(358e) Comparison of Bench-Scale Fixed-Bed Units for Chemical-Looping Combustion

Abstract:

The
objective of this work is to develop a detailed dynamic model for chemical-looping
combustion in a fixed-bed reactor. The key properties desired of this model are
that it can be practically implemented in different fixed-bed reactors using
the same reaction kinetics network. Experimental data from published fixed-bed chemical-looping
reactors are used for model development and validation.

Fixed-bed
reactors are typically used to evaluate the properties of oxygen carriers in
repeated oxidation/reduction cycles [1-4]. Fuel and air are sequentially fed to
the reactor to simulate chemical-looping combustion oxidation and reduction
cycles. Inert gas is fed between cycles to isolate reduction and oxidation
steps.

Comparison
of the results from various laboratories reveals inconsistencies regarding the
apparent kinetics, and process time scales. Hence, it is useful to use a dynamic
PFR model to perform a model-based comparison, including the most important
reactions, and time-space effects on volumetric flow, pressure drop, reaction
extent and selectivity.

Reactions
involved in the chemical-looping reduction step are listed below:

Kinetics
for the first four reduction reactions are taken from Iliuta et al. [5].
Kinetics for steam reforming [6,7], dry reforming [8,9], CH₄
pyrolysis [10,11], water gas shift [6,7], reverse methanation [6,7], reverse
Boudounard reaction [12], and carbon gasification by steam [12] are taken from the
literature.

Figure 1
presents model predictions of the profiles of H₂ and CO selectivity,
oxygen carrier conversion and CH₄ to carbon conversion for the fixed-bed
reactor of Ref [5]. An interesting observation is that there is an optimal
point at relatively low reaction times (~10 s) and in the mid-zone (~3.5 mm) of
the reactor, where the process reaches to a H₂ and CO selectivity
extremum, at high OC conversions (~65%) and negligible carbon formation (~0).

                  (a)                                                                                                            (b)

Figure
1:
Prediction of spatial and time profiles in chemical-looping reduction using
Ni/Al₂O₃ as oxygen carrier at 800 °C in a fixed-bed
reactor; (a) H₂ and CO selectivity and (b) oxygen carrier and CH₄
to C conversion.

A thorough
review of the kinetics of chemical-looping reactions is carried out, focusing
mainly on Ni- and Fe-based oxygen carriers. Comparison of experimental data with
the results from the dynamic model allowed to identify the most important
chemical-looping reaction, to select a finite number of kinetic networks, and
to obtain kinetic parameters capable of describing yields and selectivities of
different experimental setups.

In this
presentation, the optimized kinetic network along with the fixed-bed model will
be presented. After implementing this model into different fixed-bed designs, a
comparison of the predicted results and the experimental results for these
different reactors will be illustrated and discussed.

Acknowledgement:
NSF Career Award
No. 1054718

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