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Why green ammonia may not be that green


Ammonia has been in the news for its suitability as a hydrogen carrier and fuel, in addition to being a essential ingredient in fertilizer. Existing distribution networks and the ease of converting ammonia gas into a liquid make ammonia one cost effective way to transport renewable energy. For a given volume, ammonia – a molecule made up of three hydrogen atoms and one nitrogen atom – contains about 50% more hydrogen atoms than hydrogen itself.

Because ammonia contains only hydrogen and nitrogen, it does not emit carbon dioxide when used. If made using green hydrogen (produced with renewable energy), its production also emits no carbon dioxide. Therefore, green ammonia could help achieve a zero world, particularly as a fuel for long-haul transport and heavy industry.

Green ammonia produced with renewable energy can be used for power generation, energy storage and transport, fuel and fertilizer.
Source: Davos Agenda/World Economic Forum, CC BY-NC-ND

Australia is well positioned to develop a major renewable hydrogen export industry, potentially using green ammonia. Proposed projects include Cape Hardie, Collinsville, Australian renewable energy hub, HyNQ, H2Tas And Gibson Island.

Unfortunately, just because ammonia doesn’t contain carbon doesn’t make it good for the environment. It is a source of nitrogen pollution, which has many harmful effects on the environment. Ironically, despite Australia’s natural advantage in green ammonia production, we have the largest nitrogen footprint per capita in the world.

But rarely do proponents of green ammonia assess its environmental sustainability more critically than just zero claims.

Read more: Green ammonia could reduce agriculture emissions – and power the ships of the future

Crossing planetary boundaries

The Stockholm Resilience Center defines nine processes that regulate the Earth to support life as we know it. The boundaries of a “safe operating space” are defined for each process.

It probably won’t surprise you that climate change is one of the boundaries we’ve crossed. You could be forgiven for thinking this has been our biggest impact on the planet. But that ‘honor’ goes to biogeochemical streams of nutrients, mostly as a result of nitrogen fertilizers.

Why? Well, it comes back to ammonia.

Image showing the nine planetary boundaries from the Stockholm Resilience Centre
The planetary boundaries of the Stockholm Resilience Centre. (BII stands for Biodiversity Intactness Index. E/MSY is the number of extinctions per million species years, a common measure of extinction rates. P stands for phosphorus and N for nitrogen.)
Source: Azote for Stockholm Resilience Center, based on analysis in Wang-Erlandsson et al 2022, CC BY-NC-ND

Read more: You’ve heard of a carbon footprint – now it’s time to take steps to reduce your nitrogen footprint

Unbalance the nitrogen cycle

Nitrogen forms 78% of our atmosphere as dinitrogen gas (N₂). However, in this form it is inaccessible to living organisms.

To sustain life, nitrogen must be converted to reactive forms such as ammonia. Once it reaches ecosystems, ammonia undergoes a series of chemical transformations. Eventually it breaks down back into N₂ and the cycle can begin again.

For millions of years, the processes in each step of the cycle have been in equilibrium. However, industrial fertilizer production in the 20th century has unbalanced this cycle.

On the positive side, ammonia has been a panacea for growing food. An estimated 3.5 billion people are fed by chemicals fertilizers. This means that almost half of the world’s population would go hungry if synthetic ammonia were not available.

The downside is that too much reactive nitrogen ends up in the environment – ​​more than that twice the recommended planetary limit.

In the case of excess nitrogen, crossing the planetary boundary has already had huge consequences. It has led to ecosystem degradation, photochemical smog, acid rain and health problems such as respiratory disease and cancer, algal blooms leading to fish kills, such as the recent one at Menindee Lakesdamage to the Great Barrier Reef and greenhouse gases far more potent than carbon dioxide.

Producing additional ammonia as a sustainable energy carrier could exacerbate these problems.

Read more: Nitrogen pollution: the forgotten element of climate change

A leakage problem

Some estimate that ammonia supply chains are leaking it into the environment at rates of up to 6%. However, research is limited. More commonly understood natural gas supply chains can yield a margin of approx 2.6%.

Replacing fossil fuels with ammonia for long haul trucks and shipping can reduce the carbon footprint of transportation (which is responsible for 37% of total carbon dioxide emissions). But if 2.6% of ammonia leaked from the supply chain, we would estimation this could triple the reactive nitrogen flow, further crossing the planetary boundary. With a leakage percentage of 6%, this can be four times the case.

A natural gas terminal
An estimated 2.6% of natural gas in the supply chain leaks to the environment and we can reasonably expect a similar leak for ammonia.
Norbert Fellechner/EPA/AAP

Read more: Global shipping is under pressure to quickly stop using heavy fuel oil – it’s not easy, but changes are coming

We all know how climate change has changed our world – and that is “only” 1.2 times the planet’s carbon limit. The use of green ammonia as a sustainable energy carrier can have an even greater impact on the nitrogen planetary boundary.

A supplement to ammonia

Another sustainable energy carrier can be made from green hydrogen: methanol. It is a molecule of four hydrogen atoms and some carbon and oxygen atoms.

Like ammonia, methanol can be used as a fuel. It could replace petrochemical raw materials used in industrial processes and manufacturing. Our research shows that methanol could also enable biotechnology to better integrate with industrial processes.

More importantly, methanol does not affect the nitrogen cycle. As long as it is made using a renewable carbon source, such as carbon dioxide direct aerial shot, there is a net zero impact on the environment. International Energy Agency projections show that the United States is putting more emphasis on green methanol, while Australia and Europe are more focused on green ammonia.

Environmental sustainability means more than net zero. In the case of green ammonia, holistic thinking is needed so that we don’t solve one problem just to make another worse.

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