# Experimental Study of Entrainment and Mixing of Renewable Active Particles in Fluidized Beds
## 1 Introduction  
Nowadays, the waste-to-energy principle becomes popular due to the energy problem and the global awareness of the dangers of greenhouse emissions. Fluidized bed conversion is a clean energy technology that utilizes a wide variety of solid hydrocarboneous fuels such as coal, biomass, and municipal solid waste. Fluidized beds have excellent mixing and heat transfer characteristics [1]. However, their operating parameters were originally optimized for coal. The current trend is to partially or fully replace fossil fuels by renewable fuels such as solid refused fuel (SRF) and biomass. The energy content of these renewable fuels is competitive to lignite as shown in Table 1. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline  & **SRF** & **Bark** & **Sunflower shell** & **Wheat straw** & **Lignite** \\ \hline Calorific value, MJ/kg & 13.6–14.9 [2] & 16.0–17.5 [3] & 17.4–18.8 [4] & 18.5–19.8 [5] & 23.6–24.6 [6] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of the calorific values of renewable fuels and lignite. The challenge is that these renewable fuels have different physical, chemical, and geometrical properties. Thus, to reach the efficient processing of these heterogeneous fuels in fluidized beds, there is a need to understand their fluid-dynamics and mixing characteristics. There are plenty of literature studies that investigated the thermochemical conversion of renewable fuels [7; 8; 9; 10; 11; 12; 13; 14; 15]. However, only a few of them addressed their fluid dynamical context. A recent paper [16] studied the vertical arrangement of SRF in binary-mixture bubbling fluidized beds. The authors reported the PI-segregation phenomena, that is, a simultaneous separation from the bed body regime towards the top and bottom regimes at low velocities. [PERSON] et al. [17] and [PERSON] et al. [18] investigated the effect of particle size distribution of the bed material on the gas yield of biomass gasification. They concluded that wider particle size distribution (PSD) enhances the gas yield due to the higher carbon conversion efficiency. [PERSON] et al. [19] studied the effect of the hydrodynamic parameters on char mixing in a dual fluidized bed biomass gasification plant. They used bronze and polyethylene particles as representatives of the bed and char particles, respectively, in their cold flow model. They measured the char concentration in the recirculation stream by taking samples from the return leg. They reported that the char concentration in the recirculation stream decreases with increasing the bed height. And, rising the fluidization rate increases the char concentration in the recirculation stream. [PERSON] et al. [20] used a density-similar magnetic particle tracer to simulate the mixing and segregation of different active particles in a cold-flow bubbling fluidized bed. They identified three fuel segregation regimes based on the fluidization velocity and the density of the tracer particle. Earlier, [PERSON] et al. [21] investigated the transport velocity of coarse particles in a multi-solid fluidized bed system. They reported that the transport velocity of coarse particles decreases with increasing the solids circulation rate. Concluding over the literature survey, we can say that the study of the fluid dynamic characteristics of coarse active particles was not covered with sufficient details. For example, the majority of the literature measurements were performed on representative particles (spherical tracer or spherical beads). However, the real heterogeneous properties of the active particles are more complex and are expected to have some differences in behavior compared to the representative spherical particles. In the present study, entrainment and mixing measurements are performed on SRF, bark, sunflower shell, and wheat shell. The results of these nonspherical and heterogenous are presented and analyzed. The entrainment results of the investigated fuel particles were plotted on the Haider-Levelspiel chart to obtain the representative sphericity values.