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| Fig. 1: Block diagram of SMR Haber-Bosch Process. (Image Source: K. Bailey, after Lin et al. [2]) |
In 1909, Fritz Haber and Carl Bosch transformed society through the development of a process that enabled large-scale ammonia production for the first time. [1] The so-called Haber-Bosch process now accounts for over 98% of ammonia production worldwide. [2] This ammonia has been a key ingredient in fertilizers that have helped expand the agricultural industry and therefore grow our global population from two billion to eight billion people today. [1]
Currently, the most popular method of ammonia production is through steam methane reforming (SMR), which is depicted in Fig. 1. [2] First, the methane feedstock undergoes desulfurization. [2] This natural gas and steam are then passed through a primary reformer to convert the natural gas into carbon oxides, hydrogen, and water. [2] In the secondary reformer, nitrogen from air is added to the gas mixture to create a 3:1 hydrogen-to-nitrogen ratio. [2] Any carbon monoxide is converted to carbon dioxide via shift conversion, and the carbon dioxide is then removed by solvent scrubbing. [2] The following methanation unit reacts any remaining carbon oxides with hydrogen to form methane and water in order to protect the oxygen-sensitive Haber-Bosch catalyst. This gas mixture is compressed and sent into the ammonia synthesis loop. [2] After the ammonia is produced, it is separated from unwanted gasses, such as extra reactants and carbon oxides, often in a flash separator. [2]
While the Haber-Bosch process was revolutionary in ammonia production, there are some drawbacks, including its large energy consumption and carbon footprint. Overall, this process is responsible for 2% of global energy consumption and 1-2% of global greenhouse gas emissions. [2]
Table 1 below shows total energy consumption and theoretical energy consumption of the Haber-Bosch process in 2021. Haber-Bosch energy consumption was estimated as 2% of global energy consumption. The theoretical minimum of energy required for the Haber-Bosch process was calculated by using the heats of formation at 298°K and 1 atm of the reaction [3]
| 3 2 |
H2 | + | 1 2 |
N2 | → | NH3 |
This reaction is exothermic, so it produces heat and has a negative energy input alone. However, there needs to be a sufficient source of hydrogen, which is often obtained via steam reforming. Taking into account steam reforming, the new chemical reaction is
| 1 2 |
N2 | + | 3 8 |
CH4 | + | 3 4 |
H2O | → | NH3 | + | 3 8 |
CO2 |
This process is endothermic, requiring 16.49 kJ/mol NH3, as calculated from the heats of formation of the reactants and products at 298°K and 1 atm. [3]
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| Table 1: Total energy consumption and Haber-Bosch energy consumption. [3,5,6] |
The required reaction conditions for the Haber-Bosch process make it very energy-intensive. In the ammonia-production stage, hydrogen and ammonia react at 15-25 MPa and 400-450 degrees Celsius with a typical iron-based catalyst. [1] These high temperature and pressure conditions require a large input of energy and are part of the reason why the actual Haber-Bosch energy consumption, 11.90 EJ, deviates from the theoretical minimum, 0.15 EJ, calculated at ambient conditions. The theoretical minimum is also assuming 100% conversion from reactant to product, which is often not the actual case. Finally, the theoretical minimum does not account for any cost from separating the ammonia from other products and reactants. Finding a way to conduct this process at less harsh conditions or developing a different, but still cost-effective, method of obtaining hydrogen reactant could decrease the energy required for this process.
The Haber-Bosch process has been targeted by researchers due to its high energy requirement and greenhouse gas emissions. It is essential for the world to continue to meet the high energy demands of the Haber-Bosch process because otherwise, ammonia production would stall, and there would not be enough fertilizer to grow the food necessary to feed the growing human population.
Some ideas to make this process more sustainable include integrating renewable energy sources, which would decrease the amount of fossil fuels needed to provide the energy input required for this process. [2] There are also potential improvements in the separation steps that can be explored, such as incorporating membranes, using catalytic membrane reactors, and replacing reaction-condensation with reaction-adsorption/absorption columns. [2]
Furthermore, finding new materials that can catalyze the reaction at lower temperatures and pressures could save energy costs. The most used catalysts in industrial Haber-Bosch processes currently are iron oxide catalysts, due to their low cost. [4] Iron alloys, such as Fe-Co and Fe-Ni, have higher catalytic activity but are more expensive. [4] Ruthenium-based catalysts are also used, but are less common because they are more expensive and less widely available than iron. [4] Cobalt and nickel-based catalysts have potential to replace ruthenium because they are less expensive than ruthenium and have high catalytic activity at lower temperatures and pressures. [4] However, iron oxides still dominate due to their lower costs, so a material that could modify or replace these iron oxides at a low cost would be necessary to have universal impact on the Haber-Bosch process conditions.
In summary, the Haber-Bosch process has been instrumental in supporting human population growth over the past century. On the other hand, it has also been a large contributor to worldwide greenhouse gas emissions because it requires a large amount of energy, and there is a large demand for ammonia. I am optimistic that future research can help make this process more sustainable or lead to different processes for ammonia production .
© Kathleen Bailey. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] C. Smith, A. K. Hill, and L. Torrente-Murciano, "Current and Future Role of Haber-Bosch Ammonia in a Carbon-Free Energy Landscape," Energy Environ. Sci. 13, 331 (2020).
[2] B. Lin et al., "Perspective on Intensification of Haber-Bosch to Enable Ammonia Production Under Milder Conditions," ACS Sustain. Chem. Eng. 11, 9880 (2023).
[3] Koretsky, M. D., "Engineering and Chemical Thermodynamics (2nd ed.)," Wiley. 13, (2012).
[4] M. R. Moghadam, A. Bazmandegan-Shamili, amd H. Bagheri, "The Current Methods of Ammonia Synthesis by Haber-Bosch Process," in Progresses in Ammonia: Science, Technology and Membranes: Production, Recovery, Purification and Storage, ed. by A. Basile and M. R. Rahimpour (Elsevier, 2024).
[5] "BP Statistical Review of World Energy 2022," British Petroleum, June 2022.
[6] "Mineral Commodity Summaries 2023," US Geological Survey, January 2023.