Dry ice method could offset ship CO2

The seafloor. Credit: NS Energy

A consortium of global companies is positing dry ice as a long-lasting carbon-capture solution for ship carbon dioxide (CO2) emissions, allowing vessels to burn conventional hydrocarbon fuels while at sea.

The solution comprises a sequence of vehicles, made from frozen and compressed CO2, dropped from the vessel on a downward trajectory. As Henrik O. Madsen, head of the project steering committee, explained, large areas of the sea floor, particularly in deeper areas such as the abyssal plain, are comprised of soft sand. This surface would allow vessels to drop their frozen, hydrodynamically streamlined CO2 vehicles, analogous to torpedoes.

During the initial phase of the descent, Madsen explains, there is some small degree of CO2 evaporation, up to 2% per vehicle. However, “… below 500 m, the water pressure is more than 50 bar; CO2 cannot exist as a gas,” he says.

After the ice projectile reaches a descent of 500 m under the waves, it reaches a terminal velocity of 25 m/s, sufficient speed to embed into and submerge in the seabed on impact, akin to a tank shell. Here, seabed pressure and sand would act on the dry ice chunks to form ‘hydrate cement’, a form of containment around the vehicle, preventing CO2 from leaking out into the ocean.

Crucial to the system, Madsen argues, is the fact that it does not require any consumables or additional chemical agents to be carried on the vessel, making for a sensible alternative to other plausible shipboard carbon capture (CCS) systems.

In theory, the operation imposes a 10% energy penalty on a vessel burning low-sulphur fuel oil. However, Madsen notes, the process is 3% more efficient on a liquefied natural gas (LNG)-powered vessel, incorporating a heat exchanger from tank-to-engine to partially offset the energy cost of freezing the CO2 exhaust. The energy cost of the process “still compares very favourably with what might be an investment of 300% or more for an alternative fuel system.”

There are a number of circumstances, which would preclude the dropping of the vehicles, such as a hard slope in the angle of the sea floor, pipelines, or a rocky surface. The ice projectiles cannot be launched in water depths of less than 500m, as this risks them not reaching sufficient speed to safely embed. The project will run throughout 2020, in which the group will conduct feasibility studies and hopes to secure approval from the IMO, with considerations relating to MARPOL Annex VI: dumping.

Ultimately, Madsen suggests, incorporating the technique alongside the use of electro-fuels could allow shipping to become carbon-negative. However, “… the idea is that we can use existing ships and propulsion systems, as well as existing fuel infrastructure, while introducing no heat sources or high pressure, chemicals, or consumables.”

A demonstration model, however, is not included in DecarbonICE’s 2020 time frame. “There are still questions – for example, ‘do we need extra personnel on board?’” says Madsen. “We are not the ones who will make prototypes, but we have partners who will pick it up and take it further. Shipping companies will have to challenge us over whether this is possible on real ships, and in relation to real freight.”

The initiative will require a much wider extent of hydrodynamic knowledge than exists today, with the assumption that vessels will carry an echo-sounder on-board to ascertain the suitability of the seabed surface. The sheer volume of frozen dry ice which will need to be thrown overboard puts shipping’s atmospheric carbon emissions into perspective, Madsen notes, with some larger bulk carriers potentially dropping 2-tonne vehicles every 15 minutes.