Chemical looping combustion : fuel conversion with inherent CO2 capture

Abstract

Chemical looping combustion (CLC) is a new concept for fuel energy conversion with CO2 capture. In CLC, fuel combustion is split into seperate reduction and oxidation processes, in which a solid carrier is reduced and oxidized, respectively. The carrier is continuously recirculated between the two vessels, and hence direct contact between air and suel is avoided. As a result, a stoichiometric amount of oxygen is transferred to the fuel by a regenerable solid intermediate, and CLC is thus a varient of oxy-fuel combustion. In principle, pure CO2 can be obtained from the reduction exhaust by condensation of the produced water vapor. The termodynamic potential and feasibility of CLC has been studied by means of process simulatons and experimental studies of oxygen carriers. Process simulations have focused on parameter sensitivity studies of CLC implemented in 3 power cycles; CLC-Combined Cycle, CLC-Humid Air Turbine and CLC-Integrated Steam Generation. Simulations indicate that overall fuel conversion ratio, oxidation temperature and operating pressure are among the most imortant process parameters in CLC. A promising thermodynamic potentail of CLC has been found, with efficiencies comparable to, - or better than existing technologies for CO2 capture. The proposed oxygen carrier nickel oxide on nickel spinel (NiONiA1) has been studied in reduction with hydrogen, methane and methane/steam as well as oxidation with dry air. It has been found that at atmosphereic pressure and temperatures above 600° C, solid reduction with dry methane occurs with overall fuel conversion of 92%. Steam methane reforming is observed along with methane cracking as side reactions, yealding an overall selectivity of 90% with regard to solid reduction. If steam is added to the reactant fuel, coking can be avoided. A methodology for long term investigation of solid chemical activity in a batch reactor is proposed. The method is based on time variables for oxidaton. The results for NiONiA1 do not rule out CLC as a viable alternative for CO2 capture, but long term durability studies along with realistic testing of the carrier in a continuous rig is needed to firmly conclude. For comparative purposes a perovskite was synthesized and tested in CLC, under similar conditions as NiONiA1. The results indicate that in a moving bed CLC application, perovskites have inherent disadvantages as compared to simpler compounds, by virtue of low relative oxygen content.