Study of the thermochemistry for oxygen production for a solar sulfur-ammonia

The purpose of this study was to investigate the oxygen sub-cycle of a solar- thermochemical water-splitting cycle for hydrogen production. The study focuses on the thermal decomposition of molten salts in a sulfur-ammonia cycle, which evolves ammonia and SO₃ in two reactors. The molten salts proposed are a mixture of (NH₄)₂SO₄, K₂SO₄, K₂S₂O7, Na₂SO₄, and Na₂S₂O7. For this cycle to work, the salts must remain liquid with low viscosity for pumping, and ammonia and SO₃ must be released separately. Therefore, melting temperatures and the viscosity of various salt mixtures were measured. Thermogravimetric, differential thermal (TG/DTA), residual gas (RGA), and chemical analyses were employed to study the salt decomposition. The mixtures of K₂SO₄+4K₂S₂O7+Na₂SO₄+ 4Na₂S₂O7 and K₂SO₄+ 9K₂S₂O7+Na₂SO₄+ 9Na₂S₂O7 with melting temperatures 343±11°C and 332±7.8°C, respectively, would be appropriate for the streams between the reactors. The TG/ DTA showed that (NH₄)₂SO₄+2K₂SO₄+8K₂S₂O7 decomposition resulted in separate ammonia (269°C) and SO₃ release (373°C). The RGA data for (NH₄)₂SO₄+K₂SO₄+4K₂S₂O7+Na₂SO₄+ 4Na₂S₂O7 decomposition suggested that 25°C separated the end of ammonia (475°C) and the start of SO₃ release (500°C). This was confirmed by monitoring the pH of a solution through which the evolved gas was bubbled. The mass balance experiment showed that, when held at 475°C for 60 minutes, 21% of the ammonia expected to be evolved was released. The addition of 10g of water to 2g of salt mixtures resulted in 58% of the ammonia expected being released. The viscosity of K₂SO₄+4K₂S₂O7+Na₂SO₄+4Na₂S₂O7 and K₂SO₄+9K₂S₂O7+Na₂SO₄+9Na₂S₂O7 ranged from 2.6cP to 9.3cP between 393°C and 510°C, which means that they could be pumpe