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Carnival Corporation: A Deep Commitment to Sustainability (Reuters)

AAQS System Animation

The basic system is a pump, two pipes and an exhaust gas cleaning tower.
Seawater is pumped to the tower, which replaces the engine exhaust silencer.
Seawater is sprayed into the tower, where it cleans the exhaust from the ship’s engines, removing SO_x, mainly SO₂.
The seawater drains from the tower and (in port) through a filter before discharge.
Any remaining SO₂ is measured by a gas analyzer above the tower.
The seawater used is measured continuously for pH, turbidity, and PAH (oil content) in three locations (inlet, after tower, and before discharge).

The Advanced Air Quality Systems remove sulfur (SO_x) from the air emissions, preventing the airborne part of the sulfur cycle.
The Advanced Air Quality Systems also remove a high percentage of particulate matter (PM) with good effectiveness against the 10- and 2.5-micron particles and superfines.
Diesel particulate filters (“dry filters”) also reduce PM, with particular effectiveness reducing PAH.

Advanced Air Quality Systems with HFO have produced better air emissions than MGO, with much lower cost.
Advanced Air Quality Systems with HFO have typically lower sulfur (SO₂) emission levels, fewer ultrafine PMs and fewer PAHs than MGO.
Marine diesel engines are factory configured for HFO and are less efficient on MGO, leading to less CO₂ release per unit kilowatt (kW) power generated than using MGO.
MGO has a higher greenhouse gas emissions (GHG) penalty in production, as more refining is needed.

Advanced Air Quality Systems release miniscule amounts of material in the water, which is not harmful, and in the case of sulfur, it is naturally occurring in the ocean.
The concern from sulfur and PM/PAHs is from airborne impact.
The sea is the planet’s natural reservoir for sulfur and there is no environmental impact from discharging sulfates.
The small amount of soot discharged contains minute amounts of PAHs and metals, which are measured in ppb (parts per billion) and are within all major water discharge standards. This means negligible environmental impact.

Typically this is not smoke but a “white plume,” which is condensation of the water vapor entrained in the exhaust from the scrubbing process. This normally dissipates a few meters beyond the stack.
Occasionally we also see some “blue plume,” which can be more persistent. This is from light effects on remaining compounds in the exhaust, and we are working to further reduce this as well.

This is bubbles and foam caused by the agitation of seawater from the Advanced Air Quality Systems discharge, or “surface effects.”
This may also often be seen from the engine cooling water discharge.
When running Advanced Air Quality Systems outside of U.S. waters without dilution water, as in Canada or Europe, you will normally see a few bubbles and occasionally also a light film.
When running Advanced Air Quality Systems in U.S. ports, with dilution water, normally a longer discharge plume two to three meters beneath the surface may be seen, or a small cloud of microbubbles.
Occasionally excess gas in the system combined with high levels of dilution water will also create a surface agitation that creates seafoam, a natural phenomenon from agitated seawater that is often also found at the beach. We have observed that the foam dissipates quickly but when it breaks down a residual thin surface film is often temporarily visible.
High organic content in the water (e.g. algae) may lead to a brown or black foam appearance. This is most common in mid-summer to late summer as water temperatures warm up.
Lab tests of these discolored “surface effects” have typically shown zero or only trace evidence of oil, but often have high organic content.

Open loop brings in seawater for “scrubbing” the exhaust and discharges it immediately back to sea, filtered or unfiltered.
Closed loop recirculates the seawater, adding chemicals as the seawater alkalinity becomes saturated. Some seawater is bled off to a storage tank, where it is filtered and dewatered for later discharge at sea or sludge offload to designated facilities.
The most common closed-loop chemical is caustic soda, which is highly toxic and requires careful storage and handling by trained personnel.
Closed loop exhaust plume is usually much more visible than open loop.