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Influence of the hydrothermal dewatering on the combustion characteristics of Chinese low-rank coals
Applied Thermal Engineering 90 (2015) 174e181
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Research paper
Influence of the hydrothermal dewatering on the combustion characteristics of Chinese low-rank coals Lichao Ge a, b, Yanwei Zhang b, Chang Xu a, *, Zhihua Wang b, Junhu Zhou b, Kefa Cen b a b
College of Energy and Electrical Engineering, Hohai University, Nanjing 211100, China State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
h i g h l i g h t s Typical Chinese lignites with various ranks are upgraded by hydrothermal dewatering. Upgraded coals exhibit chemical compositions comparable with that of bituminous coal. FTIR show the change of microstructure and improvement in coal rank after upgrading. Upgraded coals exhibit difficulty in ignition but combust easily. More evident effects are obtained for raw brown coal with relative lower rank.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 January 2013 Accepted 8 July 2015 Available online 16 July 2015
This study investigates the influence of hydrothermal dewatering performed at different temperatures on the combustion characteristics of Chinese low-rank coals with different coalification maturities. It was found that the upgrading process significantly decreased the inherent moisture and oxygen content, increased the calorific value and fixed carbon content, and promoted the damage of the hydrophilic oxygen functional groups. The results of oxygen/carbon atomic ratio indicated that the upgrading process converted the low-rank coals near to high-rank coals which can also be gained using the Fourier transform infrared spectroscopy. The thermogravimetric analysis showed that the combustion processes of upgraded coals were delayed toward the high temperature region, and the upgraded coals had higher ignition and burnout temperature. On the other hand, based on the higher average combustion rate and comprehensive combustion parameter, the upgraded coals performed better compared with raw brown coals and the Da Tong bituminous coal. In ignition segment, the activation energy increased after treatment but decreased in the combustion stage. The changes in coal compositions, microstructure, rank, and combustion characteristics were more notable as the temperature in hydrothermal dewatering increased from 250 to 300 C or coals of lower ranks were used. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Low-rank coal Upgrading Hydrothermal dewatering FTIR Combustion
1. Introduction Low-rank coals (LRCs), which include lignite, brown coals and sub-bituminous coals [1], are an important feedstock for energy and chemical products. However, the current global coal market is dominated by high-rank coals (HRCs) which account for only half of the total coal deposits [2]. As the world's largest coal producer and consumer [3], China depends heavily on coal [4]. 70% of China's primary energy demand is supplied by coal, and coal-fired power
* Corresponding author. Tel.: þ86 13951792223; fax: þ86 25 83786510. E-mail address: [email protected] (C. Xu). http://dx.doi.org/10.1016/j.applthermaleng.2015.07.015 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
plants account for over 97% in the total thermal power capacity [5]. Unfortunately, the country is rich in LRC reserves. The reserves of lignite are about 41.18% in the amount in China [1]. Therefore, the exploitation of LRCs is necessary for the growing energy demands and the depletion of HRCs. The advantages of LRCs include high reactivity, abundance, and low market price [6], however, LRCs have not been utilized as much as HRCs [7] because they have many drawbacks, such as high moisture, low energy density, and high spontaneous combustion tendency [8]. In order to overcome these drawbacks and make LRCs' characteristics comparable with those of HRCs. LRCs should be treated by an upgrading process before use. The first step of the upgrading process is dewatering.
L. Ge et al. / Applied Thermal Engineering 90 (2015) 174e181
Hydrothermal dewatering (HTD) process, also known as hot water drying, is a typical non-evaporative dewatering process. HTD has two major advantages: it can remove water in the liquid form, saving the latent heat of vaporization; it can damage oxygen functional groups, preventing the upgraded products from reabsorbing water. Due to its high temperature and pressure, HTD has been considered as the simulation of the coal forming process in nature. The literature contains some investigations about the effects of HTD processing variables on the properties of the final products [9,10]. Morimoto et al. [11] studied the yield of the products during the HTD process, and found that the upgraded coal showed less oxygen content, higher calorific value, and higher gasification reactivity than the raw coal. Yu et al. [12] studied the effect of HTD on the slurryability of brown coals, and found that the maximum solid concentration of coal water slurry increased. Wu et al. [13] studied the gas-, solid-, and liquid-phase products obtained after HTD, and analyzed the occurrence of carbon and its migration mechanism during HTD. In recent years, there has been a growing interest of HTD in coal treatment and a number of potential applications of hydrothermal processing have been investigated, such as preparing coal water slurry with the solid and liquid products of HTD [14], improving the space time yield of brown coal liquefaction reactor [15], upgrading and dewatering of raw tropical peat [16], dewatering of sludge [17], generating alternative solid fuel from paper sludge [18], and producing liquid and solid biofuels [19,20]. However, only few studies investigated the changes in coal chemical properties and microstructures during the HTD upgrading process, which are closely related to some combustion problems. For example, the less volatile matter release may lead to an unstable flame, and the oxidation of carbon may result in a decrease in the burnout efficiency. In this context, the combustion characteristics of LRC before and after the upgrading process should be compared to improve burning efficiency, reduce emissions, and evaluate the design and construction of a boiler. In this study, three types of brown coals (with different coal ranks) produced in China were processed by HTD at different temperatures. The changes in the coal composition and microstructure of LRC by HTD were investigated with emphases on the improvement of coal rank and on the influence of HTD on the combustion characteristics of LRC. 2. Experimental 2.1. Coal sample Three brown coals, namely, Zhao Tong (ZT), Yi Min (YM), and Zhun Dong (ZD), were investigated in this study. They were formed in different geological ages (ZT in Tertiary, YM in late Jurassic, and ZD in early Jurassic) and produced in three different areas in China. The HRC used in this study was Da Tong (DT) bituminous coal. The raw coals were crushed and sieved through 2.5-mm meshes before the upgrading process. The raw and upgraded coal samples were milled for 30 s in a ring mill and sieved through 74-mm meshes for the analysis. 2.2. HTD upgrading process HTD process was performed in a bench scale hydrothermal reaction system (WeiBa WHFS-2), which had a 2-L cylindrical autoclave with the maximum pressure of 25 MPa and the maximum temperature of 350 C [12], as shown in Fig. 1. A mixture of brown coal and deionized water (under a proportion of 1:2.5) was added into the autoclave. The sealed autoclave was injected with N2 to remove air and the pressure in the autoclave was set to be 4 MPa for
175
2 h to ensure the absence of leaks. After the N2 was released, the temperature in the autoclave was increased by 3 C/min up to the preset values (250 and 300 C) and maintained for 1 h. The mixture was stirred at 100 rpm during the entire process. The autoclave was cooled to ambient temperature by an internal cooling coil with tap water after the reaction was completed. The upgraded coals were separated from the mixture by a qualitative filter paper and dried in air for approximately one week. The dried updated coals were then milled. All upgraded coals were abbreviated to “initials-treatment temperature”, such as “YM-250” and “YM-300”. 2.3. Coal composition analysis The C, H, N and S contents of the coal samples were measured using an LECO-CHNS 932 Elemental Analyzer, whereas the O content was calculated by difference. The proximate analysis data was determined using a 5E-MACIII Infrared Rapid Analyzer, and the calorific value (CV) was obtained using the adiabatic bomb calorimetric method following ISO1928. 2.4. Fourier transform infrared spectroscopy (FTIR) All the spectra of both raw and upgraded coals were generated via standard procedures with KBr pellets by using a Nicolet NEXUS670 FTIR instrument. The measuring region ranged from 4000 to 400 cm1, and the spectra were generated by collecting 32 scans at a resolution of 4 cm1. Approximately 1 mg of ground coal was mixed with 100 mg of dried KBr powder, and the mixture was further ground with a mortar and a pestle under a baking lamp to avoid water absorption. The ground mixture was then pressed into a pellet with a hydraulic tabletting machine. OMNIC software 6.1a (Nicolet) was used for data preprocessing, and the spectral analysis processes were performed using the professional software PeakFit® (Version 4.12). 2.5. Thermo-gravimetric analysis (TGA) TGA analysis was conducted using the non-isothermal method in a TGA-SDTA 851e apparatus produced by METTLER TOLEDO. 5 mg of coal sample were placed in an aluminum crucible and heated at 25 C/min to the temperature until the weightlessness curve unchanged. The flow rate of standard air was kept at 60 mL/ min. The combustion characteristics investigated in this study include ignition (Ti), burnout (Tb) and peak (Tmax) temperatures, and the maximum (kmax) and average (kmean) combustion rates. They reflect the thermal behavior and burnout property during the combustion process and can be derived from the thermogravimetric and differential (TG-DTG) curves. Therefore, they were adopted to evaluate the combustion process. An index S, which is defined in Eq. (1) was used to compare the combustion characteristics of different coal samples. The coals with a higher S value have better combustion performance.
S¼
kmax kmean Ti2 Tb
(1)
3. Results and discussion 3.1. Influence of upgrading on coal composition The moisture content (Mt) of the raw brown coals not processed by HTDd53.11 wt% for ZT, 30.92 wt% for YM, and 24.94 wt% for ZDdis dramatically larger than that of DT bituminous coal (4.59 wt
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Controller ć
Magnetic Stirrer Thermocouple
Cooling Water Gas
Gas Intake Valve
Gas Release Valve Heating Coil
N2 Coal+Water
Fig. 1. The schematic diagram of hydrothermal dewatering experimental apparatus.
%), as shown in Table 1. After HTD processing at 250 C, Mt of the brown coals significantly decreased to 10.52 wt% for ZT-250, 10.01 wt% for YM-250, and 11.12 wt% for ZD-250. The Mt of brown coals processed by HTD at 300 C decreased to 6.74 wt% for ZT-300, 5.71 wt% for YM-300, and 10.36 wt% for ZD-300. In HTD, the moisture in the brown coals was removed in three ways: the thermal effects, the carbon dioxide generated by the thermal destruction of functional groups, and the shrinkage and collapse effects of the coal. The volatile matter (Vd, dried basis calculated from Table 1) of the coals decreased from 45.51 wt% (ZT) to 39.10 wt % (ZT-250) and 35.51 wt% (ZT-300), from 38.30 wt% (YM) to 36.42 wt% (YM-250) and 33.59 wt% (YM-300), and from 30.86 wt% (ZD) to 30.35 wt% (ZD-250) and 28.49 wt% (ZD-300). Whereas the fixed carbon content and CV of all upgraded coals significantly increased. This provided an evident that the active chemical components in the volatile matter had gradually transferred into the stable compositions in the coal structure. The oxygen content (Odaf) of the coals was reduced from 25.52 wt% (ZT) to 20.39 wt% (ZT-250) and 17.65 wt% (ZT-300), from 19.79 wt% (YM) to 16.48 wt% (YM250) and 15.48 wt% (YM-300), and from 18.24 wt% (ZD) to 16.48 wt % (ZD-250) and 15.22 wt% (ZD-300). The decrease in the atomic
ratio of oxygen to carbon (AO/C), which is a measure of coal rank, indicated an improvement in the coalification rank near that of bituminous coal. Among the three brown coals used in this study, ZT had the lowest coal rank and ZD had the highest. It can be seen from the yield of solid products (Table 1) that aside from moisture, several materials in the coal were reduced or destroyed, possibly affecting the microstructure and combustion process of the coal. Based on the aforementioned results, effects of HTD on the raw coals of lower rank are more obvious than those of higher rank, which may also be caused by the high treatment temperature. 3.2. Influence of upgrading on coal structure Although the general shapes of the spectrograms of the treated coals were similar to each other, their peaks did not show such multi as raw coal, particularly for some acromion and envelope peaks, as shown in Fig. 2. This indicates that the functional groups of the coals were destroyed or disappeared. The infrared spectra of the coals were divided into four regions [21]: the peaks appearing in the range of 3600e3000 cm1 are classified as hydroxy, the peaks in 3000e2700 cm1 belong to aliphatic, the peaks in
Table 1 Results of the upgrading influence on coal composition. Coal
Condition
Yield of solid products (wt%)
Proximate analysis (air dried basis, wt%) Moisture
Ash
Volatile
Fixed carbon
ZT
RAW HTD-250 HTD-300 RAW HTD-250 HTD-300 RAW HTD-250 HTD-300 RAW
e 85.91 78.02 e 92.38 88.87 e 97.11 93.27 e
31.84 9.00 6.18 13.88 7.59 4.22 11.55 10.04 9.07 4.59
15.65 24.15 25.05 10.67 12.60 12.50 3.84 3.72 3.53 6.92
31.02 35.58 33.32 32.98 33.66 32.17 27.30 27.30 25.91 24.54
21.49 31.27 35.45 42.47 46.15 51.11 57.31 58.94 61.49 63.95
YM
ZD
DT a
AO/C: oxygen/carbon atomic ratio.
Total moisture (wt%)
Calorific value (J/g)
Ultimate analysis (dry ash free basis, wt%) C
H
N
S
O
53.11 10.52 6.74 30.92 10.01 5.71 24.94 11.12 10.36 4.59
13,358 19,383 21,022 22,319 24,404 25,470 24,828 25,340 26,081 29,453
63.21 71.58 74.51 72.27 77.01 78.12 76.44 77.95 79.16 83.98
8.28 5.68 5.50 5.78 5.11 4.92 3.64 3.99 4.03 4.44
2.11 1.60 1.63 1.95 1.34 1.32 1.24 1.23 1.19 1.22
0.88 0.75 0.71 0.21 0.06 0.16 0.44 0.35 0.40 1.29
25.52 20.39 17.65 19.79 16.48 15.48 18.24 16.48 15.22 9.07
AO/Ca
0.30 0.21 0.18 0.21 0.16 0.15 0.18 0.16 0.14 0.08
L. Ge et al. / Applied Thermal Engineering 90 (2015) 174e181
0.6 0.5
Absorption intensity
Cal C Cal Hal H Hal ¼ * C H C Cal
ZD-RAW HTD-250 HTD-300
fa ¼ 1
0.4 0.3 0.2 0.1 0.0 4000 3600 3200 2800 2400 2000 1600 1200 800
400
-1
Wave (cm ) Fig. 2. The infrared spectrum of ZD raw and upgraded coals.
1800e1000 cm1 correspond to functional groups and aromatic, and the peaks in 900e700 cm1 are considered as aromatic in the low wave region and ash matter. Additional information about the assignments of the coal spectra are shown in Refs. [22,23]. Fig. 3 shows the band reconstructions of the four parts of ZD raw brown coal using the overlapping peak resolved method with PeakFit®. Other samples were treated under the same condition. The position, width, and integral intensity of the infrared bands were obtained. The value of many parameters [24,25] related to the microstructure, the physicochemical characteristics, and the rank of coals can be calculated from the intensities of the infrared spectrum absorptions, as shown in Table 2.
AC¼O I1640e1700 cm1 ¼ Aar I1500e1630 cm1
cm1
where Cal/C is the ratio of aliphatic carbon to the total carbon in coal; Hal =H ¼ I3000e2700 cm1 =ðI900e700 cm1 þ I3000e2700 cm1 Þ is the percentage of aliphatic hydrogen in total hydrogen; H/C is the hydrogen/carbon atomic ratio calculated from the elemental analysis; and Hal/Cal is the hydrogen/carbon atomic ratio in aromatic clusters, which has a value of 1.8 for coals [32]. The fa values of the raw coals in Table 2 demonstrate that ZT and ZD had the lowest and highest coal ranks, respectively. These results are in agreement with the geological age and AO/C ratio values. Because the dehydration, decarboxylation, and demethanation reactions in HTD upgrading process simulate the coal forming in nature, the coal rank was improved and to a greater extent when the treatment temperature was increased to 300 C. However, the effect of the temperature was limited, indicating that the upgrading process had more impact on the changes in coal structure and on the improvement of rank than the related conditions. Moreover, the lower the original coal rank, the more significant the effects of the upgrading process. For example, fa was improved from 0.261 (ZT) to 0.525 (ZT-250) and 0.541 (ZT-300), from 0.580 (YM) to 0.637 (YM250) and 0.638 (YM-300), and from 0.721 (ZD) to 0.724 (ZD-250) and 0.727 (ZD-300). Wu et al. [13] investigated the changes in the structural features of Xi Meng brown coal during HTD by solid-state nuclear magnetic resonance spectroscopy, and also found the same results. The aliphatic components decreased whereas aromatic components increased after HTD, indicated coal rank improved. Oxygen-containing groups significantly decreased in number, and the productions of CO2 and CO (the main gaseous products) were attributed to the decomposition of carbonyl and carboxyl groups. 3.3. Influence of upgrading on coal combustion
(3)
cm1
The methyl/methylene (CH3/CH2) ratio (Eq. (3)) measures the length of aliphatic chains and degree of branching of aliphatic side chains attached to the macromolecular structure of the coal [26]. Higher CH3/CH2 values indicate longer and less branched aliphatic chains, the reduction and destruction of the bridge bond connecting with CH2, and an improvement in the coal rank [21].
Aar I1500e1600 cm1 ¼ Aal I2800e3000 cm1
(5)
(2)
The carbonyl/aromatic (AC ¼ O/Aar) ratio, which represents the dissociation and shift of oxygen functional groups is defined in Eq. (2). Lower AC ¼ O/Aar values indicate a decrease in the carbonyl or carboxyl groups to aromatic carbon groups, demonstrating the reduction and destruction of oxygen functional groups during the upgrading process.
CH3 I2955e2960 ¼ CH2 I2920e2945
177
(4)
The aromatic/aliphatic (Aar/Aal), which is another parameter related to the aromaticity and grade of coal rank, is defined in Eq. (4) [27]. Higher Aar/Aal values indicate higher aromaticity and maturation [28]. The apparent aromaticity (fa), which is considered as the measurement of the coal rank [29], can be calculated following the method reported by Brown and Ladner [30], as shown in Eq. (5) [31].
The TG curves shifted toward the high temperature region and the entire coal combustion process was delayed because of the change in coal rank [33], as shown in Fig. 4. Similar to the results above, the influence of upgrading on the combustion characteristics was more obvious when the samples with low rank were used. HRC (e.g., DT) typically exhibits a slight increase in mass through adsorption before the evaporation of the volatile [34]. A similar phenomenon was observed in upgraded LRC, particularly for ZT brown coal, providing another evidence of effect of upgrading on coal rank. Two obvious peaks were observed in the DTG curves for all coal samples. The first peak at approximately 55 C can be attributed to the significant decrease in water evaporation after upgrading process, which was consistent with the tendency of moisture based on the industrial analysis. The second peak in the range of 350e450 C can be attributed to the slight increase in char combustion, which consequently became more concentrated and intense. This result demonstrates that several volatile matters turned into char structures during the upgrading process, thus enhancing the char combustion process. Besides, there was another combustion peak around 300 C for ZT-250, which was ascribed to the combustion of volatile matter [35,36] due to its highest volatile matter content (35.58 wt%, Table 1). This provided another evidence that some unstable active compositions in the volatile matter gradually transferred into stable inert components. Because of the relatively mild temperature of 250 C, the transformation process had not finished completely, the active unstable chemical compositions probably produced the separate combustion peak prior to the char
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L. Ge et al. / Applied Thermal Engineering 90 (2015) 174e181
0.035
0.10
Raw IR spectra Reconstruction
0.030 Raw IR spectra Reconstruction
Absorption intensity
Absorption intensity
0.08 0.06 0.04 0.02
0.025 0.020 0.015 0.010 0.005
0.00
0.000 3000
3100
3200
3300
3400
3500
3600
2700
-1
2750
2800
2850
2900
-1
2950
3000
Wave (cm )
Wave (cm )
(a) 3600–3000 cm-1
(b) 3000–2700 cm-1
0.08
Raw IR spectra Reconstruction
0.006 Raw IR spectra Reconstruction
Absorption intensity
Absorption intensity
0.06
0.04
0.02
0.004
0.002
0.000
0.00 1000
1200
1400
1600
1800
740
760
780
-1
800
820
840
Wave (cm )
-1
Wave (cm )
(c) 1800–1000 cm-1
(d) 900–700 cm-1
Fig. 3. Resolving and construction of FTIR peaks of ZD raw brown coal.
Table 2 Results of the upgrading influence on coal structure. Coal
Location
Geological age
Condition
Carbonyl/aromatic ratio
Methyl/methylene ratio
Aromatic/aliphatic ratio
Apparent aromaticity
ZT
Southwestern China
Tertiary
YM
Northeastern China
Late Jurassic
ZD
Northwestern China
Early Jurassic
RAW HTD-250 HTD-300 RAW HTD-250 HTD-300 RAW HTD-250 HTD-300
0.527 0.352 0.268 0.384 0.253 0.213 0.465 0.360 0.235
0.168 0.199 0.446 0.170 0.345 0.410 0.297 0.355 0.369
5.760 5.800 5.883 5.852 5.972 6.227 12.673 14.209 14.540
0.261 0.525 0.541 0.580 0.637 0.638 0.721 0.724 0.727
combustion peak. While with the treatment temperature increased further to 300 C, the transformation process was nearly finished, the volatile matter stayed in the stable state, just like the raw coal. YM and ZD brown coals had lower volatile content and relatively higher coal rank, so their combustion peaks of volatile matter were contained in the char combustion peak. Detailed information of the combustion characteristic parameters obtained from the TG-DTG curves is shown in Table 3. The upgrading process increased Ti, and the higher temperature in the upgrading process resulted in higher Ti. For example, Ti increased
from 287.3 C (ZT) to 293.8 C (ZT-250) and 313.3 C (ZT-300). The increase in Ti indicates a decrease or transfer of the volatile matter. This decrease or transfer could prevent the spontaneous combustion tendency of raw brown coal and indicates that the upgraded products were similar to HRC as the Ti of HRC is usually high (463.6 C for DT bituminous coal). As Tb increased, burnout property decreased. Tmax showed a slight increase except ZT brown coal (possibly due to its high moisture content and low coal rank). kmax, kmean and S increased, indicating that the upgraded products retained the advantages of brown coal in terms of combustion
L. Ge et al. / Applied Thermal Engineering 90 (2015) 174e181
100
179
0.00
80 -1
Weight loss rate (min )
-0.04
Weight (%)
60
ZT-RAW ZT-250 ZT-300 DT-RAW
40 20
-0.08 ZT-RAW ZT-250 ZT-300 DT-RAW
-0.12 -0.16 -0.20
0
0
100
200
300
400
500
600
700
0
100
200
300
400
500
600
700
600
700
600
700
Temperature (℃)
Temperature (℃)
100
0.00 80
Weight (%)
-1
Weight loss rate (min )
-0.05
60 YM-RAW YM-250 YM-300 DT-RAW
40
20
-0.10 YM-RAW YM-250 YM-300 DT-RAW
-0.15 -0.20 -0.25
0 0
100
200
300
400
500
600
700
800
0
100
200
Temperature (℃)
100
400
500
0.00
-1
Weight loss rate (min )
80
Weight (%)
300
Temperature (℃)
60
ZD-RAW ZD-250 ZD-300 DT-RAW
40
20
0
-0.05 -0.10
ZD-RAW ZD-250 ZD-300 DT-RAW
-0.15 -0.20 -0.25
0
100
200
300
400
500
600
700
Temperature (℃)
0
100
200
300
400
500
Temperature (℃)
Fig. 4. The TG and DTG curves of raw and upgraded coals as well as bituminous coal.
characteristics, even though Ti, Tmax and Tb increased. And as a result of relative high ignition and burnout temperature as well as the limited increment of combustion rate, the S value of ZD showed a slightly decrease. The above-mentioned trends are more obvious for young brown coals than for old brown coals. For example, Ti increased from 287.3 to 313.3 C (ZT) at 300 C, and from 337.4 to 356.3 C (YM) but only from 404.3 to 415.3 C (ZD). These changes demonstrate that the upgraded coals experienced difficulty in ignition but performed combustion easily because of their high CV,
less moisture content, and combustion characteristics similar to those of HRC. This results were consistent with Umar et al. [35], who studied the changes of combustion characteristics of three Indonesian LRCs with upgraded brown coal (UBC) process. They found that Ti of the upgraded coals increased, whereas Tmax is not so changed, and kmax significantly increased. They also treated the brown coal with hot water drying (HWD) and steam drying (SD), and the combustion characteristics of HWD and SD processes were much better than
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L. Ge et al. / Applied Thermal Engineering 90 (2015) 174e181
Table 3 Results of the upgrading influence on combustion characteristic parameters. Coal
Condition
Ignition temperature/ C
Peak temperature/ C
Burnout temperature/ C
Maximum combustion rate/min1
Average combustion rate/min1
Sa*109/(min2*K3)
ZT
RAW HTD-250 HTD-300 RAW HTD-250 HTD-300 RAW HTD-250 HTD-300 RAW
287.3 293.8 313.3 337.4 345.9 356.3 404.3 410.6 415.3 463.6
368.3 345.8 335.0 385.0 392.5 400.0 464.2 468.3 476.7 547.5
536.0 579.0 588.0 499.5 512.0 517.0 521.0 530.5 539.5 620.5
0.1155 0.1459 0.1866 0.2081 0.2289 0.2488 0.2152 0.2215 0.2226 0.1769
0.0529 0.0531 0.0568 0.1003 0.1014 0.1089 0.1452 0.1473 0.1478 0.1166
0.1382 0.1550 0.1838 0.3671 0.3787 0.4131 0.3669 0.3649 0.3536 0.1547
YM
ZD
DT a
S: comprehensive combustion characteristic parameter.
those of the UBC process due to it conducted at lower temperature and pressure [36]. Besides, there were three DTG peaks for all of the brown coals in their study. The first and second peaks appeared at around 60 C and 330 C were due to the vaporization of moisture and combustion of volatile matter, and the third peak at around 420 C represented the char combustion. After upgrading, the combustion peak of volatile matter decreased a little, whereas the char combustion process was strengthened obviously. The different combustion performances of coal can be explained intrinsically using the analysis of combustion kinetics. This analysis aims to solve the “kinetic triplet” kinetic model, pre-exponential factor (A), and Arrhenius activation energy (E). CoatseRedfern integral method was adopted in this study.
n ¼ 1;
lnð1 aÞ AR E ¼ ln ln bE RT T2 "
ns1;
ln
(6)
# 1 ð1 aÞð1nÞ AR E ¼ ln bE RT T 2 ð1 nÞ
(7)
where T is the process temperature (K), b is the heating rate (K/min), A is the frequency factor (s1), E is the activation energy (kJ/mol), R is the universal gas constant equal to 8.314 J/(mol K), and a is the conversion ratio.
a¼
m0 mt m0 m∞
(8)
where m0 refers to the initial mass, m∞ is the final mass, and mt is the mass in each time in the TG data. CoatseRedfern integral method is based on the fact that the kinetic model obeys the order of reaction, and the order n is usually regarded as 1 in the coal combustion process. A straight line can be obtained from Eq. (6), in which 1=T is equal to X and ln½lnð1 aÞ=T 2 is equal to Y. Kinetic parameters E and A were obtained from the slope and intercept of the line, respectively. As can be seen in Table 4, the activation energy increased in the first stage, and decreased in the combustion stage. These results indicate that LRC experienced difficulty in ignition but performed combustion easily. Raw coals easily caught fire because of their high volatile contents. However, the upgrading process reduced the volatile matter or turned it into passive component. Moreover, the activity functional groups were damaged in the treatment process. All of these changes resulted in the contraction of the coal structure, as well as an increase in density and coal rank, consequently increasing the E values of ignition. However, the upgraded coals maintained the high-reactivity advantages of brown coal (high porosity and surface area, low coalification, and abundant functional groups), which caused a decrease in E during the combustion stage. These changes were slightly more obvious at 300 C. For both segments, the change in E value was more obvious for ZT and YM than that for ZD. The increase in activation energy during the ignition stage and the decrease in activation energy during the combustion stage indicate an important advantage of the utilization of upgraded coals.
Table 4 Results of the upgrading influence on kinetics parameters of coal combustion. Coal
Condition
Temperature region/ C
Key temperaturea/ C
Activation energy/(kJ/mol)
Pre-exponential factor./s1
ZT
RAW
188e331 331e403 228e300 300e372 243e337 337e398 212e351 351e418 243e360 360e434 284e368 368e463 255e410 410e481 277e439 439e484 289e423 423e495
287.3 368.3 293.8 345.8 313.3 335.0 337.4 385.0 345.9 392.5 356.3 400.0 404.3 464.2 410.6 468.3 415.3 476.7
49.47 84.54 104.40 77.61 148.92 76.77 108.97 105.85 119.51 101.87 131.54 96.72 110.79 116.53 122.31 115.75 137.55 109.74
5.4E 2.6E 4.1E 2.1E 3.1E 1.3E 5.3E 2.2E 3.1E 8.1E 2.1E 2.4E 1.1E 6.9E 6.6E 1.3E 9.7E 3.9E
HTD-250 HTD-300 YM
RAW HTD-250 HTD-300
ZD
RAW HTD-250 HTD-300
a
The key temperatures in this table referred to ignition temperature and peak temperature respectively.
þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ
1 2 6 4 10 4 6 6 7 5 8 5 6 5 6 6 7 5
Correlation coefficient 0.9996 0.9993 0.9991 0.9782 0.9976 0.9786 0.9811 0.9940 0.9848 0.9858 0.9704 0.9663 0.9686 0.9987 0.9497 0.9999 0.9534 0.9998
L. Ge et al. / Applied Thermal Engineering 90 (2015) 174e181
4. Conclusion 1. Coalification was simulated in the HTD process, and consequently, the coal structure was broken, damaged, or developed to the depth direction. The inherent moisture of upgraded coals dramatically decreased. The chemical compositions of upgraded coals were comparable to those of HRC. The AO/C atomic ratio and FTIR results indicate improvement in coal rank, which consequently made the combustion process shift towards high temperature region. Results of the combustion characteristic parameters and activation energy indicate that the upgraded coals retained the advantages of brown coal in the combustion process. Moreover, the results tended to be strengthened when the treatment temperature was increased from 250 to 300 C. However, the effect of treatment temperature was limited, indicating that the upgrading process itself had more impact on the improvement in coal rank and on the changes in the coal combustion characteristics compared with the related conditions. 2. For the three studied brown coals with different ranks, more obvious effects on coal composition, microstructure, and combustion characteristics were obtained when the brown coal of lower rank was used,. Acknowledgements The authors gratefully acknowledge the financial support provided by the National Basic Research Program of China (Grant No. 2012CB214906), and the Fundamental Research Funds for the Central Universities of China (Grant No. 2015B02414). References [1] Z. Rao, Y. Zhao, C. Huang, C. Duan, J. He, Recent developments in drying and dewatering for low rank coals, Prog. Energy Combust. Sci. 46 (2015) 1e11. [2] M. Nonaka, T. Hirajima, K. Sasaki, Upgrading of low rank coal and woody biomass mixture by hydrothermal treatment, Fuel 90 (8) (2011) 2578e2584. [3] W. Li, P.L. Younger, Y. Cheng, B. Zhang, H. Zhou, Q. Liu, T. Dai, S. Kong, K. Jin, Q. Yang, Addressing the CO2 emissions of the world's largest coal producer and consumer: lessons from the Haishiwan Coalfield, China, Energy 80 (2015) 400e413. [4] Z. Chen, Inflationary effect of coal price change on the Chinese economy, Appl. Energy 114 (2014) 301e309. [5] W. Chen, R. Xu, Clean coal technology development in China, Energy Policy 38 (5) (2010) 2123e2130. [6] J. Yu, A. Tahmasebi, Y. Han, F. Yin, X. Li, A review on water in low rank coals: the existence, interaction with coal structure and effects on coal utilization, Fuel Process. Technol. 106 (2013) 9e20. [7] M. Sakaguchi, K. Laursen, H. Nakagawa, K. Miura, Hydrothermal upgrading of Loy Yang brown coal e effect of upgrading conditions on the characteristics of the products, Fuel Process. Technol. 89 (4) (2008) 391e396. [8] Y. Fei, A. Abd Aziz, S. Nasir, W.R. Jackson, M. Marshall, J. Hulston, A.L. Chaffee, The spontaneous combustion behavior of some low rank coals and a range of dried products, Fuel 88 (9) (2009) 1650e1655. [9] G. Favas, W.R. Jackson, Hydrothermal dewatering of lower rank coals. 1. Effects of process conditions on the properties of dried product, Fuel 82 (1) (2003) 53e57. [10] H. Katalambula, R. Gupta, Low-grade coals: a review of some prospective upgrading technologies, Energy Fuels 23 (7) (2009) 3392e3405. [11] M. Morimoto, H. Nakagawa, K. Miura, Low rank coal upgrading in a flow of hot water, Energy Fuels 23 (9) (2009) 4533e4539.
181
[12] Y. Yu, J. Liu, R. Wang, J. Zhou, K. Cen, Effect of hydrothermal dewatering on the slurryability of brown coals, Energy Convers. Manag. 57 (2012) 8e12. [13] J. Wu, J. Liu, X. Zhang, Z. Wang, J. Zhou, K. Cen, Chemical and structural changes in XiMeng lignite and its carbon migration during hydrothermal dewatering, Fuel 148 (2015) 139e144. [14] Y. Yu, J. Liu, K. Cen, Properties of coal water slurry prepared with the solid and liquid products of hydrothermal dewatering of brown coal, Ind. Eng. Chem. Res. 53 (11) (2014) 4511e4517. [15] T. Inoue, O. Okuma, K. Masuda, M. Yasumuro, K. Miura, Hydrothermal treatment of brown coal to improve the space time yield of a direct liquefaction reactor, Energy Fuels 26 (4) (2012) 2198e2203. [16] A.T. Mursito, T. Hirajima, K. Sasaki, Upgrading and dewatering of raw tropical peat by hydrothermal treatment, Fuel 89 (3) (2010) 635e641. [17] J. Yu, M. Guo, X. Xu, B. Guan, The role of temperature and CaCl2 in activated sludge dewatering under hydrothermal treatment, Water Res. 50 (2014) 10e17. [18] C. Areeprasert, P. Zhao, D. Ma, Y. Shen, K. Yoshikawa, Alternative solid fuel production from paper sludge employing hydrothermal treatment, Energy Fuels 28 (2) (2014) 1198e1206. [19] I. Espinosa-Gonzalez, A. Parashar, D.C. Bressler, Hydrothermal treatment of oleaginous yeast for the recovery of free fatty acids for use in advanced biofuel production, J. Biotechnol. 187 (2014) 10e15. [20] P. Zhao, Y. Shen, S. Ge, Z. Chen, K. Yoshikawa, Clean solid biofuel production from high moisture content waste biomass employing hydrothermal treatment, Appl. Energy 131 (2014) 345e367. ~ oz, R. Moliner, FTIR study of the evolution of coal structure [21] J.V. Ibarra, E. Mun during the coalification process, Org. Geochem. 24 (6-7) (1996) 725e735. [22] M.A. Ahmed, M.J. Blesa, R. Juan, R.E. Vandenberghe, Characterization of an Egyptian coal by Mossbauer and FT-IR spectroscopy, Fuel 82 (14) (2003) 1825e1829. [23] E.L. Zodrow, J.A. D'Angelo, R. Helleur, Z. Sim unek, Functional groups and common pyrolysate products of Odontopteris cantabrica (index fossil for the Cantabrian Substage, Carboniferous), Int. J. Coal Geol. 100 (2012) 40e50. [24] E.L. Zodrow, M. Mastalerz, Reconstruction of light environment for Pennsylvanian marattialean ferns: Insights from FTIR analysis of living Cyathea caracasana, Bull. Geosci. 85 (2) (2010) 361e365. [25] E.L. Zodrow, J.A. D'Angelo, M. Mastalerz, C.J. Cleal, D. Keefe, Phytochemistry of the fossilizedecuticle frond Macroneuropteris macrophylla (Pennsylvanian seed fern, Canada), Int. J. Coal Geol. 84 (2) (2010) 71e82. [26] R. Lin, G. Patrick Ritz, Studying individual macerals using i.r. microspectrometry, and implications on oil versus gas/condensate proneness and “low-rank” generation, Org. Geochem. 20 (6) (1993) 695e706. [27] R.P. Suggate, W.W. Dickinson, Carbon NMR of coals: the effects of coal type and rank, Int. J. Coal Geol. 57 (1) (2004) 1e22. [28] J. Kister, M. Guiliano, C. Largeau, S. Derenne, E. Casadevall, Characterization of chemical structure, degree of maturation and oil potential of Torbanites (type I kerogens) by quantitative FT-i.r. spectroscopy, Fuel 69 (11) (1990) 1356e1361. [29] J.A. Orrego-Ruiz, R. Cabanzo, E. Mejía-Ospino, Study of Colombian coals using photoacoustic Fourier transform infrared spectroscopy, Int. J. Coal Geol. 85 (3e4) (2011) 307e310. [30] J.K. Brown, W.R. Ladner, A study of the hydrogen distribution in coal-like materials by high-resolution nuclear magnetic resonance spectroscopy. II. A comparison with infrared measurement and the conversion to carbon structure, Fuel 39 (1960) 87e96. [31] J.M. Jones, M. Pourkashanian, C.D. Rena, A. Williams, Modelling the relationship of coal structure to char porosity, Fuel 78 (14) (1999) 1737e1744. [32] J.V. Ibarra, R. Moliner, A.J. Bonet, FT-i.r. investigation on char formation during the early stages of coal pyrolysis, Fuel 73 (6) (1994) 918e924. [33] C. Moon, Y. Sung, S. Ahn, T. Kim, G. Choi, D. Kim, Thermochemical and combustion behaviors of coals of different ranks and their blends for pulverizedcoal combustion, Appl. Therm. Eng. 54 (1) (2013) 111e119. [34] I.W. Smith, R.J. Tyler, Internal burning of pulverized semieanthracite: the relation between particle structure and reactivity, Fuel 51 (4) (1972) 312e321. [35] D.F. Umar, H. Usui, B. Daulay, Change of combustion characteristics of Indonesian low rank coal due to upgraded brown coal process, Fuel Process. Technol. 87 (11) (2006) 1007e1011. [36] D.F. Umar, B. Santoso, H. Usui, The effect of upgrading processes on combustion characteristics of Berau coal, Energy Fuels 21 (6) (2007) 3385e3387.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research paper
Influence of the hydrothermal dewatering on the combustion characteristics of Chinese low-rank coals Lichao Ge a, b, Yanwei Zhang b, Chang Xu a, *, Zhihua Wang b, Junhu Zhou b, Kefa Cen b a b
College of Energy and Electrical Engineering, Hohai University, Nanjing 211100, China State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
h i g h l i g h t s Typical Chinese lignites with various ranks are upgraded by hydrothermal dewatering. Upgraded coals exhibit chemical compositions comparable with that of bituminous coal. FTIR show the change of microstructure and improvement in coal rank after upgrading. Upgraded coals exhibit difficulty in ignition but combust easily. More evident effects are obtained for raw brown coal with relative lower rank.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 January 2013 Accepted 8 July 2015 Available online 16 July 2015
This study investigates the influence of hydrothermal dewatering performed at different temperatures on the combustion characteristics of Chinese low-rank coals with different coalification maturities. It was found that the upgrading process significantly decreased the inherent moisture and oxygen content, increased the calorific value and fixed carbon content, and promoted the damage of the hydrophilic oxygen functional groups. The results of oxygen/carbon atomic ratio indicated that the upgrading process converted the low-rank coals near to high-rank coals which can also be gained using the Fourier transform infrared spectroscopy. The thermogravimetric analysis showed that the combustion processes of upgraded coals were delayed toward the high temperature region, and the upgraded coals had higher ignition and burnout temperature. On the other hand, based on the higher average combustion rate and comprehensive combustion parameter, the upgraded coals performed better compared with raw brown coals and the Da Tong bituminous coal. In ignition segment, the activation energy increased after treatment but decreased in the combustion stage. The changes in coal compositions, microstructure, rank, and combustion characteristics were more notable as the temperature in hydrothermal dewatering increased from 250 to 300 C or coals of lower ranks were used. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Low-rank coal Upgrading Hydrothermal dewatering FTIR Combustion
1. Introduction Low-rank coals (LRCs), which include lignite, brown coals and sub-bituminous coals [1], are an important feedstock for energy and chemical products. However, the current global coal market is dominated by high-rank coals (HRCs) which account for only half of the total coal deposits [2]. As the world's largest coal producer and consumer [3], China depends heavily on coal [4]. 70% of China's primary energy demand is supplied by coal, and coal-fired power
* Corresponding author. Tel.: þ86 13951792223; fax: þ86 25 83786510. E-mail address: [email protected] (C. Xu). http://dx.doi.org/10.1016/j.applthermaleng.2015.07.015 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
plants account for over 97% in the total thermal power capacity [5]. Unfortunately, the country is rich in LRC reserves. The reserves of lignite are about 41.18% in the amount in China [1]. Therefore, the exploitation of LRCs is necessary for the growing energy demands and the depletion of HRCs. The advantages of LRCs include high reactivity, abundance, and low market price [6], however, LRCs have not been utilized as much as HRCs [7] because they have many drawbacks, such as high moisture, low energy density, and high spontaneous combustion tendency [8]. In order to overcome these drawbacks and make LRCs' characteristics comparable with those of HRCs. LRCs should be treated by an upgrading process before use. The first step of the upgrading process is dewatering.
L. Ge et al. / Applied Thermal Engineering 90 (2015) 174e181
Hydrothermal dewatering (HTD) process, also known as hot water drying, is a typical non-evaporative dewatering process. HTD has two major advantages: it can remove water in the liquid form, saving the latent heat of vaporization; it can damage oxygen functional groups, preventing the upgraded products from reabsorbing water. Due to its high temperature and pressure, HTD has been considered as the simulation of the coal forming process in nature. The literature contains some investigations about the effects of HTD processing variables on the properties of the final products [9,10]. Morimoto et al. [11] studied the yield of the products during the HTD process, and found that the upgraded coal showed less oxygen content, higher calorific value, and higher gasification reactivity than the raw coal. Yu et al. [12] studied the effect of HTD on the slurryability of brown coals, and found that the maximum solid concentration of coal water slurry increased. Wu et al. [13] studied the gas-, solid-, and liquid-phase products obtained after HTD, and analyzed the occurrence of carbon and its migration mechanism during HTD. In recent years, there has been a growing interest of HTD in coal treatment and a number of potential applications of hydrothermal processing have been investigated, such as preparing coal water slurry with the solid and liquid products of HTD [14], improving the space time yield of brown coal liquefaction reactor [15], upgrading and dewatering of raw tropical peat [16], dewatering of sludge [17], generating alternative solid fuel from paper sludge [18], and producing liquid and solid biofuels [19,20]. However, only few studies investigated the changes in coal chemical properties and microstructures during the HTD upgrading process, which are closely related to some combustion problems. For example, the less volatile matter release may lead to an unstable flame, and the oxidation of carbon may result in a decrease in the burnout efficiency. In this context, the combustion characteristics of LRC before and after the upgrading process should be compared to improve burning efficiency, reduce emissions, and evaluate the design and construction of a boiler. In this study, three types of brown coals (with different coal ranks) produced in China were processed by HTD at different temperatures. The changes in the coal composition and microstructure of LRC by HTD were investigated with emphases on the improvement of coal rank and on the influence of HTD on the combustion characteristics of LRC. 2. Experimental 2.1. Coal sample Three brown coals, namely, Zhao Tong (ZT), Yi Min (YM), and Zhun Dong (ZD), were investigated in this study. They were formed in different geological ages (ZT in Tertiary, YM in late Jurassic, and ZD in early Jurassic) and produced in three different areas in China. The HRC used in this study was Da Tong (DT) bituminous coal. The raw coals were crushed and sieved through 2.5-mm meshes before the upgrading process. The raw and upgraded coal samples were milled for 30 s in a ring mill and sieved through 74-mm meshes for the analysis. 2.2. HTD upgrading process HTD process was performed in a bench scale hydrothermal reaction system (WeiBa WHFS-2), which had a 2-L cylindrical autoclave with the maximum pressure of 25 MPa and the maximum temperature of 350 C [12], as shown in Fig. 1. A mixture of brown coal and deionized water (under a proportion of 1:2.5) was added into the autoclave. The sealed autoclave was injected with N2 to remove air and the pressure in the autoclave was set to be 4 MPa for
175
2 h to ensure the absence of leaks. After the N2 was released, the temperature in the autoclave was increased by 3 C/min up to the preset values (250 and 300 C) and maintained for 1 h. The mixture was stirred at 100 rpm during the entire process. The autoclave was cooled to ambient temperature by an internal cooling coil with tap water after the reaction was completed. The upgraded coals were separated from the mixture by a qualitative filter paper and dried in air for approximately one week. The dried updated coals were then milled. All upgraded coals were abbreviated to “initials-treatment temperature”, such as “YM-250” and “YM-300”. 2.3. Coal composition analysis The C, H, N and S contents of the coal samples were measured using an LECO-CHNS 932 Elemental Analyzer, whereas the O content was calculated by difference. The proximate analysis data was determined using a 5E-MACIII Infrared Rapid Analyzer, and the calorific value (CV) was obtained using the adiabatic bomb calorimetric method following ISO1928. 2.4. Fourier transform infrared spectroscopy (FTIR) All the spectra of both raw and upgraded coals were generated via standard procedures with KBr pellets by using a Nicolet NEXUS670 FTIR instrument. The measuring region ranged from 4000 to 400 cm1, and the spectra were generated by collecting 32 scans at a resolution of 4 cm1. Approximately 1 mg of ground coal was mixed with 100 mg of dried KBr powder, and the mixture was further ground with a mortar and a pestle under a baking lamp to avoid water absorption. The ground mixture was then pressed into a pellet with a hydraulic tabletting machine. OMNIC software 6.1a (Nicolet) was used for data preprocessing, and the spectral analysis processes were performed using the professional software PeakFit® (Version 4.12). 2.5. Thermo-gravimetric analysis (TGA) TGA analysis was conducted using the non-isothermal method in a TGA-SDTA 851e apparatus produced by METTLER TOLEDO. 5 mg of coal sample were placed in an aluminum crucible and heated at 25 C/min to the temperature until the weightlessness curve unchanged. The flow rate of standard air was kept at 60 mL/ min. The combustion characteristics investigated in this study include ignition (Ti), burnout (Tb) and peak (Tmax) temperatures, and the maximum (kmax) and average (kmean) combustion rates. They reflect the thermal behavior and burnout property during the combustion process and can be derived from the thermogravimetric and differential (TG-DTG) curves. Therefore, they were adopted to evaluate the combustion process. An index S, which is defined in Eq. (1) was used to compare the combustion characteristics of different coal samples. The coals with a higher S value have better combustion performance.
S¼
kmax kmean Ti2 Tb
(1)
3. Results and discussion 3.1. Influence of upgrading on coal composition The moisture content (Mt) of the raw brown coals not processed by HTDd53.11 wt% for ZT, 30.92 wt% for YM, and 24.94 wt% for ZDdis dramatically larger than that of DT bituminous coal (4.59 wt
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L. Ge et al. / Applied Thermal Engineering 90 (2015) 174e181
Controller ć
Magnetic Stirrer Thermocouple
Cooling Water Gas
Gas Intake Valve
Gas Release Valve Heating Coil
N2 Coal+Water
Fig. 1. The schematic diagram of hydrothermal dewatering experimental apparatus.
%), as shown in Table 1. After HTD processing at 250 C, Mt of the brown coals significantly decreased to 10.52 wt% for ZT-250, 10.01 wt% for YM-250, and 11.12 wt% for ZD-250. The Mt of brown coals processed by HTD at 300 C decreased to 6.74 wt% for ZT-300, 5.71 wt% for YM-300, and 10.36 wt% for ZD-300. In HTD, the moisture in the brown coals was removed in three ways: the thermal effects, the carbon dioxide generated by the thermal destruction of functional groups, and the shrinkage and collapse effects of the coal. The volatile matter (Vd, dried basis calculated from Table 1) of the coals decreased from 45.51 wt% (ZT) to 39.10 wt % (ZT-250) and 35.51 wt% (ZT-300), from 38.30 wt% (YM) to 36.42 wt% (YM-250) and 33.59 wt% (YM-300), and from 30.86 wt% (ZD) to 30.35 wt% (ZD-250) and 28.49 wt% (ZD-300). Whereas the fixed carbon content and CV of all upgraded coals significantly increased. This provided an evident that the active chemical components in the volatile matter had gradually transferred into the stable compositions in the coal structure. The oxygen content (Odaf) of the coals was reduced from 25.52 wt% (ZT) to 20.39 wt% (ZT-250) and 17.65 wt% (ZT-300), from 19.79 wt% (YM) to 16.48 wt% (YM250) and 15.48 wt% (YM-300), and from 18.24 wt% (ZD) to 16.48 wt % (ZD-250) and 15.22 wt% (ZD-300). The decrease in the atomic
ratio of oxygen to carbon (AO/C), which is a measure of coal rank, indicated an improvement in the coalification rank near that of bituminous coal. Among the three brown coals used in this study, ZT had the lowest coal rank and ZD had the highest. It can be seen from the yield of solid products (Table 1) that aside from moisture, several materials in the coal were reduced or destroyed, possibly affecting the microstructure and combustion process of the coal. Based on the aforementioned results, effects of HTD on the raw coals of lower rank are more obvious than those of higher rank, which may also be caused by the high treatment temperature. 3.2. Influence of upgrading on coal structure Although the general shapes of the spectrograms of the treated coals were similar to each other, their peaks did not show such multi as raw coal, particularly for some acromion and envelope peaks, as shown in Fig. 2. This indicates that the functional groups of the coals were destroyed or disappeared. The infrared spectra of the coals were divided into four regions [21]: the peaks appearing in the range of 3600e3000 cm1 are classified as hydroxy, the peaks in 3000e2700 cm1 belong to aliphatic, the peaks in
Table 1 Results of the upgrading influence on coal composition. Coal
Condition
Yield of solid products (wt%)
Proximate analysis (air dried basis, wt%) Moisture
Ash
Volatile
Fixed carbon
ZT
RAW HTD-250 HTD-300 RAW HTD-250 HTD-300 RAW HTD-250 HTD-300 RAW
e 85.91 78.02 e 92.38 88.87 e 97.11 93.27 e
31.84 9.00 6.18 13.88 7.59 4.22 11.55 10.04 9.07 4.59
15.65 24.15 25.05 10.67 12.60 12.50 3.84 3.72 3.53 6.92
31.02 35.58 33.32 32.98 33.66 32.17 27.30 27.30 25.91 24.54
21.49 31.27 35.45 42.47 46.15 51.11 57.31 58.94 61.49 63.95
YM
ZD
DT a
AO/C: oxygen/carbon atomic ratio.
Total moisture (wt%)
Calorific value (J/g)
Ultimate analysis (dry ash free basis, wt%) C
H
N
S
O
53.11 10.52 6.74 30.92 10.01 5.71 24.94 11.12 10.36 4.59
13,358 19,383 21,022 22,319 24,404 25,470 24,828 25,340 26,081 29,453
63.21 71.58 74.51 72.27 77.01 78.12 76.44 77.95 79.16 83.98
8.28 5.68 5.50 5.78 5.11 4.92 3.64 3.99 4.03 4.44
2.11 1.60 1.63 1.95 1.34 1.32 1.24 1.23 1.19 1.22
0.88 0.75 0.71 0.21 0.06 0.16 0.44 0.35 0.40 1.29
25.52 20.39 17.65 19.79 16.48 15.48 18.24 16.48 15.22 9.07
AO/Ca
0.30 0.21 0.18 0.21 0.16 0.15 0.18 0.16 0.14 0.08
L. Ge et al. / Applied Thermal Engineering 90 (2015) 174e181
0.6 0.5
Absorption intensity
Cal C Cal Hal H Hal ¼ * C H C Cal
ZD-RAW HTD-250 HTD-300
fa ¼ 1
0.4 0.3 0.2 0.1 0.0 4000 3600 3200 2800 2400 2000 1600 1200 800
400
-1
Wave (cm ) Fig. 2. The infrared spectrum of ZD raw and upgraded coals.
1800e1000 cm1 correspond to functional groups and aromatic, and the peaks in 900e700 cm1 are considered as aromatic in the low wave region and ash matter. Additional information about the assignments of the coal spectra are shown in Refs. [22,23]. Fig. 3 shows the band reconstructions of the four parts of ZD raw brown coal using the overlapping peak resolved method with PeakFit®. Other samples were treated under the same condition. The position, width, and integral intensity of the infrared bands were obtained. The value of many parameters [24,25] related to the microstructure, the physicochemical characteristics, and the rank of coals can be calculated from the intensities of the infrared spectrum absorptions, as shown in Table 2.
AC¼O I1640e1700 cm1 ¼ Aar I1500e1630 cm1
cm1
where Cal/C is the ratio of aliphatic carbon to the total carbon in coal; Hal =H ¼ I3000e2700 cm1 =ðI900e700 cm1 þ I3000e2700 cm1 Þ is the percentage of aliphatic hydrogen in total hydrogen; H/C is the hydrogen/carbon atomic ratio calculated from the elemental analysis; and Hal/Cal is the hydrogen/carbon atomic ratio in aromatic clusters, which has a value of 1.8 for coals [32]. The fa values of the raw coals in Table 2 demonstrate that ZT and ZD had the lowest and highest coal ranks, respectively. These results are in agreement with the geological age and AO/C ratio values. Because the dehydration, decarboxylation, and demethanation reactions in HTD upgrading process simulate the coal forming in nature, the coal rank was improved and to a greater extent when the treatment temperature was increased to 300 C. However, the effect of the temperature was limited, indicating that the upgrading process had more impact on the changes in coal structure and on the improvement of rank than the related conditions. Moreover, the lower the original coal rank, the more significant the effects of the upgrading process. For example, fa was improved from 0.261 (ZT) to 0.525 (ZT-250) and 0.541 (ZT-300), from 0.580 (YM) to 0.637 (YM250) and 0.638 (YM-300), and from 0.721 (ZD) to 0.724 (ZD-250) and 0.727 (ZD-300). Wu et al. [13] investigated the changes in the structural features of Xi Meng brown coal during HTD by solid-state nuclear magnetic resonance spectroscopy, and also found the same results. The aliphatic components decreased whereas aromatic components increased after HTD, indicated coal rank improved. Oxygen-containing groups significantly decreased in number, and the productions of CO2 and CO (the main gaseous products) were attributed to the decomposition of carbonyl and carboxyl groups. 3.3. Influence of upgrading on coal combustion
(3)
cm1
The methyl/methylene (CH3/CH2) ratio (Eq. (3)) measures the length of aliphatic chains and degree of branching of aliphatic side chains attached to the macromolecular structure of the coal [26]. Higher CH3/CH2 values indicate longer and less branched aliphatic chains, the reduction and destruction of the bridge bond connecting with CH2, and an improvement in the coal rank [21].
Aar I1500e1600 cm1 ¼ Aal I2800e3000 cm1
(5)
(2)
The carbonyl/aromatic (AC ¼ O/Aar) ratio, which represents the dissociation and shift of oxygen functional groups is defined in Eq. (2). Lower AC ¼ O/Aar values indicate a decrease in the carbonyl or carboxyl groups to aromatic carbon groups, demonstrating the reduction and destruction of oxygen functional groups during the upgrading process.
CH3 I2955e2960 ¼ CH2 I2920e2945
177
(4)
The aromatic/aliphatic (Aar/Aal), which is another parameter related to the aromaticity and grade of coal rank, is defined in Eq. (4) [27]. Higher Aar/Aal values indicate higher aromaticity and maturation [28]. The apparent aromaticity (fa), which is considered as the measurement of the coal rank [29], can be calculated following the method reported by Brown and Ladner [30], as shown in Eq. (5) [31].
The TG curves shifted toward the high temperature region and the entire coal combustion process was delayed because of the change in coal rank [33], as shown in Fig. 4. Similar to the results above, the influence of upgrading on the combustion characteristics was more obvious when the samples with low rank were used. HRC (e.g., DT) typically exhibits a slight increase in mass through adsorption before the evaporation of the volatile [34]. A similar phenomenon was observed in upgraded LRC, particularly for ZT brown coal, providing another evidence of effect of upgrading on coal rank. Two obvious peaks were observed in the DTG curves for all coal samples. The first peak at approximately 55 C can be attributed to the significant decrease in water evaporation after upgrading process, which was consistent with the tendency of moisture based on the industrial analysis. The second peak in the range of 350e450 C can be attributed to the slight increase in char combustion, which consequently became more concentrated and intense. This result demonstrates that several volatile matters turned into char structures during the upgrading process, thus enhancing the char combustion process. Besides, there was another combustion peak around 300 C for ZT-250, which was ascribed to the combustion of volatile matter [35,36] due to its highest volatile matter content (35.58 wt%, Table 1). This provided another evidence that some unstable active compositions in the volatile matter gradually transferred into stable inert components. Because of the relatively mild temperature of 250 C, the transformation process had not finished completely, the active unstable chemical compositions probably produced the separate combustion peak prior to the char
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L. Ge et al. / Applied Thermal Engineering 90 (2015) 174e181
0.035
0.10
Raw IR spectra Reconstruction
0.030 Raw IR spectra Reconstruction
Absorption intensity
Absorption intensity
0.08 0.06 0.04 0.02
0.025 0.020 0.015 0.010 0.005
0.00
0.000 3000
3100
3200
3300
3400
3500
3600
2700
-1
2750
2800
2850
2900
-1
2950
3000
Wave (cm )
Wave (cm )
(a) 3600–3000 cm-1
(b) 3000–2700 cm-1
0.08
Raw IR spectra Reconstruction
0.006 Raw IR spectra Reconstruction
Absorption intensity
Absorption intensity
0.06
0.04
0.02
0.004
0.002
0.000
0.00 1000
1200
1400
1600
1800
740
760
780
-1
800
820
840
Wave (cm )
-1
Wave (cm )
(c) 1800–1000 cm-1
(d) 900–700 cm-1
Fig. 3. Resolving and construction of FTIR peaks of ZD raw brown coal.
Table 2 Results of the upgrading influence on coal structure. Coal
Location
Geological age
Condition
Carbonyl/aromatic ratio
Methyl/methylene ratio
Aromatic/aliphatic ratio
Apparent aromaticity
ZT
Southwestern China
Tertiary
YM
Northeastern China
Late Jurassic
ZD
Northwestern China
Early Jurassic
RAW HTD-250 HTD-300 RAW HTD-250 HTD-300 RAW HTD-250 HTD-300
0.527 0.352 0.268 0.384 0.253 0.213 0.465 0.360 0.235
0.168 0.199 0.446 0.170 0.345 0.410 0.297 0.355 0.369
5.760 5.800 5.883 5.852 5.972 6.227 12.673 14.209 14.540
0.261 0.525 0.541 0.580 0.637 0.638 0.721 0.724 0.727
combustion peak. While with the treatment temperature increased further to 300 C, the transformation process was nearly finished, the volatile matter stayed in the stable state, just like the raw coal. YM and ZD brown coals had lower volatile content and relatively higher coal rank, so their combustion peaks of volatile matter were contained in the char combustion peak. Detailed information of the combustion characteristic parameters obtained from the TG-DTG curves is shown in Table 3. The upgrading process increased Ti, and the higher temperature in the upgrading process resulted in higher Ti. For example, Ti increased
from 287.3 C (ZT) to 293.8 C (ZT-250) and 313.3 C (ZT-300). The increase in Ti indicates a decrease or transfer of the volatile matter. This decrease or transfer could prevent the spontaneous combustion tendency of raw brown coal and indicates that the upgraded products were similar to HRC as the Ti of HRC is usually high (463.6 C for DT bituminous coal). As Tb increased, burnout property decreased. Tmax showed a slight increase except ZT brown coal (possibly due to its high moisture content and low coal rank). kmax, kmean and S increased, indicating that the upgraded products retained the advantages of brown coal in terms of combustion
L. Ge et al. / Applied Thermal Engineering 90 (2015) 174e181
100
179
0.00
80 -1
Weight loss rate (min )
-0.04
Weight (%)
60
ZT-RAW ZT-250 ZT-300 DT-RAW
40 20
-0.08 ZT-RAW ZT-250 ZT-300 DT-RAW
-0.12 -0.16 -0.20
0
0
100
200
300
400
500
600
700
0
100
200
300
400
500
600
700
600
700
600
700
Temperature (℃)
Temperature (℃)
100
0.00 80
Weight (%)
-1
Weight loss rate (min )
-0.05
60 YM-RAW YM-250 YM-300 DT-RAW
40
20
-0.10 YM-RAW YM-250 YM-300 DT-RAW
-0.15 -0.20 -0.25
0 0
100
200
300
400
500
600
700
800
0
100
200
Temperature (℃)
100
400
500
0.00
-1
Weight loss rate (min )
80
Weight (%)
300
Temperature (℃)
60
ZD-RAW ZD-250 ZD-300 DT-RAW
40
20
0
-0.05 -0.10
ZD-RAW ZD-250 ZD-300 DT-RAW
-0.15 -0.20 -0.25
0
100
200
300
400
500
600
700
Temperature (℃)
0
100
200
300
400
500
Temperature (℃)
Fig. 4. The TG and DTG curves of raw and upgraded coals as well as bituminous coal.
characteristics, even though Ti, Tmax and Tb increased. And as a result of relative high ignition and burnout temperature as well as the limited increment of combustion rate, the S value of ZD showed a slightly decrease. The above-mentioned trends are more obvious for young brown coals than for old brown coals. For example, Ti increased from 287.3 to 313.3 C (ZT) at 300 C, and from 337.4 to 356.3 C (YM) but only from 404.3 to 415.3 C (ZD). These changes demonstrate that the upgraded coals experienced difficulty in ignition but performed combustion easily because of their high CV,
less moisture content, and combustion characteristics similar to those of HRC. This results were consistent with Umar et al. [35], who studied the changes of combustion characteristics of three Indonesian LRCs with upgraded brown coal (UBC) process. They found that Ti of the upgraded coals increased, whereas Tmax is not so changed, and kmax significantly increased. They also treated the brown coal with hot water drying (HWD) and steam drying (SD), and the combustion characteristics of HWD and SD processes were much better than
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L. Ge et al. / Applied Thermal Engineering 90 (2015) 174e181
Table 3 Results of the upgrading influence on combustion characteristic parameters. Coal
Condition
Ignition temperature/ C
Peak temperature/ C
Burnout temperature/ C
Maximum combustion rate/min1
Average combustion rate/min1
Sa*109/(min2*K3)
ZT
RAW HTD-250 HTD-300 RAW HTD-250 HTD-300 RAW HTD-250 HTD-300 RAW
287.3 293.8 313.3 337.4 345.9 356.3 404.3 410.6 415.3 463.6
368.3 345.8 335.0 385.0 392.5 400.0 464.2 468.3 476.7 547.5
536.0 579.0 588.0 499.5 512.0 517.0 521.0 530.5 539.5 620.5
0.1155 0.1459 0.1866 0.2081 0.2289 0.2488 0.2152 0.2215 0.2226 0.1769
0.0529 0.0531 0.0568 0.1003 0.1014 0.1089 0.1452 0.1473 0.1478 0.1166
0.1382 0.1550 0.1838 0.3671 0.3787 0.4131 0.3669 0.3649 0.3536 0.1547
YM
ZD
DT a
S: comprehensive combustion characteristic parameter.
those of the UBC process due to it conducted at lower temperature and pressure [36]. Besides, there were three DTG peaks for all of the brown coals in their study. The first and second peaks appeared at around 60 C and 330 C were due to the vaporization of moisture and combustion of volatile matter, and the third peak at around 420 C represented the char combustion. After upgrading, the combustion peak of volatile matter decreased a little, whereas the char combustion process was strengthened obviously. The different combustion performances of coal can be explained intrinsically using the analysis of combustion kinetics. This analysis aims to solve the “kinetic triplet” kinetic model, pre-exponential factor (A), and Arrhenius activation energy (E). CoatseRedfern integral method was adopted in this study.
n ¼ 1;
lnð1 aÞ AR E ¼ ln ln bE RT T2 "
ns1;
ln
(6)
# 1 ð1 aÞð1nÞ AR E ¼ ln bE RT T 2 ð1 nÞ
(7)
where T is the process temperature (K), b is the heating rate (K/min), A is the frequency factor (s1), E is the activation energy (kJ/mol), R is the universal gas constant equal to 8.314 J/(mol K), and a is the conversion ratio.
a¼
m0 mt m0 m∞
(8)
where m0 refers to the initial mass, m∞ is the final mass, and mt is the mass in each time in the TG data. CoatseRedfern integral method is based on the fact that the kinetic model obeys the order of reaction, and the order n is usually regarded as 1 in the coal combustion process. A straight line can be obtained from Eq. (6), in which 1=T is equal to X and ln½lnð1 aÞ=T 2 is equal to Y. Kinetic parameters E and A were obtained from the slope and intercept of the line, respectively. As can be seen in Table 4, the activation energy increased in the first stage, and decreased in the combustion stage. These results indicate that LRC experienced difficulty in ignition but performed combustion easily. Raw coals easily caught fire because of their high volatile contents. However, the upgrading process reduced the volatile matter or turned it into passive component. Moreover, the activity functional groups were damaged in the treatment process. All of these changes resulted in the contraction of the coal structure, as well as an increase in density and coal rank, consequently increasing the E values of ignition. However, the upgraded coals maintained the high-reactivity advantages of brown coal (high porosity and surface area, low coalification, and abundant functional groups), which caused a decrease in E during the combustion stage. These changes were slightly more obvious at 300 C. For both segments, the change in E value was more obvious for ZT and YM than that for ZD. The increase in activation energy during the ignition stage and the decrease in activation energy during the combustion stage indicate an important advantage of the utilization of upgraded coals.
Table 4 Results of the upgrading influence on kinetics parameters of coal combustion. Coal
Condition
Temperature region/ C
Key temperaturea/ C
Activation energy/(kJ/mol)
Pre-exponential factor./s1
ZT
RAW
188e331 331e403 228e300 300e372 243e337 337e398 212e351 351e418 243e360 360e434 284e368 368e463 255e410 410e481 277e439 439e484 289e423 423e495
287.3 368.3 293.8 345.8 313.3 335.0 337.4 385.0 345.9 392.5 356.3 400.0 404.3 464.2 410.6 468.3 415.3 476.7
49.47 84.54 104.40 77.61 148.92 76.77 108.97 105.85 119.51 101.87 131.54 96.72 110.79 116.53 122.31 115.75 137.55 109.74
5.4E 2.6E 4.1E 2.1E 3.1E 1.3E 5.3E 2.2E 3.1E 8.1E 2.1E 2.4E 1.1E 6.9E 6.6E 1.3E 9.7E 3.9E
HTD-250 HTD-300 YM
RAW HTD-250 HTD-300
ZD
RAW HTD-250 HTD-300
a
The key temperatures in this table referred to ignition temperature and peak temperature respectively.
þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ þ
1 2 6 4 10 4 6 6 7 5 8 5 6 5 6 6 7 5
Correlation coefficient 0.9996 0.9993 0.9991 0.9782 0.9976 0.9786 0.9811 0.9940 0.9848 0.9858 0.9704 0.9663 0.9686 0.9987 0.9497 0.9999 0.9534 0.9998
L. Ge et al. / Applied Thermal Engineering 90 (2015) 174e181
4. Conclusion 1. Coalification was simulated in the HTD process, and consequently, the coal structure was broken, damaged, or developed to the depth direction. The inherent moisture of upgraded coals dramatically decreased. The chemical compositions of upgraded coals were comparable to those of HRC. The AO/C atomic ratio and FTIR results indicate improvement in coal rank, which consequently made the combustion process shift towards high temperature region. Results of the combustion characteristic parameters and activation energy indicate that the upgraded coals retained the advantages of brown coal in the combustion process. Moreover, the results tended to be strengthened when the treatment temperature was increased from 250 to 300 C. However, the effect of treatment temperature was limited, indicating that the upgrading process itself had more impact on the improvement in coal rank and on the changes in the coal combustion characteristics compared with the related conditions. 2. For the three studied brown coals with different ranks, more obvious effects on coal composition, microstructure, and combustion characteristics were obtained when the brown coal of lower rank was used,. Acknowledgements The authors gratefully acknowledge the financial support provided by the National Basic Research Program of China (Grant No. 2012CB214906), and the Fundamental Research Funds for the Central Universities of China (Grant No. 2015B02414). References [1] Z. Rao, Y. Zhao, C. Huang, C. Duan, J. He, Recent developments in drying and dewatering for low rank coals, Prog. Energy Combust. Sci. 46 (2015) 1e11. [2] M. Nonaka, T. Hirajima, K. Sasaki, Upgrading of low rank coal and woody biomass mixture by hydrothermal treatment, Fuel 90 (8) (2011) 2578e2584. [3] W. Li, P.L. Younger, Y. Cheng, B. Zhang, H. Zhou, Q. Liu, T. Dai, S. Kong, K. Jin, Q. Yang, Addressing the CO2 emissions of the world's largest coal producer and consumer: lessons from the Haishiwan Coalfield, China, Energy 80 (2015) 400e413. [4] Z. Chen, Inflationary effect of coal price change on the Chinese economy, Appl. Energy 114 (2014) 301e309. [5] W. Chen, R. Xu, Clean coal technology development in China, Energy Policy 38 (5) (2010) 2123e2130. [6] J. Yu, A. Tahmasebi, Y. Han, F. Yin, X. Li, A review on water in low rank coals: the existence, interaction with coal structure and effects on coal utilization, Fuel Process. Technol. 106 (2013) 9e20. [7] M. Sakaguchi, K. Laursen, H. Nakagawa, K. Miura, Hydrothermal upgrading of Loy Yang brown coal e effect of upgrading conditions on the characteristics of the products, Fuel Process. Technol. 89 (4) (2008) 391e396. [8] Y. Fei, A. Abd Aziz, S. Nasir, W.R. Jackson, M. Marshall, J. Hulston, A.L. Chaffee, The spontaneous combustion behavior of some low rank coals and a range of dried products, Fuel 88 (9) (2009) 1650e1655. [9] G. Favas, W.R. Jackson, Hydrothermal dewatering of lower rank coals. 1. Effects of process conditions on the properties of dried product, Fuel 82 (1) (2003) 53e57. [10] H. Katalambula, R. Gupta, Low-grade coals: a review of some prospective upgrading technologies, Energy Fuels 23 (7) (2009) 3392e3405. [11] M. Morimoto, H. Nakagawa, K. Miura, Low rank coal upgrading in a flow of hot water, Energy Fuels 23 (9) (2009) 4533e4539.
181
[12] Y. Yu, J. Liu, R. Wang, J. Zhou, K. Cen, Effect of hydrothermal dewatering on the slurryability of brown coals, Energy Convers. Manag. 57 (2012) 8e12. [13] J. Wu, J. Liu, X. Zhang, Z. Wang, J. Zhou, K. Cen, Chemical and structural changes in XiMeng lignite and its carbon migration during hydrothermal dewatering, Fuel 148 (2015) 139e144. [14] Y. Yu, J. Liu, K. Cen, Properties of coal water slurry prepared with the solid and liquid products of hydrothermal dewatering of brown coal, Ind. Eng. Chem. Res. 53 (11) (2014) 4511e4517. [15] T. Inoue, O. Okuma, K. Masuda, M. Yasumuro, K. Miura, Hydrothermal treatment of brown coal to improve the space time yield of a direct liquefaction reactor, Energy Fuels 26 (4) (2012) 2198e2203. [16] A.T. Mursito, T. Hirajima, K. Sasaki, Upgrading and dewatering of raw tropical peat by hydrothermal treatment, Fuel 89 (3) (2010) 635e641. [17] J. Yu, M. Guo, X. Xu, B. Guan, The role of temperature and CaCl2 in activated sludge dewatering under hydrothermal treatment, Water Res. 50 (2014) 10e17. [18] C. Areeprasert, P. Zhao, D. Ma, Y. Shen, K. Yoshikawa, Alternative solid fuel production from paper sludge employing hydrothermal treatment, Energy Fuels 28 (2) (2014) 1198e1206. [19] I. Espinosa-Gonzalez, A. Parashar, D.C. Bressler, Hydrothermal treatment of oleaginous yeast for the recovery of free fatty acids for use in advanced biofuel production, J. Biotechnol. 187 (2014) 10e15. [20] P. Zhao, Y. Shen, S. Ge, Z. Chen, K. Yoshikawa, Clean solid biofuel production from high moisture content waste biomass employing hydrothermal treatment, Appl. Energy 131 (2014) 345e367. ~ oz, R. Moliner, FTIR study of the evolution of coal structure [21] J.V. Ibarra, E. Mun during the coalification process, Org. Geochem. 24 (6-7) (1996) 725e735. [22] M.A. Ahmed, M.J. Blesa, R. Juan, R.E. Vandenberghe, Characterization of an Egyptian coal by Mossbauer and FT-IR spectroscopy, Fuel 82 (14) (2003) 1825e1829. [23] E.L. Zodrow, J.A. D'Angelo, R. Helleur, Z. Sim unek, Functional groups and common pyrolysate products of Odontopteris cantabrica (index fossil for the Cantabrian Substage, Carboniferous), Int. J. Coal Geol. 100 (2012) 40e50. [24] E.L. Zodrow, M. Mastalerz, Reconstruction of light environment for Pennsylvanian marattialean ferns: Insights from FTIR analysis of living Cyathea caracasana, Bull. Geosci. 85 (2) (2010) 361e365. [25] E.L. Zodrow, J.A. D'Angelo, M. Mastalerz, C.J. Cleal, D. Keefe, Phytochemistry of the fossilizedecuticle frond Macroneuropteris macrophylla (Pennsylvanian seed fern, Canada), Int. J. Coal Geol. 84 (2) (2010) 71e82. [26] R. Lin, G. Patrick Ritz, Studying individual macerals using i.r. microspectrometry, and implications on oil versus gas/condensate proneness and “low-rank” generation, Org. Geochem. 20 (6) (1993) 695e706. [27] R.P. Suggate, W.W. Dickinson, Carbon NMR of coals: the effects of coal type and rank, Int. J. Coal Geol. 57 (1) (2004) 1e22. [28] J. Kister, M. Guiliano, C. Largeau, S. Derenne, E. Casadevall, Characterization of chemical structure, degree of maturation and oil potential of Torbanites (type I kerogens) by quantitative FT-i.r. spectroscopy, Fuel 69 (11) (1990) 1356e1361. [29] J.A. Orrego-Ruiz, R. Cabanzo, E. Mejía-Ospino, Study of Colombian coals using photoacoustic Fourier transform infrared spectroscopy, Int. J. Coal Geol. 85 (3e4) (2011) 307e310. [30] J.K. Brown, W.R. Ladner, A study of the hydrogen distribution in coal-like materials by high-resolution nuclear magnetic resonance spectroscopy. II. A comparison with infrared measurement and the conversion to carbon structure, Fuel 39 (1960) 87e96. [31] J.M. Jones, M. Pourkashanian, C.D. Rena, A. Williams, Modelling the relationship of coal structure to char porosity, Fuel 78 (14) (1999) 1737e1744. [32] J.V. Ibarra, R. Moliner, A.J. Bonet, FT-i.r. investigation on char formation during the early stages of coal pyrolysis, Fuel 73 (6) (1994) 918e924. [33] C. Moon, Y. Sung, S. Ahn, T. Kim, G. Choi, D. Kim, Thermochemical and combustion behaviors of coals of different ranks and their blends for pulverizedcoal combustion, Appl. Therm. Eng. 54 (1) (2013) 111e119. [34] I.W. Smith, R.J. Tyler, Internal burning of pulverized semieanthracite: the relation between particle structure and reactivity, Fuel 51 (4) (1972) 312e321. [35] D.F. Umar, H. Usui, B. Daulay, Change of combustion characteristics of Indonesian low rank coal due to upgraded brown coal process, Fuel Process. Technol. 87 (11) (2006) 1007e1011. [36] D.F. Umar, B. Santoso, H. Usui, The effect of upgrading processes on combustion characteristics of Berau coal, Energy Fuels 21 (6) (2007) 3385e3387.