Recently, interest has grown in chemical looping combustion (CLC) because it is seen as a technique that may allow cost-effective carbon capture and storage (CCS). In CLC the overall reaction by which chemical energy is released is between a hydrocarbon and air as in conventional combustors. However, the reaction is completed in two separate oxidation and reduction steps occurring in different reaction vessels. In the oxidizer (or air reactor) an oxygen carrier, usually a metal, is exothermically oxidized in air resulting in an oxide and a hot air stream (oxygen depleted). The exhaust gasses may be expanded through a turbine to produce work, while the oxide passes to the reduction vessel (or fuel reactor). Here, it reacts with the fuel, is reduced and the metal regenerated. The metal then returns to the oxidizer to complete the loop. The exhaust gasses from the reducer contain only carbon dioxide and water so that, after expansion and work extraction, the water may be condensed leaving a stream of pure CO2 ready for storage. Hydrocarbon fuels will continue to be used for decades, so, in the face of ambitious emission reduction targets, CCS is an important technology and methods, such as CLC, that offer automatic CO2 separation (so-called inherent carbon capture) are particularly attractive. Despite this obvious advantage CLC was not originally conceived for the purposes of CCS, but rather as a means to produce pure carbon dioxide free from contamination by inert gases such as nitrogen. In the context of power generation it was then proposed as a means to improve the exergetic efficiency of energy conversion processes using hydrocarbons. Combustion is usually a highly irreversible process and necessitates the rejection of large quantities of heat from power cycles leading to the low thermal efficiency of gas turbines and the like. The two-stage reaction approach of CLC can reduce the irreversibility and the extent of heat rejection and hence provide improved cycle efficiency. Ideally, both goals would be simultaneously achieved thereby offsetting both the cost of carbon capture and of compression, transportation and storage. In the paper we present a thermodynamic analysis of CLC to illustrate its potential for improving efficiency. We will then develop a methodology for selecting oxygen carriers based on their thermodynamic properties and review several candidate materials. In particular, we will compare, from a thermodynamic perspective, solid phase oxygen carriers as used in fluidised bed based reaction systems and the liquid/vapour phase carriers previously suggested by the authors. Finally, comments on practical implementations of CLC in power plant will be presented.
DETERMINATION OF REACTIVITY AND THERMODYNAMIC ANALYSIS OF NICKEL-BASED OXYGEN CARRIERS FOR CHEMICAL-LOOPING COMBUSTION
