Ammonia has been manufactured for the past century using the Haber-Bosch process into which are fed nitrogen and hydrogen gas streams. At the right temperature and pressure, and in the presence of a catalyst, ammonia is produced. Traditionally, Haber-Bosch processes are fueled by hydrocarbons and release a lot of carbon dioxide. Blue ammonia is defined as ammonia produced by this traditional process however with the addition of CCS (Carbon Capture and Storage) to sequester all of the carbon dioxide generated rather than allowing it to enter the atmosphere. Achieving effective and permanent CCS will be very challenging due to the technical issues, energy consumption and high cost. There are some proposals to “sequester” the CO2 into existing oil wells and in doing so, achieve enhanced oil recovery (EOR) however this would seem to be counterproductive from an environmental point of view as it releases more hydrocarbons to be burnt in the future. Therefore blue ammonia production will probably be an interim solution only, until green ammonia production can be brought on line.
Green ammonia can be manufactured in the Haber-Bosch process using renewable energy rather than hydrocarbons. This will result in no CO2 emissions. Renewable electrical energy is used to separate hydrogen from water with electrolysers and separate nitrogen from air with air separation units. The bulk of the energy input is used to generate the hydrogen whilst a minor part is used for the air separation and Haber-Bosch processes. There are a number of pilot green ammonia plants operating in several countries.
It is likely that any new large scale green ammonia production plants will operate on the Haber-Bosch system. These systems operate most efficiently and produce lowest cost ammonia when they are continuously supplied with renewable electrical energy. Therefore the best forms of continuous renewable energy for green ammonia production are likely to be hydroelectricity, geothermal electricity or similar. Manufacturing ammonia from intermittent forms of renewable electrical energy (solar, wind, wave, tidal etc.) will be viable, but less efficient. It may be possible to use a combination of two or more of these intermittent sources to provide a virtually continuous supply and thereby improve the efficiency.
Green ammonia is the most likely contender to fulfil future global non-nuclear, carbon-free energy requirements in areas including:
– A vector to transport hydrogen over long distances
– An internationally traded energy commodity
– Grid scale energy storage
In the next instalment we will discuss ammonia as an internationally traded energy commodity.