SCR Catalytic Converter Types for Power Generation Engines

SCR catalytic converters are used as part of selective catalytic reduction systems to reduce emissions of nitrogen oxides (NOx) from power generation engines, including reciprocating engines and gas turbines. These engines can run on a range of liquid and gaseous fuels, such as Diesel, compressed natural gas, syngas, hydrotreated vegetable oil (HVO), and biogas. They are frequently used to generate electricity, either as sources of prime power or as backup in the event of mains failure.

Nitrogen oxides are formed under conditions of high temperature and pressure during combustion processes and are harmful to human health, in particular to the lungs and breathing tracts. It is common for power generation engines to be required to minimise their NOx emissions. This may be a condition of their permit to operate, although some operators also seek to reduce pollution as part of their corporate social responsibility targets.

SCR catalytic converter

The most effective technology for reducing NOx emissions from combustion fumes, where there is excess oxygen in the flue gas, is selective catalytic reduction. This involves mixing a ‘reductant’ containing hydrogen atoms, such as AdBlue, with the exhaust gas and then passing this mixture through a special type of catalytic converter. The catalytic converter causes the nitrogen oxides and reductant to break down, allowing the resulting atoms to react with each other to produce harmless nitrogen and water.

SCR Catalytic Converters for Power Generation Engines

Catalytic converters are based on a ‘substrate’, a structure containing lots of small channels through which the flue gas passes. The walls of these channels provide a very large surface area. Substrates may be made from ceramic material, or very thin metal foil. The choice of substrate affects important factors such as pressure drop, flow distribution, durability, and the size of the catalytic converter that can be produced. Different substrate materials also offer different advantages depending on the requirements of the power generation engine.

Ceramic and Metal Substrates

Ceramic substrates are extruded through a mould, which imposes a size limitation. The largest size possible is about 300 × 300 mm in cross-section. This is not large enough for a large generator engine, so multiple substrates are normally arranged within a housing. There are two main types of ceramic substrate used for SCR applications. The first is made from a type of ceramic that incorporates the catalytic material, meaning that it is homogeneous. The second has a coating containing catalytic material.

The advantage of incorporating the catalytic material into the substrate is that it remains catalytically active even if some abrasion of the surface occurs due to dust in the flue gas. With a coated substrate, however, catalytic activity is lost if the coating wears away. Applying a catalytic coating also has several advantages. It allows a wider choice of catalytic materials, including materials that may be more efficient than those that can be incorporated into the substrate itself. If these catalytic materials are relatively expensive, a smaller quantity is needed to produce a thin coating than would be required if they were incorporated throughout a homogeneous substrate.

ceramic catalytic converters

Metal substrates are made from very thin metal foil, with alternating layers that are either corrugated or flat. They can be round, square, or rectangular, but the round type is the strongest and normally provides the most efficient flow distribution. Although metal substrates are more expensive than ceramic substrates, they have two advantages. First, the ultra-thin metal foil, typically 0.05 mm, results in a lower pressure drop. Second, because no tooling is required, metal substrates can be manufactured in almost any size required, even up to a metre in diameter. Top-quality metal substrates are held together by vacuum brazing, while a cheaper type relies on the coating to act as a kind of glue.

Vanadium and Zeolite SCR Coatings

There are two main types of coatings used in an SCR Catalytic Converter: those containing vanadium pentoxide and a more modern alternative based on zeolites. Both types of coating can be applied to either ceramic or metal substrates.

Vanadium pentoxide coatings, which also normally incorporate tungsten and titanium dioxide, have been available for many years and are very effective as long as the temperature does not exceed about 500 °C. They are also resistant to most substances that poison catalysts, such as sulphur. However, vanadium pentoxide has been found to be carcinogenic, so this type of coating is now falling out of favour.

The more modern alternative to vanadium pentoxide coatings is based on zeolites. These coatings are safer and also operate over a wider temperature range. The two most popular zeolites used are known as ‘beta’ and ‘SSZ13’, and they are usually doped with either copper or iron. Copper-doped zeolites are most effective at lower temperatures, between 200 and 400 °C, whereas iron-doped zeolites are better suited to higher temperatures, between 400 and 650 °C.

The choice of SCR catalytic converter for a power generation engine depends on the specific operating conditions and emissions requirements. Ceramic and metal substrates each offer different advantages, while the choice between homogeneous and coated substrates and between vanadium and zeolite-based coatings can further affect performance, durability, and operating range. Selecting the appropriate combination of substrate and coating is key to achieving reliable NOx reduction under the required operating conditions.