NOx Reduction by Low Pressure Steam Addition

A steam superheater furnace on the Gulf coast failed NOx/CO compliance. It has twin celled radiant boxes with a common convection section and 10 fan. The cells are horizontally fired from the side walls, centered about 13" above the inside floor, using 64 NO combination gas-oil burners. Only gas firing is practiced. A 4 foot high target radiant wall is placed 4' 4" feet in front of the burner. The lowest of the radiant tubes located adjacent to the side walls is 6' 1" above the inside floor. Previous EPA sanctioned testing indicated 0.19 Lb NOx / MMBtu HHV and 0 ppmVCO at 3.8% oxygen dry; an identical furnace tested O.11 Lb / MMBtu HHV and 613 ppmV CO @ 2.7% all measured at the stack. A maintenance and air register adjustment program achieved 0.11 to 0.13 Lb NOx / MMBtu HHV and 15-19 ppmV CO at 1.8-2.2 % oxygen dry measured at the arch. Compliance required less than 0.10 Lb NOx / MMBtu HHV. The pressure control valve on the 150 PSIG steam header to the oil guns was replaced by a globe valve and the steam-oil mixer removed from each oil gun. Operation was requested to maintain the heater steady as feasible without effecting the overall performance of the unit. The test team controlled only the air register positions and the steam header pressure. The 0.10 NOx value was achieved at steam header pressure of 6.4 PSIG using 0.32 Ib steam per Ib fuel gas and 0.08 NOx value at 13.1 PSIG using 0.57 Ib steam per Ib fuel gas for oxygen concentrations of 1.6 to 1.8% dry at the arch. These results should not be generalized to other burn volume-sink surface spatial arrangements, burner configurations or other oxygen levels nor is directly applicable to adjusting for humidity.