The removal of NOx from an engine exhaust, via SCR is a chemical process. For the chemical reaction (the neutralisation of acidic NOx with basic ammonia NH3) to occur, the appropriate conditions must be provided. For example, there must be sufficient ammonia (NH3) in both quantity and spatial distribution matched to inlet NOx in combination with sufficient catalyst to meet the required target emissions levels. Of key importance is the temperature as the SCR process requires operation within a specific range.
This temperature window is dependent on several factors such as fuel type and sulphur content and is typically in the range of 300 – 500℃ (but can be lower e.g. 270℃ for lower sulphur fuels). If the exhaust temperature falls below the specific range for its operation – the efficiency of the SCR reactions drops and fouling of catalyst (with ammonium sulphate salts) may occur. Without appropriate management, in addition to the NOx emissions, unreacted ammonia (NH3) will also be released to the atmosphere.
SCR systems can tolerate limited periods of low temperature operation (with fouling). With higher temperatures these ammonium salts return to the gaseous phase, with the ammonia available to neutralise NOx.
Therefore, managing the temperature and performance conditions is critical for the efficient and effective use of deNOx SCR. Minimum temperatures of operation and/or recovery temperature capability can be achieved through multiple, known technologies, each with its own advantages for a given application including energy cost, modification of target performance levels, catalyst volume, etc. Examples include:
- Heating low temperature exhaust gas (via external sources such as burners),
- Engine management where exhaust gas temperatures are maintained at lower loads,
- Engine/battery systems that continue to maintain minimum exhaust gas temperatures with excess power (not needed a low load demand) being stored electrically,
- More advanced monitoring equipment (CEMs) that informs the operator of real-world emissions offering significant improvement on the test cycle approach.
In conclusion, when vessels operate at lower load (and lower temperatures) such as close to shore this presents challenges to deNOx technology. However, the industry has established tools to improve net performance and ensure lower NOx emissions close to shore.