Australian broadacre farmers use about 1.9 million tonnes of urea per year – most of it (1.7 million tonnes) is imported. Disruptions to supply chains and price spikes have highlighted a dire need for alternative fertiliser sources. Additionally, urea makes up the grains sector’s largest single source of greenhouse gas (GHG) emissions
A way to overcome these issues in Australia is emerging by innovating the way ammonia is made.
To accelerate innovation on behalf of the Australian grains sector, the GRDC is partnering in the ‘Hydrogen to Ammonia’ project alongside the CSIRO, Orica, and the Australian Renewable Energy Agency (ARENA).
Under development is an exciting new technology that promises to produce ammonia using only water, air and solar photovoltaic (PV) energy. While the viability of the new method has been proven – and patents obtained on key innovations – ongoing research is next attempting to scale up and lower the cost of the new ammonia reactors.
How it works
Currently, the most economical way to make ammonia on an industrial scale is the Haber-Bosch process. It uses extremely high pressure and iron catalysts to force hydrogen to react with atmospheric nitrogen gas. However, the hydrogen is derived from fossil fuels (primarily natural gas) and results in carbon dioxide emissions that account for 1.2 per cent of the global total.
The new technology removes the need for natural gas as a hydrogen source and dramatically reduces the pressure needed to combine it with nitrogen.
The process works by using solar PV energy to source hydrogen directly from water. A polymer electrolyte membrane (PEM) is used within an electrolyser to drive the hydrogen component of water across the membrane by a partial pressure difference. In the presence of a catalyst, the permeated hydrogen then reacts with nitrogen supplied to the other side of the membrane.
Successful laboratory-scale work resulted in the technology being scaled up. This involved commercial viability by optimising the design of the reactor and catalysts.
CSIRO reports that two stations have been constructed to test different reactor designs, catalysts and the interfaces between catalyst and PEM membrane. These stations can run tests at temperatures as high as 500oC and pressures of 30 bar.
These experiments can be operated for long periods of time, while the ammonia synthesis rates are continuously monitored. Additionally, the test stations are connected to each other and can exchange their product streams to investigate separation and recycling of unreacted gases.
Overall, synthesis rates are looking promising. In a single pass, the test reactors achieve a hydrogen to ammonia conversion rate of up to 11.5 per cent and an ammonia content of 11.3 per cent of volume.
In contrast, a traditional Haber-Bosch reactor achieves 15 per cent conversion rates in a single pass but requires pressure in the reactor to be five times higher (150 bar).
Overall, the development work by CSIRO and Orica demonstrates the technical feasible of creating a new distributed ammonia production system that is wholly renewable. However, to be cost effective, additional development work is needed.
A key focus is on low-cost substitutes materials for the key reactor components, such as the catalyst and membrane. CSIRO has reported encouraging results using cheaper materials, achieving conversions rates of hydrogen to ammonia as high as 10 per cent using lower temperatures.
Ways are also being found to optimise how the ammonia is separated and how the unreacted inputs are recycled.
More broadly, this work is occurring within a greater push to roll out a hydrogen economy, with many green technologies reaching maturity at the same time. In the National Hydrogen Roadmap report, CSIRO examined the wider uses of hydrogen as a fuel and estimated that renewable hydrogen could match gas and battery storage on cost by 2025.
However, most planned production of green hydrogen and ammonia in Australia is not expected to become operational until around 2030.
Additionally, there are regulatory and policy considerations that are creating a “market pull” for greener technologies and they too will ultimately impact delivery and costs. Despite the risks, the GRDC is keen to see technological innovation used to shield grain growers from fertiliser supply uncertainties, untenable commodity price spikes and inflationary impacts.
More information: Justin Crosby, firstname.lastname@example.org