Ammonia, a main component of many fertilizers, could play a key role in a carbon-free fuel system as a convenient way to transport and store clean hydrogen. The chemical, made of hydrogen and nitrogen (NH3), can also be burned as a fuel without carbon emissions. However, new research led by Princeton University illustrates that, although it may not be a source of carbon pollution, the widespread use of ammonia in the energy sector could pose a serious risk to the nitrogen cycle and climate without appropriate engineering precautions.
Publication their findings November 6 in PNAS, the interdisciplinary team of 12 researchers found that a well-designed ammonia economy could help the world achieve its decarbonization goals and ensure a sustainable energy future. On the other hand, a poorly managed ammonia economy could increase nitrous oxide (N2O), a long-lived greenhouse gas about 300 times more powerful than CO2 and a major contributor to the thinning of the stratospheric ozone layer. Could cause substantial emissions of nitrogen oxides (NOx), a class of pollutants that contribute to the formation of smog and acid rain. And it could directly leak fugitive ammonia emissions into the environment, also forming air pollutants, affecting water quality and stressing ecosystems by disrupting the global nitrogen cycle.
Fortunately, researchers found that the potential negative impacts of an ammonia economy can be minimized with proactive engineering practices. They argued that now is the time to start seriously preparing for an ammonia economy, addressing potential hot spots for ammonia fuel before its widespread use.
"We know that an ammonia economy of some scale is likely to occur," said the research leader. Amilcare PorporatoProfessor Thomas J. Wu '94 of Civil and Environmental Engineering and the High Meadows Environmental Institute. โAnd if we are proactive and forward-looking in our approach, an ammonia economy could be a great thing. But we can't afford to take the risks of ammonia lightly. โWe cannot afford to be careless.โ
Translating ammonia from agriculture to energy
As interest in hydrogen as a zero-carbon fuel has grown, so has an uncomfortable reality: It is notoriously difficult to store and transport over long distances. The tiny molecule must be stored at temperatures below -253 degrees Celsius or at pressures of up to 700 times atmospheric pressure, conditions that are unviable for widespread transport and prone to leaks.
Ammonia, on the other hand, is much easier to liquefy, transport, and store, and can be moved similarly to propane tanks.
Furthermore, since the beginning of the 20th century there has been an established process for converting hydrogen to ammonia.th century. Known as the Haber-Bosch process, the reaction combines atmospheric nitrogen with hydrogen to form ammonia. While the process was originally developed as a cost-effective way to convert atmospheric nitrogen into ammonia for use in fertilizers, cleaning products and even explosives, the energy sector has viewed the Haber-Bosch process as a way to store and transport fuel from hydrogen. in the form of ammonia.
Ammonia synthesis is inherently energy-intensive and CO-free fossil fuels2 Capture is currently used to meet almost all of its raw material and energy demands. But as the researchers noted in their paper, if the new electricity-driven processes currently in development can replace conventional fossil fuel-derived ammonia synthesis, then the Haber-Bosch process, or a completely different process, could be used widely. to convert clean hydrogen into ammonia, which in turn can be burned as a carbon-neutral fuel.
"Ammonia is an easy way to transport hydrogen over long distances, and its widespread use in agriculture means there is already established infrastructure to produce and transport ammonia," he said. Matteo Bertagnipostdoctoral researcher at the High Meadows Environmental Institute working on the Carbon mitigation initiative. "Hence, hydrogen could be created in a resource-rich area, transformed into ammonia, and then transported to anywhere in the world where it is needed."
Ammonia's transportability is especially attractive to industries that rely on long-distance transportation, such as shipping, and countries with limited available space for renewable resources. Japan, for example, already has a national energy strategy that incorporates the use of ammonia as a clean fuel. The simple storage requirements mean that ammonia could also be used as a vessel for long-term energy storage, complementing or even replacing batteries.
โAt first glance, ammonia seems like an ideal cure for the decarbonization problem,โ Porporato said. "But almost all medications come with a number of possible side effects."
'Look before you leap'
In theory, burning ammonia should produce only harmless nitrogen gas (N2) and water as products. But in practice, Michael E. Muellerassociate president and professor of mechanical and aerospace engineering, stated that the combustion of ammonia can release harmful NOx and N.2Oh pollutants.
Most of N2O emissions from ammonia combustion are the result of interruptions in the combustion process. "NORTH2O is essentially an intermediate species in the combustion process,โ Mueller said. โIf the combustion process is allowed to finish, then there will essentially be no N2Or emissions.โ
However, Mueller said that under certain conditions, such as when a turbine ramps up or down or if hot combustion gases hit cold walls, the ammonia combustion process can be disrupted and N2O emissions can accumulate rapidly.
For example, the researchers found that if ammonia fuel achieves market penetration equivalent to about 5% of current global primary energy demand (which would require 1.6 billion metric tons of ammonia production, or ten times the current production levels), and if 1% of the nitrogen in that ammonia is lost as N2Or, then the combustion of ammonia could produce greenhouse gas emissions equivalent to 15% of current emissions from fossil fuels. The greenhouse gas intensity of such a loss rate would mean that burning ammonia fuel would be more polluting than coal.
Like N in ammonia2Or emissions, Robert Socolowprofessor of mechanical and aerospace engineering, emeritus and senior scholar at Princeton, said the widespread use of ammonia in the energy sector will add to all the other impacts fertilizers have already had on the global nitrogen cycle.
in a seminal article Published in 1999, Socolow analyzed the environmental impacts of widespread use of nitrogen-enriched fertilizers in the food system to promote crop growth, writing that โexcess fixed nitrogen, in various forms, increases the greenhouse effectโฆpollutes the drinking water, acidifies rainโฆ and stresses ecosystems.โ
As the energy sector looks toward ammonia as a fuel, Socolow said it can learn from the use of ammonia as a fertilizer in agriculture. He urged those working in the energy sector to consult the decades of work by ecologists and agricultural scientists to understand the role of excess nitrogen in disrupting natural systems.
"You can make ammonia, but not the way you want," said Socolow, whose 2004 paper with Esteban PacalaFrederick D. Petrie Professor Emeritus of Ecology and Evolutionary Biology, the stabilization wedges It has become the basis of modern climate policy. "It's important that we look before we leap."
A roadmap for a sustainable ammonia economy
While the environmental consequences of an ammonia economy gone wrong are serious, the researchers emphasized that the potential obstacles they identified can be resolved through proactive engineering.
"I interpret this document as a manual for engineers," Mueller said. "By identifying the worst-case scenario for an ammonia economy, we are really identifying what we need to consider as we develop, design and optimize new ammonia-based energy systems."
For example, Mueller said there are alternative combustion strategies that could help minimize unwanted NO.x and N.2Or emissions. While each strategy has its own pros and cons, he said taking the time now to evaluate candidate systems for emissions mitigation will ensure combustion systems are prepared to operate optimally on ammonia fuel.
Another option for accessing ammonia energy involves partially or fully splitting ammonia into hydrogen and atmospheric nitrogen through a process known as cracking. Ammonia cracking, a line of research that they are actively pursuing Emily Carter, could help make the fuel composition more favorable for combustion or even avoid the environmental concerns of burning ammonia by regenerating the hydrogen fuel at the point of use. Carter is the Gerhard R. Andlinger Professor of Energy and Environment and senior strategic advisor and associate laboratory director for applied materials and sustainability sciences at the Princeton Plasma Physics Laboratory (PPPL).
Furthermore, several industrial-scale technologies already exist to convert unwanted NO.x combustion emissions back to N2 through a process known as selective catalytic reduction. These technologies could be easy to transfer to ammonia-based fuel applications. And as a bonus, many of them rely on ammonia as a raw material to remove NO.x โ something that would already be in abundance in an ammonia-based system.
Beyond engineering practices that could be developed to minimize the environmental impacts of an ammonia economy, Porporato said future work will also look beyond engineering approaches to identify policies and regulatory strategies that would ensure the best-case scenario for the fuel. of ammonia.
"Imagine the problems we could have avoided if we had known the risks and environmental impacts of burning fossil fuels before the Industrial Revolution began," Porporato said. โWith the ammonia economy, we have the opportunity to learn from our carbon-emitting past. โWe have the opportunity to solve the challenges we have identified before they become a problem in the real world.โ
Paper, "Minimize the impacts of the ammonia economy on the nitrogen cycle and climateโ, was published on November 6 in PNAS. In addition to Porporato, Bertagni, Mueller, Socolow, and Carter, co-authors include J. Mark P. Martirez of the Princeton Plasma Physics Laboratory (PPPL); Chris Greig, Yiguang Ju, Sankaran Sundaresan, Mark Zondlo and Rui Wang of Princeton University; and Tim Lieuwen of the Georgia Institute of Technology. The research was supported by the U.S. Department of Energy, the National Science Foundation, the BP-funded Princeton University Carbon Mitigation Initiative, and the Moore Foundation.
