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The use of ammonia (NH3) as a hydrogen vector can potentially enable renewable energy export from Australia to markets in Asia and Europe. With a higher hydrogen density than liquid H2, plus existing production and transport infrastructure, and well-developed safety practices and standards, the financial and regulatory barriers to this industry are lower than for liquid H2 transport. The only significant technical barrier which remains, however, is the efficient utilisation of ammonia fuel at or near the point of use, either directly or through the production of H2.
For H2 production from NH3, the purity of the product H2 is a prime consideration. As NH3 can degrade the acidic polymer electrolyte in PEM fuel cells, the relevant purity standard for mobile PEM fuel cell applications (ISO14687-2) sets a maximum NH3 concentration of just 0.1 ppmv. Furthermore, while the standard for stationary PEM fuel cells (14687-3) allows for up to 50% N2, the mobile PEMFC standard allows for just 100 ppmv N2, due to the significant energy penalty that N2 introduces during compression. NH3 cracking reactors must therefore couple catalytic ammonia decomposition with separation and purification of the H2product.
Of currently available H2 separation technologies, metal membranes show particular promise as they combine infinite H2 selectivity (i.e., a 100% pure H2 product, assuming a defect-free membrane), high temperature operation (comparable to that required for NH3 decomposition) and tolerance to NH3-containing feeds. CSIRO has recently commenced a project to demonstrate a pilot-scale ammonia cracking reactor which will integrate vanadium-based membranes with a cracking catalyst to produce high-purity H2 directly from NH3, at a rate of at least 5 kg H2 per day. Partnerships with industrial gas producers and FCV developers will see this H2product compressed, distributed and dispensed into FCVs to illustrate the product’s fitness for purpose.
Identification and validation of catalyst and membrane materials, manufacturing scale-up, system design and progress towards the goal of 5 kg H2 per day will be discussed.