Chemical looping combustion for CO2 capture
Today the international society has broadly accepted that an anthropogenic influence on the greenhouse effect and thus on the continued global warming exists. This influence is primarily caused by extensive utilization of fossil fuels and the related emission of so called greenhouse gases, with CO2 being their main representative. Even so, the worldwide energy demand is expected to increase and since carbon free technologies are still away from being mature and economical competitive, fossil fuel applications will still cover the major part of the energy demand, at least within the next decades.
To meet this contradicting situation, the European Union proposed a strategy with the overall goal of limitation of greenhouse gas emissions while, at the same time, increasing energy demands are covered. Among others like, expansion of renewable energy sources or increase of end-use efficiencies, one part of this strategy is fossil fuel conversion combined with capture and storage of formed CO2, hence the implementation of the so called carbon capture and storage (CCS) technology. The idea is to deposit concentrated CO2 in save geological storages like gas fields instead of emitting it uncontrolled into the atmosphere. Several projects are currently running to demonstrate the feasibility of carbon sequestration and long-term stability of the storages. On the other hand, research goes on to find cost-effective measures to obtain pure CO2 from power stations.
One of the most promising capture technologies is chemical looping combustion (CLC). While other capture technologies exhibit the need of cost and energy extensive gas-gas separation steps, CLC separates CO2 inherent to the process via unmixed combustion. Oxygen is transported in terms of an oxygen carrier (generally a metal oxide) from the combustion air stream to the fuel stream and thus air nitrogen and fuel are never mixed (Figure 1). The chemical reactions with oxygen carrier take place in two separated reactors. Oxidation of the oxygen carrier is performed within the so called air reactor, which is supplied with combustion air and reduction takes place inside the fuel reactor, where fuel is fed into the system. The total amount of heat released from the two reactors equals the heat released from ordinary combustion of the fuel fed. Most of the proposed CLC applications are using well-established boiler technology very similar to (dual) fluidized bed boilers, which also means that costs can be assessed with great accuracy.
Figure 1- CLC concept
In conclusion, CLC features up to 100% CO2 capture efficiency, a highly concentrated stream of CO2 ready for sequestration, no NOx emissions, and no costs or energy penalties for gas separation. Furthermore CLC uses well-established boiler technology, which means that costs can be assessed with great accuracy. Therefore, CLC is estimated to achieve CO2 capture cost reductions of 40 to 50% compared to today’s best available carbon capture technologies, namely post combustion amine and oxyfuel combustion.
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