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Research Papers

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

[+] Author and Article Information
Adam Noglik, Martin Roeb, Thomas Rzepczyk, Christian Sattler, Robert Pitz-Paal

German Aerospace Center (DLR), Institute of Technical Thermodynamics, Solar Research, 51170 Köln, Germany

Jim Hinkley

CSIRO Energy Centre, Mayfield West NSW 2304, Australia

J. Sol. Energy Eng 131(1), 011003 (Jan 06, 2009) (7 pages) doi:10.1115/1.3027505 History: Received July 05, 2007; Revised February 29, 2008; Published January 06, 2009

## Abstract

A critical step of sulfur based thermochemical cycles for hydrogen production from water is the endothermic decomposition of sulfuric acid. The necessary heat can be provided by concentrated solar radiation. A solar receiver-reactor has been developed and built to investigate this process. Experiments with the test reactor were carried out in the DLR solar furnace in Cologne and confirmed the technical feasibility of a receiver-reactor containing porous ceramics for the decomposition of sulfuric acid. The receiver-reactor and strategy of operation were iteratively optimized with respect to chemical conversion and reactor efficiency. Parametric studies were conducted with varying partial pressure of $SO3$, residence time, absorber temperature, the presence of catalyst, and the performance of different catalysts to quantify their influence on chemical conversion and reactor efficiency. The absorber temperature distribution was found to be the most crucial process parameter. Conversions close to the equilibrium—in some cases exceeding 90%—were achieved with a platinum catalyst. Thermal efficiencies of up to 35% for the foam vaporizer and 30% for the overall reactor were achieved. Enclosing the absorber in a cavity and using separate chambers for the evaporation and the $SO3$-decomposition were identified as potential measures to improve the reactor.

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## Figures

Figure 1

Scheme of the iodine-sulfur cycle with concentrated solar radiation as the only energy source

Figure 2

Thermodynamic equilibrium of the H2SO4- and SO3-decomposition depending on the temperature at 1bar(11)

Figure 3

Maximum achievable conversion at 1bar for different initial mole fractions (x) of SO3 in nitrogen

Figure 4

Conversion of SO3 as a function of reaction time and temperature in the presence and absence of catalyst; r(SO3)=k*c(SO3)n; (a) without catalyst: n=1, EA=278kJ∕mol, (b) catalytic: n=0.45, EA=75kJ∕mol(7)

Figure 5

General design of the reactor

Figure 6

Foam vaporizer and inlet for sulfuric acid

Figure 7

Position of some representative thermocouples in the absorber

Figure 8

Reactor during operation in the solar furnace

Figure 9

Temperature distribution on the foam vaporizer (Tmax,FV=990°C) and on the honeycomb absorber (two maxima at 1178°C and 1140°C), respectively

Figure 10

Temperature distribution on the quartz pane

Figure 11

Temperature distribution in the honeycomb at selected locations

Figure 12

Approaching thermal equilibrium (temperatures measured in the honeycomb absorber by thermocouples)

Figure 13

Temperature distribution in the honeycomb structure in axial direction (temperatures measured in the honeycomb absorber by thermocouples)

Figure 14

Measured conversion of SO3 compared to the thermodynamic equilibrium, as a function of operating temperature and residence time (Pt catalyst)

Figure 15

Measured conversion depending on the residence time with platinum catalyst at Toperat=1200°C

Figure 16

Comparison of the measured conversions with and without a platinum catalyst

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