The SCR system is a flue gas filter installed in the gas duct, upstream the stack. Inside of the SCR are hundreds of thousands of catalyst coated channels, where the gas passes through. Inside the channels, the catalyst promotes NOx molecules to react with ammonia injected upstream the SCR, to form harmless Nitrogen and water. For the reaction to occur, the surface of the channels needs to be kept clean however during operation, particles from the combustion process tend to adhere to the catalyst surface, preventing said reactions.
After some time of operation, whole channels are plugged with particles, creating a pressure drop and prompting the flue gas to flow with higher velocities through the remaining open channels. This reduces the efficiency of the SCR, increases the consumption of Ammonia and can also errode the catalyst, leading to costly replacements.