# Experimental Investigation of Oxygen Carrier Aided Combustion (OCAC) with Methane and PSA Off-Gas
## 2 Materials and Methods
### Bed Material  
The most commonly used bed material in biomass fluidized bed boilers is silica sand, which is inert towards fuel and air but acts as an ash absorbent and thermal flywheel. In these experiments, silica sand from Baskarp, Sweden, was used. The second material used was Norwegian rock ilmenite concentrate, supplied by Titania A/S. Ilmenite is a mineral ore rich in iron and titanium oxides. The third material evaluated in this work is a synthetic oxygen carrier referred to as C28 (CaMn\({}_{0.775}\)Mg\({}_{0.1}\)Ti\({}_{0.125}\)O\({}_{3-\delta}\)) which was manufactured by VITO in Belgium by spray drying [18]. The C28 material used in these experiments was recovered from the fuel reactor after experiments in a 10 kW\({}_{\text{th}}\) CLC reactor used for gaseous fuels [15]. The elemental composition for sand and ilmenite is reported in Table 2.  
Sand could be assumed to be an inert bed material, but to make sure that this is the case, the reactivity of sand was tested in a separate experiment at high temperature where syngas was mixed with nitrogen [29]. This experiment showed that the sand was inert and did not show any tendencies of gas-solid reactions with the CO or H\({}_{2}\). Ilmenite and C28 are, however, oxygen carriers and these are oxidized/reduced when reacting with the oxygen/fuel which has already been shown in numerous CLC experiments. The general reaction between an oxygen carrier (Me\({}_{x}\)O\({}_{y}\)) and a hydrocarbon fuel (C\({}_{j}\)H\({}_{2\delta}\)) is shown in Equation (3). \[(2j+k)\text{Me}_{x}\text{O}_{y}+\text{C}_{j}\text{H}_{2k}\rightarrow(2j+k) \text{Me}_{x}\text{O}_{y-1}+\text{k}\text{H}_{2}\text{O}+\text{CO}_{2} \tag{3}\]  
The reaction between methane and the most oxidized state of ilmenite, which is pseudobrookite (Fe\({}_{2}\)TiO\({}_{5}\)) and rutile (TiO\({}_{2}\)) can, for example, be described according to Equation (4). \[\frac{1}{4}\text{CH}_{4}+\text{Fe}_{2}\text{TiO}_{5}+\text{TiO}_{2} \rightarrow 2\text{FeTiO}_{3}+\frac{1}{4}\text{CO}_{2}+\frac{1}{2}\text{H}_{2}\text{O} \,\Delta\text{H}=35\text{ kJ}/\text{mol}\,\text{O} \tag{4}\]  
The reoxidation of ilmenite can be described by Equation (5). It should be noted that other reactions can occur with ilmenite as well, such as redox reactions with magnetite (Fe\({}_{3}\)O\({}_{4}\)) and hematite (Fe\({}_{2}\)O\({}_{3}\)). \[2\text{FeTiO}_{3}+\frac{1}{2}\text{O}_{2}\ \rightarrow\text{Fe}_{2}\text{ TiO}_{5}+\text{TiO}_{2}\ \Delta\text{H}=-235\text{ kJ}/\text{mol}\,\text{O} \tag{5}\]  
The reduction of C28 with methane can be defined according to Equation (6). \[\frac{1}{4}\text{CH}_{4}+\text{ CaMn}_{0.775}\text{Mg}_{0.1}\text{Ti}_{0.125} \text{O}_{3-\delta}\ \rightarrow\text{ CaMn}_{0.775}\text{Mg}_{0.1}\text{Ti}_{0.125}\text{O}_{3-\delta-1}+\frac{1}{2} \text{H}_{2}\text{O}+\ \frac{1}{4}\text{CO}_{2} \tag{6}\]  
Equation (7) describes the reoxidation of C28. \[\frac{1}{2}\text{O}_{2}\ +\text{ CaMn}_{0.775}\text{Mg}_{0.1}\text{Ti}_{0.125}\text{O}_{3-\delta-1} \ \rightarrow\text{ CaMn}_{0.775}\text{Mg}_{0.1}\text{Ti}_{0.125}\text{O}_{3-\delta} \tag{7}\]  
All three bed materials were sieved to the size interval 90-212 \(\upmu\)m and from that sample the particle size distribution was estimated (see Figure 3). Some differences can be observed between the different bed materials where sand has predominantly particles with a size in the lower range of the chosen sieving size interval. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Element [wt-\%]** & **Fe** & **Ti** & **Mg** & **Si** & **Al** & **Ca** & **Mn** & **K** & **Cu** & **Total** \\ \hline Silica sand & 0.35 & & & 42.30 & 2.59 & & 0.89 & 1.66 & & 47.80 \\ Ilmenite & 36.46 & 26.89 & 2.16 & 0.93 & 0.34 & 0.23 & 0.04 & 0.02 & 0.01 & 67.27 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Elemental composition of the used Baskarp sand and Norwegian rock ilmenite [28] given in wt-%. Balance is oxygen.