# Experimental Investigation of Oxygen Carrier Aided Combustion (OCAC) with Methane and PSA Off-Gas
## 3 Results
### Experiments with Methane
Some NO\({}_{2}\) is also likely present albeit in smaller concentration, typically NO\({}_{2}\) is less than 5% of the total NO\({}_{\rm x}\)[33] meaning that the NO concentration should be close to the NO\({}_{\rm x}\) concentration in the furnace and at the flue gas stack. The data for MP7 with sand as a bed material at 850 \({}^{\circ}\)C are missing. It can be observed that the measured concentrations of NO are overall low, especially at high temperature. It can also be observed that almost no NO at all is detected in the bed. It may seem contradictory at first that more NO is formed at a lower temperature, since the formation of thermal NO\({}_{\rm x}\) is expected to be the dominating mechanism for NO formation, since no nitrogen is present in the fuel and no prompt NO can be expected to be formed in this oxygen-rich atmosphere. The tendency for higher NO formation at moderate temperatures could, however, be explained by knowing where in the bed, as well as how, the fuel conversion takes place. The highest NO concentrations are measured above the bed in the cases where a significant part of the methane fuel conversion takes place above the sand bed, i.e., at a bed temperature of 750 \({}^{\circ}\)C. This was also the temperature level where flames were most visible and the most audible and visible explosions took place above the bed, as observed from the top of the reactor. Fuel-NOx formation can be excluded as the  
\begin{table}
\begin{tabular}{c|c|c|c|c|c|c|c|c|c|c|c|c|c} \hline \multicolumn{1}{c}{} & \multicolumn{4}{c}{**Sand**} & \multicolumn{4}{c}{**Ilmenite**} & \multicolumn{4}{c}{**C28**} \\ \cline{2-13} \multicolumn{1}{c}{} & 600\({}^{\circ}\)C & 750\({}^{\circ}\)C & 800\({}^{\circ}\)C & 850\({}^{\circ  
fuel does not contain any nitrogen compounds, and since the system operates with excess oxygen, any significant prompt NOx formation is unlikely. However, the NO formation can be explained by local hotspots from uncontrolled flame combustion in the freeboard where the thermal inertia is low. Considering the composition of the gas mixture, a very significant increase in gas temperature is expected as the gas ignites. Since thermal NOx formation increases exponentially with temperature, the flame temperature should be controlled. In the bed, where the heat transfer between gas and solids, as well as the thermal inertia, is high, no hotspots are expected and NO formation is not expected, which is confirmed by low NO concentration in that zone for all bed materials regardless of the amount of fuel conversion in the bed. However, high fuel conversion in the bed gives less NO formation in the freeboard and, thus, lower overall emissions, which is also seen by the performed measurements. The measured freeboard gas temperature is close to the temperature of the bed and, thus, is clearly lower than the temperature at which thermal NO formation takes place, according to the Zeldovich mechanism, where local temperatures of at least 1300 \({}^{\circ}\)C are required [34]. The dominating heat transfer mechanism to the thermocouple is radiation from reactor walls and the bed material, as well as direct heat transfer from the walls. With the short path lengths, the emissivity of gas radiation should be less than 0.05, and convection heat transfer from gas is also small. This would mean that local high gas temperatures are poorly visible in the measurement. Thus, the absence of a rise in temperature from MP4 to MP5 in Figure 4, despite significant fuel conversion taking place--Figure 5--could be explained by a combination of falling wall temperatures and poor heat transfer from the gas. It should also be added that there is a difference between flames produced in a fluidized bed and flame combustion with a gas burner for example. There is no sustained flame in the fluidized bed and local measurements cannot be made to characterize the flame and determine local temperatures and gas compositions, but the work must rely on global measurements.