# Experimental Investigation of Oxygen Carrier Aided Combustion (OCAC) with Methane and PSA Off-Gas
## 1 Introduction  
In bubbling fluidized bed reactors with silica sand, it has been observed that most of the methane is converted in the freeboard [1; 2] when fluidizing with air and methane/natural gas at moderate temperatures. Methane is a very stable gaseous fuel. This is connected to the fact that it is sterically hindered, due to the symmetrical tetrahedral structure and the activation of the C-H bond with a dissociation energy of 439 kJ/mol [3]. Thus, ignition requires harsh conditions [4]. In a bubbling fluidized bed, the solid particles effectively hinder the combustion of methane. There are two main reasons for this. Firstly, due to the presence of particles, heat is transferred to the surrounding particles in contact with the bubble [5], which otherwise could result in a local temperature increase, which could lead to ignition. Secondly, it has been identified that free radicals (H, O, OH and HO\({}_{2}\)) are quenched by a high concentration of inert particles [6]. The radicals are required intermediates for homogeneous reactions, which result in an inhibition of the gas combustion in the dense bed [6] because the bed particles have a large surface area where radical recombination occurs [7]. It has been observed that there is a critical bubble size for the air-fuel-mixture with light hydrocarbons that is required before ignition may occur [7]. When using propane as fuel, it was shown that no combustion occurred in the emulsion phase [7]. Temperature has been identified as a key parameter where an increased dense bed temperature causes the methane conversion to gradually move downwards into the dense bed [2]. It was observedthat smaller and smaller bubbles are ignited when the bed temperature is increased [7] and since the bubble size is increasing when passing through the bed, this can be used to explain why the combustion reaction is moving downwards into the bed at higher temperatures. At a high enough bed temperature, it was observed that bubbles ignited very soon after being formed at the distributor plate [7]. Another interesting observation which has been made is that decreasing the gas flow rate shows an effect similar to that of increasing the bed temperature [2]. The studies that have been conducted so far use measurements of temperature and gas concentration along the length of the reactor to verify where methane conversion takes place [1,2]. The studies mentioned using low gas velocities, mostly in the range 2-2.5 times the minimum fluidization velocity. Another possible solution to move the methane conversion downwards into the dense bed could be to replace the inert bed material with active oxygen carrier particles. The underlying idea behind this is that the gas-solid reaction between methane and the oxygen carrier is not inhibited by temperature as much as the homogeneous reaction. Oxygen carrier materials have been tested for several years for the carbon capture technology Chemical Looping Combustion (CLC), where it has been shown that it is possible to convert methane in the fuel reactor where no gaseous oxygen is present [8]. These findings led to the introduction of the technology of Oxygen Carrier Aided Combustion (OCAC), where oxygen carrier materials are used with the intention of improving the oxygen distribution in fluidized combustion units. The technology was proposed in 2013, when it was successfully tested in a 12 MWth CFB unit for biomass combustion by the partial substitution of sand with ilmenite (iron-titanium oxides) [9]. Since then, ilmenite has been used in large-scale CFB plants for biomass combustion, where it has been observed to significantly reduce the need for excess air [10]. Ilmenite is often regarded as the primary choice among oxygen carrier materials. This can be explained by its mechanical durability, non-toxicity and relatively low cost [11]. However, in the development of Chemical Looping Combustion systems, more than 900 oxygen carrier materials have been examined [12] including materials manufactured specifically for high reactivity towards certain fuels.