Exhaust Gas Cleaning System

An exhaust gas cleaning system, universally called a scrubber, is fitted to a ship's exhaust so that it can meet the international limits on sulphur oxide emissions without switching to expensive low-sulphur fuel. Since the IMO 2020 rule capped the sulphur content of marine fuel at 0.50 percent, and far lower inside emission control areas, ships have had two main routes to compliance: burn compliant low-sulphur fuel, or continue burning cheaper high-sulphur heavy fuel oil and clean the exhaust with a scrubber. The scrubber sprays the exhaust with an alkaline wash water that absorbs the sulphur oxides and captures much of the particulate matter before the gas is released.

Scrubbers come in three broad configurations. An open-loop system draws in seawater, whose natural alkalinity neutralises the sulphur, and discharges the wash water back to the sea after treatment. A closed-loop system recirculates fresh water dosed with an alkaline chemical and retains the residues for disposal ashore, discharging little or nothing to the sea. Hybrid systems can operate in either mode, running closed-loop where open-loop discharges are restricted. The choice affects both cost and where the ship can legally operate.

Scrubbers are commercially attractive when the price gap between high- and low-sulphur fuel is wide enough to repay the substantial installation cost, but they are also controversial. Open-loop systems shift a pollutant from the air into the sea, and a number of ports and coastal states have banned or restricted the discharge of open-loop wash water in their waters, which complicates voyage planning. Critics also note that scrubbers do nothing to reduce carbon dioxide, so they address the sulphur rules but not the industry's climate obligations under measures such as the EEXI and CII. As a result, scrubbers are one of several compliance strategies rather than a universal solution, and their economics shift with fuel prices and regulation.