How the Haber–Bosch Process Makes Fertilizer From Air
The Haber–Bosch process combines nitrogen from air with hydrogen over an iron catalyst at high heat and pressure, making ammonia for synthetic fertilizer.
The Haber–Bosch process is an industrial method for making ammonia (NH₃) from nitrogen and hydrogen gas. Nitrogen is taken from the air, and hydrogen usually comes from natural gas. The two gases are passed over an iron catalyst at about 400 to 500 °C and roughly 150 to 300 times normal air pressure. Most synthetic nitrogen fertilizer begins as ammonia made this way.
What goes into the process?
Nitrogen makes up about 78 percent of dry air. The plant draws in air, cools it until it liquefies, and separates nitrogen from oxygen by their different boiling points. Hydrogen is the harder input. Most plants in the world make it by steam reforming natural gas: methane reacts with steam over a nickel catalyst and yields carbon monoxide and hydrogen. A second step turns much of that carbon monoxide into carbon dioxide and adds more hydrogen.
How does the converter turn gas into ammonia?
The heart of the plant is the converter, a thick-walled steel vessel that holds beds of iron catalyst with small amounts of promoters such as potassium and aluminum oxide. A nitrogen molecule is held together by a triple bond, one of the strongest bonds in common chemistry. On the iron surface that bond loosens, so nitrogen atoms can meet hydrogen and form nitrogen–hydrogen bonds. The new ammonia molecules then leave the surface and join the gas stream.
The reaction can run in either direction, and the forward direction releases heat. Each pass through the catalyst converts only part of the gas, often well under half. So the gas leaving the converter is cooled, ammonia condenses into a liquid and is drawn off, and the unreacted nitrogen and hydrogen are compressed and fed back in. This recycle loop, more than any single pass, makes the plant economical.
| Condition | Effect on ammonia yield | Effect on reaction speed |
|---|---|---|
| Higher pressure | More ammonia | Faster |
| Higher temperature | Less ammonia | Faster |
| Lower temperature | More ammonia | Slower |
The table shows the central compromise. Low temperature and high pressure favor a higher yield, but at low temperature the catalyst is too slow to be useful. Engineers settle near 400 to 500 °C, hot enough for a practical rate and cool enough to keep the yield acceptable. Pressure stays high because it helps both yield and speed, at the cost of heavy vessels and large compressors. Like the enzymes in living cells, the catalyst lowers the energy barrier without being used up.
Why does the process matter for food?
Plants need nitrogen to build proteins, nucleic acids and chlorophyll, but they cannot use the nitrogen gas that makes up most of the air. Before the early 20th century, farms relied on manure, on crop rotations with legumes whose root bacteria fix nitrogen, and on mined deposits such as Chilean sodium nitrate. Fritz Haber showed the ammonia reaction in a laboratory in 1909. Carl Bosch then scaled it up for the chemical company BASF, and the first large plant opened in 1913. Haber received the 1918 Nobel Prize in Chemistry, and Bosch shared the 1931 prize.
A widely cited 2008 estimate puts the share of the world's population whose food depends on synthetic nitrogen fertilizer at about 48 percent. The figure depends on the assumptions in the calculation, so treat it as an approximation. Hydroponic growers face a related question about supplying nutrients, which is covered in hydroponics.
What are the costs and trade-offs?
The process is energy-hungry. Estimates put its share of world energy use at about 1 to 2 percent, most of it spent making hydrogen and compressing gas. Its main drawbacks are:
- Carbon: hydrogen from natural gas releases carbon dioxide. Splitting water with electricity avoids that only if the electricity is low-carbon.
- Runoff: nitrogen that crops do not absorb can wash into rivers and lakes, feeding algal blooms that use up the oxygen in the water.
- Price exposure: fertilizer costs rise and fall with natural gas prices, which affects the farmers who buy it.
Making hydrogen with less carbon, and applying fertilizer more precisely, are active areas of research and policy. The chemistry of the bond that the catalyst breaks is covered in chemical bonding.
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agriculture catalysis chemistry food science history