Alky Heater (HM-2) NOx Reduction and Efficiency Improvement Project
Furnace Improvements (FIS) was hired to perform a turnkey NOx Reduction revamp and efficiency improvement on an Alky Unit Heater (HM-2). The targeted NOx emissions rate was 0.035 lb/MMBtu (HHV). The Alky Heater is a vertical cylindrical heater with a design heat duty of 55 MMBtu/hr.
FIS analyzed the operating data for the HM-2 heater provided by client. Following observations were recorded:
Flue gas O2 measurements were in the range of 6-10% in the firebox. Higher O2 of about 16.5% was recorded just below the convection section, due to an air leakage in the heater.
Heater draft at arch was as high as (-0.53) to (-0.74) inch WC.
The burner flames were yellow, bushy and non-uniform.
NOx levels were moderately high on an average of 87 ppm at the burner level and 100 ppm in the stack.
The convection section fin heat transfer efficiency was between 40-50%.
The heater floor was in bad shape, there was no stack damper to adjust the draft inside heater.
The heater was operating at an average efficiency of 66%.
Thus, FIS suggested an option of installing Ultra Low NOx Burners plus Convection section upgradation. FIS’ scope included ECPM (Engineering, Procurement, Construction and Management) services for installation of new convection section, transition pieces, stack and damper, floor, burners.
Modification of Existing Floor:
The existing floor was in bad shape as it was warped, and it needed to be replaced. The burners were fitted on a smaller burner circle diameter (increased burner-tube distance) to increase the reliability of the heater by minimizing the flame impingement.
Installation of Eight New Ultra Low NOx Burners (ULNB):
FIS rerated the heater using ultra-low NOx natural draft gas fired burners to reduce NOx emissions. The burners were fired vertically upwards and the NOx emissions were reduced to 0.025 lb/MMBtu. The burners were provided with individual plenums and fitted with pneumatic control dampers to control the excess air.
Installation of New Stack and Damper:
FIS replaced the existing stack and installed O2 and temperature indicators for better control of the oxygen levels and temperatures exiting the stack. The new stack was provided with ladders and platforms. The existing stack did not have damper for draft control. A new pneumatically operated stack damper was provided. The stack damper was provided to ensure that the draft at arch is maintained at the design level of (-0.1) inch WC under all operating conditions.
Installation of New Convection Module:
Existing convection section heat transfer surface was fouled decreasing the heater efficiency. The flue gas temperature decreased to 475oF from 673oF (average operating temperature) by replacing the convection section. FIS supplied and installed an upgraded convection section.
After the revamp, the efficiency of the heater increased by 20% (as compared to operating).
Flue gas temperature leaving stack was alleviated by almost 200°F (compared to operating).
Fuel savings of 1.07 M$/year were estimated.
The project payout was estimated to be less than 2 years.