Abstract:
A steam generator includes a furnace configured and adapted to generate steam from heat transfer from furnace exit gases from the combustion of municipal solid waste fuel, where tempering air is used for at least two purposes. First, to prevent or minimize corrosion of heat transfer surfaces, specifically a plurality of platens and at least one superheater disposed within an upper portion of the furnace or backpass and second to push and exert a force on the hot flue gas to change the flow pattern and improve superheater metal temperatures.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a municipal solid waste fuel steam generator having a tempering air system to improve temperature profiles and to minimize corrosion of heat exchange elements of the superheat and steam generator in the steam generator. 
       BACKGROUND 
       [0002]    Waste-to-energy or Energy-from-waste plants generate high-energy steam in boilers by combusting waste materials. Municipal solid waste fuel steam generators use solid municipal waste, such as refuse, as fuel to generate steam. The steam is commonly used to drive high-pressure steam turbines in order to generate electricity or provide steam to a steam user “host” or district heating system. When operated in an environmentally friendly way, solid waste fuel steam generators can solve two problems at the same time: they benefit the environment by reducing the demand on landfills for disposal of solid waste and they generate much needed power. 
         [0003]    Many steam generators, including those adapted to burn fossil fuels, municipal solid waste, and other fuels, include a superheater downstream from the combustion zone. Steam within the superheater reaches a superheated state as the passing combustion gases release (i.e., transfer) heat into the superheater. This superheated steam is typically used to power high-pressure turbines. The material demands on the superheater in these extreme temperatures are great. One way to increase the life of superheaters is to include a bank of waterwall furnace platens upstream from the superheater. The waterwall furnace platens circulate relatively cool fluids, and thereby absorb some of the heat from the combustion gases before the hot gases reach the superheater. An example of such waterwall furnace platens is found in Applicant&#39;s U.S. Pat. No. 8,096,268, the teachings of which are incorporated herein by reference. This has the effect of lowering the furnace gas exit temperature in front of the superheater, extending its useable life. This arrangement ultimately reduces corrosion rate and extends the useful life of superheaters and can be operated in a way that maintains overall thermal efficiency for longer operating periods. Unfortunately, the effectiveness of such a system is highly dependent on the flue gas circulation and distribution within the steam generator. 
         [0004]    In municipal solid waste fuel steam generators the combustion gases at the superheater are much more corrosive at temperatures above 1,400 degrees than in steam generators using coal and other fuels. This leads to the need for frequent repair or replacement of the superheater, which results in additional expense and downtime both scheduled and unscheduled. Attempts have been made to protect and increase the life of the superheater, such as tubes made of costly high alloy material and/or protecting the tubes with alloy shields in the path of the hot combustion gases or making the overall height of the furnace greater in order to reduce the furnace flue gas exit temperature. Gas flow develops from the grate and passes up between the lower arches. The secondary air pushes the gas column together as it flows past the lower arches. Overall, the flow is towards the rear wall due to the geometry of the lower furnace arches. As the gases continue to flow upwards, there is flow separation as the gases come off of the nose arch, leading to a large recirculation zone along the front of the furnace below the nose arch. There is up flow in only about one-half of the furnace. The column of gases formed in the upper furnace is directed towards the front wall above the nose arch. Eventually, the column is deflected by the roof and flows across the superheater to the outlet of the model. When platens are present significantly different flows in the upper furnace occur because the platens cause a recirculation zone below the roof in the front corner. The effect of the recirculation zone is to push the column more towards the superheater and the gases cross the superheater at a lower elevation. In order to reduce corrosion of superheaters in existing facilities, it is impractical and extremely difficult to increase the size of the furnace adequately due to space limitations. Shielding of the superheater likewise does not reduce the furnace flue gas exit temperatures in the vicinity of the superheater. 
       SUMMARY 
       [0005]    The purpose and advantages of the present invention will be set forth in and become apparent from the description that follows. Additional advantages of the invention will be realized and attained by the methods and systems particularly pointed out in the written description and claims hereof, as well as from the appended drawings. 
         [0006]    To achieve these and other advantages and in accordance with the purpose of the invention, as embodied herein, the invention includes a steam generator including a furnace configured and adapted to generate a stream of combustion gases from combustion of municipal solid waste fuel. The steam generator also includes at least one superheater disposed within an upper portion of the furnace, downstream of a combustion zone, or proximate a backpass of the furnace. The superheater is configured and adapted to superheat fluids within the superheater by facilitating heat transfer between fluids within the superheater and furnace exit gases outside the superheater. Preferably at least one waterwall furnace platen is disposed within the furnace upstream from the superheater. The waterwall furnace platen is configured and adapted to lower furnace exit gas temperature at the superheater by facilitating heat transfer between fluids within the waterwall furnace platen and furnace exit gases outside the waterwall furnace platen. Positioned upstream of the superheater is at least one tempering air system being adapted and configured to redistribute and push approximately 51% of the flue gas within the cross section of the platens. Additionally, the superheater inlet velocity profile RMS deviation was reduced from 81% to 51% with the addition of the tempering air introduced through nozzles positioned below the furnace bull nose. An extension of the bull nose also improves flue gas distribution, with a 12-inch extension exerting a moderate force on the column of gases flowing up the furnace. 
         [0007]    In accordance with another aspect of the invention, the superheater and waterwall furnace platen are preferably in fluid communication with each other as part of a thermal hydraulic circuit. In another preferred embodiment, the tempering air system is comprised of a plurality of nozzles that are in fluid connection with a common header that supplies ambient air obtained from outside the furnace or recycled flue gas obtained from the outlet of the air pollution equipment. The air supplied by a fan will be ducted to a distribution header connected to the plurality of nozzles located below the platens in the rear wall below the superheater, preferably below the bull nose. The ductwork can include an expansion loop or joint configured and adapted to accommodate for the thermal expansion and flexing of the ductwork. The air pressure and flow will be controlled by damper control located between the fan discharge and the distribution header. The cool air is injected into the flue gas at an angle and at a pressure to push and redistribute and promote equal distribution of the flue gas through the platens so that maximum cooling of the Furnace Exit Flue gases occur before the gases enter the superheater. The waterwall furnace platen(s) system components can further include mechanical means operably connected to vibrate during operation to reduce residue build-up on exterior surfaces of the platen(s) components exposed to the flue gas. It is also contemplated that the waterwall furnace platen(s) can include a piping header expansion loop configured and adapted to accommodate for thermal expansion and flexing of external supply headers. 
         [0008]    The waterwall furnace platen preferably includes a bank of tubes configured with membrane construction and adapted to facilitate heat transfer between a fluid circulating within the tubes and a stream of furnace exit gases passing outside the tubes. The bank of tubes includes a corrosion resistant material. The corrosion resistant material may include an overlay disposed on an exterior surface of the bank of tubes or the header and preferably includes an Inconel® alloy. The overlay can include Inconel® 625. In a preferred embodiment, the waterwall furnace platen can be configured and adapted to be positioned within the furnace to allow maintenance access to the superheater. It is also the intent of this invention to place the waterwall furnace platen away from the superheater, not directly in front as typical waterwall screens are placed. It is important that the waterwall furnace platens are positioned several feet away especially when super heater performance is dependent on radiant heat and luminous light from the furnace combustion process. 
         [0009]    The tempering air system of the present invention is intended to be suitable for use in new steam generation plants or existing steam generating plants fitted with furnace platen addition. A steam generation plant in accordance with the invention includes a furnace configured and adapted to generate a stream of combustion gases from combustion of municipal solid waste fuel. At least one superheater is disposed within or proximate an upper portion or backpass of the furnace. The superheater is configured and adapted to facilitate heat transfer between fluids within the superheater and furnace exit gases outside the superheater. Preferably, the tempering air system is positioned up stream of the furnace platens and superheater. The combination of the tempering air system and the waterwall furnace platen is configured and adapted to lower furnace exit gas temperature at the superheater by facilitating heat transfer between fluids within the waterwall furnace platen and furnace exit gases upstream of the superheater and supplying streams of cool air to ensure the hot furnace gases are directed toward and cool in the waterwall platen before contact with the superheater. A steam powered system is in fluid communication with the superheater. 
         [0010]    It is contemplated that the steam powered system can include a high-pressure turbine. It is also contemplated that the steam powered system can include a steam host or district heating system. That at least one superheater and waterwall furnace platen can be in fluid communication with each other as part of a thermal hydraulic circuit. 
         [0011]    The invention further includes a system for generating steam in a thermal hydraulic circuit of a municipal solid waste fuel steam generator. The system includes at least one superheater configured and adapted to facilitate heat transfer between fluids within the superheater and furnace exit gases outside the superheater. A drum is in fluid communication with the superheater for separating vapor and liquid to supply saturated steam to the superheater. At least one waterwall furnace platen can be in fluid communication with the drum. 
         [0012]    It is further contemplated that the invention also includes a method of reducing corrosion of superheaters in municipal solid waste fuel steam generators. The method includes providing a waterwall platen system with a tempering air system upstream of a superheater, where the tempering air system is configured and adapted to equally distribute the furnace exit gas into the platen and lower the temperature of that gas prior to contact with the superheater. A waterwall furnace platen downstream of the tempering air system and upstream of the superheater can be used to further reduce the furnace exit gas temperature. The method involves supplying the cool air from the ambient air outside the furnace, or recirculated air from the outlet of the flue gas cleaning system, through a header that then distributes the air through a series of nozzles. The method also includes circulating a fluid through a bank of tubes in the waterwall furnace platen to cool a stream of furnace exit gases before the superheater. Determining the amount of tempering air to add is accomplished by measuring the temperatures of the inlet and outlet flue gas temperatures of each section of the superheaters and using averages for the primary and secondary superheater temperatures. The addition of the platens results in a large change in the flue gas temperature profile in the upper furnace and causes the gases to be much cooler, especially near the front, close to the roof. The region of higher temperature gases is pushed down towards the lower half of the superheaters. And, the hottest gases are adjacent to the front of the superheater in the region of the lower bends. The influence of the platens on the flow patterns would be the result of a change in density of the flue gases as they cool and the resistance to flow due to the platens. Pursuant to my invention, the addition of tempering air would again change the temperature profile. The action of the tempering air is sufficient to push the hottest gases towards the front wall and away from the superheaters. And, extending the nose arch (bull nose) by up to 12 inches can further affect the flue gas temperatures by redistributing the hot gases more evenly. My invention forces the hot flue gas to move toward the front wall and away from the superheaters resulting in a much cooler temperatures in the lower half of the superheaters. 
         [0013]    A method of retrofitting a municipal solid waste fuel steam generator to reduce corrosion of a superheater is also contemplated in accordance with the invention. The method includes providing an tempering air system upstream of at least one waterwall furnace platen upstream of the superheater can also be part of the method of retrofitting. The waterwall furnace platen can be configured and adapted to lower furnace exit gas temperature at a superheater by facilitating heat transfer between fluids within the waterwall furnace platen and furnace exit gases outside the superheater. An access is opened through the upper furnace roof of the steam generator and at least one furnace wall. The waterwall furnace platen is mounted through the upper furnace roof and at least one furnace wall. The method also includes operably connecting the tempering air system header to a source of ambient or recirculated air. The waterwall furnace platen is operably connected to a drum of the steam generator. 
         [0014]    The method of retrofitting can further include measuring temperatures within an existing municipal solid waste fuel steam generator, wherein the steps of providing an tempering air system and a waterwall furnace platen includes configuring one or both of these added systems to reduce corrosion of the superheater based on the temperatures measured in the existing municipal solid waste fuel steam generator. Additionally, the bull nose (nose arch) can be extended to further redistribute the hot flue gas and to protect the superheaters. 
         [0015]    There is also a method of enhancing NO x  control in a municipal solid waste fuel steam generator, in accordance with the invention. The method includes providing a furnace configured and adapted to generate a stream of furnace exit gas from combustion of municipal solid waste fuel, the furnace having at least one superheater disposed proximate a backpass, or downstream of a combustion zone, and an tempering air system disposed within the furnace upstream from the furnace waterwall platen and the superheater. The furnace is operated to generate steam from the super heater. The method further includes enhancing NO x  reduction by using the tempering air system to supply cooling air that mixes with the furnace exit gases to lower furnace exit gas temperature at the superheater. 
         [0016]    A method of lowering reactant requirements for selective non-catalytic reduction of NO x  (SNCR) in a municipal solid waste fuel steam generator is also contemplated as being within the scope of the invention. This method includes providing a furnace configured and adapted to generate a stream of furnace exit gas from combustion of municipal solid waste fuel that provides an enhanced and stable temperature environment within the furnace for SNCR, the furnace having at least one superheater disposed proximate a backpass, or downstream of a combustion zone, and an tempering air system working in conjunction with at least one waterwall furnace platen disposed within the furnace, both of which are upstream from the superheater and when both are employed, the tempering air system is positioned upstream of the waterwall platen and the superheater. A selective non-catalytic reduction system is provided, operably connected to reduce NO x  emissions from furnace exit gas. The method includes operating the furnace to generate steam from the super heater and operating the selective non-catalytic reduction system by reacting with the furnace exit gas with a required amount of reactant to reduce NO x  emissions to within acceptable limits. The method also includes the step of reducing the required amount of reactant by supplying cooling air to mix with the furnace exit gases to lower furnace exit gas temperature at the superheater. It is contemplated that the reactant can be urea, ammonia, or any other suitable reactant. It is also possible to use the method with selective catalytic reduction systems (SCR). 
         [0017]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention claimed. The accompanying drawings, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the method and system of the invention. Together with the description, the drawings serve to explain the principles of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0018]      FIG. 1  is a cross-sectional side elevation view of a portion of one possible representative embodiment of a steam generator in accordance with the present invention, showing the furnace, air tempering system, waterwall furnace platens, superheater, and boiler bank. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. The method and corresponding steps of the invention will be described in conjunction with the detailed description of the system. The devices and methods presented herein may be used for reducing the corrosion of superheaters in municipal solid waste fuel steam generators. The present invention is well suited to decrease furnace exit gas temperature and improve furnace gas flow distribution. 
         [0020]      FIG. 1  shows a cross-sectional side elevation view of an exemplary embodiment of a steam generator for generating steam from combusting municipal solid waste as fuel in accordance with the invention, which is designated  100 . In accordance with the invention, a steam generator includes a furnace configured and adapted to generate a stream of combustion gases from combustion of municipal solid waste fuel. At least one superheater is disposed within an upper portion of the furnace, downstream of a combustion zone, or proximate a backpass thereof. Those skilled in the art will readily appreciate other suitable locations for the superheater. The superheater is configured and adapted to superheat fluids within the superheater by facilitating heat transfer between fluids within the superheater and furnace exit gases outside the superheater. 
         [0021]    An tempering air system is positioned near the superheater, preferably upstream, and is adapted to lower furnace exit gas temperature at the superheater by injecting or supplying cooling air through a plurality of nozzles connected to a common header. Also shown are waterwall furnace platens disposed within the furnace upstream from the superheater. The waterwall furnace platen is configured and adapted facilitating heat transfer between fluids within the waterwall furnace platen and furnace exit gases outside the waterwall furnace platen. 
         [0022]    For purposes of illustration and not limitation, as embodied herein and as depicted in  FIG. 1 , a municipal solid waste fuel furnace  102  generates a stream of combustion gases by combustion of solid waste as fuel. The solid waste can be refuse or other waste material, for example. The combustion gas flows upward into upper furnace  104 , where it is diverted through superheater  108 . 
         [0023]    Downstream from the backpass  114 , boiler banks  110  include tubes that contain a water/steam mixture. Water entering boiler banks  110  is converted into saturated steam as passing combustion gases exchange heat with the water through the tube walls of boiler banks  110 . The saturated steam is passed through drum  112  and into superheater  108 . Superheater  108  is located upstream from boiler banks  110 , and is thus located in a hotter portion of upper furnace  104 . Saturated steam entering superheater  108  is superheated by exchanging heat through the tube walls of superheater  108  with combustion gases exiting the furnace. The superheated steam can then be used as a power source, for example by passing through a high-pressure steam turbine (not shown), or by being distributed to a steam host or district heating system. 
         [0024]    The combustion gases within upper furnace  104  are particularly corrosive because of the municipal solid waste used as fuel. In typical municipal solid waste fuel steam generators, the corrosive environment can quickly corrode the metallic tubes of the superheater. However, superheater  108  provided herein experiences significantly less corrosion than in known municipal solid waste steam generators. This decrease in corrosion is attributable to the inclusion of an tempering air system  200  positioned up stream of the superheater  108 . As mentioned, the tempering air system can be used alone or more preferably in conjunction with waterwall furnace platens  106 , which are located upstream from both the tempering air system  200  and superheater  108 . Tempering air system  200  is preferably comprised of a plurality of nozzles  201  spaced along a header  203 . These nozzles are in fluid communication with the internal chamber of the header  203  that is supplied with cooling air supplied from duct  202 . Duct  202  extends through a furnace wall and is in fluid communication with connecting duct (not shown) that is connected to a fan, pump, compressor, or other device that acquires ambient air and forces it to flow within duct  202  into header  203  and through nozzles  201  where it is then supplied through furnace wall at the nose arch  204  to and mixes with furnace exit gases. Mixing of the cooling air from nozzles  201  with the flowing furnace exit gases causes the furnace exit gas temperature to drop prior to contacting superheater  108 . In some cases it is beneficial to increase the length or size of the nose arch by at least 12 inches. 
         [0025]    Waterwall furnace platens  106  include tubes with flowing water. As combustion gases pass outside waterwall furnace platens  106 , some of the heat is exchanged with the water in the tubes of waterwall furnace platens  106 . This preheats the water, and can even convert some or all of the water into steam, which is passed through drum  112 . The tempering air system  200  and the waterwall platens simultaneously cause the temperature of the furnace exit gases to decrease so that the exterior temperatures of superheater  108  are significantly lower than they would be without either the tempering air system  200  or the waterwall furnace platens  106 . The lower gas temperatures around the tubes of superheater  108  substantially lower the corrosive action of the combustion gases on superheater  108 , which significantly increases the useable life of superheater  108 . 
         [0026]    Waterwall platens  106  preferably include a membrane between each pair of adjacent tubes. The membrane is preferably steel, however, any suitable material can be used. The membranes lend extra rigidity to platen  106  without significant negative effects on the flow or heat transfer characteristics of platen  106 . 
         [0027]    Waterwall furnace platens  106  can be protected against the corrosive environment of upper furnace  104  by using corrosion resistant materials. It is possible to use a protective overlay on the tubes and membrane of waterwall furnace platens  106  to reduce corrosion wear. Nickel alloys are particularly well suited for use as overlays in the application described herein. Commercial alloys that can be used in overlays for waterwall furnace platen  106  include Inconel® alloys available from Special Metals Corporation, Huntington, W.Va., USA. Inconel® 625 is a particularly well-suited overlay material. However, those skilled in the art will readily appreciate that any other suitable overlay material can be used without departing from the spirit and scope of the invention. Moreover, those skilled in the art will readily appreciate that the tube banks of waterwall furnace platens  106  can be made entirely from corrosion resistant materials without departing from the spirit and scope of the invention. 
         [0028]    Waterwall furnace platens  106 , superheater  108 , and boiler banks  110  are all parts of a single thermal hydraulic circuit. Liquid water is circulated naturally, or by any other suitable means, through waterwall furnace platens  106 , where heat is added to the water as described above. Water also passes through boiler banks  110 , where heat is added to convert liquid water into saturated steam. Steam and/or water from waterwall furnace platens  106  and boiler banks  110  is mixed in drum  112 , which separates steam from liquid water and passes the steam into superheater  108  in a manner known in the art. In superheater  108 , additional heat is added to superheat the steam, as described above. The steam is then used, for example, in a high-pressure steam turbine to generate mechanical power, or for distribution to a steam host or district heating system as described above. The turbine or heating system depletes substantial amounts of energy from the steam, which is then condensed back into liquid water. The liquid water is pumped or otherwise circulated back into boiler drum  112 , waterwall furnace platens  106  and boiler bank  110  to continue the cycle. 
         [0029]    While steam generator  100  has been described above as including a single closed thermal hydraulic circuit, those skilled in the art will readily appreciate that it is possible for waterwall furnace platens  106  to be part of a separate circuit. It is also possible for steam generator  100  to operate as an open circuit rather than a closed circuit, in which case liquid water could be supplied from an external source to waterwall furnace platens  106  and/or boiler banks  110 , and could be returned to the environment after use in the turbine or heating system. Furthermore, while waterwall furnace platens  106  and boiler bank  110  have been described above as operating in parallel, those skilled in the art will readily appreciate how to operate them in series. Those skilled in the art will readily appreciate that these and other suitable variations on the thermal hydraulic circuit can be practiced without departing from the spirit and scope of the invention. 
         [0030]    Although  FIG. 1  shows a specific cross-section with specific spacing between waterwall furnace platens  106 , the tempering air system  200  and the superheater  108 , those skilled in the art will readily appreciate that any suitable configuration can be used to allow access to the various components within the upper furnace without departing from the spirit and scope of the invention. 
         [0031]    Additionally, mechanical devices  116  can be placed on exterior surfaces of waterwall furnace platen  106 . In operation, vibrating wrappers or other suitable devices can remove or prevent formation of residue build-up on exterior surfaces of waterwall furnace platens  106 . Those skilled in the art will readily appreciate that vibration means are optional and that any suitable mechanical rapper or vibration means can be used without departing from the spirit and scope of the invention. 
         [0032]    In further accordance with the invention, a method is provided for reducing corrosion of superheaters in municipal solid waste fuel steam generators. The method includes providing an tempering air system upstream of the superheater and preferably, as shown in  FIG. 1 , downstream of waterwall furnace platens. Both the tempering air system and the waterwall furnace platen is configured and adapted to lower furnace exit gas temperature at the superheater by providing cooling air to mix with furnace exit gases and facilitating heat transfer between fluids within the waterwall furnace platen and furnace exit gases outside the superheater. The method also includes circulating a fluid through a bank of tubes in the waterwall furnace platen to cool a stream of furnace exit gases outside the tubes. 
         [0033]    The invention also includes a method of retrofitting a municipal solid waste fuel steam generator to reduce corrosion of a superheater. The method includes providing an tempering air system configured and adapted to lower furnace exit gas temperature at a superheater in a municipal solid waste fuel steam generator by supplying cooling air to mix with and cool down furnace exit gases before contacting the superheater. The method further includes opening an access through the roof of the upper furnace of the steam generator and at least one furnace wall, mounting the tempering air system within the upper furnace and operably connecting the tempering air system to duct work outside the furnace and to a source of flowing ambient air, preferably a blower, compressor, pump or like device capable of taking ambient air and forcing into the duct and into the header where it exits into the upper furnace through a plurality of nozzles mounted on the header. The size and configuration of existing municipal solid waste fuel steam generators varies from plant to plant, thus it is often desirable to measure operating temperatures within the existing steam generator to provide data useful in configuring the tempering air system alone or in combination with waterwall furnace platens for use in retrofitting. Those skilled in the art will readily appreciate how to take such measurements and apply them to configure the tempering air system according to the invention. 
         [0034]    When waterwall furnace platens are part of the retrofit in addition to the tempering air system, an access must be opened on the upper furnace roof and at least one furnace wall location to allow the waterwall furnace platen to be mounted in the upper furnace in a location suitable to reduce corrosion on the superheater, as described above. The fluid passages of the waterwall furnace platen must be connected to a thermal/hydraulic circuit of the steam generator. Typically this involves connecting the waterwall furnace platen to a drum in the upper furnace, as described above. Those skilled in the art will readily appreciate other suitable variations of the retrofitting process that can be practiced based on individual steam generator designs without departing from the spirit and scope of the invention. 
         [0035]    The circuit described above reduces corrosion of the superheater  108  without substantially impacting thermal efficiency of steam generator  100  when compared with known municipal solid waste fuel steam generators. Moreover, the configuration of steam generator  100  with the tempering air system  200  and waterwall furnace platens  106  provides reduced furnace exit gas temperature and improved furnace exit gas flow distribution over known municipal solid waste fuel steam generators. 
         [0036]    It has been mentioned above that one traditional approach to the problem of corrosion of superheaters in municipal solid waste fuel steam generators is to simply increase the height of the upper furnace to reduce temperatures at the superheater. Steam generator  100  can be used in lieu of increasing the furnace height. Thus if an existing furnace is retrofitted according to the invention, a significant saving of space results. In cases where increasing the size of the furnace is not possible or practical due to space limitations, retrofitting the furnace according to the invention may nonetheless be possible. 
         [0037]    An additional unexpected result is that municipal solid waste fuel steam generation units using both an tempering air system and waterwall furnace platens according to the invention can operate for up to 33% longer than the ordinary period between cleanings, which varies from site to site. This is a substantial benefit, as the cleaning process for municipal solid waste fuel steam generators is expensive and involved due to the variety of materials combusted. Additionally, cleaning of the steam generator has proven to be easier and up to 50% faster as a result of using the present invention. The ease in cleaning is due to changes in the make up of the ash and residue due to the lower temperatures of the furnace exit temperatures contacting the superheater. 
         [0038]    The methods and systems of the present invention, as described above and shown in the drawings, provide for a municipal solid waste fuel steam generator with reduced corrosive action on the superheater. It will be apparent to those skilled in the art that various modifications and variations can be made in the device and method of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention include modifications and variations that are within the scope of the appended claims and their equivalents.