Side-alternativer
CO2 transport by ship
Transporting CO2 by ship is a practical solution when volumes are too small or the distance of transport too far for pipeline transport to be an economical or practical solution. As a commercial commodity in food production, cooling and for other industrial purposes, CO2 is often transported by ship today. The market for CO2 in Europe is almost 3 million tons a year. A large part of this is transported by ship.
In CCS, emissions from several small sources may be collected by ship and transported to a suitable storage site, if location makes it impractical to build a common pipeline system for these sources. Transport by ship may also be a solution when there is a large distance from a big emission source or a cluster of several emission source to the nearest suitable storage site. Transport by ship is estimated to be more economical than transport by pipeline at distances over 1000 - 1500 km.
Contents
CO2 transport by ship
 |
|
A CO2 transport ship (Source: Yara) |
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
 |
|
There is a market for industrial CO2 in the food industry |
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.
See also
References
IPCC, 2005: IPCC Special Report on Carbon Dioxide Capture and Storage
