Hundred years ago, ammonia came to humanity’s rescue as a fertiliser and averted a major food crisis. Now it is poised to return in a new avatar as fuel and bail us out of the looming disaster due to climate change. Green Ammonia produced using renewable energy holds much promise to decarbonise the world economy in the coming decades.
The Haber-Bosch process for the synthesis of ammonia is widely regarded as the most important invention of the 20thCentury. As the Germans put it dramatically, ammonia made it possible to make “bread from air”. But, for ammonia-based fertilisers, the explosive growth of population in the last century would not have been possible. Now as humanity is facing yet another gargantuan crisis of climate change, ammonia is emerging as the saviour yet again in a new avatar.
Green Ammonia
Currently, almost all the ammonia in the world is derived from fossil fuels. Ammonia manufacture emits more carbon dioxide than any other process in the chemical industry. When green hydrogen, produced by electrolysis of water using renewable energy, is used to synthesise ammonia, the latter also acquires the “green” label. Both hydrogen and ammonia can be used as fuels without emitting carbon dioxide and thus hold the key to a decarbonised world.
Energy Carrier
As a carrier of energy, ammonia has several advantages over hydrogen. Whereas liquid hydrogen has to be stored at a very energy intensive minus 253 degrees C, liquid ammonia needs only minus 33 degrees C. Ammonia is much less flammable than hydrogen. Liquid ammonia has higher volumetric energy density at 3.75 kWh/L compared to 2 kWh/L for liquid hydrogen. Unlike hydrogen, ammonia has been widely transported for decades by ships, rail cars, road tankers and pipelines. It is the second most widely produced commodity chemical in the world. The infrastructure for distributing ammonia is already robust and is very well understood. Several ports across the world have terminals that are fully capable of despatching and receiving liquid ammonia. The energy from ammonia can be tapped in two ways – direct combustion or through a fuel cell.
Direct Combustion
The earliest idea of using ammonia as a fuel in an internal combustion engine appear in patents as far back as 1905, around the same time when the Haber-Bosch process was being developed. The diesel shortage during World War II resulted in the first practical use of ammonia as a motor fuel by buses in Belgium in 1943. Interest in using ammonia as a direct fuel has revived in recent years and considerable research has been carried out in optimising the performance of ammonia-fuelled engines. Ammonia’s poor combustion characteristics are a major stumbling block in its widespread deployment as an engine fuel. These include high auto-ignition temperature, low flame speed, narrow flammability limits and high heat of vapourisation. Compression ratios of 35:1 and more are required for successful use of ammonia in a compression-ignition engine. To lower the compression ratios, ammonia has been co-combusted with other fuels like diesel and dimethyl ether. Hydrogen has also been successfully used as a complementary fuel. Luckily, hydrogen can be produced by in-situ cracking of ammonia.
NOx emission is the other major problem of using ammonia as an engine fuel. If combustion temperature is lowered to reduce NOx level, concentration of unburned ammonia goes up. NOx levels in ammonia engine exhaust are as high as 1000 ppm. Researchers are trying different fuel injection techniques to simultaneously reduce NOx and unburned ammonia. Another post-combustion technique is to use selective catalytic reduction and reduce both NOx and ammonia in the exhaust gases.
Fuel Cells
An alternative way of recovering the chemical energy from ammonia is through fuel cells. Fuel cells can be indirect or direct. The indirect method uses in-situ thermal cracking of ammonia to yield hydrogen, which is subsequently processed in a fuel cell to produce power. The second method, as the name suggests, directly processes ammonia inside the fuel cell. Direct ammonia fuel cells are of many types depending on the electrolyte used. The earliest direct ammonia fuel cells were Alkaline Fuel Cells (AFC), which initially used KOH solution and subsequently molten mixtures of NaOH and KOH as the electrolyte. Oxygen from the air supplied to the cathode generate hydroxyl ions, which are transported through the electrolyte to the anode, where they encounter ammonia. The ammonia reacts with the hydroxyl ions to form nitrogen, water and free electrons, which circulate to the cathode through the external electrical load producing power. Noble metals like platinum and ruthenium are used for the electrodes. The main drawback of these cells is the absorption of carbon dioxide in the air by the alkaline electrolytes and consequent drop in efficiency.
SOFC
Solid Oxide Fuel Cells (SOFC) use several mixed conducting ceramics as the electrolyte and operate at high temperatures, typically 650 to 1000 degrees C. A commonly used lectrolyte is yttria-stabilised zirconia. They do not require noble metals as electrodes; instead cheaper nickel-based materials are used. Because of their high efficiency, they have been extensively researched in recent years and a commercial breakthrough is expected very soon. SOFCs come in twovariants: SOFC-O and SOFC-H. Oxygen anions are transported through the electrolyte in SOFC-O, whereas the SOFC-H uses protons as charge carriers. Ammonia is introduced at the anode where it splits into hydrogen and nitrogen. Air is introduced at the cathode. In SOFC-O, the oxygen at the cathode is reduced to oxygen anions,which travel through the electrolyte to the anode where they react with hydrogen to form water. Since water is formed at the anode, where ammonia is present, side reactions occur leading to nitric oxide generation. In SOFC-H, hydrogen at the anode is stripped off electrons to yield protons, which travel through the electrolyte to the cathode to react with the oxygen to produce water. Nitric Oxide formation is liminated in SOFC-H. Also, 20-30 percent higher peak power output has been achieved in SOFC-H compared to SOFC-O.
Going Green
The shipping industry, which currently accounts for nearly three percent of the global carbon dioxide emissions, has been quick to embrace ammonia as an alternate fuel in its efforts to decarbonise its operations. Five leading shipping companies have come together to sign a MOU to establish a supply chain for green ammonia. If all long-distance shipping were to convert to ammonia, the global production would have to nearly treble from the present 180 million tonnes per annum. Wartsila and MAN, the major engine manufacturers, have announced plans to undertake long term trials of ammonia in a marine combustion engine. Wartsila hopes to have its engine installed on a ship by 2023. Yara, the world’s leading producer of ammonia is betting big on going green. It has announced plans to set up the first large-scale green ammonia project in Europe. Other countries like USA and Saudi Arabia, have also chalked out plans to manufacture green ammonia. The Indian government has plans to invite bids for setting up green ammonia projects with a view to reduce dependence on fossil fuels.
Epilogue
As nations compete with each other in announcing roadmaps for decarbonisation of their economy by middle of this century, hydrogen and ammonia will play a stellar role in fructification of these ambitions plans.
Readers’ responses may be sent to:
k.sahasranaman@gmail.com or
chemindigest@gmail.com
