Hello I’m looking to remove NOx….. that’s usually how the conversation starts. Often, the process engineer or plant manager doesn’t have much more information available during the initial discussion.
This is unfortunate, as NOx scrubbers have many configurations depending on the exact gas composition and scrubbing liquids to be used. Over the years we’ve learned the proper questions to ask in order to ensure that the end result is a system that will meet the reduction needs in a cost efficient manner. There are many ways in which NOx can be chemically treated – each having its advantages and its drawbacks.
NOx refers to both the NO and NO2 contained in exhaust gases – the byproduct of a combustion process such as vent gas from a boiler. Nitrous Oxide (NO) is a colorless gas and is only sparingly soluble in water. NO is inherently difficult to scrub, and can generally only be removed by use of a strong oxidizing agent.
Nitrogen Dioxide (NO2) is orange in appearance, ranging from a reddish orange to almost brown depending on the amount of NO2 present and the gas temperature. It is moderately soluble in water, and can be converted to N2 by use of an appropriate reducing agent.
When designing a NOx mitigation system, the ratio of NO to NO2 has an impact on the optimal design approach. For NO heavy systems, a two stage solution is employed. In the first stage NO is oxidized to NO2. Strong oxidizers are preferable, treatment options include Ozone (O3) injection or Sodium Chlorite, NaClO2. The use of Sodium Chlorite is more cost effective as ozone generation systems are historically cost prohibitive.
For NaClO2 based systems, the NO is oxidized in the presence of an acid to NO2 as follows:
10 NO + 2 H2SO4 + 5 NaClO2 →10 NO2 + 2 Na2SO4
+ NaCl + 4 HCL
Of course, some clients are hesitant to utilize such a strong chemicals to treat their NO. Thankfully, other chemistries can be utilized under certain conditions. Two alternatives are Hydrogen Peroxide (H2O2) and Sodium Hydroxide (NaOH).
For either of these alternatives to be practical, there needs to be some NO2 present in addition to the NO. Ideally the NO2: NO molar ratio should be 1:1 or greater, since the NO and NO2 react in equal proportion, as follows:
NO + NO2 + 2 H2O2 → 2 HNO3 + H2O
for hydrogen peroxide
NO + NO2 + 2 NaOH → 2 NaNO2 + H2O
It should be noted that of these two options, with regards to dispositioning the wastewater, many facilities find it easier to deal with the nitric acid (HNO3) than the sodium nitrates (NaNO3).
Additionally, hydrogen peroxide systems require smaller towers than caustic systems. This is due to the both lower L/G ratio needed and lower resonance time required in order to ensure full scrubbing.
In summary, given the various ways in which NOx can be treated, it’s best if you let a CR Clean Air process expert put their knowledge to work for you!
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