Experimental evaluation of lateral mixing of bulk solids in a fluid-dynamically down-scaled bubbling fluidized bed Erik Sette ⁎, David Pallarès, Filip Johnsson Chalmers University of Technology, Dept. of Energy and Environment, SE-41296 Göteborg, Sweden abstract article info Article history: Received 14 January 2014 Received in revised form 24 April 2014 Accepted 26 April 2014 Available online 5 May 2014 Keywords: Fluidized bed Solid lateral mixing Scaling An indirect tracking method for bed material using magnetic separation was applied to a fluid-dynamically down-scaled fluidized bed, to evaluate the influences of different parameters on the lateral dispersion coeffi- cients of the bed material. Solutions to the transient diffusion equation were fitted to the experimental data and showed that the dispersion approach could be used to describe the lateral mixing of solids at the macroscopic level. The values obtained for the dispersion coefficient were scaled-up to be relevant to large-scale boilers oper- ated under high-temperature conditions. The scaled-up lateral solid dispersion coefficients were in the order of 10 −2 m 2 /s, i.e., two orders of magnitude greater than those reported in the literature for smaller sized fluidized and/or fluidized beds operated under ambient-temperature conditions. This paper also considers the mixing phenomena at the mesoscopic level, applying the so-called “mixing cell” concept to elucidate how the mixing of solids is dependent upon the flow characteristics around the main bubble paths. © 2014 Elsevier B.V. All rights reserved. 1. Introduction In most large-scale chemical processes, such as the combustion and gasification of solid fuels using the fluidized-bed technology, the mixing of inert solids is of major importance [1]. Mixing governs not only how fast the fuel is mixed throughout the unit (mainly through solid–solid interactions), but also how good the contact is between the fuel and the gas phase. In addition, an increase in lateral mixing of the bulk bed material creates a more homogeneous temperature field across the cross-section of the bed owing to the strong thermal inertia of the solids. Thus, the mixing of the solids controls the mass and heat transfer, which in combination with the chemical kinetics governs the conversion of fuel in combustion and gasification processes. Large-scale fluidized bed units can be operated under either bubbling or circulating condi- tions [2]. Regardless of the mode of operation, large-scale fluidized bed boilers for solid fuel conversion are generally operated with a dense bottom bed [3]. It is important to understand the mixing phe- nomena in these beds, so as to develop fluidized-bed modeling that is applicable to both combustion and gasification systems. Dense bottom beds in large-scale units have a low aspect ratio of b 1 [4], which consid- ered in combination with lateral solids mixing (being at least one order of magnitude lower than that in the vertical direction) [5,6] means that the mixing of solids in the lateral direction is a limiting process that requires closer investigation. In fluidized-bed combustion, the limitations associated with solids mixing may result in large variations in temperature, and consequently, variations in the combustion rate, across the furnace. This is critical when burning highly volatile fuels, which typically engender strongly reducing conditions at the furnace walls that hold the fuel inlets and re- gions of oxidation at locations further away from the fuel entrance. These spatial differences in oxidation/reduction may result in high levels of emissions of unwanted species, corrosion, and hot-spots [7]. To prevent these effects, fluidized-bed boilers are operated at higher level of excess air and reduced steam temperatures, as compared with boilers that burn low-grade fuels. Thus, although fuel flexibility is one of the main advantages of the fluidized-bed technology, possibilities remain to improve the efficiency of combustion of low-grade fuels by decreasing the level of excess air and increasing the steam temperature, which can be achieved by improving the lateral mixing of solids. In contrast to direct combustion, the performance of dual fluidized bed systems, such as indirect gasification [8] and chemical looping combus- tion (CLC) of solid fuels [9], may benefit from limited lateral mixing of solids. Thus, moderate levels of lateral mixing in indirect gasifier beds and in CLC fuel reactor beds increase the fuel residence time, thereby minimizing losses of unconverted char to the secondary reactor [8,10]. However, the rate of mixing of bed materials in such units has to be suf- ficiently high to maintain the optimal bed temperature across the bed, which enhances the relatively slow and temperature-sensitive reaction rates of gasification. In summary, there is a need to improve current un- derstanding of the phenomena that govern the process of lateral mixing of solids in fluidized beds. One of the difficulties encountered when interpreting the results of studies reported in the literature that have focused on the lateral mixing of solids is that they often applied gas distributors with a pressure drop that is considerably greater than that typically employed in large-scale Powder Technology 263 (2014) 74–80 ⁎ Corresponding author. Tel.:+46 31 772 1446. E-mail address: sette@chalmers.se (E. Sette). http://dx.doi.org/10.1016/j.powtec.2014.04.091 0032-5910/© 2014 Elsevier B.V. All rights reserved. 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