Development and Test Operation of a Demonstration Plant for Sulfuric Acid Splitting at the DLR Concentrating Solar Power Tower Facility

Sulfuric acid splitting is a key step of the hybrid sulfur cycle (HyS) for solar thermochemical hydrogen production. This exothermal reaction can be divided into two steps: firstly, the evaporation of liquid sulfuric acid (H2SO4) at about 400 °C forming sulfur trioxide (SO3), and secondly, the decomposition of SO3 to sulfur dioxide (SO2) and oxygen (O2) at 800 – 1000 °C. While the first sub-reaction has fast kinetics, the second one is rather slow and requires the introduction of catalysts to achieve sufficient conversion. Since 2004 the concept of a solar receiver-reactor for sulfuric acid splitting has been developed by DLR and operated in its solar furnace during the European projects HYTHEC and HycycleS. In the follow-up European project SOL2HY2, a scale-up of this concept has been designed, developing a solar tower demonstration plant. This demonstrator has a design flow rate of 1 l/min of sulfuric acid (50 w%) and consist of four main components arranged in series and connected by Joule heated piping: a 60 kW electrical evaporator for vaporization of the liquid acid, a solar receiver for superheating the SO3 to about 1000 °C, an adiabatic reactor with a fixed bed of an iron(III) oxide catalyst and a scrubber. The Joule heated evaporator consists of six vertical steel tubes with a siliconized silicon carbide (SiSiC) inner tubes filled with porous SiSiC foam structures for enhanced heat transfer. Liquid acid is injected at the bottom and vaporizes while passing upwards through the pipes. The vapors are collected in a steel manifold and, subsequently, conveyed to the solar receiver with an outer shell also made of steel. Concentrated radiation from the solar field passes through a quartz glass window closing the receiver and heats up a sectioned absorber composed of porous SiSiC foam structures. After superheating in the receiver, the process gas passed through the catalyst bed for an adiabatic reaction forming SO2. Before neutralization with sodium hydroxide solution in the scrubber, the SO2 concentration is measured by a customized gas analysis system via UV/Vis spectroscopy. The pilot plant was constructed and assembled on the research platform of the DLR concentrating solar power tower facility in Juelich, Germany. The layout of the plant was accompanied and supported by thermo-mechanical modelling of the most important components like the evaporator and solar receiver. Initial operation of the demonstrator was performed with air and water as process fluids. During water operation, the solar receiver reached the predicted design parameters achieving a gas outlet temperature of 1000 °C at an absorber front temperature of 1200 °C and a solar power on aperture of 50 kW. In the adiabatic reactor, however, temperatures of only about 400 °C were measured which are too low for SO3 decomposition. Therefore, the system was modified placing the catalytic fixed bed directly behind the solar absorber in the outlet section of the receiver. In the following test runs, the temperatures of this adiabatic reaction zone were sufficiently high with a minimum temperature in excess of 750 °C below which the catalyst would be deactivated due to sulfate formation. As a result, testing could proceed with sulfuric acid as the feed successfully demonstrating decomposition of SO3. A detailed analysis of all results of the systematic on-sun test series is given in the present paper