Flue Gas Desulphurisation: How Ships Cut Sulphur Emissions

The shipping industry faces mounting pressure to reduce air pollution, and one technology has become central to that fight: flue gas desulphurisation. As global regulations tighten around sulphur oxide emissions, vessel operators are turning to FGD systems—commonly called scrubbers—to meet compliance requirements without completely overhauling their fuel strategies. These systems represent a pragmatic middle ground between switching to expensive low-sulphur fuel and retrofitting entire fleets with alternative propulsion technologies.

Understanding Flue Gas Desulphurisation Technology

Flue gas desulphurisation works by chemically treating exhaust gases before they exit a ship’s smokestack. When heavy fuel oil burns in marine engines, it produces sulphur dioxide—a harmful pollutant that contributes to acid rain and respiratory disease. An FGD system captures these sulphur oxides and converts them into harmless compounds through a chemical reaction.

The process typically involves spraying alkaline seawater or a freshwater-based alkaline solution into the exhaust stream. This liquid reacts with sulphur dioxide, neutralizing it through oxidation and reduction reactions. The treated gas then exits the funnel significantly cleaner, while the chemical byproducts—primarily calcium sulphate and calcium sulphite—are either discharged overboard or collected for disposal. Modern systems achieve sulphur reduction rates exceeding 95 percent, making them highly effective at meeting the International Maritime Organization’s strict emission standards.

There are two main types of flue gas desulphurisation systems deployed on vessels. Open-loop scrubbers use seawater as the alkaline medium, relying on the ocean’s natural buffering capacity. Closed-loop systems recirculate freshwater mixed with alkaline reagents, offering greater control over discharge composition but requiring onboard chemical storage and more complex waste handling. Some operators install hybrid systems that switch between modes depending on port regulations and water conditions.

Real-World Application and Industry Adoption

Since the International Maritime Organization implemented its 0.5 percent sulphur cap in January 2020, flue gas desulphurisation has become a cornerstone technology for compliance. Container ships, bulk carriers, and tankers operating on long-haul routes have increasingly adopted scrubber technology rather than pay premium prices for compliant fuels. A vessel equipped with an FGD system can continue burning cheaper high-sulphur fuel oil while meeting emission regulations, creating significant operational cost savings over a system’s lifetime.

The installation process typically occurs during scheduled drydocking, though retrofit kits have become more streamlined in recent years. A complete system includes the scrubber tower, circulation pumps, chemical dosing equipment, and monitoring instrumentation. Operators must factor in capital expenditure—ranging from $1 million to $3 million depending on engine size and system complexity—against fuel cost differentials. For many operators, the payback period falls between three and five years, making the investment financially attractive despite upfront costs.

Port state control inspections now routinely verify FGD system compliance and performance. Vessels must maintain detailed records of scrubber operations, wash-water discharge monitoring, and chemical consumption. This regulatory scrutiny has driven manufacturers to develop increasingly reliable systems with better data logging and remote diagnostics capabilities.

Challenges and Industry Evolution

Despite widespread adoption, flue gas desulphurisation technology faces legitimate criticism. Environmental groups worry about the composition of scrubber wash water, particularly from open-loop systems, which can alter local seawater chemistry near ports. Some jurisdictions, including California and several European ports, have restricted or banned open-loop scrubber discharge, forcing operators to invest in closed-loop alternatives or abandon the technology entirely in certain regions.

Technical challenges persist as well. Corrosion from acidic exhaust gases requires specialized materials and regular maintenance. System reliability directly impacts vessel availability, and mechanical failures can result in regulatory violations if the ship operates without functioning scrubber equipment. Additionally, the technology adds weight and consumes valuable deck space—considerations that matter on capacity-constrained vessels.

The industry continues evolving as alternative fuels gain traction. Some operators question whether investing heavily in FGD systems makes sense when liquefied natural gas and methanol-powered vessels are becoming commercially viable. This uncertainty has slowed new installations among shipowners planning major fleet renewals.

Flue gas desulphurisation remains the most deployed emissions control technology in commercial shipping today, with thousands of vessels operating scrubber systems worldwide. As regulations evolve and fuel alternatives mature, the role of FGD technology will likely shift, but its contribution to cleaner maritime air quality over the past five years has been undeniable and measurable.