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

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 NOx emissions not exceeding 0.025 lb/MMBtu (HHV) and CO emissions not exceeding 0.01 lb/MMBtu (HHV) in the natural draft heater.