Chemical Looping Process - A Novel Technology for Inherent CO2 Capture

Chemical looping is a novel energy technology that undertakes inherent CO2 capture and has better energy efficiency than existing CO2 capture technologies. Metal oxides (also known as oxygen carriers) are applied for the chemical looping process in place of air to offer oxygen for fuel combustion, thus enhancing CO2 purity in the effluent stream. This article introduces the major aspects of chemical looping technology, including the oxygen carrier, reactor and fuel. Fe-, Ni- and Cu-based metal oxides were found to be suitable oxygen carriers for the chemical looping process based on the evaluation indexes of oxygen capacity, cost, reactivity, and mechanical strength. The modification of oxygen carriers by supporting materials is the typical research regime for enhancing mechanical strength and reactivity. The reactor systems currently used for the chemical looping process include fluidized and moving bed reactors. Fluidized bed reactors are well developed for gaseous fuel combustion applications, while moving bed reactors operated with a counter-flow mode between the solid fuel and oxygen carriers would enhance combustion efficiency, and reduce the operating solid inventory of oxygen carriers. Gaseous fuels, including natural gas and syngas gasified from coal, could achieve complete fuel conversion and high CO2 yield for different combinations of oxygen carrier and reactor system. Gasification of solid fuels, including coal, petroleum coke and biomass in the fuel reactor, is a critical step that might reduce overall combustion efficiency. To improve solid fuel combustion and CO2 yield, modification of the reactor design or operational conditions are necessary for the combustion of solid fuels by a chemical looping process.