The versatility of the molecule has led to its distribution at a commercial and global scale, with a market of 180 million tonnes produced annually worldwide, (13) making it the second most commercialized chemical in the world.
Moreover, liquid ammonia has a greater volumetric hydrogen density than liquid hydrogen itself (i.e., liquid hydrogen at 20 K has approximately 70 kg of H 2/m 3, while liquid ammonia at 300 K and 1.0 MPa has 106 kg of H 2/m 3 (12)). As a product of nitrogen and hydrogen, ammonia can be stored under easy conditions (i.e., refrigerated at −33 ☌ at atmospheric pressure or at 0.8–1.0 MPa under atmospheric temperature), thus making it a versatile, easy to store medium. (10) Moreover, other barriers, such as its high diffusivity, low energy volumetric density, high flammability range, etc., still make it non-economically viable for a zero-carbon transition, thus requiring another molecule.Īmmonia has been recently presented by authors and global organizations as a zero-carbon molecule that can provide the required energy storage medium for renewable sources. Thus, storage costs are between 26 and 30 times higher for hydrogen than for ammonia. In comparison to other molecules, such as ammonia, hydrogen storage can only be cheaper than the use of ammonia when salt caves are employed, (10) although this only applies over limited periods of time. However, the creation of a “hydrogen economy” faces several constraints that still require the development of equipment and, overall, infrastructure. (9) Some scenarios (9) foresee the use of hydrogen in up to 50% end-use demand applications by the middle of the 21st century. Numerous paths for its production, distribution, and consumption present a complex variation and trade-off between costs, emissions, scalability, and requirements. Hydrogen, the most common molecule in the universe, is currently under assessment everywhere. Herein, this work sets the principles, research, practicalities, and future views of a transition toward a future where ammonia will be a major energy player. Further, the fundamental boundaries of the use of the molecule are expanded to real technical issues for all potential technologies capable of using it for energy purposes, legal barriers that will be faced to achieve its deployment, safety and environmental considerations that impose a critical aspect for acceptance and wellbeing, and economic implications for the use of ammonia across all aspects approached for the production and implementation of this chemical as a fueling source. State-of-the-art knowledge provided by academics actively engaged with the topic at various fronts also enables a clear vision of the progress in each of the branches of ammonia as an energy carrier. Different from other works, this review seeks to provide a holistic perspective of ammonia as a chemical that presents benefits and constraints for storing energy from sustainable sources. Fundamentals of combustion and practical cases for the recovery of energy of ammonia are also addressed, thus providing a complete view of what potentially could become a vector of crucial importance to the mitigation of carbon emissions. The review starts from the importance of ammonia as an energy vector, moving through all of the steps in the production, distribution, utilization, safety, legal considerations, and economic aspects of the use of such a molecule to support the future energy mix. On that basis, this review has approached the subject gathering inputs from scientists actively working on the topic. Therefore, current efforts in this field are leading scientists, industries, and governments to seriously invest efforts in the development of holistic solutions capable of making ammonia a viable fuel for the transition toward a clean future.
Still, the current need to decarbonize our economy makes the search of new methods crucial to use chemicals, such as ammonia, that can be produced and employed without incurring in the emission of carbon oxides. However, the particular characteristics of the molecule always made it a chemical with low, if any, benefit once compared to conventional fossil fuels. Ammonia, a molecule that is gaining more interest as a fueling vector, has been considered as a candidate to power transport, produce energy, and support heating applications for decades.
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