A vanadium alloy membrane is tipped to transform the hydrogen separation process, and enable the use of ammonia as a means of carrying the ultralight element. Andrew Dykes speaks with the CSIRO team to learn more.
Hydrogen production should not be of interest to oil and gas operators purely for its role in creating other products. With the so-called hydrogen economy expected to grow, refiners, gas transport fleets and many other hydrocarbon-based businesses stand to gain from the substantial opportunities.
In particular, the transport of hydrogen has proved to be a difficult problem to solve. Although gaseous hydrogen can be transported by pipeline, it has a tendency to damage steel, and needs considerable pipe wall thickness to ensure it does not escape. The US Department of Energy (DoE) notes in a briefing on the fuel that
“the high initial capital costs of new pipeline construction constitute a major barrier to expanding hydrogen pipeline delivery infrastructure.”
It is for that reason that InnovOil took notice of a new project led by Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO). Under a new two-year project announced in May 2017, a research group will work to develop and demonstrate a hydrogen production system that can deliver at least 5kg per day of high-purity hydrogen, directly from ammonia.
This somewhat inverts the conventional wisdom on ammonia production – hydrogen, usually derived from natural gases, is typically combined with nitrogen to produce ammonia for fertiliser – but the logic is sound. Ammonia (NH3) has a high capacity for storing hydrogen atoms – 17.6% by weight, and at a volumetric density 45% greater than liquid H2. It has often been proposed as a carrier method, given that it is stable and can be stored in pressure tanks in much the same way as propane or other fuels. Cracking it also produces nitrogen – a non-toxic, non-greenhouse gas (GHG).
Yet the large amount of energy needed to create and/or separate ammonia molecules and unfavourable economics has discounted any further practical use – until now.
Ain’t got that swing
A technique called pressure swing absorption (PSA) is the benchmark for hydrogen purification, and delivers H2 pure enough for use in proton exchange membrane (PEM) fuel cells. Yet PSA systems are bulky, have a large number of moving parts, and as a batch process, require duplication of all components to enable continuous production.
The key to the CSIRO project rests in a different approach based on a proprietary membrane separator technology designed by Dr Michael Dolan. The thin metal membrane allows hydrogen to pass while blocking all other gases, and using decomposed ammonia feedstock, enables H2 conversion in a single step. Moreover, unlike a PSA system, it permits a smaller plant – with no moving parts – to work in continuous operations.
The membrane is made of a vanadium-based alloy. Dolan explained more about it to InnovOil via email: “Our design philosophy has been to use inexpensive materials and mass-production techniques (like metal tube extrusion and electroplating) as much as possible. The membrane substrate itself is a dense tube of a permeable, inexpensive [vanadium] alloy which is drawn down to a wall thickness of ~0.2 mm, and diameter of 10 mm. A catalytic layer is then deposited on the inner and outer surfaces.”
The membrane separates gaseous molecules at temperatures of 300-400˚C. Ammonia is first vaporised, then passed over a catalyst which decomposes it into nitrogen and hydrogen. The gas is pressurised through the membrane to extract high-purity H2 from the mixture.
There is of course an efficiency penalty for this; the system requires heat to drive the endothermic decomposition process, and the loss of pressure means the hydrogen must then be fed into a compressor for use in fuel cell applications (although it could be used at ambient pressures in stationary power generation).
Dolan added: “As with most gas separation processes, the recovery of the valuable product (H2) is never 100%. We will typically operate with around 85% recovery, but this depends on residence time and desired output.” That said, there are methods of improving its efficiency. “The unrecovered energy will not be wasted. The off-gas, which contains mostly nitrogen (N2) with unrecovered H2 and unreacted NH3, can be combusted to create the heat required for ammonia decomposition, or it can be fed to a second device, like a high-temperature fuel cell, internal combustion engine or turbine for power generation,” Dolan confirmed.
Selling out the palladium
Overall, CSIRO is confident that this membrane will enable it to produce a separation system at a lower cost than the competing palladium-based membrane technology in use today.
By how much lower, Dolan said, would be difficult to confirm at this stage, but that the issue was much more about performance and purity.
“It really depends on the geometry of the palladium [Pd] membranes and the intended application. PEM fuel cells require H2 which contains no more than 100 ppbv of ammonia. Even very small defects in a membrane will mean this limit is exceeded, and the fuel won’t be suitable for use in a PEM fuel cell.”
“Our membranes are thicker (200 micrometres) than the supported Pd-based membranes which are being commercialised elsewhere (< 10 micrometres),” This thickness of the vanadium membrane eliminates the potential for defects, meaning the CSIRO system can meet the required PEM purity standard. “Thinner membranes are more susceptible to defects, either during fabrication, or over time. The likelihood of these defects can be mitigated by making thicker membranes, but the high cost of Pd (currently US$28,000 per kg) makes this cost-prohibitive,” he added.”
Its main benefit will be in unlocking existing infrastructure for hydrogen export and transport. CSIRO’s goal is to enable the membrane technology to fill the gap between hydrogen production, distribution and delivery. Separator systems would be deployed in the form of a modular, scalable unit that can be used at, or near, a refuelling station or production site. “Because membranes are a modular technology, the scale-up issues are minimised. To make more H2 we just put more membrane tubes into the plant,” Dolan said.
Having begun the two-year endeavour, Dolan and his team are now working on the first 5kg per day demonstrator plant. “This plant will also include compression, and the resulting H2 will be distributed to several FCEV manufacturers for demonstration in their vehicles,” he said. The project has received a A$1.7 million (US$1.3 million) grant from Australia’s Science and Industry Endowment Fund (SIEF), which will be matched by CSIRO, as well as some high-profile public support from the likes of industry members such as BOC, Hyundai, Toyota and Renewable Hydrogen.
Once this first project is complete, Dolan and the team have their eyes on wider commercialisation. “We’ll then be looking to undertake trials in Korea and Europe at larger scales, probably around 100 kg per day,” he said. From here, the only limit would appear to be which sources of energy are most useful to hydrogen production. CSIRO has said it will investigate all stages of the technology chain, from solar photovoltaics (PV), concentrated solar power (CSP), through to grid management, water electrolysis and ammonia synthesis to identify the best way forward.
CSIRO chief executive Dr Larry Marshall stated: “This is a watershed moment for energy, and we look forward to applying CSIRO innovation to enable this exciting renewably sourced fuel and energy storage medium a smoother path to market.”
The oil and gas industry too is now waking up to the potential of hydrogen as a commercial opportunity, and investment in technologies such as CSIRO’s now will play a key role in the development of a supporting economy. Dolan believes the change is already visible; “Renewable H2 will become more important in the longer term, and most of the major established oil and gas companies have already announced major investment and partnerships to facilitate this transition… Using ammonia as a hydrogen carrier is one of the greatest emerging opportunities.”