Burning fuel oil in the ship's engines generally causes emissions into the air. This cannot be fully avoided. Harmful emissions mainly are sulphur oxides (SOx), nitrogen oxides (NOx) and particles. These exhaust gas components cause health problems and damage the environment and therefore need to be reduced.
Already back in 1995, TT-Line started to operate the environmentally friendly new ships Robin Hood and Nils Dacke which were dubbed 'Green Ships'. Among a lot of new features these two sister ships have diesel-electric propulsion and were burning clean low sulphur fuel. This concept was repeated and refined by the installation of podded propulsors with the next ship generation Nils Holgersson and Peter Pan coming into service 2001.
Diesel-electric propulsion systems are distinguished by diesel engines just driving electric generators, thus providing electric energy for the ship's power demand. This means that also the propellers are driven by electric motors. Because in this configuration the diesel engines can always run on constant speed, NOx emissions, among others, remain relatively constant even under not optimal operating conditions.
The International Maritime Organisation (IMO) through its MARPOL-Convention nowadays imposes strict rules for air emissions from ships. SOx is addressed by limiting the sulphur content in the fuel because sulphur oxides are generated from that sulphur during the combustion process. NOx is regulated for new engines by setting limits for its concentration in the exhaust gas.
For ships operating in the Baltic Sea which is one of several defined SOx Emission Control Areas (SECA), MARPOL rules require from 1st January 2015 on to use fuel which may not contain more than 0.1 % sulphur. This represents a reduction by 90 % compared to the limit valid before and an enormous challenge for shipping.
The easiest way to meet these requirements is to use low sulphur fuel oil which is a distillate fuel different to the residual fuels of before. Because such fuel is significantly more expensive, operating costs increase resulting in increased costs for transportation. One alternative is to install exhaust gas cleaning technology on board. In this case fuel oil with higher sulphur content can still be used. But such installation must be technically possible and economically feasible.
TT-Line decided to install an exhaust gas scrubbing system on our ship Nils Dacke in summer 2014. The scrubber removes SOx and particles from exhaust gases. This technology is in a developing stage for the operation on board ships. This pilot installation is one of the first hybrid wet scrubber systems ever installed on such a type of ship. It was co-financed by the EU within the TEN-T project "Green Bridge on Nordic Corridor".
For the future, another promising way to reduce sulphur emissions to almost zero is changing to alternative fuels like natural gas in liquefied form (LNG). But due to high costs for retrofits on existing ships we see this as an alternative for new ships only.