Ammonia is currently the most widely produced chemical in the world, used primarily as a source of nitrogen fertilizer. Its production is also a major contributor to greenhouse gas emissions, the highest of the entire chemical industry.
Now, a team of MIT researchers has developed an innovative way to produce ammonia without the need for traditional fossil fuel-powered chemical plants, which require high temperatures and pressures. Instead, they’ve come up with a way to use the Earth itself as a geochemical reactor to produce ammonia underground. These processes take advantage of the Earth’s natural temperature and pressure, which are freely available and emission-free, as well as the reactivity of minerals present underground.
The method the team came up with was to pump water underground into an area of iron-rich bedrock. The water would contain a nitrogen source and metal catalyst particles that would react with the iron to produce clean hydrogen, which would then react with the nitrogen to form ammonia. A second well would then be used to pump the ammonia back to the surface.
The process, which has been demonstrated in the lab but not yet in the natural environment, was published today in the journal Joule . Co-authors of the paper are Iwnetim Abate and Ju Li, professors of materials science and engineering at MIT, postdoctoral researcher Yifan Gao, and five others at MIT.
“When we first produced ammonia from rocks in the lab, we were very excited,” Gao recalls. “We realized that this was a completely new and previously unreported approach to ammonia synthesis.”
The standard method for producing ammonia is called the Haber-Bosch process, which was developed in Germany in the early 20th century to replace natural nitrogen sources such as mined bat dung deposits that were gradually depleting. But the Haber-Bosch process is very energy intensive; it requires temperatures of 400 degrees Celsius and pressures of 200 atmospheres, and huge equipment to be effective. In some parts of the world, such as sub-Saharan Africa and Southeast Asia, few or no such plants are operating. This has resulted in fertilizer shortages and extremely high prices in these regions, limiting agricultural production.
The Haber-Bosch process is “good.” “It works,” Abate said. “Without it, we wouldn’t be able to feed 200 million of the world’s 8 billion people today,” he said, referring to the part of the world’s population that uses ammonia-based fertilizer to produce food. But given emissions and energy needs, a better process is needed, he said.
About 20 percent of greenhouse gas emissions from Haber-Bosch plants come from burning fuel to generate heat. Hydrogen production accounts for the remaining 80 percent. But ammonia, the moleculeNH3, is made up of only nitrogen and hydrogen. There’s no carbon in the recipe, so where do the carbon emissions come from? The standard way to produce the hydrogen we need is to steam methane, splitting the gas into pure hydrogen for use, and releasing the carbon dioxide into the air.
There are other ways to produce hydrogen with low or no emissions, such as using solar or wind power to split water into oxygen and hydrogen, but these methods can be costly. That’s why Abate and his team have been working on developing a system to produce what’s called geological hydrogen. In several places around the world, including Africa, hydrogen has been found to be produced naturally underground through a chemical reaction between water and iron-rich rocks. These natural hydrogen pockets could be harnessed in a similar way to natural methane reservoirs, but the extent and locations of such deposits are still largely unexplored.
Abate realised he could create or enhance this process by injecting water mixed with catalytic particles of copper and nickel to speed up the process into the earth, where iron-rich rocks already exist. “We can use the Earth as a factory to produce clean hydrogen,” he said.
He recalls being thinking about the emissions issues arising from producing hydrogen to make ammonia. “I had an ‘aha moment.’ So I thought, what if I combined this geological hydrogen production process with the Haber-Bosch ammonia production process?”
That would solve the biggest problem with underground hydrogen production: how to capture and store the gas after production. Hydrogen is a very small molecule (the smallest of all molecules) and is difficult to contain. But if the entire Haber-Bosch process were carried out underground, the only substance that would need to be brought above ground would be ammonia, which is easier to capture, store and transport.
The only additional ingredient needed to complete the process is a nitrogen source, such as nitrates or nitrogen gas, added to the mixture of water and catalyst injected underground. Hydrogen is then released from the water molecules as they interact with the iron-rich rocks, allowing it to instantly combine with nitrogen atoms also present in the water; the deep underground environment provides the high temperatures and pressures required for the Haber-Bosch process. Ammonia is pumped from a second well near the injection well and delivered to tanks above ground.
“We call it geochemical ammonia because we’re using the temperature, pressure, chemicals and rocks below the surface to directly produce ammonia,” Abate said.
While transporting hydrogen requires expensive equipment for cooling and liquefaction, and there are virtually no pipelines for transporting hydrogen (except near refineries), transporting ammonia is easier and cheaper. It costs about one-sixth the cost of transporting hydrogen, and there are already more than 5,000 miles of ammonia pipelines and 10,000 stations installed in the United States. Moreover, Abate explains that unlike hydrogen, ammonia already has a large commercial market, and production is expected to double or triple by 2050, as it is used not only as a fertilizer but also as a feedstock for various chemical processes.
For example, ammonia can be burned directly in gas turbines, engines and industrial furnaces, providing a carbon-free alternative to fossil fuels. It is used as an alternative fuel, as a space propellant, in marine and aviation transport.
Another advantage of geothermal ammonia is that it can use untreated wastewater, such as agricultural runoff, which tends to be high in nitrogen, as a source of water, treating it in the process. “You’re solving the wastewater treatment problem and at the same time creating something valuable out of this waste,” Abbate said.
The process “involves no direct carbon emissions and offers a potential pathway to reduce global carbon dioxide emissions by up to 1 percent,” Gao added. To get to this point, he said, the team “overcame many challenges and learned from many failed experiments. For example, we tested a wide range of conditions and catalysts before identifying the most effective one.”
The project has received seed funding under the Center for Electrification and Decarbonization of Industry, a flagship project of MIT’s Climate Grand Challenges program. “I don’t think there’s been any precedent for intentionally using the Earth as a chemical reactor, and that’s one of the key innovations of this approach,” said Professor Yetmin Chan, co-director of the center. Chan emphasizes that although this is a geological process, it happens very rapidly, not on geological timescales. “The reaction was essentially over within a few hours,” he said. “The reaction happens so quickly that it answers one of the key questions: ‘Do we have to wait until geological time?’ The answer is absolutely ‘no,'”
“The innovative thinking from this group is invaluable to MIT’s ability to create impact at scale,” said Professor Elsa Olivetti, mission director of MIT’s newly formed Climate Project. “Combining these exciting results with a deep understanding of the geology surrounding hydrogen deposits, for example, represents the Institute-wide efforts that Climate Project aims to support.”
“This is a major advance for the future of sustainable development,” said Jeffrey Ellis, a geologist at the U.S. Geological Survey who was not involved in the study. “Clearly more work is needed to validate this in pilot studies and scale it up to commercial scale, but this concept has proven to be truly innovative,” he added. The approach of using Fe2 +to design a system to optimize natural denitrification is innovative and could lead to further innovation in this direction.
Because the initial work on the process has been done in a lab, the next step will be to demonstrate the process using an actual underground facility. “We believe that such experiments can be carried out within the next year or two,” Abate said. This could pave the way for applying similar approaches to other chemical manufacturing processes, he added.
The team has filed a patent and aims to bring the process to market.
“Going forward, we will focus on optimizing the process conditions and scaling up the experiments, with the goal of achieving practical application to geological ammonia in the near future,” Gao said.
The research team also included MIT’s Ming Lai, Bachu Sravan Kumar, Hugh Smith, Sok Hee Han, and Lokesh Sanghavatula. Additional funding was provided by the National Science Foundation and made possible in part by use of the MIT.nano facility.
