Evaluation of Moisture Readsorption and Combustion Characteristics

Nov 12, 2014 - properties of the upgraded lignite would partially offset the upgrading effect. Asphalt ... additive in lignite thermal upgrading to pr...
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Evaluation of Moisture Readsorption and Combustion Characteristics of a Lignite Thermally Upgraded with the Addition of Asphalt Jinping Zhang, Cheng Zhang,* Yongqi Qiu, Lei Chen, Peng Tan, and Gang Chen* State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China ABSTRACT: Thermal upgrading is a promising way to use lignite efficiently and safely. However, moisture readsorption properties of the upgraded lignite would partially offset the upgrading effect. Asphalt is so repellent that it could be used as an additive in lignite thermal upgrading to prevent moisture readsorption of upgraded lignite. In this study, a Chinese lignite upgraded at various temperatures (200−500 °C) with various concentrations of asphalt addition (0−10 wt %) was thoroughly investigated. The changes in chemical structures of the upgraded lignites were investigated using Fourier transform infrared spectroscopy (FTIR). The changes in physical structures were analyzed by N2 adsorption isotherm and scanning electron microscopy (SEM). The moisture readsorption and combustion characteristics of the upgraded lignites were studied using a constant temperature/humidity chamber and a thermogravimetric analyer (TGA), respectively. The results indicate that the abundance of oxygen-containing groups (i.e., hydroxyl and carboxyl) decreased with the increasing upgrading temperature, while it was not significantly influenced by the addition of asphalt. The pore volume and surface area of the upgraded lignites increased with the increasing temperature, while the pore diameter decreased. SEM images revealed that some pores in upgraded lignites were covered by asphalt and the surface became much smoother with the increasing asphalt concentration, which resulted in the decrease of the pore volume and surface area. The moisture readsorption ratio and spontaneous combustion tendency to the upgraded lignites decreased with the increasing temperature, while the influence of asphalt on the moisture readsorption and combustion characteristics was temperature-dependent. process.10 Consequently, the advantages of thermal upgrading would partially be limited by the moisture readsorption. Therefore, it is necessary to develop methods to prevent moisture readsorption during the storage of the upgraded lignite. A considerable number of studies on moisture readsorption and spontaneous combustion characteristics of upgraded coals have been conducted in previous reports. Li et al.9 found that the moisture readsorption rate of dried coals depended upon the drying temperature, coal particle size, and relative humidity of the atmosphere. Choi et al.11 verified that a coal−oil slurry dewatering process reduced the moisture readsorption ratio and spontaneous combustion tendency to upgraded lignite. It is generally concluded that the equilibrium moisture content (EMC, the content of moisture absorbed by the coal when it reaches a dynamic equilibrium with the surroundings) is a major indicator for moisture readsorption property and is mainly determined by the pore structure of the coal and hydrophilic groups (i.e., carboxyl, hydroxyl, and carbonyl) on the coal surface,2,12 while the spontaneous combustion tendency to coal is found to be a function of the EMC and oxygen-containing groups in the coal.13 Umar et al.14 studied the combustion characteristics of some Indonesian raw and upgraded coals produced by an upgraded brown coal (UBC) process using differential thermal analysis−thermogravimetry (DTA−TG) and observed that the ignition temperature of the

1. INTRODUCTION It is well-accepted that low-rank coals (e.g., sub-bituminous coal and lignite) have a large reserve in the world and particularly play an important role in supplying primary energy in the developing countries, such as China.1 The price of lignite is approximately one-third of that of bituminous coal, thereby attracting lots of attention worldwide.2 However, lignite has a high moisture content (25−65 wt %) and low heating value, which resulted in a low combustion efficiency and high transportation costs.3,4 In addition, lignite is rich in volatile matter and oxygen-containing groups, which increase the tendency toward spontaneous combustion. This shortage poses challenges to the storage and transportation of lignite, thereby limiting its use.5 Therefore, techniques that can increase the combustion efficiency and lower the spontaneous combustion tendency of lignite are in demand. Lignite upgrading is such a promising way for the purpose to use lignite more efficiently and safely.6 Thus far, many upgrading techniques have been developed, among which thermal upgrading is most widely used.6,7 Thermal upgrading would cause significant changes in physical and chemical structures of lignite, which could improve the heating value and reduce the spontaneous combustion tendency.8 However, it is reported that the upgraded lignite may further absorb moisture from the surroundings, because of a large number of holes and crevices existing on the coal surface. It is called the moisture readsorption property of upgraded lignite.9 The problem of moisture readsorption not only reduces the heating value of upgraded lignite but increases the spontaneous combustion tendency as a result of the heat release and temperature rise during the moisture readsorption © 2014 American Chemical Society

Received: August 27, 2014 Revised: November 7, 2014 Published: November 12, 2014 7680

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Table 1. Properties of ZT Lignite proximate analysis (wt %)

a

ultimate analysis (wt %, ad)

sample

Mara

Madb

VMad

Aad

FCad

LHVc (MJ/kg)

C

H

Od

N

S

ZT lignite

52.3

9.60

44.01

15.37

31.02

16.46

46.31

3.97

22.50

1.36

0.89

ar = as-received basis. bad = air-dried basis. cLHV = low heating value. dBy difference. with a Parr-6300 bomb calorimeter (Parr Corporation). Each test was repeated 3 times, and the results are tabulated in Table 1. 2.2. Thermal Upgrading Experiment. The thermal upgrading experiment was conducted in a fixed-bed reactor (Figure 1). The tube

upgraded coals increased, whereas the maximum combustion rate was not significantly changed. Asphalt is characterized as adhesive, insulative, waterproof, and rustproof; thus, it is widely used in different fields. In construction, it is used as a waterproof layer, thermal insulating layer, etc. In transportation, it can be used as paving, railway rustproof material, etc. In agriculture, asphalt is usually used for heat preservation, moisture evaporation, and fertilizer loss reduction of soil. In the electrical industry, it is used as insulation material, especially for underground and underwater cables.15 In view of the hydrophobic property, asphalt could be used as a material to prevent the moisture readsorption of the upgraded lignite in the process of upgrading. Choi11 and Karthikeyan16 have proven that asphalt could cover pores of the coal during the heating and cooling process and suppress the active functional groups from reacting with moisture and oxygen in the air, thus reduced the moisture readsorption ratio and the spontaneous combustion tendency of upgraded coals. However, systematical work has to be performed to investigate the effects of asphalt addition and upgrading temperature on the chemical and physical structures, moisture readsorption, and combustion characteristics of the upgraded lignite. In addition, how the change of chemical and physical structures influences the moisture readsorption ratio and combustion characteristics of upgraded lignite would be of importance to the subsequent use of upgraded lignite. In this work, a Chinese Zhaotong (ZT) lignite was studied in a lab-scale fixed-bed reactor. The lignite was thermally upgraded under various temperatures (200−500 °C) by coating with various concentrations of asphalt (0−10 wt %). Moisture readsorption and combustion characteristics of the upgraded lignites were analyzed subsequently. Fourier transform infrared spectroscopy (FTIR) and N2 adsorption isotherm were used to identify the changes in the physical−chemical structures of upgraded lignites.

Figure 1. Schematic diagram of a fixed-bed reactor. furnace was first washed in a N2 flow and then heated to a designated temperature (e.g., 200, 300, 400, or 500 °C) at a flow rate of 2 L/min. In each experiment, 1.0 g of coal sample was placed in a porcelain crucible, centered in the tube, and held for 30 min. Then, the upgraded lignites were pushed to the end of the tube and cooled to ambient temperature in N2 before being collected in a sealed bag. The upgraded lignites were named after the upgrading temperature and/or the asphalt concentration. For example, T200 referred to the upgraded lignites at 200 °C, C-2% referred to the upgraded lignite with 2 wt % asphalt coating, and T200−C-2% referred to the upgraded lignite at 200 °C with 2 wt % asphalt coating. The prepared samples were dried at 105 °C for 24 h to remove the adsorbed moisture during the storage before the FTIR analysis, N2 adsorption isotherm, scanning electron microscopy (SEM) image analysis, moisture readsorption, and thermogravimetry−differential thermogravimetry (TG−DTG) analysis. 2.3. Upgraded Lignite Characterizations. 2.3.1. Characterization of Chemical Structures. The surface chemical structure of the coal samples was characterized by a Vertex 70 FTIR spectrometer (Bruker Corporation). KBr pellets were prepared by grinding 100 mg of KBr with 1 mg of dried coal sample. FTIR spectra of coal samples for the 400−4000 cm−1 region were studied by curve-fitting analysis using the Origin8.0 data-processing program (OriginLab Corporation). Prior to FTIR measurements, a reference spectrum was obtained from pure KBr pellets.17 Accordingly, the band area ratios of oxygencontaining groups and aromatic carbon groups (Car) (ACOOH/ACar and AOH/ACar) could be calculated.18−20 2.3.2. Characterization of Physical Properties. To obtain the variation of physical properties of the upgraded lignites, an ASAP2020 N2 adsorption apparatus (Micromeritics Instrment Corporation) was used to measure the pore structures at 77 K. The dried sample was degassed in the adsorption system at 200 °C to a final pressure of 1.33 × 10−4 Pa. The surface area was calculated by the Brunauer−Emmett− Teller (BET) method, while the pore diameter and pore volume were determined by the Barrett−Joyner−Halenda (BJH) model.20 JSM-5610LV SEM (NEC Electronics Corporation) was used to characterize the morphological change of the coal samples. The SEM images were captured at an accelerating voltage of 25 kV. The samples were coated with gold before measurements to improve resolution. 2.4. Analysis of Moisture Readsorption. The moisture readsorption of the raw and upgraded lignites was characterized in a HWS-150 constant temperature/humidity chamber (Shanghai Senxin Experimental Instrument Corporation). In each experiment, 5 g of coal sample was uniformly distributed in a glass weighing bottle with a

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. A lignite from ZT coalfield in China was used in this study. The ZT coalfield is located in northeastern Yunnan, China. It has a total area of 142 km2 and a retained reserve of around 8 billion tons of lignite. The coal sample was air-dried, milled, and then sieved into a particle size smaller than 0.1 mm. The coal samples were prepared by coating with asphalt in various mass ratios. First, asphalt of a predetermined amount was dissolved in kerosene. Then, the coal sample was added to the solution to form a slurry (mass ratio of the solution/lignite was adjusted to 1:1). Subsequently, the slurry was stirred by a DF-101B heat-collected magnetic stirrer at room temperature under atmospheric pressure for 0.5 h. Finally, the slurry was kept in an oven at 130 °C for 3 h to remove the remaining solvent.11 The concentration of asphalt in the final solid sample was set as 2, 5, and 10 wt %. The carbon, hydrogen, and nitrogen contents of the ZT lignite were measured by a VARIO CHN MAX Elementar (GmbH Corporation). The sulfur content was determined on RAPID CS CUBE Elementar (GmbH Corporation). The proximate analysis was conducted using TGA2000 (Las Navas Corporation). The heating value was obtained 7681

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Figure 2. (a) Proximate analysis and (b) LHV of the raw and upgraded lignites.

Figure 3. FTIR spectra of (a) C-0% at different temperatures and (b) T500 with various asphalt concentrations.

3. RESULTS AND DISCUSSION

thickness of around 5 mm, which was maintained in the chamber at 30 °C with a relative humidity (RH) of 80%. The weight changes were monitored every 2 h for the first 10 h and then every 12 h for the following 5 days. The moisture readsorption ratio was defined in eq 1

ε = [(m1 − m0)/m0] × 100%

3.1. Upgraded Lignite Characterization. Figure 2 shows the results of proximate analysis and low heating value (LHV) of the raw and upgraded lignites. As shown in Figure 2a, with the increasing upgrading temperature, the moisture (M) and volatile matter (VM) contents in upgraded lignite decreased, while ash (A) and fixed carbon (FC) contents increased. It resulted from the decomposition of unstable substances and oxygen-containing groups during thermal upgrading.26 For the lignites upgraded at the same temperature, M gradually decreased with the increase of the asphalt concentration, because asphalt is so hydrophobic to prevent the adsorption of moisture on the surface of coal samples. The VM content of the upgraded lignites with asphalt addition was higher than those without asphalt addition, which is caused by the high content of VM in asphalt. However, it is also found that, at the same temperature, some VM in upgraded lignites decreased slightly with the increasing of asphalt addition. This phenomenon might be because asphalt coating on lignite plugged the pore structures and inhibited the releasing of volatile matter in lignite during the test of VM. Thus, part of VM in upgraded lignite was tested as FC and resulted in the decrease of VM for upgrading lignite. It can be certified by the increase of combustible matter

(1)

where m0 and m1 referred to the weight of the sample before and after adsorbing moisture in the chamber, respectively. 2.5. Analysis of Combustion Characteristics. The combustion characteristics of the raw and upgraded lignites were investigated using a Diamond TG/DTA system (PerkinElmer Instruments Corporation). For all experiments, About 5 mg of sample was heated at a rate of 10 °C/min over a temperature range of ambient to 900 °C with an air flow rate of 100 mL/min. Many parameters related to the coal combustion behavior can be derived from the TG−DTG curves.21−23 The ignition temperature (Tig) is the most relevant index for lignite because of its high intensity of spontaneous combustion. In general, those coals with a lower ignition temperature are considered as easier to burn.14,24 The maximum combustion rate (Rmax), obtained from DTG peaks, indicates the combustion intensity of coal.14 Tmax (the temperature at which Rmax occurs) and the reactivity index R (defined in eq 2) are used to characterize the reactivity of upgraded lignite. τ0.5 in eq 2 represents the time required to reach the conversion of 50 wt % of coal.25

R=

0.5 τ0.5

(2) 7682

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Figure 4. Changes of carboxyl and hydroxyl groups of (a) C-0%, (b) C-5%, and (c) T300 and T500.

(VM and FC) of upgraded lignite with the increase of the asphalt concentration. As seen in Figure 2b, the LHV of the upgraded lignites was higher than that of raw lignite and increased with the upgrading temperature. For example, the LHV increased from 17.96 MJ/ kg in T200−C-0% to 20.63 MJ/kg in T500−C-0%. The improvement in LHV at 200 °C is mainly due to the removal of moisture in lignite, which implies that LHV decreases during the moisture readsorption process, consequently weakening the upgrading effect.16 The improvement in LHV at the temperature higher than 200 °C resulted from the release of volatile matter, decomposition of functional groups, and depolymerization reactions.21,27 The effect of asphalt addition on the LHV depended upon its concentration and the upgrading temperature. For the same concentration of asphalt addition, the LHV showed no obvious difference at the temperatures of 200−400 °C; however, a sharp rise was observed at 500 °C. For the same upgrading temperature, with respect to that of C-0%, the LHV decreased when the asphalt concentration is 2 wt % and then increased at 5−10 wt %. This was supposed that the addition of asphalt during upgrading could influence the LHV of upgraded lignite on two sides. On the one side, the asphalt covered the lignite surface and prohibited the release of volatile matter during the thermal upgrading, resulting in the VM being higher than that of upgraded lignite without asphalt addition. Thus, the testing LHV of upgraded lignite decreased. On the other side, the LHV of asphalt is twice as high as the LHV of ZT lignite that the testing LHV of their mixture could be increased. At a lower asphalt concentration (2 wt %), the decrease effect is higher than the increase effect; therefore, the LHV was lower than that

of upgrade lignite without asphalt addition. When a larger amount of asphalt was added (5−10 wt %), the increase effect was higher than the decrease effect; thus, the LHV of the upgraded lignite with 5−10 wt % asphalt addition was higher. 3.2. Effects of the Temperature and Asphalt Concentration on Chemical Structure Changes. The FTIR spectra of the raw and upgraded lignites are depicted in Figure 3. The shapes of infrared spectra for all samples are similar except the changes in absorption intensities of corresponding peaks. Generally, oxygen-containing groups on the coal surface are believed to be active sites, which relate to the hydrophily and combustion reactivity of coals.18,20,28 It is reported that carboxyl and hydroxyl groups are the oxygen-containing groups best contributing to the EMC of low-rank coals.26 The absorption peaks of carboxyl and hydroxyl groups are located in the region of 1700−1760 and 3000−3700 cm−1, while the 1610 cm−1 band corresponds to the aromatic carbon (Car).19,28 As clarified in the literature,20 band area ratios ACOOH/ACar and AOH/ACar represent the relative content of carboxyl and hydroxyl groups. The evolution of ACOOH/ACar and AOH/ACar with different upgrading temperatures and asphalt concentrations is shown in Figure 4. Panels a and b of Figure 4 demonstrate that ACOOH/ACar decreased from 200 to 500 °C and AOH/ACar decreased from 200 to 400 °C. This is because the chemical bonds connecting carboxyl and hydroxyl groups to the carbon matrix broke down when the temperature increased to a certain value.20 The trends of ACOOH/ACar showed a slight breakdown of carboxyl groups from 105 to 200 °C, with a more intense breakdown thereafter, and decomposed completely when the temperature approached 500 °C. Differently, hydroxyl groups started decomposing at 7683

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Figure 5. Pore structure of (a) C-0%, (b) C-5%, and (c) T500.

Figure 6. SEM images of the raw and upgraded lignites (magnification of 5000×).

the temperature below 200 °C. The increasing of AOH/ACar from 400 to 500 °C is probably contributed by the intense decomposition of Car.28 Figure 4c shows that AOH/ACar decreased when the asphalt was added during the upgrading; however, it changed slightly with further increasing of the asphalt concentration from 2 to

10 wt %. The decrease in AOH/ACar when asphalt was added in lignite is mainly attributed to the low content of hydroxyl groups and the high content of aromatic carbon (Car) in asphalt.29 While the small increase of the proportion of asphalt to lignite (≤10 wt %) could not cause a significant change in AOH/ACar. Additionally, asphalt is reported to contain some 7684

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Figure 7. Moisture readsorption ratios of (a) C-0% and (b) C-5%.

carboxyl groups, which caused the result that ACOOH/ACar changed negligibly with asphalt addition.29 3.3. Effects of the Temperature and Asphalt Concentration on Physical Property Changes. The impact of the upgrading temperature and asphalt concentration on the pore structures of the lignite, such as BET surface area (SBET), pore diameter (Dm), and pore volume (Vm), are depicted in Figure 5. As seen in panels a and b of Figure 5, SBET and Vm increased, while Dm decreased in parallel with the increasing temperature, especially at 400−500 °C. It is caused by the opening of micropores by the rapid release of volatile matter from the particles during the heating process, the breaking of large individual particles into small particles, and the collapse of pores via partial melting or softening deformation.20,30,31 This is in agreement with the changes of SEM images revealed in Figure 6. The raw lignite particles had rough and loose surfaces (Figure 6a). At 300 °C, cracks and fine particles were generated (Figure 6b). At 500 °C, large numbers of micropores developed and the cracks were expanded and deepened (Figure 6c). Figure 5c illustrates the effect of the asphalt concentration on the pore structure of T500. It can be observed that SBET and Vm decreased, while Dm increased gradually with the increasing asphalt concentration, indicating that the addition of asphalt blocks the fine pores more easily than the larger pores and retrained the development of meso- and micropores.11 This is also certified by the SEM images (panels c−f of Figure 6). The upgraded lignite surface turned smoother and flatter with the increase of the asphalt concentration, suggesting that more pores were plugged by asphalt coating on the lignite surface during the thermal upgrading.11,16 3.4. Analysis of Moisture Readsorption Characteristics. 3.4.1. Effect of the Upgrading Temperature on the Moisture Readsorption Ratio. Figure 7 shows the moisture readsorption behaviors of the raw and upgraded lignites. The moisture readsorption ratio (ε) rapidly increased during the first 10 h and gradually reached equilibrium at a certain level thereafter. Raw lignite showed the highest equilibrium moisture content (EMC = 19.7%), and EMC decreased from 18.2% in T200−C-0% to 14.6% in T500−C-0%. This is because hydroxyl and carboxyl groups decomposed during the thermal upgrading and restrained the interaction of water molecules with the lignite surface by hydrogen bond with carboxyl and hydroxyl groups (Figure 4a).2 On the other hand, lignite has a gel structure that is subject to irreversible changes upon thermal upgrading, and the shrinkages would damage the diffusion path

and inhibit the diffusion of moisture to the interior of the coal particles.32,33 Similarly, EMC in C-5% showed a prominent drop at 500 °C (12.2%) compared to those upgraded at 200−400 °C. 3.4.2. Effect of the Asphalt Concentration on the Moisture Readsorption Ratio. Figure 8 shows the effect of the asphalt

Figure 8. EMC of the raw and upgraded lignites.

concentration on the moisture readsorption characteristics. The asphalt performed two opposite effects on the EMC. One is the coating effect, which could reduce the EMC (asphalt coated on the lignite surface and reduced the surface area, thus preventing the ambient moisture from being absorbed to the surface and further entering in the interior of the particles). The other is the polarity effect, which could enhance the EMC (the small amount of polar groups contained in asphalt, such as COOH, would attract moisture in the surroundings34,35). For T200 and T300, the EMC was increased at 2 wt % and decreased when the asphalt concentration was ≥5 wt %. This is because the decomposition of oxygen-containing groups in the lignite was weak at T ≤ 300 °C. When lignite was coated with asphalt at a low concentration (2 wt %), the polarity effect of asphalt was higher than the coating effect; therefore, the EMC of C-2% increased with respect to that of C-0%. However, when the addition of asphalt increased to 5−10 wt %, the coating effect became more predominant, which resulted in the decrease of EMC. However, for T400 and T500, EMC decreased with the increasing asphalt concentration. In comparison to T500−C7685

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Figure 9. TG−DTG curves of (a) C-0% and (b) C-5%.

Rmax relates to the maximum combustion rate, while Tmax and R refer to coal reactivity. Reactive coal has lower Tmax and higher R.13 As seen in Table 2, Rmax and R of T200 and T300 were higher than those of T400 and T500, while Tmax of T200 and T300 was lower than that of T400 and T500. It indicates that the lignites upgraded at 200−300 °C could burn more intensively than those upgraded at 400−500 °C.14 In addition, combustion reactivity of the lignite was enhanced when upgraded at 200−300 °C and reduced thereafter. The enhancement in combustion reactivity at 200−300 °C is caused by the removal of moisture and absorbed gas. However, the decrease in combustion reactivity at 400−500 °C resulted from the intensive decomposition of the combustion active sites (carboxyl and hydroxyl groups) on the lignite surface. 3.5.2. Effect of the Asphalt Concentration on the Combustion Characteristics. For T300, Tig increased with the increasing asphalt concentration, indicating that the addition of asphalt during the upgrading could reduce the spontaneous combustion tendency to lignite. It resulted from the smaller surface area and lower porosity of the lignite caused by asphalt coating, which restrained the active sites on the lignite surface from contacting oxygen surroundings.11 The increasing amount of asphalt addition also resulted in the increase of Rmax, implying that the upgraded lignites with more asphalt addition burn more intensively, which has probably arisen from the higher heating value in asphalt. Additionally, R decreased, while Tmax increased slightly as the asphalt concentration increased, suggesting that the combustion reactivity of the upgraded lignite was reduced by the addition of asphalt. Tig and Tmax of T500 slightly increased with the increase of the asphalt concentration, whereas the changes in Rmax and R were negligible. All of these slight changes occurred for the reason that most oxygen-containing groups in the lignite had decomposed at 500 °C, and as a result, seldom active sites were open to contact with oxygen in the air. Overall, chemical and physical structures of the lignite changed so slightly at T ≤ 300 °C that the addition of asphalt could reduce the moisture readsorption ratio and spontaneous combustion tendency. However, the asphalt showed a smaller effect on the combustion characteristics when upgraded at 400−500 °C, because the oxygen-containing group is close to decomposing completely.

0%, the decrement rates of T500−C-2%, T500−C-5%, and T500−C-10% were 9.8, 16.7, and 30.7%, respectively. This is mainly because oxygen-containing groups were intensively decomposed, so that the attraction force to the moisture molecule decreased, and as a result, the coating effect played a predominant role. 3.5. Analysis of Combustion Characteristics. The TG− DTG curves of raw and upgraded lignites are shown in Figure 9. TG profiles of C-0% and C-5% shifted to the higher temperature zone with the increasing upgrading temperature, suggesting that the devolatilization temperatures of the upgraded lignites were increased. This implies that the thermal stability of the lignite was improved, which resulted from the increase of the ash content and decrease of volatile matter during the upgrading (Figure 2a).13,23 In addition, the raw lignite had a substantial moisture removal stage (the first peak on the DTG curve), while it was not obvious for upgraded lignite.14,36 3.5.1. Effect of the Upgrading Temperature on the Combustion Characteristics. Combustion parameters of the raw and upgraded lignites based on TG−DTG analysis are given in Table 2. It can be observed that Tig of C-0% and C-5% increased with the increasing upgrading temperature, illustrating that the spontaneous combustion tendency to lignite was decreased by the thermal upgrading. This mainly resulted from the release of volatile matter and decomposition of oxygencontaining groups during the upgrading.22 Table 2. Combustion Parameters Based on the TG−DTG Analysis sample

Tig (°C)

Rmax (% min−1)

Tmax (°C)

R

raw coal T200−C-0% T300−C-0% T400−C-0% T500−C-0% T300−C-2% T500−C-2% T200−C-5% T300−C-5% T400−C-5% T500−C-5% T300−C-10% T500−C-10%

284 295 303 305 316 308 319 315 316 319 321 322 322

5.87 6.85 7.52 5.51 5.45 7.58 5.42 7.52 7.80 5.69 5.24 8.30 5.13

348 354 356 357 363 361 366 362 365 370 371 369 372

0.0154 0.0157 0.0155 0.0142 0.0133 0.0152 0.0133 0.0152 0.0150 0.0142 0.0131 0.0148 0.0129 7686

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4. CONCLUSION ZT lignite was chosen to investigate the moisture readsorption and combustion characteristics after thermal upgrading at 200− 500 °C with asphalt addition. Changes of the chemical and physical structures of the upgraded lignites were studied in detail. Effects of the upgrading temperature and asphalt concentration on moisture readsorption and combustion characteristics were obtained. The moisture and volatile matter in the upgraded lignites decreased, while the fixed carbon and ash increased with the increasing upgrading temperature, resulted in the improvement of the LHV. The addition of asphalt also resulted in an increase of the LHV of the upgraded lignites when the concentration is higher than 5 wt %. Hydroxyl and carboxyl groups in the lignite started decomposing at T ≥ 200 °C, and a higher temperature resulted in a more intensive decomposition reaction. The addition of asphalt showed less influence on variation of hydroxyl and carboxyl groups. The surface area of the upgraded lignite increased with the increasing upgrading temperature and decreased with the increasing asphalt concentration. The moisture readsorption ratio of the upgraded lignites decreased with the increasing of the upgrading temperature, which was mainly contributed by the decomposition of hydroxyl and carboxyl groups. The effect of asphalt on the moisture readsorption characteristics was temperature-dependent. When upgraded at 200−300 °C, the moisture readsorption ratio was not reduced until the asphalt concentration added up to 5 wt %. When upgraded at 400−500 °C, the moisture readsorption ratio decreased with the increasing asphalt concentration. The spontaneous combustion tendency to the lignite decreased with the increasing upgrading temperature and asphalt concentration. The effect of asphalt on the combustion characteristics was also temperature-dependent. The addition of asphalt improved the combustion behaviors of lignite significantly by thermal upgrading at 300 °C, while it performed negligible influence on the combustion behavior when the temperature rose to 500 °C because of the occurrence of intensive decomposing of oxygen-containing groups.

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: 86-27-87542417-8301. Fax: 86-27-87545526. Email: [email protected]. *Telephone: 86-27-87542417-8301. Fax: 86-27-87545526. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The project was financially supported by the National Natural Science Foundation of China (51006042), the Natural Science Foundation of Hubei Province (2012FFB02602), the Project on the Integration of Industry, Education, and Research of Guangdong Province (2012B091100173), and the Independent Innovation Fund of Huazhong University of Science and Technology (HUST) (2013QN082). The technical support from the Analytical and Testing Center of HUST is greatly appreciated. 7687

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Energy & Fuels

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