Ultra-Low NOx Burner Installation for Crude Heater Unit D
A refinery in East Texas has a cabin type Crude Heater (Unit D) with a top mounted convection section, off takes and stack. The heater was designed for a duty of 98.92 MMBtu/hr: 98.21 MMBtu/hr process duty and 0.71 MMBtu/hr steam superheating duty. The crude heating was done in the convection first and then in the radiant section. The flue gases after preheating the process feed were used to superheat the steam.
The firing rate of the heater was 114.74 MMBtu/hr at 25% excess air. The firing in the heater was achieved by 10 natural draft combination burners located on the floor. Design heat release of the burners was 13.19 MMBtu/hr. The heater had flame impingement issues and the estimated flame lengths were 19-20 ft long.
Client requested Furnace Improvements (FIS) to revamp the heaters with targeted NOx emissions of 0.025 lb/ MMBtu. FIS recommended the following:
Maximum flame length of the burners should be limited to ⅓ to ½ of the heater height. This ensures that the combustion is complete in the firebox without any flame impingement on the tubes.
Burner heat release must be limited to 7-7.5 MMBtu/hr, as its flame length can be restricted to 12-13 ft.
Ultra-Low NOx burners require more fuel gas staging and leads to longer flames. Thus, it was recommended to operate the heater at 15% excess air.
Volumetric heat release of the heater was very high and higher heat concentration was observed in the center. Thus, 16 ULNB Burners were installed. By increasing the number of burners, the heat release could be spread evenly in the firebox.
The heater was successfully revamped in July 2004.