Exhaust gas cleaning systems (EGCS) are also referred to as marine scrubbers and are installed aboard large ships in order to reduce emissions from their large diesel engines.
Legislation introduced by the International Maritime Organisation (IMO) limits the production of sulphur gas (SOx) and other particulates, to a maximum of 0.5%. In some regions, this limit has been set even lower, at 0.1%. EGCS are being implemented as a cost-effective solution in order to meet these legislative requirements, both as new-build and retrofit to existing vessels.
In a simple exhaust gas cleaning system, the exhaust gases are passed through a spray of seawater. The composition of seawater, naturally containing bicarbonate, is usually enough to neutralise any acidic gases present although the seawater can also be dosed with an alkaline addition if required.
There are a variety of EGCS designs. At the simplest, there is a ‘once-through’ system, where a continuous stream of seawater is pumped into the scrubber from the ocean and discharged straight back again after any sludge or solids are removed. A ‘recirculated’, or closed-loop system, ensures there are no continuous discharges back to the ocean, and the scrubbing water is recirculated through the tower. This system requires additional holding and treatment tanks, adding cost and taking up more space within the ship. Finally, a ‘hybrid’ system combines both approaches and can offer cost-effective operating costs if the ship is travelling through regions with different sulphur limits.
Parts of the system handling seawater should ideally be constructed from super duplex stainless steels, with a pitting resistance equivalent number (PREN) >40. Alternatively, super austenitic stainless steels, such as 6Mo (254SMO) also meet this level of corrosion performance but at a higher cost. Polymer piping can be used, but not if passing through the ship’s bulkhead for structural safety reasons.
The use of super duplex stainless steels may be restricted in the scrubber tower if there is no bypass to divert the hot exhaust gases during periods of non-operation. This means that more expensive materials will need to be utilised, capable of operating from 250-450degC, such as nickel-based alloys like C-276, C-22 or Alloy 59.
Pumps, valves and other equipment items included within the overall system will also utilise super duplex stainless steels to resist corrosion by the seawater. Langley Alloys carries a complete stock of SAF2507 (S32750, F53, 1.4410), S32760 (F55, 1.4501) and Ferralium 255-SD50 (F61, 1.4507) in bar form up to 16” (406.4mm) diameter and plate up to 3” (76.2mm) thickness.