NOx exhaust treatment

The use of catalytic converters in automobiles makes plans to control mobile NO_x emissions very limited.

As a result, stationary sources of NOx emissions are now subject to more stringent standards. Includes plants that produce nitric acid, nitrate materials such as fertilizers, and explosives. Industrial production (metallurgical plants, glass manufacturing, cement kilns, generators, etc.) where high processing temperatures are used.

NOx is the main pollutant substance in the atmosphere, and is agents of acid rain, photochemical smog and ozone accumulation.

The main oxides are nitrogen monoxide (NO) and nitrogen dioxide (NO₂) both of which are corrosive and dangerous to health.

Man-made sources of NOx emissions need to be treated

Fuel Burning

At high temperatures, oxygen and nitrogen present in the air combine to form nitrogen oxides. Typical exhaust samples contain 100-1500 ppm NOx.

Nitric acid production

The unrecoverable exhaust gas in the final absorber typically contains 2-3% NOx based on the weight of acid produced.

Chemical process

Many processes in which nitric acid, nitrate or nitrite are used as reagents result in the generation of NOx emissions. For example, manufacturing explosives, plastics, dyes, etc

High temperature process

Processes in which materials are manufactured at high temperatures such as glass manufacturing, electric furnaces and cement kilns generate NOx emissions.

Man-made sources of NOx emissions

Natural sources of NOx emissions need to be treated

Pollution due to volcanic activity

Volcanic activity spews out huge amounts of pollutants including NOx (mainly N₂O₂, NO)

Pollution caused by forest fires

Due to natural causes as well as unconscious human activities, pollutants such as smoke, dust, SOx, NOx, CO…

Pollution caused by decomposition of organic substances in nature

Due to the fermentation process of organic matter in landfills, swamps, animal carcasses, etc.

Natural sources of NOx emissions

NOx emission treatment technologies

Selective non-catalytic reduction (SNCR) for treating NOx emissions

Bysupplying NH₃ gas into the high-temperature exhaust gas stream. NO is selectively reduced to N₂ without a catalyst.

The reaction takes place very quickly at temperatures above 900⁰C. Maximum NOx removal efficiency is achieved at temperatures around 1000⁰C. The greater the amount of NH₃ introduced, the higher the treatment efficiency.

Below is the simple equation of the reaction using NH₃:

4NO + 4 NH₃ + O₂ → 4 N₂ + 6 H₂O

If the amount of NH₃ added increases, the treatment efficiency can be higher, but at the same time, the amount of unreacted NH₃ also increases to form NH₄ HSO₄.

This problem appears in the case of catalytic NOx reduction with NH₃. For boilers, the retention time at different NOx treatment temperature ranges is very short, the treatment efficiency reaches 30 – 50%.

Using urea (CO(NH₂)₂) as a reducing agent has been put into practical use. The reaction is as follows:

CO(NH₂)₂ + 2NO + 1/2O₂ → 2N₂ + 2H₂O + CO₂

The urea solution is sprayed in mist form into the furnace. The investment and operating costs of this method are the lowest. Treatment with urea solution is simple and safe.

Selective catalytic reduction with reducing agent NH₃ (SCR)

NH₃ is a reducing agent capable of selectively reacting with NO and NO₂ at temperatures higher than 232⁰C. The reduction process is carried out on the catalytic surface to form nitrogen and water according to the following reactions:

When there is oxygen

4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O

2NO₂ + 4NH₃ + O₂ → 3N₂ + 6H₂O

When there is no oxygen

6NO + 4NH₃ → 5N₂ + 6H₂O

6NO₂ + 8NH₃ → 7N₂ + 12H₂O

Some catalytic groups commonly used in this process include:

• Noble metal catalyst group: Pt, Rodi, Pt-Ro, Pt/Al₂O₃,…reaction temperature from 200 -300⁰C
• Metal oxide catalyst group: Fe2O3/Cr2O3, V2O5/TiO2,… reaction temperature from 300-4500C.

Recently, because NH₃ is a chemical with a strong and unpleasant odor, urea solution is used to replace it.

In addition, people also use a number of other methods to treat NOx such as: adsorbing NOx with silicagel, alumogel, activated carbon…