Program Background and Project Benefits
Advanced combustion power generation from fossil fuels involves combustion in a high-oxygen (O2) concentration environment rather than air. This type of system eliminates introduction of most, if not all, of the nitrogen (N2) found in air into the combustion process, generating flue gas composed of CO2, water (H2O), trace contaminants from the fuel, and other gas constituents that infiltrated the combustion system. The high concentration of CO2 (≈60 percent) and absence of nitrogen in the flue gas simplify separation of CO2 from the flue gas for storage or beneficial use. Thus, oxygen-fired combustion is an alternative approach to post-combustion capture for Carbon Capture and Storage (CCS) for coal-fired systems. However, the appeal of oxygen-fired combustion is tempered by a number of challenges, namely capital cost, energy consumption, and operational challenges associated with supplying O2 to the combustion system, air infiltration into the combustion system that dilutes the flue gas with N2, and excess O2 contained in the concentrated CO2 stream. These factors mean oxygen-fired combustion systems are not cost-effective at their current level of development. Advanced combustion system performance can be improved either by lowering the cost of oxygen supplied to the system or by increasing the overall system efficiency. The Advanced Combustion Systems Program targets both of these possible improvements through sponsored cost-shared research into two key technologies: (1) Oxy-combustion, and (2) Chemical Looping Combustion (CLC).
In chemical looping systems, oxygen is introduced to the system via oxidation-reduction 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) that does not take part in the reactions. For a typical CLC process, combustion is split into separate oxidation and reduction reactions that take place in multiple reactors. The metal oxide supplies oxygen for combustion in the fuel reactor, operated at elevated temperature, and is reduced by the fuel. The reaction in the fuel reactor can be exothermic or endo¬thermic, depending on the fuel and the oxygen carrier. The combustion product from the fuel reactor is a highly concentrated CO2 and H2O stream that can be purified, compressed, and sent to storage or for beneficial use. The reduced metal carrier is then sent to the air reactor, which also operates at elevated temperatures, where it is regenerated to its oxidized state. The air reactor produces hot flue gas, which is used to create steam that drives a turbine, generating power.
Current CLC R&D efforts are focused on development and refinement of oxygen carriers with sufficient oxygen capacity that can withstand the harsh environment associated with CLC operation, development of effective and sustainable solids circulation and separation techniques, reactor design to support fuel and oxygen carrier choices, effective heat recovery and integration, and overall system design and optimization.
Alstom is continuing development of a limestone chemical looping combustion (LCL-C™) system. The LCL-C™ power production system has the potential to use smaller, less-expensive, more efficient reactors than conventional systems with carbon capture, producing a nearly pure CO2 stream without the need for an oxygen-production plant. These factors have the ability to drive down the cost of electricity and the cost of CO2 capture relative to conventional coal-fired power plants with CCS. This project will focus on addressing technology gaps identified for the 1-MWe prototype system, with emphasis on solids handling and carrier capacity. Successful prototype testing of this CLC system including validation of projected full-scale cost and performance will provide a basis for design, development, and testing of a larger pilot-scale prototype, furthering progress toward full-scale demonstration.
This project is a continuation of an ongoing effort that has been performed by Alstom under "Alstom's Chemical Looping Combustion Prototype for CO2 Capture from Existing Pulverized Coal Fired Power Plants" (Contract No.: DE-NT0005286.) A Fact Sheet for the original project provides more detailed discussion of the work.