7 May 2018

Green Ammonia – a significant contributor to a low-carbon energy system?

What will our future energy systems look like? How will we continue to provide easy access to affordable energy, and avoid the causes of climate change? It’s time to talk about “Green Ammonia”.

While the move away from coal represents good progress in reducing carbon emissions from the power sector, renewable sources still only account for around 20% of our electricity generation. And similar progress in other energy-intensive sectors such as industry, transport and heat remains, as yet, elusive.

How can we store energy in sufficient quantities?

Renewable power sources offer carbon-free energy, but the problem is their intermittency: we can’t control when the sun shines, or when the wind blows – yet we want (and are used to) the freedom to choose when we use this energy. Energy storage is often presented as the solution to this intermittency problem, but the challenge is to store energy in sufficient quantities – and at low enough costs – to meet our needs. Energy is, of course, already stored in vast quantities today – it is just that the energy stores we are used to come in the form of fossil fuels such as oil (and its derivatives) and natural gas. These chemical energy vectors are ubiquitous for a reason – carbon based fuels are stable, energy dense, and are easy to store and transport.

The issue lies with the CO2 emitted when we burn them.

Ammonia (NH3) is a promising candidate

So, what if there was an alternative? What if we could synthesise a carbon-free fuel, using renewable energy, which could be used to store and transport that energy in bulk, without the carbon emissions associated with its fossil-based counterparts?

Ammonia (NH3) is a promising candidate for just such a carbon-free, synthetic fuel. An ammonia molecule is made up of one nitrogen atom and three hydrogen atoms – in some ways similar to natural gas (methane, CH4 ) which has one carbon atom plus four hydrogen atoms – and can be synthesised from abundant raw materials, namely air and water.

Nitrogen comprises 78% of the atmosphere, and may be readily separated out from air; water may be split back into its constituent elements via an electrochemical process called electrolysis – you may even have done this experiment in school with a beaker of water, two electrodes, and a battery.

Once the hydrogen and nitrogen are produced, they can be combined in a reaction called the Haber-Bosch process to produce ammonia. If renewable energy is used to power these processes, then that energy becomes locked up in the ammonia molecule, without any direct carbon emissions.

“Green Ammonia”

The final step, energy release from this “Green Ammonia”, can be achieved by cracking the ammonia back into nitrogen and hydrogen, and then using the hydrogen in a fuel cell – such as in a fuel cell electric vehicle. Another way is to use combustion – in exactly the same way as we burn carbon-based fuels today – such as in a gas turbine, for example.

In this way, Green Ammonia offers the enticing prospect of reducing carbon emissions not just in electrical power generation, which has so often been the limit of our current best efforts to decarbonise, but also other sectors such as transport and industry.

The infrastructure is already widespread

The beauty of all this is that the technology required to put it into practice already exists at the required scale: industrial air separation processes to produce nitrogen are routine; water electrolysis was performed on an industrial basis before steam methane reforming became a cheaper source of hydrogen. Fritz Haber won his Nobel Prize “for the synthesis of ammonia from its elements” in 1918, and today the Haber-Bosch process accounts for 180 million tons of ammonia production each year. The infrastructure required to store and transport it safely is already widespread.

To better understand the prospects and challenges surrounding the use of ammonia in energy systems, Siemens is leading a collaborative project to build and test an ammonia-based energy storage system at the Rutherford Appleton Laboratory (RAL) in the UK.

Testing a demonstration system

Green Ammonia has the opportunity to play a vital part of a future, low-carbon energy system. Our work at the RAL is now centred on learning about the practical aspects of operating a Green Ammonia plant, by testing a demonstration system , and discussing the economic prospects for Green Ammonia.

Related links:

The Guardian: Siemens pilots the use of ammonia for green energy storage

Hydrogen – the green energy of the future

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