Article pubs.acs.org/EF
Study of Chemical Structure Changes of Chinese Lignite upon Drying in Superheated Steam, Microwave, and Hot Air Arash Tahmasebi,†,‡ Jianglong Yu,*,†,‡ Yanna Han,† Fengkui Yin,† Sankar Bhattacharya,§ and David Stokie§ †
Key Laboratory of Advanced Coal and Coking Technology Liaoning, School of Chemical Engineering, University of Science and Technology Liaoning, Number 185 Qianshan Zhong Road, Anshan 114051, People’s Republic of China ‡ Thermal Energy Research Centre, Shenyang Aerospace University, Number 37 Daoyi Nan Avenue, Shenyang 110136, People’s Republic of China § Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia ABSTRACT: Chemical changes of Chinese lignite upon drying in superheated steam, microwave, and hot air have been studied in this paper using the Fourier transform infrared (FTIR) spectroscopy technique. The infrared (IR) spectra of raw and dried samples were curve-fitted to a series of bands in aliphatic hydrogen (3000−2800 cm−1) and carbonyl absorption (1850− 1500 cm−1) zones. It has been found that aliphatic hydrogen absorbance decreased slightly with an increasing temperature during superheated steam drying, while absorption of carboxyl (COOH) and carbonyl (CO) groups decreased drastically, indicative of the loss of oxygen functionalities with an increasing drying temperature. During steam drying, aromatic carbon and aromatic ring stretch absorption remained relatively unchanged up to 250 °C and decreased significantly thereafter because of some pyrolysis reactions that took place at higher drying temperatures. Microwave heating of lignite resulted in a significant decrease in the concentration of oxygen-containing functional groups. Aromatic carbon remained relatively unchanged under microwave drying conditions, while aliphatic hydrogen decreased slightly. The aromaticity of coal calculated from curve-fitted spectra of deconvoluted peaks showed a progressive increase with an increasing drying intensity under both steam and microwave drying conditions. Under air drying conditions, aliphatic hydrogen absorbance decreased drastically at 250 °C, while aromatic carbon remained unchanged. It was observed that oxidation in air mainly took place on aliphatic hydrogen sites, especially on methylene groups. Changes of carboxyl and carbonyl groups during air-dried samples showed a different trend compared to those dried in steam and microwave, increasing gradually up to 150 °C and then a sharp increase at 200 °C. The absorption of these groups decreased significantly at an increased air temperature up to 250 °C. hydroxyl, and carbonyl).24 Elimination of polar groups will cause a decrease in the oxygen content and moisture-holding capacity of the coal.25−27 Oxygen is removed in the form of water, carbon dioxide, and carbon monoxide during decomposition of oxygen functional groups.27 Hydrophobicity of low-rank coals increases and their self-ignition tendency decreases with the removal of oxygen-containing functional groups.26,28 On the other hand, drying of coal in hot air will cause the oxidation of its organic structure. Oxidation affects coal properties, such as softening and swelling properties, heat of combustion, calorific values, and coking and caking characteristics, which decrease significantly.29−31 The main effects of oxygen on the coal structure are the increase in oxygen functionalities with a simultaneous decrease in the aliphatic hydrogen content.32,33 Peroxides, hydroperoxides, and hydroxyl species form as a result of the reaction between molecular oxygen and aliphatic groups.34,35 Peroxide and hydroperoxide groups start to oxidize to hydroxyl, carbonyl (ketones), and carboxyl groups at low temperatures.29,34,36,37 Ketonic carbonyl and carboxyl groups decompose at higher temperatures to produce lower molecular-weight structures, such as carbon dioxide.38,39
1. INTRODUCTION An estimated 45% of the world’s coal reserves consist of low-rank coals.1 Use of low-rank coals is important because of the advantages of low mining cost, high reactivity, high amount of volatiles, and low amount of pollution-forming impurities, such as sulfur, nitrogen, and heavy metals.2,3 However, high moisture content (25−60%) in low-rank coals exerts significant effects in their use processes.4 The high moisture content results in lower efficiency, increased transportation cost, and higher CO2 emission.5 Low-rank coals have difficulties in transportation and storage and also tend to spontaneously combust.6 Therefore, it is necessary to reduce their moisture content to increase the product quality.7,8 Various drying and upgrading technologies have been developed to decrease the moisture content and increase the calorific value of low-rank coals.9−13 Among these technologies, superheated steam drying7,14−19 and microwave drying20−23 are promising technologies compared to conventional hot air or hot flue gas drying. Low-rank coals undergo chemical structure changes during the drying process. Chemical changes in the coal structure are important regarding its behavior in downstream applications, such as combustion, gasification, pyrolysis, and liquefaction. The change in the chemical nature of the coal surface is reported to be predominantly in the formation and/or destruction of the oxygen-containing polar functional groups (including carboxyl, © 2012 American Chemical Society
Received: April 1, 2012 Revised: May 24, 2012 Published: May 29, 2012 3651
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2. EXPERIMENTAL SECTION
Fourier transform infrared (FTIR) spectroscopy is a useful method for determining the chemical changes during drying and oxidation of coal.35−37,40−42 Despite the importance and significance of superheated steam fluidized-bed and microwave drying technologies in upgrading and use of low-rank coals, very little has been reported on chemical changes during drying of low-rank coals in these processes. Furthermore, the effects of air drying on the low-rank coal structure reported in previous works mostly covers the lowtemperature oxidation aspect of coal, and a deep insight into the effects of air on the lignite chemical structure at high temperatures is necessary. In this study, the chemical changes in the low-rank coal organic structure during superheated steam fluidized-bed and microwave drying are reported and compared to conventional hot-air fluidized-bed drying by use of the FTIR technique.
2.1. Superheated Steam Fluidized-Bed Drying Experiments. A Chinese lignite coal, Shenhua No. 6, was used in drying experiments, and its proximate analysis data and sulfur content are presented in Table 1. Samples were crushed and sieved to less than 1 mm for steam drying experiments. An experimental setup was designed and manufactured for steam fluidized-bed drying experiments (Figure 1), comprising a drying reactor, an electrically heated furnace, a water pump, and a controller. Drying temperatures varied in these experiments were 150, 200, 250, and 300 °C, and 3 g of samples was dried for 10 min before being subjected to FTIR measurement. The final moisture content of samples after drying was almost zero (samples were almost completely dried). The higher temperatures (250 and 300 °C) have been used in the experiments primarily for comparison purposes, while the lower temperatures (150−200 °C) are more likely to be the temperatures to be employed in practice. 2.2. Microwave Drying Experiments. Microwave drying experiments were carried out in a domestic microwave oven (Midea) with technical features of ∼220 V, 50 Hz, and 700 W and a microwave frequency of 2450 Hz. Three different microwave output power levels of 380, 540, and 700 W were used for this study. The dimensions of the drying chamber used for drying were 305 × 204 × 305 mm and consisted of a rotating glass plate with a diameter of 270 mm at the base of the microwave oven. A 150−500 μm size fraction of samples was dried for 10 min at different output power levels, and the dried samples were subjected to FTIR analysis. Similar to steam drying experiments, the final moisture content of samples after 10 min of drying was near
Table 1. Proximate Analysis of the Lignite Sample Used in Drying Experimentsa proximate analysis (%) moisture (ar)
volatile matter (ad)
fixed carbon (ad)
ash content (ad)
39.04
38.59
56.37
5.03
a
ar, as received; ad, air dried.
Figure 1. Schematic diagram of the steam fluidized-bed drying experimental setup. 3652
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Figure 2. Curve-fitted spectra of the 1850−1500 cm−1 zone for superheated steam-dried coal at 300 °C.
Figure 3. FTIR spectra of as-received coal and steam-dried samples at 150, 200, 250, and 300 °C.
zero (samples were almost completely dried). At each time interval, the coal sample was taken out from the microwave oven and the temperature of the samples was immediately measured using a thin and sensitive thermocouple, and thus, the temperature change versus time during drying was obtained. 2.3. Air Fluidized-Bed Drying Experiments. Air drying experiments were carried out in a fluidized-bed dryer setup comprising of a quartz reactor, an electrically heated furnace, and a controller. Similar samples to those used in superheated steam fluidized-bed and microwave drying (Chinese Shenhua No. 6) were used in air drying experiments. A total of 2 g of the as-received sample with a 224−355 μm particle size was placed in the quartz fluidized-bed reactor and heated in an electric furnace for 10 min. Air was used as the heat carrier, and fluidizing gas and samples were dried at 150, 200, and 250 °C prior to FTIR analysis. 2.4. FTIR Spectroscopy. Infrared (IR) spectra of raw coal and dried samples were obtained with a Thermo Fisher Nicolet IS5 mid-FTIR spectrometer. KBr pellets were prepared by grinding around 2.5 mg of dried coal with 200 mg of KBr. IR spectra of the lignite sample for the 4000−400 cm−1 region were studied by curve-fitting analysis using a commercially available data-processing program (OriginPro, OriginLab Corporation). The assignment of the bands in the IR spectra was made according to the literature.36,41,43−46 The number of bands and peak positions were obtained by examining second derivatives of the spectral data. Gaussian and Lorentzian functions were used as mathematical functions for band shapes at aliphatic hydrogen and carbonyl stretching regions.36,45 An example of the curve-fitted spectra for 300 °C superheated steam-dried coal is shown in Figure 2. The coefficient of determination (R2) was the primary criterion in examining the goodness of curve fitting. R2 values of the curve-fitting analysis in all cases of steam, microwave, and air drying and in both aliphatic C−H stretching (3000− 2800 cm−1) and carbonyl adsorption (1850−1500 cm−1) zones were in the range of 0.994−0.999, indicating that IR spectra of all samples were curve-fitted with high accuracy.
Figure 4. Changes of aliphatic structures for steam-dried samples as a function of the temperature: (a) raw coal, (b) 150 °C, (c) 200 °C, (d) 250 °C, and (e) 300 °C.
(near 2865 and 2850 cm−1, respectively), and methane (C−H) stretching (near 2897 cm−1) as a function of the drying temperature in superheated steam is shown in Figure 4. As seen, the absorption of aliphatic C−H stretching decreased slightly with an increasing drying temperature and the decrease is more significant at 300 °C. Shui et al.47 also reported the decrease in aliphatic groups of steam-treated coal compared to those of raw coal, suggesting that steam treatment can break some weak aliphatic C−H bands, resulting in the decrease of the volatile yield of the steam-treated coal. The 3000−2800 cm−1 zone was curve-fitted to the above-mentioned five bands, and the 1850− 1500 cm−1 zone was curve-fitted to a series of seven bands corresponding to carboxyl groups and quinines (1770−1650 cm−1), aromatic carbon (around 1615 cm−1), and carboxylate and aromatic ring stretch groups (1560−1490 cm−1). From the curve-fitted bands in these two zones, a series of parameters were defined as ratios of deconvoluted peak areas in these zones to
3. RESULTS AND DISCUSSION 3.1. Superheated Steam Fluidized-Bed Drying. Figure 3 shows the IR spectra of as-received and steam-dried samples at 150, 200, 250, and 300 °C with original particle sizes of 0−1 mm. Changes in absorption of functional groups in the regions of 3000−2800 cm−1 (aliphatic C−H stretching) and 1850− 1500 cm−1 (carbonyl adsorption) were investigated. Adsorption of five bands in the aliphatic hydrogen absorption region attributed to asymmetric methyl (−CH3) and methylene (−CH2−) stretching (near 2955 and 2922 cm−1, respectively), symmetric methyl (−CH3) and methylene (−CH2−) stretching 3653
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drying temperature under superheated steam conditions. The methyl/methylene absorption ratio was considered as an estimation of the length of aliphatic chains of coal.45,48 This ratio increased with an increasing drying temperature to 250 °C, suggesting the preferential decomposition of methylene aliphatic groups.49 This was suggested to be due to the loss of alkyl chains and conversion of the hydroaromatic methylene structure to aromatic rings.50,51 At temperatures higher than 250 °C because of the very high drying intensity, methyl groups also started to convert to aromatic structures, which decreased the CH3/CH2 ratio. The aromatic carbon (Car)/aliphatic hydrogen content (Hal) ratio provided a measurement of the aromatic structure evolution during drying. This ratio increased progressively with an increasing drying temperature, indicating the evolution of aromatic structures at the expense of aliphatic hydrogen decomposition. Similar results have been reported by Ohki et al.49 This ratio increased significantly at temperatures higher than 250 °C because of the higher drying intensity. The ratio of aromatic carbon/carboxylic groups as defined in eq 3 was reported as a suitable index to investigate the maturation of organic matter because two mechanisms of aromatization and oxygen removal are combined in this ratio.50,52,53 This ratio increased progressively with the drying temperature, indicating the combined effect of drying on both the aromatization of the coal organic structure and oxygen removal. Figures 5−8 show the evolution of carboxyl (COOH), carbonyl (CO), carboxylate (COO−) and aromatic ring stretch, and aromatic carbon (CC) group absorption, respectively, as a function of the drying temperature. As seen, the absorption of carboxyl and carbonyl groups decreased drastically with the increasing drying temperature in superheated steam (Figures 5 and 6), indicating the progressive loss of oxygen functional groups
quantify the chemical changes in aliphatic hydrogen and oxygen functional groups. The defined parameters were as follows: 2955 cm−1 band/2922 cm−1 band
CH3/CH 2 : Car /Hal :
(1)
1600 cm−1 band/2965 − 2850 cm−1 zone (2)
Car /COOH + Car :
1600 cm−1 band/1710 cm−1 band
+ 1600 cm−1 band
CO/Car :
(3)
1750 − 1650 cm−1 zone/1610 cm−1 band (4)
COOH/Car :
1710 cm
−1
band/1600 cm
−1
band
(5)
Equations 1−5 define the ratios of methyl/methylene, aromatic carbon/aliphatic hydrogen, aromatic carbon/carboxylic group, carbonyl/aromatic, and carboxyl/aromatic, respectively. Table 2 summarizes the evolution of these parameters as a function of the Table 2. Structural Parameters Derived from Curve-Fitted Analysis of FTIR Spectra of Superheated Steam-Dried Coal Samples drying temperature (°C)
CH3/ CH2
Car/ Hal
Car/COOH + Car
CO /Car
COOH/ Car
raw coal 150 200 250 300
0.2 0.21 0.19 0.25 0.16
3.12 4.16 4.13 4.35 5
0.68 0.72 0.75 0.76 0.77
1.18 0.87 0.77 0.70 0.67
0.47 0.37 0.35 0.32 0.30
Figure 5. Changes of carboxyl (COOH) group (1710 cm−1) absorption as a function of the drying temperature in steam, microwave, and air. 3654
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Figure 6. Comparison of carbonyl (CO) group (1750−1650 cm−1) absorption as a function of the drying temperature in steam, microwave, and air.
with an increasing drying intensity,24,26,54 which decreased the moisture holding capacity25,27,54,55 and self-ignition tendency26 of lignite. Shui et al.47 also reported the decrease in carbonyl bands of steam-treated coals at 250 °C. The progressive decomposition of carboxyl groups is also consistent with results reported by Murakami et al.,56 who observed the monotonic decrease of these groups with the temperature. The carboxylate and aromatic ring stretch absorption remained relatively constant up to 250 °C but decreased significantly thereafter because of the very high drying temperature (Figure 7). A similar trend was observed in aromatic carbon (CC) absorption (Figure 8). It seemed that, at 300 °C, some pyrolysis reactions took place that altered the coal organic structure. A higher thermal stability of carboxylate groups compared to its acid form (carboxyl groups) has been reported by several researchers.56−58 The question may arise whether the reduction in absorption bands at 1600−1500 cm−1 was related to decomposition of carboxylate (COO−) groups or aromatic ring stretch. Because the aromatic carbon (CC) also started to decrease at temperatures higher than 250 °C, it was evident that the aromatic ring groups are less stable than the carboxylate groups and the decrease in the 1600−1500 cm−1 zone at 300 °C was related to decomposition of the aromatic ring stretch. This conclusion is consistent with the results reported by Murakami et al.,56 who observed that aromatic ring groups start to decompose drastically at 250 °C but carboxylate groups are stable up to 350 °C. Ozaki et al.57 and Schafer59 reported the decomposition of carboxylate groups at temperatures around 400 and 450 °C, respectively.
The ratios of carbonyl/aromatic (CO/Car) and carboxyl/ aromatic (COOH/Car), which were defined in eqs 4 and 5, respectively, indicated the progressive decrease in absorption of carbonyl and carboxyl functional groups and the increase in aromaticity of steam-treated lignite (Table 2). Decomposition of oxygen functional groups was reported to result in evolution of water, carbon dioxide, and carbon monoxide at higher drying temperatures24,60−62 From the above-mentioned results, it can be concluded that drying of low-rank coals in a superheated steam fluidized-bed dryer up to 300 °C increased the aromaticity of low-rank coal and decreased the oxygen-containing functional groups significantly but some aromatic carbon and aromatic ring stretches were decomposed at very high drying temperatures (above 250 °C). It seemed that 250 °C was the maximum and optimal drying temperature for steam drying, considering the fact that it caused a significant decrease in oxygen functionalities and the decomposition of the coal organic structure was not yet started at this temperature. 3.2. Microwave Drying. Microwave drying has some unique features that are absent in conventional heating. Some of these important features include very high drying rates compared to conventional heating, selective heating of moist areas, and an additional mechanism of moisture transport because of internal evaporation.21,63 It seems that selective heating of microwave has some unique effect on the coal chemical structure in a sense that organic structures of coal less likely undergo chemical changes because of a lower dielectric constant compared to moist areas. Dielectric constants of 78 and 1.8 are reported for liquid water and coal organic matter, respectively,64,65 indicating the fact that 3655
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Figure 7. Comparison of carboxylate (COO−) and aromatic ring stretch group (1600−1500 cm−1) absorption as a function of the drying temperature in steam, microwave, and air.
increase in aromaticity of coal during microwave drying. This increase was more significant at 700 W because of the conversion of both methylene and methyl structures to aromatic rings. A progressive decrease in aromatic carbon/carboxylic group, carbonyl/aromatic, and carboxyl/aromatic ratios indicated the decomposition of oxygen functional groups in low-rank coal under microwave drying conditions. The changes in absorption of carboxyl (COOH), carbonyl (CO), carboxylate (COO−) and aromatic ring stretch, and aromatic carbon (CC), which correspond to 1710, 1750− 1650, 1600−1500, and 1610 cm−1 bands, respectively, are shown in Figures 5−8. The drastic loss of carboxyl and carbonyl groups with an increasing drying intensity was obvious, and these results were consistent with curve-fitted analysis results, as discussed above. The removal of oxygen functional groups (decrease in the coal oxygen content) caused the increase in hydrophobicity and loss of the colloid structure of low-rank coals.24,66,67 Carboxylate groups and aromatic ring stretch increased slightly possibly because of the interaction of carboxylic acid with coal mineral matter or conversion of aliphatic hydrogen to aromatic structures. Aromatic carbon (CC) absorption remained relatively unchanged under microwave drying conditions even at the maximum output power level of 700 W. This observation indicated the importance of the selective heating feature of microwave radiation compared to conventional drying methods. The organic structure of coal underwent minimum changes under microwave radiation (because of a low dielectric constant), but water was removed and oxygen functionalities were
water molecules will absorb microwave significantly more than coal organic matter. Figure 9 shows the IR spectra obtained from raw coal and microwave-dried samples at 380, 540, and 700 W microwave output powers. The maximum temperature of 170 °C was recorded during drying of lignite samples at 700 W microwave power,21 and the temperatures corresponding to 380 and 540 W obtained from linear regression were 101 and 135 °C, respectively. Similar to steam-dried samples, the aliphatic hydrogen (3000−2800 cm−1) and carbonyl (1850−1500 cm−1) adsorption zones were curve-fitted to 5 and 7 bands, respectively. Absorption of five bands in the aliphatic hydrogen region (3000−2800 cm−1) as a function of the microwave output power is shown in Figure 10. Absorption of these bands remained relatively unchanged up to 540 W and decreased thereafter. The main decrease was observed in asymmetric and symmetric methylene (2923 and 2851 cm−1, respectively) and asymmetric methyl (2955 cm−1) at 700 W. The evolution of parameters defined in eqs 1−5 as a function of the microwave power is presented in Table 3. The methyl/methylene ratio increased slightly up to 540 W and decreased at 700 W. This observation indicated that methylene groups were lost in small amounts at lower drying intensities because of conversion of hydroaromatic methylene structures to aromatic rings.50,51 At 700 W, methyl groups decomposed and converted to aromatic rings as a result of more intensive drying conditions. A progressive increase in the aromatic carbon (Car)/aliphatic hydrogen content (Hal) ratio with an increasing microwave output power indicated the 3656
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Figure 8. Comparison of aromatic carbon (CC) group (1610 cm−1) absorption as a function of the drying temperature in steam, microwave, and air.
Figure 10. Changes of aliphatic structures for microwave-dried samples as a function of the microwave output power: (a) raw coal, (b) 380 W, (c) 540 W, and (d) 700 W.
Figure 9. FTIR spectra of as-received coal and microwave-dried samples at 380, 540, and 700 W.
decomposed selectively. The maximum temperature of 170 °C at 700 W was not high enough to cause the significant changes in the organic structure of coal; therefore, it can be concluded that microwave drying is a very promising technology in a sense that its energy is used to selectively remove water and oxygen functionalities from the coal structure, while the coal organic
structure remains relatively unchanged under microwave radiation. 3.3. Air Drying. IR spectra of raw and air-dried coal at 150, 200, and 250 °C were curve-fitted to five and seven bands at aliphatic hydrogen (3000−2800 cm−1) and carbonyl adsorption 3657
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groups showed a slight increase in absorption up to 150 °C,72 while carboxylate group absorption decreased slightly in this temperature range. A further increase of the drying temperature to 200 °C resulted in a drastic increase in absorption of all of these three groups. The aromatic carbon band showed negligible changes during drying at different temperatures. This result confirmed the abovementioned observation that oxidation took place on aliphatic moieties (side chains and bridge bonds) and the aromatic nucleus remained stable. This observation was consistent with results reported in the literature.29,36,38,39,73 Because the aromatic carbon (Car) remained almost invariable during the oxidation process, the ratio of oxygen-containing functional groups to aromatic carbon provided a quantitative study on their evolution during oxidation.36,45 Curve-fitting analysis of the carbonyl adsorption region (Table 4) showed that the ratios of carboxyl/aromatic carbon (COOH/Car) and carbonyl/aromatic carbon (CO/Car) increased significantly up to 200 °C, showing the generation of oxygen functional groups, as shown in Figures 5 and 6. This study showed that carboxylic acids and carbonyl groups were preferentially formed during intensive oxidation at the expense of aliphatic structures. Carboxyl and carbonyl groups (including aldehyde, ketone, and acid) arose from oxidization of methylene bridges in aromatic units.36,45,48,71 Absorption of carboxylate groups showed similar trends to those of carboxyl and carbonyl groups. Carboxylates could be formed from the interaction between carboxylic acid and coal mineral matter.45 Increasing the drying temperature to 250 °C resulted in significant decomposition of oxygen functionalities (Figures 5 and 6). It is believed that some combustion processes took place at this temperature. These results were consistent with those by Saikia et al.,74 who reported the loss of O−H and CO groups during thermal treatment of Indian coals at 250 °C in air. Decomposition of carboxyl groups to carbon dioxide and carbonyl groups to carbon monoxide at high temperatures has been reported in the literature.24,75−78 Carboxyl groups can easily dehydrate to generate anhydride and decarboxylate, leading to the formation of carbon dioxide.29,79 These reactions at higher temperatures under air drying conditions resulted in a decrease in coal volatile matter and an increase in the ash content.45 It can be concluded that air drying is suitable for low-rank coal drying only up to 150 °C. Drying in air at 200 °C resulted in a significant increase in oxygen functional groups, which is not favorable for downstream applications. The treatment of low-rank coals in air at 250 °C leads to significant decomposition of the aliphatic hydrogen structure of coal.
Table 3. Structural Parameters Derived from Curve-Fitted Analysis of FTIR Spectra of the Microwave-Dried Coal Samples microwave power (W)
CH3/ CH2
Car/ Hal
raw coal 380 540 700
0.2 0.23 0.26 0.24
3.23 3.28 3.36 4.23
Car/COOH + Car CO/Car COOH/Car 0.68 0.74 0.82 0.85
1.18 0.82 0.55 0.36
0.47 0.35 0.22 0.18
Figure 11. Changes of aliphatic structures for raw coal and air-dried sample at 250 °C.
Table 4. FTIR Analysis on Oxidation Parameters as a Function of the Temperature in Hot Air Drying drying temperature (°C)
CH3/ CH2
Car/ COOH + Car
CO /Car
COOH/ Car
raw coal 150 200 250
0.18 0.28 0.47 0.73
0.74 0.67 0.61 0.73
0.65 0.69 1.15 0.44
0.3 0.49 0.76 0.41
(1850−1500 cm−1) zones, respectively, as described in previous sections. Absorption of five bands in the aliphatic hydrogen region as a function of the drying temperature for raw coal and dried sample at 250 °C is shown in Figure 11. As seen, a drastic decrease in aliphatic structures took place when coal was dried in air at 250 °C. This observation implied that oxygen in drying air mainly reacted with aliphatic sites in the coal structure.36,68−70 Table 4 shows the evolution of parameters defined in eqs 1−5 as a function of the drying temperature in hot air. The drastic increase of the methyl/methylene ratio showed a significant disappearance of methylene groups as a result of oxidation. Oxygen absorption on methylene groups has been reported in the literature as the main mechanism of coal oxidation.41,45,71 Oxidation of aliphatic parts in the organic structure of coals resulted in the formation of oxygen functional groups.36,45 The evolution of carboxyl (COOH), carbonyl (CO), carboxylate (COO−) and aromatic ring stretch, and aromatic carbon (CC) corresponding to 1710, 1750−1650, 1600−1500, and 1610 cm−1, respectively, is shown in Figures 5−8. The evolution of these groups showed different trends compared to those of superheated steam- and microwave-dried samples. Carboxyl and carbonyl
4. CONCLUSION Superheated steam fluidized-bed drying resulted in a slight decrease in aliphatic hydrogen at high temperatures, suggesting that superheated steam treatment causes the breakage of weak C−H structures. The absorption of carboxyl (COOH) and carbonyl (CO) groups decreased with an increasing drying temperature, indicative of the progressive loss of oxygen functional groups with an increasing drying intensity. Aromatic ring stretch and aromatic carbon remained relatively unchanged up to 250 °C and decreased significantly thereafter. Carboxylate groups seemed to be persistent up to about 400 °C. Steam drying up to 300 °C progressively increased the aromaticity of coal. It is therefore suggested that the optimum drying temperature for lignite in superheated steam fluidized-bed drying was 250 °C, under which a significant decrease in oxygen function groups takes place but the organic structure of coal undergoes minimum 3658
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changes. Drying temperatures higher that 250 °C significantly modified the organic structure of coal. Aromatic carbon remained relatively unchanged during microwave drying even at the maximum output power of 700 W, while the oxygen functionalities (carboxyl and carbonyl) decreased progressively with increasing drying intensity. The loss of oxygen functional groups resulted in a progressive increase in aromaticity of coal with increasing microwave power level. The absorption of the aliphatic hydrogen content decreased significantly when coal was heated in air at 250 °C. The aromatic carbon remained unchanged during air drying, suggesting that oxygen in air reacted mainly with the aliphatic structure to produce oxygen functional groups. A progressive decrease in the CH3/CH2 ratio indicated that methylene structures were the most active sites on the coal structure to react with oxygen. Carboxyl and carbonyl groups increased significantly with an increasing drying temperature from 150 to 200 °C. Above this temperature, oxidation took place and oxygen functional groups decomposed significantly to produce water, CO, and CO2. The optimum drying temperature of lignite in air was 150 °C, above which oxygen functional groups and aliphatic structures undergo significant changes.
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AUTHOR INFORMATION
Corresponding Author
*Fax: +86-412-5929105. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (21176109 and 20976106). The authors gratefully acknowledge the financial support of the International Ph.D. Research Scholarship (2009) from the Liaoning Provincial Government of China and the financial support of the Australia− China Joint Coordination Group on Clean Coal Technology Research and Development Grants of the Australian Government.
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