Ship Based Transportation
Liquefied natural gas and petroleum gases such as propane and butane are routinely transported by marine tankers; this trade already takes place on a very large scale. Carbon dioxide is transported in the same way, but on a small scale because of limited demand. The properties of liquefied carbon dioxide are not greatly different from those of liquefied petroleum gases, and the technology can be scaled up to large carbon dioxide carriers.
Carbon dioxide is continuously captured at the plant on land, but the cycle of ship transport is discrete, and so a marine transportation system includes temporary storage on land and a loading facility. The capacity, service speed, number of ships and shipping schedule will be planned, taking into consideration, the capture rate of CO2, transport distance, and social and technical restrictions. This issue is, of course, not specific to the case of CO2 transport; CO2 transportation by ship has a number of similarities to liquefied petroleum gas (LPG) transportation by ship.
What happens at the delivery point depends on the CO2 storage system. If the delivery point is onshore, the CO2 is unloaded from the ships into temporary storage tanks. If the delivery point is offshore – as in the ocean storage option – ships might unload to a platform, to a floating storage facility (similar to a floating production and storage facility routinely applied to offshore petroleum production), to a single-buoy mooring or directly to a storage system.
The use of ships for transporting CO2 across the sea is today in an embryonic stage. Worldwide there are only four small ships used for this purpose, as of 2010. These ships transport liquefied CO2 from large point sources of concentrated carbon dioxide such as ammonia plants in northern Europe to coastal distribution terminals in the consuming regions. From these distribution terminals CO2 is transported to the customers either by tanker trucks or in pressurized cylinders.
Design work is ongoing in Norway and Japan for larger CO2 ships and their associated liquefaction and intermediate storage facilities.Existing CO2 ships are designed for transporting CO2 in the liquid phase at a pressure between 1.4 to 1.7 MPa and at a temperature range of -25°C to -30°C. The liquid phase gives a high density of the CO2, i.e. 1100 kg/m3, but due to the high pressure, the tanker size and thereby the capacity for existing CO2 ships is relatively low, i.e. between 850-1,400 tons of CO2. This capacity is too small to effectively transport the amounts of CO2 that is associated with CCS.
For LPG, there are three types of ship design: low temperature type, which is designed to keep LPG liquid at a low temperature and atmospheric pressure, the pressure type which is designed against the boiling point of LPG maintaining the LPG liquid at ambient temperature and, the semi-ref type which is a combination of both, i.e. the ship is both pressurized and cooled. Seen from a bulk-transportation perspective, the low temperature type is preferable due to fact that this design does not require pressurized tankers. Existing low temperature ships have a capacity of up to 80,000 m3. This option is, however, not possible for CO2 ships since CO2 at atmospheric pressure can only exist in gaseous or solid phase, but not as a liquid. The best option for CO2 for bulk transportation is the semi-ref type design. A standard semi-ref LPG ship has a capacity of 22,000 m3, i.e. 24,000 ton CO2, and is designed for a pressure of 0.7 MPa and a temperature of -50°C. Such a ship should be suitable for CO2 transportation.