CO2 capture will initially be carried at large point sources such as power stations to reduce costs. In most cases it is unlikely that these sources of CO2 will be near to anywhere that could be suitable for storage of CO2. This means the CO2 will require transportation to the storage sites. Transport refers to the movement of the CO2 after the capture process until it is safely stored in a suitable location. The main complication with CO2 transport is how it behaves, as small temperature and pressure changes can result in CO2 rapidly changing phase. CO2 may be transported in all four phases: gas, liquid, solid as well as a supercritical fluid depending on the temperature and pressure conditions. There are currently two methods used to transport large volumes of CO2 – by pipelines or by storage vessels.
On a commercial scale CO2 can be transported via tanks, pipeline and ships. For large scale CO2 capture and storage projects, it is more economic to consider using pipelines as these will be dealing with continuous supplies of CO2 over large distances from source to storage. Public perception is that there is significant experience with pipeline design and that CO2 is relatively benign. Those in the industry know that this is not the case and that special design considerations need to be implemented for pipelining CO2.
Differences between existing and new CO2 pipelines Cosham and Eiber (2008)
CO2 transport is complicated by the presence of impurities within the captured CO2. To understand the risks associated with transporting CO2 over long distances, it is important to be aware of the physical properties of both the CO2 and any present impurities. In its pure state CO2 is a colourless, odourless, non-flammable and non-toxic substance. Impurities influence the hydraulic parameters such as the pressure and temperature conditions, but also the density and viscosity of the fluid, depending on what impurities are present. Hydrogen or nitrogen can cause higher pressure and temperature drops, although depending on the mixture of impurities this can change. Excessive water content in CO2 can cause formation of highly corrosive carbonic acid which can in turn corrode and alter the integrity of the pipelines. While the solubility of water in pure CO2 is well known as a function for pressure and temperature, few data are available for the effect of trace chemicals on solubility.
The most efficient method of transporting CO2 in pipelines is during its supercritical state also known as when it is a dense phase liquid. In this form, it has the density of a liquid but the viscosity of a gas. This allows more CO2 to be transported in higher levels in smaller pipelines. Comparisons have been made between CO2 pipelines and existing natural gas pipelines. This is to obtain the risks associated with the transport of CO2 and as such whether it should be regulated as a dangerous fluid under the Pipeline Safety Regulations. However, unlike natural gas, CO2 does not require ignition for it to cause harm. It most likely will be transported at higher pressure than that of natural gas which changes the transport properties as well as the corrosive behaviour of the CO2 – this is to prevent phase changes during transportation. Because the supercritical CO2 behaves as a liquid in the pipeline, pumps, rather than compressors, are used at CO2 pipeline booster stations. Fractures in CO2 pipelines do not propagate in the same manner as natural gas pipelines. In addition, leakages of CO2 do not behave in the same manner as natural gas leaks. CO2 can accumulate in low-lying areas such as depressions near pipelines.
Pipeline Components and Durability
When the distance needed to transport the CO2 is over several km, carbon steel is the most cost effective choice of material for pipelines. Carbon steel requires that the CO2 is dried to eliminate any free water. Water in the pipeline is removed or the use of inhibitors to reduce corrosion caused by the free water. Carbonic acid can lead to corrosion rates of up to 1-2mm within two weeks on standard carbon steel pipelines.
The solvent properties of supercritical CO2 on its own can damage some elastomers commonly used in valves, gaskets, coatings and O-rings used for sealing purposes. Elastomers are permeable to CO2 and can allow the CO2 diffuse into it, resulting in a pressure release which may cause explosive decompression and blistering. While operating under pressurised conditions, no harmful effects are noted on pipelines. Problems arise when there is rapid decompression. As the pressure outside the elastomer falls below that of the gas contained in the elastomer, the gas begins to expand and move towards the surface, leading to fractures or ruptures. Some synthetic lubricants can harden in the presence of CO2. When the structural integrity of a pipeline is compromised, there is a chance of a failure. This is when there is an uncontrolled release of CO2 from the transport pipeline and is known as a blowout.
The CO2 will convert from the supercritical state to vapour phase as it expands. When the CO2 is rapidly released, there is a strong cooling as the CO2 expands. This is known as the Joule-Thomson effect. Once the CO2 streams falls beneath the triple point temperature and pressure (216.55 K and 517 kPa), solid dry ice particles can form. This cold CO2 condenses water in the atmosphere, resulting in a white cloud.
CO2 pipelines are currently in existence. The majority occur within North America. Those that are operational are mainly part of the oil industry as CO2 can be used in enhance oil recovery (EOR). EOR is a general term for techniques which involve improving the amount of crude oil extracted from an oil field. CO2 has been used in EOR since the early 1960’s. During EOR, more than half of the injected CO2 returns to the surface with the produced oil. The recovered of the CO2 is normally injected back into the subsurface to reduce costs. The remainder CO2 is trapped in the subsurface by various mechanisms.