Adam Noglik

Solar Thermochemical Generation of Hydrogen: Development of a Receiver Reactor for the Decomposition of Sulfuric Acid

A key step in the sulfur based thermochemical cycles for hydrogen production is the highly endothermic decomposition of sulfuric acid at temperatures between 800 °C and 1200 °C. This reaction can be carried out in a receiver-reactor which is irradiated with concentrated solar radiation from a heliostat field. To investigate this process a test reactor was developed and built. The reaction takes place on the surface of a catalytically coated porous absorber irradiated through a quartz pane of the receiver-reactor. This concept has the advantage of a minimum number of heat transferring steps. Experiments with the test reactor were performed in the DLR solar furnace in Cologne. Firstly the feasibility of a solar decomposition of sulfuric acid in a receiver-reactor containing volumetric absorbers was investigated and proven. Then the reactor was qualified at different operating points. Finally the receiver-reactor and strategy of operation was iteratively optimized with respect to chemical conversion and reactor efficiency. Several test series were performed with variation of the absorber temperature, the mass flow and the dilution rate. Partial pressure of SO3 , residence time, absorber temperature, and the kind of catalyst applied were identified and quantified as parameters with the most relevant influence on chemical conversion and reactor efficiency. The operation behavior observed and the detailed knowledge of dependencies of different operation parameters assist in evaluating the potential of scaling up the described technology.Copyright

Simulation of a Volumetric Solar Receiver-Reactor for Hydrogen Producing Thermochemical Cycles

Sulfur based thermochemical cycles for hydrogen generation from water have one reaction step in common which is the decomposition of sulfuric acid as one of the most energy consuming steps. The present work deals with the development of a dynamic mathematical model of a solar reactor for this key step. One of the core parts of the model is a part model of the reaction kinetics of the decomposition of sulfur trioxide, which is based on experiments investigating the kinetics of the used catalyst platinum coated on a ceramic solar absorber. Other part models describe e. g. the absorption of solar radiation, heat conduction in the absorber, convection between gas and the absorber walls and energy losses due to heat radiation. A comprehensive validation of the reactor model is performed using measured data which is gained in experiments with a prototype reactor. The operating behavior of the real reactor is compared with the results of the numerical simulation with the model. The validation is in particular done by reproducing the influences of individual parameters on the chemical conversion and the reactor efficiency. The relative deviations between experimental data and simulation results are mostly within the range of measurement accuracy. In particular the good agreement of calculated values of the derived parameters SO3 conversion and reactor efficiency with those determined from the experiments qualify the model for optimization purposes.Copyright

Experimental Study on Sulfur Trioxide Decomposition in a Volumetric Solar Receiver-Reactor

Process conditions for the direct solar decomposition of sulfur trioxide have been investigated and optimised by using a receiver-reactor in a solar furnace. This decomposition reaction is a key step to couple concentrated solar radiation or solar high temperature heat into promising sulfur based thermochemical cycles for solar production of hydrogen from water. After proof-of-principle a modified design of the reactor was applied. A separated chamber for the evaporation of the sulfuric acid, which is the precursor of sulfur trioxide in the mentioned thermochemical cycles, a higher mass flow of reactants, an independent control and optimisation of the decomposition reactor were possible. Higher mass flows of the reactants improve the reactor efficiency because energy losses are almost independent of the mass flow due to the predominant contribution of re-radiation losses. The influence of absorber temperature, mass flow, reactant initial concentration, acid concentration, and residence time on sulfur trioxide conversion and reactor efficiency have been investigated systematically. The experimental investigations was accompanied by energy balancing of the reactor for typical operational points. The absorber temperature turned out to be most important parameter with respect to both conversion and efficiency. When the reactor was applied for solar sulfur trioxide decomposition only, reactor efficiencies of up to 40% were achieved at average absorber temperature well below 1000 °C. High conversions almost up to the maximum achievable conversion determined by thermodynamic equilibrium were achieved. As the reradiation of the absorber is the main contribution to energy losses of the reactor a cavity design is predicted to be the preferable way to further raise the efficiency.Copyright

NUMERICAL OPTIMIZATION OF A VOLUMETRIC SOLAR RECEIVER-REACTOR FOR THERMOCHEMICAL HYDROGEN GENERATION VIA DECOMPOSITION OF SULFUR TRIOXIDE

A basic concept for a receiver-reactor for solar sulfuric acid decomposition as the key step of thermochemical cycles for hydrogen production has been developed and realized. A prototype reactor has been built and is specialized for the second part of the reaction, the decomposition of sulfur trioxide. For a detailed understanding of the operational behaviour of the developed reactor type a mathematical model was developed. The reactor model was validated using experimental data from the prototype reactor test operation. The present work deals with the optimization of process and design parameters and the evaluation of the achievable performance of the reactor type. Furthermore the reactor model is used for numerical simulations to predict operational points, which are not easy to realize in experiments due to hardware limitations, to save the experimental effort, and to predict the performance of a large-scale reactor on a solar tower. The results of the simulation confirm a central finding of the experiments: Depending on the operation conditions an optimum of reactor efficiency emerges if one parameter is varied. This is in particular true for the absorber temperature. Two oppositional effects compensate each other in a way that the reactor efficiency exhibits a maximum at a certain temperature: by increasing process temperature the reradiation losses increase disproportionately high whereas the chemical conversion decreases when lowering the temperature. Beyond that influences of other operational parameters like feed mass flow, residence time, and initial concentration of the acid were also analyzed. In a scale-up study the reactor was simulated as being part of the aperture area of a large scale tower receiver. The main differences to the prototype system are the diminished gradients of solar flux on the receiver front face and the reduced thermal conduction losses due to the presence of several neighbor modules at comparable temperature level. This leads to higher chemical conversions and better efficiencies. Reactor efficiencies up to 75% are predicted. Even higher efficiencies are possible if re-radiation losses can be decreased, e.g. by considering a cavity design.Copyright