On-board carbon capture and storage (OCCS), coupled with CO2 utilisation in the form of downstream product displacement, can outperform permanent storage in life cycle assessment (LCA), a study by the Global Centre for Maritime Decarbonisation (GCMD) explained.
OCCS can be an effective mid-term option for shipping to decarbonise and to reduce "tank-to-wake" emissions, with most ships still expected to be operating on conventional fuels.
The study, Project Captured, used data from the world's first ship-to-ship (STS) offloading of liquefied CO2 captured on board a ship with an OCCS system. The liquefied CO2 would eventually be transported overland to be used downstream as industry feedstock.
Without a waste heat recovery system (WHRS), this pilot achieved 7.9pc greenhouse gas (GHG) savings with a 10.7pc onboard capture rate, resulting in 582 kg of CO2 equivalent (kgCO2e) per tonne of CO2 captured and offloaded. GHG savings was equal to 0.84t of CO2 emission avoided per tonne of CO2 captured and offloaded.
Another 375 kgCO2e/t of CO2 captured and offloaded was introduced from offloading and transport, particularly from truck transport of more than 2,200km, which contributed to half of the captured emissions at 194 kgCO2e/t. Hose purging before and after offloading resulted in the second-highest CO2 emissions of 99 kgCO2e/t, which was 26pc of transport emissions.
In the real-life pilot, industry utilisation of captured CO2 was key in realising emissions savings. Emissions captured through the production of post-carbonated slag (PCS) and precipitated calcium carbonate (PCC) stood at 951 kgCO2e/t.
Optimised value chains
Total value-chain GHG savings can rise to 17.8pc, when inefficiencies are cut by installing a WHRS, using a pipeline to transport CO2, and aligning offloaded CO2 volume and tank capacity, as demonstrated in the study's hypothetical scenarios with optimised value chains. Savings are equivalent to about 2t of CO2 reduced per tonne or CO2 captured and offloaded from the ship. The fuel penalty also fell from 5pc to 1.5pc, compared with a baseline scenario of the ship operating without OCCS.
Another key finding reflected that the usage of CO2 in a carbon capture and utilisation (CCU) value chain can avoid more GHG emissions, compared to permanent storage in the carbon capture and storage (CCS) pathway.
If shipowners can optimise the CCU pathway, higher GHG savings of 68-71pc are possible, depending on whether the PCS output is used for steel sintering or concrete production. The GHG savings from the utilisation of CO2 is significantly higher than a 21pc emissions savings measured in the CCS pathway, where captured CO2 was permanently stored.
"Project Captured shows that onboard carbon capture, when thoughtfully integrated with utilisation pathways, can deliver real emissions reductions today while we continue to scale up low- and zero-carbon fuels," said GCMD's chief executive officer, Lynn Loo.
"If our frameworks continue to ignore avoided emissions and displaced carbon, we risk disincentivising investments in solutions that can meaningfully bend the emissions curve," said Loo, underscoring the importance of how GHG reductions are measured and accounted for.
Current GHG accounting methods by the International Maritime Organization (IMO) give shipowners limited incentive to invest in OCCS, since formal frameworks do not account for avoided GHG emissions in the case where highly emissive products are displaced by captured CO2 equivalents.
The study suggests that future LCA guidelines by the IMO should address cross-sector allocation of both emissions and avoided emissions, to prevent double counting and encourage more shipowners to invest in OCCS.
More countries are making progress in the final utilisation and storage of CO2. Indonesia intends to support carbon utilisation initiatives, and Denmark has awarded the first licence of permanent CO2 storage in the country. Japanese shipping firms like Kline have also received more liquefied CO2 carriers to meet the growing demand for carbon capture projects.
