(23d) Selective Catalytic Reduction Reactors and Their Uses

Bryan Pourchot

Sales & Service Engineer

Haldor Topsoe, Inc.

The burning of fossil fuels and other materials create the means for a civilization and economy to flourish.  With this burning comes the consequence of releasing large amounts of compounds which can effect global climate change and the air quality of the civilization it supports.  The compounds to be discussed during this presentation, and a technology to combat them, comprise NO_x.  NO_x represents the compounds NO and NO₂.  NO_x can react with other chemicals in the air to create nitric acid, ozone, and other chemicals which can damage lung tissue and cause a variety of other health problems.  NO_x can also destroy ozone in the stratosphere, increasing the amount of UV light let down to the Earth’s surface causing, among other ailments, skin cancer.

 One way of destroying NO_x is by the implementation of a Selective Catalytic Reduction (SCR) System at the exhaust side of processes which are responsible for the creation of NO_x.  There are numerous applications for SCRs from things as small as the exhaust on a car engine to things as large as a coal power plant.  A SCR will convert NO_x, through a reaction with ammonia (NH₃) or another reducing agent, into the benign compounds of diatomic nitrogen (N₂) and water (H₂O).  A SCR is capable of achieving very low levels of NO_x emissions, on the nature of ≤ 1ppm, coming from a multitude of processes while expelling a minimum of unreacted reducing agent out of the exhaust, also on the nature of ≤ 1ppm.  This allows many of the processes to comply with the ever challenging requirements of the Environmental Protection Agency (EPA) on NO_x and ammonia emissions. 

 This presentation will discuss the chemistry behind an SCR, the structure of some common SCR reactors, the types of processes that use or can use a SCR, and some of the design considerations and issues inherent to the process.