Could ammonia, what most of us think of as a typical household cleaning agent, be our new sustainable source of carbon-free fuel?
a NEMS iNsight by Özlem Yetkinoglu Sobhi
Ammonia is a long-known substance and is mentioned in scripts as early as the 13th century by alchemists. It is common in nature both in mother earth and in the outer planets. It occurs in small quantities in nature, being produced from nitrogenous animal and vegetable matter.
Gaseous ammonia was first isolated in the late 1700. However, modern ammonia production started at the beginning of 1900. Today ammonia is one of the most widely produced inorganic chemicals and in 2016 globally 175 million tonnes of ammonia were produced. It is mainly used in agriculture as fertilizer. It is also used in cleaning products and as a refrigerant in industrial coolers.
Pure ammonia is gas in standard temperature and pressure. It is pressurized to liquid form for transport and storage. Due to its chemical properties, it can be easily liquified and is transported at moderate pressure. Today, it is safely and efficiently transported globally via road trailers, railroads, ships, river barges and pipelines to serve the needs of mainly the fertilizer industries and other industries. It can be stored in an inexpensive and lightweight container over a long time.
Ammonia consists of 3 Hydrogen atoms, 1 Nitrogen atom, and has the chemical formula NH3. This means that it is carbon-free, thus unlike fossil fuels, no CO2 emissions are formed when combusted.
Ammonia properties such as being a carbon-free, widely produced, easy to store and transport has been attracting the attention of many who tries to solve the global climate crises. The research has been going on for decades, but the rate has accelerated in recent years.
The possibilities to convert ammonia into useful energy are many. Ammonia can be used directly as a fuel in internal combustion engines and can be used in fuel cells either as direct fuel or indirectly. This means that it can fuel cars, trains, ships, and industrial engines. It can also be used in gas turbines. Due to its hydrogen content, it is also a good candidate as a Hydrogen carrier. 1 liter of ammonia contains more hydrogen than 1 liter of liquid hydrogen. Compared to hydrogen, ammonia is also way easier to transport, and store and liquid ammonia has higher energy density than hydrogen. Hydrogen is an excellent fuel as only water is formed after combustion, but transport and onboard storage are difficult. The idea is to extract hydrogen from ammonia when it is going to be used at the destination.
The shipping industry being more aware of its carbon footprint today, is also beginning to turn eyes on ammonia as an alternative fuel to reduce their carbon impact. We see that investments are being made to utilize ammonia both in internal combustion engines and in fuel cells. We see such developments in the Norwegian offshore oil and gas industry also.
Ammonia (NH3) does not contain any carbon atoms but this does not necessarily mean that it is carbon-free. How ammonia is produced is detrimental in its carbon print. However, the commercial production of ammonia today is not green. Today a century-old recipe is used to make ammonia; a process called Haber-Bosch. Hydrogen and Nitrogen are the two elements required. Air is used as the source for Nitrogen and Hydrogen is stripped from natural gas or coal. Mainly natural gas (methane) is used as a source and in simple terms, hydrogen is separated from carbon atoms in high temperature via a process called steam reforming. CO2 is produced as a result of the reaction and as a result of heat generated for the steam. Steam reforming is the cheapest way to produce hydrogen. In addition, Haber-Bosch is a high-pressure and temperature process and generating high pressure and temperatures requires energy. It is estimated that 1% of global CO2 emissions come from ammonia production.
Ammonia can also be produced in a greener way. Reverse fuel cells combined with renewable power and hydrogen extracted from a non-CO2 emitting source and processes are an alternative. Hydrogen is produced from water with fuel cells which do not generate any CO2 emissions. Reverse fuel cells are slower compared to the Haber-Bosch process and therefore pilot plans still prefer to use Haber-Bosch to combine Nitrogen and Hydrogen, but with a greener hydrogen and a renewable power supply. Further techniques are also being studied to combine nitrogen and hydrogen to form ammonia in a greener way.
There are also challenges with the combustion of ammonia which is classified as a non-flammable gas. It has a very high ignition temperature. This high temperature combined with its nitrogen content leads to NOx emissions which is a toxic gas and subject to strict regulations. Recent developments in Japan point to catalytic combustion to overcome these issues. Copper oxide catalysts supported on aluminum silicates and silicon oxides have successively achieved low NOx combustion. If ammonia is combusted in the presence of this catalyst, it was observed that NH3 remains highly active in selective N2 production, suppressing NOx formation. This catalyst is also very attractive as it can be produced easily and at low cost. Other technical solutions are also studied to solve the issues related to NH3 combustion, such as natural gas dual fuel gas turbines where ammonia is mixed with natural gas to reduce the ignition temperature.
The combustion problem can be avoided if ammonia is used in fuel cells. Ammonia can be directly used in fuel cells by using Solid Oxygen Fuel Cells (SOFC). These fuel cells operate at high temperatures and are the most promising candidate among all fuel cells. They are not at a stage for commercialization yet, but major developments are underway. Alkaline and Alkaline Membrane Direct Ammonia Fuel Cells also use ammonia directly, however, challenges remain in these types of fuel cells as well. Proton Exchange Membrane (PEM) fuel cells are also tested with catalysts and alternative membranes for the purpose of using ammonia directly. Proton Exchange Membrane (PEM) fuel cells where the ammonia is fully separated, and the hydrogen is then used to generate electric power is commercially available and is used in marine vessels.
Another property of ammonia is that it is toxic and highly corrosive and therefore requires special handling. However, the agriculture industry has long experience with handling ammonia which can be transferred to the fuel industry.
The Future of Ammonia
Technical challenges exist for using ammonia as a fuel. Moreover, modern ammonia production has a noteworthy carbon footprint. Recent developments in research and development show an acceleration in the direction for solving these issues. We are maybe not there today but technical changes can be achieved with R&D. We already see reassuring results. Ammonia does absolutely stand out as a promising alternative fuel and hydrogen carrier and can be one of the players to solve today’s climate crisis. It should not be seen as a competitor to hydrogen or other alternative fuels but as a part of the solution. We need to consider all possible solutions to solve our global issue. For example, electric cars might work very well in Norway due to the renewable national electricity grid, however, they might not work as good if one plugs her car in Poland where electricity comes mainly from coal. In such area’s hydrogen cars (using green hydrogen) can be a greener option and the high cost of transporting hydrogen can be solved by transporting it in the form of ammonia.
It is also important to develop alternative fuel sources by considering their global effect in other industries. Today, the main application of ammonia is in the agriculture industry as a fertilizer due to its Nitrogen content. Nitrogen is an essential nutrient for vegetation as it is the main building block of proteins. Today 80% of the ammonia produced in the industry is used as a fertilizer. Commercial production of ammonia has allowed a significant increase in food production for the growing world population and it is important that developments in the fuel industry do not threaten world food security.