Power-to-X processes based on PEM water electrolyzers: A review of process integration and flexible operation

Hydrogen produced with renewable electricity may be used to synthesize valuable chemical products, thus aiding the decarbonization of the chemical industry. Thanks to their fast dynamics, Polymer Electrolyte Membrane Water Electrolyzers (PEM-WEs) appear promising for coupling with fluctuating power inputs, e.g., from a grid balancing request or standalone renewable power supply. However, the flexibility of downstream synthesis processes under time-variable hydrogen flow rates is still unclear, and expensive hydrogen storage could be needed as buffer. We review the state of the technology with a focus on the potential for flexible operation of the key components of the main Power-to-X processes: hydrogen production via PEM-WEs, carbon capture, nitrogen production, and the synthesis of methane, methanol, syngas, and ammonia. Flexibility concepts of the units are described and the characteristic times for start-up and load variations, ramp rate limits, and operating ranges are collected. Opportunities for heat and mass integration are discussed.