How to make greener ammonia with tribology?

A bit of context

Ammonia synthesis is estimated to generate ~1% of the world CO₂ emissions [2]. This single molecule is mass produced (~175 Mt) and the Haber-Bosch process employed is fossil fuel hungry! High pressure, high temperature and natural gas are required in this process. You may wonder who on earth needs to clean their house so often. Indeed ammonia is used as a cleaner by many households. But that’s not the heart of the challenge: ammonia is mainly used to produce fertilizers and approximately half of the world food production relies on ammonia based fertilizers.

To make things clear: any idea to produce ammonia with less CO₂ emission is highly welcome. Han et al. [3] propose a very interesting device which generates ammonia in ambient conditions. The process they describe is also self powered. It uses the N₂ storage pressure to generate high voltage current through triboelectric generators mounted on a Tesla turbine. Therefore, the prerequisite to ammonia production with this device is high pressure N₂ that can be extracted from the air.

Tesla turbine

Let’s break it down into its different components. First, there is the Tesla turbine (see Figure 2). This kind of turbine is based on the fluid flow boundary layer forming at the surface of the disc. In most fluid flows, the flow velocity at the surface is considered null and it increases if you measure the flow just above the surface to rapidly reach the velocity of the ambient flow. The layer in which the velocity gradient occurs is the boundary layer. Therefore, when a high velocity flow is applied on a surface, a high shear occurs. It is the shearing due to the high velocity N₂ that set the discs in motion.

Triboelectric nanogenerator

The Tesla turbine shaft is connected to the triboelectric nanogenerator (TENG) rotor (see Figure 3). This rotor is basically an FR4 PCB with precharged Kapton sectors. On its side, the stator has a copper circuit on it. The two electric terminals of the stator are linked to 4 copper sectors each. A large potential difference is created between the two terminals when the rotor is set in motion by the Tesla turbine: this creates triboelectricity. According to Han et al. [3], a 0.12 MPa pressure N₂ inflow allows the TENG for generating 2.5 mW at a high voltage.

Reactor

This high voltage is precisely what is needed by the second device of this process: the ammonia generator (see Figure 4). Han et al. connected the TENG to the ammonia generator: the turbine exiting N₂ is applied at the inlet of the tank whereas the terminals of the TENG are linked to the terminals of the discharge circuit via a diode bridge. The tank is partially filled with water. The two electrodes of the generator are a tantalum sheet and a steel needle. The tantalum sheet is immersed whereas the steel needle is separated from the water by the N₂ flow forming bubbles under the water level. As the high voltage is applied to the electrodes, N₂ corona discharges occur between the needle tip and the bubble surface. According to the authors N⁺ and N₂⁺ are generated by the discharges. These ions recombine with water to form NH₃, the molecule sought-after. To better understand the discharge generating conditions, the authors used COMSOL simulations.

Can it save the world?

While this process operates at the lab scale for now, it is a straightforward way to generate ammonia. The traditional way to generate it actually seems overly complicated compared to this process. Ambient temperature, ambient pressure, not fossil fuel involved as long as the high pressure N₂ is produced with renewable or nuclear energy. It just feels like an easy way to reduce by ~1% the world CO₂ emissions.

The authors did not mention any major concern as to why their device could not make it to industrial scale. There are surely obstacles to using Han’s et al. process at larger scales but nothing insurmontable is foreseen at this stage. A full life cycle assessment would be very intersting. Overall, it remains a very inspiring work that shows that widely used processes can also be replaced by newer, cleaner and more advanced ones. Some still need to be invented, some others seem ready to take over the world!

[1] Photo by Patrick Hendry on Unsplash

[2] https://cen.acs.org/environment/green-chemistry/Industrial-ammonia-production-emits-CO2/97/i24

[3] Han, K., Luo, J., Chen, J. et al. Self-powered ammonia synthesis under ambient conditions via N₂ discharge driven by Tesla turbine triboelectric nanogenerators. Microsyst Nanoeng 7, 7 (2021). https://doi.org/10.1038/s41378-020-00235-w