# Experimental Investigation of Oxygen Carrier Aided Combustion (OCAC) with Methane and PSA Off-Gas
## 3 Results
### Experiments with Methane
In cases where methane is completely converted, the concentration of CO\({}_{2}\) is equal to the inlet concentration of CH\({}_{4}\) at the inlet when taking into account the condensed steam, which does not reach the gas analyser. In the comparison between C28 and sand, it can, for example, be observed that the CO\({}_{2}\) concentration is higher in the bed for all the measurement points in the bed temperature  
Figure 6: Measured gas concentration of CH4 (in %, dry) at different distances from the distributor plate for all three bed materials where each subfigure presents a measurement at a certain bed temperature. interval 750-900 \({}^{\circ}\)C. To summarize, it can be clearly be seen that more fuel conversion takes place in the bed with the two oxygen carrier materials compared to sand. Some of the differences in methane conversion and CO\({}_{2}\) formation can be explained by the CO concentration measurements (see Figure 7). It can be observed that the CO concentration in the bed is overall highest in the system with ilmenite as a bed material. The concentration of CO is overall low in the freeboard, especially with sand and ilmenite. In the bed with C28, the CO concentration is generally the lowest in the bed but the highest above the bed. It is apparent that some of the CH\({}_{4}\) is converted to CO in the bed but that most of it is then converted to CO\({}_{2}\) in the lower part of the freeboard. The CO formation is generally small for all bed materials at high temperature, which aligns well with the previous observations. It should also be mentioned that the measured H\({}_{2}\) concentrations are overall low and have been neglected in this analysis. In addition, the dependency on the air-to-fuel ratio was investigated where the ratio was varied between 1 to 1.2 for the three bed materials at 750 \({}^{\circ}\)C. The results are presented in Table 5. It can be observed that the air-to-fuel ratio had a limited impact on the conversion of methane in the bed in general for the three bed materials although some improvement in fuel conversion is observed when increasing the AFR. It can also be seen that the same trend with a lower methane concentration with ilmenite and C28 as a bed material compared to sand is observed at each measurement point for other air-to-fuel ratios within the tested interval with the exception of MP1, where the methane concentration was slightly lower with sand as a bed material compared to using ilmenite. It should be noted that the choice of bed temperature for the variation in the AFR could influence the results since the difference in fuel conversion between sand and the two oxygen carriers is observed to be more pronounced at higher bed temperatures. Figure 7: Measured gas concentration of CO (in %, dry) at different distances from the distributor plate and at different bed temperatures for all three bed materials. The grey area indicates the bed where the dotted line indicates the estimated bed height for the respective bed materials. It should be mentioned that the system with and has a higher methane conversion in the freeboard than the system with C28. The results for sand and C28 presented in Table 4 show that the methane concentration is higher with C28 as bed material at MP5 even though more methane conversion takes place in the bed (see MP4). The same trend is observed when looking at the methane concentration at bed temperatures 800-850 \({}^{\circ}\)C.  
This could potentially be caused by the fact that the concentration of the methane is lower in that region, which results in it being more difficult to have the fuel ignition taking place in the gas phase, compared to the system with sand, where the gas mixture is closer to stoichiometric conditions. In addition to the results related to fuel conversion, the NO emissions were estimated, and the results are presented in Table 6. NO emissions from the process are based on the measured concentration of NO in the dry flue gas.