The common approach to hydrogen economy is illustrated in Fig. 5.2 and consists of the following steps: production of hydrogen from primary energy sources at some locations, hydrogen distribution (infrastructure not yet developed), and hydrogen delivery on vehicles where it is used for power generation/propulsion. Our aim here is to assess the total cost of hydrogen per unit mass at the utilization point (i.e., on the vehicle) for two alternative layouts of the hydrogen transportation economy. The first layout was just introduced and illustrated in Fig.5.2. The second layout refers to the hydrogen from ammonia option and will be introduced later.
Fig 5.2 The layout of hydrogen economy for transportation.
For a preliminary analysis, let us assume that hydrogen is produced from liquid water, either by electrolysis or by thermo-chemical splitting. The corresponding reaction and the associated (ideal) reaction heat are
Here, in the ideal situation for every kmol of produced hydrogen one needs AHh o = 286 MJ/kmol H2 of energy to split the water molecule. Obviously, the real energy needed to drive this process is larger than the theoretical one due to the imperfections. After produced, the hydrogen has to be stored in buffers at the production place, and then charged on vehicles specialized for pressurized or cryogenic hydrogen transport, then is distributed to fueling stations, and finally delivered to the consumers vehicles. Along this complex chain the cost of hydrogen will increase with amounts proportional to the energy specific to each phase. We will later study the distribution costs and discuss the total cost of hydrogen delivery.
We now introduce the second alternative where, instead of hydrogen, ammonia is produced at a synthesis plant then buffered locally, then distributed to fueling stations, and charged on vehicles where it is reformed to hydrogen by thermal cracking of the ammonia molecule. This scenario is illustrated in Fig. 5.3. Only ~12% from ammonia's HHV is needed for reforming. There is enough heat on board of a vehicle: the most advanced H2ICEs have efficiency of 50-60% and the most advanced fuel cell systems of 60-70%; the rest of the hydrogen energy is dissipated as heat. Thus, the onboard reforming process is "for free". Exception may make only PEM fuel cell system for which the heat being rejected at low temperature is unsuitable for ammonia cracking. In this case, which is not analyzed now, a small part of the produced hydrogen can be combusted for generating the reformation heat.
Furthermore, ammonia is industrially produced from hydrogen and nitrogen via the well-established Haber-Boch process. Hydrogen can be obtained from water (gasification, thermochemical water splitting, or electrolysis) according to the reaction (5.1), while the nitrogen comes from the atmospheric air. In order to obtain an ammonia quantity corresponding to 1 mole of hydrogen, the following reactions have to be considered (ideal case)
|H2O ^ H2 + 1/2O2 + 286 MJ I H2 +13 N2 ^ 2/3 NH3 - 30.7 MJ
Thus the energy needed to produce 1 mole of hydrogen embedded in ammonia is 286-30.7 MJ or AH^ = 255.3 Mj/kmol H2. Therefore, on a mass basis the cost of NH3 over the cost of H2 can be estimated as proportional to the energy for their synthesis as
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