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Ammonia produces just nitrogen and water vapour when burned and, unlike hydrogen, it is relatively easy to store in liquid form. That means transporting ammonia will not require costly new infrastructure, says John Fleming of SilverEagles Energy in Lubbock, Texas.
Fleming and Tim Maxwell at Texas Tech University, also in Lubbock, are developing a system to produce ammonia that can be installed in filling stations. Powered by mains electricity, it first produces hydrogen from water using electrolysis, then combines it with nitrogen from the air to produce ammonia.
To achieve this, the researchers have adapted the Haber-Bosch process used to make ammonia industrially. Their version works on a small scale and can make ammonia fairly cheaply.
In their system, a piston rapidly compresses hydrogen and nitrogen, heating the gases to 400 °C. The mixture is fed into a chamber containing an iron oxide catalyst, which sparks a reaction that further heats the gases and generates ammonia. In a third chamber, the mixture decompresses and cools down to room temperature. As it does so, it pushes against another piston, from which mechanical energy is recovered and fed back to the compressor, significantly cutting the process’s power consumption.
Finally, a heat pump cools the mixture down to around -75 °C, liquefying the ammonia for collection.
The team say the whole system could fit within a standard container and could therefore be transported by truck for installation at filling stations, where it could make between 4000 and 40,000 litres of ammonia per day. Maxwell adds that the system has a modular design, so it can easily be scaled up to produce more. The ammonia could be made for just 20 cents per litre, they claim.