John Morgan investigates the development and verification of carbon capture and storage flow measurement technologies.
Although Carbon Capture and Storage (CCS) is very much an emerging technology and yet to be demonstrated fully and at scale, it is regarded as one of the best potential solutions to tackle the problem of climate change from the burning of fossil fuels.
CCS will play a prominent role in the abatement of anthropogenic carbon dioxide (CO2) emissions as part of a secure and sustainable global energy supply. Forthcoming EU legislation will also make CCS compulsory for all new power plants and combustion plants.
One of the many technical challenges to be overcome in establishing CCS as a practical operational process is effective measurement and monitoring. This will be a key element of the CCS regulatory framework (as reflected in the European Union CCS Directive), which calls for CO2 flow measurement, composition measurement, leak detection and quantification across the full CCS chain. Accurate measurement will therefore be essential for environmental and safety needs and will be fundamental in reducing financial exposure in CO2 trading schemes.
To this end, the European Union Emissions Trading Scheme’s (EU ETS) monitoring and reporting guidelines set stringent measurement and monitoring criteria.
Transportation of CO2 to its final storage destination is claimed to be more economical by pipeline, especially if cluster networks are formed for multiple emitters. Typically, dedicated CO2 pipelines are designed to transport CO2 well above critical pressure, so that it will be in its dense phase, which means pipelines will be operating away from the phase boundary. In many instances, however, existing pipelines will be utilised for transportation which does not permit operations at levels well above critical pressures.
Compared to other substances that are transported by pipeline (e.g. oil, natural gas and water), CO2 is unusual because its critical temperature lies close to ambient temperature, which is the normal operating condition and the region where most industrial processes are carried out. This means that even small changes in pressure and temperature may lead to rapid and substantial changes in the physical properties of CO2, such as phase, density and compressibility.
There is therefore not only a risk of changing between phases, but also, when operating on or close to a phase boundary, multiphase flow conditions can arise. The occurrence of phase changes and multiphase flow occurring at measurement points will have a detrimental effect on measurement accuracy, especially when most measurement devices are designed to operate only in single phase, either gas or liquid.
Measurement would be more accurate if CO2 was being transported in only gaseous phase or single phase at all times and throughout the transportation system; however, CO2 is usually only transported in the gaseous phase when low volumes are involved or if the pipeline is being reused. In addition, if the transported fluid contained 100% CO2, e.g. no impurities, then higher accuracy could be achieved.
Unfortunately, this ideal scenario simply does not exist. This means that measurement challenges including flow measurement, composition measurement, physical properties measurement, leakage detection and quantification must be addressed urgently. One significant reason for this is that impurities readily enter pipeline systems that have inputs from multiple capture plants with varying compositions of CO2 streams.
Flow metering will be necessary for regulatory measurement under the EU ETS. This includes custody transfer/fiscal metering, where there is a transfer of ownership in the pipeline, leakage detection and metering of the various processes across the CCS network, including controlling the volume of CO2 being injected into the geological storage formation. Under the EU ETS, the mass of annually transferred CO2 is required to be determined within a maximum uncertainty of less than 1.5%. For custody transfer purposes, the accuracy requirements may be even more stringent.
To put the importance of accurate flow measurement into perspective, consider the UK’s largest power station, which emits approximately 22 million tonnes of CO2 per annum. Each percentage of uncertainty in flow measurement could result in a €1.98 million exposure in the trading scheme, based on a carbon trading price of €9/tonne as on November 12, 2012.
It is also clearly understood that there is an urgent need to address the issues surrounding flow measurement in CO2 transportation. One of the primary outcomes of such a programme would be the fiscal metering of CO2 with a maximum measurement uncertainty of ± 1.5%. Of course, in order to meet these targets, the behaviour of CO2 in transportation systems has to be understood. The specifications of the fluid and accurate accounting of the CO2 through all sections of CCS schemes must also be achieved.
Composition measurement is necessary to determine the concentration of CO2 and to detect contaminants present in the CO2 stream. This information is essential in order to understand the physical properties, chemistry and behaviour of the CO2 mixture. Chemical analysis of injected fluid and gas analysis, using gas chromatographs and spectrometers, can be used for composition analysis.
The composition of the CO2 stream will affect the density, compressibility and phase envelope of the gas or liquid. In order to establish the necessary pressures and temperatures required to maintain a stable phase and economical transfer, knowledge of the composition is vital.
Without knowledge of the composition it would be extremely difficult to plan the CCS processes and achieve the flow conditions necessary to maintain a stable phase and ensure safe and economical transportation through the pipelines.
Properties and behaviour
The presence of contaminants in the CO2 stream will significantly alter the physical properties from those of pure CO2, and it is the physical properties of the CO2 stream that dictate its behaviour under different processes and conditions. The overall effect of impurities in the CO2 stream is to shift the phase boundary and create two-phase regions with the associated impact on flow measurement as described above.
Although equations of state models exist for calculating the physical properties of pure CO2, the best currently available models that include the contaminants likely in CCS streams have uncertainties of at least 10% in density. Without accurate knowledge of density, however, it will not be possible to convert volumetric flow to mass flow. Clearly, such models are unacceptable for converting from volumetric to mass flow when trying to meet the ±1.5% uncertainty target.
Further work, including modelling and experimental research, is therefore required to obtain the necessary chemistry and physical properties data to allow the planning and design of CCS schemes. In particular, the development and validation of robust equations of state from CO2 mixtures is essential.
Leak detection and quantification
It is also vital that appropriate measurements are in place to detect and quantify a leakage if it should occur across the CCS network. This includes leakages from above surface pipelines, buried pipelines, subsea pipelines and from the geological storage formations; however, although there are many technologies in place for detecting leakage from the storage formation, the real challenge is to quantify any and all leakage - and this is an area which requires urgent attention.
For pipelines, it is likely that a combination of methods will be used to detect CO2 leakage. These will include internal measurements such as flow, pressure and temperature, along with external methods such as the screening and sampling of the surrounding environments. In serious cases, the leakage of CO2 may be clearly visible, either by the formation of CO2 clouds in the atmosphere, or the presence of solid CO2 deposits on the ground.
Measurement, monitoring and verification (MMV) is a key requisite for the optimal operation and management of a CO2 sequestration site, but it presents challenges that are site-specific and ever changing. Individual CCS projects will therefore need site-specific MMV processes, to ensure the captured CO2 will be injected safely and permanently for sequestration. It is almost impossible to recommend a set of MMV tools that could be applied universally to all CO2 storage sites because every geological storage site presents unique structures, conditions and challenges.
The future of measurement
In CCS schemes, sampling, physical properties, flow measurement, leakage detection, and quantification data are interdependent parameters. Sampling the composition of the CO2 stream will provide the necessary data to calculate the physical properties, which will then be used for flow measurement and control calculations. In particular, it will allow determination of the necessary pumping pressures and conditioning required to maintain a stable, safe and economical flow across the network. This includes using validated algorithms, models and equations of state, which will underpin online physical properties calculation software and computers.
In order to ensure effective control and fluid management of the overall system, and a smooth transition from the point of capture through to transportation and injection into the storage formation, it will be necessary to have sophisticated, online, interfaced measurement systems to allow constant monitoring and tracking of changes throughout the network. The use of standardised methods, industry standards, best practise guidelines, and traceable validated measurement equipment will help minimise inconsistencies between measurement points and duty holders.
It is evident that there are a number of potential issues associated with the measurement of CO2 which require addressing to support CCS schemes. The current situation shows that there is little validated data available to support the performance and accuracy of various flow measurement technologies for use in CCS schemes. This reflects the lack of test and calibration facilities available worldwide to support the necessary research, development and verification of CCS flow measurement technologies.
Specifically, there are no validated primary reference standards for flow measurement of CO2. Particularly, there is inadequate knowledge of the physical properties and phase envelopes relating to CO2 mixtures in CCS schemes, yet such data will be essential for controlling CCS processes and for planning and designing suitable and accurate measurement systems.
For the full scale industrial deployment of CCS schemes, it is essential that accurate and robust monitoring programmes are in place, which means much work has to be done now to address the various measurement, monitoring and verification issues across the full CCS chain.
The UK government’s national measurement system recognised this requirement and invested in innovative research and development facilities which are addressing the key metrology issues within CCS schemes.
This work includes the development of an infrastructure to incorporate CO2 flow measurement standards with three separate CO2 test facilities. These facilities will provide a platform for research and development work on measurement devices and components across a wide range of conditions that are relevant to CCS schemes, and will develop and test recommended practises for full scale deployment, supporting national and international standardisation.
The practical measurement challenges being addressed through these state of the art facilities are being complemented by the application of novel detailed Computational Fluid Dynamics (CFD) modelling of the various CCS process stages. Such modelling may be applied as part of the design phase to help anticipate how equipment will perform and will enable problems to be identified. CFD modelling therefore provides an invaluable tool in the development phase to assist designers in identifying critical areas and undertaking design changes.
Published: 07th Mar 2013 in AWE International