Marine Scrubbers Sail Ahead With Specialist Stainless Steels
In 2020, the International Maritime Organisation (IMO) introduced new standards intended to reduce sulfur emissions that cause acid rain and air pollution. Their main impact is that the sulfur content in the fuel oil used by merchant vessels is limited to 0.5% globally and 0.1 % in designated Emission Control Areas (ECAs) – the Baltic Sea area, the North Sea area, the United States, Canada, and the United States Caribbean Sea area.
Currently, vessel operators have two main options. They can switch to very low sulfur fuel oil (VLSFO) that contains less than the 0.5 % sulfur limit, but which is more expensive. Or they can install a scrubber that washes the vessel’s exhaust gases so that it can continue to run on high-sulfur fuel (HFO).
Generally, the shipping industry uses wet scrubbers (see Figure 1). These exhaust gas cleaning (EGC) systems wash the exhaust gas stream by forcing it into contact with water to remove the sulfur dioxide (SO2), a toxic gas that is directly harmful to human health.
There are three main design arrangements for marine EGCs:
• Open-loop: This uses seawater to scrub the exhaust gas to remove SO2. The wash water is treated and discharged back to sea.
• Closed-loop: Fresh water is used in a closed circuit that is treated with an alkaline chemical such as caustic soda.
• Hybrid: This can be used in either open- or closed-loop mode according to operational needs.
Figure 1 – Open-Loop EGC system (credit: Exhaust Gas Cleaning Association).
Material selection is critical for reliability
The conditions inside a scrubber are very harsh, with a combination of acids, elevated temperatures and a high chloride content in the wash water. To ensure the desired 20-year service life, high-alloyed stainless steels and nickel base alloys are required. It can be devastating to select a material with insufficient corrosion resistance. Yet the safest choice is seldom the most economic one.
Corrosion types and service conditions
There are three main types of corrosion risk found in marine scrubbers:
• Caused by a critical combination of chlorides, high temperatures and low pH
• Stainless steel is protected against corrosion by a natural, self-forming passive oxide film on its surface. If a local breakdown of this protective film occurs it causes the formation of pits
• Caused by the critical combination of chlorides, high temperatures and low pH and the presence of crevices
• Typically occurs in very acidic conditions
• Uniform corrosion rate across the surface
At the inlet, where the uncleaned gas meets the wash water, there is a mix of wet and dry conditions, with condensing acids from gas and chloride-rich wash water and elevated temperatures. These conditions are very aggressive with a risk of uniform and pitting corrosion.
Within the scrubber body there is a slightly acidic wash water combined with a lower temperature than the inlet. The main risk here is pitting corrosion. Similar conditions apply for other parts of the scrubber system, such as packing material, demisters, tubing and spray nozzles
Crevice corrosion may occur in areas where two surfaces are in close contact, such as flanges, packing material and under deposits.
The influence of scrubber type on material selection
In the open-loop design, the presence of chlorides typically requires the use of high-alloyed materials. In the closed-loop design, where water is recirculated and treated with caustic chemicals, the corrosivity depends on the quality of the wash water. Typically, the conditions are less aggressive than in the open-loop design.
Since the hybrid design alternates between open- and closed-loop operation, materials must be selected according to the most aggressive operating conditions. Therefore, the same materials are used as in an open-loop scrubber.
Operational experience and field testing are vital
Outokumpu has been supplying stainless steel for marine EGCs since 2006. It carries out extensive R&D activities including laboratory testing and long-term field testing in cooperation with marine scrubber OEMs and their customers. The results show that material selection must always be done on a case by case basis.
For the scrubber body, Ultra 254 SMO is one of the most widely used grades. This is a 6% molybdenum and nitrogen-alloyed austenitic stainless steel with extremely high resistance to both uniform and localized corrosion. It was developed initially for oil and gas offshore platforms and the pulp and paper industry.
However, in some cases it is possible to use a 25Cr super duplex stainless steel such as Forta SDX 2507. This offers similar corrosion resistance to Ultra 254 SMO and improved mechanical strength. It is well proven in extremely corrosive environments such as desalination, chemical, or offshore subsea applications.
Standard 22Cr duplex stainless steel can also be used successfully in less aggressive applications such as closed-loop designs and some parts of open-loop scrubbers. A typical example is Forta DX 2205, which is the most popular duplex product on the market. It offers very good resistance to uniform and localized corrosion in combination with high mechanical strength.
Ultra 654 SMO – an alternative for scrubber inlets
As the entry point for hot exhaust gas, the scrubber inlet is a particularly demanding application. Traditionally, Alloy 31 has been the material of choice. It is an iron-nickel-chromium-molybdenum alloy with nitrogen addition developed to fill the gap between special alloyed austenitic stainless steels and nickel alloys.
The test program has established that Ultra 654 SMO offers a viable and more cost-effective alternative. This is a 7% molybdenum and very high nitrogen-alloyed austenitic grade that is the most corrosion resistant stainless steel in the world.
Ultra 654 SMO has a higher mechanical strength than Alloy 31. If the design of the inlet enables this higher strength to be utilized then it could offer weight savings between 15 to 25%. Furthermore, the lower nickel content of Ultra 654 SMO also makes it cost-effective and more stable in price.
Outokumpu currently supplies stainless steel material for use in the construction of around 60% of the marine scrubbers installed every year. Two typical examples include:
Ecospray Technologies S.r.l. Italy (Figure 2) needed a highly corrosion resistant material for its exhaust gas cleaning scrubbers. Ultra 254 SMO proved to be the best combination of performance, price and dimensions.
Figure 2 – Ecospray Technologies scrubber in Ultra 254 SMO.
ContiOcean Environment Tech Co., Ltd. delivers open-loop scrubbers to cargo ships of various size classes. Forta SDX 2507 provides the necessary corrosion resistance for the harsh conditions in the scrubber body (see Figure 3).
Figure 1 – Open-loop marine scrubber manufactured by ContiOcean Environment Tech Co., Ltd in super duplex stainless steel
Summary – key facts about materials for marine scrubbers
• Material selection is the most critical factor in helping ship owners to ensure the reliability of their exhaust gas cleaning (EGC) systems over a 20-year service life
• A wide range of stainless steels can be used
• Conditions and corrosivity vary depending on location and type of scrubber
• Materials selection should always be made according to the specific operating conditions
• Duplex grades can often replace the dominating austenitic choices
• For the toughest situation found in the scrubber inlet Ultra 654 SMO offers a very competitive alternative to Alloy 31