Upgrading of Lignite in a Tunnel-Type Microwave Oven
Lignite with high moisture content was upgraded in a tunnel-type microwave oven. The physicochemical characterizations of the upgraded lignite were investigated with chemical composition analysis, X-ray photoelectron spectroscopy, N2 adsorption porosimetr
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Abstract
Lignite with high moisture content was upgraded in a tunnel-type microwave oven. The physicochemical characterizations of the upgraded lignite were investigated with chemical composition analysis, X-ray photoelectron spectroscopy, N2 adsorption porosimetry, and scanning electron microscopy. The calorific value of the upgraded lignite increased as the moisture content decreased. The carboxyl and hydroxyl groups with high hydrophilicity reduced on the upgraded lignite surfaces, indicating that the coal rank was upgraded after microwave irradiation. The specific surface area of upgraded lignite first decreased and then increased with enhancing microwave power. Particle surfaces of upgraded lignite become compact, indicating the improvement of grinding characteristics. Therefore, upgrading of lignite with microwave irradiation was beneficial for its long-distance transportation and combustion properties.
Keywords
Lignite
1
Upgrading
Microwave irradiation
Introduction
Lignite is an abundant fossil resource source for both energy and chemical and will play an important role in meeting the world’s energy needs in the future. It is estimated that nearly half of the world’s coal reserves consist of lignite [1]. However, the high moisture content in lignite results in lower calorific value, lower efficiency, increased transportation cost, and higher carbon dioxide emissions has limited its usage. Therefore, moisture in lignite must be removed for its use as a source of highly efficient energy [2]. Microwaves are electromagnetic waves with frequencies that range from 300 MHz to 300 GHz [3]. Microwave heating is a kind of dielectric heating, offering a number of unique advantages such as noncontact heating, energy transfer instead of heat transfer, rapid heating, selective heating, volumetric heating, and quick start-up and stopping
[4, 5]. In recent several decades, there has been a growing interest and acceptance of microwave treatment of coal. Different materials exhibit different characteristics when subjected to microwave radiation. The polar molecules, such as moisture in coal, are a good absorber of microwave energy. One of the most important possible applications of microwave heating in coal field is dewatering. Microwave drying of a subbituminous coal was investigated by Pickles [6], including the effects of microwave power (160–560 W), sample mass (10–30 g), and some addition of magnetite to the coal. Tahmasebi [7] investigated effects of microwave power (217–1200 W), sample mass (1–30 g), and coal ash contents on microwave drying characteristics of coal. Cheng [8] investigated the effect of microwave power (280–700 W) on unit energy consumption for inherent moisture removal of air-dried coal by microwave heating. Cheng [9] employed microwave irradiation to improve the properties of air-dried
F. Zhou J. Cheng (&) J. Liu J. Zhou K. Cen State Key Lab of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China e-mail: [email protected] © Springer Science+Business Media Singapore a
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