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Marine scrubbers: what they are, how they work, and the role of nozzles

Magazine, PNR Service Lab

Marine scrubbers: what they are, how they work, and the role of nozzles

In recent years, the term “scrubber” has become familiar even outside the maritime industry, driven by growing attention to shipping-related pollution. But what exactly are marine scrubbers, how do they work, and why — from a technical standpoint — does their effectiveness depend so heavily on nozzles?

A marine scrubber, or EGCS (Exhaust Gas Cleaning System), is a system installed on board ships to reduce sulfur oxides (SOx) and particulate matter present in marine engine exhaust gas before it is released into the atmosphere.

Their widespread adoption is largely tied to IMO (International Maritime Organization) regulations limiting the maximum sulfur content of marine fuels: ships that do not use low-sulfur fuel must install an abatement system to comply with these limits.

Put simply, a scrubber consists of a tower through which the engine’s exhaust gas passes. This is known as the scrubber tower.

Inside the tower, a washing liquid — typically seawater or freshwater treated with an alkaline reagent — is atomized in contact with the exhaust. The liquid chemically absorbs sulfur oxides and captures part of the particulate matter, before being treated and either discharged or recirculated.

A marine scrubber, or EGCS (Exhaust Gas Cleaning System), is a system installed on board ships to reduce sulfur oxides (SOx) and particulate matter present in marine engine exhaust gas before it is released into the atmosphere.

Marine scrubber vs. EGCS: is there a difference?

“Marine scrubber” and “EGCS” (Exhaust Gas Cleaning System) refer to the same system — but they’re not used in exactly the same contexts.

“Marine scrubber” is the term most commonly used commercially and in everyday industry conversation. “EGCS” is the term you’ll find in class documentation, project specifications and technical datasheets — it’s the formal designation used in IMO guidelines and by classification societies.

An EGCS includes, beyond the scrubber tower and its atomizing nozzles, a washwater treatment system (for open-loop and hybrid systems, covered below), recirculation pumps, the continuous emissions monitoring instrumentation (CEMS) required by regulation, and the control systems that regulate washing liquid flow rate according to engine load.

How a scrubber works: the main system types

There are three main marine scrubber configurations:

Across all three configurations, the underlying physical principle is the same:the contact surface between the washing liquid and the exhaust gas determines pollutant removal efficiency. The finer and more uniform the atomization of the liquid, the greater the surface area available for the chemical reaction with SOx and for particulate capture.

OPEN-LOOP SCRUBBERS

use seawater directly as the washing liquid, relying on its natural alkalinity to neutralize SOx. After treatment, the water is discharged back to sea within regulatory limits.

CLOSED-LOOP SCRUBBERS

use freshwater treated with an alkaline reagent (typically caustic soda), which is recirculated on board. This is the required solution in port areas and marine zones where direct discharge to sea is not permitted.

HYBRID SCRUBBERS

combine both modes, allowing a vessel to operate open-loop at sea and switch to closed-loop near the coast or in port

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The role of nozzles and of spray lances in marine scrubbers

This is where the least visible, yet technically most critical, component of a scrubber comes into play: the nozzle system that atomizes the washing liquid inside the tower.

A nozzle not correctly sized for the scrubber’s operating conditions — exhaust gas flow rate, available pressure, washing liquid properties — can significantly reduce abatement efficiency, regardless of the quality of the rest of the system. Key factors include:

Droplet spectrum

atomization that is too coarse reduces the contact surface available for the chemical reaction; atomization that is too fine can instead lead to excessive energy consumption for liquid pressurization.

Tower cross-section coverage

the liquid must be distributed evenly across the entire cross-section of the scrubber tower, avoiding untreated gas pockets.

Material resistance

nozzles are continuously exposed to seawater, alkaline reagents, and hot, corrosive gas, which is why material selection (corrosion-resistant alloys) is critical to system lifespan.

PNR Italia designs and supplies nozzles and spray lances for marine scrubbing systems, with a droplet spectrum calibrated to each installation’s specific conditions and materials selected to withstand the most aggressive environments. The same technical approach underpins our solutions for onshore industrial desulfurization (DeSOx), an application where the operating principles — and to a large extent the nozzle technology itself — are substantially the same.

To find out more about the solutions offered by PNR Italia specifically designed for the maritime sector, visit the page “Shipbuilding Industry”. For a direct comparison with the DeSOx application on shore, the section dedicated to desulphurization on the Air pollution control page provides a comprehensive overview of the processes and products of PNR Italy.

Ask the expert

Two questions for Stefano Bissaco, Sales Manager and cooling and gas treatment expert at PNR Italia, on the technical challenges of nozzles in EGCS systems.

What are the most suitable materials for nozzles in a marine scrubber, given the extremely corrosive environment they operate in?

A scrubber nozzle lives every day with seawater, alkaline reagents, and hot, acidic exhaust gas: it’s one of the most aggressive applications we work with. For this reason, we typically use highly corrosion-resistant alloys, such as super duplex steels or nickel-based alloys, avoiding standard materials that would degrade within a few months.

Material selection is never standard: it depends on the scrubber configuration (open-loop, closed-loop, or hybrid) and the specific composition of the washing liquid used on board.

How do you prevent nozzle clogging when using untreated seawater, as in open-loop systems?

It’s one of the most common issues. Untreated seawater contains sediment, marine organisms, and particulate matter that can clog nozzles with narrow passages. The design solution works in two ways: choosing geometries with larger free passage, which are less prone to clogging than high-pressure nozzles with small orifices; and correctly sizing the flow rate and operating pressure at the design stage to maintain a flow velocity that limits deposit buildup.

Scheduled maintenance is still necessary, but a nozzle properly engineered for the application significantly reduces the frequency of interventions.

Frequently asked questions

How does a scrubber work?

A marine scrubber passes the engine’s exhaust gas through a scrubber tower, where a liquid — seawater or treated freshwater — is atomized in contact with the exhaust. The liquid chemically absorbs sulfur oxides and captures part of the particulate matter before the gas is released into the atmosphere.

What is a scrubber tower?

It’s the structure inside which the exhaust gas washing process takes place: exhaust enters at the bottom and rises through the tower, meeting the washing liquid atomized by nozzles installed along its cross-section.

What’s the difference between open-loop, closed-loop, and hybrid scrubbers?

An open-loop scrubber uses seawater as the washing liquid and discharges it back to sea after treatment. A closed-loop scrubber uses freshwater treated with an alkaline reagent, recirculated on board. A hybrid scrubber combines both modes, allowing the vessel to adapt to different operating areas.

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Spare parts for systems – whether existing, retrofitted, or new projects involving scrubbing, EGCS, and CO2 capture: Our technical team is on hand at every stage.  Send us a message, and we’ll get back to you as soon as possible.

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