While simultaneously decarbonising energy production, the world’s energy needs are increasing at pace. In this article, David Bowers, Product Manager Process & DP Flow, Spirit IT at ABB Measurement & Analytics, explores what is driving interest in green hydrogen, what the process entails, and how modern measurement instruments and analysers designs are adapting to meet the challenges of producing it on a large scale.
The true benefit of hydrogen is that it is a clean-burning fuel that helps to reduce noxious gases in heavily polluted areas however, traditional methods of large-scale hydrogen production for energy tend to rely on the burning of fossil fuels. This is known as “grey hydrogen” or is referred to as “blue hydrogen” when captured through carbon capture and storage (CCS). Whilst hydrogen is less harmful to the environment than traditional fossil fuels, its impact is not negligible, particularly when fossil fuels are used to produce it.
The emergence of clean or ‘green hydrogen’, which is produced through an electrolysis process using renewable electricity, and produces little or no carbon emissions, is an area where there is significant potential to both bring down renewable energy costs and reduce the carbon impact of energy generation.
Source: ABB
As a clean burning fuel hydrogen produces only water as a by-product. With more renewable energy sources being brought online all the time, large scale production of green hydrogen presents increasing opportunities to reduce emissions, while also creating an abundant supply of hydrogen for use in industrial processes.
Green hydrogen production is not without its challenges. Processes must be controlled with a high degree of precision to ensure effective and safe operation, while achieving maximum productivity, efficiency and purity of the hydrogen gas end product.
Types of electrolysis
The three most commonly used technologies to produce green hydrogen are alkaline electrolyte cells (AEC), polymer electrolyte membranes (PEM/PEMEC) and solid oxide electrolysers (SOE/SOEC). AEC electrolysers tend to have the lowest CAPEX of the three, and are arguably the most established of the technologies. Each has its advantages and disadvantages, for instance PEM allows a faster ramp-up, while SOE provides high efficiency, but also requires high operating temperatures.
Common to all three methods is the need for measurement. Controlling a hydrogen electrolyser of any kind is crucial to ensure safe operation, efficient power to hydrogen conversion, and adequate purity quality control. Measurement of temperature is also required to avoid over-heating in the electrolyser stack, irrespective of the method used.
Smart measurement devices
Continuous measurement of a vast array of parameters relating to electrolysis process performance is made possible thanks to recent advances in measurement technologies. Digital-enabled devices, connected via the cloud, provide greater accuracy and faster response times than their analogue equivalents.
This can help operators to detect anomalies and potential faults before they risk disrupting the process or causing damage to equipment. Remote monitoring allows parameters to be analysed without the need for physical inspection, allowing maintenance teams to use their time more efficiently, and allocate duties precisely where they are needed, when they are needed.
Gas analysis
Electrolysis creates chemical reactions, which can cause small concentrations of oxygen to build up in the hydrogen stream, and vice-versa. The electrolyser stack assembly can also potentially leak gas from one side of the electrolyser cell to the other. This can produce hazardous conditions, and as such, gas analysers must be sufficiently sensitive to detect potential safety risks and trigger a safe shutdown if required.
Source: ABB
ABB’s EasyLine EL3060 gas analyser, for instance, is designed specifically for hazardous area applications, and incorporates different options for accurately measuring hydrogen down to -0-1 vol.-% to 0-10 vol.%. Protective features include the use of a flameproof control unit housing, while a touch screen allows safe operation without needing to open the housing.
Liquid level measurement
As well as measuring for potential gas leakages within the electrolyser, and cross-contamination between the oxygen and hydrogen streams, it is also important to detect and measure the presence and volume of liquids. Raw hydrogen gas contains electrolyte vapors which must be separated from the gas. Typically there will be an initial knock-down phase separator to allow gas and liquid separation after the electrolyser. The hydrogen is then cooled, and a second separation removes the condensate.
The gas solution is pumped to the separator and then recirculated back to the electrolyser. To avoid the pump running dry and hydrogen therefore entering the pump, resulting in it flowing to the wrong part of the electrolyser, it is vital to monitor the water in the knock-down phase separator. A sufficiently low level should trip the recirculation pump, and extremely low levels should trigger a safe shutdown of the electrolyser and initiate a nitrogen gas purge. Level monitors used in this part of the process should be ATEX-certified.
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 measurement
Overheating is a risk in any electrolyser. Renewable sources of energy, such as solar and wind, are variable in their nature, and sudden ramp ups can result in the electrolyser drawing more current, and creating more heat. To mitigate this, water is pumped around the electrolyser to cool it, but in order to trigger that action, more measurement is required. Platinum resistance thermometers, which detect temperature by measuring electrical resistance, are typically used here.
Temperature measurement is also critical at the de-oxo stage, where traces of oxygen in the hydrogen are converted to water in an exothermic catalytic reaction. It is important to ensure that the reaction remains under control and that the temperature does not exceed safe levels.
Pressure measurement
Certain types of electrolyser operate at elevated pressure. This is important if the gas is to be used at high pressure, as pumping at higher pressures is much more energy efficient compared to compressing the hydrogen gas after the electrolyser. As with many processes, over-pressurisation of vessels and equipment can create potentially hazardous conditions, and so accurate pressure measurement is vital to ensure safe operation.
Intricate gauges and red valve on a sophisticated industrial pipeline.
One issue that can affect pressure transmitters in hydrogen production is that of hydrogen permeation, whereby a hydrogen molecule splits into two hydrogen ions, which then penetrate the pressure transmitter diaphragm. This is a leading cause of pressure transmitter failure. In response, ABB has developed an “H-Shield” titanium-based binary nano coating on its transmitters, which forms a protective coating across the surface of the diaphragm. This provides high resistance against the permeation of hydrogen ions, while still offering sufficient flexibility for the diaphragm to move in response to changing pressure conditions.
Conclusion
In the pursuit of sustainable energy, there is no doubt hydrogen will become an important part of a more sustainable energy mix in the coming years. Advances in instrumentation technology are helping to provide reliable monitoring of the myriad of process variables involved to make the process safe, effective and financially viable.
For more information about measurement technologies for hydrogen applications, visit https://bit.ly/ABBMeas_Hydrogen.
