Controlled Removal of Trace O2 in an H2 Environment of a Small-scale Electrolyser

Abstract

Power-to-gas (PtG) technology represents a promising route in providing long-term energy security and involves pairing volatile sources of renewable energy (solar and wind energy) with alkaline water electrolysis (AWE) to create a wide range of products based on intermediate hydrogen gas for end-users across various industries such as in fuels, chemicals and power sectors. However, PtG currently lacks the technical adaptability to match the intermittency of these sources caused by daily and seasonal weather patterns, and suffers from a low turndown ratio (TDR) as a consequence.

Zero Emission Fuels (ZEF) B.V. aim to adapt this process to create an autonomous, dynamically operated solar panel add-on known as a micro-plant that will produce methanol from CO2 and H2O in the atmosphere. In doing so, the company looks to extend the TDR, but requires the micro-plant and by extension, its AWE unit to operate in wider partial load ranges, in which lower current densities dominate. These cause a rise in gas crossover within the electrolyser, creating impure and potentially flammable H2-O2 mixtures in the outlet that hinder the functionality of the micro-plant. Specifically within the AWE unit, the risk of oxygen crossover has been qualitatively identified as a likely safety hazard. Therefore, the focus of this thesis was set on studying the feasible development of a prototype (gas scavenger) that can remove trace O2 within the techno-economic scope of ZEF’s micro-plant and electrolyser.

To determine a viable method that meets ZEF’s constraints on cost, size, weight and efficiency, industry techniques were studied that either purify hydrogen gas streams or destructively remove flammable H2-O2 mixtures. Based on a subsequent numerical estimation on each method’s limiting factors, a local combustion process in the form of a micro-combustor was identified to be the most suitable method.

To quantitatively estimate the safety risk crossover poses, two models were developed: a non-dimensional model to analyse how 50 bar pressure influences crossover during operation, and a two-dimensional, transient diffusion model to analyse the extent of trace O2 contamination overnight. Results indicate that to mitigate dangerous impurity levels, a gas scavenger needs to be implemented: (a) above a 0.1% difference in pressure between electrolyser half cells during operation, and (b) upon start-up to remove overnight gas accumulation that may not pose an immediate severe risk at 0.079 mol% contamination relative to amount of stored hydrogen, but has the potential to build-up on a monthly timescale.

Finally, the feasibility of the micro-combustor was preliminarily characterised at 50 bar and 80 ⁰C through flame simulations. The variability in low amounts of trace oxygen content show that the mixture is normally non-flammable. Further studies on geometry design and control strategies of the micro-combustor must consider how to achieve optimal ignition conditions, determined to be at richer fuel mixtures between 휙 = 1.03−1.40, and how to contain the flame within a well-defined region. Initial suggestions include using a small volume to induce ignition and relying on wall effects to promote flame quenching.