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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