Abstract:
A biogas flare system for burning biogas generated primarily by a landfill includes at least one burner for igniting a mixture of biogas and air. A main supply line supplies a mixture of biogas and air to the burner. A biogas supply line feeds biogas into the main supply line. An air supply line feeds air into the main supply line. A mixer structure mixes the biogas and air prior to the mixture being supplied to the burner.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     This invention relates to a system for flaring biogas generated by landfill sites or waste water facilities, and, more particularly, to a system that decreases harmful combustion products. 
     In landfills and waste water treatment, oftentimes it is necessary to dispose of waste gases, such as methane, generated by the disposal and decay of biological products. Flaring systems are used to burn off or combust such biogases to prevent environmental, explosion, and worker safety hazards. Various flare units are utilized to combust the biogas. Assignee of this application manufactured a unit having a stack with a plurality of burners located therein. The burners are fed via a supply line containing biogas. The biogas is fed directly to the burners without any premixture of air. The tip of each of the burners is disposed in an aperture formed in a false bottom within a stack. The false bottom is insulated with refractory or other suitable heat-resistant material to ensure that excess heat generated by flames extending from the burner tip is not transferred to the burner manifold located below the false bottom within the stack. An annular gap exists between the burner tip and the aperture formed in the false bottom. Air from a chamber below the false bottom flows upwardly through these annular gaps and is utilized to accomplish the combustion of the biogas exiting the burner tip, and further to potentially quench the temperature in the stack if necessary to reduce and control the heat generated within the stack. The air is drawn into the chamber below the false bottom via dampers positioned in the outer wall of the stack. The dampers can be actuated to control the combustion and quench air that flows to the flame via the annular apertures in the false bottom. 
     This biogas flaring system suffers from various disadvantages. First, it is difficult to finely adjust the amount of combustion air utilized in the process by utilizing the air delivery structures of the prior art system. More specifically, a correct premixture of air and fuel, prior to combustion, can reduce the emissions of various harmful gases, such as nitric/nitrous oxide (NOx) and carbon monoxide (CO). The prior air supply structures do not allow a proper premixing of air with fuel prior to combustion. Further, if the biogas must seek combustion air within the stack, flames will often extend upwardly from the burner tip to substantial heights, thus requiring a substantial height of the stack to conceal the flames. 
     In prior systems, each flame generated by a burner tip is generally unrestricted after it exits the burner tip, and oftentimes flows in a nonturbulent manner. This type of flame structure can result in an unstable flare system which can generate a significant amount of combustion instability noise. Added to the noise generated by combustion instability is the noise of the quench air flowing through the blades of the dampers located in the stack wall of the prior art system. 
     Therefore, a flaring system is needed which alleviates the problems of the prior art discussed above. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a flaring system that reduces the emission of nitric oxide. 
     It is a further object of the present invention to provide a flaring system which reduces the emission of carbon monoxide even at lower combustion temperatures. 
     A still further object of the present invention is to provide a flaring system that decreases the flame length to decrease the size of stack required. 
     Another object of the present invention is to provide a flaring system that reduces noise resulting from combustion and noise resulting from air flowing across the damper blades and into the stack. 
     Yet another object of the present invention is to provide a flaring system that increases flame temperature resulting in an increase in destruction efficiency in unburned hydrocarbons. 
     Accordingly, the present invention provides for at least one burner for igniting a mixture of biogas and air. A main supply line supplies the mixture to the burner. A biogas supply line feeds into the main supply line. An air supply line also feeds into the main supply line. A mixer structure is utilized to ensure that the biogas and air are mixed prior to being supplied to the burner. 
     The invention also provides for a flame stability device for use in conjunction with the burner. The device includes an enclosure generally surrounding and extending upwardly from a burner tip. The enclosure has an inner surface that is exposed to a flame generated from the burner tip. A stability surface extends generally from the inner surface to the burner tip. The stability surface surrounds the burner tip and creates a turbulent zone also surrounding the burner tip. The flame generated by the burner tip reattaches to the inner surface above the stability surface. 
     The invention further provides for an ignition arrangement for a plurality of burners. The arrangement includes at least one enclosure surrounding one of the burners and extending upwardly from the burner tip. A pilot is used to ignite the enclosed burner. An ignition port extends from the enclosed burner to at least one adjacent burner such that when the pilot lights the enclosed burner, combustion gases from the enclosed burner travel through the ignition port to ignite the adjacent burner. 
     Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings which form a part of this specification and are to be read in conjunction therewith and in which like reference numerals are used to indicate like parts in the various views: 
     FIG. 1 is a side elevational view of a biogas flare system embodying the principles of this invention, parts being broken away and shown in cross section to reveal details of construction; 
     FIG. 2 is a cross-sectional view taken generally along line  2 — 2  of FIG.  1  and showing the arrangement of a plurality of burners utilized in the flaring system of the present invention; 
     FIG. 3 is an enlarged view of a portion of the central area in FIG. 2, and showing the ignition ports extending from a main burner to adjacent burners; 
     FIG. 4 is a cross-sectional view taken generally along line  4 — 4  of FIG.  3  and showing a flame stability device associated with a burner; and 
     FIG. 5 is a top perspective view of two flame stability devices according to the present invention shown installed on two adjacent burners; 
     FIG. 6 is a graph depicting experimental results at a biogas (or fuel) flow rate of 1,500 standard cubic feet per minute (scfm) for a particular gas makeup; 
     FIG. 7 is a graph depicting experimental results at a flow rate of 500 scfm for the same gas as in FIG. 6; 
     FIG. 8 is a graph depicting experimental results at a flow rate of 500 scfm for a different gas makeup; 
     FIG. 9 is a graph depicting experimental results at a flow rate of 1,000 scfm for a still further gas makeup; and 
     FIG. 10 is a graph depicting experimental results at a flow rate of 500 scfm for the same gas as in FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings in greater detail, and initially to FIGS. 1-3, a biogas flaring system designated by the reference numeral  10  as shown. System  10  includes a biogas supply line  12  and an air supply line  14 , which feed into a main supply line  16 . Biogas in supply line  12  is introduced into the line from the landfill or waste water site where it has been collected utilizing methods and structures well known in the art. Air is introduced into supply line  14  via use of a variable speed fan  18  shown diagrammatically in FIG.  1 . After air and biogas are introduced into main supply line  16 , they are forced through a static mixer  20  disposed in line  16 . Mixer  20  typically is of a corrugated plate variety and ensures adequate interaction between the biogas and air. One type of static mixer that has been found suitable is a mixer identified by the model number SMF-LF, manufactured by Koch Engineering Company, Inc., of Wichita, Kans. 
     The amount of air and biogas entering main supply line  16  from supply lines  12  and  14  is controlled by a controller  22 . More specifically, controller  22  can actuate and control variable speed fan  18  and also possibly a variable speed fan (not shown) or valve coupled to line  12  in a manner well-known in the art. Controller  22  can be utilized to adjust the ratio of biogas to air, as will be more fully described below. One suitable type of controller for adjusting the biogas/air ratio is identified by the model number TSX 3721001, manufactured by Modicon of Palatine, Ill. 
     After gas exits mixer  20 , it flows to a burner manifold  24  disposed in a generally cylindrical shell or stack  26 . Stack  26  has an open top where combustion gases generated in the stack are emitted into the environment. Located adjacent the lower end of stack  26  is a plurality of motorized dampers  28 . Dampers  28  are of a construction well-known in the art and are utilized to supply quench air to stack  26 , as will be more fully described below. Additionally, dampers  28  can also be electrically controlled by controller  22 . A suitable construction for dampers  28  can include a plurality of mutually actuated blades, or further, a single blade-type actuation mechanism. 
     Extending upwardly from burner manifold  24  is either one or a plurality of burners  30  and  32 . More specifically, the burners are arranged in a pattern such that there is a central burner  30  and secondary burners  32  disposed and generally surrounding central burner  30 , as best shown in FIGS. 2,  3 , and  5 . The mixture of air and biogas supplied to manifold  24  is equally divided and supplied to burners  30  and  32 . 
     With reference to FIG. 4, each burner includes a burner tip  34  to which the biogas/air mixture is supplied and from which a flame extends upwardly. Associated with each burner tip is a generally cylindrical flame stability device or tile  36 . Stability devices  36  generally surround burner tips  34  and extend upwardly therefrom. Each device  36  has a generally annular primary stability surface  38 , an intermediate generally annular ridge  40  extending inwardly from an inner surface  42  of device  36 , and a top generally annular lip  44  extending inwardly from inner surface  42 . Ridge or ring  40  forms a generally annular primary retention surface  46  on its lower end, and a generally annular secondary stability surface  48  on its upper end. Additionally, lip  44  forms a generally annular secondary retention surface  50  adjacent its lower surface. 
     Primary stability surface  38  and primary retention surface  46  cooperate with inner surface  42  to form a generally cylindrical primary stability zone  52 . Secondary stability surface  48  and secondary retention surface  50  cooperate with inner surface  42  to form a secondary stability zone  54 . The purpose of annular surfaces  38 ,  46 ,  48 , and  50  and zones  52  and  54  will be more fully described below. Stability devices  36  can be made of any suitable heat-resistant material, for instance, a ceramic refractory, or high grade stainless steel. One such suitable material is identified by the trademark THERMBOND®, available from John Zink Company (a division of Koch-Glitsch, Inc.), of Tulsa, Okla. 
     With reference to FIGS. 2 through 5, central burner  30  has a plurality of ignition ports  56  extending from its stability device  36  to the stability devices  36  of secondary burners  32 . Ignition ports  56  are in the form of tubes, which can be made of the same material as devices  36 . Each tube  56  defines an inner bore  60  which serves to spatially connect central burner  30  with each of secondary burners  32 . Ports  56  are utilized to light secondary burners  32  after central burner  30  has been lit. More specifically, combustion gases in central burner  30  flow through bore  60  to ignite the adjacent burners, as will be more fully described below. Central burner  30  is lit utilizing a pilot assembly  62  which can be actuated externally of shell  26 . Again, controller  22  can be utilized to automatically actuate pilot assembly  62 , in a manner as is well-known in the art. 
     In operation, the premixing of the biogas with air in mixer  20  provides a significant advantage over prior art flare systems. More specifically, it has been found that the premixing of biogas and air prior to ignition in a burner can significantly reduce the nitric oxide and carbon monoxide emissions. More specifically, experimental data has shown that a primary air/fuel mixture can reduce nitric oxide by a factor of five to ten when compared with a conventional raw gas landfill flare. Additionally, typically carbon monoxide emissions dramatically increase as the temperature inside a conventional biogas flare decreases below approximately 1500° F. Premixing can allow the carbon monoxide emissions to remain very low, even if the temperatures in the stack decrease below 1500° F. The proper ratio of biogas to air is governed by controller  22  and is dependent upon the makeup of the biogas being flared. FIGS. 6-10 reflect experimental emissions data of the invention for various flow rates of various biogas/air mixtures for various compositions of gas compared to a standard prior art nonpremix burner. In the figures: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 NOx = 
                 nitric oxide 
               
               
                 CO = 
                 carbon monoxide 
               
               
                 EA = 
                 excess air 
               
               
                 TNG = 
                 Tulsa Natural Gas (93.4%-CH 4 ; 2.7%-C 2 H 6 ; 0.6%-C 3 H 8 ; 0.2%- 
               
               
                   
                 C 4 H 10 ; 2.4%-N 2 ; 0.7%-CO 2 ) 
               
               
                 CO 2 =   
                 carbon dioxide 
               
               
                 Std. = 
                 prior nonpremix burner 
               
               
                 burner 
               
               
                   
               
             
          
         
       
     
     Generally, it is advantageous to have a ratio of biogas to air that has approximately 20% or greater excess air; further, a range of 20% to 50% excess air is preferable. Controller  22  is utilized in a manner well-known in the art to accomplish these ratios. It has also been found that premixing of air with biogas prior to combustion substantially reduces the soot formation in the flame resulting in a flame with a lower radiant fraction. 
     The premixing has been found to decrease the flame height within the stack by approximately thirty to fifty percent (30%-50%) as compared with conventional biogas flare systems. 
     Stability devices or tiles  36  are utilized to aid ignition of the system and provide flame stability. Devices  36  also reduce noise by blocking or shielding the combustion noise. More specifically, with reference to FIG. 4, stability zones  52  and  54  create generally annular turbulent areas  66  at locations surrounding burner flame  68 . These turbulent areas  66  increase the turbulent burning velocity, thus increasing the stability of the flame. In order to maximize the turbulence and hence flame stability within areas  66 , it has been found advantageous to have the width w p  and w s  of primary and secondary stability surfaces  38  and  48  designed such that the reattachment of the flame occurs near locations  70  and  72  which are below the locations of primary and secondary retention surfaces  46  and  50 , respectively, as best shown in FIG.  4 . It has been found advantageous to have the height h p  of primary stability zone approximately seven to ten times the width w p  of primary stability surface  38 . Further, it has been found advantageous to have the height h s  of secondary stability zone  54  seven to ten times the width w s  of secondary stability surface  48 . The ratios of these dimensions tend to allow the reattachment of the gas prior to the primary and secondary retention surfaces  46  and  50 . Preferably, a positive pressure is maintained in the primary stability zone  52 . The positive pressure in primary stability zone  52  operates to force combustion gases through ignition ports  56  to light secondary burners  32 . More specifically, once central burner  30  is lit utilizing pilot assembly  62 , the positive pressure within primary stability zone  52  forces hot combustion gases from central burner  30  through ignition ports  56  to ignite biogas/air mixtures flowing through secondary burners  32 . In this manner, each of secondary burners  32  can be easily lit simply by lighting central burner  30 . 
     In addition to devices  36  reducing combustion noise via shielding within stack  26 , the premixing of air and biogas also reduces the amount of air that must flow through dampers  28  so as to reduce the noise generated at dampers  28 . More specifically, because the air is premixed with the fuel, there is no necessity for combustion air to flow though dampers  28 , and only quench air flows through dampers  28 . Dampers  28  can also be used and controlled by controller  22  in response to temperature sensed via thermocouple  64 . The purpose of controlling the temperature inside the unit is to help reduce emissions and control potentially harmful structural temperatures and flame height. 
     From the foregoing, it will be seen that this invention is one well-adapted to attain all the ends and objects hereinabove set forth together with other advantages which are obvious and which are inherent to the structure. It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims since many possible embodiments may be made of the invention without departing from the scope thereof. It is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.