# Experimental Investigation of Oxygen Carrier Aided Combustion (OCAC) with Methane and PSA Off-Gas
## 2 Materials and Methods  
### Reactor System  
The experiments were carried out on a laboratory-scale high temperature 253 MA steel reactor (see Figure 1). The reactor is 1.27 m high and has a 77.9 mm inside diameter. The reactor is placed in an electrically heated furnace with three different heating zones, which can be controlled separately by adjusting targeted furnace temperature in each zone. The fluidization gas consists of air and fuel gases, which are mixed in a T-connection outside of the furnace before entering the wind box. The fuel gas consists of either 99.5% methane or a gas mixture representing a typical PSA off-gas composition. The gas mixture entering the wind box then passes through a 5 mm thick hole plate with 61 0.6 mm holes to the bubbling fluidized bed. The bottom plate was designed to provide sufficient pressure drop to enable a good gas distribution. The fuel gas flow was controlled using mass flow controllers (MFCs). In order to ensure that the fuel is converted inside the bed and nowhere else it is important to consider the risk of fuel ignition in the wind box of the reactor. The temperature at which this can occur is best described by the so called autoginition temperature, which is the lowest temperature at which a certain substance spontaneously ignites without the presence of an external source of ignition, such as a flame or spark. The fuel/air mixtures, which result in the lowest autoignition temperature, are different for different fuels, but the autoignition temperature is approximately 500 \({}^{\circ}\)C for hydrogen, 580 \({}^{\circ}\)C for methane and 609 \({}^{\circ}\)C for carbon monoxide [27]. As soon as the ignition reaction is initiated, the exothermic process rapidly increases the temperature which, in turn, causes positive feedback for the combustion of the fuel. The wind box temperature should therefore be limited. Thus, a cooling coil with a 6 mm outer diameter is placed in the wind box. Cooling air is flowing through the coil at high velocity and the coil makes several passes along the walls of the wind box (see Figure 1). The air flow in the cooling coil is controlled manually with the goal to keep the wind box temperature below 500 \({}^{\circ}\)C, in order to reduce the risk of fuel ignition in the wind box. The approach to premix air and fuel in the wind box has been used also in previous experiments to study methane combustion in a BFB [1,2]. Figure 1: Schematic illustration of the BFB reactor system with gas sampling tubes to the left and the inclined tubes for the pressure transducers and thermocouples to the right, each corresponding to a certain measurement point (MP0 to MP8). Due to the size of the reactor its surface area to volume ratio is low, resulting in significant heat losses to the surroundings. Consequently, the reactor is placed in an electrically heated furnace with three different heating zones, which can be controlled separately to reach the target temperature for the system. The temperature is measured in the wind box as well as at the nine vertical positions (MP0-MP8)--see Table 1--by using thermocouples measuring the temperature approximately 1 cm from the inside reactor wall. The thermocouples, type K, tolerance class 1, have a common reference point and are connected to an NI 9213 measurement module where the measurement is done in high-resolution mode with a measurement accuracy of < 0.02 \({}^{\circ}\)C. The second measurement point located approximately 9 cm above the distribution plate, MP2, was chosen as representative for the bed temperature and was thus closely monitored during the experiments. The tubes where the thermocouples are introduced are also used to measure the pressure. Gas present in this volume is in connection with pressure transducers, which are used to measure the pressure. It is also possible to use the same tube connections to flush the tubes with nitrogen, to remove accumulated bed material.