Solar Hydrogen Production from a ZnO/Zn Thermo-chemical Cycle

Participants:

• PSI (CH)

• ETH (CH)

Contact:

• Anton Meier, anton.meier@psi.ch
• Christian Wieckert, Christian.wieckert@psi.ch
• Aldo Steinfeld, aldo.steinfeld@eth.ch

Funding:

• BFE-Swiss Federal Office of Energy, PSI, ETH

Duration:

• January 1, 2003 - December, 2007

Schematic of the solar chemical reactor configuration: 1 = rotating cavity lined with sintered ZnO tiles,
2 = 80%Al2O3-20%SiO2 insulation,
3 = 95%Al2O3-5%Y2O3 CMC,
4 = alumina fibres, 5 = Al reactor shell, 6 = aperture, 7 = quartz window, 8 = dynamic feeder,
9 = front cone, 10 = rotary joint,
11 = chain wheel.

Solar power input, cavity temperature, and O2 mass flow rate in the product gases, with one feed-cycle of 398 g of ZnO. Evidence for ablation mode: ZnO dissociation reaches its maximum rate indicated by the peak O2 rate before the cavity wall temperature attains its stationary value of 1850 K.

Schematic of the “Solar TG” reactor used for testing quench units for Zn(g)-O2 separation.

• Abanades S., Flamant G. (2006) Thermochemical hy-drogen production from a two-step solar-driven water-splitting cycle based on cerium oxides, Solar Energy 80(12), 1611-1623.

• Karlsson M., Alxneit I., Ruetten F., Wuillemin D., Tschudi H.R. (2007) A Compact Setup to Study Homogeneous Nucleation and Condensation, Rev. Sci. Instrum. 78, 31102.

Background:

Hydrogen production from water using solar energy in a two-step thermochemical cycle is being investigated. The first, endothermic step is the thermal dissociation of ZnO(s) into Zn(g) and O2 at temperatures above 2000 K using concentrated solar energy as the source of process heat. The second, non-solar, exothermic step is hydrolysis of Zn at 700 K to form H2 and ZnO(s); the latter separates naturally and is recycled to the first step. H2 and O2 are derived in different steps, thereby eliminating the need for high-temperature gas separation.

Objectives:

• Solar chemical reactor technology for the production of Zn by thermal dissociation of ZnO
• solar chemical reactor modeling using CFD and Monte Carlo ray-tracing simulations;
• basic research on the reoxidation and quenching of Zn(g)
• production of H2 by hydrolysis of Zn.

Achievements in 2006

The engineering design of the solar chemical reactor for thermal dissociation of ZnO at above 2000 K has been improved, eliminating the materials problems in the previous reactor. It features a rotating cavity-receiver lined with ZnO particles that are held by centrifugal force. With this arrangement, ZnO is directly exposed to concentrated solar radiation and serves simultaneously the functions of radiant absorber, chemical reactant, and thermal insulator. The multi-layer cavity is made of sintered ZnO tiles placed on top of a porous 80%Al2O3-20%SiO2 insulation and reinforced by a 95%Al2O3-5%Y2O3 ceramic matrix composite, providing mechanical, chemical, and thermal stability and a diffusion barrier for product gases. 3D CFD was employed to determine the optimal flow configuration for an aerodynamic protection of the quartz window against condensable Zn(g). Experimentation was carried out in PSI’s high flux solar simulator with a 10 kWth reactor prototype subjected to radiative heat fluxes over the aperture exceeding 3000 suns (mean) and 5880 suns (peak). The reactor was operated in a transient ablation mode with semi-batch feed cycles of ZnO particles, characterized by a rate of heat transfer – predominantly by radiation – to the layer of ZnO particles undergoing endothermic dissociation that proceeded faster than the rate of heat transfer – predominantly by conduction – through the cavity walls.

Part of the continued reactor development work is aimed at optimizing the quench unit for avoiding Zn re-oxidation. Sepa-ration by rapidly quenching the gaseous products is a promising strategy requiring control of complex physical and chemical processes such as homogeneous nucleation of zinc vapor and oxidation of zinc droplets. Two complementary laboratory ex-periments have been set up:

• A test rig for studying the homogeneous nucleation and condensation of zinc vapor using adiabatic expansion in a Laval nozzle will be put into operation in spring 2007. It is based on a much simpler device previously used to verify the measurement principle by reevaluating the homogeneous nucleation of n-butanol

• A “Solar TG” (Solar Thermal Gravimeter; see ‎Figure 4.10) is equipped with a balance allowing for continuously monitoring the mass loss of thermally dissociated ZnO using concentrated radiation. The product gas contain-ing Zn(g) and O2 exits the reactor through a quench device that is tested under different conditions for maximum Zn yield. Work is in progress.

A comprehensive model describing the complex phenomena occurring during quenching of a Zn(g)-O2 mixture has been implemented. The model analyzes the chemical reactions in a mixture of zinc vapor, oxygen and inert gas subjected to a temporal temperature gradient. It predicts key properties of the product, such as chemical composition, i.e. average degree of oxidation, or particle size distribution. The model allows integrating the results of both experimental studies into a common framework and is used to identify and assess possible quenching strategies for the gaseous product exiting the solar ZnO dissociation reactor.