The evidence supporting damage to our environment by our carbon-based economy is becoming increasingly compelling. Australia has ratified the Kyoto Protocol, is committed to introducing an emissions trading scheme by 2011, and to reducing emissions by 60 per cent of 2000 levels by 2050, as well as ensuring that 20 per cent of our electricity is generated from renewable sources by 2020.

The quest for sequestration

The Government White Paper, Carbon Pollution Reduction Scheme (CPRS): Australia’s Low Pollution Future, defines sequestration as the long term storage of carbon dioxide in forests, soils, oceans or underground in depleted oil and gas reservoirs, coal seams and saline aquifers. Geosequestration is the process of storing liquefied carbon dioxide underground, as opposed to in forests, soils and oceans.

Geosequestration is an alternative to renewable energy sources that allows energy to be produced from the combustion of coal and natural gas with much less contribution to atmospheric CO2 levels. Carbon capture, transportation and storage (CCTS) seeks to reduce CO2 emissions by capturing CO2 from fossil-fired power stations and other sources, and then transporting it for geosequestration. There is a variety of technologies and CO2 is captured either before or after combustion to prevent it from entering the atmosphere.

Mission for lower emissions: tackling the unknown

The future is not at all clear. Governments around the world are adopting a two tier approach to the reduction of CO2 emissions – a more demanding one if there is a large scale global commitment by both developing and developed countries, and a less demanding one if left to instigate reductions on a national level. A further complication is that strongly affected industries (in reality only coal-fired power stations) and emissions-intensive trade exposed industries will receive special treatment. The global financial crisis has further confused matters.

The extent to which renewable energy sources on the one hand and CCTS on the other will contribute to the required reductions in CO2 emissions is still unclear. The Greenpeace 2008 report, False Hope – Why carbon capture and storage won’t save the climate, claims that CCTS is too expensive and risky and will not be proven in time to help.

The approaches adopted by emitters when the CPRS begins in 2011 will also depend upon the price of CO2 permits. The White Paper proposes a ‘cap and trade’ scheme. The cap refers to the limit imposed on emissions, which in turn imposes a limit on the number of CO2 permits made available, while trading in permits allows a price to be determined by the market for the restricted number of permits available. Emitters are only likely to adopt a particular CO2 emissions reduction strategy if the cost per tonne is below the price of a CO2 permit. Otherwise it will be more attractive to simply purchase CO2 permits to cover the emissions.

Pipelines to the rescue

Given that sources of CO2 and the geological formations suitable for its geosequestration are not often co-located, it is almost inevitable that pipelines will be used for its transport.

The pipeline connecting the CO2 sources (emitters) to the sinks (geosequestration sites) is the one element of the overall geosequestration system that is well proven. At the source end, the technology for capturing CO2 from coal-fired power stations is understood but not fully optimised for the highest possible efficiency. There is currently no full scale power station operating with the technology, although there are demonstration projects. At the sink end, there are very significant risks in the event of leakage that might currently be uninsurable.

Further development work is required in addition to further legislation. In both cases, the uncertainties and risks associated with early entry into CCTS are likely to ensure that emitters will be extremely cautious before committing to such an approach. Projects that utilise CO2 for enhanced oil recovery will most likely be the first to use CCTS because of the economic return.

Location, location, location

As part of the Geodisc Program, the Australian Petroleum Cooperative Research Centre, Geoscience Australia and the University of New South Wales completed an analysis of potential storage sites of CO2 and compared them with the main CO2 producing locations.

Victoria and Western Australia are fortunate in having suitable sinks within 100 km of their major sources. Queensland has sinks up to 500 km from the major sources, and suitable sinks for South Australia and New South Wales are located more than 500 km from major emission sources.

Australia produces approximately 564 MMt/a of CO2 and even geosequestration of a fraction of this would require a CO2 pipeline infrastructure larger than that of the existing natural gas pipeline infrastructure, representing an enormous potential opportunity for the pipeline industry.

Where to start?

Item AS2885.0 states that the AS2885 suite of standards covering pipelines is not expressly intended for application to non-hydrocarbon transport. However, its application to such transport is not precluded provided that special consideration is given to the application.

The APIA Research and Standards Committee commissioned a review of AS2885 earlier this year to determine how the standard could be applied to CO2 pipelines and what further work was required. The review concluded that while there are a number of matters that should be identified in the standard for use with CO2 pipelines, there are no substantial technical omissions. The review recommended that an informative appendix to AS2885.1 is to be developed for CO2 as soon as is practical to do so, and that the current revision of AS2885.3 include guidance on specific operating and maintenance matters for pipelines transporting CO2.

AS2885.1 contains all of the elements required to design a CO2 pipeline. However, the differences in properties between CO2 and the hydrocarbon gases and liquids that designers are more familiar with in this country must be given special consideration.

Teaching an old pipe new tricks

Although CO2 pipelines have been in operation for many years as part of enhanced oil recovery and they have much in common with natural gas pipelines and high vapour pressure liquid pipelines, there are some important differences that must be considered during design and operation.

Wet and wild

Notably, CO2 at high pressures has the potential to be extremely corrosive if free water is present, it has some challenges in fracture control caused by its decompression behaviour, and there are different risks to natural gas, since CO2 is non-flammable but heavier than air.

The corrosion of carbon steel pipelines by CO2 is well understood because of the role played by CO2 in the corrosion of water wet natural gas pipelines. Although the models used for estimation of CO2 corrosion rates in natural gas pipelines are not accurate at the much higher CO2 partial pressures of CO2 pipelines, it is known that corrosion rates in the presence of water are very high. The use of carbon steel will likely require reliable water removal from the CO2.

Threat of fracture

The possibility of both brittle fracture and ductile fracture propagation must be eliminated in the pipeline design and CO2 provides additional challenges over natural gas in both respects. The pressurisation during fill and depressurisation of CO2 can cause very low temperatures, which will require attention to the brittle ductile transition temperature of the pipeline steel and may require the use of special low temperature steels at certain locations.

Ductile fracture avoidance requires special consideration in CO2 pipelines because of the high pressure two phase region encountered on depressurisation. The wall thickness or toughness requirements of steel required to arrest a ductile fracture in a CO2 pipeline will be significantly higher than for a natural gas pipeline operating at the same pressure, unless external crack arrestors are utilised.

The heavier than air, non-flammable and asphyxiant properties of CO2 all require special consideration in the design of a CO2 pipeline. Safety and risk management aspects of design will be determined by dispersion modelling rather than by energy release rate calculations, as is the case with natural gas pipelines.

Positive prospects for pipelines

The pipeline is already a proven method suitable for low risk transportation of CO2 over the required distances. All design issues associated with the different properties in comparison with natural gas can be overcome using present day knowledge. The remaining impediments to the use of CO2 pipelines include the developments required at each end of the pipeline for capture and storage; the requirement for suitable economic incentives; and, a legal framework for CCTS.

It is not clear how the expense of a CO2 pipeline infrastructure will be supported, or even if large scale CCTS is supportable, but this will become clearer as the market establishes a price for CO2 permits.

What is evident, however, is that the CPRS represents opportunities for pipeliners, whether via emphasis on natural gas-fired power stations, use of coal seam gas to produce LNG for export, or in the longer term, the possibility of CO2 transfer to geosequestration sites.