# Experimental Investigation of Oxygen Carrier Aided Combustion (OCAC) with Methane and PSA Off-Gas
## 2 Materials and Methods
### Experimental Procedure  
The reactor setup enables continuous measurements of pressure and temperature for all measurement positions. The gas composition can be measured continuously for one vertical position at a time. An air flow is constantly fed to the reactor to prevent bed material from falling through the distribution plate and to avoid local hotspots in the bed. Fuel was added to the system once the bed temperature had reached its setpoint value and measurement data were then extracted for 1 min once steady state operation was observed with respect to measured pressure, temperature and gas concentrations. The chosen temperature intervals were chosen to cover a range from a low temperature (600 \({}^{\circ}\)C) at which little or no fuel conversion could be expected to take place, to a high temperature where complete if not close to complete fuel conversion occurs. This should enable a good comparison between the different bed materials as well as providing insights about the temperature dependence. The target was to maintain a wind box temperature not exceeding 500 \({}^{\circ}\)C to avoid fuel ignition before the gas reaches the bed. When operating at a bed temperature of at least 750 \({}^{\circ}\)C fuel ignition took place above the bed with all three bed materials. Figure 3: Particle size distribution for used bed materials before operation. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Position** & **Sand** & **Ilmenite** & **C28** \\ \hline Mean particle diameter (\(\mu\)m) & 130 & 163 & 149 \\ Bulk density (kg/m\({}^{3}\)) & 1366 & 2189 & 1905 \\ Minimum fluidization & 0.54 & 1.37 & 1.00 \\ velocity u\({}_{\text{mf}}\) (cm/s) & 42.6 & 96.4 & 73.2 \\ Solids inventory (g) & 854 & 1369 & 1191 \\ Fuel input (kW) & 0.82 & 0.82 & 0.82 \\ Specific bed mass (kg/kW\({}_{\text{th}}\)) & 1.05 & 1.68 & 1.46 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Experimental parameters for the used bed materials. Each bed material was pre-heated in the reactor system at 900 \({}^{\circ}\)C in the presence of air for at least one hour before the start of the experiments. This was mainly done to ensure that there was no net absorption of oxygen in the system with ilmenite, which otherwise could occur when using fresh ilmenite particles at low temperatures. When the gas sampling probe was placed at a certain height, the gas concentration at that point is estimated, as well as the average temperature and pressure at all measurement points. As a result, several average temperatures and pressures are recorded and the one presented in this article for a certain position is the average of those values. The targeted superficial gas velocity is estimated based on the targeted bed temperature. The air-to-fuel ratio (AFR) is defined as the ratio of the volumetric flow rate of air fed to the fluidized bed to the volumetric flowrate, corresponding to the stoichiometric combustion of methane. The stoichiometric amount of air is based on the amount of oxygen needed to convert the methane fed to the reactor, according to Equation (8):  
\[\mathrm{CH_{4}+2\;O_{2}\;\rightarrow\;CO_{2}+2\;H_{2}O\;\Delta H_{298^{\circ} C}=-891\;kJ/mol} \tag{8}\]  
If CO rather than CO\({}_{2}\) is detected, it indicates that the combustion is incomplete. When PSA off-gas is used, the amount of air is, in addition to the required oxygen to convert the methane, estimated based on the oxygen needed to burn H\({}_{2}\) and CO, according to Equations (9) and (10). \[\mathrm{H_{2}+\frac{1}{2}\;O_{2}\;\rightarrow\;H_{2}O\;\Delta H_{298^{\circ} C}=-286\;kJ/mol} \tag{9}\]  
\[\mathrm{CO+\frac{1}{2}\;O_{2}\;\rightarrow\;CO_{2}\;\Delta H_{298^{\circ}C}=-2 83\;kJ/mol} \tag{10}\]  
The gas composition of the simulated PSA off-gas is presented in Table 4.  
The measurements with both methane and PSA off-gas were made with an air-to-fuel ratio of 1.05 and a superficial gas velocity of approximately 0.2 m/s. In the experiments with methane as fuel, the air-to-fuel ratio was also varied from 1 to 1.2 at 750 \({}^{\circ}\)C for all three bed materials.