Introduction

CO2 is transported in three states: gas, liquid and solid. Commercial-scale transport uses tanks, pipelines and ships for gaseous and liquid carbon dioxide.

Gas transported at close to atmospheric pressure occupies such a large volume that very large facilities are needed. Gas occupies less volume if it is compressed, and compressed gas is transported by pipeline. Volume can be further reduced by liquefaction, solidification or hydration. Liquefaction is an established technology for gas transport by ship as LPG

CO2 pipeline operators have established minimum specifications for composition. Box 4.1 gives an example from the Canyon Reef project (Section 4.2.2.1). This specification is for gas for an enhanced oil recovery (EOR) project, and parts of it would not necessarily apply to a CO2 storage project. A low nitrogen content is important for EOR, but would not be so significant for CCS. A CO2 pipeline through populated areas might have a lower specified maximum H2S content.

Dry carbon dioxide does not corrode the carbon-manganese steels generally used for pipelines, as long as the relative humidity is less than 60% (see, for example, Rogers and Mayhew, 1980); this conclusion continues to apply in the presence of N2, NOx and SO contaminants. Seiersten (2001) wrote:

"The corrosion rate of carbon steel in dry supercritical CO2 is low. For AISI 1080 values around 0.01 mm yr-1 have been measured at 90-120 bar and 160°C-180°C for 200 days. Short-

term tests confirm this. In a test conducted at 3°C and 22°C at 140 bar CO2, and 800 to 1000 ppm H2S, the corrosion rate for X-60 carbon steel was measured at less than 0.5 ^m yr-1 (0.0005 mm yr-1). Field experience also indicates very few problems with transportation of high-pressure dry CO2 in carbon steel pipelines. During 12 years, the corrosion rate in an operating pipeline amounts to 0.25-2.5 ^m yr-1 (0.00025 to (0.0025 mm yr-1)".

The water solubility limit in high-pressure CO2 (500 bar) is 5000 ppm at 75°C and 2000 ppm at 30°C. Methane lowers the solubility limit, and H2S, O2 and N2 may have the same effect.

Corrosion rates are much higher if free water is present; hydrates might also form. Seiersten (2001) measured a corrosion rate of 0.7 mm yr-1 corrosion rate in 150 to 300 hours exposure at 40°C in water equilibrated with CO2 at 95 bar, and higher rates at lower pressures. She found little difference between carbon-manganese steel (American Petroleum Institute grade X65) and 0.5 chromium corrosion-resistant alloy. It is unlikely to be practicable to transport wet CO2 in low-alloy carbon steel pipelines because of this high corrosion rate. If the CO2 cannot be dried, it may be necessary to build the pipeline of a corrosion-resistant alloy ('stainless steel'). This is an established technology. However the cost of steel has greatly increased recently and this may not be economical.

Once the CO2 has been dried and meets the transportation criteria, the CO2 is measured and transported to the final use site. All the pipelines have state-of-the-art metering systems that accurately account for sales and deliveries on to and out of each line, and SCADA (Supervisory Control and Data Acquisition) systems for measuring pressure drops, and redundancies built in to allow for emergencies. In the USA, these pipelines are governed by Department of Transportation regulations. Movement of CO2 is best accomplished under high pressure: the choice of operating pressure is discussed in an example below, and the reader is referred to Annex I for a discussion of the physical properties of CO2.

4.2.2 Exis ting experience

Table 4.1 lists existing long-distance CO2 pipelines. Most of the projects listed below are described in greater detail in a report by the UK Department of Trade and Industry (2002). While there are CO2 pipelines outside the USA, the Permian Basin contains over 90% of the active CO2 floods in the world (O&GJ, April 15, 2002, EOR Survey). Since then, well over 1600 km of new CO2 pipelines has been built to service enhanced oil recovery (EOR) in west Texas and nearby states.

4.2.2.1 Canyon Reef

The first large CO2 pipeline in the USA was the Canyon Reef Carriers, built in 1970 by the SACROC Unit in Scurry County, Texas. Its 352 km moved 12,000 tonnes of anthropogenically produced CO2 daily (4.4 Mt yr-1) from Shell Oil Company gas processing plants in the Texas Val Verde basin.

4.2.2.2 Bravo Dome Pipeline

Oxy Permian constructed this 508 mm (20-inch) line connecting the Bravo Dome CO2 field with other major pipelines. It is capable of carrying 7.3 MtCO2 yr-1 and is operated by Kinder Morgan.

4.2.2.3 Cortez Pipeline

Built in 1982 to supply CO2 from the McElmo Dome in S.E. Colorado, the 762 mm (30-inch), 803 km pipeline carries approximately 20 Mt CO2 yr-1 to the CO2 hub at Denver City, Texas. The line starts near Cortez, Colorado, and crosses the Rocky Mountains, where it interconnects with other CO2 lines. In the present context, recall that one 1000 MW coal-fired

Box 4.1 Specimen CO2 quality specifications

The Product delivered by Seller or Seller's representative to Buyer at the Canyon Reef Carriers Delivery Meter shall meet the following specifications, which herein are collectively called 'Quality Specifications':

(a) Carbon Dioxide. Product shall contain at least ninety-five mole percent (95%) of Carbon Dioxide as measured at the SACROC delivery meter.

(b) Water. Product shall contain no free water, and shall not contain more than 0.48 9 m-3 in the vapour phase.

(c) Hydrogen Sulphide. Product shall not contain more than fifteen hundred (1500) parts per million, by weight, of hydrogen sulphide.

(d) Total Sulphur. Product shall not contain more than fourteen hundred and fifty (1450) parts per million, by weight, of total sulphur.

(e) Temperature. Product shall not exceed a temperature of 48.9 oC.

(f) Nitrogen. Product shall not contain more than four mole percent (4%) of nitrogen.

(g) Hydrocarbons. Product shall not contain more than five mole percent (5%) of hydrocarbons and the dew point of Product (with respect to such hydrocarbons) shall not exceed -28.9 oC.

(h) Oxygen. Product shall not contain more than ten (10) parts per million, by weight, of oxygen.

(i) Glycol. Product shall not contain more than 4 x 10-5 L m-3 of glycol and at no time shall such glycol be present in a liquid state at the pressure and temperature conditions of the pipeline.

Table 4.1 Existing long-distance CO2 pipelines (Gale and Davison, 2002) and CO2 pipelines in North America (Courtesy of Oil and Gas Journal).

Pipeline

Location

Operator

Capacity

Length

Year finished

Origin of CO2

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