Thermochemical Production of Sulfur as Solar fuel: Design and development of moving bed heat exchanger for sulfuric acid evaporation heated by ceramic particles

Abstract

In the context of solar energy systems, concentrated solar thermal power plants offer sustainable, low carbon energy technologies to generate baseload electricity. In next generation receivers developed at DLR, granular particles are used as thermal energy storage medium as they may store and transport the heat at high temperatures. In this presented work, a new concept of a moving bed heat exchanger (MBHE) is developed for the sulfuric acid vaporization with ceramic particles as the heat transfer media. Based on a literature Research several concepts are compared eventually selecting a tube and shell ountercurrent heat-exchanger. As the hot ceramic particles flow downwards along the tubes, the heat from them is used to evaporate sulfuric acid (H2SO4) into Sulfur trioxide and water (SO3+H2O) inside the tube-type reactor, which is a section in the process of thermo-chemical production of sulfur. Being a highly corrosive fluid by nature, precautions were taken in dealing with the material selection of the tubes and the tube inlets where direct contact with the sulfuric acid is expected. Based on the correlations of flow boiling heat transfer coefficient and particle bed heat transfer coefficient, a system model was developed and implemented with the commercial software package Engineering Equation Solver (EES) which is coupled with a program for thermal design of the MBHE. The complete model is discretized into four zones depending on the energy balance. As the mass flow rate plays a key role, different associated parameters are analyzed in order to design the geometry of the system (e.g. tube length, diameter and arrangement). When the particles reach the bottom of the MBHE, they cool down and exit from a hopper. This hopper is designed considering the rheological properties of the ceramic particles and a suitable feeder is selected, which is connected below the hopper to regulate the mass flow rates of the particles. Finally, for a selected mass flow rate a complete design of the sulfuric acid evaporator is implemented in the CAD software Autodesk Inventor®.