Reactor choices for chemical looping combustion (CLC) — Dependencies on materials characteristics

CATEGORY: CHEMICAL LOOPING
Energy Procedia, Volume 37, 2013, Pages 567–574
GHGT-11
Reactor choices for chemical looping combustion (CLC) — Dependencies on materials characteristics
Erin Kimball (a), Arnold Lambert (b), Anita Fossdal (c), Rebecca Leenman (a), Elodie Comte (b), W.A.P. van den Bos (a), Richard Blom (c)
a TNO, P.O. Box 6012, NL-2600 JA Delft, The Netherlands
b IFPEN, Rond-point de l’échangeur de Solaize, 69360 Solaize, France
c SINTEF, Pb. 124 Blindern, N-0314 Oslo Norway
Abstract
The physio-chemical stability of the oxygen carrier material during chemical looping combustion (CLC) operation is crucial. In the present paper we discuss the challenges connected to operating a metal oxide base material in a cyclic manner between oxidizing and reducing atmospheres. Especially, focus has been put on the phase changes occurring within the oxygen carrier particles leading to changes in particle volume during operation and consequently, with time, also particle disintegration. Particle sintering may also occur for some oxygen carrier materials in their reduced form. These challenges have been exemplified through lab-scale CLC experiments carried out both in fixed bed and fluidized bed reactors.
Introduction
CLC is a cyclic process where a metal oxide first is used to combust a fuel, and then the reduced metal oxide is re-oxidized in air before a new cycle can be carried out (see Figure 1). Such a red-ox cycle can in principle be carried out in two ways; either i) by moving the metal oxide between static gas streams or ii) by keeping the metal oxide static while switching the gas streams. Option i) is in most cases implemented with a dual circulating fluidized bed (CFB) reactor setup where the metal oxide powder circulates between a fuel reactor, in which the combustion takes place, and an air reactor, where re-oxidation takes place [6][7] CFB reactors have recently gained by far the most attention within the CLC community since this reactor type already has commercial applications for combustion processes (boilers) and within refinery processes, such as fluidized catalytic cracking (FCC). Option ii) most often involves one or more fixed bed reactors where complex valving sequences assure cyclic gas feeding to the reactors and optimal gas separation. Initial CLC experiments were carried out in single fixed bed reactors [3][8].
In the present contribution we will discuss, in more general terms, the properties of the oxygen carrier materials in connection to the kind of reactor used for the CLC process. We will also present data from real CLC experiments conducted in various reactor types using NiO/NiAl2O4 [10] as oxygen carrier (OC) and discuss the material deactivation and particle degradation observed in terms of changes in the particle properties during red-ox cycling at the relevant conditions. The Cu based material Cu0.95Fe1.05AlO4 [9] has been included due to its extreme changes in particle morphology upon red-ox cycling. Also, the changes in particle morphologies upon unplanned stops and failures in the CLC process will be discussed.  
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