study of chemical structure changes of chinese lignite upon drying

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Study of Chemical Structure Changes of Chinese Lignite upon Dryingin 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 andTechnology Liaoning, Number 185 Qianshan Zhong Road, Anshan 114051, People s Republic of ChinaThermal Energy Research Centre, Shenyang Aerospace University, Number 37 Daoyi Nan Avenue, Shenyang 110136,Peoples Republic of ChinaDepartment 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 studiedin this paper using the Fourier transform infrared (FTIR) spectroscopy technique. The infrared (IR) spectra of raw and driedsamples were curve-tted to a series of bands in aliphatic hydrogen (30002800 cm1) and carbonyl absorption (18501500 cm1) zones. It has been found that aliphatic hydrogen absorbance decreased slightly with an increasing temperature duringsuperheated steam drying, while absorption of carboxyl (COOH) and carbonyl (CO) groups decreased drastically, indicativeof the loss of oxygen functionalities with an increasing drying temperature. During steam drying, aromatic carbon and aromaticring stretch absorption remained relatively unchanged up to 250 C and decreased signicantly thereafter because of somepyrolysis reactions that took place at higher drying temperatures. Microwave heating of lignite resulted in a signi cant decrease inthe concentration of oxygen-containing functional groups. Aromatic carbon remained relatively unchanged under microwavedrying conditions, while aliphatic hydrogen decreased slightly. The aromaticity of coal calculated from curve-tted spectra ofdeconvoluted peaks showed a progressive increase with an increasing drying intensity under both steam and microwave dryingconditions. Under air drying conditions, aliphatic hydrogen absorbance decreased drastically at 250 C, while aromatic carbonremained unchanged. It was observed that oxidation in air mainly took place on aliphatic hydrogen sites, especially on methylenegroups. Changes of carboxyl and carbonyl groups during air-dried samples showed a different trend compared to those dried insteam and microwave, increasing gradually up to 150 C and then a sharp increase at 200 C. The absorption of these groupsdecreased signicantly at an increased air temperature up to 250 C.

1. INTRODUCTION

An estimated 45% of the worlds coal reserves consist of low-rankcoals.1 Use of low-rank coals is important because of theadvantages of low mining cost, high reactivity, high amount of

volatiles, and low amount of pollution-forming impurities, suchas sulfur, nitrogen, and heavy metals.2,3 However, high moisturecontent (2560%)in low-rank coals exerts signicant effects intheir use processes.4 The high moisture content results in lowerefficiency, increased transportation cost, and higher CO2emission.5 Low-rank coals have difficulties in transportationand 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 technologieshave been developed to decrease the moisture content andincrease the caloric value of low-rank coals.913 Among thesetechnologies, superheated steam drying7,1419 and microwavedrying2023 are promising technologies compared to conven-tional hot air or hot ue gas drying.

Low-rank coals undergo chemical structure changes during thedrying process. Chemical changes in the coal structure areimportant regarding its behavior in downstream applications,such as combustion, gasication, pyrolysis, and liquefaction. Thechange in the chemical nature of the coal surface is reported to bepredominantly in the formation and/or destruction of theoxygen-containing polar functional groups (including carboxyl,

hydroxyl, and carbonyl).24 Elimination of polar groups will causea decreaseinthe oxygen content and moisture-holding capacityof the coal.2527 Oxygen is removed in the form of water, carbondioxide, and carbonmonoxide during decomposition of oxygenfunctional groups.27 Hydrophobicity of low-rank coals increasesand their self-ignition tendency decreases with the removal ofoxygen-containing functional groups.26,28

On the other hand, drying of coal in hot air will cause theoxidation of its organic structure. Oxidation affects coalproperties, such as softening and swelling properties, heat ofcombustion, caloric values, and cokingand caking character-istics, which decrease signicantly.2931 The main effects ofoxygen on the coal structure are the increase in oxygen

functionalities with a simultaneous decrease in the aliphatichydrogen content.32,33 Peroxides, hydroperoxides, and hydroxylspecies form as a result o f the reaction between molecular oxygenand aliphatic groups.34,35 Peroxide and hydroperoxide groupsstart to oxidize to hydroxyl, carbonyl (ketones), and carboxylgroups at low temperatures.29,34,36,37 Ketonic carbonyl andcarboxyl groups decompose at higher temperatures to producelower molecular-weight structures, such as carbon dioxide.38,39

Received: April 1, 2012Revised: May 24, 2012Published: May 29, 2012

Article

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2012 American Chemical Society 3651 dx.doi.org/10.1021/ef300559b| Energy Fuels 2012, 26, 36513660
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Fourier transform infrared (FTIR) spectroscopy is a usefulmethod for determiningthechemical changes during drying andoxidation of coal.3537,4042

Despite the importance and signicance of superheated steamuidized-bed and microwave drying technologies in upgradingand use of low-rank coals, very little has been reported onchemical changes during drying of low-rank coals in these

processes. Furthermore, the eff

ects of air drying on the low-rankcoal structure reported in previous works mostly covers the low-temperature oxidation aspect of coal, and a deep insight into theeffects of air on the lignite chemical structure at hightemperatures is necessary. In this study, the chemical changesin the low-rank coal organic structure during superheated steamuidized-bed and microwave drying are reported and comparedto conventional hot-air uidized-bed drying by use of the FTIRtechnique.

2. EXPERIMENTAL SECTION

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 inTable1. Samples were crushed and sieved to less than 1 mm for steamdrying experiments. An experimental setup was designed andmanufactured for steam uidized-bed drying experiments (Figure 1),comprising a drying reactor, an electrically heated furnace, a water

pump,and a controller. Drying temperatures varied in these experimentswere 150, 200, 250, and 300 C, and 3 g of samples was dried for 10 minbefore being subjected to FTIR measurement. The nal moisturecontent of samples after drying was almost zero (samples were almostcompletely dried). The higher temperatures (250 and 300 C) havebeen used in the experiments primarily for comparison purposes,while the lower temperatures (150200 C) are more likely to be thetemperatures to be employed in practice.

2.2. Microwave Drying Experiments. Microwave drying experi-ments were carried out in a domestic microwave oven (Midea) withtechnical features of 220 V, 50 Hz, and 700 W and a microwavefrequencyof 2450 Hz. Three different microwave outputpower levels of380, 540, and 700 W were used for this study. The dimensions of thedrying chamber used for drying were 305 204 305 mm andconsisted of a rotating glass plate with a diameter of 270 mm at the baseof the microwave oven. A 150500 m size fraction of samples wasdried for 10 min at different output power levels, and the dried sampleswere subjected to FTIR analysis. Similar to steam drying experiments,the nal moisture content of samples after 10 min of drying was near

Table 1. ProximateAnalysis of the Lignite Sample Used inDrying Experimentsa

proximate analysis (%)

moisture (ar) volatile matter (ad)

xed carbon (ad) ash content (ad)39.04 38.59 56.37 5.03

aar, as received; ad, air dried.

Figure 1.Schematic diagram of the steam uidized-bed drying experimental setup.

Energy & Fuels Article

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zero (samples were almost completely dried). At each time interval, the

coal sample was taken out from the microwave oven and thetemperature of the samples was immediately measured using a thinand sensitive thermocouple, and thus, the temperature change versustime during drying was obtained.

2.3. Air Fluidized-Bed Drying Experiments. Air drying experi-ments were carried out in a uidized-bed dryer setup comprising of aquartz reactor, an electrically heated furnace, and a controller. Similarsamples to those used in superheated steam uidized-bed andmicrowave drying (Chinese Shenhua No. 6) were used in air dryingexperiments. A total of 2 g of the as-received sample with a 224355 mparticle size was placed in the quartz uidized-bed reactor and heated inan electric furnace for 10 min. Air was used as the heat carrier, anduidizing gas and samples were dried at 150, 200, and 250 C prior toFTIR analysis.

2.4. FTIR Spectroscopy. Infrared (IR) spectra of raw coal and driedsamples were obtained with a Thermo Fisher Nicolet IS5 mid-FTIR

spectrometer. KBr pellets were prepared by grinding around 2.5 mg ofdried coal with 200 mg of KBr. IR spectra of the lignite sample for the4000400 cm1 region were studied by curve-tting analysis using acommercially available data-processing program (OriginPro, OriginLabCorporation). The assignment ofthebands in the IR spectra was madeaccording to the literature.36,41,4346 The number of bands and peakpositions were obtained by examining second derivatives of the spectraldata. Gaussian and Lorentzian functions were used as mathematicalfunctions for band shapes at aliphatic hydrogen and carbonyl stretchingregions.36,45 An example of the curve-tted spectra for 300 Csuperheated steam-dried coal is shown in Figure 2. The coefficient ofdetermination (R2) was the primary criterionin examining thegoodnessof curve tting. R2values of the curve-tting analysis in allcases of steam,microwave, and air drying and in both aliphatic CH stretching (30002800 cm1) and carbonyl adsorption (18501500 cm1) zones were inthe range of 0.9940.999, indicating that IR spectra of all samples were

curve-tted with high accuracy.

3. RESULTS AND DISCUSSION

3.1. Superheated Steam Fluidized-Bed Drying.Figure3shows the IR spectra of as-received and steam-dried samples at150, 200, 250, and 300C with original particle sizes of 01 mm.Changes in absorption of functional groups in the regions of30002800 cm1 (aliphatic CH stretching) and 18501500 cm1 (carbonyl adsorption) were investigated. Adsorptionof ve bands in the aliphatic hydrogen absorption regionattributed to asymmetric methyl (CH3) and methylene(CH2) stretching (near 2955 and 2922 cm

1, respectively),symmetric methyl (CH3) and methylene (CH2) stretching

(near 2865 and 2850 cm1, respectively), and methane (CH)stretching (near 2897 cm1) as a function of the dryingtemperature in superheated steam is shown in Figure 4. Asseen, the absorption of aliphatic CH stretching decreased

slightly with an increasing drying temperature and the decrease ismore signicant at300 C.Shuietal.47 alsoreported the decreasein aliphatic groups of steam-treated coal compared to those ofraw coal, suggesting that steam treatment can break some weakaliphatic CH bands, resulting in the decrease of the volatile

yield of the steam-treated coal. The 30002800 cm1 zone wascurve-tted to the above-mentioned ve bands, and the 18501500 cm1 zone was curve-tted to a series of seven bands corre-sponding to carboxyl groups and quinines (17701650 cm1),aromatic carbon (around 1615 cm1), and carboxylate andaromatic ring stretch groups (15601490 cm1). From thecurve-tted bands in these two zones, a series of parameters weredened as ratios of deconvoluted peak areas in these zones to

Figure 2. Curve-tted spectra of the 18501500 cm1 zone forsuperheated 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.

Figure 4.Changes of aliphatic structures for steam-dried samples as afunction of the temperature: (a) raw coal, (b) 150 C, (c) 200 C, (d)250 C, and (e) 300 C.




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quantify the chemical changes in aliphatic hydrogen and oxygenfunctional groups. The dened parameters were as follows:

CH /CH : 2955 cm band/2922 cm band3 21 1

(1)

C /H : 1600 cm band /2965 2850 cm zonear al1 1

(2)

+

+

C /COOH C : 1600 cm band /1710 cm band

1600 cm band

ar ar1 1

1 (3)

C O /C : 1750 1650 cm zone /1610 cm bandar1 1

(4)

COOH/C : 1710 cm band/1600 cm bandar

1 1(5)

Equations15dene the ratios of methyl/methylene, aromaticcarbon/aliphatic hydrogen, aromatic carbon/carboxylic group,carbonyl/aromatic, and carboxyl/aromatic, respectively. Table2summarizes the evolution of these parameters as a function of the

drying temperature under superheated steam conditions. Themethyl/methylene absorption ratio was considered as an estima-tion of the length of aliphatic chains of coal.45,48 This ratioincreased with an increasing drying temperature to 250 C,suggesting the preferential decomposition of methylene aliphaticgroups.49 This was suggested to be due to the loss of alkyl chainsand conversion of the hydroaromatic methylene structure toaromatic rings.50,51At temperatures higherthan 250 C because ofthe very high drying intensity, methyl groups also started toconvert to aromatic structures, which decreased the CH3/CH2ratio. The aromatic carbon (Car)/aliphatic hydrogen content (Hal)ratio provided a measurement of the aromatic structure evolutionduring drying. This ratio increased progressivelywithan increasingdrying temperature, indicatingthe evolution of aromatic structuresat the expenseof aliphatichydrogen decomposition. Similar resultshave been reported by Ohki et al.49 This ratio increased signi-cantly at temperatures higher than 250C because of the higherdrying intensity. The ratio of aromatic carbon/carboxylic groupsas dened in eq3was reported as a suitable index to investigatethe maturation of organic matter because two mechanisms ofaromatization and oxygen removal are combined in thisratio.50,52,53 This ratio increased progressively with the drying

temperature, indicating the combined effect of drying on both thearomatization of the coal organic structure and oxygen removal.

Figures 58 show the evolution of carboxyl (COOH),carbonyl(CO), carboxylate (COO) and aromatic ring stretch,and aromatic carbon (CC) group absorption, respectively, as afunction of the drying temperature. As seen, the absorption ofcarboxyl and carbonyl groups decreased drastically with theincreasing drying temperature in superheated steam (Figures5and 6), indicating the progressive loss of oxygen functional groups

Table 2. Structural Parameters Derived from Curve-Fitted

Analysis of FTIR Spectra of Superheated Steam-Dried CoalSamples

drying temperature(C)

CH3/CH2

Car/Hal

Car/COOH +Car

CO/Car

COOH/Car

raw coal 0.2 3.12 0.68 1.18 0.47

150 0.21 4.16 0.72 0.87 0.37

200 0.19 4.13 0.75 0.77 0.35

250 0.25 4.35 0.76 0.70 0.32

300 0.16 5 0.77 0.67 0.30

Figure 5.Changes of carboxyl (COOH) group (1710 cm1) absorption as a function of the drying temperature in steam, microwave, and air.




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with an increasing drying intensity,24,26,54 which decreasedthemoisture holding capacity25,27,54,55 and self-ignition tendency26 oflignite. Shui et al.47 also reported the decrease in carbonylbands ofsteam-treated coals at 250C. The progressive decomposition ofcarboxyl groups is also consistent with results reported byMurakami et al.,56who observed the monotonic decrease of thesegroups with the temperature. The carboxylate and aromatic ringstretch absorption remained relatively constant up to 250 C butdecreased signicantly thereafter because of the very high dryingtemperature (Figure7). A similar trend was observed in aromaticcarbon (CC) absorption (Figure8). It seemed that, at 300C,some pyrolysis reactions took place that altered the coal organicstructure. A higher thermal stability of carboxylate groupscompared to its acidform(carboxyl groups) has been reported

by several researchers.5658 The question may arise whether the

reduction in absorption bands at 1600

1500 cm1

was related todecomposition of carboxylate (COO) groups or aromatic ringstretch. Because the aromatic carbon (CC) also started todecrease at temperatures higher than 250 C, it was evidentthat the aromatic ring groups are less stable than the carboxylategroups and the decrease in the 16001500 cm1 zone at 300 C

was related to decomposition of the aromatic ring stretch. Thisconclusion is consistent with the results reported by Murakamiet al.,56 who observed that aromatic ring groups start todecompose drastically at 250 C but carboxylate groups arestable up to 350 C. Ozaki et al.57 and Schafer59 reported thedecomposition of carboxylate groups at temperatures around 400and 450 C, respectively.

The ratios of carbonyl/aromatic (CO/Car) and carboxyl/aromatic (COOH/Car), which were dened in eqs 4 and 5,

respectively, indicated the progressive decrease in absorption ofcarbonyl and carboxyl functional groups and the increase inaromaticity of steam-treated lignite (Table2). Decomposition ofoxygen functional groups was reported to result in evolution of

water, carbondioxide, and carbon monoxide at higher dryingtemperatures24,6062 From the above-mentioned results, it can

be concluded that drying of low-rank coals in a superheatedsteamuidized-bed dryer up to 300 C increased the aromaticityof low-rank coal and decreased the oxygen-containing functionalgroups signicantly but some aromatic carbon and aromatic ringstretches were decomposed at very high drying temperatures(above 250 C). It seemed that 250 C was the maximum andoptimal drying temperature for steamdrying, consideringthe factthat it caused a signicant decrease in oxygen functionalities and

the decomposition of the coal organic structure was not yetstarted at this temperature.

3.2. Microwave Drying. Microwave drying hassome uniquefeatures that are absent in conventional heating. Some of theseimportant features include very high drying rates compared toconventional heating, selective heating of moist areas, and anadditional mechanism of moisture transport because of internalevaporation.21,63 It seems that selective heating of microwave hassome unique effect on the coal chemical structure in a sense thatorganic 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 waterand coal organic matter, respectively,64,65 indicating the fact that

Figure 6.Comparison of carbonyl (CO) group (17501650 cm1) absorption as a function of the drying temperature in steam, microwave, and air.




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water molecules will absorb microwave signicantly more thancoal organic matter.

Figure9shows the IR spectra obtained from raw coal andmicrowave-dried samples at 380, 540, and 700 W microwaveoutput powers. The maximum temperature of 170 C wasrecordedduring drying of lignite samples at 700 W microwavepower,21 and the temperatures corresponding to 380 and 540 Wobtained from linear regression were 101 and 135 C, respec-tively. Similar to steam-dried samples, the aliphatic hydrogen(30002800 cm1) andcarbonyl (18501500 cm1) adsorptionzones were curve-tted to 5 and 7 bands, respectively.

Absorption of ve bands in the aliphatic hydrogen region(30002800 cm1) as a function of the microwave output poweris shown in Figure 10. Absorption of these bands remainedrelatively unchanged up to 540 W and decreased thereafter. The

main decrease was observed in asymmetric and symmetricmethylene (2923 and 2851 cm1, respectively) and asymmetricmethyl (2955 cm1) at 700 W. The evolution of parametersdened in eqs 15as a function of the microwave power ispresented in Table 3. The methyl/methylene ratio increasedslightly up to 540 W and decreased at 700 W. This observationindicated that methylene groups were lost in small amounts atlower drying intensities because of conversion of hydroaromaticmethylene structures to aromatic rings.50,51 At 700 W, methylgroups decomposed andconverted to aromatic rings as a resultofmore intensive drying conditions. A progressive increase in thearomatic carbon (Car)/aliphatic hydrogen content (Hal) ratio

with an increasing microwave output power indicated the

increase in aromaticity of coal during microwave drying. Thisincrease was more signicant at 700 W because of the conversionof both methylene and methyl structures to aromatic rings. Aprogressive decrease in aromatic carbon/carboxylic group,carbonyl/aromatic, and carboxyl/aromatic ratios indicated thedecomposition of oxygen functional groups in low-rank coalunder microwave drying conditions.

The changes in absorption of carboxyl (COOH), carbonyl(CO), carboxylate (COO) and aromatic ring stretch, andaromatic carbon (CC), which correspond to 1710, 17501650, 16001500, and 1610 cm1bands, respectively, are shownin Figures58. The drastic loss of carboxyl and carbonyl groups

with an increasing drying intensity was obvious, and these resultswere consistent with curve-tted analysis results, as discussedabove. The removal of oxygen functional groups (decrease in the

coal oxygen content) caused the increase inhydrophobicity andloss of the colloid structure of low-rank coals.24,66,67 Carboxylategroups and aromatic ring stretch increased slightly possibly

because of the interaction of carboxylic acid with coal mineralmatter or conversion of aliphatic hydrogen to aromaticstructures. Aromatic carbon (CC) absorption remainedrelatively unchanged under microwave drying conditions evenat the maximum output power level of 700 W. This observationindicated the importance of the selective heating feature ofmicrowave radiation compared to conventional drying methods.The organic structure of coal underwent minimum changesunder microwaveradiation(because of a low dielectricconstant),

but water was removed and oxygen functionalities were

Figure 7. Comparison of carboxylate (COO) andaromaticring stretch group (16001500cm1) absorption as a function of thedrying temperature insteam, microwave, and air.




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decomposed selectively. The maximum temperature of 170 C at700 W was not high enough to cause the signicant changes inthe organic structure of coal; therefore, it can be concluded thatmicrowave drying is a very promising technology in a sense thatits energy is used to selectively remove water and oxygenfunctionalities from the coal structure, while the coal organic

structure remains relatively unchanged under microwaveradiation.

3.3. Air Drying.IR spectra of raw and air-dried coal at 150,200, and 250 C were curve-tted to ve and seven bands ataliphatic hydrogen (30002800 cm1) and carbonyl adsorption

Figure 8.Comparison of aromatic carbon (CC) group (1610 cm1) absorption as a function of the drying temperature in steam, microwave, and air.

Figure 9. FTIR spectra of as-received coal and microwave-dried samplesat 380, 540, and 700 W.

Figure 10.Changes of aliphatic structures for microwave-dried samplesas a function of the microwave output power: (a) raw coal, (b) 380 W,(c) 540 W, and (d) 700 W.




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(18501500 cm1) zones, respectively, as described in previoussections. Absorption of ve bands in the aliphatic hydrogenregion as a function of the drying temperature for raw coal anddried sample at 250 C is shown in Figure11. As seen, a drasticdecrease in aliphatic structures took place when coal was dried inair at 250C. This observation implied that oxygen in dryingairmainly reacted with aliphatic sites in the coal structure.36,6870

Table4shows the evolution of parameters dened in eqs15as

a function of the drying temperature in hot air. The drasticincrease of the methyl/methylene ratio showed a signicantdisappearance of methylene groups as a result of oxidation.Oxygen absorption on methylene groups has been reportedinthe literature as the main mechanism of coal oxidation.41,45,71

Oxidation of aliphatic parts in the organic structure of coalsresulted in the formation of oxygen functional groups.36,45 Theevolution of carboxyl (COOH), carbonyl (CO), carboxylate(COO) andaromaticring stretch, andaromaticcarbon (CC)corresponding to 1710, 17501650, 16001500, and 1610 cm1,respectively, is shown in Figures 58. The evolution of thesegroups showed different trends compared to those of superheatedsteam- and microwave-dried samples. Carboxyl and carbonyl

groupsshoweda slight increase in absorption up to 150C,72whilecarboxylate groupabsorption decreased slightly in this temperaturerange. A further increase of the drying temperature to 200 Cresulted in a drastic increase in absorption of all of these threegroups.

The aromatic carbon band showed negligible changes duringdrying at different temperatures. This result conrmed the above-mentioned observation that oxidation took place on aliphaticmoieties (side chains and bridge bonds) and the aromatic nucleusremained stable. This observation was consistent with resultsreported in the literature.29,36,38,39,73 Because the aromatic carbon(Car) remained almost invariable during the oxidation process, theratio of oxygen-containing functional groups to aromatic carbonprovided a quantitative study on their evolution duringoxidation.36,45 Curve-tting analysis of the carbonyl adsorptionregion (Table 4) showed that the ratios of carboxyl/aromaticcarbon (COOH/Car) and carbonyl/aromatic carbon (CO/Car)increased signicantly up to 200 C, showing the generation ofoxygen functional groups, as shown in Figures5and6. This studyshowed that carboxylic acids and carbonyl groups werepreferentially formed during intensive oxidation at the expense ofaliphatic structures. Carboxyl and carbonyl groups (including

aldehyde, ketone, and acid)arosefrom oxidization of methylenebridges in aromatic units.36,45,48,71 Absorption of carboxylategroups showed similar trends to those of carboxyl and carbonylgroups. Carboxylates could be formed from the interaction

between carboxylic acid and coal mineral matter.45

Increasing the drying temperature to 250 C resulted insignicant decomposition of oxygen functionalities (Figures 5and6). It is believed that some combustion processes took placeat this temperature. These results were consistent with those bySaikia et al.,74who reported the loss of OH and CO groupsduring thermal treatment of Indian coals at 250 C in air.Decomposition of carboxyl groups to carbon dioxide andcarbonyl groups to carbon monoxideat high temperatures has

been reported in the literature.24,7578 Carboxyl groups can easily

dehydrate to generate anhydrideanddecarboxylate, leading tothe formation of carbon dioxide.29,79 These reactions at highertemperatures under airdrying conditions resulted in a decreaseincoal volatile matter and an increase in the ash content.45 It can beconcluded that air drying is suitable for low-rank coal drying onlyup to 150 C. Drying in air at 200 C resulted in a signicantincrease in oxygen functional groups, which is not favorable fordownstream applications. The treatment of low-rank coals in airat 250 C leads to signicant decomposition of the aliphatichydrogen structure of coal.

4. CONCLUSION

Superheated steam uidized-bed drying resulted in a slightdecrease in aliphatic hydrogen at high temperatures, suggesting

that superheated steam treatment causes the breakage of weakCH structures. The absorption of carboxyl (COOH) andcarbonyl (CO) groups decreased with an increasing dryingtemperature, indicative of the progressive loss of oxygenfunctional groups with an increasing drying intensity. Aromaticring stretch and aromatic carbon remained relatively unchangedup to 250 C and decreased signicantly thereafter. Carboxylategroups seemed to be persistent upto about 400 C. Steam dryingup to 300C progressively increased the aromaticity of coal. It istherefore suggested that the optimum drying temperature forlignite in superheated steam uidized-bed drying was 250 C,under which a signicant decrease in oxygen function groupstakes place but the organic structure of coal undergoes minimum

Table 3. Structural Parameters Derived from Curve-FittedAnalysis of FTIR Spectra of the Microwave-Dried CoalSamples

microwave power(W)

CH3/CH2

Car/Hal

Car/COOH +Car CO/Car COOH/Car

raw coal 0.2 3.23 0.68 1.18 0.47

380 0.23 3.28 0.74 0.82 0.35

540 0.26 3.36 0.82 0.55 0.22700 0.24 4.23 0.85 0.36 0.18

Figure 11. Changes of aliphatic structures for raw coal and air-driedsample at 250 C.

Table 4. FTIR Analysis on Oxidation Parameters as aFunction of the Temperature in Hot Air Drying

drying temperature(C)

CH3/CH2

Car/ COOH +Car

CO/Car

COOH/Car

raw coal 0.18 0.74 0.65 0.3

150 0.28 0.67 0.69 0.49

200 0.47 0.61 1.15 0.76

250 0.73 0.73 0.44 0.41




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changes. Drying temperatures higher that 250 C signicantlymodied the organic structure of coal.

Aromatic carbon remained relatively unchanged during micro-wave drying even at the maximum output power of 700 W, whilethe oxygen functionalities (carboxyl and carbonyl) decreasedprogressively with increasing drying intensity. The loss of oxygenfunctional groups resulted in a progressive increase in aromaticityof coal with increasing microwave power level.

The absorption of the aliphatic hydrogen content decreasedsignicantly when coal was heated in air at 250C. The aromaticcarbon remained unchanged during air drying, suggesting thatoxygen in air reacted mainly with the aliphatic structure toproduce oxygen functional groups. A progressive decrease in theCH3/CH2 ratio indicated that methylene structures were themost active sites on the coal structure to react with oxygen.Carboxyl and carbonyl groups increased signicantly with anincreasing drying temperature from 150 to 200 C. Above thistemperature, oxidation took place and oxygen functional groupsdecomposed signicantly to produce water, CO, and CO2. Theoptimum drying temperature of lignite in air was 150 C, above

which oxygen functional groups and aliphatic structures undergosignicant changes.

AUTHOR INFORMATION

Corresponding Author

*Fax: +86-412-5929105. E-mail:[email protected].

Notes

The authors declare no competing nancial interest.

ACKNOWLEDGMENTS

This study was supported by the National Natural ScienceFoundation of China (21176109 and 20976106). The authorsgratefully acknowledge the nancial support of the InternationalPh.D. Research Scholarship (2009) from the Liaoning Provincial

Government of China and thenancial support of the Australia

China Joint Coordination Group on Clean Coal TechnologyResearch and Development Grants of the Australian Govern-ment.

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study of chemical structure changes of chinese lignite upon drying

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