Captura y Almacenamiento de CO₂

4. How can CO₂ be transported once it is captured?

• 4.1 What are the methods of CO₂ transport?
• 4.2 How expensive is CO₂ transport?

4.1 What are the methods of CO₂ transport?

The source document for this Digest states:

Except when plants are located directly above a geological storage site, captured CO₂ must be transported from the point of capture to a storage site. This section reviews the principal methods of CO₂ transport and assesses the health, safety and environment aspects, and costs.

Methods of CO₂ transport

Pipelines today operate as a mature market technology and are the most common method for transporting CO₂. Gaseous CO₂ is typically compressed to a pressure above 8 MPa in order to avoid two-phase flow regimes and increase the density of the CO₂, thereby making it easier and less costly to transport. CO₂ also can be transported as a liquid in ships, road or rail tankers that carry CO₂ in insulated tanks at a temperature well below ambient, and at much lower pressures.

The first long-distance CO₂ pipeline came into operation in the early 1970s. In the United States, over 2,500 km of pipeline transports more than 40 MtCO₂ per year from natural and anthropogenic sources, mainly to sites in Texas, where the CO₂ is used for EOR. These pipelines operate in the ‘dense phase’ mode (in which there is a continuous progression from gas to liquid, without a distinct phase change), and at ambient temperature and high pressure. In most of these pipelines, the flow is driven by compressors at the upstream end, although some pipelines have intermediate (booster) compressor stations.

In some situations or locations, transport of CO₂ by ship may be economically more attractive, particularly when the CO₂ has to be moved over large distances or overseas. Liquefied petroleum gases (LPG, principally propane and butane) are transported on a large commercial scale by marine tankers. CO₂ can be transported by ship in much the same way (typically at 0.7 MPa pressure), but this currently takes place on a small scale because of limited demand. The properties of liquefied CO₂ are similar to those of LPG, and the technology could be scaled up to large CO₂ carriers if a demand for such systems were to materialize.

Road and rail tankers also are technically feasible options. These systems transport CO₂ at a temperature of -20ºC and at 2 MPa pressure. However, they are uneconomical compared to pipelines and ships, except on a very small scale, and are unlikely to be relevant to large-scale CCS.

Environment, safety and risk aspects

Just as there are standards for natural gas admitted to pipelines, so minimum standards for ‘pipeline quality’ CO₂ should emerge as the CO₂ pipeline infrastructure develops further. Current standards, developed largely in the context of EOR applications, are not necessarily identical to what would be required for CCS. A low-nitrogen content is important for EOR, but would not be so significant for CCS. However, a CO₂ pipeline through populated areas might need a lower specified maximum H2S content. Pipeline transport of CO₂ through populated areas also requires detailed route selection, over-pressure protection, leak detection and other design factors. However, no major obstacles to pipeline design for CCS are foreseen.

CO₂ could leak to the atmosphere during transport, although leakage losses from pipelines are very small. Dry (moisture-free) CO₂ is not corrosive to the carbon-manganese steels customarily used for pipelines, even if the CO₂ contains contaminants such as oxygen, hydrogen sulphide, and sulphur or nitrogen oxides. Moisture-laden CO₂, on the other hand, is highly corrosive, so a CO₂ pipeline in this case would have to be made from a corrosion-resistant alloy, or be internally clad with an alloy or a continuous polymer coating. Some pipelines are made from corrosion-resistant alloys, although the cost of materials is several times larger than carbon- manganese steels. For ships, the total loss to the atmosphere is between 3 and 4% per 1000 km, counting both boil-off and the exhaust from ship engines. Boil-off could be reduced by capture and liquefaction, and recapture would reduce the loss to 1 to 2% per 1000 km. Accidents can also occur. In the case of existing CO₂ pipelines, which are mostly in areas of low population density, there have been fewer than one reported incident per year (0.0003 per km-year) and no injuries or fatalities. This is consistent with experience with hydrocarbon pipelines, and the impact would probably not be more severe than for natural gas accidents. In marine transportation, hydrocarbon gas tankers are potentially dangerous, but the recognized hazard has led to standards for design, construction and operation, and serious incidents are rare.

Source & ©: IPCC  Carbon Dioxide Capture and Storage: Technical Summary (2005)
4. Transport of CO2, p. 29

4.2 How expensive is CO₂ transport?

The source document for this Digest states:

Transport cost in pipelines

Cost with distance for pipelines and ships

Costs have been estimated for both pipeline and marine transportation of CO₂. In every case the costs depend strongly on the distance and the quantity transported. In the case of pipelines, the costs depend on whether the pipeline is onshore or offshore, whether the area is heavily congested, and whether there are mountains, large rivers, or frozen ground on the route. All these factors could double the cost per unit length, with even larger increases for pipelines in populated areas. Any additional costs for recompression (booster pump stations) that may be needed for longer pipelines would be counted as part of transport costs. Such costs are relatively small and not included in the estimates presented here.

Figure TS.5 shows the cost of pipeline transport for a nominal distance of 250 km. This is typically 1–8 US$/tCO₂ (4–30 US$/tC). The figure also shows how pipeline cost depends on the CO₂ mass flow rate. Steel cost accounts for a significant fraction of the cost of a pipeline, so fluctuations in such cost (such as the doubling in the years from 2003 to 2005) could affect overall pipeline economics.

In ship transport, the tanker volume and the characteristics of the loading and unloading systems are some of the key factors determining the overall transport cost.

The costs associated with CO₂ compression and liquefaction are accounted for in the capture costs presented earlier. Figure TS.6 compares pipeline and marine transportation costs, and shows the break-even distance. If the marine option is available, it is typically cheaper than pipelines for distances greater than approximately 1000 km and for amounts smaller than a few million tonnes of CO₂ per year. In ocean storage the most suitable transport system depends on the injection method: from a stationary floating vessel, a moving ship, or a pipeline from shore.

Source & ©: IPCC (WGI)  Intergovernmental Panel on Climate Change IPCC
Carbon Dioxide Capture and Storage: Technical Summary (2005)
4. Transport of CO2, p. 30