Decarbonisation Technology August 2025 Issue

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Process stage

Figure 3 Flow velocity profile across AEL process stages

Figure 4 Temperature profile across AEL process stages

• Control system behaviour: Flow may be adjusted automatically for purity, pressure, or analyser requirements. • Gas withdrawal rates: Increased compressor demand increases velocity, while blockages or recirculation reduce it. Temperature varies in different phases in the process Internal temperatures range from 40-90°C, peaking during electrolysis and dropping after drying (see Figure 4 ). Analysers must handle high temperatures or be protected by conditioning: • Start-up: Temperature is near ambient (20-30°C) with no active reactions. • Hydrogen ramp-up: Heat rises quickly from electrochemical activity. • Inside the electrolyser: Temperatures peak at 60-90°C during sustained operation. • After the separator: Slight cooling occurs as gas and liquid separate. • After the dryer: Active/passive cooling lowers temperatures, often <40°C. • Steady production: High temperature (~85°C) is maintained for efficiency. • Shutdown: Heating stops, system cools back to ambient. These temperature shifts directly influence gas density requiring real-time temperature compensation in gas analysers. Cross-gas contamination risks Maintaining H₂ and O₂ separation is vital for both safety and gas purity. In AEL systems, gases are generated on opposite sides of a diaphragm,

but leaks or diffusion can still cause cross- contamination (see Figure 5 ). Online analysers must detect ppm-level impurities to prevent explosive mixtures and ensure system reliability. Why it is potentially hazardous • Even small concentrations of O₂ in H₂ (or vice versa) can lead to flammable or explosive mixtures, especially if combined with pressure drops or vacuum formation. • H 2 mixed with O₂ at concentrations above 4% can become explosive under normal atmospheric conditions. Common contamination scenarios include • Membrane or diaphragm permeation: Ageing or damaged membranes can become permeable, allowing gas diffusion due to pressure differences. • Sudden pressure changes or vacuum: Rapid pressure shifts or vacuum can draw gas across compartments, breaching separation. • Inadequate separation or design: Poor system design, bubble crossover, or separator wear can lead to gas mixing. • Downstream equipment leaks or backflow: Leaks or reverse flow in compressors, dryers, or piping during shutdowns may mix H₂ and O₂. Impact on measurement and product purity • Online analysers must be able to detect low- range O₂ in H₂ or H₂ in O₂ to ensure gas streams are safe and within specification. • Failure to monitor cross-contamination can result in product rejections, explosion