Ammonia Production via a 2-Step Al2O3/AlN Thermochemical Cycle

The production of ammonia via a 2-step cyclic process is proposed as an alternative to its conventional production by the Haber-Bosch process. The first, endothermic step is the production of AlN by carbothermic reduction of Al₂O₃ in a N₂ atmosphere at above 1500°C. The second, exothermic step is the steam-hydrolysis of AlN to produce NH₃ and reform Al₂O₃; the latter is recycled to the first step. Further, the endothermic reduction step can be carried out using concentrated solar energy as the source of process heat. Figure 1 illustrates the proposed cycle.

1st step: Al₂O₃ + 3C + N₂ → 2AlN + 3CO (1a)

Al₂O₃ + 3CH₄ + N₂ → 2AlN + 6H₂+ 3CO (1b)

2nd step: 2AlN + 3H₂O → Al₂O₃ + 2NH₃(2)

Fig. 1: Scheme of the 2-step cyclic solar thermochemical cyclic process for ammonia production. The first, endothermic step is the solar production of AlN by carbothermic reduction of Al₂O₃ in a N₂ atmosphere. The second, exothermic step is the steam-hydrolysis of AlN to produce NH₃ and to reform Al₂O₃; the latter is recycled to the first step.

The proposed 2-step process offers the following four-fold advantages:

it eliminates the need for high pressure, minimizing costs and safety concerns;
it eliminates the need for catalysts; minimizing costs associated with their production and recycling;
it eliminates the need for hydrogen as feedstock, reducing energy consumption and associated CO₂ emissions.
It eliminates concomitant CO₂ emissions derived from fossil-fueled endothermic processes.

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