“…Very few studies [9,32] take this feature into account; consequently, it is often overlooked in experimental and simulation studies. Second, the re-distribution of the reducing gas by the raceway is often simplified [32] or neglected [28] owing to a lack of understanding regarding its formation, shape, and size. Recently, the importance of the raceway size [13,[33][34][35] and shape [33,36] in the development of the gas and fines flow behaviour has been recognized and new methods, such as X-ray tomography [15,18], DEM [37][38][39][40][41][42], Kinetic Theory of Granular Gases (KTGG) [43], isostress boundary models [23,24], and fractal theory [44,45] are being employed to gain additional insights.…”