Global efforts to achieve zero carbon emissions from industry are gathering pace as a variety of environmental, economic and geopolitical factors are driving the development of sustainable energy sources such as green hydrogen. The latest generation of smart instruments and analyzers are helping to enhance the efficiency, safety and viability of green hydrogen production. Sensors, analyzers and transmitters help with measuring conductivity, temperature, level, pressure and flow.
With the combined concerns of disruptive climate change and energy security, countries around the globe are focusing on reducing the production of greenhouse gases such as carbon dioxide (CO2) and methane and finding ways to shift from fossil fuels to more sustainable alternatives. Last year’s COP26 summit in Glasgow encouraged countries to draw up ambitious emissions reduction targets for 2030 with the aim of reaching net zero carbon emissions by the middle of the century. Projections estimate the global hydrogen economy will be worth $2.5 trillion and create 30 million jobs by 2050. As a way of maximizing efficiency and safety and providing the data needed to inform decision making,
Achieving these targets will mean moving from conventional energy sources, such as coal, oil and gas, to renewable sources that produce minimal emissions and do not depend on a handful of countries for supply.
Hydrogen for sustainable fuel, feedstock for fertilizer, plastics
The criticality of energy supplies for everything from leisure through to industrial use requires sources that are consistent, reliable and scalable. Although renewable sources, such as wind and solar, can help reduce emissions, they are intermittent, and it is difficult to store the electricity produced.
However, despite their environmental impact, fossil fuels have continued to be used as the mainstay of energy supplies. This is because they offer advantages such as higher energy density, can be stored to meet seasonal demand and their potential to be used as a chemical feed stock for industrial processes that depend on carbon.
The growing viability of hydrogen as an energy source is changing this. Hydrogen offers many of the advantages of renewables and fossil fuels – it can be produced with low or zero emissions, can be stored and transported, is clean burning producing and is reactive for use in further chemical processing or production.
As such, it is considered one of the key fuels to help de-carbonize energy use. It can be used as fuel for transport and electricity peaking plants, while burning hydrogen also can provide heat for many types of industries and residential and commercial buildings. Hydrogen can act as a feedstock for chemicals, such as fertilizers, fuel refining and plastics.
Hydrogen production by color classifications, hydrogen economy future
The production of hydrogen is well understood, and a number of processes can be used. These vary in the chemical origin of the hydrogen and the renewability of their electricity source.
Hydrogen production is generally classified as green, gray, blue, brown, or white depending on the method used. Green hydrogen, the most ecologically friendly type, is produced by electrolysis using renewables or nuclear energy.
If hydrogen is to make a significant contribution to mitigating climate change, its production must be based on zero-carbon electrolysis powered by renewable energy sources. The International Energy Agency (IEA) estimates if net zero emission is achieved by 2050, total hydrogen demand from industry will have expanded by 44% by 2030, with low carbon hydrogen making up 21 million tonnes, according to an The IEA September 2022 tracking report on hydrogen. Some progress is being made in increasing hydrogen production, with nearly 70 MW of electrolysis capacity installed in 2020, doubling the previous year’s record, IEA said.
3 methods to optimize electrolyzer performance with controls, instrumentation
As a multi-stage process, green hydrogen production requires accurate measurements to ensure safe and efficient operation. ISO22734:2019 (Hydrogen generators using water electrolysis – Industrial, commercial and residential applications) stipulates the main parameters that need to be measured during hydrogen production processes to help maintain control and avoid potential issues that could affect efficiency or safety.
To produce green hydrogen, there are three main electrolysis methods in use today.
Alkaline electrolysis (AEC) is a mature, commercial technology. To maximize the conductivity of the electrolyte used to produce hydrogen, AEC electrolyzers uses an alkaline solution of 25-30 wt% potassium hydroxide (KOH), known as lye. The highly alkaline nature of the electrolyte means any instrument that comes into contact with it must be corrosion proof. With features including a PVDF body and Hastelloy C electrodes, an industrial conductivity sensor is ideally suited for aggressive applications such as high concentration KOH measurement.
The PEM (Proton Exchange Membrane) electrolyzer uses pure water as an electrolyte solution, avoiding the need to recover and recycle the potassium hydroxide electrolyte solution needed with alkaline electrolyzers. The purity of the water is key, with reverse osmosis and ion exchange resins being used to deionize the water to a conductivity of less than 0.1 mS/m. Designed for use in ultra-pure water applications, a 2-electrode conductivity cell can ensure water conductivity is maintained at this level, with virtually no requirement for maintenance.
Solid Oxide Electrolysis cells (SOEs) use ceramics as the electrolyte and have low material costs. Operating at high temperatures and with a high degree of electrical efficiency, they use steam for the electrolysis process and so require a heat source. Using steam rather than makeup water to supply the electrolyzer, SOE electrolyzers have different instrumentation requirements from AEC and PEM electrolyzers, demanding accurate measurement of flow, pressure and temperature with smart instrumentation.
Controlling hydrogen electrolysis reactions requires accurate gas analyzers
Process control of a hydrogen electrolysis process performs three main functions – safe operation, efficient power to hydrogen conversion and gas purity control.
One challenge in the electrolysis process is the potential for small concentrations of oxygen to build up in the hydrogen stream and hydrogen to build up in the oxygen stream. The electrolyzer stack assembly can leak gas from one side of the electrolyzer cell to the other. ISO22734 defines this as a fault condition.
To avoid this, hydrogen electrolyzers require sensitive gas analyzers that can measure traces of hydrogen in the oxygen stream and vice versa to very low levels.
Raw hydrogen gas also contains electrolyte vapors from the electrolyzer cell. A knock-down phase separator allows gas and liquid separation after the electrolyzer. Monitoring the liquid level in the knock-down phase separator is critical as a very low level would shut down the electrolyzer and trigger a nitrogen gas purge.
Level measurement, temperature control for hydrogen measurements
Magnetic level instruments, including magnetic switches and sensors, can be used to measure low and high levels in the phase separator. By isolating the device from the process medium, magnetic level measurement offers an ideal non-contact solution for measuring levels in the phase separator, while also eliminating the need for costly seals, diaphragms, and process connections commonly associated with point level switch technology. Set points can be adjusted without any changes to process piping, resulting in level switches that are quickly deployed, readily adjustable and easy to maintain.
Temperature control is also critical. Variable electricity supply from renewable sources may cause the electrolyzer to increase production, drawing more current and raising the temperature. Continuously measuring the stack temperature will enable effective control of cooling to maintain levels within safe limits.
Combining a platinum resistance thermometer with an appropriate transmitter will provide the measurements needed and a solution for triggering preventive measures in the event of an alarm. Where features such as continuous sensor monitoring and self-monitoring are also included, there is the added possibility of gathering additional information about supply voltage and issues such as wire breaks or corrosion.
The same technologies can be applied to monitor and control temperatures in at the de-oxo stage, where traces of oxygen in the hydrogen are converted to water in an exothermic catalytic reaction to create the final hydrogen product. It is essential to monitor the temperature to ensure the reaction remains under control and conditions remain within safe limits.
Pressure measurements, pumping liquid water supply
Some types of electrolyzers are designed to operate at elevated pressure. The ability to accurately measure pressure levels is especially important if the gas is to be used at high pressure, as pumping the liquid water feed to the electrolyzer to an elevated pressure such as 30 bar is less costly and much less energy intensive than compressing the hydrogen gas from atmospheric pressure to 30 bar after the electrolyzer. Installing a digital pressure transmitter in the water circuit to continuously monitor pressure can help optimize pumping performance.
Accurate and reliable pressure measurement is important in maintaining process safety by preventing over-pressurization of the electrolyzer and ensuring hydrogen and oxygen gases generated by the electrolyzer can flow away without obstruction.
Pressure transmitters measure the pressure of oxygen and hydrogen gases. Certification by TUV NORD for use in process safety control systems according to the IEC61508 standards series on functional safety helps to protect pressurized electrolyzers.
Another issue that can affect pressure transmitters in hydrogen applications is the problem of hydrogen permeation. Caused by hydrogen molecules passing through the pressure transmitter diaphragm and diffusing into the pressure transmitter’s fill fluid, hydrogen permeation can impair transmitter performance until failure occurs. Putting a titanium-based binary nano coating provides highest resistance against hydrogen ion permeation, while enabling the pressure transmitter diaphragm to respond to changing pressure conditions.
Smart measurement adds automation, control efficiency
Today’s smart digital measurement technologies provide greater accuracy, range and depth of information that can be used to assess process performance and the status of the measurement devices. Features such as remote connectivity help to make diagnostics information more usable, enabling engineers to perform actions such as fault tracing or changes to an instrument’s configuration without having to be present. Greater predictivity facilitates proactive maintenance, avoiding unnecessary downtime and minimizing the risk of potential damage to key process plant or impaired hydrogen quality.
Digital instruments offer enhanced simplicity, making it easier for operators at any level of experience to access or relay key operational and maintenance-related data using familiar technologies, such as QR codes.
Automation can help develop the hydrogen economy
The development of energy sources such as green hydrogen is predicted to play a growing role in achieving net zero carbon goals, with projections estimating the global hydrogen economy will be worth $2.5 trillion and create 30 million jobs by 2050. As a way of maximizing efficiency and safety and providing the data needed to inform decision making, smart instruments are almost certainly set to play a major part in this growth.
