Exhaust gas cleaning systems (EGCS), also called marine scrubbers, are installed aboard large ships to reduce emissions from their diesel engines.

Legislation from the International Maritime Organisation (IMO) limits sulphur gas (SOx) and other particulates to a maximum of 0.5%, and to 0.1% in some regions. EGCS are a cost-effective way to meet these requirements, both on new-build vessels and as retrofits.

How an EGCS works

In a simple system, the exhaust gases pass through a spray of seawater. Seawater naturally contains bicarbonate, usually enough to neutralise the acidic gases, though it can also be dosed with an alkaline addition if needed.

System types

  • Once-through: a continuous stream of seawater is pumped into the scrubber from the ocean and discharged straight back after sludge and solids are removed.
  • Recirculated (closed-loop): no continuous discharge to the ocean; the scrubbing water is recirculated through the tower. It needs additional holding and treatment tanks, adding cost and taking up space.
  • Hybrid: combines both approaches and can offer cost-effective running for ships travelling through regions with different sulphur limits.

Material selection

Parts handling seawater are ideally built from super duplex stainless steels, with a pitting resistance equivalent number (PREN) greater than 40. Super austenitic stainless steels such as 6Mo (254SMO) meet the same corrosion performance but at higher cost. Polymer piping can be used, but not where it passes through the ship's bulkhead, for structural safety reasons.

Super duplex use may be restricted in the scrubber tower itself if there is no bypass to divert the hot exhaust gases during periods of non-operation. In that case, more expensive materials capable of operating from 250-450degC are needed, such as nickel-based alloys like C-276, C-22 or Alloy 59.

Pumps, valves and other equipment in the system also use super duplex stainless steels to resist seawater corrosion. We carry 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.