bed reactor and above all, an iron oxide material without the stabilizing Ce0.5Zr0.5O2. While these material choices, born from experimental difficulties, were not anticipated, the kinetic model with parameters from the TGA measurements did yield good results in the final reactor model with only slight modifications. This success raises the question for future work, how exact and faithful to reaction mechanisms a kinetic model for the CWGSR has to be, in order to model the reactor behaviour.
Future work could comprise of a more rigorous model discrimination, incorporate steam as an educt in the experimental setup and consider a more detailed oxidation model of iron, e. g.
the oxidation range of wustite. Also, methods for the optimal design of experiments could be employed to greatly reduce the number of experiments and/or reduce the parameter confidence intervals. An interesting point of study is also the performance of the kinetic model in case multiple reaction occur simultaneously, which can also occur in the CWGSR.
The presented results directly benefits only the user of the same stabilized iron oxide mate-rial as used in this work. Users of similar iron oxide based matemate-rials or model based analysers can use the presented data as a basis, especially since a full model with all stepwise reduction and oxidations of iron / iron oxide is hard to find in the literature. The presented method is of interest to everybody with the goal to measure the rate of gas-solid reactions.
The second objective was the experimental analysis of the CWGSR. Reactor models have been published in the past and have been used to make predictions on the favourable operating mode. But these models were either of lower complexity and/or have not been backed up by experimental investigations.
A test stand was constructed, able to operate a tubular fixed bed reactor at 750◦C, supply CO/CO2/H2/H2O/N2in a variety of mixtures, change flow direction during operation and mea-sure the reactor outlet gas concentrations continuously. Experiments of increasing complexity were conducted to understand the reactor as well as the test plant.
Basic assumptions of previous models could be confirmed: Two distinct reaction zones do form in the fixed bed and their position/movement can be controlled by the CWGSR operating conditions. But the benefits of the in the literature predicted, favourable operating modes, based on the concept of flow reversal mode and short cycle durations, could not be validated experimentally. This failure could, however, not be attributed to the CWGSR itself, but to shortcomings in the experimental setup.
The construction, testing and operation of the test stand, as well as the preparation of the fixed bed was the most time and resource consuming part of this work. Iteration loops caused
by erroneous estimated flow ranges, test stand retrofits or fixed bed material problems caused the low number of data sets in this work. But time and effort needed for experimental studies are also the reason for the scarcity of published results and the lack of results similar to this work in the literature.
Future work can improve on many points of the experimental setup. Stability and repro-ducibility is of foremost concern, e. g. with regards to the fixed bed, the steam source or temperature measurements in the fixed bed. This will increase the ability to confirm model predicted, favourable operating modes of the reactor. With the background of this experi-mental study, a mostly, or even purely, experiexperi-mentally driven analysis and optimisation of the CWGSR is deemed futile.
The third objective was to formulate a reactor model that can simulate the experiments of the CWGSR test stand and reproduce its results. Thus creating a validated model which can be used for further analysis of the CWGSR.
Based on mass balances of gas and solid phase and the reaction model from the first part of this work, a dynamic, one-dimensional, isothermal model was build and implemented in Matlab.
Experiments could be reproduced with good quality. The model is also consistent with pre-viously published models and predicts the same favourable operating modes, while providing more information than the previously used shortcut models.
After exploring alternatives, the numerical solution of the model was done via a straight-forward discretisation of the involved differential equations and a solution of the system, one time step after the other. While being robust, this method of solving the system is not suited for directly calculating cyclic steady states. In some cases, several hundred operating cycles have to be simulated to reach a cyclic steady state. Since the analysis and especially a numeri-cal optimisation of the CWGSR will involve computing large numbers of cyclic steady states, better solutions methods have to be employed.
The next step for a model expansion would be the introduction of energy balances to reflect the influence of temperature effects on the reactor. Effects of heat accumulation in the reactor by recuperation have already been predicted. But experimental validation is lagging behind.
This work succeeded in proposing a new, validated model of the CWGSR. The model has a higher degree of detail than previously published models. It allows for better planning of future experiments and a more detailed, model based analysis of this reactor type.
A. Appendix: Estimation of Various Parameters
A.1. Weight Loss of Stabilised Iron Oxide
Effect of Iron Oxide Reduction on Sample Weight
A complete reduction of FeO4/3 to Fe corresponds to the loss of4/3mol O per mol of FeO3/2. The stabilised material consists of 20 m% CeO2ZrO2. Therefore the anticipated weight loss of the oxygen storage material, based on its initial mass, i. e. its freshly synthesised state, is estimated by Eq. A.1. Similar calculations lead to the other expected weight losses given in Tab. A.1.
0.8 gFeO3/2
ginitialmass· 1
90(g/mol)FeO3/2 ·4/3molOloss
1molFe2O3 ·16 gOloss
molOloss ≈0.19 gOloss
ginitialmass (A.1)
[gOloss/ginitialmass] Reduction to Fe3O4 FeO Fe
Reduction from Fe2O3 0.024 0.071 0.213
Fe3O4 0.047 0.190
FeO 0.142
Table A.1.: Theoretical weight losses through reduction of stabilised iron oxide material in gOloss/ginitialmass.
Effect of CeO
2Reduction on Sample Weight
A possible reduction of 1 mol CeO2 to CeO3/2 corresponds to the loss of1/2mol O per mol of the compound CeO2ZrO2. The stabilised material consists of 20 m% CeO2ZrO2. Therefore
the anticipated weight loss of the oxygen storage material, based on its inital mass, i. e. its freshly synthesised state, is:
0.2gCeO2ZrO2
ginitialmass· 1
295(g/mol)CeO2ZrO2 · 1/2molOloss
1molCeO2ZrO2 ·16 gOloss
molOloss ≈0.005 gOloss
ginitialmass (A.2)