Imitating Nature to Produce Ammonia
Ed Brown | February 09, 2017Nearly 500 million tons of ammonia are produced annually using the Haber-Bosch process. Ammonia plays a major role in the worldwide production of fertilizers and consumes about 1% of the world’s energy for its production. One problem is that high pressure and temperature are needed to drive the chemical reaction.
Ammonia consists of one nitrogen atom and three hydrogen atoms. The process requires breaking apart the two nitrogen atoms bonded together in nitrogen gas. Then the nitrogen must be "reduced" by adding electrons and protons to it in the form of hydrogen. In the Haber-Bosch process, hydrogen and nitrogen gases are pumped over beds of metal catalysts at pressures up to 250 times atmosphere and temperatures up to 932° F.
In biology, conversion of gaseous nitrogen to ammonia is called nitrogen "fixation." This can be done with enzymes called nitrogenases, which are used by some bacteria.
University of Utah scientists built a fuel cell system that replicates the biological process of nitrogen fixation at room temperature, by using nitrogenase and hydrogenase.
The cell consists of two compartments, connected via carbon paper electrodes. In one vial, hydrogen gas is oxidized by hydrogenase and electrons are carried to the anode. In the other, electrons come off the cathode and are combined with nitrogen, via nitrogenase, to create ammonia.
During the process the electrons move from the anode to the cathode through an electrical circuit, while protons (oxidized hydrogen atoms) travel through a membrane between the two chambers, supplying the hydrogen atoms need to synthesize the ammonia. The movement of the electrons forms an electrical current, which is the source of the small amount of electrical power generated by the reaction.
Before the process can be scaled for industrial production several challenges need to be overcome.
One is that nitrogenase not commercially available and must be handled in an oxygen-free environment. Another problem is the use of adenosine triphosphate (ATP), a source of energy in cells and in nitrogen fixation, which is chemically expensive.
Therefore, the next step toward commercializing the process is to reengineer the reaction to circumvent the need for ATP, says Ross Milton, one of the researchers. The most important aspect of the work is the production of ammonia without the massive energy drain required by the industry-standard process, says Milton.
"The real thing is not the quantity of ammonia produced, but that it's possible to make electricity at the same time," he says.
I have solar but instead of batteries I would like to store energy as ammonia. Is there an economical solution for doing this. Thanks
The ultimate goal of this research should be to produce ammonia on a massive scale as a fuel for combustion engines: the energy content per weight of liquid ammonia (@ 20°C and 10 bar pressure) is roughly half of that of gasoline. Handling this fuel would be comparable to handling LPG.
Since the fuel does not have any carbonatoms, exhaust gases (after passing over an exhaust catalyst) would consist of nitrogen gas, oxygen gas and water vapour which would radically solve the CO2-climate change issue. Converting an engine to use NH3 as a fuel should be fairly easy, so there is no need to replace over 800 million combustion engines worldwide. They should only be converted to the new fuel. Another advantage is that this fuel can be produced anywhere in the world.
In short: not only is agriculture waiting for this but so too is the transport sector.