# Experimental Investigation of Oxygen Carrier Aided Combustion (OCAC) with Methane and PSA Off-Gas
## 3 Results
### Experiments with Methane
This could, therefore, limit the possibility to observe changes in measured temperature based on temperature differences in the gas, especially in the freeboard where the presence of particles is scarce. In order to get a better idea about the differences between the three bed materials, gas concentrations measurements should, therefore, be consulted. In relation to the gas concentration measurements, it is somewhat uncertain how representative the measured value for the average gas concentration in the bed is. There are two main reasons for this. The first reason for this is related to the nature of fluidized beds, where the gas concentration can be assumed to be different in the emulsion phase and in the bubble phase, at least according to the two-phase theory for fluidization. As a result of the presence of these two theoretically distinguishable phases, it could be expected that the probe manages to sample more gas from the emulsion phase where the gas is flowing upwards with a lower velocity. It is, in this experiment, unknown from which phase the extracted gas originates from (emulsion phase, bubble phase or both) which could have an impact on the measured concentration. It should also be mentioned that the gas velocity is the same for the different bed materials and the particle size is similar. A quick estimation of bubble diameters, bubble velocities and bubble fractions based on equations provided by [PERSON] and [PERSON] [31] show that only minor differences between the three bed materials can be expected. The same conclusion is drawn when comparing the apparent overall coefficients of gas interchange between bubble and emulsion phases based on the equations presented by [PERSON] and [PERSON] [32]. This indicates that the three cases should have a similar share of extracted gas from bubble and emulsion phase and that the mass transfer between bubble and emulsion phase should be similar. In summary, the results for different bed materials should be useful for comparison. The second reason is that the measured gas concentration is affected by the depth at which the gas sampling takes place. This was evaluated in separate experiment with sand as bed material where the gas sampling point was varied between one side of the reactor wall to the other. In this experiment it could be observed that less fuel conversion took place in the middle of the tube where a higher CH\({}_{4}\) concentration and lower CO\({}_{2}\) concentrations was observed, compared to close to the reactor walls where more fuel was converted. This could, for example, be explained by the fact that bubbles are more easily formed in the middle of the bed and gas present in the bubbles has lower residence time in the bed. At 900 \({}^{\circ}\)C, the difference in measured CO\({}_{2}\) concentration was quite significant at MP3, 6.9% in the middle compared to around 9.6% by the reactor walls. These measurements supported the choice to place the gas sampling location at a point approximately halfway into the middle of the tube, which should provide an estimated gas concentration, which is the most representative overall. It should be said that, at 600 \({}^{\circ}\)C, the measured concentrations were already very similar at MP1 and throughout the whole bed. This indicates that the gases were well mixed when entering the bed and a larger concentration difference over the cross section when the in-bed fuel conversion is higher. The measured concentration should, nevertheless, provide for a good comparison between different cases and it is also observed overall that the measured concentrations align well with the measured values in the freeboard which supports the conclusion that the measured values are reliable. A first observation, which can be seen in Figure 5, is that the methane concentration in the bed decreases with increasing bed temperature, thus displaying that more combustion of methane takes place in the lower part of the reactor at higher bed temperatures.