Although progress has been made reducing fossil
fuel consumption, crude oil and natural gas will be a significant energy source
for many decades to come. Therefore, reducing associated CO2
emissions is crucial to controlling the release of GHGs into the atmosphere. CCUS
is regarded as one of the best solutions to slowing climate change.
DR. CHRIS MILLS, TÜV SÜD
National Engineering Laboratory
Carbon capture utilization and storage (CCUS)
is regarded as a key enabling technology for the large-scale displacement of
natural gas by hydrogen in gas grids. While the production of hydrogen via electrolysis
of water is the optimum solution and already widely used for small-scale
production, large-scale production via electrolysis is not prevalent at
present. Instead, large-scale production will be based on stream reforming of
natural gas. Although this is a mature technology, the by-product is carbon
dioxide (CO2). By incorporating CCUS into this process, the
production would be significantly cleaner and help reduce anthropogenic CO2
One of the many technical challenges to be
overcome in establishing CCUS as a practical operational process is effective
measurement and monitoring. Accurate measurement will be essential for
environmental and safety needs and fundamental in reducing financial exposure
in CO2 trading schemes. However, there are a large number of
potential measurement challenges expected in CCUS, due to both the physical properties
of carbon dioxide (CO2) and the processes involved in CCUS projects.
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, Fig. 1.
Compared to other substances that are transported by pipeline
(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 physical
properties of CO2 (phase, density, compressibility).
In CCUS 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. 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
significant detrimental effect on measurement accuracy, where flow meters are
designed to operate in one specific phase only.
Another major challenge for measurement will be coping
with impurities in the CO2 stream, which will be present and vary
depending on the capture process, capture technology and fuel source used. Even
trace levels of 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 CCUS processes and undertake accurate
flow measurement. Specifying the optimum flow metering system is dependent on
the operating phase.
There are a number of other factors that may affect
the measurement of CO2. 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. Three principal areas are essential to monitor CO2 across the
Sampling of the CO2 mixture. Sampling of the CO2 stream will be necessary to determine the
CO2 concentration and for the regulatory reporting of other non-CO2
components in the CO2 stream. Sampling points will be necessary at
the capture plant and at various points throughout the transportation network
where the composition can vary. It will be necessary to undertake continuous
sampling using continuous emissions measurement systems (CEMS).
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 CCUS network and for flow measurement purposes.
Determining the physical properties. There will be a need to establish new equations of state and phase
diagrams, due to to the many different impurities in CO2 streams that
are likely to arise in CCUS 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
the processing and transport of CO2.
Another issue with relying on physical properties
software modelling is that 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
(hardware/software) to minimize inconsistencies and ensure a uniform approach throughout
industry. This would be a fundamental requirement in cases where different
parties are sharing the same CCUS network.
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 CCUS chain.
For example, the draft CCUS Monitoring and Reporting guidelines under
the European Union Emissions Trading Scheme require that the overall
measurement uncertainty, i.e., for the combination of flowmeter and composition
analyzer, be carried out within measurement uncertainty levels of 2.5%.
To achieve such levels, it will be essential to
install the correct type of flowmeter 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 flowmeter selected to measure in the
supercritical phase, to ensure the flowmeter 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. Flowmeters should be
calibrated at traceable laboratories in CO2 under the conditions and
ranges for which they will be required to operate. Any secondary instruments
used to convert into mass flow, such as pressure, temperature and density
instruments, should be calibrated and traceable to national standards and
located as close as possible to the flowmeter. However, there is a lack of
traceable calibration facilities in the world that can offer CO2 as
the test medium. The few facilities that do exist are limited to gas phase CO2.
There are a number of potential flowmeters that may be
suitable for use in CCUS schemes, some of these have already been used to
measure CO2-rich mixtures in enhanced oil recovery (EOR) schemes and
CCUS pilot plants. However, 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 FLOWMETERS
The following provides a brief overview on some of the
types of flow metering technologies and meters that are potentially suitable
for CCUS applications.
Differential pressure flowmeters. Orifice plate meters have a long track record of measuring
CO2 and are used widely in EOR applications. They can be used over a
wide range of pipe diameters. Good knowledge of density and viscosity (when
used under stable, single-phase conditions) will provide accurate measurement
with uncertainty levels of ±1%. They are of robust design but intrusive in the
pipeline, so they incur pressure drops across meter, thus necessitating the need
for careful positioning in the CCUS pipeline to avoid phase changes.
Venturi meter and V-cone meters are other types of
Differential Pressure flowmeters that could be used in CO2
applications. They are of robust design and can be used over a wide range of
pipe diameter sizes. However, there is a lack of experience for these meter types.
They induce lower pressure drops than orifice plates but are typically less
accurate. However, in optimum conditions, they can achieve ±1% measurement
uncertainty under stable, single-phase flow.
Volumetric flowmeters. Turbine meters have a long track record of measuring
CO2 and experience in EOR applications, with reported measurement
uncertainty within ±1%. They can be manufactured for any given diameter of
pipe. They have a large number of moving parts, but are considered robust, reliable
and have a good track record in single-phase flow. Turbine meters will work in
single-phase gas, liquid or supercritical fluid, if of the correct design,
i.e., a liquid turbine meter can only be used for liquid applications, and is
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. They are
sensitive to pulsations and need to be calibrated in the viscosity and
conditions of use.
The vast majority of ultrasonic flowmeters use either time-of-flight
(ToF) techniques to determine fluid flowrate. Traditionally, ultrasonic ToF
meters have not been intended for CO2-rich applications. This is due
to the relatively large amplitude loss of the ultrasound waves, referred to as
attenuation, that occurs in CO2. The attenuation of an ultrasonic
pulse comes from either classic absorption or from relaxation processes. Classical
absorption is based on the effects of viscosity and thermal conductivity. The
relaxation processes that lead to attenuation are based on the exchange of energy
between molecular vibrations and translations.
For CO2, it is the relaxation processes
that are the main contributors in terms of causing the meter to lose signal. Over
recent years, an ultrasonic meter has been developed to overcome the issues
caused by the attenuation. 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 CCUS measurement system.
Mass flowmeter. Coriolis meters have a demonstrated track record in EOR applications and
are used for the custody transfer measurement of gaseous and supercritical
ethylene. Recent developments suggest that selected meters may be able to
operate and measure in two-phase conditions, although not to the levels of
accuracy required for CCUS regulatory measurement. Their main advantage is the
ability to provide a direct mass flow measurement. Coriolis meters are limited
to a pipeline diameter of 16 in., which means that for large pipelines, a split
manifold to accommodate a number of meters in parallel will be required. In
this situation, consideration would have to be given to pressure drop and the
impact on pipeline and process conditions.
Integrated measurement system. All of the various measurement parameters in the CCUS scheme
will be interdependent of 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 from point of capture of the CO2, through its transportation
to injection into the storage formation, it will be necessary to have smart
measurement systems and interfaces in place to provide online tracking of the
This could comprise CEMS systems, which feed into a physical
properties calculation tool, based on industry standards that generate 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
an integrated measurement/tracking system in a shared pipeline.
Across the complete CCUS network, accurate measurement
of CO2 at temperatures, pressures, flowrates and fluid phases will
be required. These measurements require validation through a credible
traceability chain. This traceability chain will provide the confidence in
meter performance, financial and fiscal transactions and critically, environmental
compliance. With the renewed interest in CCUS and its role in enabling
large-scale displacement of natural gas by hydrogen in gas grids, it is
essential that a robust metrology framework be implemented and deployed as soon
as possible. The success of CCUS depends on it. WO
MILLS is a senior
consultant engineer at TÜV SÜD
National Engineering Laboratory, a provider of technical consultancy,
research, testing and program management services. Part of the TÜV SÜD Group,
the company is also a global center of excellence for flow measurement and
fluid flow systems. The laboratory also serves as the UK’s National Measurement
Institute for Flow Measurement. www.tuvsud.com/en-gb/nel