Measurement and Testing
An international standard for CO2 purity could support safe and efficient CCS operations
Jun 22 2020 Read 1005 Times
Author: Stephen B. Harrison on behalf of sbh4 GmbH
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The concept of Carbon Capture and Storage (CCS) is to recover carbon dioxide (CO2) from industrial process gas emissions and inject the CO2 deep into the ground for long term storage. Atmospheric emissions of CO2 from existing combustion processes, such as cement production or electrical power generation can thereby be reduced to slow down climate change.
Several new projects have also been proposed to construct steam methane reformers (SMR), or auto thermal reformers (ATR) to produce large quantities of hydrogen for heating and mobility applications. CCS is an integral part of these schemes to ensure that they produce blue hydrogen and play a role in sustainable decarbonisation.
SMRs and ATRs are most commonly fed with natural gas which is rich in methane. When the natural gas rises from the underground gas reservoir, it is generally accompanied by large quantities of CO2 which has existed with the methane in the underground gas field for thousands of years. Many CCS schemes plan to use depleted natural gas reservoirs for the long-term storage of CO2, thereby refilling them with a gas that they previously contained. So, the CCS process can be thought of as returning CO2 to its underground home.
The UK has spearheaded extraction of fossil fuel reserves from the North Sea for many years. The country is also planning CCS schemes which return CO2 to underground storage locations in sectors of the North Sea. For example, the Fergus gas terminal at Peterhead in Scotland would be integral to the Acorn project, which UK Chancellor Rishi Sunak referred to during his annual budget speech in March 2020. In this case, the flow direction of the existing natural gas pipeline would be reversed to pump carbon dioxide from onshore sources, such as Ineos’s Grangemouth refinery, out to the gas fields under the North Sea.
In Norway, Equinor are playing a key role in the existing Northern Lights project which is collecting CO2 from cement production and other heavy industries in the Oslo Fjord. Tanker ships transport liquefied carbon dioxide to offshore gas processing plants where the CO2 is compressed to high pressure and injected underground. In future, it is proposed that the CO2 collection network will be expanded to include a wide range of sources from Norway and potentially other nations.
Before we store CO2 deep underground, we must ensure that important gas purity criteria have been met. However, at present there is no common standard to define the quality of CO2 that should be used in CCS projects. Many of the brightest minds in gas purity assay are working hard to address this gap.
Gas purity standards in Hydrogen energy and biogas can guide the way
The purity of Hydrogen for use in fuel cell electric vehicles is subject to an international standard, namely the ‘ISO14678:2019 Hydrogen fuel quality – product specification’. Impurities such as carbon monoxide and hydrogen sulphide are capped at levels that will guarantee the hydrogen is compatible with standard modern fuel cells and does not poison the sensitive catalysts. The maximum combined argon and nitrogen concentration is also specified to avoid the long-term accumulation of these inert air gases in the fuel cell which would result in a potentially dangerous loss of vehicle power.
Biomethane is another area where gas purity standards are important. Firstly, the methane concentration and the energy value of the gas mixture must be known for fair valuation of the gas and accurate commercial invoicing. On the other hand, harmful impurities such as siloxanes which can cause corrosion in valves and gas compression equipment by the formation of sandy solids must be detected to ensure that they are present at very low levels to avoid damage to the gas distribution infrastructure.
The examples above focus on hydrogen and methane but there are transferrable lessons to CCS because these gas standards need to specify maximum concentrations of similar impurities which could be harmful to the application. There may also similarities in the most suitable analytical methods to conduct analysis of trace impurities. So, in the consideration of a future standardised purity specification for CO2 in CCS applications, there may be some parallels to learn from.
The importance of carbon dioxide purity in CCS applications
In the realm of international metrology, many issues are being considered as the debate about the requirement for a CCS CO2 standard is taking place. Dr Arul Murugan, Senior Research Scientist for Energy Gases at NPL in the UK says that “in some CCS schemes, the idea is to liquify the carbon dioxide either for immediate storage or to enable its transportation by ship to an offshore platform where it will be further processed. Incondensable gases such as nitrogen or methane could reduce the efficiency of this process by increasing the required energy input. Furthermore, these gases do not behave in the same way as CO2 when injected underground and they take up valuable storage space”.
Murugan adds that “in other CCS schemes, the proposal is to compress CO2 to a high pressure so that it can be cost effectively transported in long distance pipelines before being injected into suitable geological structures deep underground. These compressor stations and pipelines are highly valuable assets which must be protected.” If there are combinations of gases in the CO2, that can result in corrosion, such as ammonia and moisture or hydrogen sulphide and moisture they may cause irreversible damage to the pipeline or even the storage site itself. This corrosion of the CCS infrastructure would be costly to repair. Corrosion could also pose a safety risk if it went unnoticed and caused a pipeline rupture. In these cases, detection of these trace contaminants is essential to prevent problems escalating.
Process performance and gas distribution asset integrity are not the only reasons for careful analysis and control of CCS CO2 purity. The safety of the personnel operating the CCS equipment and the general public are also of paramount importance. CO2 intended for CCS may also contain trace levels of highly toxic chemicals such as mercury or hydrogen cyanide. Whilst operators cannot always prevent these molecules being present at tiny levels, they can monitor their concentrations to ensure that they exist in the gas only in minute traces which would ensure that any potential CO2 leak from the CCS processing equipment or storage site does not pose a health risk. Murugan continues to say that “with all these operational, health and safety considerations in mind, my Energy Gases Standards team at NPL are starting to develop the analytical methods and traceable reference materials required for performing these important purity analyses”.
Many nations in may also wish to participate in CCS schemes, but if their country does not have the appropriate geological profile, they must rely on exporting their CO2. So, for the trade in CO2 for CCS will inevitably become international. This is a key driver for the development of an international standard for CCS CO2 purity to ensure a harmonised approach and consistent levels of safety. The standard will also lock in parameters that players in the CCS industry can design around to ensure optimal performance of their equipment.
Accurate gas analysis closes the loop
Internationally acclaimed metrological bodies, such as NPL, play a role beyond defining the composition of gas standards for new applications. They also produce high precision gas standard mixture cylinders that contain the required chemical species at the relevant concentrations. These certified gas mixtures can be used to calibrate gas analysers in the field which are in service to measure the CO2 purity.
CCS asset owners need to be sure that their gas storage terminals, compression stations and distribution pipelines will not be attacked by corrosive impurities in the CO2. Speaking as Head of Product Management for the continuous gas analyser product range within ABB’s Measurement & Analytics business line, Steve Gibbons says that “our LGR-ICOS gas analyser is ideal to monitor trace levels of many gases, such as moisture, oxygen and hydrogen sulphide. Oxygen is a tracer that is used to provide early warning of pipeline leaks. Hydrogen sulphide is toxic and when combined with water, it causes corrosion problems. So, in one instrument we have the capability to address several concerns that these trace impurities raise”. Laser techniques are highly responsive to small concentration changes, so they can rapidly enable mitigating process control interventions to deescalate potential issues before they become major problems.
Operators also want to be sure that the inert gases, which rob power from the gas compressor, are present only at minimal concentrations. Methane and nitrogen are the most common inert gases found in carbon dioxide. Gibbons says that “in some cases, we would recommend a TCD gas analyser, such as the Caldos27 to measure nitrogen or an NDIR analyser like the Uras26 to measure methane. Or, the LGR-ICOS can also be used to measure methane alongside the reactive gases”.
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