How Can We Calibrate Flow Meters for CO2 Custody Transfer?

Carbon capture, utilisation, and storage (CCUS) is emerging as a key technology to enable the energy transition. Its role in reducing emissions from industries such as steel, power generation, and cement manufacturing is well established. But what may not be immediately clear is how crucial accurate CO₂ flow measurement is to the successful implementation of CCUS.

In this post, I summarise some key aspects of CO₂ flow measurement, focusing on calibration challenges and the implications for CCUS projects that are discussed in more detail in this webinar that you can watch now.

What is Carbon Capture, Utilisation, and Storage (CCUS)?

CCUS stands for carbon capture, utilisation, and storage. It’s the process of capturing CO₂ from industrial sources, which would otherwise be emitted into the atmosphere. The captured CO₂ is then transported, often over long distances, to be either utilised in various industrial processes or permanently stored underground.

The Role of CCUS in Climate Change Mitigation

The primary goal of CCUS is to mitigate the quantity of CO₂ emissions from industrial sources, thus helping us achieve net zero CO₂ emissions by 2050 and limiting our impact on global temperatures. According to the Intergovernmental Panel on Climate Change (IPCC)¹, CCUS is a key technology in our fight against climate change. Governments worldwide, including the UK, are investing heavily in CCUS projects. The UK government alone has committed a billion pounds to two CCUS clusters, with more clusters expected by 2030.

The Importance of Accurate CO₂ Flow Measurement

Accurately measuring the flow of captured CO₂ is key to ensuring the success of CCUS projects². Without precise measurements, industries cannot accurately report the quantities of CO₂ captured for emission trading schemes or meet regulatory compliance. For example, emission trading schemes in the UK and EU require flow measurements to have an uncertainty of no more than 2.5% of the mass fraction of CO₂.

CO₂ is often transported through pipelines at varying pressures and phases as a high-pressure gas up to 40 to 50 bar or compressed into liquid or dense phase (also known as supercritical) which makes flow measurement a complex task.

In terms of thermodynamics there are only three fluid phases of matter. Gas and liquid phases are commonly used in day-to-day operation and the third is supercritical. This means that the fluid is above its critical pressure and critical temperature. However, an engineering term is often used to cover the liquid and part of the supercritical phase. This “dense phase” does not yet have a harmonised definition but is usually described as a single-phase fluid state with a density above 500 kg/m3.

The Challenges of Calibrating Flow Meters for CO₂

When it comes to calibrating flow meters for CO₂ custody transfer, we face several key challenges:

• Limited Calibration Facilities: Including TÜV SÜD, there are only three accredited labs globally that can calibrate flow meters for gas-phase CO₂, with none currently accredited for handling liquid or dense phase CO₂. While using CO₂ as a calibration fluid would be ideal, the lack of suitable facilities makes this impractical for now.
• Complex Behaviour of CO₂: CO₂ gas, liquid, and dense phase require different calibration methods. The dense phase exhibits unusual properties that complicate flow measurement further.
• Use of Alternative Fluids: Because of the challenges in calibrating with CO₂ directly, industries often use alternative fluids for calibration. While common in other sectors such as oil and gas, the use of substitute fluids introduces its own set of uncertainties, particularly when dealing with CO₂ in its supercritical state.

If you want to learn more about this topic, watch Dr Chris Mills’ webinar, ‘Can we calibrate in one fluid and operate in another?’.

Experimental Insights from TÜV SÜD into the Transferability of Flow Meter Calibration from Alternative Fluids to CO₂

For the regulations and contractual framework to be successful, we need robust flow measurement of liquid and dense phase CO₂. From our experience of existing sectors, this is usually completed by calibrating the flow meters in a fluid which is as close to the live fluid as possible.

At TÜV SÜD, we’ve been conducting extensive research to understand if different fluids (eg water, oil and gas) can be used for the calibration of CO₂ flow meters.

The study involved a comprehensive experimental campaign using a transfer package of five different flow meters:

• Orifice plate
• Two clamp-on ultrasonic meters
• Helical turbine meter
• Coriolis meter

Experimental Process:

1. Build transfer package
2. Calibrate all meters in water facility
3. Apply water calibration correction to future tests
4. Test all meters in EPAT oil facility
5. Test all meters in liquid and dense phase CO₂
6. Assess transferability of water calibration to CO₂

Our findings suggest that calibrating Coriolis meters with water can be extended to liquid and dense phase CO₂ with an associated uncertainty of approximately 0.35%. This is a promising result, given that the required measurement uncertainty for emission trading schemes is 2.5%. However, more research is needed to validate these findings across different meter sizes, pressures, and fluid compositions.

Update: Further research into this topic was published after the webinar in June 2024 ³ and again in October 2024 ⁴ which suggests that a water calibration can transfer to liquid or dense phase CO₂ provided that the appropriate meter corrections are applied and, if required, that the fluid density is measured directly.

Preliminary results on a turbine meter and clamp-on ultrasonics suggest that a calibration in CO₂ may be necessary when using them for custody transfer applications. As with the Coriolis meter, more work is needed to validate this result.

Gas phase testing has also shown encouraging results. Experiments conducted across multiple laboratories and with various meter sizes suggest that Coriolis meters could be calibrated using alternative fluids such as nitrogen or natural gas for applications with gaseous CO₂. For more info, please read ‘Experimental evaluation of various flow meters using gaseous CO₂’ (Flow Measurement and Instrumentation, Volume 99, October 2024, 102672).

Impurities in Captured CO₂ & their Effect on Flow Measurement

In real-world applications, CO₂ won’t be pure. It may contain small amounts of other gases, such as nitrogen or methane, which can affect flow measurement. While our preliminary small-scale testing on the Coriolis meter suggests that slight impurities may not significantly alter measurement accuracy, more research is needed to fully understand their impact under varying conditions.

From these recent studies, we have learnt that impurities have a large effect on the fluid properties of the CO₂ mixture, this results in difficulties in determining the density of the fluid by using equations of state alone. In our work we have seen that relying solely on equations of state for determining the density in the presence of impurities can result in large errors. Therefore, for the time being we urge caution in this area and suggest the use if direct measurement of the density where it can impact the overall flow measurement.

Addressing Industry Needs: TÜV SÜD's New Testing Facilities

To address the challenges of CO₂ flow meter calibration, TÜV SÜD is developing a new testing facility in East Kilbride, which will be operational in 2025. This facility will allow testing at pressures up to 200 bar and temperatures of up to 50°C, offering the capability to use custom CO₂ mixtures. This is a critical step forward in ensuring more accurate and reliable CO₂ flow measurements for CCUS applications.

The Future of Calibrating Flow Meters for CO₂ Custody Transfer

As the CCUS industry continues to grow, the need for accurate and traceable flow measurement standards becomes increasingly urgent. With UK CO₂ pipelines projected to handle 23 megatonnes per year by 2035, and similar developments expected worldwide, the limitations of current calibration capabilities must be addressed. Future research areas will include:

1. Improved density measurement techniques for supercritical CO₂
2. Long-term stability of flow meters in CO₂ service
3. Standardisation of calibration procedures for CO₂ flow meters

While significant progress has been made in CO₂ flow measurement, there is still a long way to go. Calibration and accurate measurement are essential to making CCUS a viable and scalable solution to climate change. Continued investment in research and infrastructure, like TÜV SÜD’s new facility, is key to overcoming these challenges.

TÜV SÜD National Engineering Laboratory is the UK’s Designated Institute for Flow Measurement, under contract from DSIT, and part of the UK’s National Measurement System.

Want to Learn More About CalibratING Flow Meters for CO₂ Custody Transfer?

Watch the recording of my webinar where I explore this topic in more detail.

Carbon Capture, Utilisation and Storage (CCUS) has been identified as a key tool to effectively reduce carbon dioxide emissions in a timely manner in order to meet our net zero goals. CCUS development has been gaining momentum in recent years with a near doubling in capture capacity in development since 2021 and with the implementation of the UK and EU Emissions Trading Schemes (ETS) this trend is likely to continue. However, for these schemes to be effectively implemented accurate measurement of the CO₂ captured and stored is needed and custody transfer standard of flow measurement with its routine calibration requirements will important.

With the rapid development of this new area of flow measurement many challenges have been posed which ultimately need addressing.

FOCUS TOPICS

• Do existing flow measurement technologies work in CO₂?
• How can we calibrate flow meters for use in CO₂?
• What are the effects of impurities in the CO₂ stream on flow measurement?

References

1 https://www.ipcc.ch/report/sixth-assessment-report-working-group-3/

2 Toward standardized measurement of CO₂ transfer in the CCS chain Nexus Volume 1, Issue 2, 18 June 2024, 100013 - Gabriele Chinello, TÜV SÜD et al

3 Investigation of the transferability of calibration between alternative fluids for liquid and dense phase carbon dioxide flow measurement Flow Measurement and Instrumentation, Volume 98, September 2024, 102644 - Russell Brown, TÜV SÜD, Gabriele Chinello, TÜV SÜD

4 Can a water flow calibration transfer to dense phase carbon dioxide with the presence of impurities? R. Brown, TÜV SÜD, G. Chinello, TÜV SÜD, T. Davies, TÜV SÜD

SPEAKER

Russell Brown
CCUS Flow Measurement Consultant

Russell is a Flow Measurement consultant here at TÜV SÜD specialising in the measurement challenges of Carbon Dioxide for the CCUS Industry. In his time at TÜV SÜD, Russell has gained experience in the testing a calibration of high pressure dry gas, wet gas and multiphase flow meters. More recently he has been worked on the design, upgrade, and commissioning of our newest CO₂ flow facilities and the subsequent research campaigns carried out using these facilities.