Hydrogen emissions from an electrolysis unit

Abstract

Hydrogen is expected to play a vital role as an energy carrier in the future decarbonized system. Currently, a large portion of the hydrogen produced comes from the steam reforming of methane present in natural gas, which produces significant amounts of carbon dioxide emissions. However, with the growing need to reduce greenhouse gas emissions, a shift from fossil fuels to renewable energy sources is required. As a result, there is a growing emphasis on green hydrogen, i.e., hydrogen generated using renewable power. Green hydrogen has been growing at an exponential rate since 2020 and is expected to account for the majority of hydrogen production by 2050. However, a few studies have recently suggested that hydrogen may indirectly contribute to global warming. It is believed that hydrogen delays the decomposition of methane, a strong greenhouse gas, and thus extends its lifetime in the atmosphere. If green hydrogen is to be the primary fuel in the energy transition, hydrogen emissions from an electrolysis unit should be investigated.

This project focuses on identifying the sources of hydrogen emissions from an electrolysis unit. The goal is to comprehend the depth of this potential issue and investigate possible solutions. This project is carried out in collaboration with Worley, a market leader in the design, construction, and delivery of green H2 facilities. The leakage estimates for the green hydrogen alkaline electrolysis plant are based on Worley’s in-house data. Venting during startup and shutdowns when power is unavailable, as well as hydrogen crossover in the electrolyzer, have been identified as two major contributors to hydrogen emissions. Solutions such as flaring systems to combust the vented hydrogen and battery energy systems to reduce frequent shutdowns and startups are investigated. To reduce emissions from hydrogen crossover, a reactor is modeled to explore the catalytic recombination of hydrogen and oxygen. These solutions are subjected to a techno-economic analysis to determine their viability.

Flare systems and battery energy systems are both deemed feasible. In the long
run, however, installing a battery energy system would be preferable to combusting the hydrogen product. In comparison to other battery technologies such as Li-ion and lead-acid batteries, vanadium redox flow battery systems have been found to provide the maximum incentives and highest optimal capacities at the lowest overall costs. To avoid emissions from hydrogen crossover in a low-pressure alkaline electrolysis unit, the most cost-effective design involves a single-stage compression followed by a scrubber, heater, and reactor. However, this design is still costly because the annualized costs are four times greater than the costs offset by emissions reductions per year. Governments can encourage the adoption of such solutions by providing financial incentives to businesses.