Designing a compact SCR system
SCR systems for generators are bigger than many people realise. This can lead to problems with installation, since the designers of SCR systems for generators don’t always take space constraints into consideration. However, SCR technology is now widely used in the automotive sector, where compactness is paramount. The innovations that have arisen as a result can be applied to large engines too.
What makes up an SCR system?
The two main functional parts of the system are the catalytic converter and the reductant injection system, both of which have a large impact on the size of the system. The catalytic converter provides a large surface area coated in a catalytically active material. When this material comes into contact with pollutants, it causes a chemical reaction which turns them into harmless substances. The larger the surface area, the better the performance.
The physical structure of a catalytic converter is provided by a honeycomb-like ‘substrate’. Substrates are often described in terms of the number of cells per square inch or CPSI. The greater this number, the greater the surface area per unit of volume. In the automotive sector it is common to use substrates with 200 cpsi, and these provide approximately 2.65 m2 of surface area per litre of volume. However, some of the substrates used in generator SCR systems have as few as 34 cpsi, and these provide only 0.8 m2 per litre of volume. In order to provide the same surface area as a 200 cpsi substrate, they need to be more than three times the size! Substates with lower cell densities are used on large engines simply because they are often found in power stations and are therefore readily available.
Reductant injection and size
One issue that has major implications for compactness is the size of the reductant injection system. SCR systems work by mixing ammonia with the exhaust gas, which reacts with the nitrogen oxides as the mixture passes through the catalytic converter. In most cases it is not actually ammonia which is injected into the exhaust system but a solution of urea in water (which is much safer than ammonia), such as Adblue (called DEF in the USA) which consists of 32.5 % urea.
One of the many disadvantages of using urea solution, however, is that it doesn’t convert to ammonia gas instantly. It has to undergo two processes known as ‘hydrolysis’ (in which the water is removed) and ‘thermolysis’ which is the reaction where urea breaks down into ammonia and carbon dioxide. As a rule, these reactions take about 0.2 seconds, meaning that we need to allow enough volume in the exhaust between the point where the urea solution is injected and the catalytic converter to accommodate 0.2 seconds’ worth of exhaust gas. On a large diesel engine the exhaust gas flow is measured in cubic meters per second, meaning that it will require a lot of space. The ammonia also needs to mix well with the exhaust gas and a big volume helps in this respect too.
So how do exhaust designers in the automotive sector get around this problem? Well, one useful trick is to use what is known as a ‘hydrolysis catalytic converter’. This promotes the reaction whereby water is extracted from the urea solution. The mixing of the ammonia and the exhaust gas can also be improved by careful design of the chamber in which this occurs. A bit of ingenuity here can save an awful lot of precious space.
Another approach is to convert the urea solution into ammonia before it is injected into the exhaust. This is quite easy to achieve in a separate reactor, which takes up a lot less room. It also gives the advantage that the SCR system can now work at a lower temperature, since the conversion of urea solution into ammonia is not reliant on the temperature of the exhaust gas.
So what’s the real reason that SCR systems on generators are often so huge? Simply put, the designers haven’t done their homework! With a little science and creative thinking, a compact SCR system can be created.