> Transport by ship

CO₂ transport by ship

When transported by ship, the CO₂ is cooled to allow it to stay in liquid form at lower pressure. On LPG carriers this is usally achieved at between -50°C and 700 KPa and -54°C at 600 KPa, when transporting 22 000 m3. Typically, pressures and temperatures are lower when transporting larger volumes, and higher when the volumes are small. Similar numbers apply for CO₂.

Some net CO₂ loss must be expected in long distance transport by ship. This results partly from boil-off and partly from the exhaust from the ship's engines. Boil-off occurs because heat is transferred from the environment through the walls of the tank and boils some of the CO₂. This causes pressure in the tanks to increase, and the boil-off gas must therefore be vented, which is usually done together with the exhaust gas of the ships's engines. However, these emissions can be avoided by use of a refrigeration unit to capture and liquify the boil-off gas.

The total loss to the atmosphere is between 3 and 4% per 1000 km. This figure can be reduced by recapturing boil-off gas and by changing to low or zero emission fuels in the ship's engines.

Existing experience

Experience with large scale transport of CO₂ by ship is limited. While there is a considerable market for food-grade CO₂, transport volumes are small. 70 percent of the CO₂ traded in the commodities market is transported by truck. Still, CO₂ is transported on small ships from large point sources of CO₂ to coastal distribution terminals, and there is ongoing design work on larger CO₂ ships and their associated liquefication and storage facilities.

Sea transport of LNG and LPG also provides considerable experience that can be used in CO₂ transport. A notable difference between CO₂ and hydrocarbon gases is that CO₂ cannot be liquefied by lowering the temperature alone. In stead, it goes directly to its solid phase, also known as dry ice. CDO2 must therefore be transported on semi-refrigerated carriers where the tanks are both pressurized and refrigerated. Carriers of this type is already in use for transport of CO₂ LPG.

Safety

The risks involved in transport by ship is low. New tankers are generally well designed to avoid loss of cargo in the case of a collision, stranding or fire. There has been no accidental losses of cargo from LNG tankers. Should an accident happen to a liquid CO₂ tanker, liquefied CO₂ might be released onto the surface of the sea. Release of liquid CO₂ will cause freezing and may cause local acidification of the water, damaging local ecosystems. The environmental effects of such an event are not fully known, and requires further study. However, the long-term environmental impacts of a liquid CO₂ spill are anticipated to be less than that of crude oil spills.

Source: Zero Emission Resource Organisation

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