Ammonia (NH3) is a chemical compound made up of one nitrogen atom and three hydrogen atoms. Although it is a gas at room temperature, it is much more readily liquified than hydrogen, enabling large quantities of energy to be stored and transported. It can be directly used to produce electricity in some fuel cells, converted back into hydrogen, or burned in an internal combustion engine. These properties could make it a vital fuel for applications including shipping, aviation, grid energy storage and energy exports.
There are many established industrial uses for ammonia. Hundreds of millions of tons are produced annually, consuming about 2 percent of the world’s fossil fuel energy. About 80 percent of this ammonia is used in fertilizers, while 5 percent is used in explosives. Other uses include cleaning, refrigeration and the production of many nitrogenous compounds. Although ammonia is highly toxic, these well-established industries mean there are proven methods for its safe handling.
Challenges for Hydrogen Storage
Hydrogen is sometimes seen as the ideal medium to store and transport renewable or nuclear energy. Using electrolysis, electricity can be used to split water into oxygen and hydrogen. Large quantities of hydrogen can then be stored much more cheaply than storing the equivalent quantity of energy in a battery. When the energy is required, the hydrogen can either be used to produce electricity again using a fuel cell or burned to generate heat. In theory, the only emissions are water, although combustion in air may also result in nitrous oxides. One problem with this hydrogen economy is that more energy is lost when storing electricity as hydrogen than when batteries are used. This means that hydrogen is likely to be used when direct electrification is not possible rather than becoming the primary energy carrier.
Another reason that hydrogen isn’t more widely used is that it is very bulky, and when highly compressed the tanks are very heavy. Local storage of large quantities to buffer demand in the grid should not be an issue, as underground salt caverns can be used. These huge man-made caves, constructed by dissolving salt formations, are currently used to store natural gas. If the hydrogen needs to be transported, then it must be compressed, and this is where it starts to become problematic. Due to its extremely low density and boiling temperature, very high pressures are required to achieve even modest energy density. For road tankers, 200 bar is the standard transport pressure, but tanks are typically only emptied to 40 bar. This means that a standard tube-trailer truck that carries 3,000 kg of natural gas only delivers 2,400 kg. When transporting hydrogen, these trucks can deliver just 250 kg, and even when upgraded to 500 bar, they only deliver 1,000 kg. Liquefaction typically requires more than 30 percent of the hydrogen’s energy content. Liquefaction is also impractical for many applications as it requires cryogenic cooling and creates boil-off losses.
For small vehicles such as cars, hydrogen gas can be stored at very high pressure, typically 700 bar, in composite tanks. However, the need to contain such high pressures means that the promise of extended range and reduced weight is largely lost. For example, a passenger car requires just 5 kg of hydrogen for a typical 500 km range, yet the high-pressure tanks needed to contain it typically weigh around 90 kg. The manufacture of these composite tanks also creates significant greenhouse emissions. Producing the tanks for a Toyota Mirai produces 5.6 t CO2-Eq, or around 37 g/km. If ships and aircraft are to achieve a useful range, the bulk and mass of tanks becomes a major issue. Even when hydrogen is liquified, the fuel tanks require four times the volume of kerosine. When it comes to the type of large-scale energy trading currently utilizing oil tankers, hydrogen becomes completely uneconomic.
Ammonia Storage
The challenges with storing hydrogen are driving industry to look at ammonia as a more convenient storage medium. In its pure form, ammonia is a gas at room temperature. For some industrial uses, as well as domestic cleaning, ammonia may be dissolved into water, but this makes it unsuitable for use as a fuel. Pure ammonia can be liquified relatively easily, requiring just 10 bar pressure at room temperature, to give ammonia an energy density of 14 MJ/L. This is far easier to achieve than the 700 bar required just to compress hydrogen, and even cryogenically cooled liquid hydrogen only manages an energy density of 10 MJ/L. The specific energy of ammonia is 23 MJ/kg. Although the specific energy of pure hydrogen is 142 MJ/kg, this ignores the mass of the high-pressure tanks. When these are included, the figure for hydrogen falls to just 8 MJ/kg.
Safety Issues
Ammonia gas is highly toxic. In the quantities required for energy use, ammonia leaks present a significant danger in urban environments. For example, when a tanker carrying ammonia crashed in Houston in 1976, the fumes killed five people and injured 178. Such disasters are rare, and ammonia is regarded as safe for industrial applications such as shipping and grid energy storage. However, it is unlikely that ammonia will prove safe enough to be used in private vehicles and homes.
How Ammonia Is Produced
Most ammonia is currently produced using the Haber–Bosch process to fix nitrogen from the air by reacting it with hydrogen at temperatures of over 450 °C and pressures of up to 200 bar. This is a very energy- and carbon-intensive process. The hydrogen is typically extracted from natural gas, coal or oil, releasing CO2 in the process. However, the price of green hydrogen, produced by electrolysis using renewable energy, is falling rapidly and is expected to drop below the price of hydrogen from fossil fuels by 2030. Current plans for renewable ammonia production involve using green hydrogen, followed by the conventional Haber–Bosch process. Achieving a steady state process takes hours or even days, meaning that ammonia is only suitable for long-term storage.
Electrochemical synthesis is an alternative, cutting-edge process to produce renewable ammonia in a single reactor with inputs of water and nitrogen. Different electrolytes can produce the synthesis at temperatures between 20 °C and 800 °C. Although currently at an early stage, it is expected that this process will eventually be simpler while having a similar energy consumption to the electrolyzer and Haber–Bosch process.
Conversion Back to Hydrogen
A number of ammonia-cracking methods have been developed to extract hydrogen. The proton exchange membrane fuel cells (PEMFCs) used in electric vehicles require very high-purity hydrogen and they will be permanently damaged by small quantities of ammonia. It is therefore vital that cracking achieves near perfect conversion. Although nickel heterogeneous catalysts are well established, they require temperatures of 900 °C to achieve full conversion. Alternative catalysts such as ruthenium, sodium and lithium can achieve conversion at lower temperatures. However, subsequent scrubbing processes, such as bubbling through water, may also be required to remove any trace of ammonia.
Using ammonia to store electricity results in a round-trip energy efficiency similar to that of liquid hydrogen, approximately 30 percent less efficient than when hydrogen is stored at low pressure. Currently this is typically 11 to 19 percent, although it could be as high as 36 to 50 percent if waste heat is utilized for district heating.
Ammonia Fuel Cells
Two of the most established hydrogen fuel cell technologies are alkaline and PEMFCs. Alkaline fuel cells achieve higher efficiencies but must operate at a relatively steady output. PEMFCs are less efficient but can respond rapidly to changes in demand, making them better suited to use in vehicles and some grid load leveling applications. Neither of these technologies can directly utilize ammonia. The newer solid oxide fuel cells (SOFCs) can use either pure hydrogen or ammonia directly while operating at very high efficiency and low cost. However, SOFCs do not have the flexibility of PEMFCs and operate at high temperatures.
Direct Fuel Replacement in Internal Combustion Engines and Gas Turbines
Ammonia can also be burned in conventional internal combustion engines and gas turbines with only minor modification. These engines do, however, produce emissions. At low combustion inlet temperatures there are significant unburned ammonia emissions, as well as some nitrous oxides (NOx). As inlet temperatures increase, unburned ammonia emissions reduce but NOx emissions increase. For example, one study found that for a micro gas turbine, above 580 °C there was virtually no unburned ammonia emitted but the outlet gas contained around 1,000 ppm NOx.
Combustion efficiency can be improved by co-firing with another more readily combustible fuel. This has been demonstrated with ammonia co-fired with methane, hydrogen, coal and other more conventional fuels. Hydrogen co-firing is of particular interest since partial cracking of the ammonia can produce both fuel streams from a single fuel source. Research into low-NOx ammonia combustion is ongoing.
Applications
Ammonia is set to play a key role in the increasingly diverse decarbonized energy system. It is the leading technology for long distance transport and trade in energy. For example, Australia has a huge surplus of wind and solar energy to export from its relatively small and isolated population. Shipping ammonia is currently the only viable option for Australia to export large quantities of renewable energy around the world. Ammonia is also the frontrunner in attempts to decarbonize shipping. For countries that become dependent on energy imports, ammonia could be used in several ways, being first converted to hydrogen, or burned in engines to power vehicles or generate electricity.