A scrubber hoisted on a ship as close as possible to the chimneys

A "scrubber" is a system for filtering exhaust gases from heavy-fueled ships' engines.


Sailing propulsion

Historically, in the merchant marine, the last clean energy to have been used was sailing, scientifically called sailing propulsion.

Sailing propulsion was replaced by a stable source of energy, coal, which when burning produced steam from water. The elevation of the pressure is used by a steam motor to transmit energy to the vessel propeller. We know today that the combustion of coal produces noxious fumes for man and his environment.

The coal was then replaced by diesel engines, which operates in the maritime heavy fuel oil, that is to say a fuel even less refined (less pure) than the diesel fuel of land vehicles. So even more polluting and illustrated on numbers of postcards, films and other images of the last century and the first two decades of the current century, by thick black smokes over the chimneys.


Ship exhaust fumes without "scrubbers"




The propellant quota of an energy, a key indicator in maritime transport


Faced with the growing imperative of finding so-called "clean" energies, all sectors of the economy are dedicated to finding alternative energy that is less polluting than those currently in place - in transportation in general and in the marine in particular, the 3 main factors to be taken into account in this search for new energy are the immense distances to be covered (intercontinental journeys), the weight of the ship itself (several hundreds of thousands of tons) and the carrying capacity (number of passengers for the cruise, number of containers, weight or volume of the goods for the bulk, etc ...).

Unfortunately, the marine has not yet found propulsion energy that can equal the energy conversion efficiency of carbon energy. Therefore, pending a more than hypothetical return to sailing propulsion, the vast majority of ships in service (99.7% in 2017) of the world fleet and frequenting waters with regulated air discharge (and running on heavy fuel) is seen obliged to have "scrubbers", a filtration system inserted between the exit of the exhaust pipes of the engine and the apparent chimneys of the ships. How does it work ?



How does a scrubber work ?

A "scrubber" is a chamber capable of withstanding a pressure of between 150 and 200 bar, in which several systems are simultaneously operating to "wash" the gases evolved by the combustion of the heavy fuel engine. This washing is done first with clear water and then with a glycol-based additive.

The "scrubber" fits just before the exhaust outlets 

The scrubbers are to be inserted in the exhaust line of the combustion gases, they are between the engine and the apparent outputs of the chimneys of a vessel. Contrary to what one might think, a "scrubber" still weighs between 15 and 20 tons. And on passenger ships, there is hardly any place other than the highest possible near the final exit of the chimneys. The installation of "scrubbers" has a significant impact on the stability of vessels. When vessels are equipped with an automatic calculation system, there is no option but a reconfiguration of it.


On a classic ferry, it is estimated that the "scrubbers" trap about 500 tons of CO² (carbon dioxide) year round. On a last-generation container ship, the same catch is estimated at more than 2,000 tons a year. A "scrubber" is therefore good for the environment, since it limits the emission of greenhouse gases. But it is also good for human health and not only, since it also traps:

- sulfur releases (SOx)

- releases of nitrogen oxide (NOx)

Hence the promulgation of laws at European level to protect human health in addition to the protection of the environment by establishing SECA zones. On this subject, read the article
"SECA" zone, what is it ?


Functional diagram of a "scrubber"


The "scrubber" remains only a waiting solution 

But in practice, the installation of "scrubbers" is only an alternative solution while waiting for a much more effective solution to fight against global warming. Indeed, studies have shown that "scrubbers" only capture about 20% of CO² emissions from vessels: this is insufficient to hope to stem the decrease in the quality of the air we breathe daily.

Worse, the non-harmonization of legislation in the four corners of the planet makes it possible to see the appearance of so-called “open-loop” “scrubbers”: which means neither more nor less than the sulfur captured by ten tons by the “scrubber system" is discharged directly into the sea, with no obligation to store on board for destruction / recycling on shore. Direct consequence : accelerated acidification of the already poor oceans.



What alternatives to diesel for maritime transport?

Some shipowners then turned to another fossil fuel wrongly considered less polluting : LNG (or Liquefied Natural Gas).



Why is LNG considered less polluting?

Because unlike diesel / heavy fuel oil, there is no release of soot, dust or smoke. And the combustion of LNG produces 20% less CO² during its combustion than heavy fuel oil / marine diesel. On this specific point of CO² emissions, a ship equipped with a scrubber or an LNG fuel ship are more or less equal in terms of CO² emissions. But the comparison stops there.



Indeed, the other negative points of LNG are not lacking :

  • as fossil fuel, even if over the same distance traveled by the ship, LNG emits less CO² (20% only), it still produces some (80%),
  • as a fossil fuel, LNG is, like oil, in limited quantity on Earth. Like oil, some extraction fields are easy to access and do not require heavy recourse to oil to extract it, but like oil, the more LNG we consume, the more it will be necessary to go search in hard to reach places. We are not even talking about shale gas, which through excessive use of water is leading ecological nonsense.
  • LNG, as its name suggests, is a liquefied gas and it is precisely its liquefaction process which is a very, very large consumer of energy.
  • studies carried out as part of the search for alternative resources to heavy fuel oil have shown that "accessible" (easy tp roduce) LNG reserves would be exhausted by 2070 at the latest ! !

The only positive point of the conversion from heavy fuel oil to LNG maritime transport is that the technology on board LNG ships is the same for later switching to bio-LNG, otherwise called bio-methane. Bio-LNG is a gas obtained from waste, and therefore much less polluting, without any comparison, with the current LNG of fossil origin. But the whole sector is almost to be built...

This in no way solves our inability to consume LESS energy year after year ! !



What are the other alternatives ?

Today there is none : the propulsion quota of heavy fuel oil / marine diesel / land diesel is unmatched, both at sea and on land.



On the other hand, there are a number of options to be exploited in parallel :

  • reducing the speed of ships drastically reduces consumption: from 20 to 50% depending on the maritime relations in question,
  • make better use of sea currents so that ships "let themselves go", even if it means traveling a longer distance but polluting less,
  • have recourse to sailing propulsion in addition. Companies such as the French Airseas or the German SkySails are working on the manufacture of an automatic deployment system for a sail in "kite surf" format to benefit from the altitude winds naturally available above the vessel. At the end of it, an announced decrease in consumption of at least 20% or in other words, equivalent to 2 MW of the engine power of the ship.


Skysails already equips "small" general cargo vessels

Skysails Group - Airseas


But contrary to what one might think, resistance to change is strong, whatever the field concerned. Here are a few examples :

  • for industry: longer travel times, but less polluting, this is equivalent to an increase in the cost of the stock from an accounting point of view. While manufacturers have continued to put pressure on shipping companies to reduce transport times and make stopover dates more reliable, forcing them to remove from economic reality a risk that has been quite natural since the dawn of time. : maritime risk (storm, strike, collision, etc.).
  • for shipping companies, reducing the commercial speed of ships means reducing the power of on-board engines. This is not without consequence when monsters over 400 meters long and over thirty meters in height find themselves caught in storms. The drop in engine power has already been pointed out many times by sea rescue professionals. Even our national flagship, the ABEILLE LIBERTE, will not be able to prevent such a monster from running aground : the tension on the cable trailer goes beyond what any material can withstand in such a circumstance when the elements are raging furiously.
  • for ports, connecting ship engines to shoreside energy to eliminate atmospheric pollution once alongside is already a difficult solution to make it emerge in rich countries (G7), then outside this restricted circle ...


And these are just a few of the most meaningful examples ...





The operation of laying scrubbers on a ferry in pictures




Les fumées avant mise en place de "scrubbers"


Dépose de la carrosserie au niveau des cheminées


Fumées d'échappement sur un navire équipé de "scrubbers"





Related articles :

"SECA" zone, what is it ?

"Cold Ironing", what is this ?







The ABEILLE LIBERTE - IMO 9308699 - belongs to the company LES ABEILLES INTERNATIONAL based in France

"When the ABEILLE LIBERTE leaves (the port), everyone goes back (to the port)"

Olivier de Kersauson