The primary objective of this guidance document is ‘to improve the industry’s knowledge, and to assist developers and operators to carry out hazard analysis, procure and manage their CCS plant safely’. It does not provide an exhaustive set of information on CCS issues and, by its very nature as an emerging area, research and knowledge is progressing at rapid speed. It is, therefore, important for participants in the CCS industry to keep abreast of research and developments in this sector.
7.1.1 Properties of carbon dioxide
Carbon dioxide is a clear odourless gas, slightly denser than air at atmospheric conditions. It can exist as a solid, liquid or gas. Above the critical point, it can no longer exist with separate gas and liquid phases and forms a supercritical fluid with some properties of a liquid and some of a gas. Carbon dioxide has a sublimation line, where at this boundary both gaseous and solid carbon dioxide can exist and transform between the two states without an intermediate liquid phase.
Carbon dioxide and some of the impurities found in CCS carbon dioxide streams can have:
- Physiological effects. Both asphyxiant and toxicological effects.
- Ability to accumulate in confined spaces being slightly denser than air.
- Large pressure expansion ratio when changing from liquid to gas.
- Low temperature conditions during the expansion from liquid to gas.
- Incompatibilities with some elastomers, internal coatings, lubricants and greases.
- Contain impurities that affect the fundamental physical properties such as the triple point, density etc. impacting on design. In some cases, the impurities themselves may have physiological effects (e.g. sulphide).
- Corrosion can occur with some impurities such as water.
- With water within the stream, even if there is no free water, hydrates can be formed.
- Under certain conditions, it is possible to have a carbon dioxide BLEVE.
7.1.2 State of the art in CCS technology
There are a number of technological options for capturing and transporting carbon dioxide, through pre-combustion capture, post-combustion capture and oxyfuel technology. In the race to develop CCS, many companies are developing small scale and demonstration type projects. This allows real operating experience in CCS at the small to medium scale. It also allows the transfer of technology from allied industries at a similar scale of application into CCS.
In many cases there are examples of applications of similar scale to those of commercial CCS projects (e.g. compressors, rectisol carbon dioxide removal plants, EOR at Weyburn), and in others there are examples of applications at demonstration size (e.g.carbon dioxide removal plants). Coupled with the research work (e.g. oxyfuel burner rig testing), this is providing a baseline of knowledge for the increased development of commercial CCS projects. The challenge for the industry is to capitalise on the experience to date and as future demonstration plants start operating, recognising that there may be issues involved with scale up that may not, at first, be obvious.
Many parties have recognised the need to transfer information and to manage the scale up of technology sensibly. Existing knowledge sharing networks include CO2NET, the IEAand the sharing through the EU Demonstration projects programme. The plethora of demonstration projects and research also makes for inevitable overlap in research and some confused messages within the CCS industry as knowledge is rapidly overtaken. The industry does need to develop sensible standards to help CCS participants to avoid repeating design/operation mistakes. It also needs to focus on training new engineers entering the CCS field and in particular to help them recognise where their existing experience and training is useful and where it is not translatable (for example being an expert or experienced in designing a particular piece of equipment in another field does not necessarily translate into competence for CCS).
7.1.3 Onshore plant design and operation
The industrial gases industry provides useful experience of designing and operating carbon dioxide facilities. Many of the design principles which have been developed for the current carbon dioxide industry can be applied for CCS applications such as:
- Avoiding designing plant to have enclosed spaces, basements, hollows, bunded areas or dips.
- Where this is unavoidable, classifying all these areas as enclosed spaces and carrying out to determine the appropriate mitigation and procedures.
- Carbon dioxide detection systems may not always be necessary in open production areas and may need further development to tangibly reduce risk. Staff training and operating procedures in leak detection can be an appropriate option.
- Avoiding the failure of materials (metals, seals, valves, gaskets, lubricants etc.) in service by ensuring that they are tested to meet the operating conditions in the particular application they will face (where no standard tests or guidance exists) such as:
- impurities within the carbon dioxide stream;
- operating pressures to which the materials will be subjected;
- operating temperatures (particularly noting the conditions which can be extremely cold);
- cycling of plant and its materials, and
- whether you have wet or dry carbon dioxide and selecting materials appropriately for either condition.
- Recognising that in plant areas where solid formation is possible (e.g. in blow down lines), then material erosion may occur and materials should be designed to avoid this erosion or should be inspected and replaced regularly.
- External impingement is avoided in areas where blow down is expected.
- Avoiding failure of ball valves caused by expansion of liquid carbon dioxide.
- Reviewing existing codes of practice designed for the liquid carbon dioxide market and determining whether they provide useful guidance to your project’s particular conditions.
The application of principles developed in the existing carbon dioxide operations provides a useful design starting point for demonstration sized projects, allowing designers to incorporate learning from the existing carbon dioxide applications. At some stage, existing carbon dioxide standards will need to reviewed and systematically revised for the CCS industry to ensure that they cover the complete range of operating conditions encountered, including larger quantities.
From the experience and shared learning of the CCS demonstration projects, further codes of practice and design guides may be required to meet the needs of the scaled up full size industry, particularly with reference to higher pressures and quantities.
7.1.4 Onshore pipeline design and operation
Carbon dioxide pipelines are operating at the quantities and pressures expected for European CCS installations and can be considered a mature technology in the US/Canada. The hours of operation and pipeline lengths are not equivalent to the body of knowledge for natural gas, but there is no step change in scale of pipeline (in regard to quantity and pressure). US and Canadian design standards are prescriptive and classify carbon dioxide pipelines in the same way as hydrocarbons. UK and European regulations and standards allow for the design of carbon dioxide pipelines within current structures covering areas such as materials selection, blow down and emergency shutdown design. Gaps in knowledge do not prohibit design but are likely to lead to more conservative design.
The CCS industry would benefit from a consistent approach to gathering failure data of carbon dioxide pipelines to ensure that lessons are learnt across the industry, similar to the data gathering and dissemination carried out in the natural gas pipeline industry.
Fundamental to managing the hazards from a carbon dioxide pipeline is to:
- Carry out appropriate hazard analysis, including evaluating mitigation strategies.
- Carry out careful of the carbon dioxide admitted to the pipeline systems, to avoid and/or minimise the impact of corrosion or hydrate formation.
- Recognise that the most appropriate form of large scale transportation over long distances will be in the liquid form (encompassing and up to supercritical).