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This solar reactor will recycle carbon dioxide.
A
prototype solar reactor
that will break carbon dioxide into carbon monoxide and oxygen, or
split water into hydrogen and oxygen, will be started up this spring by
researchers at Sandia National Laboratories (Albuquerque, N.M.; edlinks.che.
com/7368-535). The ultimate goal of the so-called "Sunshine to
Petrol" project is to recycle sequestered CO2 and combine the
CO product with H2 to synthesize liquid fuels.
The
cylindrical reactor contains 14 alumina rings, 12-in. dia. by 1/2-in. thick, whose
perimeters support 1in.-high monolithic segments of cobalt-substituted
ferrite (COxFe3-x O4) The disks are arranged along a single axis, but two
motor-driven shafts rotate alternating disks clockwise and the rest
counter-clockwise, at a rate of about 1 rpm.
In the upper part of the reactor the ferrite segments are
heated to 1,400-1,500°C by concentrated solar energy, via a quartz window, causing the evolution of 02. However, by the time the heated
section of a disk rotates to the lower, "dark" side of the reactor, its temperature drops to about 1,100°C. This
temperature is sufficient to break down CO2 or split water, either
of which is fed into the bottom of the reactor, says Jim Miller, a principal member of
technical staff.
Oxygen released
from either reaction re-oxidizes the thermally reduced ferrite. Normally, thermal
breakdown of CO2 or water requires temperatures of "well
above 2,000°C," says Miller, "but by dividing the reaction into
two steps we can accomplish thermal splitting at much lower temperatures.
The cobalt helps by altering the thermodynamics of the reduction step."
Even more important, he adds, the counter-rotation of the disks allows the
reduction and oxidation reactions to occur simultaneously, at different
temperatures, while minimizing heat loss. "In effect," says Miller,
"we have a counter-current heat exchanger that improves the maximum
potential thermodynamic efficiency from about 35% to more than 70%."
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