GUIDANCE NOTE
Measurement of CO2 throughout the
carbon capture and storage (CCS) chain
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Measurement of CO2 throughout the carbon capture
and storage (CCS) chain
Measurement Challenges
There are a large number of potential measurement challenges
expected in CCS due to both the physical properties of carbon
dioxide (CO2) and the processes involved in CCS schemes.
CO2 is unusual because of the relationship and closeness of its
triple point and critical point to the temperatures and pressures
commonly found in industrial processes. This can be seen in the
CO2 Phase Diagram in Figure 1.
There are a number of other factors that may affect measurement.
The acoustic attenuation properties of CO2 can affect flow
measurement using ultrasonic meters. Large pipeline diameters
may limit some measurement technologies and the corrosiveness
of CO2 mixtures may, where applicable, have to be considered
during the planning of measurement systems and materials.
Measurement Needs
The main three areas will be essential to monitor CO2 across the
CCS chain:
Sampling of the CO2 mixture
Determining the physical properties
Flow measurement
Sampling of the CO2 mixture
Figure 1 - CO2 Phase Diagram
Compared to other substances that are transported by pipeline
(e.g. oil, natural gas and water) the critical point of CO2 lies close
to ambient temperature. This means that even small changes
in pressure and temperature may lead to rapid and substantial
changes in the CO2 physical properties (e.g. phase, density,
compressibility).Therefore, not only is there a risk of changing
between phases, but also when operating on or close to a phase
boundary line, multiphase flow conditions can arise. Phase changes
and multiphase flow occurring at measurement points will have a
detrimental effect on measurement accuracy, where flowmeters
are designed to operate in one specific phase only.
In CCS applications, regulating the temperature and pressure
will be a difficult undertaking, particularly over long distances.
Pipelines will span hundreds of miles, and be subjected to various
climates and conditions, which will naturally affect pressure and
temperature.
Another major challenge for measurement will be coping with
impurities in the CO2 stream. This will be inevitable as the purity
levels in the CCS capture processes are unlikely to be greater than
95%. Even trace levels of other contaminants will invalidate the
CO2 Equations of State and Phase Diagram, which are based on
pure CO2. Without knowing the exact phase envelope and physical
properties of the CO2 stream it will be extremely difficult to control
the CCS processes and undertake accurate flow measurement.
It will be necessary to design the flow metering system for the
correct physical phase it will be operating in. Accurate density
measurement will be required to allow reporting in mass CO2 units,
if the meter used measures volumetric flowrate. Accurate density
will be required to allow reporting in mass CO2 units, if the meter
used measures volumetric flowrate.
Sampling of the CO2 stream will be necessary to specify the CO2
concentration and for the regulatory reporting of other non-CO2
components in the CO2 stream. In order to measure the full suite
of components a number of different sampling technologies may
be required.
As the composition of the CO2 stream will vary continuously both
at the capture plant and throughout the transportation network
(particularly where a number of different sources with different
CO2 mixtures and impurities will be introduced to shared pipelines)
it will be necessary to undertake continuous sampling using CEMS
(Continuous Emissions Measurement Systems).
Sampling points will be necessary at the capture plant and at
various points throughout the transportation network where the
composition can vary.
Once the composition of the CO2 stream has been measured, the
physical properties can then be calculated to provide the necessary
data for handling and transporting the CO2 throughout the different
parts of the CCS network and for flow measurement purposes.
Determining the Physical Properties
There will be a need to establish new Equations of State and Phase
Diagrams to cater for the many different CO2 mixtures that are
likely to arise in CCS schemes.
Physical properties software modelling packages can be used to
generate new data for the different CO2 mixtures. However, any
such models would have to undergo validation to demonstrate
the level of accuracy, as even small errors may result in serious
problems during processing and transport of CO2.
There can be a wide variation in results between different software
packages and algorithms when used to model the same CO2
mixture. It may be necessary, therefore, to establish validated
industry standards and tools (i.e. hardware/software) to minimise
inconsistencies and ensure a uniform approach throughout
industry. This would be particularly in cases where different parties
are sharing the same CCS network.
November 2009
Flow Measurement Systems
Flow measurement, in conjunction with the CO2 concentration
derived from sampling of the CO2 stream, will be required to
calculate the transfer of CO2 on a mass basis, across the CCS
chain. The draft CCS Monitoring and Reporting guidelines under
the EU ETS require that flow measurement be carried out within
measurement uncertainty levels of 1.5%. In order to achieve such
levels it will be essential to install the correct type of flow meter at
locations along the network where the flow conditions are stable
and in the single phase under which the flow meter is designed
to operate. This may necessitate the use of gas meters at certain
locations and liquid meters at other locations along the network.
Special consideration should be given to any flow meter selected to
measure in the supercritical phase, to ensure the flow meter is suitable,
of sound design with proven accuracy within this specific phase.
To ensure and maintain a traceable measurement uncertainty for
the purpose of regulatory reporting, flow measurement systems
should be calibrated, maintained and checked at regular intervals.
Flow meters should ideally be calibrated using traceable flow
facilities in CO2 under the conditions and ranges under which they
will be required to operate. Any secondary instruments used to
convert into mass flow, such as pressure, temperature and density
instruments, should also be calibrated and traceable to national
standards and located as close as possible to the flow meter.
There are a number of potential flow meters that may be suitable
for use in CCS schemes. Some of these have already been used
to measure CO2 rich mixtures in Enhanced Oil Recovery (EOR)
schemes and CCS pilot plants, although in general there has not
been any real validation of their performance and associated
measurement uncertainty, due to the different measurement needs
and regulatory requirements.
Types of Flow Meters
The following provides a brief overview of some of the types of
flow metering technologies and meters that are potentially suitable
for CCS applications.
Differential Pressure (DP) Flow meters
Orifice Plate Meters
These devices have a long track record of measuring CO2 and are
used widely in EOR applications. Good knowledge of density and
viscosity (when used under stable, single-phase conditions) will
provide accurate measurement to measurement uncertainty levels
between ± 1%. Robust design but intrusive in pipeline so incur
pressure drops across meter, thus necessitating the need for careful
positioning in the CCS pipeline to avoid phase changes. Orifice
plates can be used over a wide range of pipe diameters.
Venturi Meters
No known experience in CO2 applications. Induce lower pressure
drops than orifice plate but less accurate. They have the potential
to achieve ± 1% measurement uncertainty under stable, singlephase flow. There is experience with this type of meter in wet gas
flow. Robust design and can be used over a wide range of pipe
diameters.
V-cone Meter
The V-cone meter is a proprietary development of the Venturi
meter. No known experience in CO2 applications. V-cone meters
induce lower pressure drops than orifice plate but less accurate.
However, they have the potential to achieve ± 1% measurement
uncertainty under stable, single-phase flow. These meters have
been used in multiphase flow and are of robust design.
Volumetric Flow Meters
Turbine Meter
Long track record of measuring CO2 and experience in EOR
applications, with reported measurement uncertainty within ±1%.
These meters consist of a large number of moving parts, but are
considered robust, reliable and have a good track record in single
phase flow. Turbine meteres will work in single phase gas, liquid or
supercritical fluid, if of the correct design. Not intended to operate
in multiphase flow. If a meter encounters a phase for which it is
not designed, there is a large risk of mechanical failure. Sensitive to
pulsations and require to be calibrated in the viscosity and conditions
of use. Can be manufactured for any given diameter of pipe.
Vortex Meters
No known experience in CO2 applications. Intended for single
phase measurement only.
Ultrasonic Meter
Ultrasonic flow meters typically use either time-of-flight (ToF) or
Doppler techniques to determine fluid flow rate.
Traditionally ultrasonic ToF meters have not been intended for
CO2 rich applications. This is due to the high levels of acoustic
attenuation present in CO2. The main effect is caused by molecular
relaxation, which dampens the ultrasound, causing the meter to
lose signal.
Over recent years an ultrasonic meter has been developed
to overcome these issues. This has included the use of more
sophisticated and powerful signal processing features and
diagnostics. Subsequently a number of field trials in CO2 rich
applications (60% and upwards) have demonstrated accurate
and comparable results with an orifice plate used as a reference.
Although further validation will be necessary and extensive
development required on the majority of meters on the market,
ultrasonic ToF meters have the potential to provide a high accuracy,
non-invasive CCS measurement system.
Mass Flow Meter
Coriolis Meter
Experience in EOR applications and used for the custody transfer
measurement of dense phase/ supercritical ethylene. Recent
development suggests that selected meters may be able to operate
and measure in two-phase conditions, although not to the levels
of accuracy required for CCS regulatory measurement. Main
advantage is ability to provide a direct mass flow measurement.
Coriolis meters are limited in pipeline diameter size, which for large
pipelines may require a manifold to branch a number of meters in
parallel. In this situation consideration would have to be given to
the impact on pipeline and process conditions.
Measurement of CO2 throughout the carbon capture
and storage (CCS) chain
Integrated Measurement System
All of the various measurements parameters in the CCS scheme will
be interdependent on one another: sampling, physical properties
and flow measurement. Sampling the composition of the CO2
stream will provide the necessary data to determine and calculate
the physical properties and phase envelope at various points. This
will allow the planning of the operational processes to determine
the necessary pumping arrangements and conditioning required
to transport the CO2 economically and safely in the pipeline.
The physical properties data and composition will feed into flow
measurement calculations to determine mass flow of the CO2.
To ensure the effective control and management of the overall system
and a smooth transition from the capture of the CO2, through to
its transportation and injection into the storage formation, it will be
necessary to have smart measurement systems and interfaces in place
to provide online tracking of the key parameters.
This could comprise CEMS systems, which feed into a physical
properties calculation tool based on industry standards that
generates the necessary data and phase envelope, which in
turn feeds into a flow measurement system. Comprehensive
measurement systems could integrate the different parameters to
cater for both the process and regulatory reporting needs, including
measurement uncertainty throughout the measurement chain.
Figure 2 shows, in simple terms, an integrated measurement/
tracking system in a shared pipeline.
Figure 2 Integrated Measurement/Tracking System
Recommended Further Reading
Hunter, L and Leslie, G.
A Study of Measurement Issues for Carbon Capture and Storage
Report 2009/54
TUV NEL Ltd, East Kilbride, Glasgow
Proposal for a Directive of the European Parliament and the
Council on the Geological Storage of Carbon Dioxide and Amending Council Directives 85/337/EEC, 96/61/EC, Directives 2000/60/
EC, 2001/80/EC,2004/35/EC, 2006/12/EC and Regulation (EC) No
1013/2006 – Brussels 23.1.2008.
Directive 2003/87/EC of the European Parliament and of the Council of 13 October 2003 establishing a scheme for greenhouse gas
emission allowance trading within the Community and amending
Council Directive 96/61/EC.
Commission Decision of 18/07/2007 establishing guidelines for the
monitoring and reporting of greenhouse gas emissions pursuant of
Directive 2003/87/EC of the European Parliament and the Council.
The purpose of this Guidance Note is to provide, in condensed
form, information on measurement methods and technologies. It was produced as part of the UK Government’s National
Measurement System.
For further information, contact:
TUV NEL, East Kilbride, GLASGOW, G75 0QF, UK
Tel: + 44 (0) 1355 220222
Email: [email protected]
www.tuvnel.com
© TUV NEL Ltd 2009
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