Ammonia Selective Catalytic Reduction

While we may not put too much daily thought into the systems that control emissions and keep our air clean, they’re an essential part of protecting our health and our environment. In many cases, selective catalytic reduction is the process that’s used to keep these emission levels low. 

Ammonia-based selective catalytic reduction has been applied to stationary-source fossil fuel combustion for nearly 50 years. It is used in the United States, as well as Japan, Europe, and a host of other countries that rely on fossil fuels and have a vested interest in protecting their land, air, and people.

In the U.S. alone, there are more than 1,000 stationary selective catalytic reduction systems in place, used across a wide range of applications and sources. 

What do you need to know about this process, and where it is used? This article will examine the selective catalytic reduction process and various applications of ammonia-based selective catalytic reduction.

What is Selective Catalytic Reduction?

What exactly does “selective catalytic reduction” refer to? It’s an advanced active emissions control system used to severely limit the amount of pollutants that are released into the air.

Selective catalytic reduction (SCR) is a reaction that uses ammonia to reduce nitrogen oxides—also known as NOx—into nitrogen (N₂) and water, which are much safer substances for both people and the environment. Because of this, it has been used in power plants and chemical refineries since the 1970s, and in vehicles since the mid-2000s to reduce NOx emissions in diesel engines. 

SCR technology allows NOx reduction reactions to happen in an oxidizing atmosphere, by using ammonia as a reductant within some kind of catalyst system. The chemical reaction, known as a reduction, takes place by converting pollutants into nitrogen, water, and trace amounts of Carbon dioxide (CO₂). 

In theory, SCR systems can reach 100 percent efficiency in NOx reduction, though in practice, commercial coal, oil, and natural gas SCR systems typically are engineered to meet efficiency control targets above 90 percent. The process of reducing Nitrogen oxides in emissions can sometimes also be referred to a DeNOx, 

Why Nitrogen Dioxide Can Be Harmful

Nitrogen dioxide (NO₂) is released into the air through the burning of fuel that powers things like cars, trucks, buses, power plants, and off-road equipment. In high concentrations, it can be damaging to people and animals, and also to the environment. NO₂ reacts with other chemicals found in the air that forms two things: particulate matter and ozone, both of which can be harmful or damaging to the respiratory system.

When we breathe air with high concentrations of NO₂, it can irritate our airways. With short-term exposure, this can aggravate respiratory diseases like asthma, and contribute to respiratory symptoms like coughing, wheezing, and difficulty breathing. In some cases, those exposed need to be hospitalized. 

With longer exposures to high levels of NO₂, people can develop asthma and increased susceptibility to respiratory infections. Especially those with asthma or other breathing issues, plus children and the elderly are all at an increased risk of health issues from exposure to NO₂.

Effects on the Environment

When we learned about acid rain in our science classes in elementary school, we learned that this was mainly caused by fuel emissions. These emissions are composed of NO₂. When nitrogen dioxide is released into the air, it interacts with water, oxygen, and other chemicals to form acid rain, which can wreak havoc on sensitive ecosystems including lakes and forests. 

The particles formed when NO₂ hits the air also make it hazy and difficult to see through, and also contribute to pollution in coastal waters. 

Essentially, the more that can be done to mitigate the release of NO₂ into the air, the better. 

Ammonia Selective Catalytic Reduction Can Help

SCR systems can reduce Nitrogen oxide levels in many sources across a variety of industries. Selective catalytic reduction has been used in:

• Utility and industrial borders
• Process heaters
• Gas turbines
• Internal combustion engines
• Chemical plants
• Steel mills
• Fluid catalytic cracking units
• Ethylene cracker furnaces
• Nitric acid plants
• Catalyst manufacturing processes
• Nitrogen fixation processes
• Solid, liquid, or gas waste incinerators

Selective catalytic reduction is the most widely-used technology to reduce Nitrogen oxide emissions in coal-fired plants because it is so successful in removing NOx from the flue gases released upon combustion. The necessary reagent for this process? Industrial ammonia. 

Ammonia is needed to act as a reduction reagent for NOx. In about 80 percent of SCR systems, anhydrous ammonia or aqueous ammonia is used. In the remaining 20 percent of cases, urea is used—but only by converting it into ammonia before injection first. For efficiency’s sake, it’s no wonder the overwhelming majority of SCR systems rely on ammonia. 

When polled, coal-fired power plants that use an SCR system to control their Nitrogen oxide emissions used anhydrous ammonia 3 times more frequently than aqueous ammonia. Half of these respondents cited cost as their primary consideration in which reagent they used, and around 25 percent listed safety as their primary concern. 

Either way, there’s no denying that when it comes to SCR DeNOx systems, using industrial ammonia is the choice that makes the most sense. 

How Airgas Specialty Products Can Help

In addition to offering a reliable, steady supply of industrial ammonia products including anhydrous ammonia and aqua ammonia, our team at Airgas Specialty Products can deliver a complete ammonia package. This includes your SCR DeNOx facility design and installation, as well as providing an ongoing ammonia supply and consulting with engineers, and even safety training to keep your team compliant and protect your processes. 

When it comes to DeNOx systems—including SCR systems—Airgas Specialty Products can contribute our expertise in any way you need it. No two facilities’ needs are alike, which means we will evaluate your individual needs and customize a solution to fit your requirements. When you need someone to handle the maintenance and repair of your ammonia system or assistance with product conversions, we are there for you.

To discover more about how our qualified experts at Airgas Specialty Products can help you limit emissions and keep your facility safe and your operation in compliance with a dependable, well-engineered SCR system and ammonia supply, contact us today.