Skip to main content
Open Access Publications from the University of California

UC San Diego

UC San Diego Electronic Theses and Dissertations bannerUC San Diego

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 pumped

Main Content
For improved accessibility of PDF content, download the file to your device.
Current View