Conversion of existing natural gas pipelines to transport CO2

Repurposing natural gas pipelines is a viable option to consider in terms of reducing CO2 emissions and implementing CCS, but it would require careful planning and execution to ensure that the process is done safely and efficiently.

CO2 can be transported in two different forms, gas phase or dense phase (supercritical fluid)

1. Supercritical CO2 fluid will have a density of liquid but a flow of gas. Supercritical CO2 transportation is more economical than gas-phase transportation.

2. Supercritical CO2 fluid needs to be transported at a minimum of 1070 psi to avoid phase change. The critical point of CO2 at 30.9oC is 1070 psi. To account for temperature fluctuations, we need to keep a minimum of 1200 psi.

3. The majority of current supercritical CO2 pipelines are operating at 1900 psi. The suggested range is 1200-2200 psi.

4. The gaseous form of CO2 transportation should be below 1070 psi. It is also dependent on the pipeline temperature. From the graph, for a pipeline operating at 10oC, the gas phase transportation should be around 600 psi. Raising the pressure above 600 psi @10oC could cause condensation and associated operational problems.

Source: Principles of General Chemistry, Bruce A. Averill, 2012

The below table summarizes the possible challenges an operator may encounter while repurposing the existing NG pipelines.

Operators should conduct internal pigging, inline inspections, and/or hydrostatic tests, MAOP must be re-established before introducing CO2 in the pipeline. In addition to technical aspects, regulatory and legal issues would also need to be addressed.