Green Ammonia Better than Hydrogen: Does NH3 Beat the Miracle Substance of Defossilization?
Climate-neutral transportation, chemicals, heating and basic industries – hydrogen is supposed to do the trick. But the light gas is making it difficult for plant manufacturers. Can material storage offer an alternative and save the energy transition?
Hydrogen, they say, is a promise: a hope that not everything has to change despite the energy transition. That there can still be gas heating, filling stations and cement in the future, and that we can continue to use existing infrastructures.
Moreover, according to almost all studies on defossilization, hydrogen is needed.
If we want to produce emission-free crude steel, polymers and building materials in the future, there is hardly any way around the lightest of all gases.
But it is also a fantastic substance: Produced in an emission-neutral way, hydrogen burns without affecting the climate. Rich in energy and eager to bind, it supplements waste gases such as carbon monoxide or dioxide to form valuable synthesis gas. Since it is already present in the gas mix, hydrogen can be added to the natural gas network and thus reduce the CO2 footprint of heating without much effort. Incidentally, hydrogen is also expected to solve one of the most urgent problems of the energy transition: the lack of base load security.
Could wind and solar power be stored as hydrogen?
Gas is an ‘old acquaintance’ for the process industry, and gas storage and pipelines are proven concepts. So, electrolysis by green power plus gas cavern makes electricity storage for the energy transition? Unfortunately, it's not that simple. Of all things, hydrogen doesn't make it easy for us when it comes to transport and storage – the apparent parade disciplines.
The molecule diffuses through or into materials, causing leaks or embrittlement. About 20 % could be fed into the natural gas grid, but a pure H2 economy would require new pipes, fittings and seals. And while the mass-specific energy density (heating value) of hydrogen is a phenomenal 33.3 kWh/kg, the volume-specific energy density is very low at three Wh/l. The core problem of the energy transition: Where to put surplus green electricity?
The gas is stored in liquid form at -253 °C, which increases the density to 71 kg/m³. In this energy-intensive process, up to 30 % of the theoretically usable calorific value is lost. Of course, storage under pressure, for example in CFRP cylinders at 700 bar, would also be conceivable – but part of the energy (around 12 %) also falls by the wayside during compression.
Power-to-gas apologists therefore have high hopes for alternative storage solutions such as metal hydride storage, in which gas is dissolved in metals or alloys under pressure. However, gas uptake and release remain slow, and large quantities of metal are needed to store small volumes of gas.
This leaves liquid organic hydrogen carriers or LOHC (Liquid Organic Hydrogen Carriers), i.e. organic compounds that can absorb and release hydrogen. In principle, almost any unsaturated carbon double or triple bond can be used for this purpose, but in practice the necessary temperatures limit the usability of hydrocarbons. For example, dibenzyltoluene (DBT, known as heat transfer oil under the trade name Marlotherm) absorbs gaseous hydrogen at around 200 °C and 5 bar overpressure using a ruthenium catalyst. About 600 liters of hydrogen gas can be stored in one liter of DBT.
Ammonia from electrolysis hydrogen: electricity storage of the future?
But perhaps there is a simpler way: ammonia (NH3) is one of the oldest ‘bulk chemicals’ and is produced on a scale of hundreds of millions of tons worldwide for the production of fertilizers. The Haber-Bosch process, still the benchmark in ammonia synthesis today, has nitrogen and hydrogen react on an iron catalyst, with the necessary H2 usually obtained by steam reforming from natural gas or coal.
What makes ammonia interesting as an energy storage medium is its ease of transport and storage, with which the industry also has decades of experience. The colorless gas has not played a major role in climate protection to date, although ammonia synthesis accounts for around 3 % of global CO2 emissions.
First ammonia projects pick up speed
That could easily change: If the H2 molecules were obtained by electrolysis rather than from fossil hydrocarbons, ammonia could become ‘green.’ For example, the U.S. gas giant Air Products, together with Saudi Arabia's ACWA Power, plans to invest five billion dollars in the production of climate-neutral ammonia by 2025.
Such projects could defossilize fertilizer production and provide an alternative storage route for renewable energy: Ammonia can be burned in modified gas and steam turbines or internal combustion engines and could become an alternative to fuel oil and natural gas for gas-fired power plants or marine propulsion.
Alternatively, the gas can also be ‘reconverted’ into electricity in fuel cells. The Norwegian shipping company Eidesvik, for example, plans to equip the supply ship Viking Energy with a 2-MW ammonia fuel cell.
From Jutland to Australia: These are the largest NH3 projects
Currently, the largest European project for green ammonia is being built in Esbjerg, Denmark, where wind power will be used to produce ammonia for ship propulsion and fertilizer. Australia is thinking even bigger: there, the 10-billion-dollar Asian Renewable Energy Hub, a wind power and photovoltaic plant with a total capacity of 9 GW, is to be supplemented by an ammonia production plant.
In principle, the hydrogen could also be recovered: The Center for Fuel Cell Technology (ZBT), for example, is working on an ‘ammonia cracker’ together with the Chair of Energy Technology at the University of Duisburg-Essen in Germany. In this process, liquid NH3 is cracked into a gas mixture with a high hydrogen content in a pressurized reactor at around 700 to 800 °C. The ammonia is then used as a fuel.
The hydrogen is then used in a fuel cell to generate electricity. The highlight of the process is that the burner that brings the ammonia reactor up to temperature is operated with residual gas from the fuel cell. According to the developers, the process thus achieves an efficiency of over 90 %.
All ammonia? What methanol and co. could do
A key aspect that makes the combination of water electrolysis and ammonia synthesis so attractive as an ‘electricity storage system’ is its good load controllability: Thyssenkrupp's alkaline water electrolysis (AWE) technology, for example, can ramp up to full load within minutes and compensate for load fluctuations within seconds, the plant manufacturers explain. In this way, production adapts to fluctuating power generation from photovoltaics and wind energy.
Of course, there would also be alternatives to ammonia: The Fraunhofer Institute for Solar Energy Systems ISE, among others, is working on the production of methanol from hydrogen and CO2. The process not only produces one of the most common basic chemicals in this way, but can also use carbon dioxide from exhaust gas streams.
The researchers are not alone in this: the work is part of the ‘Power-to-Methanol’ project, which is funded by the German Federal Ministry for Economic Affairs and Energy and led by Dechema. Industrial partners on board include Crop Energies (Südzucker Group, Clariant and Thyssenkrupp Industrial Solutions).
But methanol is not without its pitfalls: The CO2 content in the synthesis gas, for example, ages catalysts. The process is also dependent on the availability of carbon monoxide and carbon dioxide – ‘raw materials’ that, strange as it sounds, could become a rare commodity in the course of defossilization.