4.3.1 Marine transportation system
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.
4.3.2 Exis ting experience associated liquefaction and intermediate storage facilities. 4.3.3 Design
For the design of hull and tank structure of liquid gas transport ships, such as LPG carriers and LNG carriers, the International Maritime Organization adopted the International Gas Carrier Code in order to prevent the significant secondary damage from accidental damage to ships. CO2 tankers are designed and constructed under this code.
There are three types of tank structure for liquid gas transport ships: pressure type, low temperature type and semi-refrigerated type. The pressure type is designed to prevent the cargo gas from boiling under ambient air conditions. On the other hand, the low temperature type is designed to operate at a sufficiently low temperature to keep cargo gas as a liquid under the atmospheric pressure. Most small gas carriers are pressure type, and large LPG and LNG carriers are of the low temperature type. The low temperature type is suitable for mass transport because the tank size restriction is not severe. The semi-refrigerated type, including the existing CO2 carriers, is designed taking into consideration the combined conditions of temperature and pressure necessary for cargo gas to be kept as a liquid. Some tankers such as semi-refrigerated LPG carriers are designed for applicability to the range of cargo conditions between normal temperature/high pressure and low temperature/atmospheric pressure.
Annex I to this report includes the CO2 phase diagram. At atmospheric pressure, CO2 is in gas or solid phase, depending on the temperature. Lowering the temperature at atmospheric pressure cannot by itself cause CO2 to liquefy, but only to make so-called 'dry ice' or solid CO2. Liquid CO2 can only exist at a combination of low temperature and pressures well above atmospheric pressure. Hence, a CO2 cargo tank should be of the pressure-type or semi-refrigerated. The semi-refrigerated type is preferred by ship designers, and the design point of the cargo tank would be around -54 °C per 6 bar to -50°C per 7 bar, which is near the point of CO2. In a standard design, semi-refrigerated type LPG carriers operate at a design point of -50°C and 7 bar, when transporting a volume of 22,000 m3.
Carbon dioxide could leak into the atmosphere during transportation. The total loss to the atmosphere from ships is between 3 and 4% per 1000 km, counting both boil-off and exhaust from the ship's engines; both components could be reduced by capture and liquefaction, and recapture onshore would reduce the loss to 1 to 2% per 1000 km.
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. These ships transport liquefied food-grade 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
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