It was over a hundred years ago that Edgar Rice Burroughs wrote his Princess of Mars, a romantic vision of a dying planet with million year old cities on the shores of long dry seas. The world, for all this, is inhabited by the red men, the green men and the great white apes, but all live only by grace of the great canals and the continuous operation of the great atmosphere plant which continuously refreshes and restores the vital component of the planet's air.
How different is our own green earth, but not as different as one might think, ...
... for all life on our planet relies on proteins, and proteins are compounds containing nitrogen. Our planet has no shortage of nitrogen. In fact, over 3 parts in 4 of the atmosphere are nitrogen, but atmospheric nitrogen is, for the most part, biologically inert. There are some organisms, a few prokaryotic microbes, that can directly use nitrogen from the air, but the only organisms that can do so in any quantity are the Rhizobium bacteria living in root nodules of one particular class of plants.
These bacteria have their limits, and as the earth's human population has grown we have surpassed their limited ability to produce the nitrogen necessary for human life. In the 19th century, as our knowledge of soil chemistry improved, biologically accessible nitrogen was mined from islands of guano and the sands of high Andean deserts, but these supplies were limited and increasingly challenging to exploit. In Conrad's Lord Jim, the title character is offered a job running such a mine on an isolated, nearly inaccessible island, with just a pair of guns to keep the workers in line.
By the early 20th century, the world's human population had passed 1.6 billion, and was rising thanks to the fruits of the industrial, sanitary and first agricultural revolutions. Chemistry too had advanced with an increasing ability to operate at high temperatures and pressures and to work with catalysts, so it is not surprising that a chemical process was developed to produce massive quantities of biologically available nitrogen from the inert gas found in the atmosphere. This was the leading edge of the second agricultural revolution.
The first agricultural revolution was built on the scientific and industrial revolutions. The former improved crop yields by improved breeding and better understanding of soil conditions. The latter with its mass produced farming equipment, prime movers and more advanced metallurgy. Cyrus McCormick's harvester, was called Lincoln's ace in the hole, ensuring the wheat harvest even with so many men called to war.
The second agricultural revolution was built on industrial chemistry, genetics research and energy extraction, so despite warnings of a "population bomb" the agriculture is feeding six billion people today. Of these six billion, perhaps half are only alive and fed by the grace of one chemical process converting the inert nitrogen of the atmosphere into the building blocks of life using a process developed by a Nobel Prize winning war criminal.
From an extremely parochial point of view, the First World War could be seen as a battle between two Jewish chemists, each faced with the challenge of producing reactive nitrogen, which is not only essential of life, but also a major component of gunpowder and other more modern chemical explosives. In England, Chaim Weitzman, the future president of Israel, developed a process for extracting nitrogen compounds from seaweed and was rewarded with the Balfour Declaration of Palestine as a Jewish homeland.
In Germany, the chemist was Fritz Haber, who had pressed on when others had failed and developed an inorganic process to extract nitrogen from the air and turn it into ammonia. For this he won the 1920 Nobel Prize, but his work on chlorine gas used in gas warfare had turned him into a war criminal and a pariah outside of Germany. He could not accept the award at the usual ceremony and had to skulk into Stockholm on Nobel's birthday for a special presentation. His first wife, a chemist herself, had committed suicide shortly after the first use of chlorine gas.
At least in Germany he was honored. Carl Bosch turned Haber's laboratory bench work into an industrial process. The first nitrogen fixing plant in Oppau was one of the first modern chemical plants, producing ammonia from nitrogen and hydrogen in a continuous process. Today, a chemical plant is a configuration of pipes connecting tanks and cylinders and oddly mis-scaled buildings which seem to be half open and full of more pipes and tanks and cylinders. Before Haber and Bosch, chemical plants were different, producing their products in batches and operating on a much smaller scale.
For a while things went well for Haber, at least until the National Socialists came into power. It was the old story. Though he had long ago converted to Christianity, he was still a Jew. He was fired from the research center he had founded, and the tree planted in his honor was uprooted. He fled to England where Weitzman tried to help him, offering him a post in Palestine, but Haber never saw the promised land and died shortly after his exile. The Nazis prohibited his colleagues from attending his memorial service. They sent their wives, which was as close to a protest as one could find in that dark era. Bosch died later, in 1940, horrified by the Nazi war machine, sure it meant doom for Germany.
The chemical process Haber discovered and Bosch developed, combining nitrogen and hydrogen gases at high pressures and temperatures in the presence of a catalyst, evolved and improved. It always relied on atmospheric nitrogen but used a variety of processes for extracting hydrogen from water or natural gas. The pressures rose past 100 atmospheres and the temperatures rose as well. The catalysts - they had tested thousands - have been improved and new compressors and configurations developed. The plants have grown in size and efficiency. The chemical reaction itself is endothermic - it requires a certain input of energy for each molecule formed. In a modern facility, there is perhaps an additional 50% more energy required to drive the centrifugal compressor and heat the reactants. Despite the extreme conditions, it is an efficient reaction.
The first Haber process plant in Oppau
Haber's process is used world wide, and it has restructured agriculture. Modern varieties of wheat were bred to make use of the high levels of soil nitrogen available. They are short and top heavy, with more edible grain and less stalk, and optimized for double cropping and mechanical harvesting. Modern hybrid corn cannot reproduce itself, but its yield dwarfs those of the previous varieties. Rice too has been reinvented, bred for high yields and rapid growth.
In the United States, nitrogen is fixed near the Gulf of Mexico, to take advantage of the natural gas supplies there. Then the ammonia is pumped north in great pipes or carried at low temperature in trucks and ships to the grain belt where it is pumped directly into the soil using specially designed blades. Europe is dotted with nitrogen fixing plants, and the increasing agricultural yields have led to the reforesting of its wheat belt running across northern France into Denmark.
Much as life on Burrough's Mars would not exist without its atmosphere plants, human life on our planet increasingly relies on the Haber process. The choice has been between accepting the agricultural revolution or starvation. Even during the fury of The Great Cultural Revolution of the 1960s, China faced the problem of its rising population. Thirty million, perhaps fifty million, had died of hunger in the agricultural restructuring of the 1950s. Mao's government had to change direction and installed the first nitrogen fixing plants even as it shattered every other institution. Today, the people of China live on rice, wheat and the Haber process. India, the second most populous nation, is little different, though its agricultural planning is less centralized.
Nitrogen in the food supply - adapted from Enriching the Earth
Nitrogen compounds are essential for human life. Every year humans consume roughly 25 million tons of nitrogen, largely in the form of proteins. This is the primarily the product of the 60 million tons of nitrogen in the world's crops. In comparison, nitrogen collected by grazing animals and oceanic sources is negligible. As the world's population rises, the demand for nitrogen will rise with it, and so will our dependence on the Haber process.
We tend to think of animal foods as our primary source of protein, but worldwide, only 20% of human nitrogen is provided by animal foods, and these are produced at the expense of roughly half of all crops planted. Moving towards a more vegetable intensive diet could remove some of the pressure on agricultural production, but humans tend to move towards certain more desirable foods whenever they can.
Pulses, like chick peas and lentils, grow in symbiosis with nitrogen fixing bacteria, so their external nitrogen requirements are lower than those of most plants. However, this symbiosis has its price, and the pulses have lower nutritional yields and are perceived as being of lower quality cross culturally. Annual per capita consumption of pulses has declined by half in India since 1960, and in China it has declined even more. Similarly, given the choice, most humans will eat more animal products.
Traditional agricultural processes - fallowing, rotation, recycling - can only provide limited levels of useful nitrogen. The great nitrogen mines have been depleted. The Haber process is extremely efficient, but even modern chemistry has its limits. How far can we push the technology to feed a rising population? One approach is to use fertilizer more efficiently. In the west, wheat yields have continued to rise even as nitrogen fertilizer applications have steadied or fallen. China has only recently started to address the problem of balanced plant fertilization, paying more attention to available phosphorus and potassium, rather than simply increasing the levels of nitrogen. There is clearly still room for improvement.
Unlike Edgar Rice Burrough's Mars, ours is not a dying world, but in one way we are becoming more like his Mars with our rising dependence on our atmosphere plants turning atmospheric nitrogen into the stuff of life.
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If you want to know more about the Haber process itself, read on.
The modern Haber process
The chemical reaction is almost trivial. Molecular nitrogen reacts with molecular hydrogen to form ammonia, and the chemical industry takes it from there. The challenge is that nitrogen and hydrogen don't like to react, except at high temperatures and high pressures, and even then only in the presence of certain catalysts. The Haber process wasn't discovered until the 20th century, partly because the high pressure technologies necessary weren't available before then. Haber's original lab bench reactor operated at one hundred times sea level atmospheric pressure and at several hundred degrees centigrade. It was relatively inefficient, and it took tens of thousands of tests to find a catalyst to produce the yields required for commercialization. Even with an optimized catalyst, the modern process still requires recirculating the nitrogen-hydrogen mixture and continuously condensing the ammonia produced. (You can see this loop on the right half of the diagram.)
There was also the challenge of scaling the process. It is one thing to order a canister of nitrogen and a canister of hydrogen and run a prototype; it is another to produce ammonia on a commercial scale. While nitrogen is ubiquitous, it has impurities that would foul the reaction - for example, oxygen. These have to be removed. In the original plants, hydrogen was produced by reacting water with coke, purified coal, which is simply carbon. Modern plants start with natural gas, methane - a compound of carbon and hydrogen - and have it react with water. In either case, the product is hydrogen and carbon monoxide, and the latter has to be removed. (The plant described in the diagram removes the oxygen and carbon monoxide using catalysts and water to produce soda water which is then extracted leaving only the nitrogen and hydrogen.)
Operating a chemical plant at high temperature and pressure introduced all sorts of new problems. Hydrogen gas tends to dissolve in steel and make it brittle, so the reaction vessels had to be redesigned to maintain their strength. The ammonia produced had to be stored, and until the 1921 explosion at the Oppau plant which blasted a 100 meter crater, 20 meters deep, and destroyed most of the homes in the city, no one knew that ammonia was explosive. It is, but given how much ammonia is produced and how important it is, the safety record of the Haber process is impressive.
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For an excellent and wonderfully detailed history and analysis of the Haber process and our world's human nitrogen budget, get a copy of Enriching the Earth by Vaclav Smil. It was the primary source for this article, though I've read a number of accounts of Haber's work and on agriculture, and, of course, I learned about the process in high school chemistry as did so many others.