NOx Scrubbing Technology Breakthrough

Pacific Rim Design and Development and Dupont are proud to introduce a technology breakthrough in the treatment and safe elimination of problematic nitric oxide (NO) and nitrogen dioxide (NO2 ).  There are six forms of nitrogen oxide (NOx); the drawings to the right show the two most problematic forms.  These two are of particular interest because they are identified as atmospheric pollutants (greenhouse gases) and subject to regulatory control by the Environmental Protection Agency (EPA) and many other organizations.   

A new patented NOx scrubbing process described here will likely prove to be the most cost effective and best available control technology for waste gas streams containing both NO and NO2.

Although this process has broad technical applications for a number of other processes, it offers a profound and immediate improvement in the treatment of industrially created NOx waste gas. This   process has been proven beneficial for a number of industrial applications such as exhaust gas treatment in chemical milling and pickling of metals, cooled stationary source combustion process flue gas (i.e. power generation plants), tail gas from nitric acid plants and other sources of waste gas containing nitrogen oxides.

This patent pending process uses a single scrubbing stage with less than 1.5 seconds of residence time(treatment time within the scrubber) to efficiently treat more than 99% of the NOx (both NO and NO2) in an ambient temperature gas stream. This process, which utilizes chlorine dioxide (ClO2) in a new and unique way, is less expensive to install and operate than currently available industrial technologies for NOx treatment.

This process transforms the NOx into mineral acids in a two-step process that can be described in equations 1 and 2 below:

  • 5 NO + 2 ClO2 + H2O → 5 NO2 + 2 HCl       (1) 
  • 5 NO2 + ClO2 + 3 H2O → 5 HNO3 + HCl      (2)

Both of these processes proceed rapidly to completion with minimal excess ClO2 present in the reaction environment, making for the safe and efficient elimination of NOx.

Additional operational savings are possible when an optional second stage wet scrubber operating at a pH near 8 is employed.  The easier to remove, water soluble NO2 in equation 2 above can either be eliminated from the waste gas with ClO2 as described in equation 2 or removed from the waste gas stream with less expensive sodium hydroxide (NaOH) as described in equation 3 below.

  • 2 NO2 + 2 OH- → NO2- + NO3- + H2O          (3)

The nitrate ion (NO3-) and nitrite ion (NO2-) are soluble in water and therefore removed from the waste gas. This is just one of several possible NO2 treatment options for the water soluble NO2.

How can this process be less expensive AND more effective?

It is not uncommon to treat waste gas streams that contain predominately NO2 with a single stage wet scrubbing process. This works because NO2 is quite soluble in water. However waste gas streams with high concentrations of NO historically require multi stage scrubbers. Typically this waste gas is treated with a three stage wet scrubber where:

  • First stage: Converts the NO into NO2
  • Second stage: Treats the NO2
  • Third stage: Treats the odors created in the second stage

Because NO is not very soluble in water it requires a long residence time (up to 2 minutes for some manufacturers) to transform NO into NO2. The second and third stages of 3 stage scrubbers are also typically very large, therefore the cost of this equipment is significantly higher when compared to the small single stage vessel used for the 1.5 second residence time required in the process described here.

The differences in vessel costs more than compensate for the cost of ClO2 generation equipment required in this process. Operational costs for the three stage scrubbers are also higher than the single stage process described here.

Cost comparison details between this new single stage process and other multi stage processes are available upon request.

Why is this process the Best Available Control Technology?

  1. This single stage process has the ability to produce higher removal efficiency than can be obtained from conventional two, three and six stage scrubbing systems. The reason we can reach greater than 99 percent for both NO and NO2: we can cost effectively increase residence time.  Residence time is directly related to removal efficiency.  Our chemical reactions are so fast they only require 1.5 seconds to accomplish what conventional systems require 5 to 120 seconds of residence time to accomplish. Where desired, we can increase our residence time and proportionately increase our removal efficiency. Conventional NOx treatment is already enormous, doubling size is economically inferable.  
  2. The mineral acid reaction byproducts (see equations 1 and 2 above) are clean enough in the single stage process that they could have commercial value.  At the very least these waste products poise less challenge for commercial water treatment processes than the complicated chemistry typically produced in three stage scrubbing systems.  Details of various multi-stage NOx treatment process and their waste products are available upon request.

Generating Chlorine Dioxide (ClO2)

There are a number of methods for ClO2 generation. Each has advantages and disadvantages that must be considered in each application.  Typically the two and three chemical generation processes are less expensive to purchase but require the use of reactive chemistry and produce waste streams that may require further treatment or be detrimental to the larger process.

The single chemical electrochemical chlorine dioxide generation process is more expensive to purchase but produces pure, pH 7 ClO2 at a lower cost per pound than the multi-chemical processes.  This method produces two byproducts that can be used in an optional second stage treatment; NaOH, which would replace part of the purchased sodium hydroxide and a stream of chlorite/ chlorate/ chlorine dioxide that could also be dosed into the second stage wet scrubber.  

For additional information:

For additional information on ClO2 generation options or for this new NOx treatment process, contact: Robert Richardson at 530-474-4819 and see



Pacific Rim Design & Development

Robert Richardson Ph.D.               

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