A network to promote further development and scale-up of processes for CO2 capture which involve solid looping cycles operating at elevated temperatures.
The high-temperature solid looping network brings together researchers and developers of technology to capture CO2 at high temperature in cyclical processes using either circulating or fixed beds of solids. The technology has advanced considerably in the last few years and recently several large pilot plants have been constructed and brought into operation. This important step is expected to enable convincing demonstration of the potential for the technology to work at industrial scale to be done.
The network is progressively expanding participation beyond the research community, businesses, plant designers and equipment suppliers as the technology moves rapidly through pilot and industrial demonstration towards full-scale commercial deployment.
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.
Calcium Looping Cycles
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.
The favoured feedstock, limestone, is abundant and cheap but more effective and durable synthetic alternatives are also being developed. In recent years considerable advances have been made in combating loss of reactive capacity and attrition of the sorbent after a number of cycles to the point that these are no longer barriers to implementation. In addition, the use of spent sorbent to produce cement has been demonstrated.
Chemical Looping Combustion
Another 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.
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 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 consists of CO2 and H2O only and allows for subsequent water condensation, and 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. Despite this, it cannot compete in gas-fired power generation but may have a role in gas-fired steam generation such as is required for extraction of shale oil. A key objective is to develop this technology to be able to completely combust coal which, if successful, would yield a carbon dioxide capture process whose only energy penalty is the CO2 compression energy.
Fixed Bed Systems
A variety of combinations of sorption with combustion or gasification reactions is possible and such processes promise enhanced efficiency over thoseconducted in separate steps.
Pressure as well as temperature need to be varied to make such processes efficient and this can be achieved easily by using fixed beds in cyclic operation.
One example is the sorbent-enhanced water gas shift reaction,SEWGS, being tested by ECN under the EU- and industry-funded CACHET project. (Shown in photograph on the right- Photograph courtesy of ECN.)
Large-scale demonstrations such as that represented by the EU FP7 CAOLING project (www.caoling.eu) are in full swing and are expected to yield the results necessary to progress to commercial-scale plant designs.
The picture below shows the recently constructed 1,7 MWt demonstration plant at La Pereda in Spain. The project, code named CAOLING, with the main aim of building this pilot plant, is co-funded by the EU Commission. Inset in red is a picture of an earlier pilot plant at INCAR in Oviedo, Spain, clearly showing that scale-up is proceeding apace. Other locations where large pilot plants have been built are:
TU Darmstadt 1MW: chemical looping combustion/calcium looping
IFK Stuttgart 200kW: solid looping pilot
TU Vienna 120kW: chemical looping combustion
ECN (Netherlands): sorbent-enhanced water gas shift (SEWGS) pilot plant
Meeting and Summary Reports
This network builds upon four preceding international workshops on in-situ CO2 removal, ISCR. The fourth and last meeting of the ISCR series was held at Imperial College, London, in July 2008. Click here for presentations from the meeting.
For details of the network meetings and past presentations, please login to the members area.
IEAGHG plans to hold the next High Temperature Solid Looping Cycles Meeting in 2017 in Lulea in Sweden. For more details, please log in to the members area
Membership of this network is open to countries and industries actively engaged in practical research on CO2 capture, or seeking ways to promote such activities.
If you are NOT already registered on this site, click here to register for membership of this network.