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Background
High temperature solid looping cycles involve the use
of a solid reactant to transfer either CO2 or O2 from
one reactor to another. For example, CO2 at relatively
low concentration can be scrubbed from flue gases, with
the sorbent then being regenerated to yield a pure stream
of CO2. Metal oxides can transport oxygen from the air
to react with fuel, effectively “burning”
the fuel to yield a pure stream of CO2 and H2O, which
can then be easily separated and the CO2 stored. Alternatively,
there exist a number of methods of producing H2 or syngas
from hydrocarbon-based fuels, while simultaneously producing
pure CO2.
The carbonation/calcination reactions
occur at temperatures higher than those used in the
steam cycle of conventional coal-fired power plants,
so that it is theoretically possible to recover the
heat used to regenerate the sorbent at temperatures
suitable for highly efficient modern power generation.
There are energy losses due to the need to produce pure
oxygen for firing the calciner and for compression of
the captured CO2 but the process has intrinsic efficiency
advantages as additional power can be generated from
the capture system. Although the favoured feedstock,
limestone, is abundant and cheap, the processes based
on calcination/ carbonation have suffered from loss
of reactive capacity after a number of cycles and from
attrition of the sorbent material. Recent studies on
sorbent reactivation, the role of the sorbent residual
activity and the development of more durable synthetic
sorbents (together with preactivation of limestone)
have shown that the process is feasable. Integration
of the looping cycle with the production of cement (a
natural fit because the waste CaO from the calcium looping
cycle can be used as feedstock for cement manufacture,
replacing the CaCO3 generally used, directly reduces
the CO2 emissions of both industries) is a subject of
considerable interest and ongoing research.
Figure 2. CANMET Energy Technology Center mini pilot-scale
sorbent looping test facility. Courtesy of Professor
E.J. Anthony, CanMet, Canada.
The other high temperature application often referred
to as “chemical looping” combusts fuel by
reducing solid metal oxides which are then re-oxidised
in the other half of the cycle. This is effectively
a form of oxy-combustion and theoretically has the potential
to be a very efficient form of CO2 capture. While fixed-bed
reactors may be used, a circulating fluid bed system
may be more appropriate at an industrial scale. Chemical
looping combustion (CLC) , is a method of indirect combustion
where fuel and air are never mixed. The concept has
therefore been classified as “unmixed combustion”
. Metal oxides are used to selectively transport oxygen
from air to fuel in the solid phase. If a suitable metal
oxide is used as the oxygen carrier, the CLC system
can be operated in such a way that the exhaust gas of
the fuel reactor ideally consists of CO2 and H2O only
and allows for subsequent water condensation, compression,
and storage of CO2. The costly gas–gas separation
steps are inherently avoided. Therefore, CLC is one
of the most energy-efficient approaches to carbon capture
from power production or fuel upgrading.
Figure 3. 120 kW Chemical Looping
test rig (Courtesy of Tobias Proell and Christoph Pfeiffer,
TU Vienna, Austria).
There is a great deal of synergy between research
into “chemical” and “calcium”
looping cycles; attrition and mechanical stability of
particles is an important issue, as are the fluid dynamics
of the reactors used, which are themselves likely to
be very similar.
Although these processes might be classified as post
combustion or oxy-combustion for which IEA GHG networks
already exist it is felt that the technology they use
is different, somewhat specialised and hence better
dealt with in a separate network.
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