# Experimental Investigation of Oxygen Carrier Aided Combustion (OCAC) with Methane and PSA Off-Gas
## 3 Results
### Experiments with Methane
At 600 \({}^{\circ}\)C, it can be observed in Figure 6 that the difference between the three materials is small, where little/no methane conversion is observed. At all other bed temperatures (750-900 \({}^{\circ}\)C), however, it can be observed that the methane concentration is overall lower in the bed, using ilmenite and C28 as bed materials, compared to the results with sand. The reason for this difference in gas conversion can be explained by the gas-solid reaction with the oxygen carrier, which only occurs in the system with ilmenite and C28. This reaction can, therefore, be considered as especially important to obtain a high methane conversion in the bed. It should be mentioned that the reaction with the bed material itself is endothermic but the reoxidation is exothermic, so the net heat release corresponds to the heat of combustion for methane. It should be mentioned that limitations in mass transport between the bubble and emulsion phase can have a significant impact on the comparison between sand and the oxygen carriers. In the extreme case that the mass transfer from the bubbles to the emulsion phase would be very poor, and if there are no bed particles present in the bubbles, the contact between fuel and bed material would be negligible. This would have resulted in identical fuel conversion for different kinds of bed material. It can be observed that this is not the case here. It is likely that gas-solid reactions occur in the emulsion phase, between fuel and oxygen carrier. Gas-solid reactions could, however, also take place as a result of particles present in the bubbles. It should also be noted that the degree of mixing in the fluidized bed, where the experiments take place, can affect the experimental results. Since the mixing of the fluidized bed can vary between different reactors, it is important to keep this in mind when evaluating the difference between different bed materials. It is possible to estimate the degree of methane conversion based on the CH\({}_{4}\) inlet concentration, the measured concentration of CH\({}_{4}\) and the fact that 2 mol of steam is produced per mol CH\({}_{4}\) according to Equation (8). At the highest tested bed temperature (900 \({}^{\circ}\)C), for example, the methane conversion is almost complete in the bed with C28 and ilmenite (corresponding to 99.7% and 99.3% methane conversion at MP2, respectively), whereas the measured methane conversion (based on the measured CH\({}_{4}\) concentration) with sand was equal to 86.7%. The trend is the same at 850 \({}^{\circ}\)C at the same measurement point, where the methane conversion was 98.7% for C28, 90.4% ilmenite and 61.7% for sand. This displays that almost 100% oxidation of the methane can be achieved in the bed in the presence of an oxygen carrier, which cannot be said when using sand, where a significant part of the combustion of methane takes place above the bed at these bed temperatures. In the comparison between the bed materials, it should be mentioned that different bed masses are used which could have an influence on the result, where a larger specific bed inventory (see Table 3) could potentially result in a higher degree of fuel conversion. It is not possible to evaluate the effect of this in this experiment, but it should be mentioned that operation with C28 results in higher fuel conversion than ilmenite, but with a smaller specific bed inventory compared to ilmenite. Regarding the bed inventory, it should also be added that the pressure is overall higher in the bed and in the wind box in the system with ilmenite since the bed density is higher. This could cause more significant fluctuations in pressure where bursts of larger bubbles could pass through the bed. In order to confirm that the converted methane is fully oxidized to CO\({}_{2}\), and not converted to CO, the measured concentrations of CO\({}_{2}\) and CO can be consulted. The measured CO\({}_{2}\) concentrations confirm that changes in the CH\({}_{4}\) concentration results in the opposite direction for the CO\({}_{2}\) concentration.