Global shipping transports around 80% of all goods on our planet. It is a piece of our modern world puzzle that we cannot do without, and our prosperity is entirely dependent on global shipping.
We have a responsibility to ensure that the negative aspects of shipping are minimized. Among these negative aspects is the fact that shipping emits about 3% of human-made greenhouse gases and accounts for up to 15% of all local air pollution. Until 2020, part of this air pollution consisted of sulfur oxides from ship exhaust.
SINTEF Ocean works on many fronts to find solutions that minimize emissions from shipping. We pursue this goal fully aware that there are no alternatives to this mode of transport, and that all transportation will have some impact on the environment.
Introduced by the IMO’s Marine Environment Protection Committee (MEPC).
Introduced by the International Maritime Organization (IMO), which operates under the United Nations.
These guidelines have since been updated, expanded, and developed to optimize the benefits of the technology and to reduce environmental impact as much as possible.
A ban on the use of marine fuels with a sulfur content higher than 0.5% was introduced, and a stricter limit of 0.1% was set for particularly sensitive areas. Prior to this, fuels with up to 3.5% sulfur content were allowed. The new regulations represented a significant step toward reducing air pollution from ship exhaust. These rules were implemented by the IMO’s Marine Environment Protection Committee (MEPC).
A necessary evil
More than 75% of all fuel used in global shipping was—and still is— heavy fuel oil. Heavy fuel oil is a collective term for the residual product from crude oil refining, and most of it contains sulfur.
In a ship engine, this sulfur contributes to cylinder lubrication in addition to being burned into sulfur oxides (SOx). SOx is a group of compounds that cause respiratory problems in humans and animals and contribute to the acidification of waterways and soil. However, in the ocean, SOx are neutralized due to seawater’s high buffering capacity for pH, as seawater is alkaline.
Supply and demand don’t always go hand in hand
Between 2016 and 2020, there was a growing concern about whether there would be enough low-sulfur fuel available for global shipping, due to uncertainty about refineries’ capacity to remove sulfur from fuel oil. It was assumed that there would be a shortage of low-sulfur fuel, leading to high prices. Therefore, it was decided that ships could use alternative methods to remove sulfur emissions, namely scrubbing using seawater.
The original purpose of scrubbing was to remove SOx from the air, not from the sea. SOx released from a ship’s exhaust mixes with the air and is washed out by rain. The same happens with soot and particles, most of it eventually ends up in the ocean.
By scrubbing the exhaust with seawater in the funnel, the same process is essentially replicated: SOx are transformed into SO₄—sulfate—which occurs naturally in seawater. The background concentration of sulfates in seawater is approximately 2700 ppm. If all the world’s ships used fuel oil and seawater scrubbing for 150 years, the concentration would increase by less than 2 ppm.
Scrubbing improves air quality
In other words, scrubbing is a zero-sum game for the ocean, but a significant improvement for air quality. With one caveat: emissions into the sea can become concentrated in smaller areas if this is not properly managed. This is where the MARPOL regulations come into play, ensuring that such concentration does not occur.
There are two main types of scrubbing systems. One uses a closed-loop system where the wash water is treated, chemicals are added, and the water is reused. The other is an open-loop system where seawater is taken in, passed through the scrubber, cleaned, and then discharged back into the sea in accordance with MARPOL.
One of the major questions is: how much pollution is released into the sea as a result of scrubbing? It is important to emphasize that scrubbing does contribute to pollution—the question is not if, but how much. It should also be noted that the scrubber itself does not add any substances; it only uses seawater as a medium.
Polluted water in = polluted water out
This question can be addressed by sampling the water discharged from scrubbing and analyzing it for the substances of concern. For substances where the analysis shows zero, the value is set to half the detection limit. If these discharge values are then input into a dispersion model, one can visualize how the wash water may affect the seawater the ship travels through.
If we assume that the seawater taken into the ship for scrubbing is pristine, this method will attribute all pollution to the scrubbing process.
The wash water used in a scrubber is drawn from a sea chest on the side of the ship. The hull of a ship is often coated with substances designed to prevent marine growth on the hull—so-called biocides, which, as the name suggests, are intended to inhibit or kill marine organisms.
These are permitted for use on ships but also result in the release of substances into the ocean that the marine environment would be better off without. When a ship sails through already polluted waters—such as shipping lanes, port areas, and coastal zones—this pollution enters the scrubber along with the seawater. All of this introduces substances into the system, which are then discharged back into the sea.
It is important to understand that these substances are present regardless of whether the ship uses scrubbing or not, and they are released into the water as the ship moves through it.
To determine the net pollution added by exhaust gas scrubbing, the same analysis must be performed on the water going in as on the discharged water. The pollution entering the system must then be subtracted from the pollution exiting. This way, one can determine the actual contribution of the scrubbing process to marine pollution.
Emissions from alternatives to fuel oil must be mapped
The alternative to using scrubbing is to use low- or ultra-low-sulfur fuel oil—or marine diesel oil. As previously mentioned, sulfur primarily functions as a lubricant in the engine cylinder, and this property must be replaced with another substance in low- and ultra-low-sulfur fuel oils.
The current rules do not cover this, and thus we do not know which substances replace sulfur. No systematic studies have been conducted on the additional pollution that may result from these new additives used to maintain cylinder lubrication. Furthermore, all exhaust from ships is exposed to weather, wind, and precipitation—and most of it ends up in the ocean.
Removing sulfur from fuel oil is an energy-intensive process. SINTEF Ocean has examined the consequences of this in terms of greenhouse gas emissions and found that such removal can result in total emissions that are 4–5% higher than those associated with using scrubbing. Other studies have found greenhouse gas reductions of up to 25%.
Today, approximately 6,000 ships operate with open-loop scrubbers. Very few newbuilds are being ordered with such systems. According to one of the largest manufacturers of these systems, the number could grow to around 15,000 ships by 2050, assuming that onboard carbon capture and storage becomes widespread—since these systems are similar to SOx scrubbers.
SINTEF Ocean works to develop methods and technologies to reduce marine pollution from shipping, and we support the ongoing efforts to achieve this. We have been addressing these issues since our inception more than 50 years ago. Through our fact-based research we achieve results that have a real impact on the choices made by shipowners and operators worldwide. That is our promise, and we will keep it.
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