Context:
Annually, the Haber-Bosch process extracts hundreds of millions of tonnes of nitrogen from the atmosphere to produce fertilizer, contributing 165 million tonnes of reactive nitrogen to the soil. This industrial method vastly surpasses the amount of reactive nitrogen naturally replenished by biological processes, estimated between 100 and 140 million tonnes each year. The synthesis of ammonia from nitrogen and hydrogen through this process is essential, as it provides a crucial mechanism to support the increasing global food demand, which could not be met by natural processes alone.
Relevance:
GS III: Agriculture
Dimensions of the Article:
- What is the nitrogen molecule?
- How is nitrogen availed in nature?
- What is the nitrogen cycle?
- How is ammonia made?
- Haber-Bosch Process
- Downsides of Fertilizers
What is the nitrogen molecule?
- Nitrates are molecules of oxygen and nitrogen, abundant in the earth’s atmosphere. Nearly eight metric tonnes of nitrogen lie on every square metre of the earth’s surface, yet it can’t feed a single blade of grass.
- Nitrogen in the air is mostly in the form of N2. When two nitrogen atoms join together, they share three pairs of electrons to form a triple bond, rendering the molecule nearly unbreakable.
- The energy required to break the nitrogen triple bond is so high (946 kJ/mol) that molecular nitrogen is nearly inert.
- But if the bond is broken, atomic nitrogen can form ionic nitrides such as ammonia (NH3), ammonium (NH4+), or nitrates (NO3–).
- Plants need these types of nitrogen, called reactive nitrogen, to synthesise enzymes, proteins, and amino acids. Healthy plants often contain 3-4% nitrogen in their above-ground tissues, significantly more than other nutrients.
How is nitrogen availed in nature?
- Among natural things, only lightning has enough energy to destroy the N2 triple bond.
- In a lightning bolt, nitrogen in the air combines with oxygen to generate nitrogen oxides such as NO and NO2. They can then combine with water vapour to create nitric and nitrous acids (HNO3 and HNO2, respectively).
- Reactive nitrogen-rich droplets fertilize farmlands, woods, and grasslands when it rains. This pathway is estimated to replenish soil by around 10 kg of nitrogen per acre per year.
- Apart from lightning, a gentle metabolic process carried out by Azotobacter bacteria can also create reactive nitrogen.
- Some microorganisms such as Rhizobia have developed symbiotic relationships with legume plants (clover, peas, beans, alfalfa, and acacia) to provide reactive nitrogen in exchange for nutrition.
- Azolla, a species of aquatic fern with a symbiotic association with the cyanobacterium Anabaena azollae, can absorb and convert nitrogen from the air to reactive nitrogen, so dried and decaying Azolla is an effective fertilizer for farmland.
What is the nitrogen cycle?
- Plants usually get their reactive nitrogen from the soil, where they absorb minerals dissolved in water such as ammonium (NH4+) and nitrate (NO3-).
- Humans and animals need nine pre-made nitrogen-rich amino acids from plants. Nitrogen makes up approximately 2.6% of the human body.
- The nitrogen ingested by plants and animals returns to the soil through excreta and the decomposition of dead bodies. But the cycle is incomplete: some nitrogen is released back into the environment in molecular form. Nitrogen from human waste is also rarely returned to the fields.
- Although legumes can produce nitrogen independently, important food crops such as rice, wheat, corn, and potatoes and less well-known crops like cassava, bananas, and common fruits and vegetables draw nitrogen from the soil.
- As the human population multiplies, nitrogen in agricultural soil depletes faster, needing fertilizers to compensate.
- Farmers understood this early. They cultivated legumes or fertilized their crops with ammonia to increase output where possible.
- They also used ammonium-bearing minerals from volcanic eruptions and naturally occurring nitrates found in caves, walls, and rocks as fertilizer.
How is ammonia made?
- Ammonia (NH4) is made of nitrogen and hydrogen, both of which exist naturally as two-atom molecules. Under extreme heat, the molecules separate and form a compound, but it is short-lived because of the heat.
- The reversible reaction N2 + 3H2 = 2NH3 (the ‘=’ sign has been used here as a stand-in for bidirectional arrows) must be maintained in specific conditions to harvest considerable amounts of ammonia.
- The German chemist Fritz Haber heated the N2-H2 combination to various temperatures in a platinum cylinder and calculated the amount of ammonia created.
- He also used hot ammonia to decompose into nitrogen and hydrogen, attempting to approach the equilibrium point from the opposite direction.
- At 1,000 degrees Celsius, Haber found that harvestable ammonia made up just one-hundredth of 1% of the mixture — too little for commercial production.
- Then Haber wondered if pressure could be the answer. He calculated that hydrogen and nitrogen would only remain united in extreme conditions: temperatures of 200 degrees Celsius and pressures of 200 atm (that is, 200-times the average air pressure at sea level).
- But the ammonia production rate was still too slow, so Haber set about looking for a catalyst. He also realised that if he could cool the ammonia to a liquid state, he could collect most of it.
Haber-Bosch Process
The Haber-Bosch process is a critical chemical method developed in the early 20th century, pivotal in synthesizing ammonia from nitrogen and hydrogen gases under high pressures and temperatures. This process revolutionized agricultural practices by providing a steady production method for ammonia-based fertilizers, which are essential for modern agriculture.
Key Components and Development
- Initiation and Key Contributors: The process was primarily developed by Fritz Haber with significant contributions from his assistant Robert Le Rossignol, who engineered the necessary high-pressure seals, and Friedrich Kirchenbauer, who constructed the apparatus. Their teamwork laid the foundational work for Haber’s method, which he acknowledged during his Nobel Prize acceptance, sharing the honor and financial rewards with his team.
- Chemical Process: At its core, the Haber-Bosch process involves combining nitrogen (N2) from the air with hydrogen (H2) derived typically from natural gas into ammonia (NH3) under high pressures (around 200 atmospheres) and temperatures (450-500°C). This reaction occurs in the presence of a catalyst, which initially was osmium, as discovered by Haber, but due to its rarity and cost, was later replaced by a more economical iron-based catalyst developed by BASF’s Alwin Mittasch.
- Industrial Scale and Impact: Carl Bosch, an engineer at BASF, was instrumental in scaling up Haber’s laboratory setup to an industrial process. This scale-up involved significant engineering challenges, particularly in designing equipment that could handle the extreme conditions of the ammonia synthesis reaction. The successful industrialization of the process led to the opening of the world’s first large-scale ammonia production plant by BASF in 1913.
Significance and Legacy
The Haber-Bosch process has had a profound impact on the global population and food production. By providing an abundant source of nitrogen fertilizers, it has significantly boosted agricultural yield and thus supported a growing global population. However, the process is also energy-intensive, relying heavily on fossil fuels, which raises environmental concerns in terms of carbon emissions and sustainability.
Cons of Fertilizers
Even though the Haber-Bosch process had amplified the total food production in this earth immensely by offering an avenue for producing synthetic nitrogen fertilizers, it has brought forth severe environmental and social problems arising from the consumption of these fertilizers.
Environmental Effects
- Eutrophication: Nitrous compounds from excessive fertilizer applications can find their way into bodies of water; these eutrophication leads to over-enrichment of the water plants and algae with plenty of nutrients, then consume oxygen in the water, thus killing all aquatic animals, and eventually destroys natural ecosystems.
- Acid Rain: Oxides of nitrogen from fertilizer usage can become atmospheric in form and contribute toward acid rain. Acid rain is harmful to water environment, forests as well as soil and causes corrosion of natural as well as man-made structures.
- Biodiversity Loss: It leads to the imbalance of nutrients in soils as a result of high levels of nitrogen. This consequently enhances the growth of some species over others, thus minimizing biodiversity. This also minimizes ecosystem resilience to pests and diseases.
Social and Economic Concerns
- Increasing inequality in food distribution: Just because the world produces an adequate supply does not reduce hunger worldwide. Increased food production would only mean whatever increase remains is handed out to which particular region of starvation and malnutrition despite increased distribution, access, or equity in food.
- Dependence on Fertilizers Over time, dependence on synthetic fertilizers can cause the soil to degrade. As the health of the soil declines, more fertilizer applications are required, creating a cycle of dependence that may eventually be economically difficult to control for small-scale farmers.
-Source: The Hindu
