Ammonia - an ideal hydrogen storage medium

Author: Prof. Dr. Jochen Fricke, Energy Technology Cluster (As of: October 2018)

For many decades, the establishment of a global hydrogen economy has been strived for. However, so far  hydrogen storage could not be solved satisfactorily. New developments show that ammonia as a carbon-free synthetic hydrogen storage is ideally suited as a green energy carrier. Could not the CO2-free internal combustion engine with ammonia as fuel become an interesting alternative to the electric drive?

Hydrogen has the highest mass-specific energy density (calorific value) of all fuels with 33.3 kWh/kg. However, the volume-specific energy density is very low at about three Wh/liter. About 700 billion m³ of hydrogen are produced worldwide each year today. In steam reforming, the hydrogen source is usually methane, which is converted into H ₂ and CO ₂ at a pressure of 25 bar and a temperature of 900 °C.

Conventional hydrogen storage

Conventional storage of hydrogen is in the liquid state at 20 K with a density of 71 kg/m³ and a volume-specific energy density of 2.4 kWh/liter. Liquefaction costs about 30% of the calorific value. In gaseous form, hydrogen can also be stored at high pressures, for example at 700 bar in  CFRP pressure cylinders, there at densities of 40 kg/m³ and volume-specific energy densities of about 1.35 kWh/liter. The energy required for compression to 700 bar is about 12% of the hydrogen calorific value.

Advances in hydrogen storage

As the energy transition progresses and fluctuating renewable power sources such as wind energy and photovoltaics are expanded, hydrogen production for storage purposes will increase dramatically. Power-to-gas, or P2G for short, is the technology to be expanded into the GW power range here. This mostly involves the electrolysis of water. To produce 1 m³ of hydrogen at normal pressure, an electrical energy of 4.3-4.9 kWh is required - this can be compared with the hydrogen heating value of 3 kWh/m³, i.e. about one third of the electrical energy is lost in this process. A lot of development work has been put into hydrogen storage with metal hydrides over decades. Disadvantages of hydride storage are the low hydrogen/metal ratio  and the relatively slow absorption and release of the hydrogen. An advantage is the safety of the bonded hydrogen. So far, only the nickel-metal hydride battery has found widespread use.

Favorites for hydrogen storage today are LOHCs (Liquid Organic Hydrogen Carriers), primarily dibenzyltoluene (DBT), a low-cost, non-toxic, low-flammability heat transfer oil known as Marlotherm. It consists of three benzene rings and absorbs gaseous hydrogen during hydrogenation using a ruthenium catalyst at temperatures of about 200 °C  and pressures of > 5 bar. In this process, the double bonds in the benzene rings are broken, allowing the addition of up to 18 hydrogen atoms per DBT molecule. This is referred to as PerOxyDBT. About 600 liters of gaseous hydrogen can be stored in one liter of DBT; this corresponds to a storage density of about 2 kWh/kg or 2 kWh/liter of DBT. H accumulation releases heat amounting to 0.6 kWh/kg DBT. Dehydrogenation, i.e. hydrogen release, occurs by applying heat at temperatures of about 300 °C and reduced pressure. Several research institutes and industrial companies are conducting research on DBT storage and its application. The company Hydrogenious Technologies GmbH commissioned the first DBT storage facility in Erlangen in January 2016.

Ammonia as hydrogen storage

Approximately 200 million tons  of ammonia (NH ₃ ) are produced worldwide each year today, about 3/4 of which is used for fertilizer production. The energy input for ammonia production corresponds to about 2% of world energy production. In the most commonly used production process, the Haber-Bosch process, the gases nitrogen and hydrogen react with each other at about 200 bar and 450 °C on an iron catalyst according to: N ₂ +3H ₂ → 2NH ₃ The nitrogen is obtained via air liquefaction and the hydrogen via steam reforming of natural gas or coal. The gaseous reaction product NH3 is liquefied either by cooling or absorption in water. Ammonia can also be produced in a fuel cell: In this process, water is split into oxygen, H+ ions and electrons at the anode, which is coated with a catalyst. The protons diffuse through an electrolyte and a membrane to the cathode. The electrons reach it through a wire. At the cathode, nitrogen molecules are split into N atoms by catalyst, which can then react with the protons and electrons to form NH ₃ . Ammonia is gaseous under normal conditions and has a density of 0.73 kg/m³. At - 33oC it is liquid and has a density of 0.68 kg/l. Under 9 bar pressure, it can be liquefied already at 20 °C. Ammonia is toxic, but people can smell ammonia even at the lowest, harmless concentrations. Its combustion produces only nitrogen and water. The production of 1 kg of ammonia requires about 0.6 kg of methane or about 30 MJ ≈ 8.3 kWh. The heating value of ammonia is 5.2 kWh/kg. This corresponds to an efficiency for production of 63%. It should be noted: the heating value of NH3 is thus 2.6 times higher than that of PerOxyDBT - but only about half that of gasoline or diesel and about one-sixth that of liquid hydrogen.

(Editor's note: Energy is required for the conversion to ammonia and corresponding production losses must be taken into account).

Long history

Ammonia was used as an energy storage and fuel as early as 1872. At that time, streetcars in New Orleans ran on this energy source. In World War 2, Belgian buses ran on ammonia. In 1981, a Chevrolet Impala in the U.S. ran on ammonia. Today, there is worldwide activity to establish ammonia as a green fuel. For example, Siemens is leading the 300 kW NH ₃ composite project at the Rutherford Appleton Laboratory in  Oxfordshire, England. This will investigate the practical aspects of operating a demo NH ₃ energy system, but also determine the economic conditions for a green ammonia economy. There are several NH ₃ projects underway in Australia: For example, ammonia will also be generated within the international $10 billion 9 GW wind+PV Asian Renewable Energy Hub project. Yara, the world's largest ammonia producer, plans to convert its CO ₂ -heavy ammonia production to renewables, reducing its CO ₂ emissions by 50%. The state of South Australia is building an ammonia plant that will use electricity from wind and PV systems to produce fertilizer and liquid ammonia starting in 2020. The liquid ammonia can be burned in a turbine or converted to electrical energy in an NH ₃ fuel cell for grid stabilization. Many other R & D projects for generation ("Beyond Haber-Bosch"), combustion and conversion to electrical energy will be featured at the Pittsburgh Ammonia Conference.

Sources

[1] https://www.the-linde-group.com/de/images/HydrogenBrochure_DE_tcm16-10196.pdf [2] G. Do et al. React. Chem. Eng. 2016(1)313, Hydrogenation of .... Dibenzyltoluene Ammonia, Wang_et_al-2017-AIChE_Journal.pdf [3] https://royalsociety.org/~/media/events/2017/10/decarbonising-uk-energy/Rene%20Benares-Alcantara%20presentation.pdf [4] https://ammoniaindustry.com/ammonia-for-energy-storage-economic-and-technical-analysis/ [5] Haber Bosch Process BASF Video: https://www.solarify.eu/2017/09/10/ammoniak-als-energiespeicher/ [6] https://www.solarify.eu/2017/09/10/ammoniak-als-energiespeicher/ [7] https://nh3fuelassociation.org/