Energies 2023 , 16 , 280
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regions. Huge energy losses are incurred during the transportation of electricity over longer distances but a 100% efficient pipeline transport of hydrogen is feasible—making hydrogen an economically attractive alternative for transporting large-scale renewable energy over long distances. In summary, hydrogen and its derivatives will allow high penetration rates of variable renewable energies, leading to a significant reduction of CO 2 emissions (avoiding up to 60 Gt CO 2 in 2021–2050, a 6.5% of total cumulative emission reduction [41]), playing a crucial role in hard-to-decarbonize sectors, and functioning as a catalyst for sector coupling (Figure 3).
Figure3. Role of green hydrogen in a carbon-neutral future. 2.2. Green Ammonia
Ammonia as the basis for all mineral nitrogen fertilizers forms the bridge between the nitrogen in the air and the foods we consume. The production of ammonia, however, is far from being clean. The nitrogen is captured from the air but almost all of the hydrogen required is currently produced from fossil fuels. Thus, the conventional production of ammonia is a very carbon-intensive process. The process accounts for 1.3% of global CO 2 emissions from the energy system [42] of which 80% originate from the hydrogen production stage [43]. This provides room for the decarbonization of ammonia synthesis where hydrogen production can be achieved through water electrolysis using low-carbon electricity sources (green hydrogen) to react with nitrogen from the air to form green ammonia. This green process of ammonia synthesis could potentially reduce the carbon footprint of conventional ammonia production from 1.6 to 0.1 tCO 2 /tNH 3 which can further reach near zero in the future with technological advancement [44]. Ammonia has an important role to play in the carbon-neutral future scheduled for the next three to four decades. As mentioned earlier, the 1.5 °C goal will lead to significant growth in green hydrogen demand, and the transport of green hydrogen from one region to another will become a common feature in this future transition. However, it is challenging to store, handle, and transport hydrogen. Though this is achievable with compressed or liquified hydrogen at − 253 °C, the process requires huge capital investments, energy (for cooling), energy losses due to cooling, and poses safety concerns. Alternatively, it is safer, easier, and cheaper to transport and store hydrogen in the form of ammonia. This is because, relative to volume, liquid hydrogen has a lower energy density than ammonia. Also, at − 35 °C, ammonia is already in a liquified state, and can then be easily and safely transported. In addition, the required infrastructure for transporting ammonia is already in place for decades as millions of tons of ammonia are annually transported by sea. About 20 Mt of ammonia (out of the 185 Mt of production) were globally traded in 2020 [42].
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