NOX Reduction

In NO_X Reduction, Selective Catalytic (and Non-Catalytic) Reduction Systems (SCR and SNCR), you'll learn ...

• NO_X formation and importance of NO_X emission control
• Techniques to control NO_X emission by reducing NO_X generation during combustion as well as those used to covert NO_X to environmentally friendly species after generation
• Selective Non-catalytic Reduction (SNCR) Systems for post-combustion NO_X reduction
• Selective Catalytic Reduction (SCR) Systems for post-combustion NO_X reduction

Overview

Credit: 3 PDH

Length: 33 pages

NO_X (a group of highly reactive gases) form when fuels are burned at high temperatures. The two major sources of NO_X generation are transportation vehicles and stationary combustion sources such as electric utility and industrial boilers. NO_X has great health impacts, as well as environmental and agricultural impacts. Thus, NO_X emissions have been regulated by the US states and federal authorities, as well as by most developed countries.

NO_X emissions have become an important factor in the design and operation of fossil fuel burning equipment. Two techniques are normally adopted for controlling/minimizing NO_X emissions from the combustion sources. First, is to minimize the NO_X formation by influencing the combustion process in the combustion regions. The second approach relies on chemically converting NO_X emission, after its formation during the combustion process.

The first approach, which is based on design modifications to reduce NO_X formation (usually <30%) during the combustion process, burning low nitrogen fuels and reducing combustion temperature and excess air will reduce NO_X generation during combustion. In the second approach, additional systems/equipment are installed to convert NO_X into environmentally friendly species.

Post-combustion NO_X reduction techniques (the second approach), usually used for high-level NO_X reduction, are the main focus of this course. SNCR (selective non-catalytic reduction) and SCR (selective catalytic reduction) are discussed in detail, and their costs and NO_X removal capabilities are compared.

The SNCR process takes place in the boiler, immediately downstream of the combustion zone, where the flue gas temperature is high enough for the process to occur. Ammonia or urea is injected into the flue gas through multiple points, to react with NO_X, producing N₂ and water. Effects of temperature, residence time, reducing reagent, and inlet NO_X and reagent concentrations on the SNCR performance are discussed.

SCR systems, with 90% removal rate, are the best choice for large scale NO_X reduction applications. In SCR, ammonia is blended and mixed with the NO_X containing hot flue gas leaving the boiler. The mixture will then flow through one or more layer(s) of catalysts. NO_X will react with ammonia, producing N₂ and water.