Chemical Looping Combustion (CLC)

The Chemical Looping Combustion (CLC) concept is based on the transfer of oxygen from the combustion air to the fuel by means of an oxygen carrier in the form of a metal oxide, avoiding the direct contact between fuel and air. The CLC system is made of two interconnected reactors, designated as air and fuel reactors. In the fuel reactor, the fuel gas is oxidised by a metal oxide through the chemical reaction: CH₄ (CO, H₂) + MeO  →  CO₂ + H₂O (CO₂, H₂O)+ Me The exit gas stream from the fuel reactor contains CO₂ and H₂O. After water condensation, almost pure CO₂ can be obtained with little energy lost for component separation. The metal or reduced oxide, Me, is further transferred into the air reactor in which it is regenerated by taking up oxygen from the air. Me  +  ½O₂  →  MeO The flue gas leaving the air reactor contains N₂ and unused O₂. The total amount of heat evolved over the two reactors in CLC process is the same as for normal combustion, where the oxygen is in direct contact with the fuel. The significant advantage compared to normal combustion is that the CO₂ is not diluted with N₂. As opposite to other technologies proposed for CO₂ separation, this process has no significant energy penalty for the capture process, and external capture devices are avoided. Thus, the process is expected to be less costly than available technologies for CO₂ separation. A conceptual process scheme is shown in the figure below. Different metal oxides have been proposed as possible candidates for CLC process: CuO, CdO, NiO, Mn₂O₃, Fe₂O₃, and CoO. In general, these metal oxides are combined with an inert which acts as a porous support providing a higher surface area for reaction, as a binder for increasing the mechanical strength and attrition resistance, and, additionally, as an ion conductor enhancing the ion permeability in the solid. The only drawback of the overall CLC process is that the oxygen carriers are subjected to strong chemical and thermal stresses in every cycle and the performance could be poor after enough number of cycles in use. Research on chemical-looping combustion at ICB-CSIC: The work includes different projects funded by ECSC, Capture of CO₂ in Coal Combustion (CCCC), and by EU. In these projects we co-operate with Chalmers University of Technology in Sweden, Technical University of Vienna, BP (UK) and Alstom (France). The work is also supported by the Spanish Ministry of Education and Science. The objectives of this research line are:

• To develop an oxygen carrier with appropriate reduction and oxidation rates, resistant to attrition and with high durability, maintaining its chemical, structural and mechanical properties after a high number of reduction-oxidation cycles.
• To investigate the possible designs with respect to the fluidization conditions.
• To demonstrate and evaluate this new combustion technology in a laboratory-scale chemical-looping combustor.
• Main achievements of this research line are:
• To develop a Cu-based oxygen carrier without agglomeration problems and excellent properties for the CLC process.
• To demonstrate the CLC technology a prototype of 10 kWth has been designed and built in the ICB-CSIC. This pilot plant was satisfactorily run during 200 hours burning methane and using a Cu-based oxygen carrier.