How to make ammonium hydroxide
Ammonia - an ideal hydrogen storage medium
Author: Prof. Dr. Jochen Fricke, Energy Technology Cluster (as of October 2018)
The establishment of a global hydrogen economy has been sought for many decades. However, the hydrogen storage could not be solved satisfactorily so far. New developments show that ammonia, as a carbon-free synthetic hydrogen storage medium, is ideally suited as a green energy carrier. Couldn't the CO2-free combustion engine with ammonia as fuel be an interesting alternative to electric drive?
Of all fuels, hydrogen has the highest mass-specific energy density (calorific value) at 33.3 kWh / kg. However, the volume-specific energy density is very low at around three Wh / liter. Around 700 billion m³ of hydrogen are produced annually worldwide today. In steam reforming, methane is usually used as the hydrogen source, which at a pressure of 25 bar and a temperature of 900 ° C together with water vapor in H.2 and Co2 is implemented.
Conventional hydrogen storage
The conventional storage of hydrogen takes place in the liquid state at 20 K with a density of 71 kg / m³ and a volume-specific energy density of 2.4 kWh / l. Liquefaction costs around 30% of the calorific value. Hydrogen can also be stored in gaseous form at high pressures, e.g. at 700 bar in CFRP pressure bottles, 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 approx. 12% of the hydrogen calorific value.
Advances in hydrogen storage
With the progress of the energy transition and the expansion of fluctuating regenerative power sources such as wind energy and photovoltaics, 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. In most cases, the electrolysis of water is used. To produce 1 m³ of hydrogen at normal pressure, electrical energy of 4.3–4.9 kWh is required - this can be compared with the hydrogen calorific value of 3 kWh / m³, ie about a third of the electrical energy is used in this process lost.
Much development work has been put into hydrogen storage with metal hydrides over the decades. Disadvantages of the hydride storage system are the low hydrogen / metal ratio and the relatively slow absorption and release of hydrogen. The safety of the bound hydrogen is advantageous. So far, only the nickel-metal hydride battery has found widespread use.
Today's favorites for hydrogen storage are LOHCs (Liquid Organic Hydrogen Carriers), primarily dibenzyltoluene (DBT), an inexpensive, non-toxic, hardly inflammable heat transfer oil known as Marlotherm. It consists of three benzene rings and when hydrogenated using a ruthenium catalyst at temperatures of around 200 ° C and pressures of> 5 bar, it absorbs gaseous hydrogen. The double bonds in the benzene rings are broken and allow the addition of up to 18 hydrogen atoms per DBT molecule. One then speaks of PerOxyDBT. Around 600 liters of gaseous hydrogen can be stored in one liter of DBT; this corresponds to a storage density of around 2 kWh / kg or 2 kWh / liter DBT. The H accumulation releases heat of 0.6 kWh / kg DBT. Dehydration, i.e. the release of hydrogen, takes place by supplying heat at temperatures of around 300 ° C and reduced pressure. Several research institutes and industrial companies are researching DBT memories and their application. Hydrogenious Technologies GmbH put the first DBT storage system into operation in January 2016 in Erlangen.
Ammonia as a hydrogen storage medium
Around 200 million tons of ammonia (NH3) are now produced annually worldwide and about 3/4 are used for fertilizer production. The energy consumption for ammonia production corresponds to about 2% of world energy production. In the most frequently used manufacturing process, the Haber-Bosch process, the gases nitrogen and hydrogen react with one another at around 200 bar and 450 ° C over an iron catalyst according to: N2 + 3H2 → 2NH3
The nitrogen is obtained by liquefying the air and the hydrogen by steam reforming from natural gas or coal. The gaseous reaction product NH3 is liquefied either by cooling or by absorption in water. Ammonia can also be produced in a fuel cell: In this process, water is split into oxygen, H + ions and electrons on the anode coated with a catalyst. The protons diffuse through an electrolyte and a membrane to the cathode. The electrons reach this through a wire. At the cathode, nitrogen molecules are split into N atoms by means of a catalyst, which then together with the protons and electrons to form NH3 can react.
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 at 20 ° C. Ammonia is poisonous, but people can smell ammonia even in the smallest, harmless concentrations. When it is burned, only nitrogen and water are produced. About 0.6 kg of methane or around 30 MJ ≈ 8.3 kWh are required to produce 1 kg of ammonia. The calorific value of ammonia is 5.2 kWh / kg. This corresponds to a production efficiency of 63%. It can be said that the calorific value of NH3 is 2.6 times higher than that of PerOxyDBT - but only about half as high as that of gasoline or diesel and about a sixth as large as that of liquid hydrogen.
Ammonia was used as energy storage and fuel as early as 1872. At that time the trams in New Orleans ran on this energy source. Belgian buses ran on ammonia during World War II. In 1981 a Chevrolet Impala ran on ammonia in the USA.
Today there are worldwide activities to establish ammonia as a green fuel. Siemens is managing the 300 kW NH3Composite project at the Rutherford Appleton Laboratory in Oxfordshire / England. The practical aspects of operating a demo NH3-Energiesystem investigated, but also determined the economic conditions for a green ammonia economy. There are several NH in Australia3-Projects: For example, ammonia is to be generated within the international 10 billion US dollar 9 GW wind + PV project "Asian Renewable Energy Hub". Yara, the world's largest ammonia producer, is planning its CO2- to convert heavy ammonia production to renewable energies and thus reduce its CO2- Reduce emissions by 50%. The state of South Australia is building an ammonia factory that will use electricity from wind and PV systems to produce fertilizer and liquid ammonia from 2020. The liquid ammonia can be burned in a turbine or in an NH3-Fuel cell can be converted into electrical energy to stabilize the grid.
Many other R&D projects on generation (“Beyond Haber-Bosch”), combustion and conversion into electrical energy will be presented at the Pittsburgh Ammonia Conference.
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