Modeling of a new four-step magnesium–chlorine cycle with dry HCl capture for more efficient hydrogen production

• A new four-step Mg–Cl cycle is developed and simulated.
• An HCl capturing process is introduced for a decreased electrical work consumption.
• The proposed cycle is a more feasible option than the existing three-step Mg–Cl cycle.

In this study, a new four-step Mg–Cl cycle is introduced as an alternative to the conventional three-step cycle in order to capture the HCl from the hydrolysis reaction in dry form and enhance the cycle performance. Instead of direct hydrolysis of MgCl2, an intermediate step is considered to produce MgOHCl from the hydrolysis reaction where this product is then decomposed into fine MgO and dry HCl in a decomposition reactor. A separation process after the hydrolysis step is also introduced to capture HCl gas in dry form. The heat requirements of all individual cycle components are evaluated and compared with the conventional Mg–Cl cycle. Decreasing electrical work consumption enhances the exergy efficiency of the cycle and makes it a more feasible one compared to water electrolysis and the three-step Mg–Cl cycle configuration. A successful dry HCl capturing process makes this cycle 13% more efficient than water electrolysis, in terms of electricity consumption, and thermodynamically more efficient than the existing three-step configuration.