Abstract:
A method of combusting non-gaseous fuel to heat a load in a process chamber includes the steps of injecting reactant streams into a combustion chamber. The combustion chamber does not contain a load to be heated, but communicates with the process chamber through a burner port. A first reactant stream includes the non-gaseous fuel, and is injected into the combustion chamber to cause the non-gaseous fuel to volatilize in the combustion chamber. A second reactant stream, which includes a premix of gaseous fuel and primary oxidant, also is injected into the combustion chamber. The first and second reactant streams then combust together to yield products of combustion that flow through the burner port from the combustion chamber to the process chamber. Importantly, the second reactant stream is injected into the combustion chamber adjacent to the first reactant stream. This causes the second reactant stream to adjoin the first reactant stream at the onset of volatilization of the non-gaseous fuel, which is found to inhibit the production of NOx.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of provisional U.S. Patent Application 61/347,153, filed May 21, 2010, which is incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This technology relates to the combustion of non-gaseous fuel for heating a load in a process chamber. 
       BACKGROUND 
       [0003]    Non-gaseous fuels frequently contain nitrogen. Burning such fuels using traditional methods results in a large portion of the fuel-bound nitrogen combining with oxygen to form nitrogen oxides (NOx). NOx is a regulated air emission since it can have negative effects on people and/or lead to photochemical smog. 
         [0004]    The initial stages of solid fuel combustion require the addition of heat to release volatile matter from the solid particle, called devolatilization. The terms devolatilization and volatilization may generally be used interchangeably in this context. Volatile matter may be either light molecular weight species which are gaseous at room temperature or tars which are liquid at room temperature. Increasing the initial heating rate of the solid fuel accelerates the devolatilization process. In the prior art the initial heating rate has been increased by using either substantially pure oxygen or oxygen-enriched combustion air which reduces fuel NOx but increases thermal NOx. 
         [0005]    Traditional approaches to mitigation of fuel NOx have centered on spatially staging the combustion air to delay mixing, such that few oxygen radicals are available, and thus the nitrogen radicals produced predominantly combine with each other. Spatially staging the combustion air to delay mixing requires the fabrication of large, expensive structures for segmenting and subsequently delivering the combustion air. 
         [0006]    In the combustion of solid fuels in the prior art, the solid fuel is injected into a combustion zone in one or more streams of conveyance air. Combustion air is introduced into the combustion zone in one or more streams that are adjacent to and/or intersect with the streams of solid fuel and conveyance air. The inventors have observed that introducing air in this manner creates a region of locally high oxygen concentration, which favors the creation of NOx. 
       SUMMARY OF THE INVENTION 
       [0007]    The invention provides a method of combusting non-gaseous fuel to heat a load in a process chamber. The method includes the steps of injecting reactant streams into a combustion chamber. The combustion chamber does not contain a load to be heated, but communicates with the process chamber through a burner port. A first reactant stream includes the non-gaseous fuel, and is injected into the combustion chamber to cause the non-gaseous fuel to volatilize in the combustion chamber. A second reactant stream, which includes a premix of gaseous fuel and primary oxidant, is injected into the combustion chamber adjacent to the first reactant stream. This causes the second reactant stream to adjoin the first reactant stream at the onset of volatilization of the non-gaseous fuel. The first and second reactant streams then combust together to yield products of combustion that flow through the burner port from the combustion chamber to the process chamber. Causing the premix stream to adjoin the non-gaseous fuel stream at the onset of volatilization of the non-gaseous fuel is found to help inhibit the production of NOx. 
         [0008]    In a preferred mode of the method, a reactant stream of secondary oxidant is injected into the process chamber from a port that is spaced from the burner port. This further helps to inhibit the production of NOx by separating the non-gaseous fuel from the stream of secondary oxidant until the volatilization of the non-gaseous fuel is complete or nearly complete. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic view of parts of a boiler. 
           [0010]      FIG. 2  is a view taken on line  2 - 2  of  FIG. 1 , showing additional parts of the boiler. 
           [0011]      FIG. 3  is a view taken on line  3 - 3  of  FIG. 1 , showing additional parts of the boiler. 
           [0012]      FIG. 2  is a view taken on line  4 - 4  of  FIG. 1 , showing additional parts of the boiler. 
           [0013]      FIG. 5  is an enlarged partial view of the boiler of  FIG. 1 . 
           [0014]      FIG. 6  is a partial front view of parts shown in  FIG. 5 . 
           [0015]      FIGS. 7-10  are views similar to  FIG. 5 , showing alternative embodiments. 
           [0016]      FIG. 11  is a partial front view of parts shown in  FIG. 10 . 
           [0017]      FIGS. 12 and 13  are views of other alternative embodiments. 
           [0018]      FIG. 14  is a partial front view of parts shown in  FIG. 13 . 
           [0019]      FIG. 15  is a view of another alternative embodiment. 
           [0020]      FIG. 16  is a partial front view of parts shown in  FIG. 15 . 
           [0021]      FIGS. 17-19  are views of other alternative embodiments. 
           [0022]      FIG. 20  is a view of another alternative embodiment. 
           [0023]      FIGS. 21 and 22  also are views other alternative embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    The invention provides a method of delivering non-gaseous fuel to an industrial combustion process using premix, which is a mixture of oxidant and gaseous fuel. For the case of solid fuels, premix can be used as the conveyance means instead of conveyance air; for the case of liquid fuels, premix can be used as an atomizing medium. Alternatively, premix can be injected separately from the non-gaseous fuel stream(s) in one or more adjacent streams or in ported, annular, or arcuate arrangements surrounding the solid fuel delivery port. 
         [0025]    A preferred result of the invention is to establish an initial, local, high reaction rate zone upon ignition of the premix which maximizes the volatilization rate of the non-gaseous fuel. It has been shown that high temperature volatilization increases volatile yield and reduces char, thereby diminishing the nitrogen content of the char and reducing the amount of fuel NOx produced from the combustion of the non-gaseous fuel. 
         [0026]    This invention seeks to inhibit NOx formation from the combustion of the non-gaseous fuels due to the fuel NOx pathway, but it can also inhibit the thermal NOx pathway. Liquid and solid fuels used in industrial combustion commonly contain non-trivial concentrations of nitrogen atoms which are chemically bound to other atoms in the fuel; it is well known that although much of such nitrogen in these fuels, upon liberation from chemical bonds (usually to carbon and/or hydrogen atoms) and radicalization, combine with other nitrogen radicals to form diatomic nitrogen (N2), a portion of the nitrogen radicals react with oxygen radicals to form nitric oxide (NO). 
         [0027]    Recent research (by others) has indicated that localized high heat release zones obtained via oxygen injection, though potentially increasing thermal NOx, can reduce fuel NOx because alternate reaction kinetics become favorable. Due to the complex organic molecules typically present in liquid and solid fuels, the volatilization and reaction time is on the order of tens of milliseconds (10 −2  seconds). 
         [0028]    For premix combustion, ignition delay is on the order of a millisecond (10 −3  seconds), and reaction time is on the order of a microsecond (10 −6  seconds); thus, the invention makes it possible to achieve regions of higher reaction rate (and hence higher heat release) in the presence of premix than for oxygen injection. Liquid fuels are typically atomized, forming tiny liquid droplets, to increase the surface area of the liquid stream and increase vaporization rate since the fuel reacts with oxidant essentially only in the gaseous phase. The atomizing media is typically compressed air or steam; however, one embodiment of this invention would utilize a gaseous fuel-air premix as the atomizing media instead. Since the premix would ignite both at a lower temperature and not have to undergo the vaporization step required of the liquid fuel, the premix would begin reacting sooner and at a faster rate than if the liquid fuel were atomized conventionally. Thus, using premix for atomization of the liquid fuel can create a region of high heat release early in the process, leading to less production of fuel NOx. 
         [0029]    Solid fuels are typically delivered to a combustion process as particles suspended in a stream of conveyance air. The conveyance air is typically significantly less than that required for stoichiometric combustion of the solid fuel, and additional combustion air is added in the proximity of the solid fuel/conveyance air outlet. When the solid fuel is exposed to high temperature (as in a combustion chamber), a portion of the species therein volatilize and become gaseous. It is these gaseous species which subsequently combust with the conveyance and combustion air. Therefore, in another embodiment of the invention, the conveyance air is replaced with a stream of gaseous fuel-air premix. Since the premix would ignite both at a lower temperature and not have to undergo the volatilization step required of the solid fuel, the premix would begin reacting sooner and at a faster rate than if the solid fuel were conveyed conventionally. Thus, using premix for conveyance of the solid fuel can create a region of high heat release early in the process, leading to reduced production of fuel NOx. 
         [0030]    Oxygen injection requires an operationally expensive onsite source of high-purity oxygen, the associated infrastructure to deliver the oxygen to the combustion chamber, and the additional means of injection. From a design standpoint, significant trial and error may be required to arrive at an oxygen injection configuration (injection angle, velocity, etc.) for suitable fuel NOx reduction. 
         [0031]    When oxygen combustion is employed, some amount of fuel is burned at extremely high flame temperatures, potentially in a region of where substantial O2 is present. This can lead to a super-equilibra of O atoms and substantial thermal NOx production. By using a volume of air/fuel gas premix, the interface between the combustion air stream and the non-gaseous fuel can essentially be “engineered”. This means that specific temperatures and oxygen concentrations can be achieved to best release the nitrogen from the fuel, limiting fuel NOx, while not creating excessive levels of thermal NOx. 
         [0032]    In an oxy-boost or oxy-combustion environment, initial reaction of O2 with the fuel will occur at flame temperatures near 5000 F. While great for volatilization of the fuel, this is clearly a temperature region where substantial thermal NOx is created. Using a premix “buffer” between the non-gaseous fuel and the combustion air stream in accordance with the invention, one can select the proper premix ratio to achieve desired results. For instance, the premix can be designed to react at 2500 F, producing little to no thermal NOx in its own reaction, but providing substantial heat for the volatilization of the non-gaseous fuel along with a limited amount of oxygen (approx. 9% O2) in the premix products. Since the premix reaction occurs rapidly compared to other chemical processes in these flames, a specific temperature and oxygen profile can be engineered by selecting appropriate premix volumes and fuel/air ratio. These in turn will determine the reaction temperature, premix product gas O2 content and the percentage of non-gaseous fuel combusted with the premix product gas. 
         [0033]    The addition of combustion products early in the combustion zone further aid in minimizing thermal NOx produced downstream, as the products reduce the local oxygen concentrations. 
         [0034]    A preferred embodiment of the invention injects a stream of gaseous fuel/air premix into a combustion zone adjacent to a stream of solid fuel and conveyance air. The stream of premix shields the solid fuel from combustion air in a region of reacting premix, thus further heating the solid fuel and increasing the rate of volatilization. Further, since the combustion of the fuel in the premix consumes oxygen, it creates a region of locally low oxygen concentration which, instead of promoting the production of NOx, favors the recombination of nitrogen radicals into diatomic nitrogen (N2) as the solid fuel interacts with the combusting premix. 
         [0035]    The utilization of premix in combination with non-gaseous fuels creates a favorable thermochemical environment for the fuel-bound nitrogen to recombine with other nitrogen to form diatomic (N2) nitrogen instead of forming NOx, and can do so while avoiding the drawbacks of using an increased oxygen oxidant. Such drawbacks include ultra-high flame temperature, locally high oxygen concentration, associated thermal NOx creation, high capital and operating cost, and limited flexibility. 
         [0036]    The premix can be varied across a range of fuel/air ratios, which can be specifically selected based on the volatilization characteristics of the non-gaseous fuel. Additionally, premix has more flexibility to create a favorable environment for nitrogen recombination since it can provide a range of the heat required by the process. 
         [0037]      FIG. 1  is a schematic illustration of a field-erected boiler  10  that is used to create steam in an electricity-generating power plant by combusting fuel to heat liquid water. The two main portions of the boiler  10  are the firebox  12  and the convection pass  14 . Inside a series of circular tubes  16  ( FIG. 2 ), water is first directed through the convection pass  14  where the tubes  16  fill most of the rectangular cross-section, though there is some space in between the tubes  16 . After the convection pass  14 , the water is routed to a series of adjacent tubes  18  ( FIG. 3 ) along the walls of the firebox  12 . After leaving the firebox  12 , the water, now in the form of gaseous steam, continues to a turbine where it is used to generate electricity. 
         [0038]    The firebox  12 , which is sometimes referred to as the furnace, defines a process chamber  21  in which the heat of combustion is used to do work. In the given example the process chamber  21  is rectangular in cross-section, and is connected to a burner  30 , which itself is connected to sources of non-gaseous fuel  32  and gaseous fuel  34 . A blower system  36  provides the burner  30  with primary combustion air. Suitable non-gaseous fuels include coal, biomass and fuel oil. In this embodiment, solid fuels would be delivered to the burner  30  with a stream of conveyance air, but liquid fuels would not. 
         [0039]    The non-gaseous fuel in the illustrated example is pulverized coal that is transported in suspension by a relatively small quantity of flowing conveyance air. The conveyance air is preferably about 10% of that required for stoichiometric combustion of the non-gaseous fuel. The non-gaseous fuel contains about 90% of the heat input into the boiler  10 . The gaseous fuel, which is preferably natural gas, contains the balance of the heat input delivered to the boiler  10 . 
         [0040]    The non-gaseous fuel, gaseous fuel, and combustion air provided through the burner  30  combust in the firebox  12 , creating high temperature products of combustion (POC) which flow generally upward through the firebox  12 . A source of secondary oxidant, which in the illustrated example is a port  41  through which secondary combustion air is delivered from the blower system  36 , is located above the burner  30 . In other examples, the source of secondary oxidant could include a lean premix of gaseous fuel and air, exhaust from a gas turbine, recirculated flue gas, or any other available source of oxygen. The secondary oxidant provides the balance of the oxygen which is required to complete combustion of the gaseous and non-gaseous fuels. The high temperature POC transfer heat to the water tubes  18  via radiant and convective heat transfer modes in the firebox  12  before proceeding through the convection pass  14 , where they continue to transfer heat to the water tubes  16  in that portion of the boiler  10  primarily via convective heat transfer. After traveling through the convection pass  14 , the POC go to a stack where they are discharged outside of the power plant and into the atmosphere. 
         [0041]    The burner  30  in the example of  FIG. 1  shown separately in  FIGS. 5 and 6 . This embodiment of the burner  30  includes a refractory structure  100  defining a combustion chamber  101 . An open outer end portion  102  of the refractory structure  100  defines a burner port  105  centered on an axis  107 . The outer end portion  102  is internally tapered for stabilizing a flame  109  projecting outward through the burner port  105  from the combustion chamber  101  into the process chamber  21 . 
         [0042]    A duct structure  110  at the rear of the refractory structure  100  defines a combustion air plenum  111 . A group of mixer tubes  114  communicate the plenum  111  with the combustion chamber  101 , and are arranged in a circular array centered on the axis  107 . A corresponding group of branch lines  116  extend from a gaseous fuel supply line  118  into the mixer tubes  114 . A non-gaseous fuel supply line  120  extends along the axis  107  to the combustion chamber  101 . In this example, the outlet ports of the mixer tubes  114  and the non-gaseous fuel line  120  are located in a common plane  121  on a wall  122  facing axially into the combustion chamber  101  at the rear end of the combustion chamber  101 . 
         [0043]    In this configuration, the non-gaseous fuel supply line  120  delivers a stream of coal and conveyance air into the combustion chamber  101  along the axis  107 . The mixer tubes  114  receive streams of primary combustion air from the plenum  111 , and also receive streams of gaseous fuel from the branch lines  116 . Those reactants form a fuel-air premix as they flow along the mixer tubes  114 . The premix in this example is the source of primary combustion air, and is injected into the reaction chamber  101  in a circular array of streams that surround, and are adjacent to, the axially centered stream of coal and conveyance air. Combustion of all these reactants produces the flame  109  that projects outward through the burner port  105  from the combustion chamber  101  to the process chamber  21 . 
         [0044]      FIGS. 7-19  show alternative embodiments the burner. In the burner  200  of  FIG. 7 , an axially centered stream  201  of premix is injected into the combustion chamber  203  surrounded by an annular stream  205  of coal and conveyance air. These reactant streams produce a flame  209  that projects through the burner port  211  from the combustion chamber  203  to the process chamber  21 . 
         [0045]    In the burner  240  of  FIG. 8 , an axially centered stream  241  of coal and conveyance air is injected into the combustion chamber  243  surrounded by an annular stream of premix  245 . 
         [0046]    In the burner  250  of  FIG. 9 , an axially centered stream  251  of coal and conveyance air is surrounded by an annular stream  253  of premix. The annular stream  253  of premix is surrounded by an annular stream of primary combustion air  255  that enters the combustion chamber  257  through swirling vanes  258 . 
         [0047]    In the burner  270  of  FIGS. 10 and 11 , an axially centered stream of coal and conveyance air  271  is surrounded by an annular stream  273  of premix. Tangentially oriented ducts  274  convey streams  275  of secondary combustion air radially inward to form a vortex surrounding the annular stream  273  of premix. 
         [0048]    In the burner  280  of  FIG. 12 , tangentially delivered streams  281  of premix enter the combustion chamber  283  as a vortex surrounding an axially centered stream  285  of coal and conveyance air. 
         [0049]    In the burner  290  of  FIGS. 13 and 14 , non-gaseous fuel branch lines  292  extend through mixer tubes  294  to inject streams of coal and conveyance air into the reaction chamber  297 . Each stream of coal and conveyance air injected from a non-gaseous fuel branch line  292  is surrounded by a stream of premix emerging from the respective mixer tube  294 . 
         [0050]    In the burner  300  of  FIGS. 15 and 16 , premix is injected into the combustion chamber  301  through an arcuate premix port  303  located above the non-gaseous fuel port  305 . 
         [0051]    In the burner  310  of  FIG. 17 , an annular stream  311  of coal and conveyance air surrounds an axially centered stream  315  of premix that enters the combustion chamber  317  through swirling vanes  318 . 
         [0052]    In the burner  330  of  FIG. 18 , an axially centered stream  331  of coal and conveyance air is surrounded by an annular stream  335  of premix that enters the combustion chamber  337  through swirling vanes  338 . 
         [0053]    In the burner  350  of  FIG. 19 , an axially centered stream  351  of coal and conveyance air is surrounded by concentric annular streams  353  and  355  of premix and secondary air that enter the reaction chamber  357  through respective swirling vanes  358  and  360 . 
         [0054]    In each embodiment of the invention, the intention is to separate the non-gaseous fuel from the combustion air until the volatilization of the non-gaseous fuel and the recombination of the nitrogen is complete or nearly complete. In the embodiments shown as examples in  FIGS. 1-20 , this is accomplished by injecting a stream of premix into the combustion zone adjacent to the stream of non-gaseous fuel and conveyance air. By “adjacent” it is meant that no other reactant stream is injected into the combustion zone between the adjacent streams. This enables the stream of premix to adjoin, mix and interact with the stream of non-gaseous fuel and conveyance air before the combustion air can have the undesirable cooling and NOx producing effects noted above. The ports from which the adjacent streams are injected into the combustion zone are likewise adjacent to each other, and in some examples are as close together as their structures will allow. In those examples, the adjacent streams are initially separated only by the structure of their respective ports, and are thus injected into the combustion zone so as to adjoin and interact with each other as soon as possible upon entering the combustion zone. 
         [0055]    In  FIG. 20 , an axially centered stream  390  of non-gaseous fuel is surrounded by an annular stream  391  of premix. An annular stream  395  of combustion air is located radially outward. 
         [0056]    Zone 1 indicates the location where the premix combustion initiates. 
         [0057]    Zone 2 indicates where the combusting premix mixes with the solid fuel stream, causing the particles to heat up. 
         [0058]    Zone 3 indicates where the particles have been heated sufficiently to begin releasing volatile species in a gaseous phase, which subsequently begin combusting. 
         [0059]    Providing a zone 1 of combusting premix at a location adjoining zones 2 and 3 reduces the time, and thus distance, required to heat the particles and initiate volatilization, in essence moving the end of zone 3 and beginning of zone 4 closer to the point of injection of the solid fuel (toward the left in  FIG. 20 ) as compared to not providing such a zone 1 of combusting premix. 
         [0060]    Zone 4 is the high temperature/low oxygen zone which greatly promotes recombination of nitrogen radicals because of the lower concentration of oxygen radicals available for the production of NOx. 
         [0061]    Zone 5 indicates the where the completion of combustion occurs as the air mixes with the combination of the premix and solid fuel and their respective products of combustion formed thus far. 
         [0062]    In the burner  400  of  FIG. 21 , combustion air is injected into the combustion chamber  401  as an annular stream  403  surrounding an axially centered reactant stream  405 . The axially centered reactant stream  405  contains solid fuel, and contains gaseous fuel/air premix as the conveyance medium for the solid fuel. 
         [0063]    In the burner  420  of  FIG. 22 , combustion air is injected into the combustion chamber  421  as an annular stream  423  surrounding an axially centered reactant stream  425  injected from an atomizing nozzle  426 . The axially centered reactant stream  425  contains liquid fuel, and contains gaseous fuel/air premix as an atomizing medium. 
         [0064]    This written description sets forth the best mode of the invention, and describes the invention so as to enable a person of ordinary skill in the art to make and use the invention, by presenting examples of the elements recited in the claims. 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 elements that do not differ from the literal language of the claims, or if they have equivalent elements with insubstantial differences from the literal language of the claims.