Abstract:
Combustion systems having reduced nitrogen oxide emissions and methods of using the same are disclosed herein. In one embodiment, a combustion system is provided. The combustion system includes a combustion zone, which includes a burner for converting a fuel, under fuel rich conditions, to a flue gas. An intermediate staged air inlet is downstream from the combustion zone, for supplying intermediate staged air to the flue gas and producing fuel lean conditions. A reburn zone is downstream from the intermediate staged air inlet for receiving the flue gas. A process for using the combustion system and a method of reducing NO X  flowing into the reburn zone of a combustion system are also described herein.

Description:
BACKGROUND OF THE INVENTION 
     This disclosure relates generally to combustion systems for power plants, and more particularly to combustions systems having reduced nitrogen oxide emissions. 
     During a typical combustion process within a furnace or boiler, for example, a flow of combustion gas, or flue gas, is produced. Known combustion gases contain combustion products including, but not limited to, carbon, carbon dioxide, carbon monoxide, water, hydrogen, nitrogen, sulfur dioxide, chlorine, and/or mercury generated as a result of combusting fuels. Combustion gases also contain nitrogen oxides (NO X ), usually in the form of a combination of nitric oxide (NO) and nitrogen dioxide (NO 2 ). Various technologies have been applied to combustion systems to minimize the emissions of NO X , however, further improvements are needed. 
       FIG. 1  shows a prior art combustion system  100 . As shown, the prior art combustion system  100  includes a fuel lean main combustion zone  120 , a reburn zone  124 , and a burnout zone  126  stacked upwardly from the base of the prior art combustion system  100 . These different zones of the prior art combustion system  100  are enclosed within a housing  110 . Within the main combustion zone  120 , the fuel undergoes combustion and forms a flue gas that flows upwardly to the reburn zone  124 . As used herein, the term “flue gas” refers to the products of combustion, including but not limited to, carbon, carbon dioxide, carbon monoxide, water, hydrogen, nitrogen, sulfur dioxide, chlorine, NO, NO 2 , and/or mercury generated as a result of combusting fuels. Flyash is excluded from flue gas when computing residence times in the combustion system. 
     The amounts of fuel and air supplied to the main combustion zone  120  of the prior art combustion system  100  are selected to achieve fuel lean conditions therein. The term “fuel lean,” as used herein, refers to a condition having less than a stoichiometric amount of fuel available for reaction with the O 2  in the air, i.e., a stoichiometric ratio (SR) of greater than about 1.0. The exact SR in the main combustion zone  120  of the prior art combustion system  100  varies depending on the fuel type and combustion system design, but generally ranges from about 1.05 to about 1.10. Flue gas produced in the main combustion zone  120  then flows to the reburn zone  124  and fuel is added to the flue gas through one or more reburn inlets  134 . The amount of fuel added through the reburn inlets  134  is effective to produce fuel rich conditions in the reburn zone  124 . The term “fuel rich,” as used herein, refers to a condition having more than a stoichiometric amount of fuel available for reaction with the O 2  in the air, i.e., a SR of less than about 1.0. The exact SR in the reburn zone  124  of the prior art combustion system  100  varies depending on the fuel type and combustion system design but generally ranges from about 0.85 to about 0.95. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Combustion systems having reduced nitrogen oxide emissions and methods of using the same are disclosed herein. In one embodiment, a combustion system is provided. The combustion system includes a combustion zone that includes a burner for converting a fuel, under fuel rich conditions, to a flue gas. An intermediate staged air (ISA) inlet is downstream from the combustion zone, for supplying intermediate staged air to the flue gas and producing fuel lean conditions. A reburn zone is downstream from the intermediate staged air inlet for receiving the flue gas. 
     In another embodiment, a process for using a combustion system is provided. The process includes supplying a fuel and air under fuel rich conditions to a combustion zone, which has a burner, to form a flue gas. Intermediate staged air is supplied to the flue gas through an intermediate staged air inlet downstream of the combustion zone to produce fuel lean conditions. The flue gas is then channeled to a reburn zone downstream from the intermediate staged air inlet. 
     In another embodiment, a method of reducing NO X  flowing into the reburn zone of a combustion system. The method includes supplying a fuel and air under fuel rich conditions to a combustion zone that includes a burner, to form a flue gas. Intermediate staged air is then supplied to the flue gas through an intermediate staged air inlet downstream of the combustion zone to produce fuel lean conditions. The flue gas is then channeled to a reburn zone downstream from the intermediate staged air inlet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       These and other features, aspects, and advantages of the exemplary combustion system will be better understood when the following detailed description is read with reference to the accompanying drawing, in which: 
         FIG. 1  is a schematic diagram showing a side, cross-sectional view of a prior art combustion system. 
         FIG. 2  is a schematic diagram showing a side, cross-sectional view of an embodiment of a combustion system having reduced levels of nitrogen oxides. 
         FIG. 3  graphically illustrates the basic NO X  emissions curves generated by a coal over coal reburn model that was calibrated using field data, for a combustion system using traditional reburn (prior art) system, shown in  FIG. 1 , and for two embodiments of a combustion system shown in  FIG. 2 . 
         FIG. 4  graphically illustrates NO X  emissions as a function of loss on ignition (LOI) for several simulated combustion system process conditions with the ISA flow rate held constant at twelve percent (12%) of the stoichiometric air flow rate. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  shows an exemplary embodiment of the combustion system  200  that can be used for various applications such as in a fossil-fuel fired boiler, furnace, engine, incinerator, etc. One particularly suitable application of combustion system  200  is as the source of power generation in a power plant. The main combustion zone  220  is equipped with one or more main burners (not shown) such as specially designed burners for producing low levels of nitrogen oxides (NO X ). In one embodiment, the main combustion zone  220  includes two or more burners arranged in two or more rows. Fuel and primary air are supplied together to the main combustion zone  220  through one or more inlets  228 . Secondary air is also generally supplied to the main combustion zone  220  through inlets  228 . The amounts of fuel and air supplied to the main combustion zone  220  are selected to achieve fuel rich conditions therein. The exact SR in the main combustion zone  220  will vary depending on the fuel type and furnace design, but will be less than about 1.0. In one embodiment, the SR in the main combustion zone  220  is about 0.90 to about 0.95. Examples of suitable fuels for use in the main combustion zone  220  include, but are not limited to including, fossil fuels, such as lignite coal, bituminous coal, sub-bituminous coal, anthracite coal, oil, or gas, such as natural gas or gasified coal, various types of biomass, and combinations including at least one of the foregoing fuels. Any suitable form of fuel can be supplied to the main combustion zone  220 , including pulverized coal that is ground using a powdered coal mill. Within the main combustion zone  220 , the fuel undergoes combustion and forms a flue gas that flows upwardly toward the intermediate staged air zone  222 . 
     The flue gas produced in the main combustion zone  220  flows to the intermediate staged air (ISA) zone  222 . Air is added to the flue gas in this zone through one or more intermediate staged air inlets  232 . The amount of ISA supplied to zone  222  is effective to produce fuel lean conditions, i.e., SR of greater than about 1.0. In one embodiment, sufficient ISA is supplied to zone  222  to produce a SR of about 1.05 to about 1.10. Flow into the ISA inlet  232  may be regulated by an ISA damper  231 . 
     In one embodiment, the ISA inlet  232  is a burner out of service (BOOS) through which cooling air is injected. In this way, an existing furnace may be adapted to incorporate ISA by running cooling air through the existing top row of burners, making them the ISA inlets  232 . This has a minimal cost impact and avoids additional wall penetrations in the furnace of the combustion system  200 . 
     In another embodiment, the existing burners in the top row of the main combustion zone  220  are replaced with injectors specifically designed to inject ISA. In this way the velocity and mixing of the ISA in the ISA zone  222  may be better optimized for the system, but new furnace wall penetrations are not required. Alternatively, the existing burners in the top row of the main combustion zone  220  are blocked off and new injectors specifically design to inject ISA are placed at an elevation below, equal to, or above the top burner row. This does require additional wall penetrations for the ISA inlets  232 . In another embodiment, the ISA inlet  232  is above (downstream) of the upper burner row of the main combustion zone  220 . This enables the use of all of the existing burners in the main combustion zone  220 , but does require additional wall penetrations for the ISA inlets  232 . 
     The ISA supplied through the ISA inlet  232  may be in the form of cool ambient air, heated air, or both cool ambient air and heated air, with heated air being preferred. In one embodiment, the ISA is boosted such that the ISA is supplied at a relatively higher pressure. This may be accomplished using one or more rotating booster fans. The boosting of the ISA can achieve improved levels of air jet penetration and mixing in the ISA zone  222 . 
     The fuel-lean flue gas then enters the reburn zone  224  and fuel is added to the flue gas through one or more reburn inlets  234 . The fuel is typically accompanied by carrier gas. The carrier gas may be carrier air, boosted flue gas recirculation (FGR), or any other appropriate gas for the specific fuel and furnace design. The amount of fuel added through the reburn inlets  234  is effective to produce fuel rich conditions in the reburn zone  224 . The exact SR in the reburn zone  224  of the combustion system  200  varies depending on the fuel type and combustion system design but generally ranges from about 0.85 to about 0.95. 
     The flue gas formed in the reburn zone  224  then proceeds through the combustion system  200  and is subjected to optional operations and treatments. In one embodiment the flue gas formed in the reburn zone  224  flows upwardly to the burnout zone  226 , which is downstream from the reburn zone  224 . Overfire air (OFA), also known as separated overfire air (SOFA), is supplied to the burnout zone  226  through inlet  236 . OFA flow through inlet  236  may be regulated by an OFA damper  235 . The OFA restores the system to overall fuel lean conditions, i.e., SR of greater than about 1.0. The exact SR varies depending on the fuel type and furnace design. In one embodiment, the SR in the burnout zone  226  is about 1.15 to about 1.3. The OFA can be added to the burnout zone  226  at a relatively higher pressure through inlet  236 , such as with boosted overfire air (BOFA). This may be accomplished using one or more rotating booster fans. The BOFA can be in the form of cool ambient air, heated air, or both cool ambient air and heated air, with heated air being preferred. The introduction of the BOFA can achieve desired levels of air jet penetration and mixing in the burnout zone  226 . 
     Air may be fed to the various stages in the combustion system  200  from a variety of sources. In one embodiment, a windbox supplies secondary air to the main combustion zone inlets  228 , ISA to the ISA inlets  232 , and/or OFA to the OFA inlets  236  through ducting  238 . In another embodiment, air is delivered to one or more inlets  228 ,  232 , and  236  through separate ducting (not shown). Control of the flow to the various inlets may be linked, or may be independent. The source of the air and the configuration of the ducting is not critical to the combustion system  200  and may be tailored to suit the particular furnace design. 
     The flue gas in the burnout zone  226  passes downstream to an outlet  244  where the flue exits the combustion system  200 . As the flue gas passes to outlet  244 , the flue gas flows past the tip of the boiler nose  240  and can flow through one or more heat exchangers  242  to serve as a heat source. 
     The residence time of the substances flowing through various regions of the combustion system  200  varies depending on fuel and air flow rates. As used herein, the term “residence time” refers to the average time the flue gas spends in a defined region of the furnace. Operation of the exemplary furnace is conducted such that there is sufficient residence time to enable conversion of the NO X  to take place. The exact residence time required depends on the furnace design, primary fuel type, and/or reburn fuel type. In one embodiment, a residence time of flue gas in a region of the combustion system  200  between a centerline of the intermediate staged air inlet  232  and a centerline of the reburn inlet  234  is about 100 to about 400 milliseconds. In an alternative embodiment, a residence time of flue gas in a region of the combustion system  200  between the centerline of the reburn inlet  234  and a centerline of the overfire air inlet  236  is about 300 to about 1000 milliseconds. In general, fuels that devolatilize and mix quickly require relatively low average residence times. In another alternative embodiment, a residence time of the flue gas in a region of the combustion system  200  between the centerline of the OFA inlet  236  and the tip of the boiler nose  240  is greater than about 300 milliseconds. In still another alternative embodiment, a residence time of the flue gas in a region of the combustion system  200  between a centerline of a top burner row and the centerline of the tip of the boiler nose  240  (i.e., the total residence time of the combustion system) is greater than about 1,300 milliseconds. As used herein, the term “centerline” refers to an imaginary line running through the middle of an object. 
     The use of intermediate staged air in the exemplary combustion system  200  enables the main combustion zone  220  to operate at fuel rich conditions. This reduces the initial NO X  flowing into the reburn zone  224  to improve overall NO X  emissions by, for example, about 10% to about 25%, as compared to reburn without intermediate staged air. In at least some known combustion system, both air and fuel staging usually have the unwanted side effect of increasing the emissions of CO and unburned carbon in fly ash as measured by loss-on-ignition (LOI). In the exemplary embodiment, the use of ISA provides additional flexibility and control of CO and LOI while maintaining low NO X  levels. The use of ISA combined with BOFA can also help restore the CO and unburned carbon emissions to more acceptable levels by improving the penetration of air into, and mixing with, the combustion gas. This type of integrated technology can reduce NO X  emissions to less than or equal to about 200 milligram/Newton-meters cubed (mg/Nm 3 ) at about 6% O 2  dry, or about 0.163 pound/million Btu (lb/MMBtu), thus meeting the NO X  emissions requirement of the European Union Large Combustion Plant Directive (LCPD), Phase 2. The combustion system  200  also can maintain the LOI at a sufficiently low level to allow the fly ash waste to be sold in Europe. This technology also is less expensive than selective catalytic reduction (SCR) technology. The combustion system  200  is therefore a low cost alternative to the SCR technology. 
     In additional embodiments, the combustion system described above can be combined with a selective non-catalytic reduction system (SNCR) such as the SNCR systems described in U.S. Pat. No. 5,853,683. For example, a SNCR system can be disposed downstream from the combustion system. Combining the ISA, BOFA, and the SNCR technologies into one power generation unit can reduce NOx emissions to less than or equal to about 123 mg/Nm 3  at about 6% O 2  dry, or about 0.1 lb/MM Btu, which meets the requirements of the Clean Air Interstate Rule (CAIR) of the United States. As such, the combination of these technologies, in a layered NO X  control approach, can provide effective reduction of NO X  emissions with added flexibility in controlling CO and LOI. 
     The disclosure is further illustrated by the following non-limiting examples. 
     EXAMPLES 
     One embodiment of the exemplary combustion system was tested in a pre-existing wall-fired boiler. Since the upper furnace was quite large, the system was not optimized, rather the system was designed to work within existing constraints. A series of tests were performed in which bituminous coal was burned in the wall-fired boiler operating at its Maximum Continuous Rating load (MCR). There were originally four elevations of low NO X  burners. The burners in the top row were taken out of service by turning off the fuel to them during reburn operation, such that they became burners out of service (BOOS). The BOOS were converted to ISA inlets by supplying secondary cooling air through them. The secondary air injected through the BOOS served as the ISA. The ISA flow rate remained at about 12% of the stoichiometric flow rate of the total air input into the system during the series of tests. No primary air flowed though the central coal pipe in the BOOS while they were being utilized as the ISA inlets. A series of tests were performed at various burner and reburn stoichiometric ratios. In these tests, coal over coal reburn was utilized. 
       FIG. 3  shows a plot of projected NO X  emissions as a function of the percent reburn fuel, with the stoichiometric ratio entering the reburn zone (SR 1 ) as a curve characterization parameter. In all cases, the overall boiler stoichiometric ratio was held constant at about 1.15 and the ISA flow rate for the inventive cases was held constant at about 12% of the stoichiometric air flow rate. These curves were generated from a model that was calibrated using the field data generated in the tests described above. Emissions for four cases are presented. The first case is the prior art system  100  (shown in  FIG. 1 ), without ISA (Prior Art RB SR 1 =1.05). The second case corresponds to an exemplary embodiment of the combustion system  200  (shown in  FIG. 2 ) with cooling air flowing as ISA through the upper burner row taken out of service (BOOS ISA inlet SR 1 =1.07). The third and fourth cases correspond to alternative embodiments of the combustion system  200  in which the upper row burners were replaced by single-tube air injectors designed for improved mixing (Single tube ISA inlet SR 1 =1.05 and Single tube ISA inlet SR 1 =1.10). 
     All of the reburn system configurations shown were able to achieve NO X  emissions below about 200 mg/Nm 3  at about 6% O 2  dry (0.163 lb/MMBtu) level. However, the exemplary system  200  with SR 1 =1.05 was able to reach NO X  levels well below about 200 mg/Nm 3  at about 6% O 2  dry over a wide range of reburn fuel rates. This example shows that ISA in the exemplary combustion system  200  not only provides flexibility in controlling NO X  emissions, but also has the potential of improving control over LOI and CO. This additional control over LOI and the drive toward process conditions that minimize total emissions (NO X , LOI, CO) is highlighted in  FIG. 4 . 
     Figure shows a plot of NO X  emissions as a function of LOI for different reburn operating conditions for the single tube ISA inlet embodiment of the exemplary system  200  (shown in  FIG. 2 ), compared with a reburn system of the prior art system  100  (shown in  FIG. 1 ) that did not utilize ISA. The calibrated NO X  model was used to determine NO X  emissions whereas as a calibrated computational fluid dynamics (CFD) model was used to determine LOI emissions (percent in ash). Without any reburn or ISA, NOx emissions would be about 541 mg/Nm 3  at about 6% O 2  dry (0.440 lb/MMBtu) and LOI of about 1.86% (not shown). As shown in  FIG. 4 , the use of reburn enabled the best-case prior art system to yield NOx emissions of 187 mg/Nm 3  at 6% O 2  dry (0.152 lb/MMBtu) with LOI of about 2.82%. The exemplary system  200  using ISA was capable of significantly lower NO X  emissions, such as with Test 2 at about 142 mg/Nm 3  at about 6% O 2  dry (0.115 lb/MMBtu), with its higher LOI of about 4.17%. Combined with ISA this example used deep fuel staging, which shifted fuel to a higher furnace elevation, leading to less overall carbon burnout residence time and thus, higher LOI. However, the exemplary system  200  using ISA was also capable of generating both low NO X  emissions and low LOI, as shown in Test 5. This represented the best-case that was modeled, yielding NO X  emissions of about 163 mg/Nm 3  at about 6% O 2  dry (0.133 lb/MMBtu) and LOI of about 2.17%. The exemplary system  200 , using ISA, gave previously unattainable flexibility in the ability to control both LOI and NO X  in the combustion system. 
     As used herein, the terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. Moreover, the endpoints of all ranges directed to the same component or property are inclusive of the endpoint and independently combinable (e.g., “about 5 wt % to about 20 wt %,” is inclusive of the endpoints and all intermediate values of the ranges of “about 5 wt. % to about 20 wt %”). Reference throughout the specification to “one embodiment”, “another embodiment”, “an embodiment”, and so forth means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the embodiment is included in at least one embodiment described herein, and may or may not be present in other embodiments. In addition, it is to be understood that the described elements may be combined in any suitable manner in the various embodiments. It is also to be understood that the disclosure is not limited by any theories described therein. Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this invention belongs. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.