The combustion of fossil fuels in nearly pure oxygen, rather than air, presents an opportunity to simplify carbon dioxide (CO2) capture in power plant applications. Oxy-combustion power generation provides oxygen to the combustion process by separating oxygen from air. However, chemical looping systems produce oxygen internal to the process, eliminating the large capital, operating, and energy costs associated with oxygen production.
In chemical looping combustion (CLC) systems, oxygen is introduced to the system via reduction-oxidation cycling of an oxygen carrier. The oxygen carrier is usually a solid, metal-based compound. It may be in the form of a single metal oxide, such as an oxide of copper, nickel, or iron, or a metal oxide supported on a high-surface-area substrate (e.g., alumina or silica), which does not take part in the reactions. For a typical CLC process, combustion is split into separate reduction and oxidation reactions in multiple reactors. The metal oxide supplies oxygen for combustion and is reduced by the fuel in the fuel reactor, which is operated at elevated temperature.
This reaction can be exothermic or endothermic, depending on the fuel and the oxygen carrier. The combustion product from the fuel reactor is a highly concentrated CO2 and water stream that can be purified, compressed, and sent for storage or beneficial use. The reduced oxygen carrier is then sent to the air reactor, also operated at elevated temperature, where it is regenerated to its oxidized state. The air reactor produces a hot spent gas stream, which is used to produce steam to drive a turbine, generating power. Then the oxygen carrier is returned to the fuel reactor, re-starting the reduction-oxidation cycle.
Current CLC research and development (R&D) efforts are focused on developing and refining oxygen carriers with sufficient oxygen carrying capacity, durability, and production cost to minimize one of the primary CLC operating costs; developing effective solids circulation and separation techniques; improving reactor design to support fuel and oxygen carrier choices; effective heat recovery and integration; and overall system design and optimization.
Several CLC concepts are being researched around the globe at bench and small-pilot scale to prove system design and operation strategies and to estimate capital and operating costs for commercial-scale systems. Currently, the National Energy Technology Laboratory (NETL) supports several CLC projects in collaboration with industry, academia, and NETL’s Research and Innovation Center (RIC), ranging from lab- and bench-scale testing to evaluation of pilot-scale prototypes.