As most of the world strives to attain net-zero carbon dioxide emissions by mid century, no fuel is cleaner than hydrogen produced with renewable energy. But hydrogen has a low energy content—just one-tenth that of natural gas at ambient temperature and pressure and one-sixth in liquid form. Ammonia packs more energy per molecule, and it’s getting attention for its potential to carry hydrogen—and hence carbon-free energy. Liquid ammonia stores much more hydrogen than liquid hydrogen: 121 kg per m3, compared with 71 kg per m3. And liquefying ammonia is far easier and consumes much less energy.
Ammonia, however, must first solve its own carbon problem. Synthesis by the conventional process is a major contributor to climate change, accounting for 620 million tons of CO2 each year, about 1.3% of annual global anthropogenic emissions of the greenhouse gas, according to the International Energy Agency (IEA). Nearly all of the approximately 200 million tons of ammonia produced annually by that process is so-called gray ammonia. It originates with hydrogen that is steam-reformed from natural gas or coal and leaves CO2 as the byproduct. The atmosphere provides ammonia’s nitrogen content.
Substantial reductions in CO2 emissions would be achieved if the greenhouse gas were to be captured and stored—the so-called blue-ammonia method. Green ammonia—manufactured entirely without hydrocarbons—obtains its hydrogen from splitting water in electrolyzers powered by renewable electricity. Adding 10 electrolyzers of 30 MW each per month and one large carbon capture and storage plant every four months between now and 2050 would reduce emissions from ammonia production by only 70%, according to the IEA. Near-zero emissions from ammonia production would require even more rapid deployment of blue and green technologies, the agency says.
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