Patent Publication Number: US-2021180789-A1

Title: Air shield for combustor firetube stack

Description:
PRIORITY STATEMENT &amp; CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from co-pending U.S. Patent Application No. 62/949,293, entitled “Air Shield for Combustor Firetube Stack” and filed on Dec. 17, 2019, in the name of Chris Aldrich; which is hereby incorporated by reference, in entirety, for all purposes. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates, in general, to and, in particular, to the field of shielding combustor stack exhaust plumes from fugitive gases, in particular, air shield barriers for shielding combustor stack exhaust plumes from fugitive gases. 
     BACKGROUND OF THE INVENTION 
     Many tanks for storing liquids, particularly in the oil and gas industry, are associated with combustible vapors, sometimes referred to as “fugitive gases”. Examples of fugitive gases include methane, ethane, propane in addition to other hydrocarbon gases as well known to those skilled in the art. Such tanks include, but are not limited to, oil production tanks, produced water and condensate tanks and the like. Hydrocarbon vapors collect in the head space of the tank. The tanks might be vented to atmosphere, but more preferably, for environmental and possibly regulatory reasons, the vapors need to be eliminated. If the vapors are vented to atmosphere, and not eliminated through a combustion process, then this can also add to the greenhouse gas problem which is associated with climate change. 
     Such tanks are usually operated at low pressures, such as just over atmospheric, the pressure therein numbering in mere inches of water column. Optional pumps or vapor recovery units can be used to raise scavenged vapors to higher pressure, for delivery to be sent into sales gas lines for custody transfer or used on heating appliances as the burner fuel gas. Combustion of low vapor gases is typically a continuous process, in burners configured for low pressure fuel and air. 
     Natural draft burners are used in a variety of process apparatus, line heaters, reboilers, heat treaters, free water knockout drums, storage tanks and the like, commonly used in the oil and gas industry. Such equipment is often located on oil and gas sites, or other industrial sites having the possibility of fugitive hydrocarbons present, and at which ignition sources are carefully controlled. These burners can be further used in a combustor to burn off waste gases located at an oil or gas production or storage facility. One or more natural draft burners can be supported in a firetube of a combustor that can extend upwards wherein the air heated as a result of the combustion of waste gases can rise and exit into the atmosphere. 
     One problem that exists is that some fugitive emissions on an oil and gas well site can accumulate in the vicinity of a combustor and blow over and into an exhaust plume exiting from a combustor firetube and auto-ignite as a result of the temperature of the exhaust plume being higher than the auto-ignition temperature of the fugitive gas. The temperature of the exhaust plume can be in excess of 800 degrees Celsius, which is more than sufficient to auto-ignite most, if not all, fugitive gases. 
     Some well sites can have small footprints thus placing a combustor in close proximity to oil and gas production equipment or storage equipment and increasing the risk of ignition of fugitive gases. Without having a combustor on a well site to burn off the low pressure and/or low volume gases, then these gases would then be vented to the atmosphere and contribute to the accumulation of greenhouse gases in the atmosphere. 
     Another reason for having a combustor in close proximity to oil and gas production equipment or storage equipment is to minimize line loss in the pipes feeding waste gas to the combustor. This allows for the use of 3 inch or 4 inch pipe to feed the waste gas to the combustor. If the combustor is placed further away from the equipment, then the low pressure and/or low volume of the gas would require larger diameter pipe, such as 6 inch or more, to compensate for the line loss associated with longer pipe runs to feed the waste gas to the combustor. This raises other problems such as condensation forming inside the pipe, which could freeze in sub-zero degree Celsius temperatures that, in turn, would require heat tracing to be placed on the pipe to prevent the freezing. 
     It is, therefore, desirable to provide a combustor that can be placed in close proximity to oil and gas production and storage facilities and equipment for destroying waste gases that would be otherwise vented into the atmosphere and not auto-ignite fugitive gases that may be present at the site. 
     SUMMARY OF THE INVENTION 
     An air shield for shielding exhaust plumes from a combustor exhaust stack can be provided. In some embodiments, the air shield can comprise of an outer stack or jacket disposed around the exhaust stack or inner firetube of a combustor where the inner firetube extends slightly higher than the end of the outer stack, and wherein an annular chamber is formed between the inner firetube and the outer stack. In some embodiments, the annular chamber and the inner firetube are in communication with a combustor base box that can draw in air through one or more flame arrestors. In some embodiments, the combustor box can comprise a burner assembly that is configured to burn low pressure and/or low volume of fugitive gas. Some examples of suitable burner assemblies are disclosed in U.S. Pat. Nos. 9,709,266 and 10,247,411, which are incorporated by reference into this application in their entirety. 
     In some embodiments, when the burner assembly is operating, air is drawn into the combustor box through the flame arrestors to be used as combustion air for the burner assembly resulting in heated air rising in the inner firetube as a heated exhaust plume, which can reach temperatures higher than the auto-ignition temperature of any fugitive gas. Air drawn into the combustor box can also enter the annular chamber and becomes heated by the inner firetube and can then rise up the annular chamber until it exits as shielding air at the top of the outer jacket just below the top of the inner firetube. The exhaust plume exiting the inner firetube can further draw the shielding air exiting the annular chamber, which is cooler than the exhaust plume, to surround the exhaust plume as an air shield or curtain. As a result, any fugitive gases in the vicinity of the exiting exhaust plume can be prevented from being exposed to the high temperature of the exhaust plume that can cause the fugitive gas to auto-ignite. In some embodiments, the firetube can comprise one or more air inlets disposed along its length and/or around its circumference, wherein cooler shielding air can be drawn from the annular chamber into the firetube and mix with the heated air rising in the firetube, which can further cool the exhaust plume as it exits the firetube. 
     Broadly stated, in some embodiments, a method can be provided for shielding an exhaust plume exiting an outlet of a firetube of a combustor to prevent fugitive gases in the surrounding atmosphere from auto-igniting, the combustor comprising a base combustor box, the firetube comprising an inlet disposed on top of the combustor box wherein the inlet is in communication with an interior of the combustor box, a burner assembly disposed in the inlet, one or more flame arrestors disposed on the combustor box configured to allow atmospheric air to enter the combustor box for use as combustion air for the burner assembly, the method comprising: drawing in shielding air into the combustor box; conveying the shielding air from the combustor box along an exterior of the firetube up to the outlet of the firetube; and enveloping the exhaust plume with the shielding air to form an air shield therearound, whereby the air shield forms a barrier between the exhaust plume and the fugitive gases to prevent the fugitive gases from auto-igniting. 
     Broadly stated, in some embodiments, the method can further comprise drawing in a portion of the shielding air into the firetube at one or more locations disposed along the firetube. 
     Broadly stated, in some embodiments, wherein the conveying of the shielding air can comprise natural draft or convection. 
     Broadly stated, in some embodiments, an apparatus can be provided for shielding an exhaust plume exiting an outlet of a firetube of a combustor to prevent fugitive gases in the surrounding atmosphere from auto-igniting, the combustor comprising a base combustor box, a burner assembly disposed in the combustor box, one or more flame arrestors disposed on the combustor configured to allow atmospheric air to enter the combustor box for use as combustion air for the burner assembly, the apparatus comprising: an outer stack or jacket disposed around the firetube thereby forming an annular chamber therearound, the annular chamber in communication with the atmospheric air disposed in the combustor box, wherein the atmospheric air is drawn into the annular chamber to form shielding air. 
     Broadly stated, in some embodiments, an improved combustor can be provided, the combustor comprising a base combustor box, a burner assembly disposed in the combustor box, one or more flame arrestors disposed on the combustor configured to allow atmospheric air to enter the combustor box for use as combustion air for the burner assembly, the improvement comprising: an outer stack or jacket disposed around the firetube thereby forming an annular chamber therearound, the annular chamber in communication with the atmospheric air disposed in the combustor box, wherein the atmospheric air is drawn into the annular chamber to form shielding air. 
     Broadly stated, in some embodiments, a top end of the outer stack can be disposed below the outlet of the firetube. 
     Broadly stated, in some embodiments, the firetube can comprise one or more breather openings configured to allow the shielding air to be drawn into the firetube. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which: 
         FIG. 1A  is a side elevation view depicting one embodiment of a combustor with an exhaust plume air shield. 
         FIG. 1B  is a temperature legend depicting the temperatures associated with the exhaust plume of the combustor of  FIG. 1A   
         FIG. 2  is a top plan view depicting the stack of the combustor of  FIG. 1A  showing temperature measurement locations 
         FIG. 3  is a chart depicting a temperature profile of the exhaust plume of the combustor of  FIG. 1B . 
         FIG. 4A  is a top plan view depicting the combustor of  FIG. 1A . 
         FIG. 4B  is a side elevation cross section view depicting the combustor of  FIG. 4A  along section lines A-A. 
         FIG. 5A  is a front elevation view depicting the combustor base box of the combustor of  FIG. 1A . 
         FIG. 5B  is a side elevation view depicting the combustor base box of  FIG. 5A . 
         FIG. 5C  is a bottom plan view depicting the combustor base box of  FIG. 5A . 
         FIG. 5D  is a top plan view depicting the combustor base box of  FIG. 5A . 
         FIG. 5E  is a rear elevation view depicting the combustor base box of  FIG. 5A . 
         FIG. 5F  is a cross section view depicting the tubing frame of the combustor base box of  FIG. 5A . 
         FIG. 6  is a side elevation view depicting a base plate gusset of the combustor base box of  FIG. 5A . 
         FIG. 7  is a side elevation view depicting an internal gusset of the combustor base box of  FIG. 5D . 
         FIG. 8  is an elevation view depicting the assembly layout of the combustor of  FIG. 1A . 
         FIG. 9  is a top plan view depicting the outer stack of the combustor of  FIG. 8 . 
         FIG. 10  is a side elevation view depicting a lifting lug of the combustor of  FIG. 8 . 
         FIG. 11  is a side elevation view depicting a heat guard mount of the combustor of  FIG. 8 . 
         FIG. 12  is a side elevation view depicting an outer stack support gusset of the combustor of  FIG. 8 . 
         FIG. 13  is an elevation view depicting the firetube of the combustor of  FIG. 1A . 
         FIG. 14  is an elevation view depicting a breather notch of the firetube of  FIG. 13 . 
         FIG. 15  is a top plan view depicting the outer stack and firetube of the combustor of  FIG. 8 . 
         FIG. 16  is a bottom plan view depicting the outer stack and firetube of the combustor of  FIG. 15 . 
         FIG. 17  comprises side and end views depicting a support bracket. 
         FIG. 18  is a side elevation cross section view depicting the mounting of the firetube to the outer stack using the support bracket of  FIG. 17 . 
         FIG. 19  is a side elevation view depicting the top support bracket of the firetube of  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In this description, references to “one embodiment”, “an embodiment”, or “embodiments” mean that the feature or features being referred to are included in at least one embodiment of the technology. Separate references to “one embodiment”, “an embodiment”, or “embodiments” in this description do not necessarily refer to the same embodiment and are also not mutually exclusive unless so stated and/or except as will be readily apparent to those skilled in the art from the description. For example, a feature, structure, act, etc. described in one embodiment can also be included in other embodiments but is not necessarily included. Thus, the present technology can include a variety of combinations and/or integrations of the embodiments described herein. 
     Referring to the Figures, an embodiment of an improved combustor is shown. In some embodiments, combustor  10  can comprise of base box  20 , firetube  12  disposed on top of base box  20  and outer stack disposed circumferentially around firetube  12  and on top of base box  20  to form annular space  16  disposed between firetube  12  and outer stack  14 . 
     Referring to  FIGS. 5A to 5E , base box  20  can comprise a cube-like structure made of plate steel and can further comprise base box  20  disposed on base plate  36 , and further supported by gussets  40  welded therebetween on each corner thereof. Base box  20  can further comprise internal gussets  42  welded in each upper internal corner thereof, as illustrated in  FIG. 5D . In some embodiments, base box  20  can further comprise flame arrestors  22  disposed on opposing sides thereof to allow air to enter therethrough into base box  20 . In some embodiments, base box  20  can comprise access door  24  disposed on a side of base box  20  to permit ingress thereto by service personnel. Access door  24  can be attached to door frame  34 , which can be comprised of square steel tubing, with hinges and/or fasteners as well known to those skilled in the art. Base box  20  can further comprise opening disposed through top surface  21  thereof to provide communication to firetube  12  and outer stack  14  when mounted on top surface  21 . 
     Referring to  FIGS. 8 to 12 , in some embodiments, outer stack  14  can be mounted on top surface  21 , centered on opening  38 , by welding or other fastening means as well known to those skilled in the art. Outer stack  14  can be further supported on top surface  21  by gussets  46  welded therebetween in a spaced-apart configuration around the circumference of outer stack  14 . In some embodiments, outer stack  14  can have heat guard  18  mounted around the outside of outer stack  14 , as shown in  FIG. 8 , with a plurality of heat guard mounts  48 . In some embodiments, outer stack  14  can comprise a plurality of lifting lugs  44  mounted around the circumference of outer stack  14  in a spaced-apart configuration to allow combustor  10  to be picked up and moved by a crane or other suitable means. 
     Referring to  FIGS. 13 to 19 , in some embodiments, firetube  12  can be disposed within outer stack  14 , wherein firetube  12  can be attached to, and supported by, a plurality of support brackets  50  disposed at a lower end of firetube  12 , where threaded fasteners (not shown) can be used to secure firetube  12  to outer stack  14 . In some embodiments, firetube  12  can be further supported and substantially centered within outer stack  14  by a plurality of top support brackets  52  disposed around the circumference of an upper end of firetube  12  in a spaced-apart configuration. By mounting firetube  12  within outer stack  14  in this configuration, annular chamber  16  is formed, which can be in communication with the interior of base box  20 . In some embodiments, firetube  12  can extend upwards in height above the upper end of outer stack  14 . In a representative embodiment, wherein outer stack  14  can be 36 inches in diameter, nominally, and where firetube  12  can be 30 inches in diameter, nominally, firetube  12  can extend above outer stack  14  by 4 inches, approximately. 
     In some embodiments, firetube  12  can comprise a plurality of notch openings  28  disposed around the circumference thereof and at different elevations thereof to permit the influx of air from annular chamber  16  when combustor  10  is in operation. In some embodiments, notch openings  28  can be approximately rectangular in shape and be formed by cuts through firetube  12  along the sides and bottom edge of the rectangular shape and with the cut portion bent outwards along the top edge thereof. 
     Referring to  FIGS. 4A and 4B , in some embodiments, combustor  10  can comprise burner assembly  30  disposed within firetube  12  at a lower end thereof, where burner assembly  30  can be supplied by inlet waste gas to be burned supplied gas inlet  26 . When combustor  10  is in operation, burner assembly  30  can be ignited to burn waste gas entering through gas inlet  26  using air within base box  20  as combustion air, with the resulting heated air rising up firetube  12 . Air can be further drawn into base box  20  through flame arresters  22  to provide the combustion air. Air can also enter in annular chamber  16  to form shielding air  17  (as shown in  FIG. 1A ). The shielding air can rise in annular chamber  16  as a result of natural draft or convection therethrough. A portion of the shielding air can be drawn into firetube  12  through notch openings  28 , as described above. As exhaust plume  13  (as shown in  FIG. 1A ) exits the upper end of firetube  12 , shielding air  17  can be drawn up from annular chamber  16  through natural draft or convection and envelop exhaust plume  13  with a shield or curtain of cooler air, which can act as a barrier between exhaust plume  13  and any fugitive gases that may be present in the atmosphere surrounding combustor  10 . In some embodiments, combustor  10  can comprise thermocouple  32  operatively coupled to the exterior of outer stack  14 , which can be configured to measure the temperature thereof. Thermocouple  32  can be operatively coupled to a control system (not shown) wherein the control system can be configured to shut down the operation of combustor  10  should the temperature of outer stack  14  exceed a safe operating temperature as well known to those skilled in the art. 
     Referring to  FIGS. 1A, 1B, 2 and 3 , these figures illustrate the results of a test in measuring the temperature of exhaust plume at various locations radially from the center of firetube  12  and vertically from the upper end of firetube  12 . In the test, the temperature of exhaust plume  13  at the center of firetube  12  was measured to be 825 degrees Celsius right at the mouth of firetube  12 . At 36 inches higher in elevation from firetube  12 , the temperature was measured at 75 degrees Celsius. The temperature exhaust plume  13  at locations 16½ inches radially from the center of firetube  12  range from 39 degrees to 96 degrees Celsius. This illustrates that shielding air  17  can provide an air shield or curtain that can surround or envelop exhaust plume  13  where the temperature at various locations within shielding air  17  can be kept below the auto-ignition temperature of a fugitive gas. Thus, shielding air  17  can act as a barrier between exhaust plume  13  and any fugitive gas in the atmosphere surrounding combustor  10 . 
     Although a few embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications can be made to these embodiments without changing or departing from their scope, intent or functionality. The terms and expressions used in the preceding specification have been used herein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof, it being recognized that the invention is defined and limited only by the claims that follow.