute  20  and out the bottom of lower fuel chute  21 . The opening at the bottom of lower fuel chute  21  also allows the injection of the fuel into combustion chamber  1 . Alternately refractory tiles  23  can be replaced with cast refractory. Jets of recirculated flue gas are strategically located in the fuel chutes and along with the falling fuel induce a circulating flow of hot combustion gas to enter the fuel chutes from front pass  5  at the top, mix with the fuel, and exit the fuel chutes at the bottom back into combustion chamber  1 . The hot combustion gas and radiant heat from refractory tiles  23  produces a drying effect on the falling fuel. The fuel chutes described above are more specifically detailed in U.S. patent application Ser. No. 12/471,081 and 61/522,939. 
         [0022]    The outer boiler walls  3 ,  25 ,  26 ,  27  and intermediate boiler wall  4  are supported by foundation walls  2  and intermediate foundation wall  7  and rise vertically to a height up to 80 feet or more above the ground. Interior to front pass  5  are three chamber walls  24  constructed of similar steel tubes to the boiler walls with the tubes side by side in closely spaced parallel configuration forming flat panels. The chamber walls  24  are arranged parallel to the front wall of the boiler and are spaced more or less evenly between front wall  3  and intermediate wall  4 . The lower extremities of the tubes forming chamber walls  24  pass through the side walls  25  and  26 , with half of the tubes forming each chamber wall  24  coming in through left wall  25  and half through right wall  26 . The tubes comprising one half of each chamber wall  24  pass through side walls  25  or  26  horizontally or with a slightly upward angle as they extend toward the middle of front pass  5 . The more or less horizontal tubes are arrayed with one tube immediately above another so that all of the tubes forming half of one chamber wall  24  can pass through sidewall  25  or  26  between two wall tubes that have been bent apart for that purpose. When the two sets of tubes forming each half of the chamber walls  24  meet in the middle of the boiler, they turn upward and form a single panel that continues vertically and finally exits through the roof of the boiler. Chamber walls  24  are narrower than the width of the boiler such that gaps  28  exist between the outside edges of chamber walls  24  and left sidewall  25  and right sidewall  26 . In some cases, over-fired air ports  29  are formed in sidewalls  25  and  26  to inject combustion air and/or recirculated flue gas between chamber walls  24 . The arrangement of over-fired air ports  29  is typically two or three horizontal rows of ports  30  with each port in each row located approximately centered in the gap between chamber walls  24 . Vertical separation between air port rows  30  can be 3 feet to 10 feet. Gaps  28  allow for circulation of combustion gasses around and between chamber walls  24 . The arrangement of over-fired air ports  29  can induce the circulation of gasses around chamber walls  24  depending on which ports are turned on and off. For example, referring to  FIG. 4 , if over-fired air ports  29  on left wall  25  are, from front to back, off, on, off, on, and the ports  29  on the right wall are on, off, on, off, then, when viewed from above, clockwise circulation  46  will develop around front chamber wall  33 , counterclockwise circulation  47  will develop around middle chamber wall  34 , and clockwise circulation  48  will develop around rear chamber wall  35 . Other circulation patterns can be developed depending on the number of chamber walls, the number of over-fired air ports, and which ports are turned on and off. Over-fired air ports  29  are fitted with automated dampers that can be programmed to turn on and off in unison and at prescribed intervals. This will “flip-flop” the damper settings and reverse the circulation patterns to prevent a long term bias in the gas flow, heat transfer, tube fouling patterns, ash accumulation or other aspects of the boiler operation.  FIGS. 1 and 4  show three chamber walls  24  but the number may vary from one to four or more. The circulation of the gasses around the chamber walls increases the residence time of the gasses in front pass  5  and improves the mixing of combustion air and pyrolysis gasses liberated in combustion chamber  1  thereby improving the combustion and heat release and minimizing the emissions from the boiler. Circulating the combustion gasses also improves the heat transfer from the hot gas to boiler walls  3 ,  4 ,  25 ,  26 , and chamber walls  24  by increasing the gas residence time for better radiant heat transfer, and increasing the gas velocities relative to the wall surfaces for better convective heat transfer. 
         [0023]    Chamber walls  24  are made as tall as practical with the lower extremities just above the refractory walls  15  of combustion chamber  1  and extending up through the roof of the boiler  32 . The lower ends of chamber walls  24  are fed by downcomer pipes (not shown) from steam drum  43  that feed relatively cold water to vertical headers  40  that in turn feed the individual tubes forming chamber walls  24 . At the top of front pass  5 , some of the tubes comprising chamber walls  24  are bent out of the plane of the chamber walls to form front to back passage ways for the combustion gasses to pass through and exit to the rear of front pass  5 . These are chamber wall screens  39 . At this point the gasses stop circulating around chamber walls  24  and pass through and around chamber walls  24  to exit the front pass. Similarly, some of the tubes at the top of intermediate wall  4  are bent out of the plane of the wall to form rear screen  41 . The vertical location of the cluster of over-fired air ports  31  within front pass  5  can be from just above the horizontal sections of chamber walls  24  to just below the chamber wall screens  39 . After chamber wall screens  39  and rear screen  41  pass through boiler roof  32  the tubes are bent back into the plane of chamber walls  24  and intermediate wall  4 , respectively, and then terminate in collector pipes  42  that are in turn connected back to steam drum  43 . As the water in chamber walls  24  is heated it expands and becomes less dense and the heavier cold water flowing down from steam drum  43  pushes the hotter and lighter water upward creating a natural circulation through chamber walls  24 . Chamber walls  24  are arranged in a similar manner as screens in conventional boilers in that they are located in the front pass of the boiler and comprise some of the water circulation circuits of the boiler. The chamber walls are novel, however, in that they extend much lower in the boiler and are arranged so that combustion gasses can circulate around them. The chamber walls extend below the level of the superheaters, and extend below one half the boiler height. The chamber walls are also meant to take the place of the generating bank in conventional boilers therefore boiling will occur in the upper portion of chamber walls  24 . The lower portion of chamber walls  24  are filled with water and therefore will be held close to the saturation water temperature. This prevents chamber walls  24  from over-heating at their lower extremities where the combustion gasses are hottest. The high differential temperature between the gasses leaving combustion chamber  1  and the surface of the lower portion of chamber walls  24  will create a high heat flux from the combustion gases to the water in the chamber wall tubes. This will rapidly reduce the temperature of the combustion gasses as they circulate around and rise past chamber walls  24  and will prevent overheating the chamber wall tubes even as the water is boiling at the upper ends of the tubes. 
         [0024]    After flowing up and around chamber walls  24  and through chamber wall screens  39 , the combustion gasses pass through rear screen  41  and enter back pass  6  where the gasses turn and flow vertically down over the superheaters  50 . In some cases, a series of after-burner air ports  51  may be located below rear screen  41  and above superheaters  50  to inject a final amount of combustion air to complete the combustion of any syngas remaining in the flue gas stream. The arrangement of after burner ports  51  may be in one or more levels with one or more ports at each level with the ports aligned to create interlaced or circulating gas flow patterns depending on the spacing of the ports or the setting of the control dampers. Automated dampers can be installed and programmed to control the flow pattern and periodically flip-flop the arrangement as described above for side sweep ports  45  and over-fired air ports  29 . One or more gas or oil burners may be installed at this location to control final steam temperature by controlling the temperature of the flue gas entering the superheaters. The installation and arrangement of over-fired air ports  29  and after burner ports  51  depends on many factors including fuel type, emissions requirements, boiler loading, downstream processes, etc. For example, over-fired air ports  29  may be moved up or down in front pass  5  to fit specific requirements, or may be omitted altogether in favor of after burner ports  51 , and vice versa. 
         [0025]    The tubes forming superheaters  50  run horizontally back and forth across rear pass  6 , either front to back or side to side. They can be arranged conventionally with one tube above another but preferably arranged in a staggered pattern in which the combustion gas has to flow around all of the tubes. The latter arrangement is more thermally efficient but can be more difficult to clean. Being more thermally efficient, the staggered arrangement requires fewer tubes thereby reducing the cost of the boiler. Another feature of superheaters  50  are internal tube sheets  52  that channel the flue gasses across only the straight sections of the tubes. It is common that particulates in the flue gas stream preferentially erode the tubes at the bends therefore tube sheets  52  shield the tube bends and prevent their erosion. There are at least two significant advantages to the location of superheaters  50  in rear pass  6 . First, as the superheaters are located downstream from chamber walls  24 , chamber wall screens  39 , and rear wall screen  42 , the flue gas temperature entering superheaters  50  will be lower than if the superheaters were located in front pass  5  as is common practice. This will help prevent corrosion of the superheater tubes from chlorides present in the flue gas if, for example, the boiler is used to incinerate municipal waste containing plastic. A second advantage is water can be used to clean the superheater tubes as there is no danger of water interfering with the combustion process. This allows tighter spacing of the superheater tubes which can compensate for lower flue gas temperature and/or reduce the overall surface area of the superheater required. 
         [0026]    Another feature of some embodiments of the present invention is that the tubes comprising rear wall  27  act as the downcomers from steam drum  43  to lower rear wall header  53  that in turn feeds sidewall headers  10  that then feed water to front wall  3 , side walls  25  and  26 , and intermediate wall  4 . This eliminates the need for separate downcomer pipes required to feed the wall headers. It is possible that some of the tubes comprising sidewalls  25  and  26 , specifically those located in rear pass  6 , can also be used as down flow tubes if additional flow area is required. 
         [0027]    Some embodiments of invention improve on previous technology by lowering the amount of capital employed and improving the operation of power boilers. Superheaters in conventional boilers are typically placed at the top of the boiler above the bullnose and arranged with the combustion gas in cross flow. The superheater platens are typically spaced 7-12″ or more apart to minimize the potential to plug between the platens. This wide spacing increases the superheater surface area and the volume (furnace size) required to enclose the superheaters. Accumulation of ash and other deposits on the walls of a boiler are generally not a problem because the deposits will burn or melt or slough off when they reach sufficient thickness. 
         [0028]    Various embodiments provide improvements in many ways. Chamber walls  24  and intermediate wall  4  replace a typical generating bank but are widely spaced therefore, like boiler walls  3 ,  25 ,  26 , and  27 , are not prone to plugging. Chamber walls  24  and intermediate wall  4  are water filled, therefore at a lower temperature than superheaters, and are exposed on both sides to the hot combustion gasses. The combustion gasses are in more intimate contact with chamber walls  24  and intermediate wall  4  than outside walls  3 ,  25 ,  26 , and  27  therefore for all these reasons, they will transfer much more heat than the outside boiler walls. This large heat transfer rate collapses the gas temperature quickly so that by the time it enters back pass  6  the gas temperatures are lower than in a conventional boiler. This makes any accumulated material easier to remove and creates an opportunity to control the gas temperature in back pass  6  by the introduction of additional combustion air through air ports  51 . Chamber walls  24  also scrub ash and particulates from the combustion gas flowing through the boiler. The suspended ash and particulates will tend to stick to the surfaces they contact (until they accumulate sufficiently and are eventually shed) so the gasses will deposit a large part of the suspended material on chamber walls  24  before the gasses enter back pass  6 . Chamber wall screens  39  and rear wall screen  41  are widely spaced and combined with the lower gas temperature and less particulates, will be much easier to clean by sootblowers located adjacently. So the chamber walls are advantageous compared to a generating bank because they collapse gas temperature, trap particulates yet are easier to clean, reduce the required boiler volume, improve heat transfer rates, and reduce the overall cost of the boiler. 
         [0029]    Superheaters  50  located in back pass  6  are in cross flow arrangement (as in a conventional boiler) but because much of the particulates have been trapped out by chamber walls  24 , the tubes can be more closely spaced, even in a staggered arrangement, improving the heat transfer efficiency. Due to the lower gas temperature in back pass  6 , more surface area may be needed in superheaters  50  but this is offset by the more efficient tube arrangement and the improved cleanliness of superheaters  50 . With the superheaters  50  located in the back pass, and with lower gas temperature, the tubes can be cleaned more effectively with conventional sootblowers or even periodically with water. This ensures much cleaner tube surfaces and allows for a reduction in the required surface area as well as an overall reduction in the size of the boiler. With less particulates reaching the superheaters and lower gas temperature, less steam will be required to clean the superheaters when using conventional sootblowers, lowering the operating cost of the boiler. Also, the tubes comprising superheaters  50  are all the same length (as opposed to tapered pendants in a conventional boiler) allowing more economical fabrication. Utilizing rear wall  27  as downcomers (and perhaps part of side walls  25  and  26 ) eliminates the requirement for separate large diameter downcomers to feed water to the walls. The large pipes required for separate downcomers are very expensive. 
         [0030]    Finally, some embodiments of the present invention have the advantage of being ground supported. Conventional large power boilers are suspended from overhead with the thermal expansion more or less centered at the steam drum. As described above some embodiments of the invention can be much smaller than a conventional boiler for the same load rate and with intermediate wall  4  the construction is very rigid. Therefore the boiler can support the weight of steam drum  43  and it can move up and down with the boiler expansion. This eliminates the need for a very strong building surrounding the boiler. So those embodiments of the invention are less expensive to build and operate and run better than a conventional boiler of the same load rate. 
         [0031]    A second embodiment of the present invention is as a gasifier. As a gasifier, fuel enters the boiler in the same manner as described in the first embodiment, but under substoichiometric combustion only to the extent required to produce syngas. Refractory lining  15  and insulating lining  14  absorb much heat of combustion in combustion chamber  1  promoting high-temperature fuel gasification. If the gasifier is operated predominately as a syn-gas producer, it is preferred to inject pure oxygen to minimize the syn-gas dilution from nitrogen. In that case, the oxygen can be injected through stepped floor cleaners  49 , running continuously to agitate the fuel piles and inject oxygen directly into the fuel. Only enough oxygen is injected to produce the concentrated syngas production targets. Side sweep ports  45  can be adapted to inject oxygen to maintain and control the temperature in combustion chamber  1 . It is important to reduce the temperature of the syn-gas before it is pulled off the boiler so that it can be handled, cleaned, and acid gases (CO 2 , H 2 S, COS) removed. Chamber walls  24  will be very effective in reducing the gas temperature before it leaves the boiler. Over-fired air ports  29  are not required and the syn-gas will leave the boiler just downstream from rear screen  41 . As a gasifier the boiler will be configured to produce saturated steam at a rate proportional to the heat released but not used by the gasification process. If superheated steam is required, some of the syn-gas can be allowed to flow into back pass  6  where after burner ports  51  can inject air to burn those gases and produce the heat required to superheat the steam. Alternately a separate fuel, such as natural gas, can be burned at the location of afterburner ports  51  to superheat the steam. If superheated steam is not required back pass  6  and superheaters  50  are not required. 
         [0032]    Although embodiments of the present invention and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.