Onboard carbon capture: a feasible pathway to net zero emissions for shipping

-- This article was originally published by New Energy World on 25/9/2024 --

Shipping faces a formidable challenge: to reduce greenhouse gas (GHG) emissions to net zero by 2050. While several technologies are being developed, the first seagoing zero emission ship of any significant size is yet to be built. However, onboard carbon capture (OCC) could facilitate swift emissions reductions and reduce dependencies on nascent sustainable fuels, write Joan van den Akker from shipping company Conoship International and Richard L Stevenson from Scottish Carbon Capture & Storage.

To achieve net zero emissions by 2050, shipping is exploring several low- or zero-emission alternatives for fossil fuels. So far, no single solution has been found, and it seems that several parallel developments will be key to meeting the 2050 target.
 
Wind propulsion is one of the oldest methods for ship propulsion. It can significantly reduce fuel consumption but, with ships that must meet their schedules regardless of wind conditions, near-zero GHG emissions will not be achieved using wind power alone.
 
Batteries are a very efficient way to use electricity, with huge potential for short and medium-range shipping. However, the energy density of today’s batteries is not sufficient for long-distance shipping.
 
Biofuels and e-fuels are technically the ‘easiest’ way to reduce emissions, by replacing conventional fossil fuels with cleaner, sustainable equivalents such as biodiesel or e-diesel (synthetic diesel produced using renewable electricity). The availability of these fuels is extremely limited, however, and this is not expected to change soon.
 
Other technologies use hydrogen or ammonia; fuels that don’t contain carbon and will thus not cause any CO₂ emissions at the point of use. But these fuels lack appropriate engine (or fuel cell) technologies. Moreover, such fuels are only sustainable if they are made using renewable electricity. Today, most hydrogen and ammonia are derived from natural gas, so the emissions are merely shifted from the ship to the production of the fuel, rather than reduced.
 
The common limitation: availability of renewable energy
Despite technological developments, the quick uptake of most low- or zero-emission solutions for shipping all relies on the availability of a large supply of renewable energy. This energy is arguably better used elsewhere and, in any case, it is currently not available at the required scale, nor is it expected to be so any time soon. This is illustrated by recent reports that new ships in the fleet of Maersk, a company known as a strong advocate of switching to green methanol, will be fueled by LNG, because the green methanol supply chain is simply not there yet.
 
Additionally, current uncertainty regarding which fuels will become dominant in shipping is a risk: a shipowner could invest heavily in a new vessel capable of running on ammonia, but if the production of green ammonia does not pick up, it might be left with a very expensive ship and no emission reductions at all.
 
Onboard carbon capture 
One of the very few solutions to the limited availability of cleaner energy to produce zero emission fuels is to install onboard carbon capture (OCC). Here, the CO₂ produced in the ship’s engine is captured before it leaves the exhaust and stored onboard until the ship reaches a port where it can be discharged and transported on for utilisation or permanent geological storage.
 
Carbon capture technologies have been operational for many decades across power and industrial sectors and, generally, climate scientists and policymakers around the world see carbon capture (and storage) as indispensable in achieving required GHG reductions.
 
There are several advantages in installing carbon capture onboard ships that make it a valuable addition to the toolbox of solutions for shipping. First, the technology is proven on land, and there are several parties, such as the partners of the EverLoNG project, that are working to marinise it. Second, the technology is developed as an aftertreatment, so it can be added as an ‘expansion’ to a conventional propulsion system. This makes it possible to retrofit a carbon capture system to an existing ship.
 
This is very important, considering that the majority of the current order book is based on conventional fossil fuels, and ships being built today will very likely still be operational in 2050.
 
The CO₂ produced in the ship’s engine is captured before it leaves the exhaust and stored onboard until the ship reaches a port where it can be discharged and transported on for utilisation or permanent geological storage.
 
OCC also permits immediate action, allowing shipowners to take matters into their own hands instead of waiting for zero-emission fuel supply chains to develop. Moreover, even when these fuels do become available, OCC will still be an efficient way to reduce emissions. A ship fitted with a carbon capture system could even operate with net-negative emissions if it is fuelled with a biofuel or e-fuel and the resulting CO₂ is stored. Alternatively, the captured CO₂ could be used to produce synthetic fuels, creating a nearly closed, circular carbon loop.
 
So, are there no drawbacks to OCC then? Of course there are. The systems are complex and expensive, and they require additional energy to operate, leading to increased fuel consumption. And the logistics chains for offloading, transporting and permanently storing onboard captured CO₂ are not fully developed yet. None of these, however, are showstoppers. As wider CCS developments continue apace, EverLoNG is but one project tackling these issues head on to help the entire playing field move forwards.
 
Just like other low- and zero-emission technologies, carbon capture is not a silver bullet, but it is a very valuable option for shipowners and it will help accelerate the reduction of GHG emissions so that the maritime sector might achieve its 2050 goal.
 
The views and opinions expressed in this article are strictly those of the author only and are not necessarily given or endorsed by or on behalf of the Energy Institute.