ONBOARD carbon capture and storage faces significant drawbacks despite its potential for reducing emissions.

The Global Centre for Maritime Decarbonisation’s latest report shows the technology’s potential, but also its downsides.

The report found using conventional OCCS to capture 40% of the CO2 from an medium-range tanker running on heavy fuel oil could save 29% of well-to-wake greenhouse gas emissions, at a cost of between $269-$405 per tonne of CO2.

That ship could therefore avoid penalties from the International Maritime Organization’s new green rules until 2032. An liquified natural gas-fuelled ship with the same system would stay below the penalty line until 2035, and one running on 100% biofuel could even achieve negative emissions, the report found.

The 40% capture rate was considered the practical limit, since running the system uses extra fuel, producing more emissions. Capturing 90% of the carbon would use 40% more fuel.

The liquefied CO2 would also need to be stored somewhere on board, then offloaded at a port and moved to a depleted oil reservoir, perhaps thousands of kilometres away.

Onboard storage would reduce cargo capacity, and this cost was not included in the GCMD estimates. 

Burning one tonne of fuel oil releases about three tonnes of CO2. Assuming a 40% capture rate, the GCMD found that for each tonne of CO2 captured on board and stored permanently in a reservoir, the emissions avoided are just 0.77 tonnes, due to the GHG emissions across the value chain.

Costs varied depending on whether the CO2 was stored permanently in reservoirs, used to cure concrete or to make e-methanol.

It was hoped that using captured CO2 to make e-fuels for shipping would create a kind of circular economy.

But GCMD chief executive Lynn Loo said on LinkedIn that this concept “breaks down quickly unless capture rates are near 100%”.

Loo said that at 40% capture efficiency, over 93% of the initially captured CO2 is re-emitted after just three re-use cycles.

Loo said carbon crediting, a key incentive for using more expensive greener inputs, gets murky.

“Who gets the credit? Is it the shipowner capturing CO2, the e-methanol producer using it, or the end user paying the green premium?”

UCL Energy Institute associate professor Tristan Smith said the cost and complexity meant OCCS was unlikely to happen in any volume.

Smith said there was a niche for CCS, especially with LNG, as it allowed compliance with the IMO’s GHG fuel intensity targets for a short period in the 2030s.

But the cost per tonne meant it was not much cheaper than other options that were competitive during that window, such as biofuels.

“What the 40% capture rate also indicates is that it’s not going to be GFI-compliant for very long, so will need to be retrofitted to ammonia to be competitive,” Smith said, adding it would be cheaper to just go straight to ammonia.

Smith said a dual-fuel ammonia ship could run on blue ammonia, made with natural gas and carbon capture. The economies of scale for CCS were much better on land, and avoided the problem of having to store and offload CO2 from a ship.

Michael Liebreich, an energy transition consultant and the founder of Bloomberg NEF, said OCCS would always lose out to carbon capture on land.

“That is an argument for going to book-and-credit schemes,” Liebreich told Lloyd’s List.

“If you’re going to the brain ache of capturing carbon, do it somewhere where it’s really cheap, where it’s easier to put somewhere, where it’s not going anywhere, rather than on board a ship.”

Liebreich said CCS was on land was also much easier to verify.

“There’s going to be a very substantial degree of distrust if somebody says ‘I’ve just done this trip and I captured all the carbon, honestly I did’,” he said, adding the industry had “certainly cleaned a few tanks in the middle of oceans” in the past.

“A really robust book-and-credit system is probably a better thing for the industry to really insist on and develop and allow to be audited and so on.”

There were just 50 operating carbon capture projects in the world in 2024, with the capacity to store 51m tonnes of CO2 per year, according to the Global CCS Institute, an industry group.

Shipping, which makes up about 2.3% of man-made CO2 emissions, emitted 911m tonnes of CO2 in 2023, according to the International Council on Clean Transportation.