What Is the Haber Process? The Reaction That Feeds Half the World

Plants need nitrogen to build proteins and DNA, and they can take it only in fixed forms such as ammonia or nitrate. Natural fixation is slow: lightning converts a small amount, and certain bacteria, some living in nodules on the roots of peas, beans and clover, do the rest, which is why crop rotation with legumes has been standard agricultural practice for centuries. By the late nineteenth century Europe was importing enormous quantities of Chilean nitrate and Peruvian guano to keep yields up, and in 1898 William Crookes, addressing the British Association, warned that these deposits would run out and that the wheat-eating nations faced starvation unless chemistry solved the nitrogen problem. The warning was taken seriously, and it framed the work of the following decade as a race.

The chemistry combines nitrogen from the air with hydrogen, historically from coal and now almost entirely from natural gas, to make ammonia, and the difficulty is that the reaction is reversible and fights back at every stage:

• •It is exothermic, so a low temperature favours a high yield, and it is also extremely slow at low temperature, which is the central conflict
• •It produces fewer gas molecules than it consumes, so high pressure pushes it forward, and the plant must therefore contain gas at 150 to 250 atmospheres
• •The compromise is around 400 to 450 degrees, hot enough to proceed at a useful rate and cool enough to leave a workable yield, which is a practical application of Le Chatelier's principle
• •An iron catalyst, developed by Carl Bosch's colleague Alwin Mittasch after testing thousands of formulations, speeds the reaction without shifting the equilibrium
• •Only around fifteen percent of the gas converts on each pass, so the ammonia is condensed out and the unreacted gas is recycled continuously, which is what makes the overall process efficient

Fritz Haber demonstrated the reaction at laboratory scale in 1909, producing a slow drip of ammonia in a bench apparatus, and turning that into an industrial plant fell to Carl Bosch at BASF. The problems were formidable: no vessel existed that could hold hot hydrogen at two hundred atmospheres, because hydrogen at those conditions attacks steel, diffusing into it and reacting with its carbon to form methane, which cracks the metal from inside. Bosch's solution was a double-walled reactor with a soft iron liner and small holes in the outer casing to let hydrogen escape rather than build up. His team also had to design compressors, heat exchangers and a continuous recycling system on a scale nobody had attempted. The first commercial plant opened at Oppau in 1913, and the achievement was recognised with Nobel Prizes for both men, Haber in 1918 and Bosch in 1931. The phrase high pressure chemistry as an industrial discipline essentially begins here.

Ammonia is also the starting point for explosives, since nitric acid is made from it and nitrates are the basis of most military propellants and munitions. Germany's supply of Chilean nitrate was cut by the British naval blockade within months of the First World War beginning, and the country would have run out of munitions in 1915 had the new process not been scaled up rapidly to fill the gap, which is a reasonable claim to having extended the war by years. Haber's own role went further: he directed Germany's chemical weapons programme, personally supervised the first large chlorine gas attack at Ypres in April 1915, and his wife Clara Immerwahr, herself a chemist who had opposed the work, killed herself with his service pistol days afterwards. He was Jewish, was forced out of his position in 1933 and died in exile the following year, and a compound developed under his institute's earlier work on insecticides was later used in the gas chambers, killing members of his extended family.

The process consumes something like one to two percent of the world's total energy, almost all of it from natural gas, and is responsible for roughly the same share of global carbon dioxide emissions, since the hydrogen comes from splitting methane and releasing the carbon. The nitrogen that reaches fields does not all stay there: runoff carries it into rivers and coastal waters, where it feeds algal blooms that strip the water of oxygen and create dead zones, of which the one in the Gulf of Mexico is the most studied, and soil bacteria convert some of it into nitrous oxide, a greenhouse gas roughly three hundred times more potent than carbon dioxide per molecule. The human alteration of the nitrogen cycle is now larger than the natural one, and it is one of the planetary boundaries that researchers judge to have been exceeded by the widest margin. Work on green ammonia, using hydrogen from electrolysis powered by renewable electricity, and on electrochemical routes that would work at ambient conditions, is active and not yet competitive.

Combining atmospheric nitrogen with hydrogen to make ammonia required overcoming a reversible reaction that is fast only when hot and productive only when cool, solved with a compromise around 430 degrees, pressures above 150 atmospheres, an iron catalyst and continuous recycling of unreacted gas. Haber demonstrated it in 1909 and Bosch built the plant, which sustained German munitions through the First World War and now supports roughly half the world's food supply, at the cost of substantial energy use, emissions and nitrogen pollution of rivers and coasts.