Sulphur contained in the ship's fuel is converted during combustion into sulphur oxides (SOx). One way to reduce SOx is to remove it from the exhaust gas by means of cleaning technology. Basically, there are two different options to clean exhaust gases on ships.
One option, the so called wet cleaning systems are usually referred to as "scrubbers". In a scrubber the exhaust gas is led through sprayed water which absorbs the sulphur compounds. Scrubbers can be designed for either operating in an open loop process or closed loop or both.
Open loop means that sea water, which has a natural capability to absorb SOx is pumped through the scrubber and then discharged back into the sea. While in closed loop operation, water is circulated in the system but chemicals need to be added.
Another exhaust gas cleaning technology option are dry absorbers which use calcium hydroxide granulate for absorbing SOx. Gypsum is created as an end product of the process which need to be collected on board for disposal ashore.
A disadvantage of exhaust gas cleaning technology of both types is the space needed on board and the additional weight which might sum up to several hundred tonnes. Dry "scrubbers" require even more space than wet types and are heavier. Installing scrubbers on existing ships can be a challenge.
After thorough investigation TT-Line opted for the installation of a hybrid (combined open and closed loop) wet scrubber system of the Finnish company Wärtsilä. This system was installed on the RoPax vessel M/S Nils Dacke in 2014 and comprises a total of four scrubbers, one for each diesel engine. To create space for them, two newly built funnels had to be mounted on the ship, replacing old funnels.
The space needed for auxiliary equipment like pumps, separators, tanks, control cabinets etc. could only be found in two newly created machinery spaces separated from an existing cargo hold. This arrangement is reducing the ship's loading capacity but a good part of the system weight is placed low in the ship thus balancing the weight of scrubbers in the funnels.
However if properly designed and a feasible solution for installation has been found, scrubbers are a good way of reducing air emissions from ships - not only SOx but also particles. At the same time an economical sound ship operation can be secured by omitting expensive fuel types.