Design challenges for future CO2 pipelines
Overview of some design issues that need to be considered when planning, engineering and constructing pipelines to transport CO2 in its supercritical state from process facilities to remote storage
Paul Andrews and Sub Parkash, Fluor UK
John Barrie and Peter Hatcher, Fluor Canada
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Industry faces a significant challenge to reduce current levels of greenhouse gas (GHG) emissions in order to comply with upcoming legislation. Public pressure for reduced GHG emissions from industry has intensified in recent years, and industrial plant owners are now accelerating their efforts to minimise these emissions from their facilities. Scientists believe that anthropogenic (man-made) carbon dioxide (CO2) makes a larger contribution to global warming than other industrial flue gases.
Refinery and petrochemical facilities are now seeking cost-effective methods of capturing this gas and sequestering it in geologic formations. Since the market for use of CO2 is very small, there is a need for storage of this gas. In order to bring a significant benefit to the environment, large quantities of CO2 need to be captured and stored. Accordingly, CO2 capture and transport will require the construction of large diameter, dedicated pipelines. As an example, a 400 MW coal-fired plant can produce up to 8000 tonnes/day of CO2. With several thousand of these plants worldwide, the opportunity exists to capture and transport substantial amounts of CO2, should these plants install capture facilities.
Pipelines have provided significant benefits to industry and consumers, and they are the safest, least expensive mode of transportation for many commonly used liquids and gases. To provide fuel for cooking, early pipelines were made from crude material, such as bamboo used in ancient China, and wooden pipelines were used as recently as the early 1900s. Over the years, pipeline materials have improved and now transport products that currently include natural gas, water, crude oil, refined petroleum products, coal slurries, ammonia, ethylene and propylene. The types of product, fluid and gas routinely transported by pipelines have increased over recent years, and since the transport of CO2 by pipeline has been practised for almost three decades, it appears to be a prudent choice for transport. Today, there are more than 3000 km of CO2 pipelines transporting approximately 45 million tonnes of CO2 per year. Some of these pipelines have been operating since the 1980s.
Current experience with CO2 pipelines is for transport of this gas for use in enhanced oil recovery (EOR) facilities. Recent installations include the Sheep River pipeline in West Texas and the 300 km-long North Dakota to Weyburn, Saskatchewan, pipeline in North America, which has been in operation for the past eight years. Another owner has subsea pipelines in the North Sea, which inject more than a million tonnes of CO2 per year into saline aquifers, several hundred metres below the seabed.
Existing international pipeline codes are satisfactory for previous pipeline designs. However, since CO2 is transported as a supercritical fluid, unlike other pipeline products, there are suggestions that regulating authorities develop new codes or revise existing codes to ensure the safety and integrity of pipelines for this unique type of service. There is an ongoing Joint Industry Project (JIP) in Europe to develop an industry guideline for safe transportation of CO2 by pipeline.
Since CO2 has been used in beverages for many years, the public’s perception is that this gas is relatively safe to transport. This belief leads to the perception that CO2 can be processed and pipelined with no risk to people. In fact, because of this perception and because of the large quantities of this gas in transport, the consequences of potential CO2 pipeline accidents are of significant concern. There is a need for more strict regulations governing CO2 pipelines than those for natural gas pipelines.
CO2 can be fatal to humans and animals, partly due to suffocation. It also produces dangerous physiological effects when present in high levels in the blood stream. Either one of these factors, or both, can cause fatalities. The permissible exposure limit (PEL) for this gas is 5000 ppm per OSHA guidelines, and 40 000 ppm (4%) is considered to be immediately dangerous to life and health (IDLH).
A volcanic lake, Lake Nyos, in Cameroon presented recent proof of the danger created by CO2 release when gas from the bottom of this ancient lake travelled down a valley, killing approximately 1800 people and many animals in a very short period. Naturally occurring CO2 in mountainous regions can also be dangerous to hikers and skiers in these areas. Some incidents related to the food and beverage industry have been reported that were a consequence of exposure to high levels of CO2. Therefore, this gas needs to be handled and transported with a full understanding of the danger it poses, and the public needs to be adequately informed of its potential dangers.
CO2 pipelines can be an important component in the plan to reduce GHG emissions to the atmosphere. However, as with natural gas and crude oil, these pipelines pose their own risks, which must be identified and mitigated. When pipelines traverse populated areas, special attention must be directed to issues such as pipeline design factors, burial depth, overpressure protection and more sensitive leak detection. These issues are covered by codes for these types of application. However, current codes do not specifically address fluids being transported in their supercritical state.
There appears to be no recorded fatalities directly attributable to the failure of CO2 pipelines. Acid gas injection (CO2 and H2S) has been practised in Western Canada for a number of years and no major safety issues have been reported for these small pipelines, due to good design and operating practices being implemented by the owners. However, if future CO2 pipelines become as common and extensive as natural gas pipelines, similar failure statistics as for natural gas pipelines would be expected and applicable. The consequences of failure would be much more severe due to the large inventory transport without adequate dispersion mechanisms for the escaped gas. Natural gas pipeline ruptures lead to escape of the gas upwards, due to its lower than air density, and may ignite on rupture, thereby burning the released gas. However, CO2 cannot be burned and it will disperse quickly and collect in nearby depressions, which may store this gas for extended periods until wind disperses it or vegetation absorbs it.
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