CO2 purity will impact onboard containment system design

Somewhat ironically, the shipping industry’s decarbonisation and progress towards the IMO’s 2030 and 2050 goals may be hindered by a shortage of high purity CO2 and compounded by a lack of transportation infrastructure. These barriers could significantly impact the ability to produce e-fuels at scale.

While liquid carbon dioxide (LCO2) has been transported by sea for almost half a century, there is now rapidly growing demand to move large volumes of CO2 over increased distances. Forecasts for growth in demand have been predicated on the growing network of sequestration routes for CO2 captured from industrial processes.

In addition to the infrastructure demand to send CO2 for sequestration there is now a growing demand for it to be used as feedstock for synthetic fuels such as e-methanol and e-methane (e-LNG). Both are seen by many as practical, lower-risk alternative marine fuels in comparison to e-ammonia and e-hydrogen. Satisfying the demand for these fuels will create significant additional demand for a commodity which is already in short supply.

Carbon capture on board presents an interesting opportunity for a closed system whereby CO2 captured onboard ships is offloaded for use in synthetic fuel production. If those on-board capture systems can deliver pure CO2 they could offer the vessel owner a strong revenue stream in addition to the benefits of close to zero CO2 emissions.

Supply chain challenges

Transporting large volumes of CO2 by sea presents significant challenges, and we need to have a fundamental rethink on the way we approach it. The small number of vessels currently operating in this area predominantly transport high purity food grade CO2 over relatively short distances. We are entering a new era of CO2 transportation with a range of distances, end destinations and significantly a much more diverse range of sources of the cargo. Importantly, we need to recognise that as onboard carbon capture gains traction then almost every ocean-going ship type has the potential to become a CO2 carrier.

The challenges also extend to shoreside infrastructure development. Aside from the need to build an adequate shore side infrastructure to carry the cargo to and from the ship it is also essential that the Master can establish the nature of the cargo that is being loaded. This necessitates certification of the purity of the cargo preloading.

Purity considerations

There is no standard technology for carbon capture either on board or in shore based industrial processes. There are many different alternatives and the transport supply chain needs to be ready to cope with CO2 captured from all of them. Each of these technologies results in different purity levels for the CO2. Both sequestration and reuse of CO2 require high purity and as purifying CO2 carries with it a significant cost and energy penalty, it is likely that high purity CO2 will command a premium and that capture processes directly producing high purity CO2 will also present a stronger business case, particularly for on board capture.

The purity of CO2 will be dictated by the method of its capture, the source of the CO2 (for example, exhaust gases or an industrial or chemical process) and any post capture clean-up process. Impurities could include the presence of other species in the CO2 or may reflect the presence of water.

It’s important to note that this isn’t a percentage game. The biggest impact on the challenge of transport and reuse or sequestration is the nature of the impurities rather than their quantity. The significance of the impurity may be on the potential for corrosion in the containment tanks or in the loading or unloading equipment – as seen with hydrogen sulphide, sulphur oxides and water – or it may increase safety hazards in case of an escape, e.g. carbon monoxide, sulphur and nitrogen oxides.

From the perspective of the onboard containment system, a range of other so-called non-condensable gases such as nitrogen, argon, hydrogen or methane affect the storage pressure necessary for the liquid at a given temperature. When it comes to sequestration even oxygen can only be tolerated at parts per million level in former oil and gas reservoirs, as its presence can result in biological activity that will build up to clog the injection well.

Containment Tank Pressure Choices

The liquefication processes and the necessary cleaning and drying of the CO2 for it to be stored safely as a liquid can remove a large part of the impurity content in the gas. However, the presence of the residual non-condensable impurities in a CO2 cargo on either a dedicated carrier or one which employs on board carbon capture will have a significant impact on the design of the containment and reliquification plant. The size of the impact will be dependent on the nature of the impurity. Ultimately, investments in onshore equipment to achieve high purity CO2 could affect the CAPEX and OPEX of shipping , the risks of transportation, and the costs of mitigating those risks.

In a worst-case scenario, impurities could result in an increase in design pressure and hence tank weight of over 30% or require a reduction in storage temperature that would increase risks of cargo freezing but which will certainly have an impact on the energy requirement in the whole process. These challenges are particularly significant for the lower pressure transport systems which offer cheaper tanks and larger revenue earning cargoes.

Given the range of potential impurities the obvious questions are whether the Master knows what cargo is being loaded, or if the owner knows what range of cargos the ship will be expected to carry during its life. If the vessel is designed for one route with one source of CO2 (for example a dedicated vessel in a single CCUS transportation chain) these are easier questions to answer, but they limit the lifetime flexibility of the vessel.

In any case, the costs of maintaining the cargo increase with transport distance. Whilst standardisation of the storage pressure used may seem attractive it may not be the right answer for all cargo and voyage profiles. A the very least there is a need for a certification scheme which will ensure that the ship, the cargo and the ultimate destination are appropriately matched.

Houlder has seen a number of tenders for LCO2 carrier designs that fail to recognise the number of variables and rely on a repeat of previous designs for key areas such as cargo containment. This misconception could result in significant over-specification of storage tanks and reliquefication plants, requiring unnecessary CAPEX. There is also the potential of a baked-in OPEX problem resulting from significant and potentially unnecessary increases in non-revenue earning cargo weight and energy consumption.

Specific regulation is required

The burgeoning CO2 industry faces new challenges and needs regulation designed to address them. It’s not enough to simply try to push the regulation of these cargoes into the same category as others .

There is no place for binary or blanket regulations. For example, the IMO has a correspondence group that has been discussing classifying LCO2 as toxic, putting it in the same category as ammonia. While CO2 has an asphyxiation risk, the harmful concentrations and consequences of exposure are different from those of ammonia. Regulators must not use blunt instruments when regulating sophisticated technical sectors. CO2 must either maintain its non-toxic classification or a new, more nuanced designation must specifically be developed.

As with all the challenges that we face in decarbonisation, it’s important to take a system-based approach and to balance opportunities and risks throughout that system. We have all been used to seeing ships as a system, but we can no longer look at the ship system in isolation. The transportation of CO2 presents a clear example of why we need to see the ship as part of the whole logistics and value chain and make sure it fits in the most effective way.

Delivering the supply chain for CCUS is an ambitious goal but we shouldn’t forget the other significant opportunities for the reuse of CO₂ which will also require their own supply chains, To deliver this organisations such as ours need to look beyond the vessel and ensure that the marine transport is properly integrated into the whole supply chain.