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Achievements
in 2006
Roughly 30
cycles in the 1200-2300 K temperature range were selected as suitable
for concentrated solar energy using defined criteria. Primarily two-step
and three-step cycles were selected for their ease of implementation,
which implies favorable economics.
Energy and
exergy cycle efficiencies were evaluated. It was shown that exergy losses
mainly occur during the quenching step (for volatile oxides) and the
reduction step because of high operating temperatures of the solar reactor (typically
2000 K).
In order to
validate the reaction schemes, experimental studies were conducted on each
reaction involved in selected cycles (such as two and three-step metal oxide
cycles) for determining operating conditions, chemical conversion and
kinetic data. For example, experimental data have been improved for the
two-step iron oxide cycle: (1) Fe3O4 ® 3FeO + ½O2 and (2) 3FeO + H2O ® Fe3O4
+ H2 (Ref. REF _Ref161138872 \r \h [4.21]).
This cycle uses cheap and abundant chemical components but hydrolysis of
wustite (FeO) is limited because of the formation of a Fe3O4 diffusion
barrier. Nevertheless, conversion of 70% was obtained using 30-50 mm FeO
particles. Consequently, the iron oxide cycle may be an option for future
solar processes.
Innovative
cycles have been demonstrated such as the new cerium oxide cycle. The main
interest in this cycle is the high reactivity with steam of Ce2O3, the main
drawback being the partial vaporization of CeO2 during the reduction step at
reduced pressure (0.1-0.2 bar). This cycle opens the door to a new family of
mixed oxide cycles containing cerium oxides.
A lab-scale
cavity-type solar reactor for the production of reduced metal oxides was
designed and simulated. It is compatible with either volatile or
non-volatile continuous metal oxide processing and can be operated under
controlled atmosphere.
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