Article pubs.acs.org/EF
Drying Kinetics of Soft and Hard Lignite and the Surface Characteristics of Products Zhenyong Miao,†,‡ Jingyu Tian,†,‡ Qiongqiong He,*,†,‡ Keji Wan,†,‡ Guoli Zhou,§ Xuefeng Ren,‡ and Jingpeng Chen†,‡ †
School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou, Jiangsu 221008, China National Engineering Research Center of Coal Preparation & Purification, China University of Mining and Technology, Xuzhou, Jiangsu 221008, China § School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou, Henan 450001, China ‡
ABSTRACT: Hard (Shengli, SL) and soft (Zhaotong, ZT) lignite by high temperature drying processes using hot gas were explored in this study. The results showed that the temperature played a significant role in the drying process. For SL-2 mm and ZT-2 mm, the drying rate at 300 °C was 4.4 times and 13.6 times that at 150 °C, respectively, which meant that the influence of temperature was more prominent for ZT and the effects of particle size could be reduced by higher temperature. Fifteen kinds of widely used drying kinetics models were fitted with 125 drying tests at 150−300 °C with different particle sizes of different lignite ranks. The residual moisture content prediction equation was constructed based on the logarithmic model, which was associated with temperature, initial moisture content, and drying time. The volatile matters, oxygen-containing functional groups, and contact angle were analyzed to show the surface characteristics of products. SL had small changes after drying, while the volatile matters of ZT decreased 5% and the carboxyl group was significantly reduced, the contact angle was increased, and close to or even greater than SL. lignite macromolecule decomposition12,13 and Tahmasebia and his co-workers used nitrogen at 250 °C.14 Based on our previous study on lignite pyrolysis,15 the covalent bond (Cal− O) between the aliphatic carbon and oxygen atoms and the covalent bond between aliphatic carbon atoms that were relatively easy to be broken, but cleavage temperatures for these two covalent bond were still as high as 360 and 430 °C. It can be inferred that, below 300 °C, breakage of covalent bonds seldom occurs.16 However, research has also found that, at temperatures higher than 160 °C,17 lignite structure carboxyl started to decrease, leading to modification of surface characteristics. To get a fast drying rate and surface modification, drying tests at high temperatures (>200 °C) were explored in this study. In recent years, drying kinetics has made considerable progress, including empirical models, semiempirical models, and theoretical models. Tahmasebi and his co-workers18,19 found the Midilli−Kucuk and Page models fit best for the drying kinetics of lignite in nitrogen fluidized-bed (up to 250 °C) and superheated steam fluidized-bed and microwave drying, respectively; the Midilli−Kucuk model was also shown to be the best fit for air fluidized-bed drying;20 the Wang and Singh model21 was the best model describing the drying behavior of coarse lignite particles in a fixed bed at 70, 100, and 130 °C; Zhou22 found that, for Shengli (SL) lignite in hot air drying and a temperature below 200 °C, the logarithmic model had the best fit; Xiong23 illustrated that the Page model was the most suitable in the drying of Shigouyi lignite at 80−
1. INTRODUCTION The lignite resource in China is more than 130 billion tons, approximately 13% of the total coal reserve.1,2 Depending on the origin of lignite, moisture content in raw lignite varies between 30% and 65% by weight. Up to 20−25% of heat from combustion of lignite is used to remove the associated water in the combustion process.3 High water content and low calorific value are the major impediment in using lignite,3−5 and upgrading lignite by dewatering is absolutely indispensable prior to further use. Coal drying can be achieved through evaporative drying and nonevaporative drying methods.6 In our previous study,7 we tried to dewater different ranks of lignites under nonevaporative conditions and found that in mechanical thermal expression (MTE) tests the effect of expression temperature (200 and 240 °C) on moisture content reduction is evident for soft lignite (Loy Yang from Australia),7,8 but it is only marginal for hard lignite (Shengli from northeast of China). The development of hydrothermal dewatering (HTD),9,10 another popular evaporative dewatering method, can modify the surface characteristics effectively, but it is limited for its harsh experimental conditions and environmental issues in terms of wastewater. In China, the largest lignite coalfield is typical hard lignite in Inner Mongolia, to get products of hard lignite with moisture content lower than 10%, the evaporative method is necessary in the mining industry. Pusat and his co-workers11 summarized the evaporative coaldrying process systematically and comprehensively and presented the influences of the parameters (media, temperature, pressure, velocity, relative humidity, coal type, particle size, and drying method). Most of the evaporation drying research work with gases was still at low temperature to avoid © 2017 American Chemical Society
Received: September 20, 2016 Revised: January 6, 2017 Published: February 2, 2017 2439
DOI: 10.1021/acs.energyfuels.6b02419 Energy Fuels 2017, 31, 2439−2447
Article
Energy & Fuels 120 °C; Zheng and his co-workers24 developed a drying kinetic model for the drying of the single lignite from 10 to 25 mm, and it agreed well with the experimental results in the temperature range of 600−900 °C. All the models (except for model from Zheng and his co-workers24) are empirical/ semiempirical drying models based on the diffusion theory, so we also used the empirical/semiempirical models in this study. In this study, drying process and kinetics of soft and hard lignites at high temperature up to 300 °C in hot gas were reported. At the same time, the characteristics of volatile matters, organic elementals, and contact angle were also provided to quality of the products. 2.1. Coal Samples. Run of mine lignite samples from Zhaotong (ZT), Yunnan, southwest of China and Shengli (SL) coalfield mine, Inner Mongolia, northeast of China, were collected as typical soft and hard lignite. Before testing, the samples were ground to the particle size of −2 mm (or −13 mm). The SL sample was demineralized by dense medium cyclone, and named as SLJ + 0.125 mm. Ultimate and proximate analysis was carried out using Vario MACRO cube from Elementar Co. Ltd. and 5E-MAG6700 from Changsha Kaiyuan Instrument Co. Ltd., respectively. 2.2. Drying Experimental Procedure. The purpose of the experiment in this section was to study the variation of moisture content of sample at different drying temperatures. According to the results of the pyrolysis experiment of SL lignite, when the temperature was above 300 °C, pyrolysis reactions begins,15 and thus, in order to prevent loss of volatile during drying, 150, 200, 250, and 300 °C were selected as the target temperatures in this study. The samples were named as “lignite-particle size-drying temperatures”, for example, SL-2 mm-150. Drying experiments were conducted in a fixed bed reactor as shown schematically in Figure 1. Before a test, the quartz tube (2 cm in
diameter) was heated by the external oven to set temperatures of 150, 200, 250, and 300 °C at a heating rate of 10−20 °C/min and at nitrogen flowing velocity of 160 mL/min. The heating process is automatically controlled by the device proportional−integral− derivative (PID) program, and the furnace thermocouple placed inside to measure temperature of furnace and give feedback. When the temperature of oven reached up to the set point (150, 200, 250, or 300 °C), 25 g samples was rapidly injected into the reactor and a filter fixed in the quartz tube kept them on the upper part of quartz tube. Sampling points were selected at the drying time of 1, 2, 5, 10, 20, and 40 min, respectively. A separate experiment was carried out for each drying time. The repeatability tests of the experimental setup were performed before performing batch experiments. The relative standard deviation of device stability was within ±5%. Due to the addition of fresh sample, the temperature of quartz tube would decrease by 5−10 °C, and there was greater temperature drop at higher experiment temperature. The temperature of tube furnace under the PID automatic adjustment would be gradually increased to the target value. After drying experiment, the quartz tube was quickly withdrawn and cooled at an atmosphere of continues nitrogen for 30− 60 s to avoid the spontaneous combustion of high temperature lignite with air. Then, the samples were rapidly upended and poured into a weighing bottle, and when it was thoroughly cooled, the mass was weighed. The wastewater was also collected and weighed. 2.3. Characterization of Samples. 2.3.1. Moisture Content. The moisture content of dried lignite was determined by vacuum drying at 60 °C until no more mass loss, and it usually took around 14 h. 2.3.2. Volatile Matters. The proximate analysis was carried out using 5E-MAG6700 from Changsha Kaiyuan Instrument Co. Ltd.. 2.3.3. X-ray Photoelectron Spectroscopy (XPS). The surface elemental composition of the samples was determined by a Thermo Fisher ESCALAB 250Xi XPS with ground samples less than 200 mesh. Energy correction was performed to account for sample charging based on the carbon (1s) peak at 284.8 eV. The uncertainty is ±0.2 eV. The Marquardt method was used to curve resolve the carbon (1s) spectrum. 2.3.4. Contact Angle. The contact angles were measured using Germany Kruss DSA100 contact angle measurement system. Lignite particles were passed through a 325 mesh and pressed at 28 MPa for 10 min to form a pellet, 10 mm in radius, and 1 mm in thickness. Contact angle was tested by squeezing deionized water drops (10 μL) from a fixed-volume microsyringe and observing the shape of the drops on the lignite surface using a camera. By recording the form of liquid on the solid surface, the contact angle was calculated to express hydrophilic characteristics of the sample. 2.3.5. Analysis of Combustion Characteristics. The thermogravimetric analyzer (TGA) (SDT Q600, TA Instruments) was used to investigate combustion characteristics. In TGA, about 5 mg sample (
