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Comparison of Process Options for Sustainable Ammonia Production

Ammonia is one of the most produced chemicals in the world, with a production of about 150 million metric tons a year. It is critical for improved yields in modern agriculture as well as a chemical feedstock to various other processes. Today, Steam Methane Reforming (SMR), which uses fossil based natural gas as its feedstock, is the most widely used method for ammonia production. In this process, the natural gas is used to produce hydrogen, which is then reacted with nitrogen from the air to form ammonia. This technology generates a significant amount of greenhouse gases (GHGs), which has led to proposals for new processes that lower the carbon intensity of ammonia production while still maintaining process efficiency.

One approach for reducing GHGs from the conventional SMR process is either sequestration of vented CO2, carbon capture from process flue gas, or a combination of the two. These methods have sometimes been dubbed “Blue Ammonia”. Another approach, sometimes called “Green Ammonia”, utilizes water electrolysis as its source of hydrogen. The use of water electrolysis allows water and renewable sources of electricity, such as wind and solar, to supplant natural gas as the feedstock for the required hydrogen production. Two categories of electrolysis units include alkaline water electrolysis (AWE) and polymer electrolyte membrane electrolysis (PEM). While the SMR process can use air as its source of nitrogen, the AWE and PEM based technologies require pure nitrogen to be available. A third category of electrolysis, solid oxide electrolysis (SOE), can also generate pure hydrogen from renewable electricity, but unlike AWE and PEM, does not require pure nitrogen. SOE also provides additional avenues of heat integration between hydrogen production and ammonia synthesis that is lacking from the other electrolysis technologies.

A comparison is made between the various newer technologies to a conventional SMR system using a steady-state simulator. This comparison includes the configuration and requirements of each system, as well as each system’s carbon intensity and power requirements per ton of ammonia produced.

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