Greening the production and utilization of ammonia
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Energy Sector Analysis
Ammonia has the lowest source-to-tank cost among fuels producible from air and water using renewable electricity, including hydrogen gas and methanol.
Greening the production and utilization of ammonia By Tianyu Liu Feature Editor: Sabrina Sartori
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mmonia can supplement hydrogen gas as a clean fuel to combat climate change. It overcomes hindrances that currently impede the realization of the full potential of hydrogen gas, including economical storage, political commitment, and safety concerns. Ammonia, a zero-carbon-emission fuel (yielding dinitrogen and water when completely combusted), possesses unique advantages over hydrogen gas. First, ammonia production is abundant. According to the US Geological Survey, the global production of ammonia in 2019 reached 182 million metric tons, which is more than double the weight of hydrogen gas produced in the same year. Second, infrastructures for the liquefaction, storage, and transport of ammonia are already in place, since ammonia has been extensively used to manufacture agricultural fertilizers. Third, ammonia’s relatively narrow explosive limit (15–28 vol% of NH3 versus 4–75 vol% of H2) and an odor threshold as low as 0.04 ppm ease the safety control for ammonia implementation. These merits impart ammonia increasing popularity as a supplement to the hydrogen energy regime. For ammonia to become a sustainable fuel, its production must be completely changed. Conventionally, ammonia is synthesized via the Haber–Bosch process, which combines nitrogen and hydrogen gases into ammonia under temperatures of 400– 500°C and pressures of 150–250 bar on surfaces of iron-containing catalysts. The nitrogen and hydrogen gases come from the air and natural gas reforming, respectively. This long-practiced production of ammonia, however, is by no means sustainable. It consumes more than 1% of global energy and emits ~400 million tons of carbon dioxide if fossil fuels are used to heat reactors. Replacing the energy input by sustainable energy resources (e.g., sunlight and wind) is attractive to green the synthesis of ammonia. Geoffrey A. Ozin, a professor of chemistry at the University of Toronto, has envisioned a solar refinery scheme outlining possible ways for sustainable production of ammonia (see Figure). This scheme integrates solar energy into the existing infrastructures, such as those for the air-separation and the Haber–Bosch reaction. It also bridges state-of-the-art electrocatalysis and photocatalysis of nitrogen fixation. For example, in a solar ammonia refinery, the Haber–Bosch process consumes hydrogen gas generated from solar water splitting and is powered by electricity converted from sunlight. The realization of this grand scheme requires breakthroughs in multiple experimental and technical aspects, including the availability of high-performance catalysts for ammonia formation.
Developing efficient nitrogen-reduction catalysts has been a hot topic in the fields of thermal, electrochemical, and photochemical catalysis. Researchers have s
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