C1 Pipeline transport of CO2
Transport of CO2 by pipeline in the(above 7.58 MPa [1,100 psi] at ambient temperatures) is by far the most cost effective means of moving large quantities of CO2 long distances.
Although CO2 is an inert, non-flammable gas, it still has the potential to be dangerous. As CO2 is 1.5 times heavier than air it will displace oxygen in confined spaces (such as valleys). At high concentrations, CO2 can lead to a range of adverse health effects, including asphyxiation.
C1.1 Pipeline design considerations
Carbon dioxide by its chemical nature forms what is called a “dense phase” or “supercritical” state at temperatures above 88°F when compressed above 7.59 MPa (1,100 psi) (or 1,200 psi at any temperature). Dense phase is a state which is neither a true liquid nor a true vapour. It has the density near that of water (0.8 grams/cc at 27°C and 10.34 MPa), but is still quite compressible. A single cubic foot of CO2 in the dense phase is equal to about 1,200 cubic feet of the gas at ambient conditions. Thus, large quantities of CO2 can be transported in a relatively small volume while in the dense phase.
If properly dehydrated, CO2 does not aggressively attack steel (relatively non-corrosive), so that special metallurgy or exotic linings are not needed in steel pipelines. Excessive water in the stream will form carbonic acid, which will aggressively attack the steel. However, due to its compressibility, steel with a high fracture toughness is required and in larger diameters (greater than 12 inches), crack arrestors may be required. This is due to the nature of a rupture of a pipeline transporting compressible fluids, in that a crack in normal carbon steel can propagate faster than the fluid can decompress. As a result, fracture tough steel or crack arrestors are needed to avoid extensive pipe ruptures.
Compression with adequate interstage cooling is required initially to bring CO2 from a gas at ambient conditions to the dense phase. Once at dense phase, pumps can be used to raise the pressure to pipeline inlet conditions. The compression/pumping will require about 82 kWh to raise 1 tonne per hour of CO2 from atmospheric to 10.34 MPa.
As the fluid traverses down the pipeline, it will slowly lose pressure to friction. Care must be taken to maintain the CO2 in the dense phase. When the pressure drops to near critical levels, intermediate pump stations can be used to reboost the pressures to adequate levels. Reboosting of the pressure from 6.89 MPa to 10.34 MPa only requires about 2.0 kWh per tonne, as opposed to the 82 kWh per tonne for the initial compression.
C1.2 Pipeline permitting and public safety
The U.S. Pipeline and Hazardous Materials Safety Administration (PHMSA) has jurisdiction over CO2 pipelines in the USA with regard to pipeline operation and safety. The PHMSA goals are provided below.
- Increasing pipeline safety.
- Reducing environmental impact of pipeline transport.
- Helping to maintain reliability of the pipeline systems.
- Enhancing pipeline standards across national boundaries.
- Increasing preparedness.
Oil and natural gas pipelines also fall under PHMSA jurisdiction.
CO2 pipeline route selection, similar to most other pipelines transporting potentially hazardous fluids, is influenced by a number of factors. These are listed below.
- Environmental impacts and mitigation.
- Sociological impacts and mitigation.
- Public safety during construction and subsequent operation.
- Constructability and terrain.
- Access during construction and subsequent operation.
- Failure consequences.
Pipelines will generally follow existing utility easements and rights-of-way whenever possible. Regulations for route review and approval vary from jurisdiction to jurisdiction. Most jurisdictions also have a regulatory or legal process for acquiring the land and upon which to build a pipeline.
Jurisdictions also have construction and operations permitting requirements specific to their locale. These permits may also vary depending on the fluid being transported. In the USA, there is no overall nationwide permitting agency for CO2 pipelines, but there are numerous Federal and State agencies that may require permits along a given pipeline route.
C1.3 Pipeline construction
Constructing CO2 pipelines is no different than any other hazardous liquid or gas pipeline. The techniques, equipment and personnel skill requirements are those typical of the current state of the industry worldwide.
Certainmeasures may be more rigorously applied for these pipelines to ensure a higher level of integrity. This is because operational leaks in CO2 pipelines may not be readily detectable since the gas is invisible and odourless. For example, it would be common to x-ray all welds on CO2 pipelines in order to increase the possibility of detecting flaws.
It is also required to more thoroughly clean and dehydrate (remove moisture) the constructed pipeline before introducing the CO2 product into the system. As noted previously, the presence of water can lead to acid formation, which could ultimately damage the steel pipe.
C1.4 Pipeline operation
As was true for construction, the operation of a CO2 pipeline is essentially the same as that of other common pipeline systems. The safety systems, telecommunication systems, maintenance, and repair processes are characteristic of the industry. The most significant operational concern is to control the release of the CO2 from the pipeline (when required for maintenance or repair) in a safe way.