(727d) An Overview of Chemical-Looping Reduction in Fixed-Bed and Fluidized Bed Reactors Focused On Oxygen Carrier Utilization and Reactor Efficiency

Chemical-looping combustion is a method for the oxidation of
hydrocarbons with in-situ CO₂ capture. The basic concept of the
process involves two interconnected reactors, a Reducer and an Oxidizer, with
an oxygen carrier (OC) ¨C a metal/metal oxide circulating between the two. The
main principle in chemical-looping is the oxidation of hydrocarbons with a
metal oxide in a N₂-free environment. In this study, a transient
non-ideal reactor model was developed for two types of Reducers: fixed and
fluidized bed [1,2]. The models capture
the most important dynamic and spatial phenomena in CLC reduction. The mass
balance of 9 species and the energy balance are simulated in both models by the
same kinetics mechanisms and reaction constants.

In this work, Ni-based CLC tests in fixed-bed and spouted-bed CLC
reactors operated in the University of Connecticut are discussed and the simulation
of different fixed and fluidized bed unit data reported in the literature and data
of this study are presented. Comparison between experimental and predicted
results reveals that the process models and kinetics network proposed are
successful in predicting chemical-looping combustion in fixed- and fluidized
bed reactors. Through model-assisted comparison between the two types of
reactors, some substantial issues in the CLC system performance are discussed.
For instance, the comparison of carbon formation (Figure 1) between the two
types of reactors (reactor configurations of Iliuta et al. [3]) shows that carbon
deposition is not significant in the fluidized bed reactor; whereas, it is of
concern at higher conversions in a fixed-bed reactor. Based on the developed
reactor models, fixed-bed scale-up calculations are carried out. Spatial and
transient pressure and temperature variations are shown and analyzed. Other
issues, like challenges in operating CLC at minimum oxygen carrier loading
(therefore, smaller Reducer size), OC loading required for high CH₄
conversion and OC conversion, time of starting of significant carbon formation at
a given OC loading, and CO₂ selectivity of fluidized-bed and
scale-up fixed-bed units will be discussed.

Figure 1: Carbon formation of NiO based OC chemical-looping reduction
process in (a) fixed-bed and (b) fluidized bed reactors[3].

Acknowledgement:
This material is based upon work supported by the National Science Foundation
under Grant No. 1054718.

[1]      D. Kunii, O.
Levenspiel, Bubbling bed model: model for the flow of gas through a fluidized
bed, I & EC Fundamentals. 7 (1968) 446¨C452.

[2]      D.
Kunii, O. Levenspiel, Bubbling bed model for kinetic processes in fluidized
beds: Gas-Solid Mass and Heat Transfer and Catalytic Reactions, I & EC
Process Design and Development. 7 (1968) 481¨C492.

[3]      I.
Iliuta, R. Tahoces, G.S. Patience, S. Rifflart, F. Luck, Chemical-looping
combustion process : kinetics and mathematical modeling, AIChE J. 56 (2010)
1063¨C1079.