CO₂ capture using lime or metal (looping)

One of the major challenges of our time is to limit emissions of CO₂ from the burning of fossil fuels in order to prevent global warming. A number of CO₂ capture processes are currently being investigated – e.g. MEA scrubbing and oxyfuel burning. However, they have the disadvantage of large losses in efficiency of around 10-14%, which calls into question the economic viability and acceptance of these processes.

 

The carbonate looping principle represents an attractive alternative to other systems, as the energy consumption associated with it is relatively low and, for this reason, this approach is very well-suited as a retrofit technology for existing plants. This loop has already been investigated for use in integrated reforming processes. Compared to other processes, it has the advantage that the absorbent CaCO₃ (limestone) is available in large quantities in nature and is inexpensive.

 

Another alternative is the so-called chemical looping process, an innovative technology for converting fossil fuels for the purpose of energy production: This process has two reactors in place of the conventional combustion chamber. In one of the reactors – the air reactor – a metal oxide MyOx-1 (oxygen carrier) is oxidised to MyOx using oxygen, while in the other reactor – the fuel reactor – the oxidised oxygen carrier is reduced again using the fuel. The overall result is a two-stage combustion process which produces a flue gas stream containing only CO₂ and H₂O. Chemical looping is thus a flameless combustion process that does not produce any pollutants such as NO_x. The pure CO₂ that results once the water has been removed by condensation can then be compressed. The process is based on the fluidised-bed technology that is already used in steam boilers. R&D work still remains to be done on chemical looping applications for solid fuels such as coal.

Sorption processes (carbonate looping) and oxygen production processes (chemical looping) are both redox-based processes. In both processes, carrier materials are circulated in a loop. With carbonate looping, CO₂ is absorbed and then calcinated, and the carrier material (e.g. calcium) is circulated in a loop. In the case of chemical looping, a metal oxide is circulated in a loop as the oxygen carrier (alternating reduction and oxidation). Carbonate looping is a post-combustion process, while the chemical-looping process supplies oxygen for an oxyfuel process.

Both processes are currently the subject of intensive R&D work and are still at an early development stage. They will only be ready for use on a large-scale in the long term. For this reason, they are also referred to as second-generation CCS processes.

 

Carbonate looping

Carbonate looping (often also referred to as dry sorption) is one of the post-combustion processes that are capable of removing CO₂ from the flue gas of a power plant. Use is made here of the high-temperature reaction (600-700 °C) of reversible, exothermic absorption (carbonisation) of calcium oxide (CaO) in combination with the endothermic calcination of calcium carbonates (CaCO₃, approx. 900 °C). The core element of the carbonate looping process is a dual fluidised-bed reactor where the CO₂-absorbing material calcium oxide is circulated between the carbonator (CO₂ absorption) and the regenerator (CO₂ desorption) in a loop. The regeneration of the calcium oxide takes place either thermally under reduced pressure or else using a reducing agent, e.g. hydrogen. 
IIn contrast with the conventional scrubbing process, the heat of absorption can be used here in the power plant process. Another advantage of the carbonate looping process is high purity of the captured carbon dioxide that can be achieved in principle. The energy consumption and financial costs of this process arise mainly in association with the reactivation step. These are difficult to estimate at the moment as all components and materials are at the R&D stage.
Carbonate looping using CaO/CaCO₃ as the absorbent material has the advantage that the raw materials (e.g. dolomite, natural gypsum) are plentiful and inexpensive. Relatively quick and permanent deactivation of the absorbent metal oxides has been observed in the laboratory experiments currently being conducted. It is estimated that considerable amounts of material will be required to replace deactivated absorbent. The disposal of deactivated CaCO₃ is unproblematic from an environmental viewpoint as calcium carbonate can be reused as a construction material. Continuous absorption and calcination lead to deactivation of CaCO₃. For this reason, "fresh" CaCO₃ needs to be added continuously. However, this will not be a problem as CaCO₃ is available in plentiful quantities. The advantage over conventional scrubbing processes in terms of efficiency is significant. Assuming a coal-fired baseline process with an efficiency of around 46%, researchers estimate the loss in efficiency at around 7.2 percentage points (including CO₂ compression and treatment). Others estimate that the loss in efficiency (excluding CO₂ compression and treatment) will be around 3 percentage points at a capture ratio of 70 to 95%. In principle, the process is also conceivable as a retrofit option for existing power plants.

 

Chemical looping for oxyfuel processes

In the oxyfuel process, oxygen for combustion is supplied by cryogenic air separation (i.e. using refrigeration technology). The flue gas mainly consists of water and CO₂ in this process method. Chemical looping represents another method of supplying oxygen by using metal oxides as O₂ carriers. The fuel is oxidised in the reactor by a suitable oxygen carrier (generally metal oxides) instead of air. The metal oxide is regenerated in a second reactor using air as the reducing agent. The nitrogen contained in air is excluded from the actual combustion process by the metal oxide. Accordingly, a CO₂-rich flue gas can be produced by removing water by condensation.