Cross-contamination risk shapes liquid CO2 loading strategy

Mr Buschmann added, “We noticed… when we get the CO2… at -26.5°C, then we have only a temperature margin of 2.7 K,” the speaker said. After calculating heat and mass balance, the verdict was cautious: the process is “very difficult to carry out, at least not at -26.5°C,” and it would demand “really sub-cooled CO2,” which “we cannot expect… always.” The method is therefore “very uncertain.”

A different approach is then proposed that retains the pressure-control benefits of vapour return while breaking the cross-contamination pathway. In this approach, the main loading stream flows from terminal to ship as normal. A small bypass of that incoming liquid, “maybe 5%” is routed through a heat exchanger and vaporised. The warm-side duty for that exchanger is provided by the ship’s own tank vapour, which is condensed and returned to the cargo tank. A compressor lifts the shipside vapour to the pressure needed to keep the loop running. The vapour that goes back towards the terminal originates from the terminal’s own liquid, not from the ship’s original vapour space.

Mr Buschmann summarised the outcome in plain terms, “We have solved two problems. The first problem is to keep the pressure constant in both tanks, or at least… in a certain range. And the second problem is that we can load the vessel without giving the storage any contaminated vapour back.” Because the vapour returned is generated from the terminal’s own liquid on the cold side of the exchanger, he argued, “There is no reason anymore for the storage… to avoid vapour return.”

From an implementation standpoint, the proposal seeks to reuse installed shipboard assets wherever possible. “The best thing is we can actually reuse some of the equipment we have on our medium-pressure CO2 carriers,” said Mr Buschmann, pointing to the existing vapour-return compressors − “typically two of them” − adding “we need to supply an additional heat exchanger,” after which “we can carry out this process on board our ship.” He also noted the loop “works for different pressure levels… also for low pressure.”

The motivation for segregation is only now being discussed as CO2 shipping and trade becomes one of the solutions to shipping decarbonisation. Different CO2 emitters “have different chemical compositions… so there is quite a big uncertainty how to deal with this.” In the specific case where CO2 stems from combustion, “SOx and NOx” raise concerns that other sources may not share, noted Mr Buschmann.

“Emitter A does not want to take the responsibility for the specification of Emitter B… via vapour return, during loading,” noted Mr Buschmann. The proposed loop answers that objection by keeping the terminal’s product within its own system boundary.

Looking forward, Mr Buschmann linked the method to a likely operating model for early aggregation. He said there is “still no… general agreement on how the composition should be,” despite the topic being under discussion “for a few years.” In his view, an approach that avoids cross-contamination during loading “will be needed during the next year in order to have one vessel which can collect CO2 from different emitters and then bring it to the storage site.”

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