Compressed air applications in Marine
Selective Catalytic Reduction (SCR) on Ships
NOx (Nitrogen Oxides) emissions are a significant pollutant from vessels formed during the high-temperature combustion of fuel in ship engines when nitrogen in the air reacts with oxygen. Marine NOx reduction technology has become an imperative for shipyards building vessels for operation in ECAs (Emission Control Areas), and for ship operators in ECAs, since Tier III of the IMO’s MARPOL Annex VI Regulation 13 came into effect. This sets out the most stringent NOx emissions limits for marine diesel engines with a power output of more than 130 kW installed on ships. Applying to ships constructed on or after 1st January 2016, and operating in ECAs, it typically necessitates the use of marine NOx reduction technologies like Selective Catalytic Reduction (SCR).
SCR technology is one of the most effective methods of reducing NOx created by combustion engines, and most marine SCR systems rely on compressed air to operate consistently and efficiently, where it serves a number of important functions:
Compressed air is typically used for the continuous injection of a reducing agent into the hot exhaust gas stream. The compressed air helps to atomise the reducing agent, often an aqueous urea, into a fine spray. This creates a larger surface area for the urea droplets, promoting rapid evaporation of water and subsequent decomposition into ammonia. Once the hot exhaust gas stream reaches the catalyst converter - which now includes NOx and the ammonia - selective catalytic reduction reactions occur, converting NOx into harmless water (H₂O) and nitrogen (N₂).
Achieving optimum atomisation of the injected reducing agent promotes more efficient and homogenous mixing of ammonia with NOx in the exhaust gas. This enhances NOx reduction efficiency on the catalyst and reduces the formation of urea deposits on the exhaust pipe walls.
This process requires the continuous supply of low volumes of high-pressure and high-quality compressed air, as an excessively high moisture content or impurities in the compressed air, can disrupt the atomisation process of the urea and therefore the reactivity.
Many marine SCR systems will incorporate a soot blowing system to remove accumulated particulate matter, such as soot and ash from the surface of the catalyst. Periodic particulate blow-off assists the ship operator in consistently meeting the required NOx emissions regulations by maintaining catalyst performance. It also reduces pressure drop preventing excessive backpressure, and therefore optimising engine efficiency.
Compressed air is the common medium used for this process. At periodic intervals, or based on pressure monitoring, short bursts of high-pressure compressed air are released through a network of nozzles which are strategically positioned within the SCR reactor and directed at the catalyst elements. The force of the compressed air dislodges the particulate matter from the catalyst, which is then carried away by the exhaust gas flow.
Soot blowing systems operate intermittently, and the duration and frequency of each blowing cycle may depend on factors like fuel type, engine load, and level of fouling detected. This creates a fluctuating demand for compressed air. For this reason, frequency-controlled compressors are often used.
Some components within a marine SCR system may also rely on pneumatic actuators for operation, such as dampers which regulate exhaust gas flow, valves which control the flow and therefore correct dosing of the reducing agent solution, or valves which control exhaust gas bypass mechanisms.
A reliable supply of compressed air is essential for such valves and actuators to operate correctly, which can impact NOx reduction effectiveness and the avoidance of ammonia slip - essential for overall system performance and safety.
With ECAs rapidly expanding around the world, Tier III limit values are becoming increasingly commonplace, increasing the requirement for new construction projects at shipyards and ship operators to implement and operate NOx reduction technologies like SCR. A reliable supply of high-quality compressed air is essential for the effective and consistent performance of many marine SCR systems, contributing to precise reducing agent dosing, effective catalyst cleaning and overall system control, ultimately impacting a ship operator’s ability to comply with IMO’s Tier III environmental regulations.
KAESER solutions for SCR Systems in Marine
KAESER compressed air systems present the ideal solution to energy-efficiently meet the high-quality and fluctuating air demand of marine SCR systems. Delivering more compressed air for less energy, KAESER compressors can contribute to ship operators reducing their overall carbon footprint, presenting a dual emissions reductions benefit.
Compact efficiency & maximum performance
From the compact SX (T) series air-cooled rotary screw compressors with integrated refrigeration dryer, to the ASK (T) - BSD (T) water-cooled rotary screw compressors with integrated refrigeration dryer - KAESER has a range of solutions that reliably and energy-efficiently meeting the high-quality compressed air requirements of marine SCR systems.
Designed with optimum energy efficiency in mind, the SX to BSD series rotary screw compressors save energy in a number of ways. They are equipped with the energy-saving SIGMA PROFILE rotors, and a premium efficiency IE3 motor (SX/ASK series) or super premium efficiency IE4 motor (ASD/BSD series). In addition the internal SIGMA CONTROL compressor controller coordinates compressed air generation and consumption for efficient energy usage - even in partial load operation.
Ideal for marine SCR systems where space is an absolute premium, the (T) units combine a premium efficiency rotary screw compressor with an energy-saving refrigeration dryer. The small footprint of this configuration, significantly reduces the space requirement and installation expenses.
The (T) units deliver optimum-quality, dry compressed air at all times. Operating at a high level of efficiency, the refrigeration dryer is only activated when compressed air actually needs to be dried, ensuring the required compressed air quality is achieved with maximum energy efficiency.
With flow rates from 0.26 to 8.16 m3/min (9.2 to 288 cfm), and gauge working pressure 5.5 to 15 bar (80 to 217 psi), the compact SX (T) to BSD (T) series from KAESER offer maximum availability and efficiency for optimum performance. For superyachts and small cruise ships the SIGMA FREQUENCY CONTROL (SFC) versions truly come into their own. Ideal to energy-efficiently follow transient engine operation during voyaging, the ASK (T) SFC to BSD (T) SFC series models incorporate a variable-speed drive, which precisely matches compressed air output with compressed air demand, saving energy, maximising service life and enhancing reliability.
All KAESER compressors can be accepted and approved by all notable vessel classification societies, including Lloyd’s Register, CCS, ABS, DNV, Rina, Korean Register, Bureau Veritas and ClassNK.
FAQs
The SX, ASD to BSD series of rotary screw compressors are all available with an optional integrated refrigeration dryer. These compact units offer a very space-saving solution - ideal where space is at a premium.
KAESER’s integrated refrigeration dryers provide high-efficiency performance thanks to their energy-saving control, which means they are only active when compressed air actually needs to be dried.
All maintenance work can be carried out from one side of the unit with an easy to remove cover offering excellent accessibility to components.
