Hydrogen production with fixed bed chemical looping systems

Thermochemical Hydrogen Storage via the Reversible Reduction and Oxidation of Metal Oxides

“…Both steps in this cycle involve solid–gas reactions, which depend on temperature and pressure, the macro- and microstructure of the solid phase, and the heat and mass transfer characteristics of the chemical reactor configuration. The reaction rate can be limited by the surface reaction rate (adsorption, desorption, and reaction), gas-phase species transport, solid-phase species transport, and heat transfer. ,,, In this section, the kinetic rates of the reduction and oxidation steps with iron-based materials are assessed by reviewing the literature.…”

“…The cycling stability can be increased by adding support materials with a high melting point to resist sintering . The support materials can also be used to improve the mechanical stability. ,, Al 2 O 3 and ZrO 2 are among the most common support materials for iron-based oxygen carriers. ,, Other additives that have shown positive effects on stability include TiO 2 , SiO 2 , MgAl 2 O 4 , ytrria-stabilized zirconia YSZ, Cr 2 O 3 , MoO 3 , CeO 2 , and perovskites. , There are also good results with mixed oxides such as a mixture of 7.5 wt % SiO 2 , 5 wt % Al 2 O 3 , and <2.5 wt % CaO added to iron, which was stable under cycling at temperatures below 750 °C .…”

“…The challenges in translating this research to hydrogen storage are in finding additives that are (i) particularly suited for cycling stability at lower temperatures; (ii) low cost; and (iii) minimize added weight, such that the storage density of the carrier remains high. SiO 2 and Al 2 O 3 are both particularly cheap additives that have shown good potential for stabilizing chemical looping materials. ,,, …”

“…Options proposed include underground pressurized storage in the gas phase, storage in compressed hydrogen tanks, physisorption, and chemical hydrides. − An alternative approach is the thermochemical storage using a reversible metal oxide redox cycle with H 2 as a reducing agent and H 2 O as an oxidizing agent, , given by the reversible reaction and illustrated in Figure using iron oxide as an example. The oxidation reaction has been applied for high-purity hydrogen production . This storage route using iron oxide was proposed for application in fuel cell vehicles. , However, on-board storage in H 2 -fueled vehicles sets stringent requirements on energy density, process conditions, and charging/discharging response times, which are unlikely to be met with this cyclic redox process.…”