# Experimental Investigation of Oxygen Carrier Aided Combustion (OCAC) with Methane and PSA Off-Gas
## 3 Results
### Experiments with Off-Gas  
The same bed materials (sand, ilmenite and C28) were also evaluated with PSA off-gas as a fuel at five different bed temperatures (600, 650, 700, 750, 800 \({}^{\circ}\)C). The data from the experiments using PSA off-gas as fuel are also presented in Tables S4-S7 in the supplementary material. The vertical temperature profile of the systems for the experiments can be observed in Figure 9. In the experiments with off-gas as fuel, the average bed height was 15.4 cm for sand, 15.6 cm for ilmenite and 14.6 cm for C28. Figure 8: Measured dry concentrations of NO and CH\({}_{4}\) for sand at 750 \({}^{\circ}\)C and at 900 \({}^{\circ}\)C with C28 as bed material at different heights from the distribution plate. The grey area indicates approximately where the bed is located. At high temperatures, it was difficult to keep the temperature in the wind box below the limit at which the gas mixture ignites in the wind box. This occurred for all bed materials when operating at a bed temperature of 800 \({}^{\circ}\)C and with C28 at 750 \({}^{\circ}\)C. In those cases, the wind box temperature was significantly higher and complete conversion of CH\({}_{4}\), CO and H\({}_{2}\) was observed at all measurement points, which is why these results were excluded from this article. The apparent reason for this taking place is that the cooling of the wind box was not sufficiently efficient to hinder ignition at these temperature levels. For these cases, it is observed that the fuel is almost completely converted already at MP1, which makes these cases irrelevant in the analysis. The temperature profiles overall look quite similar for the three bed materials, although some differences are seen in the freeboard. In order to get a better overview of the fuel conversion in the reactor, the gas concentration measurements of fuel gas components are evaluated. A first observation, which could be made from this data, is that very little/no hydrogen is measured during any of the experiments, which indicates that hydrogen reacts quickly in presence of oxygen, as could be expected. The focus was therefore to evaluate the concentrations of CO, CH\({}_{4}\) and CO\({}_{2}\) to monitor the fuel conversion in the system. The ratio of the measured concentration of unburnt carbon in the form of CO and CH\({}_{4}\) in relation to the total measured carbon (CO, CH\({}_{4}\) and CO\({}_{2}\)) as a measure of the gas conversion. The results are presented in Figure 10, where ratio at the inlet of 40% is also included. Figure 9: Measured temperatures at different measurement points (MP0–MP8) for five different bed temperatures (from 600 to 800 \({}^{\circ}\)C) for all three bed materials. It can be observed that part of the PSA off-gas is already burnt in the bed at a bed temperature of 600 \({}^{\circ}\)C with all bed materials. It is interesting to see that some fuel conversion takes place at these moderate temperatures, which are low in relation to optimal temperatures for OCAC operation. It can also be seen that a significant part of the fuel conversion takes place above the bed and almost all fuel is converted at MP5 \(\geq\) 650 \({}^{\circ}\)C with all bed materials. Close to complete carbon conversion is observed at 700-750 \({}^{\circ}\)C at the top of the reactor with all three bed materials. The carbon conversion in the bed is also similar for the bed materials. The results in Figure 10 indicate overall small differences between the different bed materials. This suggests that the choice of bed material may not be so important to achieve high fuel conversion with PSA off-gas as fuel, as opposed to methane where there was a visible difference. At a bed temperature of 750 \({}^{\circ}\)C, for example, it is observed that the system with sand as a bed material has a lower content of unburnt carbon in the bed than the system with ilmenite at MP1-MP4.