Comparison of hydrodynamics in a gas-solids fluidized bed with binary particle systems for dry coal beneficiation

Highlights

• Gas fluidized bed with binary particle species were studied for coal beneficiation.

• Binary systems with same mean diameter and same aerodynamic diameter were compared.

• Effect of compositions in binary system on U_mf and bed expansion was studied.

• Bubble number, size and rise velocity were characterized from processed images.

• A linear relationship between the bubble size and bubble rise velocity was found.

Abstract

Fluidization behaviour of two binary particle systems, magnetite225-sand225 (M225-S225) particles with the same mean diameter (d_p) and magnetite225-sand304 (M225-S304) particles with the same aerodynamic diameter (d_a) were observed in a two-dimensional fluidized bed. The number, size and rise velocity of gas bubbles in the binary fluidized bed were characterized by image analysis. A decreasing minimum fluidization velocity (U_mf) with increasing sand volume fraction was found in the M225-S225 system while a nearly constant U_mf was found in the M225-S304 system. Increasing trends of both bubble size and bubble rise velocity along the bed height were detected and a linear correlation between the bubble size and rise velocity was found for binary systems. The higher the fraction for one type of particles in the binary system is, the closer to the corresponding single-component system the hydrodynamic phenomena and bubble properties will be in both binary systems.

Introduction

Coal is important as a steadily available source of energy worldwide due to its vast availability underground. Its market share ranks the second largest in energy source with 27.0% in 2020 (Statistical Review of World Energy | Energy economics | Home, (n.d.). https://www.bp.com/en/global/corporate/energy-economics/statistical-review-of-world-energy.html (accessed June 4, 2021). The separation of high-quality coal with less impurities is of great significance since it not only improves the combustion efficiency with a higher level of productivity, but also reduces environmental pollution due to less CO₂ and SO₂ emissions (Luo et al., 2008). Coal beneficiation processes that aim to remove impurities from raw coal have therefore become increasingly important in producing high-quality coal and reducing emission of air pollutants. There are two coal cleaning technologies: wet beneficiation and dry beneficiation (Chen and Yang, 2003). When compared to the wet beneficiation process that consumes large amounts of water, the dry coal separation technology based on gas-solids fluidization is expected to be the dominant method in the near future due to its distinct advantages such as process water conservation, low moisture content after beneficiation, lower environmental pollution and so on (Luo et al., 2008) (Luo et al., 2019).

Because raw coal typically possesses a lower density than the impurities (e.g., gauge), a gas-solids fluidized bed was taken into account for dry coal beneficiation because the overall bed density of the fluidized bed can be adjusted by varying the operating gas velocity for possible density separation (Luo et al., 2019). Too much adjustment in gas velocity would, however, cause other problems such as high energy consumption due to the high gas velocity and density disturbance by large bubbles in the coal beneficiation process (Fu et al., 2019). Therefore, a binary mixture of particles was introduced as the fluidization medium to substitute for single-component particles to help adjust the overall bed density. A wider range of raw coal can thus be chosen while avoiding energy-consuming pre-treatment processes (Fu et al., 2019). An air dense-medium fluidized bed (ADMFB) with binary particle systems is commonly used in dry coal beneficiation processes in industry. With the help of the adjusted overall bed density, the ADMFB allows impurities with higher density than the bed medium to sink to the bottom, while lighter coal floats on top of the bed (Zhenfu and Qingru, 2001, Dwari and Rao, 2007).

The selection and proportion of the binary particle system is of great importance for the separation efficiency of dry coal beneficiation in the gas-solids fluidized bed. The first priority is to achieve a desired bed density via the combination of the binary particle systems (Luo et al., 2019). In addition, the mixture density of the particle combination should be as uniform as possible throughout the whole fluidized bed so that the light coal is able to float constantly on the top of the bed (Fu et al., 2019). Therefore, the ADMFB ideally should operate under a low gas velocity in the bubbling flow regime in order to maintain the stability of the gas-solids flow (Luo et al., 2008, Fu et al., 2019, Firdaus et al., 2012, Formisani et al., 2011). The size, frequency, and distribution of the gas bubbles has a significant impact on the overall uniformity of the gas-solids flow in the fluidized bed (Formisani et al., 2011, Busciglio et al., 2012, Andreux and Chaouki, 2008, Ma et al., 2016). Moreover, the break-up and coalescence of the bubbles take place in all areas of the fluidized bed, leading to the unfavourable variation of the local bed density due to the unstable flow conditions (Busciglio et al., 2012). However, very few studies were carried out on the bubble dynamics in the fluidized bed with a binary particle system in the past. The effect of the selection and the proportion of the binary particle system on the generation and behavior of the gas bubbles needs to be investigated for the future application of the dry coal beneficiation process.

The measurement techniques used to determine the bubble properties in fluidized beds can be generally divided into intrusive ones such as probes and non-intrusive ones such as photographs (Andreux and Chaouki, 2008, Ma et al., 2016, McKeen and Pugsley, 2003, Werther and Molerus, 1973, Park et al., 1969, Busciglio et al., 2008, Agrawal et al., 2018, Lv et al., 2018, Singh et al., 2019). Compared to the intrusive methods, the visualization of the gas-solids flow by image processing is more appropriate as it avoids disturbing the local flow field. In recent years, the image visualization method had been widely applied to 2D fluidized beds with a small depth for the observation of bubbles (Busciglio et al., 2012, Ma et al., 2016, Rowe and Partridge, 1997). Muddle et al. (Mudde et al., 1994) investigated bubble behaviour of single-component particles in a 2D gas–solid fluidized bed and attempted to determine the wake angle and wake area of bubbles using image analysis technology. Busciglio et al. (Busciglio et al., 2012) carried out a series of experiments to measure bubble characteristics in binary mixtures of corundum and glass beads of different sizes but the same density by means of digital imaging.

However, research on the bubble dynamics of particle mixtures with the same size but different densities as a function of fluidized bed properties is scarce. In addition, there has been no reports on binary systems with the same aerodynamic diameter (d_a). In reality, the binary particle system with the same d_a is worth being investigated as the fluidization behavior of particles relates more closely with the aerodynamic diameter rather than the physical diameter. With the development of photograph technology, a clearer visualization of the fluidized bed can be obtained to provide a deeper understanding of the generation and growth of the bubbles under the effect of binary particles. In this work, the bubble characteristics of binary mixtures in a rectangular fluidized bed were studied and compared using image processing technology. Magnetite and sand particles were selected as the material for the binary mixtures because these two kinds of particle materials are commonly used in the ADMFB for coal beneficiation. Two binary particle systems, magnetite225-sand225 (M225-S225) particles with same mean diameter (d_p) and magnetite225-sand304 (M225-S304) particles with same aerodynamic diameter (d_a) were studied. Different compositions of the binary mixture were tested to determine the optimal fluidization condition for the coal beneficiation process. The size, distribution, rising velocity of the bubbles, and the bed expansion were measured from the processed images of the local flow field. Correlations of the bubble rise velocity and the bubble size were proposed for the binary particle systems and the effects of the excess gas velocity and particle composition are discussed. The dry coal beneficiation process requires a smooth fluidization of the binary fluidized bed where a dilute phase consist of small and stable gas bubbles and a fully mixed and uniformly expanded dense phase are favourable. Since the fluidization quality of the binary system relies heavily on the particle properties and the composition of the two types of the particles, it is of great importance to investigate the variations of the hydrodynamics such as the bubble behaviour and bed expansion with the properties of bed materials and the mixing process for a better separation of the coal.