(33d) Molten-Phase Hydrolysis Stage Analysis and Experiments for the Calcium Bromine Thermochemical Cycle

The goal of the United States Department of Energy Nuclear Hydrogen Initiative linked with the Generation IV Nuclear Energy Systems Initiative is to develop a cost-effective, proliferation-resistant, low greenhouse gas emissions, and sustainable, nuclear-based energy supply system. The Nuclear Hydrogen Initiative Thermochemical Cycle investigation of the calcium-bromine cycle at Argonne National Laboratory combines both experimental and modeling studies of a novel continuous ?hybrid? cycle for hydrogen production, where ?hybrid? means that both nuclear heat and electricity are employed. The interest in engineering the calcium-bromine cycle for continuous operation is that it permits the use of components and materials which will operate in a consistent, non-cycling chemical and thermal environment. This paper focuses on the first and important calcium bromide hydrolysis stage to generate hydrogen bromide, which when split by electrolysis, produces hydrogen.

Previous work on calcium bromide hydrolysis supported CaBr2 on calcium-titanate substrates and ran at temperatures which were kept below the melting point of the CaBr2 (728 oC). However this led to low yields and other technical barriers associated with the steam-solid-phase reaction. Initially, Argonne used the COMSOL? multi-physics partial differential equation (PDE) solver to investigate two molten calcium bromide reactor concepts operating above the melting point of the CaBr2 (750 -780 oC). In the first steam is sparged into a calcium-bromide melt, and in the second approach molten CaBr2 droplets are sprayed counter-currently to upward-flowing steam. This paper presents results for both droplet and bubble modeling analyses and this work indicated that sparging steam into a calcium bromide melt is more feasible than spraying molten calcium bromide droplets into steam. Therefore, an engineering design study was initiated which indicates that the sparging reactor concept can be readily scaled to commercial size and that the endothermic heat of reaction can be effectively supplied by heat transfer coils embedded in the melt. In addition, the sparging reactor concept can handle the presence of calcium oxide produced in the calcium bromide melt over a range of concentrations up to 25%. A bench-scale hydrolysis reactor system was constructed that contacted steam with 0.5 kg of molten CaBr2; and the successful initial results from experiments with this system are discussed in this paper. The operation of the sparing reactor is quite stable and a continuous hydrolysis reactor can be designed based on the laboratory data obtained. Recommendations for the path forward are presented.

Argonne National Laboratory is a U.S. Department of Energy laboratory managed by UChicago Argonne, LLC, under Contract No. DE-AC02-06CH11357 with the U.S. Department of Energy.