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In the previous two EESG newsletters we included first an introduction to carbon capture and storage (CCS) and then some more detail about the processes from which CO2 might be captured and the mechanisms to capture it. This time we are going to look at what is involved in getting the CO2 from the point of capture to the point of disposal. But first of all, we need to understand a little more about CO2 itself, because there are significant peculiarities with CO2 that present challenges to the Engineer.

CO2 Properties

We all probably know that CO2 is a colourless, odourless gas, which we all breathe out. When inhaled at concentrations much higher than usual, it can produce a sour taste in the mouth and a stinging sensation in the nose and throat. It causes the breathing rate to increase and confusion to set in. The density of carbon dioxide at standard temperature and pressure is about 1.98 kg/m³, or, for comparison about 1.5 times that of air. This means that any leak will tend to collect in depressions, sumps and basements. Concentrations above 5,000 ppm (the SLOT, or Specified Limit Of Toxicity) are considered very unhealthy and above about 50,000 ppm there is a Significant Likelihood Of Death (SLOD).

CO2 also has a strong Joule Thompson characteristic. The Joule Thomson effect arises if a gas or a gas mixture experiences a change of temperature during a pressure change. If one strongly compresses a gas, for example air, it warms up. Conversely, it cools down during expansion.

CO2 can exist in different phases, as shown in the following diagram:

Thus dependent on what temperature and pressure the CO2 is at, it might be a solid, liquid or gas. So can water, so what is different here? Well, at atmospheric pressure and −78.51°C, CO2 changes directly from solid to a gaseous phase (through sublimation), or from gaseous to solid (through deposition). Given that it has a strong Joule Thompson effect, if the pressure of CO2 gas in a container is suddenly released, it may rapidly cool to the point at which the remaining CO2 is solid. An operator might thus believe that a pipe is empty, whereas in reality it still contains the toxic gas.

Above the Critical Point (72.8 bar and 31°C), CO2 exhibits supercritical properties – it has no meniscus and has the density of a liquid yet the viscosity of a gas. The properties of supercritical CO2 (SCCO2) are not well known but it is a very strong solvent and is used to dissolve caffeine from coffee. It also dissolves many materials traditionally used for seals and gaskets and causes other sealing materials to explode during decompression. Bulk transport of supercritical CO2 is not presently legal in the UK because of these unknowns.

The CO2 liquid at high pressures and slightly lower temperature is described as being in ‘dense phase’ and, in practical terms, this is the region in which bulk CO2 transport operates. Liquid CO2 has low friction when passing through pipes, thus pressure losses are low: this is a real advantage when moving it over the long distances involved with offshore storage.

CO2 Compression

The CO2 is delivered from the capture plant at relatively low pressure and close to ambient temperature. In order to reach dense phase, it needs to be compressed – to at least 73bar (for pure CO2), higher if it contains impurities, like hydrogen, which it will if it has been captured from a pre-combustion plant. Because of the Joule Thompson effect it will have to be cooled, and therefore compression normally takes place in several stages with intercooling in between.

There are different types of CO2 compressor available: reciprocating, multi-casing in-line centrifugal and integral-gear centrifugal; the lattermost being preferred for Carbon Capture and Storage applications. A compressor to deliver 12,800m3/hour of CO2 at 160 bar could be expected to have 8 stages and consume 4,200kW of motor power.

During the compression process the opportunity may be taken to dry the CO2 and, of course, it will need cooling before it enters a pipe to take it off site, in order to keep it below the 31°C level, so it is not supercritical.

Liquid CO2 can also be pumped using specially designed equipment.

CO2 Pipes

The UK Health and Safety Executive have advised that pipes carrying sub-supercritical CO2 can be designed using existing codes but, to allow for the uncertainties that still exist over its behaviour, they have required it to be categorised very conservatively. On-land, the codes require the pipe to be buried and therefore measures to avoid third party intervention (such as a digger puncturing it with its bucket!) need to be taken. Off-shore, the design and construction are able to draw on the considerable experience that has been gained in the North Sea on both oil and gas pipework systems.

It is possible to transport CO2 in carbon steel pipes, provided that it is sufficiently dry that free water can be avoided, so that corrosion will not take place. Other contaminants can also have an impact on the corrosive or other behaviour of the liquid CO2 and these need to be considered carefully.

The potential for rapid cooling during expansion also carries with it the possibility that, if punctured, the temperature of the pipe in which it was contained may fall to the point at which the steel becomes brittle. Measures, such as using steels with low temperature impact properties, need to be specified.

Good design of CO2 pipes is linked to a risk assessment and dispersion modelling in order to ensure that, in the event of a pipe failure, the impact on both people and adjacent property is within acceptable limits. It is in this area of extrapolating existing models to address the potential impact of large amounts of CO2 being released that additional validation work may prove necessary. The inventory of CO2 discharged in the event of a severe leak can be managed by installing rapid shut-off intermediate sectioning valves and by keeping on-land pipe lengths as short as possible. The problem is more complex offshore but the consequences of a large leak are obviously very different.

The design and operation of CO2 pipes for use in Carbon Capture and Storage applications is a new and exciting area of Mechanical Engineering and one about which much more will be discussed over the next few years as it undoubtedly grows. CO2 disposal may even become the fourth utility along with electricity, gas and water.

Summary

CO2 has interesting properties associated with its ‘triple point’ characteristics. Given an understanding of these, it can be transported in bulk over long distances using carbon steel pipes.

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