SCR Process

Combustion processes create flue gases that become exhausted into the environment. These flue gases may contain nitrogen oxide (NOx) that is considered a pollutant and is dangerous to the environment. With the selective catalytic reaction (SCR), achieving a high separation efficiency is possible only with the aid of a catalyst. Due to the high concentration of dust, such a solution requires special precautions to keep the efficiency high and catalyst waste low. The reagent is added immediately before the catalyst using the nozzle lance in a temperature window appropriate for the reaction.

Selective catalyst reduction (SCR) involves using a reactant and a catalyst to cause a desired reaction that will change the properties of the targeted gas and reduce it. In this case, you change harmful nitrogen oxide and nitric oxide into water and clean exhaust.

There are many different types of nitrogen oxides in the environment, including nitrous oxide (N2O). Some are extremely harmful and can cause a number of health issues. In addition, nitrogen oxides can also create acid rain. The federal government has placed N2) and other nitrogen oxides as regulated greenhouse gases and has provided regulations regarding the amount that may safely be emitted in flue exhaust for commercial and industrial operations. Removing this from the exhaust stream can prevent harm to everyone.

NOx Emission Reduction

The SCR process begins by analyzing and identifying the contaminations in flue gases to determine its chemical makeup. By understanding what contaminants are present, you can select the appropriate reducing agent and temperatures to create the desired chemical reaction.

The exhaust NOx gas travels through the equipment as it encounters a reactant agent that is applied with the use of spray nozzles. The spray nozzles direct the reactant into the gas stream where the NOx gas interacts and mixes in with the reductant. During this time, a chemical reaction occurs as it then passes through a catalyst bed. The oxides become reduced leaving behind nitrogen gas and water.

Chemical Reactions in SCR

To reduce the oxides in nitrogen, the used reductant needs to fully interact with the nitrogen oxides before passing through the catalyst. The mixture must also be heated to a range around 200°C (392°F) to 400°C (752°F). The reductant relies on the oxygen present in N2O to reduce its presence in the flue gas in the catalyst. This chemical reaction helps to separate the molecules to leave behind the nitrogen and water that is later expelled.

Types of Reductants Used

There are several different reductants that can be used in the SCR process. The most common and safe reductant is aqueous ammonia. It is safe to store and handle. This reductant needs to be vaporized so it can effectively interact with the NOx. Two methods of vaporization include liquidized injection with spray nozzles or with a pre-heating process.

Urea is also another common reductant. It is the safest to use. However, for it to be used in the SCR process, urea must undergo a decomposition process. A less common reductant to use is anhydrous ammonia. This type of ammonia is extremely effective and does not require another process for it to intermix with the NOx. Yet it is very dangerous to handle and store.

Tips For Implementing SCR Systems

You have to understand the composition of the flue gas to select the right reductant, as certain reductants may cause other chemical reactions due to additional contaminants that may be present. You also should select a spray nozzle system that can inject the reductant fully into the stream so there is an even distribution. With the appropriate nozzle system, you can reduce catalyst waste while lowering the amount of nitrogen oxides that enter the environment.

SCR Products and Solutions from Lechler

Depending on the customer’s process design, Lechler provides the appropriate nozzle lances and where necessary, the injection system as well.

In order to ensure complete evaporation within the short distance, Lechler has developed twin-fluid nozzles with extremely fine droplet spectra and precise controllability. The 1AW-nozzles meet the demands placed on them excellently and have already proven their worth in use.

For more information regarding our SCR products and solutions, contact us today.