2.1 The Properties of CO2

CO2 gas is found in small proportions in the atmosphere (about 385 ppmvd1); it is assimilated by plants which in turn produce oxygen by photosynthesis. It is produced from the combustion of coal or hydrocarbons, the fermentation of liquids and the breathing of humans and animals. CO2 is also found beneath the earth's surface and emerges during volcanic activity, in hot springs and other places where the earth's crust is thin. It is found in lakes, and at great depth under the sea2. It is also commingled with oil and gas deposits.

CO2 comprises two oxygen atoms covalently bonded to a single carbon atom, with an O-C-O angle of 180°. As such, it is very stable with no known natural process other than photosynthesis capable of reducing CO2 to oxygen.

CO2 is widely used commercially. It is employed in the chemical processing industries to control reactor temperatures, to neutralise alkaline effluents, and used under supercritical conditions for purifying or dyeing polymer, animal or vegetable fibres.

In the food and beverage industries, CO2 is used for the carbonation of beverages such as soft drinks, mineral water or beer; for packaging of foodstuffs; as a cryogenic fluid in chilling or freezing operations; or as dry ice for temperature control during the distribution of foodstuffs. Caffeine can be removed from coffee using supercritical CO2.

In the medical field, CO2 produces close to physiologic atmospheres for the operation of artificial organs. CO2 is used as a component in a mixture of oxygen or air as a respiratory stimulant to promote deep breathing. It is also used for surgical dilation by intra-abdominal insufflations.

In industry, CO2 is typically used for environment protection, examples of which include its use of CO2 for red fume suppression during scrap and carbon charging, for nitrogen pick-up reduction during electric arc-furnace tapping and for bottom-stirring. In non-ferrous metallurgy, CO2 is used for fume suppression during ladle transfer of matte (copper/nickel production) or bullion (zinc/lead production). CO2 is used to enhance the recovery of oil and gas from wells where primary and secondary methods are no longer cost-effective on their own.

CO2 is also used in fire extinguishers and as 'dry ice' for stage and other effects.

2.1.1 General thermodynamics

2.1.1.1 Physical properties of pure CO2

Pure CO2 exhibits triple-point behaviour dependent on the temperature and pressure, as shown in Figure 2.1:

Figure 2.1 CO2 phase diagram

The triple point (at a pressure 5,11 bar and temperature of -56,7 °C) is defined as the temperature and pressure where three phases (gas, liquid and solid) can exist simultaneously in thermodynamic equilibrium. The solid-gas phase boundary is called the sublimation line, as a solid evaporating directly into a gas is called sublimation. Physically, this boundary implies that the gas and solid can co-exist and transform back and forth without the presence of liquid as an intermediate phase.

Above the critical point (73,8 bar and 31,1 °C), the liquid and gas phases cannot exist as separate phases, and CO2 develops supercritical properties, where it has some characteristics of a gas and others of a liquid.

In the event of an uncontrolled release of CO2 (e.g. damage to a pipe containing liquid CO2), a portion of the escaping fluid will quickly expand to CO2 gas. The temperature of the released gas will fall rapidly due to the pressure drop (Joule Thompson effect, see 2.1.1.2) and phase changes. In above ground applications some of the released CO2 will form CO2 'snow': as a result of the low temperature of the CO2, the surrounding air will also be cooled down. This could cause the water vapour in the air to condense locally, which will resemble a thick fog.

The situation for most subsea applications is different: most of the CO2 will expand to a gas as a result of expanding into the lower pressure of the water. Heat from the water will quickly be absorbed, and CO2 gas, being less dense than seawater3 (see 2.2.6) will tend to rise toward the surface. As the gas rises some of it will dissolve in the seawater (see 2.2.2), and the liquid portions will usually expand and emerge as a gas. This gas will be relatively cold, in spite of having absorbed heat from the surrounding water, and may give rise to local fogging, dependent on local climactic and wind conditions. Section 3 contains a modelled example of this dispersion.

The phase diagram, as shown in Figure 2.1, is a common way to represent the various phases of a substance and the conditions under which each phase exists. However, the graphic suggests little regarding how the changes of state for CO2 occur during transition. The CO2 pressure-enthalpy diagram (P-H), shown in Figure 2.2, or temperature-entropy (T-S) diagrams provide insight into transient conditions such as phase changes, energy transfers, and density, pressure and temperature changes during depressurisation, e.g. for a leak of CO2 from a vessel or a pipeline.

Figure 2.2 Pressure/enthalpy diagram for pure CO2

In order to understand and interpret such a diagram some basic thermodynamic theory and terms need to be established. The adiabatic (no heat exchanged) expansion of a gas may occur in a number of ways. The change in temperature experienced by the gas during expansion depends not only on the initial and final pressures, but also on the manner in which the expansion is carried out.

Isenthalpic expansion is a theoretical expansion which takes place without any change in enthalpy. In a free expansion, the gas does no work and absorbs no heat, so the internal energy is conserved. Expanded in this manner, the temperature of an ideal gas would remain constant, but the temperature of a real gas may either increase or decrease, depending on the initial temperature and pressure. This is called the Joule Thompson (JT) effect, and although usually the effect is referred to for gases, it applies equally to liquids.

2.1.1.2 Joule Thompson Effect

The JT effect refers to the change in temperature observed when a gas expands while flowing through a restriction without any heat entering or leaving the system. The change may be positive or negative and can involve a phase change as, for instance, a liquid flashes off to a gas. For each gas, there is an inversion point that depends on temperature and pressure, below which it is cooled and above which it is heated.

The amount by which the fluid cools on expansion (measured in °C/bar) is called the JT coefficient, µJT. Gaseous CO2 (taken to be at 4°C) has a particularly high µJT compared with other common gases, as shown in Table 2.14.

Table 2.1 JT coefficient for a number of fluids

Figure 2.3 Changes in temperature as a result of the JT effect for pure CO2 at 4 °C

The cooling effect of dense phase CO2 is a combination of both the phase change and flashing of the liquid to a gas. Figure 2.3 gives some examples of the change in temperature that might be expected from a differential pressure change (CO2 is taken to be liquid at 4 °C).

The value of µJT varies with the temperature of the CO2, with the higher values at lower temperatures. The effect of higher pressures at 4 °C, typical of offshore CCS applications, is shown in Figure 2.4, which indicates that the value of µJT for liquid CO2 at 4 ° varies slightly with pressure (for small changes in differential pressure). This section of the graph is exaggerated as Figure 2.5.

Figure 2.4 Changes in isothermal JT coefficient (µJT) for pure CO2 at 4 °C

Figure 2.5 Changes in isothermal JT coefficient (µJT) for pure liquid CO2 at 4 °C

For CO2 the inversion temperature, at atmospheric pressure, is 1 500 K5 (1 226,85 °C), which means that CO2 gas always cools by isenthalpic expansion for all conditions relevant for CCS applications.

Isentropic expansion takes place if the expansion process is reversible, (meaning that the gas is in thermodynamic equilibrium at all times). In this scenario, the gas does positive work during the expansion, and its temperature decreases. Here, the temperature drop will be greater than for isenthalpic expansion.

Figure 2.6 shows the pressure-density behaviour of pure CO2 during rapid decompression from 130 bar (13,1 MPa) and 5 ˚C, and it can be seen that solid, liquid and gas phases are all present.

Figure 2.6 Pressure-density behaviour during rapid decompression of pure CO2

However, impurities within the CO2 can change both the shape of the diagram and the location of the critical point (see 2.1.2).

2.1.1.3 Gaseous phase CO2

CO2 gas is colourless, heavier than air (1,521 times as heavy, with a density of about 1,98 g/litre), has an unpleasant odour, and freezes at -78,5 °C to form CO2 snow. Sub-cooling of the CO2 below this level is possible in some circumstances.

The effects of inhaling CO2 and the limit values for working when there is CO2 in the atmosphere are described in 3.2.

An escape of CO2 gas, because it is heavier than air, can accumulate in depressions in the ground and in basements or sumps. However, high pressure releases tend to become neutrally buoyant very quickly due to air entrainment. The release will disperse as a result of air movements, and models that assist in the prediction of this are described in Annex C.

2.1.1.4 Liquid phase CO2 CO2

cannot exist as a liquid at atmospheric pressure. At a pressure of anything above 5,11 bar and at a temperature between -56,6 °C and 31,1 °C it becomes liquid (see Figure 2.1). Within this 'bracket' or 'envelope' its density can rise up to 1 180 kg/m3. Were a cubic metre of liquid CO2 from a subsea pipeline at 4 °C and 200 bara to be released and expand to 1,013 bar (atmospheric pressure) and 4 °C it would occupy a volume of about 520 m3.

In practical terms the majority of the CO2 in offshore CCS applications will be in liquid form (i.e. still sub-supercritical, although close to supercritical), since it will be at high pressure (>100 bar) and low temperature (<31 °C): as a result, pipes to export it from the production to the storage sites can be much smaller than if the CO2 had been transported as a gas.

Liquid CO2 at 4 °C has a high density (~950 kg/m3) and a low viscosity (~0,11 cp), as described in 2.1.2.2. It also has a low surface tension (approx. 1,5 mN/m).

2.1.1.5 Supercritical phase

Above its critical temperature (31,1 °C) and pressure (72,9 bar), pure CO2 takes on the properties of a supercritical fluid. These properties include expanding to fill its container like a gas but with a density similar to that of a liquid. As it approaches the supercritical condition, the meniscus between liquid and gaseous CO2 disappears.

The properties of supercritical fluids lie between those of gases and liquids; a supercritical fluid has densities similar to that of liquids, while the viscosities and diffusivities are closer to that of gases. A supercritical fluid can diffuse in a solid matrix faster than a liquid, yet possess a solvent strength to extract the solute from the solid matrix.

Supercritical CO2, with a high degree of purity, is becoming an important commercial and industrial solvent due to its role in chemical extraction. This is aided by its low toxicity and relatively benign environmental impact. The solubility of solids in supercritical CO2 can be 3-10 orders of magnitude higher than in liquid CO2. The relatively low temperature of the processes, and the stability of CO2, also allow most compounds to be extracted with little damage or denaturing. This is particularly useful when extracting volatile oils and fragrances for the perfumery industry. The amount of CO2 used in these applications is very small compared to the volumes that will be produced during CCS. Whilst the fundamental physics of supercritical CO2 is understood, work is in hand to assess the impact of large releases such as might be encountered in CCS applications.

When transporting CO2 offshore for CCS applications, the CO2 will not usually be in the supercritical phase, because its temperature will be below that at which supercritical properties apply, i.e. 31,1 °C for pure CO2 (see Figure 2.1).

2.1.1.6 Solid CO2

If liquid CO2 is cooled to -78,5 °C at atmospheric pressure, it becomes solid (see Figure 2.1), and its density rises to 1 562 kg/m3. Solid CO2 has a snow-like appearance, and can be compressed into blocks to form 'dry ice'.

Once formed, solid CO2 can take a significant length of time to thaw out, at which point it vaporises to form CO2 gas.

2.1.2 Effect of impurities

2.1.2.1 Effect on phase diagram

2.1 refers to the properties of pure CO2. CO2 from CO2-capture plants will generally not be pure, and some of the impurities affect the properties of the liquid. Hydrogen, for instance, a possible impurity arising from pre-combustion capture plants, affects the triple point, and does not dissolve in the liquid until the pressure is quite high (>92 bar at 30 °C). Undissolved hydrogen may, for instance, cause cavitation in CO2 booster pumps, as two-phase flow could be experienced, where there is the potential for a loss in containment. As such, hydrogen may be seen to have the greatest potential for impact when it makes up part of the impurities in a CO2 gas stream. This might have formed in a pre-combustion capture process, for instance, in an integrated gasification combined cycle (IGCC) facility.

Modelling6 has indicated that these impurities will change both the shape of the phase diagram, shown as Figures 2.1 and 2.2, and the location of the critical point. Instead of the transition between vapour and solid being sharp, represented by a line, it becomes more gradual. This area is sometimes referred to as an 'envelope' or 'phase change envelope', in which solid, vapour and liquid CO2 co-exist.

However, other impurities may also be present. Figure 2.7 shows an extreme example of this effect for a 95 % CO2/5 % N2, a 90 % CO2/5 % N2/5 %CH4 mix, a 90 % CO2/5 % N2/5 % NO2 mix, and a 90 % CO2/5 % N2/5 % NO2 mix, compared to pure CO2. These kinds of mixtures are possible from IGCC, pre-combustion natural gas, pulverised coal and steam methane reforming (SMR) applications respectively, although commercial operations would not normally have these levels of impurities. The arrows indicate the critical point for each mixture. The results shown are indicative only.

2.1.2.2 Effect on density and viscosity of CO2

Figures 2.8 and 2.9 show the effect on density and viscosity respectively of 2 mol % impurity of a number of possible gases on pure CO2 at a pressure of 100 bara over the temperature range of 0 ˚C to 50 ˚C. These graphs have been compiled using the Peng Robinson equation of state and are for illustrative purposes only. Note: these, and other properties, need to be confirmed by experiment, because equations of state, such as Peng Robinson are not always accurate, as Figure 2.9 (which also shows the change in the density of pure CO2 with temperature according to the experimentally-derived Span-Wagner model) illustrates.

Figure 2.7 Effect of impurities on phase diagram around the critical point

Figure 2.8 Effect of 2 % impurities on the density of CO2 with changes in temperature at a pressure of 100 bar

It may be seen in Figure 2.8 that H2S has a minimal impact on density, whereas the effect of hydrogen is significant. Sulfur dioxide (SO2) also has a significant effect, often pulling the properties in the opposite direction from hydrogen.

Figure 2.9 Effect of 2 % impurities on viscosity of CO2 with changing temperature at a constant pressure of 100 bar

It may be seen from Figure 2.9 that:

- Non-linearities appear in the 25 - 35 ˚C range, because the program used (ProMax) models the supercritical fluid as a vapour. Generally, computer models do not deal well with the phase transition around the critical point. Real data are available in this region.

- H2S has a minimal impact on viscosity, whereas the effect of hydrogen is significant.

Another factor in predicting the behaviour of CO2 is the source term used. Figure 2.10 shows the same parameters (density against temperature for pure CO2 at a pressure of 100 bar) using two different equations of state, Span-Wagner and Peng-Robinson. Whilst these give generally consistent answers, it does underline the need to validate models with experimental measurements.

Figure 2.10 Effect of different models of equation of state on the density of pure CO2 at 100 bar

Graphs indicating the modelled effect (using ProMax) of impurities on the density and viscosity of CO2 with varying temperature and pressure at a fixed temperature of 4 ˚C are shown as Figures 2.11 and 2.12 respectively. Comparing this with the conclusion from Figure 2.8, it may be seen in Figure 2.9 that adding 2 mol% of H2S has a minimal impact on density, whereas the effect of hydrogen is significant. In all cases except for SO2 the effect is to reduce the density. The same conclusion may be drawn about viscosity from Figure 2.12.

Figure 2.11 Effect of 2 % impurities on the density of CO2 with differences in pressure at 4 ˚C

Figure 2.12 Effect of 2 % impurities on viscosity of CO2 with differences in pressure at 4 ˚C

The presence of hydrogen as an impurity has the greatest potential to produce two-phase flow in pumps. As the pressure reduces, or as the temperature increases, the hydrogen starts to come out of solution (the bubble point7), and the fluid being transported becomes two-phase. This situation should be avoided, because pumping the liquid CO2 becomes difficult, as cavitation will take place. As the hydrogen bubbles collapse they will cause very large local pressure transients which over time will cause damage to the impeller and possibly catastrophic failure.

Figure 2.13 shows the bubble point for a mixture of hydrogen and liquid CO2 over a range of temperatures that is appropriate to offshore CCS applications. It can be concluded that two-phase flow will not take place over a range of hydrogen concentrations at pressures above 80 bara.

Figure 2.13 Bubble point for hydrogen in CO2 for a range of hydrogen contents

Notes:

The hydrogen contents are not intended to represent the expected levels, but are chosen to demonstrate the phenomenon of theoretical changes to the bubble point.

2.1.2.3 Effect of multiple impurities

Figures 2.82.9 and 2.112.13 have considered the impact of a single impurity on the characteristics of pure CO2. Some work on the impact of multiple impurities has been carried out by Newcastle University8, further to the work shown in Figure 2.7. Six different cases were examined (see Table 2.2), as being typical of some possible compositions of CO2 (see Table 2.3, based on IPCC compositions) from the different processes. Table 6.2 also includes the Dynamis project assumed CO2 composition9, which is intended to be a practical level which could reasonably be expected to be achieved for CCS.

Fuel/capture route Post-combustion Pre-combustion Oxy-fuel
Coal Case 1 Case 3 Case 5
Natural gas Case 2 Case 4 Case 6

Table 2.2 Case definition for reference 8

Table 2.3 Impurities assumed (vol %)

The impact of these impurities on the phase envelope is shown in Figure 2.14, in addition, the result for pure CO2 is included. It may be seen that the effect of the impurities of cases 1 and 2 have little effect on the CO2, whereas those in case 3 have a significant impact, both in terms of the critical point, and opening up the phase envelope to a large two-phase area containing liquid and gaseous CO2.

Figure 2.14 Impact of impurities from Table 2.3 on the properties of CO2

1 ppmvd is 'volume parts per million dry'

2 Lakes of CO2 in the deep sea, Kenneth Nealson, Department of Earth Sciences, University of Southern California, 19th September 2006.

3 CO2 at 200 bar and 4 °C has a density of 1 050 kg/m3. Seawater does not reach this density until a depth of approximately 3 400 m, at which point the pressure would be 350 bar, and any leak in the pipeline would result in seawater ingress, not CO2 spilling on to the ocean floor.

4 Parameters are 4 °C at the 10 – 30 bar range, assembled from National Institute of Standards and Technology Material Measurement Laboratory standard reference data.

5 Perry's Chemical Engineering Handbook, McGraw-Hill, 2007.

6 Transporting the Next Generation of CO2 for Carbon, Capture and Storage: The Impact of Impurities on Supercritical CO2 Pipelines P Seevam, J Race, M Downie, Newcastle University; P Hopkins, Penspen Ltd., 7th International Pipeline Conference, 29 September - 3 October, 2008, Calgary, Canada

7 The 'bubble point' is defined as the conditions (in terms of temperature and pressure) at which the first bubble of vapour forms within a liquid and begins to rise to the top.

8 CO2 Transport UKCCSC Progress Report, Seevam, Race, Downie, Newcastle University, 2008.

9 Towards Hydrogen and Electricity Production with CO2 Capture and Storage Dynamis project thematic priority:6.1.3.2.4 Capture and sequestration of CO2, associated with cleaner fossil fuels, de Visser and Hendriks et al, Ecofys, July 2007