Enhancement of iron-based oxygen carriers through doping with tungsten oxide for chemical looping applications
Creators: name: Morales Corona, José Juan
Publisher: name: Imperial College London
Year: 2023

description: In this thesis, a series of tungsten doped iron-based oxygen carries were evaluated as potential oxygen carriers for different chemical looping applications. Chemical Looping Combustion (CLC) offer a variety of options to decarbonise different industrial sectors, such as iron and steel and hydrogen production due to the inherit CO2 separation of the process. Chemical Looping with Water Splitting (CLWS) is a chemical looping technology for hydrogen production whilst simultaneously capturing CO2. Selection of oxygen carriers (OCs) for CLWS is limited due to the thermodynamic limitations for different materials to be oxidised by steam at the relevant process temperatures. Iron based materials are one of the most widely studied options for CLC and CLWS, preferred for their relative abundance and low cost it is the most promising option for development of OCs. This work documents the development of alloyed iron and tungsten mixed oxides as OCs for several chemical looping applications. Mixed metal oxides were produced to overcome some of the disadvantages of iron oxides for CLC, which arise when the reduction of iron oxide (Fe2O3) is extended to wüstite (FeO) and iron (Fe), where agglomeration and sintering problems are the main challenge for fluidisation. The OCs were produced via different preparation techniques, that included mechanically mixing, co-precipitation, and impregnation. The OCs were characterised to evalute the effect of the synthesis conditions and tungsten loading on the material properties, and performance assessment was accomplished in a thermogravimetric analyser (TGA) and a lab-scale fluidised bed reactor (FBR) over continuous redox cycles. The use of tungsten combined with iron produced a more chemically stable material during operation in the TGA, which performed well during multiple redox cycles with no apparent decrease in the oxygen transport capacity and no apparent deactivation. The increased stability was attributed to a resulted solid solution between tungsten and the Fe2O3. This solid solution showed an enhancement on the thermal stability of the materials, as agglomeration of the particles was avoided in both reactor settings. The content of tungsten in the material was found to have a strong effect on the performance of the W-doped samples, and this was found to be in the order: 2 mol% W &gt; 5 mol% W &gt; 1 mol% W &gt; 0.5 mol% W. Although the reference samples (with no tungsten) were found to have a value of oxygen transport capacity (Ro) higher than the W-doped samples with 1 and 0.5 mol%, they were heavily agglomerated, and this value of Ro decreased with time. Furthermore, materials containing tungsten showed a resistance to carbon deposition, as carbon being deposited in the samples decreased with increasing content of tungsten to the point of being not detected for the samples with higher content of tungsten (the samples with content of 2 and 5 mol% of tungsten). A simple model, based on the effectiveness factor, to determine the kinetics was applied to the fludised bed experiments of the Fe-W mixed oxide samples. Activation energies derived from the model were in the range of 32-63 kJ∙mol-1. The source of tungsten used for the impregnation process was found to have a significant effect on the morphological properties (surface area, porosity, and pore size distributions) and the reactivity of the samples in the TGA. Tungsten addition to the material via impregnation was found to increase the measured valued of oxygen transferred per cycle and the observed rate of reduction by of 43% and 42% respectively. Kinetic parameters were isolated from the TGA experiments for most reactive of the W-impregnated samples, and the three interface random pore model was found to adequately represent the experimental data for the reaction with CO.