Open tip burner assembly for waste gas flares and methods of using the same

Abstract

A flare system operable at ultra-low nitrous oxide (NOx) emission levels without utilizing a surface burner. The flare system includes a flare stack and a burner assembly positioned in the flare stack between a closed plenum and an open exhaust chamber, the burner assembly and the flare stack defining an excess oxidant passage for routing excess oxidant between the plenum and the exhaust chamber. The burner assembly includes: a body defining a central passage through which oxidant and waste gas flow, the central passage being separated from the excess oxidant passage, a burner tip defining an open tip passage through which the oxidant and the waste gas flow from the central passage into the exhaust chamber, and a swirl plate positioned upstream of the burner tip. The swirl plate operates to direct and anchor a combustion flame ignited in the exhaust chamber.

What is claimed is: 
     
         1 . A flare system for burning off flammable gas present in a waste gas stream, the flare system comprising:
 a flare stack having a first end and a second end, the flare stack defining a closed plenum at the first end and an open exhaust chamber at the second end;   a burner assembly positioned in the flare stack between the plenum and the exhaust chamber, the burner assembly and the flare stack defining an excess oxidant passage for routing excess oxidant between the plenum and the exhaust chamber, wherein the burner assembly includes:
 a body having a first body end proximate the plenum and a second body end proximate the exhaust chamber, each body end being open, the body defining a central passage extending between the body ends through which oxidant and waste gas flow, the central passage being separated from the excess oxidant passage; 
 a burner tip connected to the second body end and defining an open tip passage through which the oxidant and the waste gas flow from the central passage into the exhaust chamber; and 
 a swirl plate positioned upstream of the burner tip; and 
   an ignition source positioned in the exhaust chamber to ignite the oxidant and the waste gas flowing through the open tip passage and create a combustion flame, wherein the swirl plate operates to direct and anchor the combustion flame.   
     
     
         2 . The flare system of  claim 1 , wherein the swirl plate includes a center retention ring, an outer ring, and swirl vanes extending radially between the center retention ring and the outer ring and set at an angle relative thereto. 
     
     
         3 . The flare system of  claim 2 , wherein each swirl vane is set at an angle between 15° to 60°, such as between 15° to 45°, relative to the center retention ring and the outer ring. 
     
     
         4 . The flare system of  claim 2 , wherein each swirl vane is welded to each of the outer ring and the center retention ring. 
     
     
         5 . The flare system of  claim 2 , wherein the swirl plate is a single monolithic plate formed by removing material to free the swirl vanes and distorting the swirl vanes into place. 
     
     
         6 . The flare system of  claim 1 , wherein the body of the burner assembly includes a mixer skirt defining the first body end and a tapered extension positioned between the mixer skirt and the burner tip. 
     
     
         7 . The flare system of  claim 6 , wherein the mixer skirt includes a static mixer positioned in the central passage and operable to mix the oxidant and the waste gas proximate the first body end. 
     
     
         8 . The flare system of  claim 7 , wherein the static mixer is grated and includes a first row of mixer vanes extending across the central passage in a first direction and a second row of mixer vanes extending across the central passage in a second direction other than the first direction. 
     
     
         9 . The flare system of  claim 8 , wherein the first direction is perpendicular to the second direction. 
     
     
         10 . The flare system of  claim 8 , wherein the first row of mixer vanes is located upstream from the second row of mixer vanes. 
     
     
         11 . The flare system of  claim 10 , wherein the mixer vanes of the first and second rows are each set at an angle relative to a central axis of the body. 
     
     
         12 . The flare system of  claim 11 , wherein the mixer vanes are each set at the angle of between 30° to 60°, such as between 40° to 50°, or 45°. 
     
     
         13 . The flare system of  claim 1 , wherein the burner assembly includes one or more waste gas rings positioned in the central passage, each waste gas ring defining a waste gas passage through which the waste gas flows in the central passage such that the waste gas and the oxidant remain separated in the central passage. 
     
     
         14 . The flare system of  claim 13 , wherein each waste gas ring includes outlets positioned proximate the second body end for injecting the waste gas into the flow of oxidant proximate the open tip passage. 
     
     
         15 . A method of operating a flare system for burning off flammable gas present in a waste gas stream, the method comprising:
 channeling a waste gas into a flare stack towards a burner assembly positioned in the flare stack;   channeling an oxidant into the flare stack towards the burner assembly;   mixing the waste gas and the oxidant using the burner assembly;   flowing the waste gas-oxidant mixture through an open tip passage of the burner assembly into an exhaust chamber of the flare stack;   igniting the mixture to create a combustion flame that produces exhaust gases that flow through the exhaust chamber and out of the flare stack;   channeling excess oxidant to the exhaust chamber to cool the combustion flame; and   controlling the combustion flame using a swirl plate positioned proximate the open tip passage, wherein the swirl plate directs and anchors the combustion flame.   
     
     
         16 . The method of  claim 15 , wherein an oxygen stack concentration in the flare stack during combustion is 10% v/v or greater, such as between 10% v/v to 15% v/v. 
     
     
         17 . The method of  claim 15 , wherein the oxidant is channeled into the flare stack at a greater flow rate than the flow rate at which the waste gas is channeled into the flare stack. 
     
     
         18 . The method of  claim 17 , further comprising controlling the oxidant/waste gas ratio in the flare stack using a controller that receives feedback signals from one or more sensors positioned in the flare stack. 
     
     
         19 . The method of  claim 15 , wherein mixing the waste gas and the oxidant using the burner assembly includes one of pre-mixing the waste gas and the oxidant upstream from the open tip passage and mixing the waste gas and the oxidant immediately prior to the open tip passage. 
     
     
         20 . A method of operating an ultra-low nitrous oxide (NO x ) emission flare system for burning off flammable gas present in a waste gas stream, the method comprising:
 channeling the waste gas stream including the flammable gas into a flare stack towards a central passage of a burner assembly positioned in the flare stack;   channeling an oxidant into the flare stack towards the central passage of the burner assembly, wherein the oxidant is channeled at a flow rate that is at least 12 times greater than a flow rate of the flammable gas;   mixing the waste gas and a first portion of the oxidant within the central passage of the burner assembly;   flowing the waste gas-oxidant mixture through an open tip passage of the burner assembly into an exhaust chamber of the flare stack, the open tip passage axially aligned with the central passage;   igniting the mixture to create a combustion flame that produces exhaust gases that flow through the exhaust chamber and out of the flare stack; and   channeling a second portion of oxidant to the exhaust chamber to cool the combustion flame and reduce an amount of NO x  in the exhaust gases to below 0.025 pounds per million BTU.