Transport of CO2 for carbon capture and storage
Newcastle University, UK, in association with Tiratsoo Technical and Houston-based Clarion Technical Conferences, and supported by the UK-based Carbon Capture and Storage Association, is organizing the first Forum on the transportation of CO2 by pipeline on 1-2 July in Newcastle. Details of the programme for the Forum are currently being finalized, and will be announced shortly at www.clarion.org.
The subject of CO2 transportation by pipeline is of widespread and increasing importance as many governments and communities world-wide come to terms with the issues of carbon capture and storage for the mitigation of climate change: the transportation aspect is often looked upon as the ‘missing link’ in a concept that is already being widely embraced. This article, prepared with the assistance of Professor Martin Downie and Dr Julia Race of Newcastle University, and Patricia Seevam of BP, provides a brief overview of the subject and an introduction to some of the technical issues involved.
Carbon capture and storage (CCS) is perceived as a short- to medium-term measure for closing the energy gap while robust carbon-neutral technologies are developed to provide power in a post fossil fuel energy era. In recent years the capture technology has developed to the point of viability and storage has been accepted to be safe and ecologically sound, but relatively little work has been carried out on CO2 transport. In the US, naturally-occurring CO2 is routinely transported for considerable distances overland, although through mostly sparsely-populated regions, for the purpose of enhanced oil recovery (EOR). There is also some limited transport of captured, or ‘anthropogenic’, CO2. In the UK a number of suitable offshore CO2 reservoirs (or ‘sinks’) have been identified in the North and Irish Seas for EOR, or simply for storage. It has been commonly assumed that the transport of CO2 to offshore sinks is straightforward, typically in the North Sea, and may even be able to utilize the existing pipeline infrastructure. However, there are significant differences between the US experience with ‘clean’ CO2, and the transport requirements for anthropogenic CO2. The UK, for instance, will be dealing with the latter, mostly from power plants, which will impose constraints on the hydraulics that have not yet been fully explored. Considerable proportions of the transport system will be subsea, for which there is as yet virtually no experience; there are questions as to the suitability of much of the existing infrastructure and the desirability of using it; and there is little experience with multi-source transport systems through densely-populated regions.
To understand the issues relating to the transport of CO2, it is necessary to be aware of its physical properties. In brief, pure CO2 is a colourless, odourless, non-toxic, and non-flammable substance which can be transported as a solid, liquid, gas, or dense-phase liquid. The most efficient state of CO2 for pipeline transport is as a dense-phase liquid as, in this phase, the fluid has the density of a liquid, but the viscosity of a gas. In this ‘supercritical’ mode, captured CO2 has to be compressed to a pressure above the critical pressure prior to transport.
CO2 that is captured from power plant and other anthropogenic sources is not pure, and the amount and type of impurities are dependent on the nature of the process and the capture technology used. Currently, there is little published work on transport of CO2 with these impurities, the main effect of which is to change physical properties such as the critical pressure, which can have a consequent dramatic impact on the CO2’s hydraulic behaviour. This in turn may change the operating regime of the pipeline, which may have to be operated at a higher pressure than would be required for pure CO2 in order to maintain it as single-phase supercritical or dense-phase.
The presence of impurities in the CO2 stream will not only have a significant effect on the hydraulic parameters such as pressure and temperature, but also on the density and viscosity of the fluid, depending on the impurities present. Some combinations, particularly if hydrogen or nitrogen are present, cause higher pressure and temperature drops for a given pipeline length than others, which has implications for the distance between compressor stations along the pipeline. The pipeline cost increases with the number of these which, in any event, are not viable for subsea pipelines. Sudden temperature drops can have potential material implications, such as embrittlement, and can also cause hydrate formation, both of which could damage the pipeline.
The solvent properties of supercritical CO2 are known to be detrimental to the elastomers commonly used in valves, gaskets, and O-rings, used for sealing purposes, and to coatings. At high pressures the supercritical CO2 diffuses into the elastomers and, when the pressure is reduced, blistering and even explosions can occur as the material decompresses. Many of the elastomeric materials currently used in oil and gas pipelines are therefore not suitable for CO2 transportation. Similarly, in-line inspection (ILI) of CO2 pipelines is problematic, as the supercritical CO2 dissolves the non-metallic components of the cleaning and ILI and tools: although high-durometer elastomers can be used to reduce the problem, they cannot eliminate it totally.
There are other important material issues that will require consideration in CO2 pipeline design. Ductile fracture propagation may be an issue, and the requirement to consider fracture propagation in CO2 pipelines is included in the federal regulations in the US. For some of the US pipelines it was concluded that the pipe material did not have sufficient toughness to arrest propagating ductile fractures and therefore crack arrestors were required along these pipelines. This experience highlights the need to define the toughness limits for equivalent pipeline networks, particularly considering the effect of impurities on the decompression behaviour of the gas, to avoid the costly requirement to fit crack arrestors. In the US the CO2 pipelines were designed-for-purpose: if pipeline re-use is to be adopted in the UK and elsewhere, existing pipelines will have to be assessed extremely carefully bearing all of these factors in mind.
In the UK, there is no experience with pipeline transportation of dense-phase CO2, and the design codes and the Pipeline Safety Regulations do not contain any classification for the material that would allow pipelines – either on- or offshore – to be designed and constructed. In the US, dense-phase CO2 pipelines have been classified as hazardous-liquid pipelines, and are therefore regulated under the Department of Transportation’s Code of Federal Regulations 49CFR195. Most operators appear to have designed their pipelines conservatively using the ASME B31.8 code for gas pipelines, as this code is more restrictive than the hazardous-liquid design code and also takes into account population density in the determination of the maximum allowable stress in the pipeline.
All of the currently operating CO2 pipelines are onshore in the US, and many are routed through sparsely-populated areas; the consequence of an accident is therefore likely to be relatively small because the CO2 will dissipate before affecting the local population. However, if networks are to be developed onshore for the collection of CO2 from industry sources in congested countries such as the UK, this will require the routeing of pipelines close to population centres. In the UK pipeline design code, the minimum distance between a pipeline and occupied buildings is defined by the substance being transported. However, supercritical CO2 has not been classified and so, in order for pipeline networks to be designed in the UK at least, regulations need to be put in place to classify supercritical CO2 and so to allow safe pipeline design both on- and offshore.
The programme for the Newcastle Forum will provide the opportunity to expand on many of these areas in detail, as well as to discuss current and future projects and research activities. It is also planned to have an open session intended to discuss formation of an academic and industry network to further progress this important topic.