Part easy, part hard.
The immediately polluting emissions can be removed from exhaust gases using scrubber technology. Basically, scrubbing is about chemically binding harmful gases as salts that can be either stored on board and processed later or flushed. The technology to do this is well tested and installed on many vessels today.
It is also possible to select a fuel that has a low sulphur content to begin with. These fuel types are considerably more expensive than heavy fuel – typically about 50 percent or so. Environmentally speaking, this is the best option as the harmful substances such as sulphur are removed and handled by the refinery.
Removing CO2 from emissions is not so easy, however. Firstly, the amount of CO2 generated by fuel combustion is quite large, since every kilo of fuel burned produces around three kilos of CO2. Using a reasonably clean fuel (i.e. low sulphur), this means combustion generates 50-100 times more CO2 than SO2. The volume alone makes scrubbing impractical and the technology to do so is not ready.
For new vessels, there are several options, depending on operating conditions, for reducing CO2 emissions. One very important component to consider is the engine, naturally. Traditional wisdom dictates you should select an engine that is efficient at the typical load the vessel will experience. This is good advice, but even so, there are still a lot of options.
- Modern diesel engines are very efficient, even if the basic technology is old. It is possible to achieve very low consumption with a new engine – even as low as 160 g/kWh. Compare this to a petrol car engine that commonly uses 400 g/kWh. The improvements to diesel engines are incremental in nature, however. There are no revolutionary leaps and bounds here, but the results are still very impressive and the price level of classic diesel engines is typically attractive.
- Selecting a dual fuel engine makes it possible for a vessel to run on natural gas (LNG) which results in lower CO2 emissions, compared to heavy fuel oil. That translates to a lowering of CO2 emissions by about a fifth, but natural gas does have some limitations that make it unsuitable for long haul vessels. Specifically, LNG takes up about twice the volume of heavy fuel and bunkering of LNG is not widely available. Operating LNG equipment is also more demanding and requires training.
- Electrical propulsion packages are also growing in number on ferries and other vessels that are able to reliably charge battery packages whenever necessary. Batteries take up space and the energy density of batteries is nearing the limits of what is possible. At the same time, the energy density of even the best batteries is about 100 times lower than that of diesel for instance – they take up a lot of space. That makes batteries a solution for short distances only.
- In the long run, nuclear propulsion might even be a possibility. That would be a zero emission solution but one that comes with a completely different set of challenges. The technology is available and well tested in military applications, but there are virtually no civilian nuclear vessels around. This is largely due to public opinion but also because implementing nuclear propulsion would mean retraining technical staff on a very large scale to handle this new technology.