Abstract:
A burner and method for oxidizing solid fuels wherein the burner has a lance having one or more nozzle feeds and one or more nozzle outlets concentrically surrounded by a primary oxidant passage which is concentrically surrounded by a secondary oxidant passage wherein the primary and secondary oxidant passages communicate at their proximal ends with a gas supply, the lance having a distal and proximal end and the one or more nozzle feeds is in communication with a gas supply.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from U.S. provisional patent application Ser. No. 61/369,894 filed on Aug. 2, 2010. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention provides for the use of a burner for carbon dioxide-oxygen-coal combustion processes. More particularly, the invention provides for a lance in connection with a burner to more effectively combust a solid fuel, such as coal, with an oxidant mixture comprising predominantly of carbon dioxide and oxygen, such as may be formed by the addition of substantially pure oxygen with flue gas recirculated from said combustion process. 
         [0003]    Air-coal burners and steam boilers for electrical power generation have been developed over many years through a process of trial and error. Typically, the motive air that is used to pulverize the coal is also used as the primary air in the air-coal burners. Primary air and pulverized coal typically flow through an annular conduit that is mesial to coaxial annular conduit for the secondary air. A helical secondary oxidant flow patter is often used to increase the mixing rate with the primary air-coal stream with the secondary air stream. 
         [0004]    In order to more cost effectively recover carbon dioxide from coal fired power stations, electrical power utilities are evaluating substituting a predominantly carbon dioxide-oxygen oxidant for the traditional air oxidant. The carbon dioxide-oxygen oxidant is formed by mixing a CO2 rich flue gas from the combustion process with industrial oxygen, and as such may also contain water vapor, nitrogen, and minor constituents arising from the combustion process. However, substitution of the nitrogen diluent in the air oxidant with carbon dioxide decreases the coal combustion flame velocity more rapidly than the coal flammability limit and flame temperature. If one tries to increase the carbon dioxide-oxygen-coal flame velocity to acceptable values by increasing the primary oxidant oxygen in carbon dioxide concentration, then the fire hazard in the coal grinding mill also increases. If one increases the flame velocity to acceptable values by increasing the oxygen in carbon dioxide concentration in the secondary oxidant stream, then excessive heat fluxes are observed in the near burner region. 
         [0005]    An additional trend is in the utilization of increasing quantities of biomass mixed with coal, or even 100% biomass firing. The biomass feed may be any organic material, including agricultural crops and agricultural wastes and residues, wood and wood wastes and residues, animal wastes, municipal wastes, algae and aquatic plants. The biomass feed may be dried, chopped, ground, or pelletized. Typical biomass feed examples include wood chips, wood charcoal, pelletized grass, and similar materials Biomass, particularly wet biomasses can further lead to reductions in flame speed and more unstable combustion conditions. As such the addition of biomass can lead combustion related problems that may not be overcome by increasing the oxygen content of the secondary air. 
         [0006]    The invention seeks to limit these concerns by adding an oxidant lance to a conventional air-coal burner assembly. 
       SUMMARY OF THE INVENTION 
       [0007]    The invention provides for the use of a predominantly carbon dioxide-oxygen oxidant in place of air oxidant in a solid fuel burner. The solid fuel herein after referred to as coal may be any solid hydrocarbon fuel, such as various grades of coal, peat, coke, or biomass The invention further provides for an improved coal burner assembly which comprises an air-coal burner equipped with an oxidant lance, thereby providing a better combustion with an oxygen-carbon dioxide-coal mixture than an air-coal mixture. 
         [0008]    In an embodiment of the invention, there is disclosed a burner for combusting fuels such as coal comprising a lance having one or more nozzle feeds and one or more nozzle outlets concentrically surrounded by a primary oxidant passage which is concentrically surrounded by a secondary oxidant passage wherein said primary and secondary oxidant passages communicate at their proximal ends with a gas supply, said lance having a distal and proximal end and said one or more nozzle feeds is in communication with a gas supply. 
         [0009]    In another embodiment of the invention there is disclosed a method for oxidizing coal comprising feeding a mixture of oxygen, carbon dioxide and coal to a burner comprising a lance having one or more nozzle feeds and one or more nozzle outlets concentrically surrounded by a primary oxidant passage which is concentrically surrounded by a secondary oxidant passage wherein said primary and secondary oxidant passages communicate at their proximal ends with a gas supply, said lance having a distal and proximal end and said one or more nozzle feeds is in communication with a gas supply. 
         [0010]    The solid fuel that can be used are selected from the group consisting of coal, coke, peat, biomass and mixtures thereof. The solid fuel is typically transported via a primary oxidant stream through the primary oxidant passages. The primary oxidant stream will have a molar oxygen concentration less than 32%, preferably less than 25% and more preferably less than 21%. 
         [0011]    The fuel and oxidant mixture comprises oxygen and carbon dioxide and is typically formed by mixing flue gases from a combustion process with industrially pure oxygen. 
         [0012]    A secondary oxidant stream is fed through the secondary oxidant passage and the secondary oxidant stream has a molar oxygen concentration less than 40%, preferably less than 30% and more preferably less than 28. 
         [0013]    The nozzles at the distal end of the lance are angled in a manner whereby the discharge streams from the lance do not intercept an outer wall of the primary oxidant passage. Further, these nozzles are positioned to direct oxygen into a flow path of the primary oxidant stream containing solid fuel. 
         [0014]    The burner further comprises a quad and the nozzles at the distal end of the lance are arranged complementary to the primary and secondary oxidant streams being directed through the quarl. In some instances, the distal end of the lance is angled so that the projected axis intercepts an outer edge of the burner quarl. 
         [0015]    The burner can be adapted for external oxidant staging. 
         [0016]    The oxidant stream that issues through the distal end of the lance is at a higher velocity than the primary oxidant. Typically, this velocity is less than four times the velocity of the primary oxidant stream. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a schematic view of an air-coal burner. 
           [0018]      FIG. 2  is a schematic A-A view of the air-coal burner of  FIG. 1 . 
           [0019]      FIG. 3  is a schematic view of an air-coal burner with oxidant lance. 
           [0020]      FIG. 4  is a schematic front view of an air-coal burner with oxidant lance. 
           [0021]      FIG. 5  is a graph showing the oxidant angle nozzle relationships. 
           [0022]      FIG. 6  is a schematic of the burner with an alternative oxidant lance embodiment. 
           [0023]      FIG. 7  is a schematic showing a nozzle for use with a lance and burner shown in  FIG. 6 . 
           [0024]      FIG. 8  is a schematic showing an alternative nozzle for use with a lance and burner as shown in  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]      FIG. 1  and  FIG. 2  show an air-coal burner. The air-coal burner is typically round in shape as seen in  FIG. 2 . Viewing the air-coal burner from the side as in  FIG. 1  shows a furnace wall  10  at the top and bottom which surrounds the air-coal burner. The air-coal burner comprises a quarl  20  which surrounds the closed end  7  of the air-coal burner. There are two openings, the first being the secondary air passage  6  which will concentrically surround the second opening, coal and primary air passage  5 . 
         [0026]      FIG. 2  shows the air-coal burner of  FIG. 1  along the A-A axis and proffers a view of the air-coal burner from its end. The furnace wall  10  is shown surrounding the quarl  20  which has two openings in its middle. The first opening is the secondary air passage  6  which concentrically surrounds the second opening, the coal and primary air passage  5 . Both these passages concentrically surround the closed end  7  of the air-coal burner. 
         [0027]      FIG. 3  shows the air-coal burner with oxidant lance of the invention. This A-A sideways view shows the quarl  40  and the burner wall  30  which in turn surround the closed end  35  of the air-coal burner with oxidant lance. Concentrically surrounding the closed end  35  is the primary oxidant passage  36  which is in fluid communication with the motive fluid or coal transport gas and coal channel  38 . The secondary oxidant passage  37  is on the outside of this primary oxidant passage  36 . 
         [0028]    The coal channel  38  and primary oxidant passage concentrically surround the closed end  35  of the air-coal burner. As seen in  FIG. 3 , the oxidant lance nozzle inlet  101  fluidly connects to the oxidant lance feed channel  39 . The outlet of the oxidant lance nozzle extends towards the front of the air-coal burner and is angled outwards from the inlet to the outlet of the oxidant lance nozzle. In  FIG. 3 , the angle is 11.2° from center. 
         [0029]      FIG. 4  is the front facing view of the air-coal burner with oxidant lance of  FIG. 3 . The quarl  40  surrounds concentrically the secondary oxidant passage  37  which in turn concentrically surrounds the primary oxidant passage  36 . Both these passages concentrically surround the oxidant lance nozzle inlets (nozzle feed) and nozzle outlets,  45  and  50  respectively. As seen in  FIG. 4 , both the oxidant lance nozzle outlets  50  and nozzle feeds  45  comprise several nozzles in concentric relation to each other. In this instance, the nozzle yaw angle is 32.1° 
         [0030]    In Table 1 below, the differences between an air-coal burner using air versus an air-coal burner using a mixture of carbon dioxide and oxygen are shown. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Comparison of Air-Coal Burner versus CO 2 —O 2 -Coal Burner 
               
             
          
           
               
                 Parameter 
                 Air 
                 CO 2 —O 2   
               
               
                   
               
               
                 Overall Burner Properties 
                   
                   
               
               
                 Primary &amp; Secondary Oxidant O 2  (vol. % O 2 ) 
                 21% 
                 28% 
               
               
                 Secondary air channel diameter (mm) 
                 142  
                 142  
               
               
                 Primary &amp; Secondary Oxidant (l/sec) 
                 157  
                 157  
               
               
                 Coal Transport Gas 
               
               
                 O 2  Concentration (mol/%) 
                 21% 
                 21% 
               
               
                 Gas flow rate (l/sec) 
                 25 
                 25 
               
               
                 Gas velocity (M/sec) 
                 13 
                 13 
               
               
                 Oxidant Lance Gas 
               
               
                 O 2  Concentration (mol/%) 
                   
                 40% 
               
               
                 Gas flow rate (l/sec) 
                   
                 28 
               
               
                 Lance feed gas velocity (l/sec) 
                   
                 13 
               
               
                 Nozzle velocity (m/sec) 
                   
                 39 
               
               
                 Primary Oxidant (Mixed Oxidant Lance and 
               
               
                 Coal Transport Gases) 
               
               
                 O 2  Concentration (mol/%) 
                 21% 
                 31% 
               
               
                 Gas flow rate (l/sec) 
                 25 
                 53 
               
               
                 Gas velocity (M/sec) 
                 13 
                 13 
               
               
                 Secondary Oxidant 
               
               
                 O 2  Concentration (mol/%) 
                 21% 
                 26% 
               
               
                 Gas flow rate (l/sec) 
                 132  
                 104  
               
               
                 Gas velocity (M/sec) 
                 20 
                 20 
               
               
                   
               
             
          
         
       
     
         [0031]      FIG. 5  is a graph showing the oxidant nozzle angle relationships. This graph plots the central angle between the nozzle inlet and outlet versus the oxidant nozzle yaw angle for four separate parameters. The parameters were determined by the ratio of primary oxidant inside diameter divided by the inside diameter of the oxidant lance. As seen from  FIGS. 3 and 4 , the nozzle yaw angle is 32.1° and the nozzle pitch angle is 11.2° for the example having a parameter of 1.57. 
         [0032]    The overall oxygen in carbon dioxide concentration was set at about 28 molar percent to yield roughly equivalent adiabatic flame temperatures for the air and carbon dioxide-oxygen oxidants. The burner outside diameter was held constant so that the carbon dioxide-oxygen burner can use the air burner mounting system. The overall oxidant flow rate was held constant which would result in an approximate 30% increase in thermal output. Similar techniques could be used for the less demanding constant thermal output basis. The coal transport gas oxygen in carbon dioxide content was set at 21 molar percent which would decrease the coal grinding mill fire hazard concern at higher oxygen concentrations. 
         [0033]    The addition of oxidant lance oxidant increased the primary oxidant oxygen in carbon dioxide content to 31 molar percent in order to roughly match the air-coal flame velocity. Sixteen oxidant lance nozzles as noted in  FIG. 4  with gas velocities about three times the transport gas-coal and mixed primary oxidant-coal velocities were provided with an elevation angle of about 11 degrees and yaw angle of 32 degrees relative to the transport gas-pulverized coal stream to ensure rapid mixing with the motive gas-coal. The primary oxidant then, with the optimum concentration for flame stability is fed to the quad. 
         [0034]    In the above embodiment of the invention a primary oxidant was thus provided by the enrichment of the coal transport gas with a lanced oxidant to a higher level of oxygen within the discharge end of the burner and, as such, delivering a ready mixed oxygen concentration suitable for a stable combustion into the burner quarl. 
         [0035]    In a further embodiment of this invention the lance nozzles  75  are located so as to inject an oxidant directly into the burner combustion space  93  within the burner quarl  60 , as shown in  FIG. 6 . For purposes of representation, the numbering scheme in  FIG. 6  is also employed in  FIG. 7  and  FIG. 8 . By introducing the lance oxidant  75 A external to the primary oxidant passage referred to in the above embodiment, concerns about elevated oxygen concentrations within a fuel containing line are alleviated. In such an embodiment as concerns over high oxidant levels are reduced industrially pure oxygen may be delivered through the lance  70  as opposed to a mixed or diluted gas. This has the advantage of reducing the size of the equipment and avoiding a gas mixing device and control thereof. Such nozzles  75  may be located on the closed end of the lance  70  or in the sides of a protruding lance or in a combination of both, as shown by example in  FIGS. 7 and 8 . The nozzles  75  are located and angled outwards such that the lance oxidant  75 A has a trajectory  90  to intercept with the expanding fuel laden transport oxidant  80  within the burner quarl  60 . By positioning the intercept to be within the burner quad  60  proximal the desired location of the flame root  92 , the local combination of the lance oxidant  75 A and transport oxidant  80  creates a primary oxidant  94  with conditions conducive for flame stabilization. 
         [0036]    The flow patterns of the expanding oxidant streams ( 75 A,  80  and  85 ) and the region for stabilization of the flame root  91  may be determined by CFD modeling or by visual observation. A further method for determining the angle of the lance oxidant nozzles  75  is for them to be angled in a divergent manner towards the outer lip or edge  65  of the burner quarl  60 . The nozzles  75  may further be angled to induce a swirling motion complimentary to the swirling motion in combustion space  93  of any of the transport and secondary streams  80  and  85 . In order to direct the lance oxidant jets  75 A into and mix with the fuel laden transport oxidant stream it is advantageous to operate the lance oxidant nozzles  75  at a greater velocity than the transport oxidant stream  80 . By orienting the lance oxidant nozzles  75  in such a divergent manner and by operation at velocities greater than that of the transport oxidant stream the fluid recirculation patterns  100  important for a stable combustion process in such a burner are reinforced. 
         [0037]    As exemplified in Tables 2a and 2b a central lance delivering relatively modest quantities of pure oxygen can deliver significant increases in the oxygen concentration when mixed with the transport oxidant. In these cases the secondary oxidant oxygen content has been maintained at a constant level of 26 mol. % which has resulted in a variation in the overall oxygen content, however the secondary oxidant oxygen content can be raised or lowered slightly to maintain an overall or global desired oxygen concentration. In these cases the lance oxidant nozzles are operated at a velocity of 45 m/s or approximately 3.5 times the velocity of the transport oxidant 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2a 
               
             
             
               
                   
               
               
                 Effect of Lance Oxidant flow on Combined Transport and Lance 
               
               
                 Stream (Primary) Composition and Global Oxygen Enrichment. 
               
             
          
           
               
                   
                 Transport 
                 Lance 
                 Primary 
                 Secondary 
                 Global 
               
               
                   
               
             
          
           
               
                 Case 1 
                   
                   
                   
                   
                   
               
               
                 O2 Concentration 
                 21 
                 100 
                 26.9 
                 26 
                 26.1 
               
               
                 (mol. %) 
               
               
                 Volumetric Flow 
                 25 
                 2.0 
                 27.0 
                 130.0 
                 157.0 
               
               
                 (l/s) 
               
               
                 Contained Oxygen 
                 5.25 
                 2.0 
                 7.3 
                 33.8 
                 41.1 
               
               
                 Flow (l/s) 
               
               
                 Case 2 
               
               
                 O2 Concentration 
                 21 
                 100 
                 31.9 
                 26 
                 27.1 
               
               
                 (mol. %) 
               
               
                 Volumetric Flow 
                 25 
                 4.0 
                 29.0 
                 128.0 
                 157.0 
               
               
                 (l/s) 
               
               
                 Contained Oxygen 
                 5.25 
                 4.0 
                 9.3 
                 33.3 
                 42.5 
               
               
                 Flow (l/s) 
               
               
                 Case 3 
               
               
                 O2 Concentration 
                 21 
                 100 
                 40.2 
                 26 
                 29.0 
               
               
                 (mol. %) 
               
               
                 Volumetric Flow 
                 25 
                 8.0 
                 33.0 
                 124.0 
                 157.0 
               
               
                 (l/s) 
               
               
                 Contained Oxygen 
                 5.25 
                 8.0 
                 13.3 
                 32.2 
                 45.5 
               
               
                 Flow (l/s) 
               
               
                   
               
               
                 Secondary Stream composition kept at constant oxygen content of 26%, overall flowrate maintained at 157 l/s and nozzle velocity of 45 m/s. 
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2b 
               
             
             
               
                   
               
               
                 Nozzle Configuration for Cases in Table 2a 
               
             
          
           
               
                   
                 Case 1 
                 Case 2 
                 Case 3 
               
               
                   
                   
               
             
          
           
               
                 Nozzle Diameter (mm) 
                 1.54 
                 2.17 
                 3.07 
               
               
                 Primary Oxidant Concentration (mol. %] 
                 26.9 
                 31.9 
                 40.2 
               
               
                 Global Oxidant Concentration (mol. %) 
                 26.1 
                 27.1 
                 29.0 
               
               
                   
               
               
                 Each case has 24 nozzles to distribute oxygen. 
               
             
          
         
       
     
         [0038]    While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims in this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the invention.