With industrial organizations focused on meeting tough environmental sustainability and decarbonization targets, the potential of carbon-free green hydrogen – produced through electrolysis and using renewable energy – is obvious. However, if that potential is to be realized, a number of significant production challenges must be overcome. In an article in Hydrogen Tech World, ‘Essential steps for safer production of green hydrogen’, Emerson’s Veronica Constantin, vice president for global sustainability, explains:
…Green hydrogen production is still in its relative infancy, which means the industry is still learning and developing, and that creates a greater safety risk in the short term. However, the availability of software tools with machine learning, AI and digital twin technology can help to significantly lower that risk by accelerating the learning process.
Hydrogen is highly flammable and the safe production of green hydrogen requires suitable control and safety systems to be in place. The article continues:
…Highly accurate measurement and control throughout the process is necessary to ensure sufficiently safe, high-pressure water flow, which prevents temperatures that could ignite hydrogen gas. Monitoring the integrity of electrolyser membranes is also required to prevent flame, accurately detect the mixture of oxygen and hydrogen, and protect against water vapour in the system. Finally, the conductivity of water supplied to electrolysis stacks must be tightly controlled and levels in separators gauged to prevent overfills and keep oxygen gas from contaminating the water system.
One challenge facing producers is that critical assets such as electrolyzer units, compressors and fire and gas safety infrastructure are often provided by different original equipment manufacturers (OEMs). Although each piece of equipment may incorporate some degree of machine safety functionality, the OEMs will often have limited visibility and understanding of plant-wide process safety. The accepted practice at large-scale plants is for all process safety functions to be centralized and managed by an integrated control and safety system (ICSS). Veronica says:
…ICSSs make it possible to operate multiple, large, interconnected electrolyser units. When combined with advanced instrumentation, including pressure, temperature and water conductivity sensors, flow meters, final control elements and gas detection sensors, the ICSS software can constantly monitor performance and equipment health. Should an issue arise, the safety system can bring operations to a safe point.
The PoSHydon project in the North Sea aims to prove the viability of offshore green hydrogen production. The ICSS provided by Emerson manages the electrolysis unit, desalination process, variable power supply and blending with natural gas for transportation via the existing gas pipeline. Digital twin technology provides an invaluable tool to analyze various ‘what if’ design scenarios, such as different rectifiers, water purification systems, or balance-of-plant design improvement ideas. The article continues:
…A digital twin can also validate the optimised control and safety schemes, including advanced control models and start/stop procedures. Digital twin technology can also prove essential in the area of regulatory compliance and validation of proposed safety concept where the electrolysis facility will be integrated within existing industrial plants.
It is vital to monitor the flow of water to the electrolyzer, as insufficient flow creates the risk of temperature increases that could ignite flammable gas. However, space for measurement devices within compact small-scale electrolyzer designs can be limited. Veronica explains:
…The latest vortex flow meters provide two independent flow measurements in one meter body and require very short straight runs to perform accurate measurements. This helps to overcome this space limitation issue, while also ensuring safe operation of the electrolyser.
Accurate and reliable temperature measurement is also important. Thermowells provide the required accuracy but add design complexity and increase the possibility of leaks. Ideally, temperature measurements should be non-intrusive. One option is to take a surface measurement of a pipe, but this does not typically provide the accuracy or repeatability required. The article explains:
…A solution to this problem is technology that measures the ambient and pipe surface temperatures and calculates the process temperature via a thermal conductivity algorithm. This offers an extremely accurate non-intrusive temperature measurement that is also very easy to implement.
Precise flow control is required, but this can be undermined by poor control valve performance. It is essential to select valves and final control elements that are reliable in high-vibration, high-pressure conditions, can operate without leaks, and ensure accurate flow control. Veronica says:
…Application-specific, high-pressure valves that use double-stage actuators and block-and-bleed designs are necessary to adhere to strict shut-off requirements, especially in case of emergencies. Superior valve packing technology with die-formed expanded graphite that achieves the highest possible emissions class for their design should be used to meet emissions compliance standards, minimise product losses and increase safety.
It is important to control the level in the hydrogen and oxygen separators to prevent overfills and hydrogen and oxygen gas carry-under from going to the water system. This is a challenging application, even for guided wave radar transmitters, which in general provide extremely accurate and reliable measurements. The article states:
…Deploying a radar with probe end projection enables accurate measurements of very low dielectric media, such as those found in this application. In addition, radar devices offering dynamic vapour compensation can eliminate accuracy errors associated with varying pressure and temperature when steam vapour is present.
The article explains that early detection of pressurized gas leaks is important, but conventional solutions, such as personal monitoring or fixed-point gas detection, detect few leaks and can also be susceptible to false alarms. Veronica says:
…The deployment of ultrasonic gas leak detection, point and open-path gas detectors increases detection efficiency and provides accurate alarms, even in external applications.
Finally, the article states that should a gas leak be ignited, it is essential to quickly locate its source and shut off the supply to that point. Hydrogen burns with a flame that is almost invisible to the naked eye, but can be seen using UV/IR flame detectors that can detect flames up to 90 meters away in just milliseconds.
Visit the Renewable Hydrogen section on Emerson.com to learn more about Emerson’s automation solutions for green hydrogen production.