In my last post I covered the very basics on how Carbon is Captured at power plants or other large point sources. The next step is the transport of CO2 (no not Storage, even though it is called Carbon Capture and Storage (CCS) and not Carbon Capture Transport and Storage (CCTS), no one knows why they missed the transport part!) from the capture point to the storage site. This normally happens through pipelines, but also through other transport media such as large ships.
If CO2 is transported via a pipeline it can either be transported as gas or as liquid or as “dense phase” (see phase diagram below). To have a predictable flow within the pipeline it is crucial that there is only a one-phase flow and not a two- or multi-phase flow. This means that the pressures and temperatures along the pipeline have to stay constant to not get both gas and dense CO2 in the pipeline.
However, of the above mentioned three options, transport in a liquid state is very uneconomic as the pipelines would need constant cooling in order to keep the CO2 in a liquid state. Thus only gaseous and supercritical (dense) phase CO2 will be transported. A pipeline with a given diameter can transport more CO2 in a dense state than in a gaseous state. However, the pressures needed for transporting dense phase CO2 are higher and thus the strength of a pipeline used for the transport of gas is not necessarily high enough to transport dense phase CO2 . This also means that pipelines that have been used to transport oil and gas from a reservoir might need a retrofit/replacement if CO2 should be stored in the depleted oil and gas reservoir. The steel used for building CO2 pipelines also needs to be able to withstand higher corrosion than normal pipeline steel as CO2 in combination with water (a possible impurity) can form a highly corrosive acid that can corrode up to 1-2mm of steel within 2 weeks.
A common problem scientists and engineers face when planning a CO2 pipeline is the fact that, depending on the mechanisms that was used to separate the CO2 from the other flue gases (see last entry), there are small amounts of other fluids/gases in the pipeline such as water or nitrogen. This means that instead of a one-phase flow multi-phase flow may occur, making the properties of flow much less predictable. Thus it is very important that the CO2 derived from the capture plants is as pure as possible and that the pipelines are dry before CO2 is flown through them the first time.
While these factors are controllable , other factors such as damage to the pipeline by external forces are not. If the structural integrity of a pipeline is compromised there is the chance that the pipeline fails and CO2 leaks through a fracture or hole. You have maybe seen images of a fractured hydrocarbon pipeline (see below) in context with oil spills. In comparison to an oil spill a CO2 leak would not be too bad: CO2 is denser than air and would flow to areas of low topography. However, wind and sunlight would quickly disperse it in the atmosphere (as can bee seen at CO2 degassing in volcanic areas).
The reason why a small leakage from a CO2 pipeline is likely to be fatal is another: Due to the depressurization of the pipeline (as CO2 leaks the pressure drops), the CO2 within the pipeline will convert from dense state into vapour phase and expand. The rapid cooling and the rising pressure due to expansion may affect the pipeline steel severely – it is likely that the fracture starts propagating along the pipeline, leading to its destruction. By adjusting the pipeline design and adding so-called crack arresters along the pipeline (every few 100m or so) the vulnerability of a CO2 pipeline can be reduced.
In general the transport of CO2 in pipelines is no problem – it has been done for 10’s of years in the US to transport CO2 from the Colorado Plateau to the Gulf of Mexico and Texas where it is used for enhanced oil recovery without major incidences. The challenge is to design new pipelines that are economically (use as few steel as possible) without compromising the safety of the pipelines.
I hope this entry gave you a quick overview what challenges CO2 transport holds. Don’t hesitate to ask any question! Short shout-out to Rachel for giving valuable insights 🙂