For certain constellations, shipping CO2 can be an interesting or even necessary option. In transportation by ship, a distinction must be made between offshore transportation on oceans and onshore transportation by river.
Numerous potential carbon storage sites are located below the seabed. As a general rule, such CO2 storage sites can also be reached by pipeline, although this may involve very high outlays, specifically for long distances, and will meet with technical constraints. In such a case, the option exists of transporting the CO -from the coast to the storage site using large tankers. In this respect, there is an analogy to transporting liquefied natural gas, because CO2 must also be liquefied for shipping. Typical parameters for the liquefied state-depending on the design of the tanks - are, for example, 7 bar and -50 °C or 20 bar and -30 °C. CO2 tankers with a capacity of some 10000 tons today already ply the oceans. Even if much larger CO2 ships are feasible in the future, it becomes clear that a very large number of ships would be necessary for the envisaged CO 2 quantities.
Transporting liquefied CO2 onshore using, for example, barges on rivers is a practicable alternative only where marginal conditions are especially favorable. Shipping can be considered only if loading an inland ship is possible close to the site of carbon capture and only if unloading is at a favorable destination. The CO2 quantities that can feasibly be transported by inland ships are altogether very limited, however.
The biggest drawback in shipping CO2 is that the CO2 must be liquefied for this purpose. Liquefaction means high additional energy consumption in the CCS process chain, since this step is being taken solely for the transportation stage. Before the CO2 is injected into the depository, it must be compressed to a supercritical pressure, and reach normal temperatures by the addition of heat. The specific, electric energy requirement for liquefaction amounts to about 0.15 kWh per kg of CO2. Furthermore, CO2 liquefaction requires an additional plant with a capacity that can far exceed previous sizes, depending on the project. For discontinuous shipping of the carbon dioxide, the liquefied CO2 must be placed in interim storage in large tanks. Liquefaction plant and tanks have considerable space requirements (e.g., in a magnitude of 1 ha for 1 million tons of CO2 per year), which must be taken into account in addition to the space needs for capturing carbon.
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