A vertically fired natural-draft petroleum-refinery crude-oil process heater was modified to reduce oxides of nitrogen (NOx) emissions by installing staged combustion air lances. Baseline NO emissions firing refinery gas before modification were measured to be 100 to 125 parts per million by volume, dry, corrected to 3 percent oxygen. Staged combustion air lances were installed to bypass combustion air around the burners to achieve staged combustion. With the modification, an average reduction of 64 percent in NO emissions was observed over a 30-day test period, with NO emissions of 25 to 50 ppm, dependent on the excess air. This method of combustion modification can be applied to most vertically fired process heaters where flame length increased by a factor of 2 to 4 would not cause flame impingement.

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