Ammonia production takes place worldwide, mostly in large-scale manufacturing plants that produce 183 million metric tonnes^[1] of ammonia (2021) annually.^[2][3] Leading producers are China (31.9%), Russia (8.7%), India (7.5%), and the United States (7.1%). 80% or more of ammonia is used as fertilizer. Ammonia is also used for the production of plastics, fibres, explosives, nitric acid (via the Ostwald process), and intermediates for dyes and pharmaceuticals. The industry contributes 1% to 2% of global CO
₂.^[4] Between 18-20 Mt of the gas is transported globally each year.^[5]

History

Dry distillation

Before the start of World War I, most ammonia was obtained by the dry distillation of nitrogenous vegetable and animal products; by the reduction of nitrous acid and nitrites with hydrogen; and also by the decomposition of ammonium salts by alkaline hydroxides or by quicklime, the salt most generally used being the chloride (sal-ammoniac).

Frank–Caro process

Adolph Frank and Nikodem Caro found that Nitrogen could be fixed by using the same calcium carbide produced to make acetylene to form calcium-cyanamide, which could then be divided with water to form ammonia. The method was developed between 1895 and 1899.

${\displaystyle {\ce {CaO + 3C <=> CaC2 + CO))}$

${\displaystyle {\ce {CaC2 + N2 <=> CaCN2 + C))}$

${\displaystyle {\ce {CaCN2 + 3H2O <=> CaCO3 + 2NH3))}$^[6]

Birkeland–Eyde process

While not strictly speaking a method of producing ammonia, nitrogen can be fixed by passing it (with oxygen) through an electric spark.

Nitrides

Heating metals such as magnesium in an atmosphere of pure nitrogen produces the nitride, which when combined with water produce the metal hydroxide and ammonia.

Haber-Bosch process

The Haber process,^[7] also called the Haber–Bosch process, is the main industrial procedure for the production of ammonia.^[8][9] The German chemists Fritz Haber and Carl Bosch developed it in the first decade of the 20th century. The process converts atmospheric nitrogen (N₂) to ammonia (NH₃) by a reaction with hydrogen (H₂) using an iron metal catalyst under high temperatures and pressures. This reaction is slightly exothermic (i.e. it releases energy), meaning that the reaction is favoured at lower temperatures^[10] and higher pressures.^[11] It decreases entropy, complicating the process. Hydrogen is produced via steam reforming, followed by an iterative closed cycle to react hydrogen with nitrogen to produce ammonia.

The primary reaction is:

${\displaystyle {\ce {N2 + 3H2 <=> 2NH3))\qquad {\Delta H^{\circ }=-92.28\;{\ce {kJ))}\ ({\Delta H_{298\mathrm {K} }^{\circ }=-46.14\;\mathrm {kJ/mol} })}$

Before the development of the Haber process, it had been difficult to produce ammonia on an industrial scale,^[12][13][14] because earlier methods, such as the Birkeland–Eyde process and the Frank–Caro process, were too inefficient.

Sustainable production

Ammonia production depends on plentiful supplies of energy. Sustainable production is possible by using non-polluting methane pyrolysis or generating hydrogen by water electrolysis with renewable energy sources.^[15] Thyssenkrupp Uhde Chlorine Engineers expanded its annual production capacity for alkaline water electrolysis to 1 gigawatt of electrolyzer capacity for this purpose.^[16]

In a hydrogen economy some hydrogen production could be diverted to feedstock use. For example, in 2002, Iceland produced 2,000 tons of hydrogen gas by electrolysis, using excess power from its hydroelectric plants, primarily for fertilizer.^[17] The Vemork hydroelectric plant in Norway used its surplus electricity output to generate renewable nitric acid from 1911 to 1971,^[18] requiring 15 mWh/ton of nitric acid. The same reaction is carried out by lightning, providing a natural source of soluble nitrates.^[19] Natural gas remains the lowest cost method.

Wastewater is often high in ammonia. Because discharging ammonia-laden water into the environment damages marine life, nitrification is often necessary to remove the ammonia.^[20] This may become a potentially sustainable source of ammonia given its abundance.^[21] Alternatively, ammonia from wastewater can be sent into an ammonia electrolyzer (ammonia electrolysis) operating with renewable energy sources to produce hydrogen and clean water.^[22] Ammonia electrolysis may require much less thermodynamic energy than water electrolysis (only 0.06 V in alkaline media).^[23]

Another option for recovering ammonia from wastewater is to use the mechanics of the ammonia-water thermal absorption cycle.^[24][25] Ammonia can thus be recovered either as a liquid or as ammonium hydroxide. The advantage of the former is that it is much easier to handle and transport, whereas the latter has commercial value at concentrations of 30 percent in solution.

Coal

Making ammonia from coal is mainly practised in China, where it is the main source.^[6] Oxygen from the air separation module is fed to the gasifier to convert coal into synthesis gas (H₂, CO, CO₂) and CH₄. Most gasifiers are based on fluidized beds that operate above atmospheric pressure and have the ability to utilize different coal feeds.

Production plants

The American Oil Co in the mid-1960s positioned a single-converter ammonia plant engineered by M. W. Kellogg at Texas City, Texas, with a capacity of 544 m.t./day. It used a single-train design that received the “Kirkpatrick Chemical Engineering Achievement Award” in 1967. The plant used a four-case centrifugal compressor to compress the syngas to a pressure of 152 bar Final compression to an operating pressure of 324 bar occurred in a reciprocating compressor. Centrifugal compressors for the synthesis loop and refrigeration services provided significant cost reductions.

Almost every plant built between 1964 and 1992 had large single-train designs with syngas manufacturing at 25–35 bar and ammonia synthesis at 150–200 bar. Braun Purifier process plants utilized a primary or tubular reformer with a low outlet temperature and high methane leakage to reduce the size and cost of the reformer. Air was added to the secondary reformer to reduce the methane content of the primary reformer exit stream to 1–2%. Excess nitrogen and other impurities were erased downstream of the methanator. Because the syngas was essentially free of impurities, two axial-flow ammonia converters were used. In early 2000 Uhde developed a process that enabled plant capacities of 3300 mtpd and more. The key innovation was a single-flow synthesis loop at medium pressure in series with a conventional high-pressure synthesis loop.^[26]

Small-scale onsite plants

In April 2017, Japanese company Tsubame BHB implemened a method of ammonia synthesis that could allow economic production at scales 1-2 orders of magnitude below than ordinary plants with utilizing electrochemical catalyst.^[27][28]

Byproducts and shortages due to shutdowns

One of the main industrial byproducts of ammonia production is CO₂. In 2018, high oil prices resulted in an extended summer shutdown of European ammonia factories causing a commercial CO₂ shortage, thus limiting production of CO₂-based products such as beer and soft drinks.^[29] This situation repeated in September 2021 due to a 250-400% increase in the wholesale price of natural gas over the course of the year.^[30][31]