NOx Scrubber

The scrubber operates at a high pH in order to drive these reactions to the left, minimizing H₂S formation. Even at pH 12.5, there will be some H₂S emissions, however. Some system suppliers routinely install an extra, caustic scrubbing stage following the NO₂ scrubber in order to capture the H₂S. Since NaOH converts absorbed H₂S back to NaHS, the blowdown from the caustic scrubbing stage can be recycled to the NO₂ scrubber. Solid NaHS or Na₂S also gives off H₂S when it reacts with atmospheric moisture and CO₂, so it is important that the area where the chemical is stored and mixed be kept under negative pressure, and vented to the scrubber.

NaHS + H₂O NaOH + H₂S

A major drawback of these scrubbers is the hydrogen sulfide odor given off by sodium
hydrosulphide.

Na₂S + H₂O NaHS + NaOH

The solid chemical used for make-up can be either sodium sulfide (Na₂S) or sodium hydrosulphide (NaHS). Because of the equilibrium between sulfide and hydrosulphide ions in solution, either of the solid chemicals will produce the same solution when dissolved in water and adjusted to the same pH:

The actual Na₂S consumption is therefore hard to predict, but a metering pump that can feed 1,3 kg of Na₂S per 1 kg of NO₂ should be more than adequate.

8 Na₂S₂O₃ + 2 NO₂ + 4 H₂O N₂ + 4 Na₂S₄O₆ + 8 NaOH
4 Na₂S₄O₆ + 2 NO₂ + 8 NaOH N₂ + 16 Na₂SO₄ + 4 H₂O
8 NO₂ + 2 Na₂S + 4 NaOH 8 NaNO₂ + Na₂S₂O₃ + 2 H₂O
2 NO₂ + 2 NaOH NaNO2 + NaNO₃ + H₂O
NO + NO₂ N₂O₃
N₂O₃ + 2 NaOH 2 NaNO₂ + H₂O

On the other hand, some Na₂S will be oxidized further, to tetrathionate, and some of the Na₂S₂O₃ and Na₂S₄O₆ will react with NO₂ to form Na₂SO₄. Some NO₂ will react with caustic to form NaNO₂ and NaNO₃. Those reactions lower the amount of Na₂S consumed per unit NO₂ absorbed.

2 NO₂ + 4 Na₂S + 4 H₂O N₂ + 4 S + 8 NaOH

So theoretically you need 770g of Na₂S to decompose 455g of NO₂.
A number of other, slower reactions also occur, however. Some of the sulfide will only
be oxidized to elemental sulfur, resulting in higher Na2S consumption.

2 NO₂ + 2 Na₂S + H₂O N₂ + Na₂S₂O₃ + 2 NaOH

The main reaction between nitrogen dioxide and sodium sulfide in the
scrubber is

Below is a summary of the reaction. This discussion is for general informational purposes only. No warranty is implied or granted with this design example. Please contact S G Plastic Construction to review the design needs of your specific project.

Recent developments have lead the use of sodium sulphide (Na₂S) for the absorption of NOx fumes. The reaction between the NOx and Na₂S is virtually instantaneous. The efficiency of absorption tops 98 % and in some instances 99%+. The emissions from the scrubber are brought well within the limits of the Air Pollution Control Act. All of the noxious brown fume is also removed.

Other methods include the use of urea and ozone. Once again these plant prove low efficiencies and high running costs.