(24a) An Assessment of Chemical Looping Combustion (CLC) for CO2 Capture In a Coal-Based IGCC Power Plant

Chemical looping combustion (CLC) is an indirect combustion process in which fuel is combusted without direct contact with air. Transfer of oxygen between air and fuel takes place with the aid of an oxygen-carrier (OC). The oxygen-carrier extracts O₂ from air in one reactor and then transfers it to fuel in a subsequent reactor. Since the fuel does not come in direct contact with air, the products of combustion contain only carbon dioxide (CO₂) and water (H₂O). A CO₂ stream of very high purity can then be obtained by condensing the water vapor. Gaseous fuels such as natural gas or syngas (CO and H₂) can be used as fuels in CLC, for inherent CO₂ capture. The focus of this paper is the use of CLC for CO₂ capture in a coal-based integrated combined cycle power plant (IGCC), in lieu of the conventional approach to capture.

CO₂ capture in conventional IGCC designs involves a water gas shift reaction to convert CO in the syngas from the gasifier to CO₂, followed by subsequent capture of CO₂ using a physical absorption solvent (Selexol or Rectisol). Alternatively, CLC can be used for CO₂ capture in a coal-based IGCC plant. The products from the CLC reactor are separate streams of oxygen-depleted air and combustion products (CO₂ and H₂O), both at high temperature and pressure. These streams can be expanded in separate turbines to generate electricity. The hot exhaust from these turbines can be used to generate steam for further electricity production. Water can be condensed from the CO₂-rich stream in order to obtain high purity CO₂ for further compression and storage.

The use of CLC in place of a conventional CO₂ capture process in an IGCC system thus involves both addition and removal of certain processes and equipment. It is important to study the advantages and disadvantages of CLC in terms of its effects on the performance of the whole system, compared to a conventional IGCC. In this paper a performance model is developed to perform a detailed thermodynamic analysis of the IGCC plant using CLC for CO₂ capture. Chemical equilibrium models are used to simulate the CLC reactions using NiO/Ni as the oxygen carrier. The effect of varying the syngas composition and process operating conditions (.e.g., temperature and pressure) also are studied. The performance results are compared to a conventional IGCC power plant with CO₂ capture in order to understand the relative advantages and disadvantages of CLC when applied to IGCC.  The needs for further model development also are elaborated.

This work is supported under a contract from the U.S. Department of Energy's National Energy Technology Laboratory (DOE/NETL)