Dewatering of fine coal slurries by selective heating with microwaves
Introduction
The presence of a variety of impurity phases (clays, quartz, pyrite, etc.) in mined coals requires extensive grinding and preparation to separate the useful carbonaceous material from the impurities [1]. In coal preparation, water is used extensively which must be removed as much as possible before coal can be shipped and burnt in power plants. Mechanical dewatering methods (e.g. centrifuges) are generally ineffective for finer coal particles [2]. Thermal dewatering methods are not cost effective since they heat both water and coals, the latter resulting in the loss of BTU value of coals. Because of these difficulties, most of the finer coal fractions are abandoned and relegated to settling ponds as slurries. These settling ponds represent serious environmental threat for the surrounding communities in addition to loss of valuable carbons. Consequently, a need exists to find an effective process for dewatering fine coal slurries.
In this work, we present the results of our studies on the dewatering of a fine coal slurry using conventional microwaves at 2.45 GHz. The basis for microwave drying is the selective absorption of microwave power by water molecules since the carboneaus matter is known to absorb microwave energy relatively poorly [3]. This process has been discussed in earlier studies by Lyons et al. [4], Lindroth [5], Chatterjee and Misra [6], Perkin [7] and Standish et al. [8] in connection with drying of coals and other porous materials and in recent studies by Marland et al. [9] in connection with coal grindability and by Lester and Kingman [10]. The power P absorbed by a material from electromagnetic (em) radiation of frequency f can be written as [3], [4], [5], [6], [7], [8], [9], [10], [11]Here E0 is the amplitude of em radiation and ε″ is the loss factor of the material at frequency f. At f = 2.45 GHz, ε″ = 10 for water compared to ε″ = 0.1 for coal, making P about 100 times more for water than that for coal [3]. This provides the basis for the selective absorption of em radiation by water present in coal, without significantly affecting the temperature of the carbon particles. In addition, microwaves penetrate deeply in the coal carbons thus heating coals inside out as compared to conventional thermal heating from the outside inwards using the standard heat transfer mechanisms. The results presented below compare conventional thermal heating with microwave heating of a fine coal slurry sample and show that energy efficiency above 80% of the theoretical limit is obtained by microwave dewatering. Results of our experimental simulations of this process for possible industrial applications are also described.
Section snippets
Pertinent experimental details
The sample of fine coal slurry used in the experiments here was obtained from Consol Energy Inc. (Pittsburgh). This represents a discarded sample of a bituminous coal sent to a settling pond as a refuse from a coal preparation plant. The slurry sample was allowed to settle in the container and excess water at the top was skimmed off. The proximate analysis of the remaining settled sample (carried out by Galbraith Laboratories, Knoxville, TN) given in Table 1, shows nearly 53% moisture and about
Thermal heating
The results of TGA studies in terms of % weight loss against temperature using samples of masses of 50, 100 and 200 mg and heating rate of 3 °C/min are shown in Fig. 1. These experiments were done in a continuous manner without taking the samples out of the balance and the data points at steps of about 3 °C are shown only for visual clarity although in the actual experiments, the data are taken every 3 s. It is evident that larger mass requires higher temperature for the same mass loss, as
Summary
The laboratory scale and bench scale studies reported here on dewatering of a fine coal slurry containing about 52% moisture using conventional microwave radiation at 2.45 GHz and 800 W have shown efficiencies ≈80% compared to theoretical maximum. About 10% of the moisture appear to remain perhaps in the capillaries of the fine coal particles. It is argued that overall characteristics of microwave dewatering represent significant advantages over dewatering by thermal heating. Using the
Disclaimer
This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, nor assume any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific
Acknowledgements
The authors thank the following persons for assistance with this project: Phillip Tucker for designing and building the electrical circuit for remote control operation of the bench scale unit, Steve Pachol for providing the coal slurry sample and Sukanya Ranganathan for the SEM studies. Financial support for this work was provided by the Center for Advanced Separation Technologies, established by a contract from the US Department of Energy (Contract # DE-FC26-02NT4160T).
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