Why should CO2 travel first class?

A 1.5°C global warming scenario outlined by the UN Intergovernmental Panel for Climate Change (IPCC) includes an annual reduction of 6 billion tonnes of CO2 through Carbon Capture, Storage, and Utilization (CCUS) by 2050. Similar scenarios from other sources arrive at roughly the same target. Following capture, the CO2 must be transported to storage or utilisation sites.

In comparison, almost 8 billion tonnes of coal have been produced worldwide this year. As a result, CO2 is set to become an important commodity in future transport systems.

As depicted in the CO2 phase diagram, CO2 can exist in four forms depending on pressure and temperature: gaseous form (CO2(g)), liquid form (CO2(l)), supercritical liquid form, and solid form (CO2(s)), more commonly known as dry ice.

For transporting large volumes, land pipelines will become the preferred mode of transportation. Pipelines can also be used for shorter distances over the sea. For long-distance sea transport of larger volumes of CO2, ships will be the primary mode. Onshore transport of smaller volumes would be by truck or train to a storage or utilisation site, or to an assembly point where the CO2 can be transferred to a pipeline.

All current studies and recommendations for sea or land transport assume that the CO2 is liquefied and transported at around -20°C and 15 atmospheres of pressure in insulated tanks. This method of transportation is well-established, similar to the transport of other gases in liquid form, such as oxygen, nitrogen, and natural gas. During transport, some CO2 boils, and a small amount is either released into the atmosphere or returned to liquid form at a separate facility.

There is substantial experience with tanker transport, which is typically safe, but accidents can happen, as shown by these photos from Mainz in 2015. In this incident, a valve on a CO2 tanker failed, leading to the release of 22 tonnes of CO2 under high pressure in narrow city streets. As CO2 is denser than atmospheric air, it sank and formed a thick layer, displacing the air. Fortunately, all residents were alerted and evacuated to upper floors, so no lives were lost.

In the Danish Energy Agency's excellent technology data report for Carbon Capture, Transport, and Storage, a semi-trailer truck with a 30-tonne liquid CO2 cargo capacity is estimated to cost EUR 660,000. This is a significant investment for the thousands of such lorries, of which there are only a small number in Europe today.

There are currently only four small ships that can carry liquid CO2. The Norwegian Northern Lights consortium has ordered three more ships with 8,000 tonnes of cargo capacity, with the first ship scheduled for delivery in 2024. A Greek ship owner has also ordered two slightly larger ships, with delivery expected in 2-3 years. If a significant portion of the captured CO2 is to be transported by sea, a considerable number of new liquefied CO2 tankers, notably larger than the current ones, will need to be constructed. Ship brokers expect a need for 50 such ships, with prices ranging from EUR 60-100 million, depending on size and operational pressure. However, building this capacity is feasible.

Surprisingly, there seems to have been little consideration given to the possibility of transporting CO2 in its solid form as dry ice under atmospheric pressure. In this state, CO2 would be entirely safe to transport. To eliminate the need for intermediate storage tanks at capture points, transhipment sites, and storage/utilization facilities, one could choose to transport dry ice in standard containers with inner passive insulation approximately 25-30 cm thick. Each container would hold just over 20 tonnes of CO2 in pellet form, which can be easily loaded and unloaded. The evaporation of CO2 would remain at a few parts per thousand per day, which can be ventilated without risk to the environment. We call this solution "decarbonICE."

To convert liquid CO2 into dry ice form, a dry ice machine is required, which consumes approximately 150-200 kWh per tonne of CO2. This cost should be weighed against the reduction in expenses for tanker trucks, ships, intermediate storage facilities, harbour facilities, and, most importantly, the transport will be completely safe.

Considering that other goods, like paper for recycling, are transported in containers via ships and trucks, this leads us to question why CO2 should receive special treatment and be transported in a more costly way.

Henrik O. Madsen, PhD, former CEO of DNV, co-Founder DecarbonICE

Mikkel Hansen, Navigator, MBA, and co-founder and CEO of Aprendio, co-Founder DecarbonICE