# Experimental Investigation of Oxygen Carrier Aided Combustion (OCAC) with Methane and PSA Off-Gas
## 3 Results
### Experiments with Methane
This is expected since methane combustion is promoted by a higher temperature in general and, therefore, there is less unreacted methane in the freeboard. This results in a temperature drop in the freeboard when little or no unreacted methane is left in the flue gas (see Figure 4) and heat is instead transferred from the reactor towards the inside of the furnace, which was approximately 50-100 \({}^{\circ}\)C cooler than the targeted bed temperature. At a bed temperature of 600 \({}^{\circ}\)C, there is almost no methane conversion taking place in any of the systems. When the bed temperature was increased further, ignition of the fuel took place in the lower part of the freeboard with a visible flame and audible sound, which occurred around 700 \({}^{\circ}\)C for all bed materials. At a bed temperature of 750 \({}^{\circ}\)C, a significant part of the methane appears to be burnt between MP4 and MP5, which is the lower part of the freeboard, which is in line with the observations in the temperature profile in Figure 4, where the temperature was more constant at 750 \({}^{\circ}\)C between MP4 and MP5 compared to higher bed temperatures. At higher bed temperatures, a clearer decrease in temperature was observed between these two measurement points. This could be due to almost no remaining unreacted methane being detected in the freeboard at bed temperatures > 750 \({}^{\circ}\)C, resulting in a flue gas with mostly inert compounds at MP6 or higher up in the freeboard. It can be noted that it was difficult to keep the wind box temperature below 500 \({}^{\circ}\)C in some cases and especially with ilmenite as a bed material. Based on the measured gas concentrations, it is, however, clear that ignition did not take place in the wind box. In the experiments with C28 as bed material, it can be observed that the measured methane concentration is lower in the lower part of the bed (MP1-MP3) compared to just above the bed at MP4 and the measured concentration is almost the same at MP5 at a bed temperature of 800-850 \({}^{\circ}\)C. Some indications of similar character can be observed with ilmenite at a bed temperature of 800-850 \({}^{\circ}\)C. This does not occur with sand as the bed material, where the methane concentration is gradually reduced along the height of the reactor. This might seem surprising at first, but this is most likely explained by the gas-solid reactions with the oxygen carrier. As described earlier, it is probable that gas from the emulsion phase is sampled compared to gas in the bubble phase. In the systems with oxygen carrier materials, gas-solid reactions are likely to occur in the emulsion phase, where gas-solid contact is better. It could, therefore, be possible to obtain a higher methane conversion in the emulsion phase if the oxygen carrier has a high reactivity with the fuel gas (such as, for example, C28). At the top of the bed the gas from emulsion phase and bubble phase is mixed and the sampled gas in the freeboard can be expected to contain more equal quantities of gas from bubble and emulsion phase. This could, therefore, explain the increased methane concentration with the oxygen carriers at MP4-MP5, where gas from the bubble phase is expected to have a higher methane concentration. The increase in methane concentration is a result of the amount of gas in the bubble/emulsion phase and the gas conversion in each phase. Nevertheless, the results support the hypothesis that the  
Figure 5: Measured gas concentration of CH\({}_{4}\) (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. gas-solid reaction takes place in the emulsion phase, which takes place to an especially high extent with C28 as the bed material. In order to simplify a comparison between the different bed materials, Figure 6 can be consulted, where the results for all bed materials are added together at the respective bed temperatures.