METHODS AND SYSTEMS FOR MINIMIZING NOx AND CO EMISSIONS IN NATURAL DRAFT HEATERS

Abstract

Systems and methods for reducing NO x and CO emissions in a natural draft heater are disclosed. For example, the disclosure provides embodiments of systems and methods for controlling a draft value within a heater shell to deliver an amount of excess air to a burner to thereby maintain at least one of NOx emissions not exceeding 0.025 lb/MMBtu (HHV) and CO emissions not exceeding 0.01 lb/MMBtu (HHV) in a natural draft heater.

That is claimed is: 
     
         1 . A natural draft heater for a transportation fuel production system, the system comprising:
 a heater shell having a base and positioned to circulate a flue gas internally therein via negative pressure during operation, the flue gas generated by combustion of a fuel within the heater shell;   one or more heating coils positioned within the heater shell, the one or more heating coils containing a process fluid and being arranged to transfer heat from the circulated flue gas to thereby heat the process fluid;   a draft sensor positioned within the heater shell to measure a negative pressure of the flue gas within the heater shell during operation of the natural draft heater;   a stack attached to the heater shell to vent at least a portion of the circulated flue gas to atmosphere, the stack including an outer shell and a split-range stack damper positioned within the outer shell to maintain a negative pressure of the flue gas when vented from the natural draft heater, the split-range stack damper including a set of inner blades, a set of outer blades, and one or more actuators arranged to actuate the set of inner blades and the set of outer blades, thereby to effectuate movement thereof;   at least one burner assembly connected proximate the base of and within the heater shell to combust the fuel when supplied thereto, thereby to generate the flue gas that transfers heat to the process fluid contained within the one or more heating coils, the at least one burner assembly having:
 a burner positioned within the at least one burner assembly to ignite the fuel when supplied to the burner, 
 a burner air sensor connected to the burner to measure a level of excess air, 
 an air plenum positioned adjacent the burner to distribute air into the burner, the air plenum including an air input to receive atmospheric air during operation, 
 a burner air register in fluid communication with the air input to direct the atmospheric air into the air plenum, the burner air register having a housing and one or more plates attached to the housing and positioned in fluid communication with the air plenum to direct air flow into the plenum, each of the one or more plates being configurable between one or more of an open position, a partially open position, and a closed position to selectively supply air to the air plenum, and 
 a fuel supply assembly including at least a fuel input conduit to deliver fuel to the burner; and 
   a controller in electrical communication with the burner air register, the burner air sensor, the draft sensor, and the one or more actuators in the split-range stack damper so that the controller receives an input signal representative of excess air level from the burner air sensor to control negative pressure of the flue gas when within the heater shell and to deliver an amount of excess air to the burner during operation, thereby to maintain at least one of NO x  emissions not exceeding 0.025 lb/MMBtu (HHV) and CO emissions not exceeding 0.01 lb/MMBtu (HHV) in the natural draft heater.   
     
     
         2 . The natural draft heater system of  claim 1 , wherein the controller receives an input signal representative of negative pressure of the flue gas from the draft sensor. 
     
     
         3 . The natural draft heater system of  claim 2 , wherein the controller provides an output signal to the one or more actuators in the split-range stack damper to adjust the positioning of the set of inner blades and the set of outer blades of the split-range stack damper and thereby maintain the negative pressure of the flue gas to within a preselected range. 
     
     
         4 . The natural draft heater system of  claim 3 , wherein the preselected range is maintained in the range of about 0.10-inches water-column to about 0.15-inches water-column. 
     
     
         5 . The natural draft heater system of  claim 3 , wherein the output signal is in the form of a current delivered to the one or more actuators, the current being in the range of about 2 mA to about 20 mA, the one or more actuators being arranged to adjust the position of at least one of the set of inner blades and the set of outer blades in response to the current received. 
     
     
         6 . The natural draft heater system of  claim 3 , wherein the controller alerts an operator to adjust configuration of the one or more plates and thereby control the excess air in the burner to a value in the range of between about 15% to about 25% by weight based on the combined weight of air and fuel needed for complete combustion. 
     
     
         7 . The natural draft heater system of  claim 1 , wherein the heater shell includes a burner section positioned proximate the base where combustion of the fuel occurs, a radiant section positioned adjacent the burner section to receive heat energy from the burner section and radiate heat energy therefrom, and a convective section positioned adjacent the radiant section to provide convection from the radiant section. 
     
     
         8 . The natural draft heater system of  claim 7 , further comprising a bridge wall connected to the heater shell and positioned between the radiant section and the convective section thereof. 
     
     
         9 . The natural draft heater system of  claim 8 , wherein the draft sensor is positioned proximate the bridge wall such that the negative pressure of the flue gas is measured proximate the location of the bridge wall. 
     
     
         10 . The natural draft heater system of  claim 1 , wherein the fuel supply system further comprises a primary manifold assembly and a second, staged manifold assembly, the fuel input conduit positioned in fluid communication with the primary manifold assembly to deliver fuel to the primary manifold assembly, the primary manifold assembly positioned in fluid communication with one or both of a burner tip of the burner and the second, staged manifold assembly to deliver fuel to one or both of the burner tip of the burner and the second, staged manifold, the second, staged manifold assembly positioned in fluid communication with another burner tip to deliver fuel to the another burner tip. 
     
     
         11 . The natural draft heater system of  claim 10 , wherein the second, staged manifold assembly includes a staged manifold valve configurable to be in a closed position to shut off fuel flow or in an at least partially open position to direct a preselected amount of fuel to the another burner tip to achieve a desired concentration of air and fuel mixture in the another burner tip. 
     
     
         12 . The natural draft heater system of  claim 1 , wherein the at least one burner assembly is a plurality of burner assemblies positioned in sequence along the base of and within the heater shell to provide substantially even heating within the heater shell, the plurality of burner assemblies are independently controlled and operated during operation of the natural draft heater, such that one or more of the plurality of burner assemblies may be taken out of service during operation of the natural draft heater while the remaining burner assemblies remain operational. 
     
     
         13 . The natural draft heater system of  claim 1 , further comprising a burner fuel tip positioned within the burner and at least partially enclosed by a riser plate positioned adjacent thereto, the riser plate having a burner riser positioned proximate a base thereof and connected thereto by one or more riser welds to allow air to enter the riser plate between riser welds during operation of the natural draft heater. 
     
     
         14 . The natural draft heater system of  claim 13 , further comprising one or more air rings positioned about the burner riser proximate the one or more riser welds to reduce air entering the riser plate. 
     
     
         15 . The natural draft heater system of  claim 14 , wherein the one or more air rings are positioned to reduce air entering the riser plate when the natural draft heater is operating at rates greater than about 40% of design capacity. 
     
     
         16 . The natural draft heater system of  claim 14 , wherein the one or more air rings are selectively installed in one of the at least one burner assembly when that one burner assembly is taken out of service during operation of the natural draft heater so as to prevent air leakage into the heater shell and thereby reduce NO x  formation in the heater shell. 
     
     
         17 . The natural draft heater system of  claim 13 , further comprising a silicone seal applied at the connection between the burner risers and the riser plates to prevent air infiltration into the burner thereby reducing NO x  formation in the heater shell. 
     
     
         18 . The natural draft heater system of  claim 1 , further comprising a flame scanner positioned adjacent the air plenum to detect the presence of a flame in the burner. 
     
     
         19 . The natural draft heater system of  claim 18 , further comprising a purge air injection input positioned adjacent the flame scanner to deliver purge air directly to the burner, the purge air being used as combustion air by the burner thereby reducing NO x  formation within the heater shell, and wherein the burner air sensor is positioned in a main duct of the burner assembly. 
     
     
         20 . The natural draft heater system of  claim 19 , wherein the flame scanner and the purge air injection input are included on alternating burner assemblies when the at least one burner assembly includes two or more burner assemblies. 
     
     
         21 . The natural draft heater system of  claim 19 , wherein the flame scanner and the purge air injection input are absent on at least one burner assembly when the at least one burner assembly includes two or more burner assemblies. 
     
     
         22 . The natural draft heater system of  claim 1 , wherein the burner air sensor is located directly adjacent the burner, and wherein the burner air sensor is configured to measure the level of excess air along a length of the burner assembly. 
     
     
         23 . A natural draft heater system comprising:
 a heater shell having a base and positioned to circulate a flue gas internally therein via negative pressure during operation, the flue gas being generated by combustion of a fuel within the heater shell, the heater shell including a burner section positioned proximate the base in a location where combustion of the fuel occurs, a radiant section positioned adjacent the burner section to receive heat energy from the burner section and radiate heat energy therefrom, a convection section positioned adjacent the radiant section to provide convention from the radiant section, and a bridge wall connected to the heater shell and positioned between the radiant section and the convection section thereof;   one or more heating coils positioned within the heater shell, the one or more heating coils including a process fluid and arranged to transfer heat from the circulated flue gas, thereby to heat the process fluid;   a draft sensor positioned within the heater shell proximate the bridge wall to measure a negative pressure of the flue gas within the heater shell and proximate the location of the bridge wall during operation of the natural draft heater;   a stack attached to the heater shell to vent at least a portion of the circulated flue gas to atmosphere during operation, the stack including an outer shell and a split-range stack damper positioned within the outer shell to maintain a negative pressure of the flue gas being vented from the natural draft heater, the split-range stack damper including a set of inner blades, a set of outer blades, and one or more actuators arranged to actuate the set of inner blades and the set of outer blades to thereby effectuate movement thereof;   at least one burner assembly connected proximate the base of and within the heater shell to combust the fuel when supplied thereto, thereby to generate the flue gas that transfers heat to the process fluid contained within the one or more heating coils, the at least one burner assembly having:
 a burner positioned within the at least one burner assembly to ignite the fuel when being supplied to the burner, 
 a burner air sensor connected to the burner to measure a level of excess air, 
 an air plenum positioned adjacent the burner to distribute air into the burner during operation, the air plenum including an air input to receive atmospheric air, 
 a burner air register in fluid communication with the air input to direct the atmospheric air into the air plenum, the burner air register having a housing and one or more plates attached to the housing and positioned in fluid communication with the air plenum to direct air flow into the plenum, each of the one or more plates being configurable between one or more of an open position, a partially open position, and a closed position to selectively supply air to the air plenum, and 
 a fuel supply system including at least a fuel input conduit to deliver fuel to the burner; and 
   a controller in electrical communication with the burner air register, the burner air sensor, the draft sensor, and the one or more actuators in the split-range stack damper, the controller to:
 receive (a) a first input signal representative of negative pressure of the flue gas from the draft sensor, (b) a second input signal representative of the level of excess air at the burner, and (c) a third input signal representative of a current setting of the burner air register, and 
 adjust one or more of the split-range stack damper or the current setting of the burner air register, based on the first input signal, the second input signal, and the third input signal, to control the negative pressure of the flue gas within the heater shell to deliver an amount of excess air to the burner to thereby maintain at least one of NO x  emissions not exceeding 0.025 lb/MMBtu (HHV) and CO emissions not exceeding 0.01 lb/MMBtu (HHV) in the natural draft heater. 
   
     
     
         24 . The natural draft heater system of  claim 1 , wherein control of the negative pressure of the flue gas within the heater shell by the controller includes:
 receiving (a) a first input signal representative of negative pressure of the flue gas from the draft sensor, (b) a second input signal representative of the level of excess air at the burner, and (c) a third input signal representative of a current setting of the burner air register; and   adjusting one or more of the split-range stack damper or the current setting of the burner air register, based on the first input signal, the second input signal, and the third input signal to control the negative pressure of the flue gas within the heater shell.   
     
     
         25 . The natural draft heater system of  claim 8 , wherein one or more of the heating coils are positioned in one or more of the radiant section or the convective section. 
     
     
         26 . The natural draft heater system of  claim 10 , further comprising one or more additional sensors positioned proximate the bridge wall and configured to measure one or more of an amount of excess oxygen, a concentration of combustible gas, flue gas temperature, or draft pressure. 
     
     
         27 . The natural draft heater system of  claim 26 , wherein the controller is in electrical communication with the one or more additional sensors and further controls the amount of excess air via one or more of the burner air register or split-range stack damper based on measurements received from the one or more additional sensors. 
     
     
         28 . The natural draft heater system of  claim 1 , wherein the at least one burner assembly includes one or more pilot burners configured to provide a continuous flame during the operation of the natural draft heater such that the burner remains firing.