Measurement and Instrumentation

CCUS measurement for low-carbon H₂ production

D. Anderson, TÜV SÜD National Engineering Laboratory, Glasgow, Scotland, UK

 

The UK has committed to drastically reducing its emissions of carbon dioxide (CO₂) and other greenhouse gases and achieving net zero emissions by 2050. To meet this target, substantial changes to the ways in which energy is generated, stored, transported and consumed are required. One potential route to decarbonization involves the use of hydrogen (H₂) as an energy vector, to power low-emissions vehicles, for heating homes and buildings, and for industrial applications.

Decarbonization with H₂ and CCUS

The H₂ pathway, as it is described in the UK Government’s Clean Growth Strategy, is attractive for several reasons. First and foremost, H₂ is clean-burning, producing only water as a byproduct, whether it is used in direct combustion or with fuel cells to generate electricity. It should be noted that H₂ itself is not a primary energy source (i.e., naturally occurring) and must be produced; however, it can be considered as a means of storing energy. For the overall process to be climate neutral, the H₂ must be produced with net zero emissions. This can be achieved in several ways, including the electrolysis of water, provided the electricity used is generated from clean sources such as solar or wind. The H₂ produced in this way is often referred to as “green” H₂.

It could be argued that using the renewable electricity directly would be more energy efficient, rather than producing green H₂, to subsequently convert it back into water. In many cases this is true, but it is important to consider that the supply of renewable electricity is generally intermittent. Solar and wind power are both subject to diurnal and annual fluctuations. At times, the supply will be insufficient to meet demand, and during periods of peak generation there can be an excess, leading to production being curtailed. Producing H₂ via electrolysis would provide an energy buffer for when demand cannot be met and could even act as a long-term storage mechanism for energy.

In some processes, it may be preferable to use H₂ produced from renewable electricity rather than to use electricity directly. Electric battery vehicles are increasingly common and represent a major step toward the decarbonization of transport. This works well for light-duty vehicles, but the weight and charging times of lithium ion batteries are prohibitive for use with heavy-duty vehicles and long-distance transportation. H₂ has a gravimetric energy density of 140 MJ/kg, which is higher than natural gas (53.6 MJ/kg) and diesel (45.6 MJ/kg), and much higher than lithium ion batteries (< 5 MJ/kg). However, in volumetric terms, H₂ is the least-dense gas and takes up more space than both natural gas and diesel. When stored as a compressed gas, the volumetric energy density of H₂ (2.7 MJ/L at 350 bar or 4.7 MJ/L at 700 bar) is still greater than that of a lithium ion battery (2.2 MJ/L), making it a serious contender for use with larger vehicles, such as HGVs.

Similarly, for the decarbonization of domestic heating, one option is to use heat pumps and electric cooking appliances. However, the replacement of natural gas with H₂ in the gas grids is also being considered in many countries, including the UK. This could potentially minimize disruption to end users and allow the gas network infrastructure, and the cumulative skills and experience of its work force, to be repurposed. It would also negate the need to upgrade the UK electricity grid to accommodate the increased electricity generation required. The Clean Growth Strategy estimates that under the electricity pathway, 647 TWh/yr of electricity would need to be generated, a 93% increase compared to the 335 TWh generated in 2018. Under the H₂ pathway, the annual electricity requirement would be similar to today, at 339 TWh. The use of H₂ could also decarbonize industrial direct flame applications, which are essential to provide many chemical products but cannot be replaced with an electrical equivalent.

A strong case can be made for the use of green H₂ in the decarbonized energy supply of the future. However, at present, most H₂ is not produced from electrolysis but from a chemical process called reforming, typically either steam methane reforming (SMR) or autothermal reforming (ATR). In these processes, methane reacts with high-temperature steam in the presence of a catalyst and at elevated pressures. Syngas is produced, which is a mixture of H₂ and carbon monoxide (CO), before a water-gas shift reaction is used to convert the CO into CO₂ and more H₂. In the final step, H₂ is separated from the CO₂ and other impurities by pressure swing adsorption (PSA), a process that exploits the differing tendencies of pressurized gases within a mixture to adsorb to solid surfaces.

Measurement challenges in CCUS

Since the feedstock includes a fossil fuel and the products include CO₂, this process is neither renewable nor carbon neutral. It is, however, a viable route to producing vast amounts of H₂, enabling the use of H₂ vehicles and appliances while the infrastructure to produce green H₂ at the scale required is developed. The key question then is how to deal with the CO₂ produced? Carbon capture, utilization and storage (CCUS) can be used to create a net-zero emissions process. The UK Government Clean Growth Strategy envisions that 700 TWh of energy could be produced from H₂ in 2050, with most H₂ produced from reforming, coupled with CCUS to keep the process carbon neutral. The H₂ produced from this route is known as “blue” H₂.

With CCUS, many potential measurement challenges are expected due to both the physical properties of CO₂ and the processes involved in CCUS projects. Crucial to the implementation of large-scale CCUS is the method by which it will be monetized, with numerous different approaches being considered, from taxation through to credit-based systems. Whichever mechanism prevails, monetization requires accurate knowledge of how much CO₂ has been sequestered, much the same as custody transfer metering in the oil and gas industry. For context, the UK Oil and Gas Authority requires measurement uncertainty of ±1% for fiscal metering of natural gas, while uncertainties of ±2.5% or less for the total mass of CO₂ measured are required under the EU Emissions Trading System (EU ETS).

In addition to the pecuniary aspects, the ability to accurately measure the flowrate of process streams at various points and reconcile this data to provide a holistic mass balance across the entire system will be important for two other reasons. The first is reservoir management, which will require knowledge of the amount of CO₂ and other process stream components fed into the geological formation. The second is safety; CO₂ is a heavy, asphyxiant gas that can readily pool upon leakage if conditions are correct, and so any breach of system integrity will need to be detected and located quickly.

CO₂ is unusual because of the closeness of its triple point and critical point to the temperatures and pressures commonly found in industrial processes. 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 physical properties of CO₂ (e.g., phase, density, compressibility) (Fig. 1).

 

 

Fig. 1. CO₂ phase envelope.

 

 

 

In CCUS applications, tightly regulating the temperature and pressure can be a difficult undertaking, particularly over long distances. Pipelines can span hundreds of miles and be subjected to various climates and conditions that affect operating pressure and temperature. When operating near a phase boundary line, there is a risk that the fluid will change phases, or even that multiphase flow conditions will arise. If this occurs at measurement points, it will have a significant detrimental effect on measurement accuracy, where flowmeters are designed to operate in one specific phase.

Impurities in CO₂ streams

Another major challenge for measurement is coping with impurities in the CO₂ stream, which vary depending on the capture process, capture technology and fuel source used. Without knowing the exact phase envelope and physical properties of the CO₂ stream, it can be extremely difficult to control the CCUS processes and undertake accurate flow measurement (Fig. 2).

Three main measurements are essential to monitor CO₂ across the CCUS chain:

1. Composition measurement of the CO₂ mixture
2. Determination of physical properties
3. Flow measurement.

Sampling of the CO₂ stream is necessary to determine the CO₂ concentration and for the regulatory reporting of other non-CO₂ components in the stream. As the composition of the stream will vary continuously, sampling points are necessary at the capture plant and at various points throughout the transportation network where the composition can vary.

 

 

Fig. 2. Phase envelopes for CO₂ with impurities.

 

 

Ensuring flow measurement certainty

After the composition of the stream has been measured, the physical properties can 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. New equations of state and phase diagrams must be established to accommodate the many different CO₂ mixtures that are likely to arise in CCUS schemes.

Physical properties software modeling packages can be used to generate new data for the different CO₂ mixtures. However, wide variation in results can exist 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 to minimize inconsistencies and ensure a uniform approach. This is particularly important in cases where different parties are sharing the same CCUS network (Fig. 3).

 

 

Fig. 3. An integrated measurement system in a shared pipeline.

 

 

Flow measurement, in conjunction with the CO₂ concentration derived from the sampling of the CO₂ stream, is required to calculate the transfer of CO₂ on a mass basis across the CCUS chain. To meet the EU ETS required measurement uncertainty of ±2.5% for the total mass of CO₂ measured, it is essential to install the correct type of flowmeter at locations along the network where the flow conditions are stable, and in the specific phase under which the flowmeter is designed to operate. This may necessitate the use of gas meters at certain locations and liquid meters at other locations along the network.

To ensure and maintain a traceable measurement uncertainty for the purpose of regulatory reporting, flow measurement systems should be calibrated, maintained and inspected at regular intervals. Flowmeters should be calibrated at traceable laboratories, using CO₂ at 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 be calibrated and traceable to national standards and located as close as possible to the flowmeter.

The ability to accurately measure the amount of CO₂ sequestered will be a fundamental foundation of large-scale CCUS, but this presents some interesting technical challenges that require an integrated approach to resolve, such as real-time determination of process stream composition, bulk flowrate and fluid properties. The essential technologies exist, but the challenges of integration and economic viability should not be underestimated.

TÜV SÜD National Engineering Laboratory operates a traceable H₂ calibration facility for domestic gas flowmeters, and a primary flow standard for validating H₂ refueling station dispensers is in development. In addition, capabilities developed for CCUS include gas flowmeter calibration with CO₂ and CO₂/N₂ mixtures at up to 1,000 m³/hr at 25 bar, as well as a facility for testing densitometers, sampling systems and various sensors with CO₂ and CCUS mixtures in liquid, gaseous or supercritical states. As the energy transition progresses, it is essential that the UK’s National Measurement System has the capability to support industry needs to meet the target of net zero emissions by 2050.

 

Dale Anderson is a Clean Fuels Engineer at TÜV SÜD National Engineering Laboratory (NEL), where his primary focus is understanding the flow measurement challenges for H₂, CO₂ and LNG. Since joining NEL, he has been involved in various projects related to the design and uncertainty assessment of physical testing facilities. Part of the TÜV SÜD Group, NEL is the UK’s Designated Institute for Flow and Density Measurement.