Marine scrubbers present a range of issues in the maritime industry, including corrosion of materials, washwater disposal into the sea, and the potential for scrubbers to remove carbon dioxide (CO2) from exhaust gases. These concerns are becoming increasingly important as the shipping industry strives to reduce its environmental impact while meeting regulatory requirements.
One of the key challenges is minimizing corrosion in scrubber materials. Scrubbers, which are used to remove sulfur from exhaust gases to meet sulfur emission regulations, are typically made from alloys of chromium, nickel, molybdenum, and nitrogen. The nickel content of these alloys can range from 7% to 60%, depending on the specific material requirements. While pure nickel is known for its corrosion resistance, it comes at a high cost, with prices starting at $17,000 per tonne (2024). As a result, scrubber manufacturers must carefully balance cost and corrosion resistance when selecting materials for their systems.
The corrosion of scrubbers depends on several factors, including the type of scrubber system (closed-loop or open-loop), the duration of operation, and the specific conditions under which the scrubber is used. Closed-loop scrubbers continuously circulate the same water, while open-loop systems use seawater, which is then discharged after treatment. Seawater is particularly corrosive, and its use in open-loop scrubbers can accelerate the rate of corrosion, leading to potential maintenance challenges. The heat generated by exhaust gases, which can exceed 250°C, also exacerbates corrosion. The temperature increase can accelerate material breakdown, particularly during the start-up and shut-down phases of scrubber operation.
The importance of high-quality welding techniques is also a rising issue. Scrubber systems are often welded together, and poor welding practices can lead to corrosion-prone areas in the scrubber, such as spatter marks, weld defects, and heat-affected zones. Therefore, well-trained welders are critical to the longevity and efficiency of scrubbers. Additionally, the design of the scrubber itself plays a significant role in its susceptibility to corrosion. Areas where water or exhaust gases can stagnate, such as crevices or joints, are more likely to develop corrosion over time.
Another significant issue associated with scrubbers is the safety of discharging scrubber washwater into the sea. Open-loop scrubbers use seawater to wash out sulfur from exhaust gases, which is then discharged into the sea after treatment. However, concerns about pollutants in this washwater have led several countries to announce bans on discharges from open-loop scrubbers in their ports. Numerous studies examine the environmental impact of scrubber washwater.
These studies primarily focus on the residence time of pollutants in the water surrounding ships equipped with scrubbers. Residence time refers to how long pollutants remain in the water before being dispersed. Factors such as the shape of the berth, tides, and currents influence this residence time. The studies include various ports, including river ports and ocean ports around the world, each with different tidal patterns and berth configurations. The results were encouraging, with pollutant concentrations in washwater being well below the maximum allowed under environmental quality standards.
For example, the highest pollutant concentration observed in the studies was only 5% of the maximum allowed amount. This suggests that even if more pollutants were discharged, they would still comply with environmental standards. Studies also found that many of the substances found in scrubber washwater are already present in the sea in trace amounts.
One of the most intriguing possibilities for removing CO2 from exhaust gases is using alkaline water in scrubbers to reduce not only sulfur oxides (SOx) but also nitrogen oxides (NOx) and CO2. This approach could help the maritime industry meet the ambitious goals of the International Maritime Organization (IMO) for 2050, which includes a significant reduction in CO2 emissions.
Alkaline water is effective at reacting with acidic gases, including CO2 and NOx, forming compounds such as carbonic acid and sodium bicarbonate. Newly proposed systems would remove SOx in the first stage using conventional seawater, and CO2 and NOx would be removed in the second stage using alkaline water. This approach takes advantage of the fact that sulfur oxides are significantly more soluble in water than CO2 or NOx, allowing for a more efficient removal process. This technology could make a significant contribution to reducing emissions from shipping.
However, scaling up this technology for use on ships presents challenges, particularly in producing large volumes of alkaline water. Although this research is still in progress, it is hoped that this technology will become available as a standard in the maritime industry to meet the IMO's 2050 emissions targets. The use of alkaline water scrubbers could be a promising solution for reducing both sulfur and CO2 emissions, making it an important area of study for the future of sustainable shipping.
The challenges associated with marine scrubbers—corrosion, washwater disposal, and CO2 removal—are significant but not insurmountable. Advances in material science, such as selecting corrosion-resistant alloys and improving welding techniques, are crucial to extending the lifespan and efficiency of scrubbers. While the disposal of scrubber washwater raises environmental concerns, studies show that pollutant concentrations are generally within acceptable limits, though regulatory changes may still impact open-loop scrubbers. Furthermore, the use of alkaline water in scrubbers holds promising potential for reducing both sulfur and CO2 emissions, aligning with the maritime industry's ambitious goals for a sustainable future. Addressing these issues will require continued research and innovation, but the development of more efficient and environmentally friendly scrubber technologies will play a key role in reducing the shipping industry's environmental footprint and meeting international regulatory standards.
