CCS involves capturing CO2 from a source such as a power plant or industrial facility and transporting it, typically via a pipeline, to a storage site where it is injected underground either for the sole purpose of storage or for Enhanced Oil Recovery (EOR).

Although CO2 transport experience exists, there are several major differences, in terms of the regulation, risk, routing, design, purpose, operation and maintenance of a CO2 pipeline network, between the current experience and the requirements of a pipeline that is transporting ‘captured carbon dioxide’.

Properties of CO2

The phase diagram for pure CO2, which contains two distinct features, the triple point (0.52 MPa, -56° C) and the critical point (74 bar, 31° C) is presented in Figure 1. In the vicinity of the triple point, CO2 can exist as one of three phases: solid, liquid or gas. The curve connecting the two points is the vapour-liquid line separating the gaseous and liquid phases. At pressures and temperatures above critical, CO2 no longer exists in distinct gaseous and liquid phases, but as a supercritical phase with the density of a liquid but the viscosity of a gas. Increases in pressure no longer produce liquids at temperatures exceeding the critical temperature. At pressures above but temperatures below critical, CO2 exists as a liquid whose density increases with decreasing temperature. For these reasons the most efficient way of transporting CO2 by pipeline is in its supercritical or dense phases in the vicinity of the critical point.

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Impurities

The current purpose of CO2 transport in the United States is for EOR which demands CO2 with high purity for both miscible and immiscible oil recovery. It should be noted that some impurities are known to decrease the minimum miscibility pressure (MMP) of the extracted fluid in immiscible recovery projects thereby making the injection process more efficient. In contrast, other impurities such as sulphur, SOx, NOx and other contaminants in the CO2 stream increase the MMP for miscible recovery and are therefore undesirable in the CO2 used for EOR. These along with a number of other impurities such as sulphur, H2 and CO are potential impurities from power stations.

The effect of impurities on pipeline transportation

Impurities in the CO2 product stream affect many aspects of CO2 pipeline transportation. This is because the presence of impurities change:

* the physical and transport properties of CO2 and affect the risk modelling, hydraulics (number of compressors, compressor power and temperature transients, etc); and, * other aspects such as fracture propagation; corrosion, due to changes in the solubility characteristics of water in CO2; non-metallic component deterioration; the formation of hydrates and clathrates; and, also the pipeline capacity itself.

Impurities such as SOx, NOx and argon in the CO2 have not been transported before in a CO2 pipeline and their effects are not fully understood. It should be noted that the composition of the capture stream for CCS varies in different literature and that the same capture technology may produce different combinations of impurities depending on the process ‘clean up’ regime.

Modelling the physical and transport properties of CO2 with impurities

In order to demonstrate some of the physical effects of impurities on the transport properties of CO2, some modelling has been conducted to calculate both the physical properties of pure and impure CO2 and also the transport properties of the CO2 stream. The physical property modelling acts as an input into the pipeline model. The flow diagram for the modelling process is shown in Figure 2. It should be noted that there is an important difference between calculating phase equilibrium compositions and calculating typical volumetric, energetic or transport properties of fluids of known composition. In the latter case (e.g. pipeline) the properties of a mixture as a whole are of interest, whereas in the former the partial properties of the individual components that make up the mixture are required and how those properties affect the mixture as a whole.

The carbon dioxide pipeline modelled has a starting pressure of 110 bar and a temperature of 50°C with an ambient temperature of 5°C. The pipeline wall thickness is 0.5 inches with an inner diameter of 15 inches. The roughness factor used was 0.0475 mm. The average flow rate is approximately 72 kg/s with an overall pipeline heat transfer of 1.135 W/m2/K.

The flow equation used is the Beggs-Brill Moody pressure gradient equation. There are several reasons for selecting this equation: as a two phase method, it can accommodate gas/liquid flow if the equation of state predicts liquid dropout; the method is known to accurately predict even small amounts of liquid dropout and it is also good for all inclination angles; and finally this method works for both single and multiphase fluids and the transition between the two regimes is continuous.

A commercially available modelling software package was used for modelling both the physical and transport properties. This software has been developed for pipeline hydraulic modelling in the oil and gas industry for and has not been used for supercritical carbon dioxide pipeline modelling.

Physical property modelling

The pressure temperature or phase diagram for pure CO2 (Figure 1) shows the phase diagram for pure CO2 generated using the software. The addition of impurities to the product CO2 stream causes a change in the resulting phase envelope. The effect of the impurities is to increase the width of the phase envelope and results in the formation of a two phase gas-liquid region. Some impurity combinations tend to cause a large increase on the envelope (e.g. hydrogen) whilst others show a much smaller increase (e.g. N2 and NO2).

The critical temperature and pressure also changes as does the area of the supercritical region with the addition of impurities. Only the CO2-NO2 combination shows a decrease in the critical temperature and pressure and a subsequent increase in the supercritical area as the amount of NO2 is increased. Hydrogen, on the other hand, shows a large increase in the width of the phase envelope and the critical temperature and pressure with increasing quantity. The increase in the critical temperature and pressure decreases the supercritical area thus reducing the optimum pipeline operating region. The reverse is true for the decrease in the critical point.

Pipeline operation within the two phase region is to be avoided due to cost and throughput optimisation, therefore the pressure has to be maintained above the critical pressure in the pipeline (i.e. the CO2 has to be kept in dense phase). Gas phase transport is disadvantaged by the low density (and consequently large pipe diameter) and high pressure drops. It is not used for pipelines of any significant capacity and length. Therefore the supercritical phase is chosen for the transport of CO2. The interaction of these impurities has also to be taken into account. Figure 3 shows an example of a ternary CO2 system (i.e. more than one impurity) and as the number of impurities increase so does the phase envelope width and also the critical temperature and pressure.

The density of CO2 changes with temperature and pressure and also with the level of impurity. Figure 4 illustrates the non-linear relationship between pressure and density for CO2 containing impurities of H2, Ar, NO2 and N2. It can be seen that there is a discontinuity in the relationship around 50 bar for pure CO2. At this point a small change in either temperature or pressure can have a large effect on the density. Below this pressure it is possible that high density liquid and low density gas could exist in the pipeline which would result in liquid slugs and potential damage to compressors. CO2 fluids with high densities are desired to reduce the pipeline size and improve throughput. This reduces the investment cost but increases the running cost and vice versa if low density CO2 is used.

Pipeline transport modelling

As stated above, the three main parameters affected by the properties of the CO2 stream are the density, pressure and temperature profile along the pipeline. In terms of the pipeline, the effect of these parameters translates into recompression distances and compressor power requirements, which in turn would have cost implications.

Conclusions

There is very little work published on CO2 pipeline transportation with the types of impurities captured from power plants. These impurities have great implications on the physical and transport properties of CO2.

The phase properties of CO2 vary with impurities, generally increasing the two phase region and changing the critical temperature and pressure of the mixture with increasing amount of impurities.

It is clear that further research work, particularly experimental, is required in these areas to further understand the implications of impurities and to ensure that pipelines transporting anthropogenic CO2 offshore are designed and operated safely and economically.