Elsevier

Fuel

Volume 172, 15 May 2016, Pages 178-186
Fuel

Density-based separation performance of a secondary air-distribution fluidized bed separator (SADFBS) for producing ultra-low-ash clean coal

https://doi.org/10.1016/j.fuel.2016.01.006Get rights and content

Highlights

  • A secondary air-distribution fluidized bed was used to produce ultra-low-ash clean coal.

  • Operational factors were determined by verifying bed stability and density uniformity.

  • Response surface methodology was used to evaluate the significance of various factors.

Abstract

Stable fluidization with consistent flow and a reasonable separating density are keys to the highly efficient separation in a secondary air-distribution fluidized bed separator (SADFBS). In this study, a binary dense medium was formed by mixing fine coal with magnetite powders. The optimum secondary air-distribution layer (SAL) height of 13 mm was obtained by verifying the density uniformity of the bed. The suitable static bed height of 60–100 mm was determined by comparing the fluctuation of bed pressure drops. In addition, the distribution characteristics of the fine coal particles in various bed layers indicated that the feeding fine coal content should be not more than 10%, which was verified by the bubbling performance in the bed. Box–Behnken Response Surface Methodology (BB-RSM) was employed to evaluate the effects of static bed height (Hs), fluidization number (N), and feeding fine coal content (P) on the combustible material recovery θ of feed coal. Based on the experimental data, a mathematical model was established to describe the relationship between combustible material recovery and the operational factors. The influential degree of various factors on θ was N > Hs > P. The separation results of SADFBS suggested that θ value was maximized when Hs = 100 mm, N = 1.45, and P = 4%, which showed a good separation performance. The ash content of clean coal reached the lowest value of 2.65% when Hs = 80 mm, N = 1.70 and P = 7%, with a yield of 64.89% and a combustible material recovery of 74.86%. The product is generally considered to be ultra-low-ash clean coal which is the raw materials for activated carbon production.

Introduction

Coal continues to play an important role as a primary fuel in energy consumption. However, direct combustion and utilization of coal increasingly wastes the coal resource and aggravates environmental pollution. To economize the coal resources and protect environment against pollution, coal separation is implemented as a clean and efficient approach that can achieve the sustainable utilization of coals.

At present, water-based separation technologies are dominant in coal preparation industry all over the world [1]. However, water-based coal separation needs consume large amounts of water. It is well known that water shortage is a global problem. Many countries (e.g. South Africa, India, Australia, and China etc.) are facing the same problem of water shortage. The application of a water-based dense medium separation and flotation is widely restricted, whereas traditional hydraulic jigging is characterized by high separating density, but low separation precision. Thus, developments of efficient dry coal beneficiation technologies are extremely urgent, including air jigging separation [2], [3], [4], FGX separation [5], triboelectrostatic separation [6], [7], and air dense medium fluidized bed (ADMFB) separation [1].

Recently, an increasing number of studies have been conducted on the dry dense medium fluidized beds [1]. Tanaka et al. [8], Kubo and Zushi [9] used the calcium carbonate particles, glass beads, and zircon sand as the dense medium in the fluidized bed and achieved satisfied separation efficiency. Azimi et al. [10] exhaustively studied the effect of operating conditions on the performance of air dense medium fluidized bed for low-ash coal beneficiation. Sahu et al. [11] reported some valuable achievements on the fluidization stability and separation results of high-ash Indian coal by an air dense-medium fluidized bed separator. Firdaus et al. [12] successfully beneficiated different size fractions of coarse coal (5–31 mm) using an air fluidized bed dry dense-medium separator with silica/zircon sands as the fluidizing medium. Researchers at the China University of Mining and Technology (CUMT) have contributed to develop dry coal beneficiation technique using an air dense medium fluidized bed since 1980s, and have achieved a series of achievements [13]. Luo and Chen [14] mixed <1 mm fine coal and 0.15–0.3 mm magnetite powder to effectively separate 6–50 mm coal with a probable error E of 0.05 g/cm3. He et al. [15] investigated the density-based segregation and separation performances of a dense medium gas–solid fluidized bed separator (DMFBS) for coal cleaning in the laboratory scale. Wei et al. [16] studied a dual-density layer fluidized bed and designed a special fluidized bed with a conical transition section for three-product separation. This bed effectively simplified the dry coal preparation of an ADMFB. Conventional fluidized bed can be combined with the vibration energy applied to strengthen the gravity field and to effectively separate <6 mm fine coal, as manifested by a higher separation efficiency and a smaller probable error E value [17], [18], [19], [20]. Fan et al. [21], [22] proposed the magnetically stabilized fluidized beds to improve the separation efficiency of 1–6 mm coal as well. Luo et al. [23], [24] examined the effects of the secondary air-distribution layer (SAL) on the fluidization characteristic. The fluctuation of the bed pressure drop was considerably more stable in such a fluidized bed than in a normal bubbling bed. Furthermore, the fluidization stability and the uniformity of bed density were significantly improved by introducing SAL into the bed. These works have greatly enhanced the adaptation and selectivity of the dense medium gas–solid fluidized bed separation technology for run-of-mine (ROM) coal.

Despite previous study has discussed the fluctuations of bed pressure drop in a secondary air-distribution fluidized bed separator (SADFBS) and the effects of fine coal on bed density. However, few studies have investigated the density uniformity, distribution characteristics of the fine coal particles in a binary dense medium, and the separation efficiency of ROM coal by the SADFBS. In the study, 0.5–1 mm fine coal was mixed into 0.074–0.3 mm magnetite powder to form a binary dense medium in SADFBS. Therefore, this paper studied the effects of SAL height on the uniformity of bed density, the variation of bed pressure drop at various static bed heights, and the effects of fine coal content on the fluidization stability. Moreover, Response Surface Methodology (RSM) approach was employed to characterize the effects of various operational factors on the separation performance of low-ash ROM coal, thereby producing ultra-low-ash clean coal. This study can provide some fundamental results for the design and optimizations of the fluidized bed separator and operational parameters.

Section snippets

Experimental apparatus

The schematic drawing of SADFBS system is illustrated in Fig. 1 and mainly consists of air supply system, airflow control system, dense medium fluidized bed, dust collection system, and data measurement devices. Dense medium particles were fluidized in a vertical cylinder with a diameter of 120 mm and a vertical height of 260 mm that was made from transparent plexiglass. The self-designed air distributors were used to achieve uniform air distribution in SADFBS, which was made of a

Effects of the SAL height Hd

The optimum value of SAL height Hd was determined by the verification of the bed density fluctuation across the whole fluidized bed. As shown in Fig. 3, the mean bed density varied in a small range of 2.06–2.10 g/cm3 at various Hd with the standard deviation of bed density Sρ of 0.041 to 0.064 g/cm3. Sρ value gradually decreased and then rapidly increased with the increase of Hd from 7 to 19 mm. The minimum Sρ value of 0.041 g/cm3 was obtained when Hd = 13 mm, indicating the optimal uniformity of the

Conclusions

In this study, the fluidization characteristics and density-based separation performance of the SADFBS were experimentally studied in detail. The optimum secondary air-distribution layer height of 13 mm was obtained with the minimum Sρ value of 0.041 g/cm3. The static bed height should be maintained within a suitable range of 60–100 mm. The significantly deep fluidized bed may result in intensive fluctuation in bed pressure drop, suggesting adverse fluidization stability. Additionally, the feeding

Acknowledgments

We gratefully acknowledge the financial support from The Natural Science Foundation of Jiangsu Province of China (No. BK20130196), Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20130095120006), China Postdoctoral Science Foundation (No. 2014T70564, No. 2013M531430) and The Postdoctoral Science Foundation of Jiangsu Province of China (No. 1302145C). A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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