Abstract:
A combustion apparatus capable of firing biomass fuel including a burner assembly which includes a biomass nozzle concentrically surrounded by a core air zone and extending axially along the length of the core air zone, the burner assembly residing within a windbox, the windbox being attached to a furnace of a boiler, and the burner assembly being connected to the furnace by a burner throat, through which air and fuel supplied to the burner assembly are emitted into the furnace.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to the field of industrial burner apparatuses for performing combustion functions for power generation. 
         [0002]    As used herein the term “biomass” describes a wide range of organic matter derived from diverse living, or recently-living organisms, such as grasses and wood products. Sources of biomass include trees, shrubs, bushes, residual vegetation from harvesting grains and vegetables. Biomass is commonly plant matter harvested to generate electricity or produce heat. Biomass may also include biodegradable wastes of organic origin that can be burned as fuel. 
         [0003]    Biomass differs from fossil fuels, which are hydrocarbons found within the top layer of the Earth&#39;s crust. Common examples of fossil fuels include coal and oil. Unlike fossil fuels, biomass fuels are generally considered CO 2  neutral and renewable resources, since CO 2  generated from biomass combustion can be removed from the atmosphere by the plants that provide the biomass. 
         [0004]    As the physical properties and chemical composition of biomass differ greatly from that of coal, biomass fuels for power generation have historically been utilized as a primary or auxiliary fuel in stoker and fluid bed style boilers. Such boilers do not rely on burners thereby enabling significantly higher furnace residence time for combustion and consequently have less stringent fuel preparation requirements. 
         [0005]    Global warming concerns relating to greenhouse gas emissions has increased the interest in developing new technologies to enable widespread use of renewable resources for power generation. One area of such interest is the use of biomass fuels in suspension firing, wherein short furnace residence times require fine particles for efficient combustion. 
         [0006]    Pulverized coal firing is the primary means of suspension firing in the power generation industry. In a first step coal is mechanically pulverized into fine particles. The particles are then subsequently conveyed via suspension in a primary air stream to a burner, wherein the burner ejects the air/fuel mixture in a furnace for combustion. Residence times are nominally 1-2 seconds, which is normally sufficient for complete pulverized coal combustion with proper particle sizing. 
         [0007]    Biomass firing in pulverized coal-fired boilers is becoming more widespread as a strategy for reducing greenhouse gases. To enable this strategy a need exists to develop a burner capable of effectively utilizing biomass fuels in suspension firing. 
         [0008]    Firing biomass fuels faces many technical challenges. As compared to bituminous coal, biomass fuels have significantly lower heating values and a higher concentration of volatile matter. Heating value is inversely proportional to moisture content, such that it amounts to 25% to 75% that of a typical bituminous coal. Biomass moisture will often be reduced prior to firing for material handling reasons and to improve process efficiency and capacity. Nevertheless, firing biomass in place of coal requires considerably more fuel mass to achieve a comparable heat output. Further, while the highly-volatile nature of biomass makes the fuel inherently easy to burn, the high moisture content can delay ignition. Delayed ignition is especially undesirable in suspension firing, 
         [0009]    Another concern with biomass fuels if that biomass is not processed to the same particle size as pulverized coal. Experience indicates successful suspension firing can be achieved with wood particles sized 0.0625 in. compared to the top size for pulverized coal of 0.012 in. Particle volume varies by the diameter cubed, thus wood particles have approximately 150 times the volume of larger coal particles used for suspension firing. The larger volume of the biomass thus requires quick ignition and rapid combustion to enable use of biomass in furnaces designed for pulverized coal firing. 
         [0010]    One known technique of utilizing biomass in suspension firing is biomass co-firing. In this technique biomass particulate is combined with pulverized coal and primary air in a single stream. The combined stream is then introduced into the furnace. This technique is however limited in practicality due to the resulting burner nozzle velocity necessary to maintain both types of particles in suspension. Excessive burner nozzle velocity results in flame instability, delayed ignition, and poor combustion performance. 
         [0011]    Thus, there remains a need to develop a means for an efficient and effective alternative to combusting coal for power generation and a means for enabling the widespread combustion of a carbon-neutral fuel for power generation applications. 
       SUMMARY OF THE INVENTION 
       [0012]    The embodiments of the present invention provide a novel combustion apparatus. More specifically, embodiments of the present invention provide a combustion apparatus capable of firing biomass fuel and alternating between biomass and coal firing, as needed, and/or combusting a combination of coal and biomass fuels concurrently. 
         [0013]    Embodiments of the present invention extend the capability of prior art burners. U.S. Pat. No. 7,430,970 to LaRue et al. (&#39;970 patent), is hereby incorporated into the by reference in the entirety. 
         [0014]    The present invention is an improvement upon prior art burners by providing a novel device for combusting renewable fuels, including, but not limited to, biomass. 
         [0015]    Embodiments of the present invention provide a superior method and apparatus for co-firing biomass in combination with pulverized coal. 
         [0016]    A combustion apparatus capable of firing biomass fuel including a burner assembly which includes a biomass nozzle concentrically surrounded by a core air zone and extending axially along the length of the core air zone, the burner assembly residing within a windbox, the windbox being attached to a furnace of a boiler, and the burner assembly being connected to the furnace by a burner throat, through which air and fuel supplied to the burner assembly are emitted into the furnace. 
         [0017]    In embodiments of the present invention, the apparatus includes a forced draft fan providing a first supply of air to the windbox, a core air duct, enclosing the core air zone, for receiving a core portion of the first supply of air, the core air duct having a core damper for regulating the core portion entering the core air duct, a core nozzle for receiving the core portion from said core air duct, the core nozzle delivering said core portion to said burner throat, a burner elbow for receiving pulverized coal and a second supply of air, the pulverized coal and said second supply of air continuing through a coal nozzle in an annulus formed between the core nozzle and the coal nozzle, the core portion serving to accelerate ignition of pulverized coal by contacting an inner cylinder of a coal jet leaving the coal nozzle, the core portion also serving to accelerate combustion. 
         [0018]    The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawing and descriptive matter in which a preferred embodiment of the invention is illustrated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0019]    In the drawings: 
           [0020]      FIG. 1 . is a schematic side elevation view of an embodiment of the present invention. 
           [0021]      FIG. 2 . is a schematic side elevation view of an alternative embodiment of the present invention. 
           [0022]      FIG. 3 . is a schematic side elevation view of an alternative embodiment of the present invention. 
           [0023]      FIG. 4 . is a schematic cross sectional view of an embodiment of the present invention which identifies the concentric zones of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    Referring now to figures, wherein like references designate the same or functionally similar elements throughout the several drawings,  FIG. 1  shows a burner assembly  1  residing within windbox  2 , which is attached to the furnace  3  of a boiler (not shown). Secondary air  22  is provided to windbox  2  by a forced draft fan (not shown) and heated by an air preheater (not shown). The burner assembly  1  is connected to furnace  3  by burner throat  4 , through which air and fuel supplied to the burner assembly  1  are emitted into the furnace  3 . A portion of the secondary air  22  constitutes core air  5 . Core air  5  enters core air duct  6  and is regulated by core air damper  7 . Core air  5  continues through the burner assembly  1  through core nozzle  8 , exiting through the burner throat  4 . 
         [0025]    Secondary air  22  is also supplied to the burner assembly (designated as secondary air to the burner assembly  9 ). Secondary air  22  enters the burner assembly  1  and travels through parallel flow paths of the inner air zone  10  and outer air zone  11 . Swirl vanes in these zones serve to swirl secondary air  22  to facilitate ignition and combustion of secondary air  22  contacting the pulverized coal stream. An air separation vane  12  at the exit of outer zone  11  acts to increase the size of an internal recirculation zone (IRZ) formed by resultant aerodynamics. Pulverized coal and primary air  13  enter burner elbow  14  and continue through coal nozzle  15 , in the annulus formed between core nozzle  8  and coal nozzle  15 . The core air  5  serves to accelerate ignition of pulverized coal by contacting the inner cylinder of the coal jet (not shown) leaving the coal nozzle  15 ; and serves to accelerate combustion by a “bellows effect” supplying air to the center of the flame. LaRue &#39;970 provides a detailed discussion on the accelerated ignition relating to core air. 
         [0026]    The burner assembly  1  according to embodiments of the present invention may be operated in combination with an over-fire-air (“OFA”) system (not shown). A portion of the secondary air  22  supplied to the furnace for combustion is supplied to the OFA system, such that the total amount of air supplied to the burner assembly  1  is less than theoretical air requirements. This produces a reducing environment in the furnace before OFA is supplied. The accelerated combustion, higher temperature flame, and larger IRZ all serve to more effectively reduce NO x  under reducing conditions. 
         [0027]    In embodiments of the present invention, biomass may be prepared for suspension firing using shredders, hammer mills and the like (not shown), collected and regulated in feed rate by a screw feeder or equivalent device (not shown) and pneumatically conveyed to the burner assembly  1  through an appropriate conduit. The conduit supplies biomass and transport air  16  through an elbow  14  whose outlet is situated at the axis of the burner  1 . 
         [0028]    In some embodiments, a reducer  17  may be used to reduce the cross-sectional area of biomass nozzle  18  as the nozzle transverses the burner elbow  14  and continues past the core air duct  16 . A reducer  17  serves to lessen the flow obstruction as the biomass nozzle  18  extends through the length of the burner assembly  1 . Near the furnace end of the burner assembly  1 , the biomass nozzle tip  19  diameter can be expanded as shown ( FIG. 1 .) to reduce the biomass exit velocity to the optimum value for combustion. In certain embodiments, this exit velocity is between about 2500 ft/min and about 5000 ft/min, and more preferably between about 3000 ft/min and 4000 ft/min. 
         [0029]    In further embodiments, core air  5  surrounding the biomass nozzle tip  19  serves to accelerate ignition of the biomass as it enters the burner throat  4 , and supplies air to feed combustion as the biomass continues into the furnace. The hot secondary core air that surrounds the biomass nozzle provides heat to enable additional moisture removal from the biomass fuel while supplying the fuel with an oxidant to facilitate ignition and combustion. This solves the problems related to delayed ignition and combustion associated with firing biomass in prior art burners. Core air damper  7  is adjusted to supply core air  5  in such quantity so as to minimize NO x  emissions when firing biomass in combination with pulverized coal. For times when biomass is not being fired, the biomass supply system (not shown) serving the burner assembly  1  is shut down and valve  23  is closed. Valve  21  is then opened and adjusted in combination with core damper  7  to supply the optimum amount of core air  5  necessary for minimizing NO x  when firing the particular coal. When the biomass is to be fired, valve  21  is shut and valve  23  is opened to admit biomass and transport air  16 . 
         [0030]    Referring now to  FIG. 4 , a schematic cross section of the burner assembly  1  of the present invention is shown wherein the five distinct zones of the burner assembly  1  are identified. A biomass zone  32  defined by biomass nozzle  18  is concentrically surround by a core air zone  44  defined the area between biomass nozzle  18  and core nozzle  8 . A coal nozzle  15  concentrically surrounds core nozzle  8  defining a first annular zone  47  wherein pulverized coal and primary air (PC/PA)  13  flows. A barrel  42  concentrically surrounds coal nozzle  15  and defines the inner air zone  10  internal to barrel  42  and an outer air zone  11  external to barrel  42 . 
         [0031]    While a preferred embodiment has been shown, alternative embodiments may also be achieved without departing from the scope of the present invention. 
         [0032]    One alternative embodiments includes a straight pipe without reducer  17  ( FIG. 2 ), and/or without expansion at the furnace end of biomass nozzle  18 . In this embodiment the alternative of a shorter or recessed biomass nozzle  18  is also shown wherein the biomass nozzle tip  19  terminates within the core nozzle  8  near the core air duct  6 . This embodiment provides the additional benefit of preheating and premixing the biomass with the core air, thereby further enabling additional moisture removal from the biomass fuel. 
         [0033]    A reducing taper may be used at the exit of the biomass nozzle  18  ( FIG. 3 ) to accelerate the biomass fuel as it enters the furnace  3  to prevent flashback into the biomass nozzle  18 . While the biomass nozzle  18  is illustrated as an open-ended nozzle in the figure, it may be readily fitted with deflectors or swirlers near the exit to increase mixing rate of biomass with core air. 
         [0034]    In other embodiments, adjustment means may be included to facilitate minor fore/aft adjustments in the end position of the biomass nozzle  18  relative to the core pipe to enable further optimization of combustion. While the biomass nozzle  18  is shown flush with the end of the core pipe in  FIG. 1 , it may also be positioned slightly further back or further forward. In certain embodiments, valve  21  may be used to admit a small amount of air, either hot secondary air or unheated air, to add air to the center of the flame while firing biomass. The purpose of this is to augment center stoichiometry for lowest NO x  (as alternative to increasing transport air quantity). 
         [0035]    Embodiments of the present invention provide a number of advantages. The biomass co-fired air jet burner according to embodiments of the present invention provides a novel, superior structure and enables a superior method for firing biomass fuels. 
         [0036]    The large core zone accommodates a biomass nozzle without changing burner size, saving the engineering and manufacturing costs normally associated with building burners of different sizes to accommodate biomass firing. 
         [0037]    The large biomass nozzle enables firing larger quantities of biomass in selected burners, such that fewer burners need be supplied to fire biomass. Biomass firing rates up to 40% of rated burner input enable boiler biomass firing rates of 20% while using only half the burners. 
         [0038]    The biomass fuel availability often varies with the seasons such that biomass firing may not be conducted continuously. In an alternative embodiment the biomass nozzle can be supplied with secondary air when not firing biomass such that both the biomass nozzle  18  and core nozzle  8  provide a combined core air jet for the combustion of pulverized coal. 
         [0039]    Also, the transport air with biomass contributes to the preferred center stoichiometry of the burner when firing biomass in combination with coal. In such case, the coal flow is reduced such that a higher PA/PC ratio is supplied to the burner. This is augmented with transport air from biomass to provide a center stoichiometry conducive to very low NO x  emissions. 
         [0040]    Further, the locating of the biomass nozzle in the core zone provides a source of hot secondary air for igniting and feeding combustion of the biomass fuel, preventing the delayed ignition experienced in prior art as well as feed combustion of the co-fired biomass fuel. 
         [0041]    While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.