Removal of Oxides of Nitrogen – N0x can be achieved by injecting Ammonia into the air and passing the resulting mixture over a catalyst.

Selective Catalytic Reduction – SCR involves the use of a catalyst that is selective to promote the reaction of Ammonia and N0x to form Nitrogen and water.

Several different forms of Ammonia can be used. That is, you can inject a water solution of Ammonia, Ammonia as a dry gas, or Urea.

How does SCR work?

Catalysts perform by lowering the temperature required to allow a chemical reaction to occur. In this case, a reduced nitrogen compound (ammonia or urea) is introduced into the air. The "oxidized" chemical (NO or NO2 or both) then reacts with the other nitrogen compound forming elemental nitrogen and water.

The common sources of neutralizing chemical are either ammonia, which is a gas at room temperature, ammonia solution or urea, which is dissolved in water and must be heated to very high temperatures and atomized into the air.

Selective Catalytic Reduction (SCR)

SCR technology is often used where high temperatures, low concentrations or high percentage of NO are encountered.

The catalyst used is normally coated on a ceramic substrate. Various configurations are used including lose pieces of ceramic media and structured honeycomb shapes. The structured shape is the most common because of its lower pressure drop. It is also easier to install and replace.

The best selection depends on the inlet gas temperature, other chemicals present, and efficiency required. Different catalysts can operate over different temperature ranges. The minimum range in temperature can be as low as 500°F / 260°C or as high as 1000°F / 530°C.

Other temperature considerations include the presence of sulfur dioxide or other impurities and the concentration of NOx.

There is a heat of reaction between ammonia and NOx. Care must always be used when designing an SCR system to avoid excessive temperature rise. Excessive is defined as anything that takes the temperature higher than the design range for the catalyst. Excessive temperatures will rapidly deteriorate the effectiveness of a catalyst.

Efficiencies of 80% - 85% can be readily achieved. Higher efficiencies can be provided by increasing the volume of catalyst used, the operating temperature or both. Branch can provide designs with high efficiency as needed.

Overall Reaction

Ammonia Reaction

NH3 + NOx + O2 → N2 + H2O + CO2

Urea Reaction

(NH2)2CO → NH3 + HNCO
HNCO + NOx + O2 → N2 + H2O + CO2

Lower Temperature Gas

Where chemical process applications involve ambient temperature or lower temperature gases, Branch can provide a package to increase the temperature of the gas using the most energy efficient method possible. The exhaust gas from the SCR can be used to preheat the inlet gas recovering most of the energy used in this process. This alternative to scrubbing has the advantage that very aggressive chemicals do not have to be handled and there is no wastewater to treat. We can work with you to determine the best overall package of either scrubbing or SCR depending on the specific conditions of your application.

NOx removal...

Branch Environmental can provide several different technologies for removing oxides of nitrogen (NOx) from air or flue gas. The best treatment method for your application will depend on the conditions of operation.

There are several techniques for removal of oxides of nitrogen (NOx) depending on the composition/temperature/removal efficiencies.

Formation of NOx

For high temperature operations, nitrogen in the air will react with oxygen to form oxides of nitrogen, NO, and NO2. The resulting NOx concentration depends on the highest temperature and how quickly the gas cools. The higher the temperature, the greater the formation of NOx. The more rapidly the gas cools, the more of the NOx is permanently formed. Sources such as boilers, incinerators, and gas turbines all create NOx.

Other operations which form NOx include chemical reactions such as those involving nitric acid.

For example; pickling lines for passivating stainless steel involve dipping the metal parts into a bath containing dilute nitric acid. The impurities are dissolved and the stainless steel receives an oxide coat which is passive and resists further corrosion. During this operation, the impurities react with the nitric acid to form oxides of nitrogen. Because of their low solubility, they come off the solution as vapor.

NO – NO2

Oxides of nitrogen include nitric oxide and nitrogen dioxide. There are other compounds which can be formed such as nitrous oxide (N2O), but these are not normally encountered in either thermal or industrial process operations.

NO is a colorless gas with virtually no water solubility. In the presence of excess oxygen, NO will slowly convert to nitrogen dioxide.

*Reference: Air Pollution Control and Design Handbook – Part 2 Pgs: 672/673.

Table 24–11

NO concentration in air (ppm) Time required for half NO to be oxidized to NO2 (min)
20,000 0.175
10,000 0.35
1,000 3.5
100 35
10 350
1 3,500

Nitrogen dioxide is also a colorless gas, but dimerizes to N2O4. As a result, the gas appears as a yellowish orange to dark red brown color gas depending on the concentration. The higher the concentration, the darker the color.

What is unique about NOx?

Many flue gas contaminants are cleaned using a variety of techniques. The most common is wet scrubbing. For example, removal of oxides of sulfur using wet scrubbers. Since scrubbers are low in initial cost, why aren't most applications handled using wet scrubbers?

NO2 has a very low solubility in water. However, NO2 will slowly dissolve.

Once NO2 dissolves, it goes through an auto-oxidation step as follows:

3NO2 + H2O →
2HNO3 + NO↑

This is the overall reaction. As you can see, the absorption of nitrogen dioxide forms byproduct nitric oxide (NO). Nitric Oxide is very low in solubility so it escapes the scrubber as part of the exhaust air.

There are techniques for interfering with this reaction including oxidation and reduction chemicals or the special wet phase catalyst packing media.

While scrubbing is obviously possible, using special chemicals or other techniques, it is a much more costly approach than a conventional scrubber. Both initial cost and operating costs are higher. Additionally, wet scrubbing is not nearly as cost effective when the inlet gas temperature is hot. Conditions involving high temperatures or low concentrations – which limit scrubbing effectiveness – are better suited to SCR technology.

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