Because of its many uses, ammonia is one of the most highly produced inorganic chemicals. Dozens of chemical plants worldwide produce ammonia. The worldwide ammonia production in 2004 was 109 million metric tonnes.[15] The People's Republic of China produced 28.4% of the worldwide production followed by India with 8.6%, Russia with 8.4%, and the United States with 8.2%.[15] About 80% or more of the ammonia produced is used for fertilizing agricultural crops.[15]
Before the start of World War I, most ammonia was obtained by the dry distillation of nitrogenous vegetable and animal waste products, including camel dung, where it was distilled by the reduction of nitrous acid and nitrites with hydrogen; in addition, it was produced by the distillation of coal, and also by the decomposition of ammonium salts by alkaline hydroxide such as quicklme, the salt most generally used being the chloride (sal-ammoniac) thus:
2 NH4Cl + 2 CaO → CaCl2 + Ca(OH)2 + 2 NH3
(Two molecules of ammonium chloride plus two calcium oxide yields calcium chloride and calcium hydroxide and two molecules of ammonia)
Today, the typical modern ammonia-producing plant first converts natural gas (i.e., methane) or liquified petroleum gas (such gases are propane and butane) or petroleum naphtha into gaseous hydrogen. The processes used in producing the hydrogen begins with removal of sulfur compounds from the natural gas (because sulfur deactivates the catalysts used in subsequent steps). Catalytic hydrogenation converts organosulfur compounds into gaseous hydrogen sulfide:
H2 + RSH → RH + H2S(g)
The hydrogen sulfide is then removed by passing the gas through beds of zinc oxide where it is absorbed and converted to solid zinc sulfide:
H2S + ZnO → ZnS + H2O
Catalytic steam reforming of the sulfur-free feedstock is then used to form hydrogen plus carbon monoxide:
CH4 + H2O → CO + 3 H2
In the next step, the water gas shift reaction is used to convert the carbon monoxide into carbon dioxide and more hydrogen:
CO + H2O → CO2 + H2
The carbon dioxide is then removed either by absorption in aqueous ethanolamine solutions or by adsorption in pres. swingadsorbers (PSA) using proprietary solid adsorption media.
The final step in producing the hydrogen is to use catalytic methanation to remove any small residual amounts of carbon monoxide or carbon dioxide from the hydrogen:
CO + 3 H2 → CH4 + H2O
CO2 + 4 H2 → CH4 + 2 H2O
To produce the desired end-product ammonia, the hydrogen is then catalytically reacted with nitrogen (derived from process air) to form anhydrous liquid ammonia. This step is known as the ammonia synthesis loop (also referred to as the haber-boch process):
3 H2 + N2 → 2 NH3
H2 required for ammonia synthesis could in principle be obtained from other sources, but these alternatives - apart from the electrolysis of water into oxygen + hydrogen - are presently impractical. At one time, most of Europe's ammonia was produced from the Hydro plant at Vemork, via the electrolysis route. Various renewable energy electricity sources are also potentially applicable.