Chemical Structure Changes Accompanying Fluidized-Bed Drying of

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Chemical Structure Changes Accompanying Fluidized-Bed Drying of Victorian Brown Coals in Superheated Steam, Nitrogen, and Hot Air Arash Tahmasebi,† Jianglong Yu,*,†,‡ and Sankar Bhattacharya§ †

Key Laboratory of Advanced Coal and Coking Technology of Liaoning Province, School of Chemical Engineering, University of Science and Technology Liaoning, No. 185 Qianshan Zhong Road, Anshan (114051), People's Republic of China ‡ Thermal Energy Research Centre, Shenyang Aerospace University, No. 37 Daoyi Nan Avenue, Shenyang (110136), People's Republic of China § Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia ABSTRACT: Chemical structure changes during drying of Victorian brown coals, that is, Loy Yang (LY) and Yallourn (YL), in hot air, nitrogen, and superheated steam are quantitatively studied using the FTIR technique. The infrared (IR) spectra of raw and dried coals were curve-fitted to a series of bands in hydroxyl groups (3500−3000 cm−1), aliphatic hydrogen (3000−2800 cm−1), and carbonyl and aromatic carbon (1850−1500 cm−1) adsorption regions. Following air drying, the IR adsorption of aliphatic structures decreased significantly, indicating that oxidation reaction mainly takes place on these structures. Carbonyl and carboxyl groups decreased up to 130 °C by 25.9% and 23.9%, respectively, and then significantly increased at higher temperatures due to oxidation. Drying of brown coals in nitrogen resulted in a significant increase in their aromaticity and a lower concentration of oxygen-containing functional groups. The loss of oxygen was confirmed by measuring the O/C ratio of raw and dried samples. The O/C ratio decreased by 30.8% and 40.7% for LY and YL coals, respectively, after drying at 200 °C for 10 min. Superheated steam fluidized-bed drying of both LY and YL coals showed the breakage of some weak aliphatic C−H structures. The decrease in adsorption of hydroxyl, carboxyl, and carbonyl groups leads to loss of oxygen in both LY and YL steam-dried coals. Superheated steam drying of brown coals showed only minor changes to the coal organic structure as the aromatic carbon content remained relatively unchanged and aliphatic structures decreased negligibly.

1. INTRODUCTION An estimated 45% of the world’s coal reserves consist of low-rank coals.1 Coal is and will continue to be an important energy source in the world in the foreseeable future. Utilization of low-rank coals is important due to the advantages of low mining cost, high reactivity, high amount of volatiles, and low amount of pollutionforming 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 utilization processes.4 Grinding, separation, and classification of high-moisture coals are difficult to handle. Low-rank coals also tend to spontaneously combust.1 The high moisture content of coals is a matter of concern in coalfired power plant efficiency.5,6 It is important to reduce their moisture content to increase the product quality.7,8 To achieve high thermal efficiency and reduce greenhouse gas emissions, drying will have to be an integral part of future advanced coal conversion technologies using the low-rank coals of high moisture content. Among various coal-drying methods, superheated steam drying is a very promising technology.7,9−15 One of the obvious advantages of superheated steam fluidizedbed drying (SSFD) is that the dryer exhaust is also steam. In air drying, the latent heat in the exhausted steam is generally difficult and expensive to recover. It is possible to recover all of the latent heat supplied in the SSFD drying from the exhaust by condensing the exhaust stream or by mechanical or thermo-compression to elevate its specific enthalpy for reuse in the dryer. If this steam is used elsewhere, the latent heat recovered is not charged to the SSFD, leading to a net energy consumption figure of the 1000−1500 kJ/kg of water removed for SSFD compared with © 2012 American Chemical Society

4000−6000 kJ/kg of water removed in a corresponding hot-air dryer.16 Other key advantages of SSFD are as follows:16 (1) A much lower chance of oxidation and, therefore, less possibility of explosion caused by possible combustion; (2) higher drying rates are possible in both constant and falling rate periods (falling rate period corresponds to the inherent water removal period), depending on the steam temperature; (3) better energy utilization can be achieved by condensing steam generated in the dryer; and (4) coal moisture is discharged as liquid water rather than as dusty vapor, and no biological or chemical treatment is needed for the condensate water. Low-rank coals are expected to undergo chemical changes during drying.17,18 Chemical changes in coal structure affect its reactivity in subsequent applications, such as combustion, gasification, pyrolysis, and liquefaction. The main changes in the chemical structure of coal are the formation and/or destruction of oxygencontaining functional groups (including carboxyl, hydroxyl, and carbonyl).17−20 Elimination of polar groups will cause a decrease in the oxygen content and moisture-holding capacity of the coal.21−23 Oxygen is removed in the form of water, carbon dioxide, and carbon monoxide.23 Using hot air as the drying gas has the potential to oxidize the organic structure of coal. Oxidation affects technological properties of coal in a sense that softening and swelling properties, heat of combustion, calorific values, and coking and caking Received: October 6, 2012 Revised: November 16, 2012 Published: November 19, 2012 154

dx.doi.org/10.1021/ef3016443 | Energy Fuels 2013, 27, 154−166

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A particle size of 1.18−2.0 mm was used for all drying experiments for both LY and YL coal samples. Seven grams of samples were dried for 10 min before they were subjected to FTIR and ultimate analysis. 2.2. FTIR Spectroscopy. Infrared spectra of raw coal and dried samples at different atmospheres were obtained with a PerkinElmer 100 FTIR spectrometer. KBr pellets were prepared by grinding around 2.5 mg of coal with 200 mg of KBr. Prior to FTIR measurements, a reference spectrum was obtained from pure KBr pellets without addition of any coal. For each raw and dried sample, three pellets were prepared, and the results reported in this study are the average of the three measurements. Infrared spectra of the brown coal samples 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 infrared spectra was made according to the literature.32,37,39−42 Initial approximation of 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 hydroxyl, aliphatic hydrogen, and carbonyl stretching regions.32,41,43 The initial set of peak parameters was left optimized until convergence of the data was achieved. An example of curve-fitted spectra for 170 °C superheated steam-dried Loy Yang coal is shown in Figure 2, and the parameters are summarized in Table 2. 2.3. Ultimate Analysis. To confirm the FTIR results on the evolution of oxygen functional groups, the ultimate analysis of LY and YL coal samples was carried out using a PerkinElmer Series II 2400 CHNS/O analyzer that operated at a combustion temperature of 925 °C and a reduction temperature of 500 °C. From the proximate analysis data, the oxygen-to-carbon ratio (O/C) was calculated.

characteristics (in the case of bituminous coals) decrease significantly.24−27 The main effects of oxygen on coal structure are an increase in oxygen functionalities with a simultaneous decrease in aliphatic hydrogen content.28,29 Peroxides and hydroperoxides and hydroxyl species form as a result of reaction between molecular oxygen and aliphatic groups.30,31 Peroxides and hydroperoxides start to oxidize to hydroxyl, carbonyl (ketones), and carboxyl groups at low temperatures.25,30,32,33 Formation of ether linkages is the dominant oxidation mechanism.31 Ketonic carbonyl and carboxyl groups decompose at higher temperatures to produce lower molecular weight structures, such as carbon dioxide.34,35 FTIR (Fourier transform infrared) is a useful method for determining the chemical changes during drying and oxidation of coal.31−33,36−38 Superheated steam fluidized-bed drying is a very promising technology in upgrading low-rank coals, such as Victorian brown coals.1 Hot air is also widely used in drying of various materials, including coal. Many researchers have studied the low-temperature oxidation that causes spontaneous combustion of coal, but little is reported in the literature on the air oxidation during hot air drying of coal. Also, very little is reported on the effect of superheated steam fluidized-bed drying on the chemical structure of Victorian brown coals. In our previous studies,17,18 we reported the changes in the chemical structure of Chinese lignite during drying. In this study, the chemical structure changes of two Victorian brown coals during drying in a laboratory-scale fluidized-bed dryer at temperatures between 100 and 200 °C using superheated steam, hot air, and nitrogen as heat carrier and fluidizing gas were investigated. The objective of this study was to study the changes in aliphatic structure and oxygen-containing functional groups of Victorian brown coals under different drying conditions and to provide implications to the design of drying processes.

3. RESULTS AND DISCUSSION 3.1. Air Fluidized-Bed Drying. Figure 3 shows the IR spectra of as-received and air-dried LY and YL coals at 100, 130, and 170 °C. Remarkable changes occur in the regions of 3500− 3000 cm−1 (hydroxyl structures), 3000−2800 cm−1 (aliphatic C−H stretching), and 1850−1500 cm−1 (carbonyl adsorption region), as can be seen in Figure 3. Adsorptions of two major bands in 3500−3000 cm−1 attributed to hydrogen-bonded water (around 3425 cm−1) and phenolic and carboxylic structures (around 3230 cm−1) were calculated by curve-fitting analysis, as described above, and the results are shown in Figures 4 and 5 for LY and YL coals, respectively. As can be seen, the adsorption of hydrogen-bonded water at 3425 cm−1 decreased significantly with increasing drying temperature. However, the phenolic and carboxylic structures adsorption increased with increasing drying temperature, indicating that drying in air at high temperatures will result in oxidation of coal organic structures and formation of these groups.17,18 Investigation of the spectra obtained for both LY and YL coal samples at different drying temperatures revealed that the adsorption of the aliphatic hydrogen region (3000−2800 cm−1) decreased with increasing oxidation temperature, whereas the adsorption of the carbonyl stretching region (1850−1500 cm−1) decreased when the coal was dried up to 130 °C and increased thereafter. Adsorption of five bands attributed to asymmetric methyl (−CH3) and methylene (−CH2−) stretching (near 2964 and 2922 cm−1, respectively), symmetric methyl (−CH3) and methylene (−CH2−) stretching (near 2870 and 2850 cm−1, respectively), and methane (C−H) stretching (near 2898 cm−1) as a function of drying temperature in air is shown in Figure 6. It can be seen that adsorption decreased monotonically up to 170 °C. This observation implies that oxygen in drying air mainly reacts with aliphatic sites in the coal structure.32,44−46 The 3000−2800 cm−1 zone was curve-fitted to the abovementioned five bands, and the 1850−1500 cm−1 zone to a series

2. EXPERIMENTAL SECTION 2.1. Fluidized-Bed Drying Experiments. Two Victorian brown coal samples, Loy Yang (LY) and Yallourn (YL), were used in the drying experiments, and their proximate and ultimate analysis data are presented in Table 1. An experimental setup designed and manufactured at

Table 1. Proximate and Ultimate Analyses of LY and YL Brown Coal Samples Used in This Study

a

coal samplea

LY

YL

moisture content, wt % (ar) ash, wt % (db) volatile matter, wt % (db) C, wt % (db) H, wt % (db) N, wt % (db) S, wt % (db) O (by diff.), wt % (db)

62.3 1.5 49.0 68.26 4.70 1.82 0.25 23.47

55.9 2.2 48.38 65.91 4.40 0.79 0.74 28.16

ar = as-received basis; db = dry basis.

the Chemical Engineering Department of Monash University (Figure 1) was used, comprising a drying reactor, an electrically heated furnace, a steam generator, a gas preheater, and an electronic balance. The inner diameter and the length of the quartz drying reactor were 40 and 600 mm, respectively. A sintered plate with a 100 μm pore size was used as the gas distributer. Hot air, superheated steam, and nitrogen were used as heat carriers and fluidizing gases. Drying temperatures used in drying experiments were 100, 130, 170, and 200 °C. A gas flow rate of 800 mL/min was used in air and nitrogen drying and a steam flow rate of 6 × 10−4 m3/s (0.482 m/s superficial velocity) in superheated steam drying experiments. 155


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Figure 1. Schematic diagram of the experimental setup: (1) steam generator, (2) gas flow meter, (3) steam valve, (4) gas preheater, (5) electric furnace, (6) drying reactor, (7) furnace temperature controller, (8) electric balance, (9) computer.

Figure 2. Curve-fitted spectra of the 1850−1500 cm−1 zone of IR spectra for steam-dried LY coal at 170 °C.

Table 2. Curve-Fitting for the 1850−1500 cm−1 Zone of the FTIR Spectra of Superheated Steam-Dried LY Coal at 170 °C

a

center (cm−1)

assignment

width (cm−1)

heighta

areab

1772 1738 1705 1652 1611 1561 1488

esters, aliphatic COOH esters, aliphatic COOH aromatic COOH highly conjugated CO aromatic CC COO− aromatic ring COO− aromatic ring

44.99 63.28 79.97 101.69 107.81 124.93 100.5

20.1 31.33 41.91 35.6 35.53 47.85 53.38

1133.78 2085.29 3200.83 3537.62 5891.85 7493.00 6728.12

Absorbance units. bIntegrated absorbance unit × cm−1.

parameters were defined as ratios of deconvoluted peak areas in these zones to quantify the chemical changes in aliphatic hydrogen and oxygen functional groups. The defined parameters are presented in Table 3.

of seven bands corresponding to carboxyl groups and quinines (1770−1650 cm−1), aromatic carbon (around 1610 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 156


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Figure 3. IR spectra of as-received and air-dried coal samples at 100, 130, and 170 °C: (a) LY coal, (b) YL coal.

Figure 4. Evolution of hydrogen-bonded water (3425 cm−1) and phenolic and carboxylic structures (3230 cm−1) during drying of LY coal in hot air, nitrogen, and superheated steam.

Oxygen adsorption on methylene groups is reported in the literature as the main mechanism of coal oxidation.37,41,48 Oxygen adsorption occurs initially on methylene groups, especially α-CH2 groups to aromatic rings. Oxidation of aliphatic parts in the organic structure of coals results in the formation of oxygen functional groups.32,41 The evolution of carboxyl (COOH), carbonyl (CO), carboxylate (COO−) and aromatic ring stretch, and aromatic carbon (CC), corresponding to 1710, 1750− 1650, 1600−1500, and 1610 cm−1, respectively, for LY and YL

These parameters define the ratios of methyl/methylene, aromatic carbon/aliphatic hydrogen, aromatic carbon/carboxylic groups, carbonyl/aromatic, and carboxyl/aromatic. Table 4 shows the evolution of parameters defined in Table 3 as a function of drying temperature in hot air for LY and YL coals. The methyl/ methylene adsorption ratio can be considered as an estimation of the length of aliphatic chains of coal.41,47 Figure 6 and the CH3/ CH2 ratio in Table 4 suggest the significant disappearance of −CH2− groups (2922 cm−1) with increasing oxidation intensity. 157


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Figure 5. Evolution of hydrogen-bonded water (3425 cm−1) and phenolic and carboxylic structures (3230 cm−1) during drying of YL coal in hot air, nitrogen, and superheated steam.

Figure 6. Evolution of aliphatic structures for air-dried coal samples as a function of drying temperature: (a) LY coal, (b) YL coal.

adsorption decreased slightly in this temperature range. A further increase of drying temperature to 170 °C resulted in a drastic increase in adsorption of all of these three groups as a result of coal oxidation.18,49 These findings were confirmed by the O/C ratio of raw and dried coal samples that are presented in Table 4. Similar to oxygen functional groups, this ratio decreased when both coals were heated to 130 °C and increased at higher temperatures. Compared to the data in our previous study,18 which investigated the oxidation of a Chinese low-rank coal (Shenhua No. 6) at temperatures above 150 °C, Victorian brown coals showed a higher reactivity during oxidation. The ratio of oxygen-containing

Table 3. Defined Parameters from Curve-Fitting Analysis in Aliphatic Hydrogen and Oxygen Functional Group Regions defined parameter CH3/CH2 Car/Hal Car/COOH + Car CO/Car COOH/Car

adsorption zone −1

2955 cm band/2922 cm−1 band 1600 cm−1 band/2965−2850 cm−1 zone 1600 cm−1 band/1710 cm−1 band + 1600 cm−1 band 1750−1650 cm−1 zone/1610 cm−1 band 1710 cm−1 band/1600 cm−1 band

coals is shown in Figures 7 and 8. Carbonyl and carboxyl decreased when coal was heated up to 130 °C. Carboxylate group 158


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Table 4. Structural Parameters Derived from Curve-Fitting Analysis of FTIR Spectra of the Air-Dried LY and YL Coals coal type

drying temp °C

CH3/CH2

Car/Hal

Car/COOH + Car

CO/Car

COOH/Car

O/C

LY

raw coal 100 130 170 raw coal 100 130 170

0.105 0.305 0.440 0.543 0.131 0.150 0.252 0.392

4.16 6.29 7.88 10.17 3.82 4.70 6.79 9.49

0.597 0.657 0.741 0.620 0.562 0.608 0.632 0.510

2.96 1.53 1.13 1.97 2.49 1.65 1.51 2.79

0.772 0.521 0.348 0.611 0.777 0.644 0.581 0.960

0.344 0.331 0.293 0.349 0.427 0.393 0.333 0.369

YL

Figure 7. Evolution of carboxyl (COOH), carbonyl (CO), carboxylate (COO−) and aromatic ring stretch, and aromatic carbon (CC) adsorption with increasing drying temperature during air drying of LY coal.

decreased up to 130 °C. However, these ratios increased significantly when coal was heated at temperatures above 170 °C in air, showing the generation of oxygen functional groups (Figures 7 and 8). Drying of LY and YL coal in air also results in the formation of phenolic and carboxylic structures at the expense of aliphatic hydrogen structures. Carboxyl and carbonyl groups (including aldehyde, ketone, and acid) form as a result of oxidization of methylene bridges in aromatic units.32,41,47,48 Adsorption of carboxylate groups showed similar trends to those for carboxyl and carbonyl groups. Carboxylates may form from the interaction between carboxylic acid and coal mineral matter.41 The aromatic carbon (Car)-to-aliphatic hydrogen content (Hal) ratio provides a measurement of aromatic structure evolution during drying. This ratio increased with drying temperature for both LY and YL coals, but the increase for YL coal is more significant than that for LY coal sample. This shows the higher reactivity of YL coal with oxygen in hot air compared with LY coal. It can be concluded that drying of LY and YL coal up to 130 °C will result in a decrease in oxygen functional groups and the changes in the organic structure of coal are in an acceptable

functional groups to aromatic carbon (Car) provides a quantitative study on their evolution during oxidation, considering the fact that aromatic carbon remained relatively invariable during the hot air drying process (Figures 7 and 8).32,41 The ratios of carbonyl/aromatic carbon (CO/Car) and carboxyl/aromatic carbon (COOH/Car) in LY and YL coals were significantly higher than those of Chinese Shenhua No. 6 coal, as calculated in our previous paper.18 The CO/Car was 2.96 and 2.49 for LY and YL coals, respectively, while this ratio for Chinese Shenhua No. 6 coal was 1.18. The COOH/Car ratio was 0.772, 0.777, and 0.47 for LY, YL, and Shenhua No. 6 coal, respectively (Table 4).18 This showed that Victorian brown coals have considerably higher oxygen functional groups compared with Chinese lignite. An insignificant change in aromatic carbon content confirmed the above-mentioned observation that oxidation takes place on aliphatic moieties (side chains and bridge bonds), and the aromatic nucleus remains stable. Similar results have been reported in the literature.25,32,34,35,50 It can be seen from Table 4 that the ratios of carboxyl/aromatic carbon (COOH/Car) and carbonyl/aromatic carbon (CO/Car) 159


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Figure 8. Evolution of carboxyl (COOH), carbonyl (CO), carboxylate (COO−) and aromatic ring stretch, and aromatic carbon (CC) adsorption with increasing drying temperature during air drying of YL coal.

Figure 9. Evolution of aliphatic structures for nitrogen-dried samples as a function of drying temperature: (a) LY coal, (b) YL coal.

in Figures 4 and 5, respectively. Adsorption of these groups decreases with increasing temperature, and this decrease was more significant at temperatures higher than 130 °C for both LY and YL coals. Similar to air-dried samples, the aliphatic hydrogen (3000−2800 cm−1) and carbonyl (1850−1500 cm−1) adsorption zones were curve-fitted to five and seven bands, respectively. Adsorption of five bands in the aliphatic hydrogen region (3000−2800 cm−1) as a function of temperature for LY and YL coals is shown in Figure 9. Adsorption of these bands decreased with increasing the drying temperature, which leads to a decrease in aliphaticity of coals after drying. The main decrease was observed in asymmetric and symmetric methylene

range, but drying of these coals at temperatures higher than 130 °C will result in oxidation and significant changes in the coal organic structure. 3.2. Nitrogen Fluidized-Bed Drying. Changes in adsorption of functional groups in the regions of 3500−3000 cm−1 (hydroxyl structures), 3000−2800 cm−1 (aliphatic C−H stretching), and 1850−1500 cm−1 (carbonyl adsorption region) of nitrogen-dried coals were investigated by curve-fitting analysis. The evolution of adsorption of bands in the 3500−3000 cm−1 zone attributed to hydrogen-bonded water (around 3425 cm−1) and phenolic and carboxylic structures (around 3230 cm−1) during nitrogen fluidized-bed drying of LY and YL coals is shown 160


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Table 5. Structural Parameters Derived from Curve-Fitting Analysis of FTIR Spectra of the Nitrogen-Dried LY and YL Coals coal type

drying temp °C

CH3/CH2

Car/Hal

Car/COOH + Car

CO/Car

COOH/Car

O/C

LY

raw coal 100 130 170 200 raw coal 100 130 170 200

0.105 0.168 0.156 0.129 0.120 0.131 0.258 0.222 0.157 0.154

4.16 4.39 4.72 6.48 8.53 3.82 6.62 8.09 9.46 10.66

0.597 0.707 0.816 0.867 0.879 0.562 0.597 0.765 0.785 0.808

2.96 0.824 0.628 0.468 0.409 2.49 1.11 0.84 0.66 0.58

0.772 0.238 0.193 0.14 0.128 0.777 0.461 0.306 0.241 0.237

0.344 0.307 0.282 0.241 0.228 0.427 0.348 0.297 0.268 0.253

YL

Figure 10. Evolution of carboxyl (COOH), carbonyl (CO), carboxylate (COO−) and aromatic ring stretch, and aromatic carbon (CC) adsorption with increasing drying temperature during nitrogen drying of LY coal.

(2922 and 2850 cm−1, respectively) and asymmetric methyl (2964 cm−1) at temperatures above 130 °C. The evolution of parameters defined in Table 3 as a function of drying temperature in nitrogen is given in Table 5. The study of the evolution of aliphatic bands with increasing drying temperature (CH3/CH2 ratio, Table 5) indicated that methylene groups were preferably lost in LY and YL coals up to 100 °C. This was probably due to a progressive loss of alkyl chains and the conversion of the hydroaromatic methylene structure to aromatic rings.43,51 This ratio decreased above 100 °C, indicating the conversion of methyl structures to aromatic rings at more intensive drying conditions. A similar trend was observed during drying of Chinese Shenhua No. 6 lignite.18 The aromatic carbon (Car)-to-aliphatic hydrogen content (Hal) ratio increased monotonically with drying temperature, indicating the evolution of the aromatic structure and decrease in aliphaticity of coal during drying. Figures 10 and 11 show the evolution of carboxyl, carbonyl, carboxylate, and

aromatic ring structures and aromatic CC groups as a function of drying temperature in a nitrogen environment for LY and YL coals, respectively. The drastic loss of carboxyl and carbonyl groups with increasing drying intensity is obvious in both coals. These results were confirmed by calculation of the O/C ratio (Table 5) derived from ultimate analysis of raw and dried coals. This ratio decreased continuously with increasing drying temperature, indicating the removal of oxygen as a result of drying. Oxygen was removed in the form of water and CO2. Removal of oxygen functional groups (decrease in coal oxygen content) causes the increase in hydrophobicity and loss of the colloid structure of low-rank coals.19,52,53 Carboxylate groups and aromatic ring stretch increased significantly, possibly due to interaction of carboxylic acid with coal mineral matter or conversion of aliphatic hydrogen to aromatic structures. The aromatic carbon (CC) adsorption remained relatively constant during drying, indicating that these groups are persistent at temperatures up to 200 °C. 161


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Figure 11. Evolution of carboxyl (COOH), carbonyl (CO), carboxylate (COO−) and aromatic ring stretch, and aromatic carbon (CC) adsorption with increasing drying temperature during nitrogen drying of YL coal.

Figure 12. IR spectra of as-received and superheated steam-dried coals at 130, 170, and 200 °C: (a) LY coal, (b) YL coal.

LY and YL coals, indicating the combined effect of drying on both the aromatization of the coal organic structure and oxygen removal. Decomposition of oxygen functional groups resulted in evolution of water, carbon dioxide, and carbon monoxide.19,56−58 From the above results, it can be concluded that drying of brown coals in an inert atmosphere, such as nitrogen up to 200 °C, increases the aromaticity of brown coal and decreases the oxygen-containing functional groups significantly.

The ratios of carbonyl/aromatic (CO/Car) and carboxyl/ aromatic (COOH/Car) (defined in Table 5) decreased during drying, indicating the progressive loss of carbonyl and carboxyl groups, while the aromaticity of nitrogen-treated brown coal (Table 5) increased. The ratio of aromatic carbon to carboxylic groups (Car/COOH + Car) is a suitable index to investigate the maturation of organic matter of coal, as two mechanisms of aromatization and oxygen removal are combined in this ratio.43,54,55 This ratio increased progressively with drying temperature for both 162


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Figure 13. Evolution of aliphatic structures for superheated steam-dried samples as a function of drying temperature: (a) LY coal, (b) YL coal.

Table 6. Structural Parameters Derived from Curve-Fitting Analysis of IR Spectra of the Superheated Steam-Dried LY and YL Coals coal type

drying temp °C

CH3/CH2

Car/Hal

Car/COOH + Car

CO/Car

COOH/Car

O/C

LY

raw coal 130 170 200 raw coal 130 170 200

0.105 0.270 0.147 0.136 0.131 0.242 0.153 0.136

4.16 5.72 6.09 7.76 3.82 7.13 7.95 8.04

0.597 0.572 0.648 0.737 0.562 0.628 0.720 0.731

2.96 2.355 1.689 1.268 2.49 1.58 1.17 1.07

0.772 0.646 0.543 0.256 0.777 0.590 0.388 0.346

0.344 0.303 0.285 0.258 0.427 0.371 0.288 0.279

YL

1850−1500 cm−1 zone to a series of seven bands, corresponding to carboxyl groups and quinines (1770−1640 cm−1), aromatic carbon (around 1610 cm−1), and carboxylate and aromatic ring stretch groups (1570−1480 cm−1). The evolution of parameters defined in Table 3 as a function of drying temperature under superheated steam is presented in Table 6. The ratio of methyl/ methylene adsorption (CH3/CH2) increased when the coal was dried with steam at 130 °C due to loss of methylene aliphatic groups.60 This was suggested to be due to conversion of the hydroaromatic methylene structure to aromatic rings.43,51 The aromatic carbon (Car)-to-aliphatic hydrogen content (Hal) ratio increased monotonically with increasing drying intensity, indicating the increase in aromaticity of steam-dried samples at the expense of aliphatic hydrogen decomposition at higher drying temperatures. Similar results were reported by Ohki et al.60 The ratio of aromatic carbon to carboxylic groups (Car/COOH + Car) increased with increasing the drying temperature (Table 6). Figures 14 and 15 show the evolution of carboxyl (COOH), carbonyl (CO), carboxylate (COO−) and aromatic ring stretch, and aromatic carbon (CC) group adsorption as a function of drying temperature for LY and YL coal, respectively. As can be seen, the adsorption of carboxyl and carbonyl groups decreased drastically with increasing drying temperature in superheated steam for LY and YL coals, indicating the progressive loss of oxygen functional groups with increasing steam drying intensity,19,22,61 which decreases the moisture-holding capacity21,23,61,62 and self-ignition tendency22 of brown coal. Similar results were observed for Chinese Shenhua No. 6 lignite.18 These results were consistent with the oxygen-to-carbon ratio (O/C) derived from

3.3. Superheated Steam Fluidized-Bed Drying. Figure 12 shows the IR spectra of as-received and steam-dried LY and YL coals at 130, 170, 250, and 200 °C with original particle sizes of 1.18−2 mm. Similar to air- and nitrogen-dried samples, changes in adsorption of functional groups in the regions of 3500−3000 cm−1 (hydroxyl structures), 3000−1800 cm−1 (aliphatic C−H stretching), and 1850−1500 cm−1 (carbonyl adsorption) were investigated. The evolution of adsorption of bands as a function of drying temperature under superheated steam conditions in the 3500−3000 cm−1 zone attributed to hydrogen-bonded water (around 3425 cm−1) and phenolic and carboxylic structures (around 3230 cm−1) during superheated steam fluidized-bed drying of LY and YL coals is shown in Figures 4 and 5, respectively. It can be seen that superheated steam drying resulted in significant loss of hydrogen-bonded water and phenolic and carboxylic acids. Adsorption of five bands in the aliphatic hydrogen adsorption region attributed to asymmetric methyl (−CH3) and methylene (−CH2−) stretching (near 2958 and 2922 cm−1, respectively), symmetric methyl (−CH3) and methylene (−CH2−) stretching (near 2870 and 2850 cm−1, respectively), and methane (C−H) stretching (near 2896 cm−1) as a function of drying temperature in superheated steam is shown in Figure 13. The adsorption of aliphatic hydrogen structures decreased slightly with increasing steam drying temperature, and the decrease is more significant at 200 °C. These results suggest that steam drying of coal results in breakage of weak aliphatic C−H structures, which decreases the volatile yield of the steam-treated coal.59 The 3000−2800 cm−1 zone was curve-fitted to the above-mentioned five bands, and the 163


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Figure 14. Evolution of carboxyl (COOH), carbonyl (CO), carboxylate (COO−) and aromatic ring stretch, and aromatic carbon (CC) adsorption with drying temperature during superheated steam drying of LY coal.

Figure 15. Evolution of carboxyl (COOH), carbonyl (CO), carboxylate (COO−) and aromatic ring stretch, and aromatic carbon (CC) adsorption with drying temperature during superheated steam drying of YL coal.

increasing steam drying intensity. The decrease in carbonyl adsorption of steam-treated coals at 250 °C has also been

ultimate analysis of raw and dried samples (Table 6). Murakami et al.63 also reported the progressive loss of carboxyl groups with 164


Energy & Fuels reported by Shui et al.59 Aromatic carbon (CC) adsorption remained relatively unchanged during steam drying. Our previous study showed that aromatic carbon and aromatic ring stretches persist up to 250 °C during superheated steam drying and decompose thereafter due to pyrolysis.18 Drying of brown coal in a steam atmosphere also resulted in decomposition of carboxylate groups, as can be seen in Figures 14 and 15. Decomposition of oxygen functional groups results in evolution of water, carbon dioxide, and carbon monoxide at higher drying temperatures.19,56−58 It can be concluded that drying of Victorian brown coals in a superheated steam fluidized-bed dryer increases the aromaticity of brown coal and decreases the oxygen-containing functional groups significantly, whereas little changes in aliphatic structures of coal took place. This implies that a higher drying temperature can be applied to brown coals without significant changes brought to the coal organic structure in superheated steam drying.

ACKNOWLEDGMENTS

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REFERENCES

This study was supported by the Natural Science Foundation of China (21176109 and 20976106). The authors gratefully acknowledge the financial support of the Australia−China Joint Coordination Group on Clean Coal Technology Research & Development Grants of the Australian Government. David Stokie of Monash University was responsible for the design of the experimental rig. We also acknowledge the funding support by the International Ph.D. Research Scholarship (2009) from Liaoning Provincial Government of China, and the Liaoning Outstanding Professorship Funding (2011) of Liaoning Province.

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4. CONCLUSION The adsorption of aliphatic hydrogen structures decreased significantly when brown coals were dried in hot air. However, the aromatic carbon adsorption remained relatively unchanged, indicating that oxygen in air reacts mainly with the aliphatic structure to produce oxygen functional groups. The continuous increase in the CH3/CH2 ratio showed that methylene structures were the most active sites on coal to react with oxygen in the air. Carbonyl and carboxyl groups decreased by 25.9% and 23.9%, respectively, when the coal was dried in air up to 130 °C and increased significantly thereafter due to the oxidation of coal organic matter. When dried in nitrogen, the ratio of aromatic carbon (Car) to aliphatic hydrogen content (Hal) increased significantly up to 200 °C, indicating the evolution of the aromatic structure and decrease in aliphaticity of coal during drying. The adsorption of carbonyl, carboxyl, phenolic, and carboxylic structures decreased significantly during nitrogen drying, showing the decomposition of oxygen functional groups. These results were consistent with the measured oxygen-to-carbon ratio (O/C) of raw and dried coal samples. The O/C ratio decreased by 30.8% and 40.7% for LY and YL coals, respectively, after drying for 10 min at 200 °C. Drying under nitrogen conditions had a combined effect of aromatization and oxygen removal on both LY and YL coals. Superheated steam fluidized-bed drying of brown coals resulted in a slight decrease in aliphatic hydrogen adsorption, suggesting the breakage of weak C−H structures. The adsorption of carboxyl, carbonyl, phenolic, and carboxylic structures decreased drastically with increasing drying temperature, indicating the progressive loss of oxygen functional groups with increasing drying intensity under superheated steam conditions. The O/C ratio decreased monotonically as a function of drying temperature. Steam drying of both LY and YL coals resulted in negligible change in the aromaticity of their structure.

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Chemical Structure Changes Accompanying Fluidized-Bed Drying of

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