Char Structural Evolution during Pyrolysis and Its Influence on

Feb 1, 2012 - Three kinds of Chinese coal and a biomass were pyrolyzed by N2 and CO2 in a bench scale fluidized bed reactor. Fourier transform ...
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Char Structural Evolution during Pyrolysis and Its Influence on Combustion Reactivity in Air and Oxy-Fuel Conditions Ben Wang, Lushi Sun,* Sheng Su,* Jun Xiang, Song Hu, and Hua Fei State Key Laboratory of Coal Combustion, HuaZhong University of Science and Technology, 430074 Wuhan, Hubei, China ABSTRACT: Three kinds of Chinese coal and a biomass were pyrolyzed by N2 and CO2 in a bench scale fluidized bed reactor. Fourier transform (FT)-Raman/infrared (IR) spectroscopy was used to identify microstructure and to evaluate the structural evolution of chars generated in N2 and CO2 environments, which are the main diluting gases of air and oxy-fuel environments. The Raman spectra were fitted with five Lorentzian bands. The reactivities of the char were measured by a thermogravimetric analyzer from room temperature to 1373 K in air and oxy-fuel conditions with O2 concentration of 21%. The derived activation energy for different samples was correlated with the Raman structural parameters. Results showed that more disordered char was formed with the pyrolysis in CO2 than that in N2, and new O-containing functional structures would be introduced into the char structure in CO2 atmosphere. The char structures became less ordered as the sample rank decreased. The reactivity of CO2 char was higher than that of N2 char, while the combustion atmospheres rarely affected the char reactivity, indicating CO2 played a more important role on the devolatization process for coal than for char combustion. The activation energy had a good linear correlation for N2 char with Raman characterizations, while the data points for CO2 char were perfectly fitted with exponential functions.

1. INTRODUCTION O2/CO2 combustion technology now has become a promising way for enrichment of CO2 in the flue gas to levels as high as 95% by volume to capture CO2 from the flue gas;1 at the same time, it can also effectively reduce pollutants such as SO2 and NO emissions. This novel combustion method has already received general concern all over the world. It has been reported that this technology has a great potential for retrofitting existing coal-fired power plants and building new power plants with several benefits.2 As combustion generally comprises the process of devolatilization, followed by the combustion of the residue char, the devolatilization process exerts its influence throughout the life of the solid particles from injection to burnout. Therefore, differences in thermal properties and chemical action between N2 and CO2 make char combustion in oxy-fuel conditions quite different from conventional conditions. The presence of CO2 in high concentrations during the initial pyrolysis stage will significantly change the distribution of alkali and alkaline earth metallic (AAEM) species. Then, these changes in the resulting char will substantially alter the coal reactivity and influence the combustion efficiency.3 Scala et al.4 found that, in the process of oxyfiring conditions, CO2 gasification contributes to a comparative extent for carbon consumption at high temperatures and low oxygen concentrations. In addition to the different gas properties, the influence of CO2 on the reactivity of coal chars is strongly dependent on the raw coal properties.3,5 However, previous research has rarely examined the effects of structural variations of chars on reactivity. Borrego et al.6 used optical microscopy and scanning electron microscopy to obtain texture and morphology information of biomass chars prepared under N2 and CO2 atmospheres, and similar characteristics and reactivity were observed. Li et al.7 applied scanning electron microscopy (SEM) to analyze the © 2012 American Chemical Society

cross-sectional and morphological characteristics of the char samples in N2 and CO2 atmospheres, and higher apparent reactivity under air conditions, as compared with oxy-fuel conditions, were observed. Nevertheless, the variations of disordered char structures in these atmospheres can be investigated with the aid of Raman spectroscopic methodology,8 which is feasible for us to realize the characterization of char structure. Sheng and Zhu9,10 applied Raman spectroscopy to characterize char structures under different heat treatment conditions and correlated the Raman parameters with char combustion reactivity. Tay et al.11 investigated the changes in char reactivity and structure during the gasification of brown coal in O2 and CO2, using the method of FT-IR. In the present work, three pulverized coal samples and a biomass were pyrolyzed under N2 and CO2 atmospheres, which are the diluting gases of air and oxy-fuel environments in a fluidized bed reactor to produce char samples. The chars were subjected to FT-Raman/IR spectroscopy analysis for the microcrystalline structure. After that, the combustion reactivity in air and oxy-fuel conditions were assessed by thermogravimetric analysis. The objective was to apply Raman spectroscopy to characterize the evolution of char microstructure in N2 and CO2 atmospheres and correlated the combustion reactivities with the structural order of different sample ranks, in addition, the transformation of char structure and surface chemistry between coal and biomass in the devolatilization process were also discussed in detail.

2. EXPERIMENTAL SECTION 2.1. Characteristics of Samples. The coals selected for this study were based on their coal ranks. Coal LNC is a high-volatile lignite, coal Received: November 4, 2011 Revised: January 31, 2012 Published: February 1, 2012 1565

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Table 1. Proximate and Ultimate Analyses of Fuels proximate analysis (wt %, as received basis)

ultimate analysis (wt %, dry and ash-free basis)

sample

moisture

volatile matter

ash

fixed carbon

C

H

N

S

O

XWJ NCP LNC RS

2.674 3.275 2.955 8.132

13.508 7.209 29.677 64.674

55.585 26.042 23.96 14.572

28.231 63.471 43.408 12.621

33.5 64.69 57.96 37.27

2.641 2.811 4.1835 5.683

0.663 0.673 1.1175 0.712

1.182 1.036 1.327 0.120

3.755 1.473 8.497 33.511

XWJ is a bituminous coal with high ash content, and coal NCP is a low-volatile anthracite. All of them are widely used in the coal-fired plants of north China. To draw a comparison with coal, a very common biomass in the rural area of China called rice straw (RS) was also selected. The samples were crushed and sieved with a fraction of particle sizes of 105−150 μm. The proximate and ultimate analysis of the sample and the elemental composition of ash are shown in Tables 1 and 2.

Table 2. Analysis of Ash (wt %) Na2O MgO Al2O3 SiO2 P2O5 SO3 K2O CaO TiO2 MnO Fe2O3

XWJ

NCP

LNC

RS

0.69 0.92 29.86 51.15 0.32 4.16 1.17 3.98 1.50 0.04 6.20

0.83 26.15 49.66 8.41 8.41 1.72 5.70 0.96 0.04 6.44

1.44 1.39 33.82 44.09 4.18 4.77 0.56 5.10 1.53 0.04 3.08

1.73 2.92 70.34 1.86 3.36 13.79 4.54 0.40 1.08

2.2. Pyrolysis Experiments. The pyrolysis experiments were carried out on a quartz fluidized bed reactor at a temperature of 1223 K, as shown in Figure 1. The reactor had two connected quartz tubes (with inner diameters of 45 and 30 mm and lengths of 0.1 and 0.28 m, respectively) with flaring to form an optimal fluidization effect. A porous quartz frit placed 650 mm from the bottom of the reactor was employed as a gas distributor. A bed of quartz sands (400−500 μm) placed above the distributor was fluidized by high purity N2 or CO2 (99.999%), and an external electric furnace supplied the heat source for the reactor. A disengaging section with a diameter of 45 mm and a length of 100 mm was constructed to avoid small particles from leaving the reactor. Because of the high heating rate in the reactor, the RS particles with high content of volatiles and low content of fixed carbon would melt into small and light particles during the devolatilization process, which could be easily elutriated out of the reactor by the fluidizing gas. So, another detachable quartz frit with a quartz handle was installed on the top of the reactor to prevent the particles from being carried out, using the method of Quyn et al.12 on a similar fluidized bed reactor. The fluidizing gas introduced from the bottom of the reactor by a thin tube with a flow rate of 800 mL/min could keep particles suspend in the gas and remove the volatiles quickly or dilute them with the fluidizing gas to minimize the reaction with nascent chars. After the flow field in the reactor became stabilized and the desired temperature of 1223 K was achieved, about 500 mg of sample was fed into the reactor per time from a thin quartz tube (not shown) in the top of the reactor. This tube ended just above the disengaging section, which could guarantee a smooth charge of solid fuel to the reaction zone. After that, the top frit was embedded onto the upper freeboard and the sample entrance was sealed with a stopper. The exhaust gases left the reactor from a side tube just below the sample entrance (with an inner diameter of 8 mm). After the devolatilization process proceeded for 10 min, the reactor was lifted out of the electric furnace and the fluidizing gas was switched to high purity argon to quench the char particles. Chars were

Figure 1. Schematic diagram of the fluidized bed reactor. then selected via a sieve with 140 meshes (corresponding to 105 μm) for further characterization and reaction. 2.3. Char Structural Characterization. The FT-IR/Raman spectra of chars were acquired using a VERTEX 70 FT-IR/Raman spectrometer. The char sample was mixed and ground with KBr to a concentration of 0.5 wt %. This diluted sample was then used to record the IR or Raman spectrum. An excitation laser with spectral resolution of 0.8 cm−1 was selected to form a laser beam (1064 nm) on the sample, and an InGaAs detector was employed to collect Raman signal in the backscattered. The laser power was controlled at about 10 mW. The Raman spectra in the range 800−2000 cm−1 were recorded to cover the first-order bands. Each spectrum was deconvoluted into five Lorentzian bands on the basis of the work of Sadezky et al.13 and Sheng et al.,9 who developed a combination of four Lorentzian bands and one Gaussian band. 2.4. Char Reactivity Measurement. Char combustion reactivity was obtained using a TGA (STA-409). The measurement was performed under nonisothermal condition following the procedure of Shim et al.14 About 1.5 mg of sample was placed in the TGA pan and heated from room temperature to 1373 K with a heating rate of 30 K/ min then held for a certain time for complete burnout. The reactivity was measured in air or O2/CO2 flows with O2 concentration of 21%, and the apparent activation energy was calculated by the non1566

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isothermal method. The specific reactivity of the char at any given time was calculated from the equation15,16 R=−

of chars by CO2. Many researchers also verified that the char− CO2 gasification reaction commences at temperatures around 973 K.17,18 3.2. Raman Spectra of Chars. The Raman spectrum mainly contains the information of widths, positions, and ratios of intensities of the G (graphite) and D (defect) bands, as shown in Figure 3. However, some structure features of lignite and biomass char remain under cover around the D and G bands because of their disordered natures. For this reason, the Raman spectra are deconvoluted into five Lorentz bands by Origin7.0/Peak Fitting Module to get detailed information about the carbon skeleton structures of chars. The parameters including peak position, full width at half-maximum (fwhm), intensity, and integrated area of each band are derived from the decomposition. The D1 band at 1350 cm−1 commonly represents the defects in graphite structure and other disordered structures. 19,20 The G band at 1580 cm −1 corresponds to stretching vibration (E2g symmetry) in the aromatic layers of graphite crystalline.21 The 1620 cm−1 band (D2) is attributed to a lattice vibration involving graphene layers, and its intensity decreases with the increase in the degree of organization.22,23 The D3 band at 1530 cm−1 is related to the small ring systems such as amorphous carbon in the chars.24,25 The D4 band at 1150 cm−1 can be observed only in very poorly organized materials, such as soot and coal chars.14,24 Figure 3 shows typical Raman spectra of the anthracite chars produced under N2 and CO2 atmospheres. The results of the spectrum deconvolution are also presented, indicating excellent agreement. Intuitively speaking, the spectra of chars in different bulk gases are similar. The intensity of D1 band is a little stronger than that of the G band, implying the poor order of both chars. At the same time, it can be seen that the discrepancies between the intensity of G band and D1 band in CO2 atmosphere is bigger than that in N2 atmosphere, indicating considerable amounts of imperfections such as heteroatoms are developed after changing the gas from N2 to CO2. 3.3. Impact of Pyrolysis Atmospheres on Char Structure Evolution. The spectrum deconvolution after curve-fitting can be quantitatively measured for the evolutions of the bands. In this paper, integrated band intensity (band area) is chosen as a combined parameter that can perfectly

1 dw w dt

where w is the ash-free weight on a dry basis of the char at any given time t.

3. RESULTS AND DISCUSSION 3.1. Pyrolysis Behavior of Fuel Samples. The total weight loss of the four char prepared in N2 and CO2 atmosphere at 1223 K in the fluidized bed reactor are shown in Figure 2. The weight losses in both atmospheres were higher

Figure 2. Weight loss of the samples in the fluidized bed reactor under CO2 and N2 atmospheres compared to the proximate volatile yields.

for the lower rank sample than for the higher rank sample. The highest weight loss was achieved by RS because it is widely accepted to behave like a brown coal with highest content of proximate volatile matter among the samples. The weight losses for all the samples in N2 and CO2 atmosphere were higher than their proximate volatiles, indicating an enhanced devolatilization in the fluidized bed due to the long residence time and the high temperature. Besides, the weight losses for all samples under CO2 were larger than that under N2. For example, the weight loss of LNC char derived in CO2 atmosphere is 41.54%, higher than 35.61% in N2 atmosphere. The increase in the weight loss in CO2 at 1223 K was attributed to the gasification

Figure 3. Curve-fitted Raman spectra using five Lorentzian bands of the char from the pyrolysis of the NCP coal in N2 and CO2 atmospheres. 1567

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total Raman intensity is higher in CO2 atmosphere than that in N2 atmosphere for the same sample. As we know, the highly disordered structures mentioned will be selectively removed when the sample is pyrolyzed in the fluidized bed. We deduce that additional new O-containing structures would be introduced into the char structure when gasified in CO2. The incoming O-containing structures would exert a resonance effect21 on the total Raman intensity and enhance the observed Raman intensity when compared with that in N2, indicating that more disordered char structures are formed in the CO2 atmosphere. In other words, the loss of O-containing functional groups and substitutional groups is more pronounced in N2 than in CO2, which is partly responsible for the observed decrease in the Raman intensity. As for the different samples, the total Raman intensities in Figure 4 increase with the decrease of sample rank in both atmospheres, implying that the char structures become less ordered as the sample rank declines. This is due to the low rank coal that generally forms the less organized char.9 As a low-rank fuel, the rice straw is widely accepted to behave like a brown coal.27,28 We expect its Raman intensity will be no less than that of XWJ char or even higher than that of LNC char because of its low rank. However, it shows just a little higher intensity than that of NCP char, which implies the RS char has been undergone drastic changes in char structure during the devolatilization process. It is generally acknowledged that alkali metal carbonates, including potassium and sodium, are excellent promoters of the pyrolysis and gasification of biomass char, and changes its overall activity.29−34 Li et al.8 believed the exchangeable ion of Na and Ca can effectively promote the repeated bond-breaking and bond-forming reactions involving radicals during pyrolysis.35,36 Analysis of Table 2 reveals that biomass has the highest content of catalytically active minerals such as

comprise the peak intensity and the full width at half-maximum. The evolution of bands could be simultaneously represented by the band area ratios and band intensity, as a result, to reflect the evolution of the ordered and amorphous carbon structure with the changes of pyrolysis atmosphere for different samples. The observed Raman intensity is affected by the light absorptivity and the Raman scattering ability of char. The decrease of Raman intensity is mainly due to the growth of aromatic ring system size and loss of O-containing functional groups.8 Electron-rich structures such as the O-containing structures tend to have high Raman scattering ability thereby increasing total Raman intensity. Ito et al.26 also suggested that the O-containing heteroaromatic structures could restrain the growth of the lamellar aromatic structures in char, enhancing the Raman intensity of the char. The Raman spectra intensity for different samples in N2 and CO2 atmospheres are shown in Figure 4. It can be seen that the

Figure 4. Total Raman peak area of different chars pyrolyzed in N2 and CO2 atmospheres.

Figure 5. Ratio of band peak areas of different chars prepared in N2 and CO2 atmospheres. (a) ID1/IG (b) IG/IAll (c) ID3/(IG + ID2 + ID3). 1568

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Figure 6. FT-IR spectra of different chars prepared in N2 and CO2 atmospheres: (a) XWJ char; (b) RS char.

compared with that in CO2. It is generally accepted that the dependence between ID1/IG and the microcrystalline planar size shows an inverse proportionality.21 The higher ID1/IG area ratio in CO2 atmosphere, therefore, implies smaller average planar size of the graphitic crystallites and higher concentration of structural defects in the char with various forms. In other words, the char structures become more disordered with the pyrolysis by CO2, which leads to the lower IG/IAll. The observed higher ID1/IG and lower IG/IALL in CO2 atmosphere is partially because the CO2 could react with char slow enough to penetrate the particles.11 Li et al.8 believed the increase in the ID/IG ratio indicates the relative increases in the concentrations of aromatic rings having six or more fused benzene rings and results from the dehydrogenation of hydroaromatics in the char. Zhu and Sheng10 also found the increase of ID1/IG and the decrease of IG/IALL to indicate the increase in the concentrations and sizes of large aromatic rings. Therefore, in addition to the annealing effect as an inert gas of N2, the effect of CO2 reacting with coal could enhance dehydrogenation of hydroaromatics and the growth of aromatic rings during the carbonization process. Borrego et al.6 also found the rearrangement of the carbonaceous structure would be equally enhanced under N2 compared to CO2. As for the combination of ID3/(IG + ID2 + ID3) (Figure 5c), for a definite coal, the ratio is higher in the case of CO2 than N2. The differences between them indicate that the relative content of the amorphous carbon and carbon active sites increase significantly. This is because the decomposition of samples by CO2 gasification promotes the transition of crystalline domains to amorphous structures. Nevertheless, it is worth noting that this ratio for LNC char increases significantly when N2 is substituted with CO2, while for the other chars, it shows slight change with different atmospheres. Borrego et al.37 compared the char obtained under conventional and oxy-fuel combustion atmospheres by a drop tube furnace. They found the oxy-fuel series showed systematically higher BET values than the combustion series. Elliot et al.40 reported the differences were greater for the high-volatile coal. Therefore, as the LNC coal contains relative higher content of volatiles than other coals and retains more fixed carbon than that of biomass to avoid excessive carbonization, CO2 atmosphere is more beneficial for the formation of the surface area and inner porous structures for LNC char. As a result, the process of polymerization and the aromatic ring growth for the formation of ordered structure will be strongly suppressed for

potassium. The amount of potassium in RS is eight times larger than that of coal. This could result in the extreme differences among chars from coal and biomass. Therefore, ring condensation reactions could be greatly enhanced by high concentrations of K and Na in biomass during the devolatilization process. This makes the resulting biomass char condensed and thus lowers observed Raman intensity. On the contrary, the effect of CO2 on the coal is thought to participate in cross-linking reactions, thereby reducing the extent of devolatilization and preventing coalescence and stacking in the structures.37 As a result, the RS char expresses a mismatched total Raman intensity with its highly disordered nature. 3.4. Ratios between the Peak Areas of Some Major Bands. The Raman intensity of the bands mentioned above and fwhm are often leveraged to represent the order of char structure. However, these Raman parameters rely on the recorded intensities of each spectrum and show relative spectra. For this reason, the band area was employed to represent a combined parameter of the peak intensity and the fwhm. Thus, band area ratios could precisely characterize the evolution of carbon crystalline structure. Figure 5 exhibits the ratios of the area of D1 band to the G band and the G band relative to the integrated area, along with peak area ratios of ID3/(IG + ID2 + I D3 ) for different samples. The index of I D1 /I G can quantitatively characterize the degree of disorder in the carbon material, while the increase of IG/IALL ratio means more organized char structures has been formed.8 As for the ID3/(IG + ID2 + ID3) ratio, it is closely related to the amorphous carbon content in carbonaceous materials, which reflects the structure evolution of chars.38,39 The relationship between the band area ratios and the sample rank in Figure 5 is independent of the pyrolysis atmospheres. The evolution of ID1/IG and ID3/(IG + ID2 + ID3) increases, while IG/IALL decreases, with decreasing sample rank. The observed increase of ID1/IG and decrease of IG/IALL in both atmospheres indicates the concentrations and sizes of large aromatic rings increase gradually, while the sharp decrease of ID3/(IG + ID2 + ID3) ratio in Figure 5c implies the amount of the amorphous structure increases considerably as the sample rank decreases. The reason is that the less organized char is derived from the lower rank coal. These variations of band area ratios are excellently consistent with the total Raman intensity for the same samples. As for the influence of different atmospheres, it is observed from Figure 5 that the ratio of the ID1/IG is lower in N2 when 1569

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Figure 7. Reactivity of the different chars (a) NCP char (b) RS char (c) XWJ char (d) LCP char in air and oxy-fuel (O2 at 21%) atmospheres.

occurred at 1593 cm−1 show that phenyl groups are conjugated with unsaturated groups or groups having lone pair electrons.43 At this time, the increased amount of aliphatic C−H groups indicates that the concentration of methylene C−H bands is higher than that in N2. This manifests that the replacement of N2 with CO2 does result in great changes in the surface chemistry for coal char. The CO2 atmosphere reduces the content of hydroxyl groups and olefinic CC bonds. Meanwhile, decreasing the extent of graphitization is evidenced by the shift to higher wave numbers of ν(O−H) bonds and the significant decrease in the amount of CC structures.44 On the other hand, the increased amount of ν(C−O) vibrations in primary C−OH and secondary C−OH and the conversion of C−O bonds to ethers structure also provide further evidence to support our earlier deduction on increasing Raman intensities. The net effect of increasing C−O bonds is to increase the degree of disorder for the carbon structure and the ratio of ID1/IG. Tay et al.11 applied FT-IR to investigate the surface chemistry of CO2 gasified Victorian brown char. They found that a wide variety of oxygencontaining groups may be present in these chars and these functional groups could give rise to the total Raman peak area. Cerfontain et al.45 considered that, during the gasification of brown coal in CO2, alkali carbonate decomposed to an active species, which is responsible for the formation of these functional structures, such as the ethers and hydroxyl groups. Figure 6b indicates the biomass char, when compared with coal char, has great differences in chemical structures. In both atmospheres, the ν(O−H) vibrations in hydroxyl groups appeared at 3450 cm−1 are slightly broader toward lower wavenumbers than that of coal, suggesting that the RS char also contains OH−ether hydrogen bonds. This is because the biomass contains higher concentration of moisture, which can promote the formation of self-associated hydroxyl groups.

the LNC char in CO2 atmospheres and thus increase the content of the amorphous carbon. 3.5. Changes in the Surface Chemistry during Devolatilization. For the purpose of investigating the changes in the surface chemistry during the devolatilization process, information on the surface chemistry of the chars is provided by FT-IR spectroscopy. The FT-IR spectra of the XWJ chars prepared in both atmospheres are shown in Figure 6a. The band with strong intensity at 1085 cm−1 denotes the presence of large amounts of primary hydroxyl groups and secondary hydroxyl groups in those chars, which can be attributed to the ν(C−O) single bond stretching vibrations. The band at 3469 cm−1 is attributable to ν(O−H) bond vibrations in hydroxyl groups. The existence of this band provides further evidence that the ν(C−O) adsorption band belongs to hydroxyl groups. The appearance of ν(CC) band stretching vibrations at 1635 cm−1 shows the presence of ethenyl and ethenylene groups in the char, indicating that the olefinic CC bonds are not conjugated with phenyl groups.41 The band at 2925 cm−1 is ascribable to νas(C−H) methylene asymmetric stretching vibrations. The appearance of ν(C−H) stretching vibrational absorption band and δ(C−H) in-plane bending vibration adsorption band shows the presence of methyl and methylene groups in coal char. The spectra of chars prepared in pure CO2 atmosphere show great distinctions from that in N2. We can find that the ν(O− H) band at 3469 cm−1 is weakened, while the band at 1053 cm−1 shows higher intensity; thus, the band at 1053 cm−1 can be ascribed to the presence of ethers groups and the thermostability is lower than of hydroxyl groups. The νas(C− O−C) vibrations can be related to asymmetric vibrations in a single graphitic sheet and between two such sheets where oxygen can act as a cross-linking agent between aromatic sheets.42 The aromatic skeletal CC vibration absorptions that 1570

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Table 3. Kinetic Parameters of the Chars under Air and Oxy-Fuel Conditions air condition

oxy-fuel condition

sample

pyrolysis atmosphere

E (kJ mol−1)

A (min−1)

R

E (kJ mol−1)

A (min−1)

R

NCP

N2 CO2 N2 CO2 N2 CO2 N2 CO2

194.78 160.11 155.81 107.99 120.78 106.60 159.45 125.59

53489.57 398.21 784.92 1.16 10.53 0.92 42907.92 249.69

0.9895 0.9979 0.9957 0.9911 0.9852 0.9912 0.9965 0.9938

202.71 160.20 155.82 110.52 121.82 107.32 163.04 122.73

132037.10 367.43 745.41 1.43 10.37 1.47 62319.32 47.00

0.9899 0.9984 0.9946 0.9931 0.9913 0.9949 0.9986 0.9924

XWJ LNC RS

an extraordinary high content of ash. Therefore, the relative concentrations of catalyst species such as Na, K, Mg, and Ca in char are much higher than the others, which can be seen in Table 2. The importance of the distribution of catalyst species is enhanced for XWJ char in the combustion processes. Because of the low content of fixed carbon and high content of ash in the char, the differences between the heating rate of the surface area and inner area of the char particle become bigger. The higher heating rate leads to the formation of more condensed structure, while the lower heating rate promotes the formation of amorphous structure. As Asadullah et al. said,48 the existence of catalytic species in condensed carbon species was less preferable, and thus, they might be enriched in the amorphous domain, which made catalyst species distribute heterogeneously. Therefore, when amorphous carbon is exhausted at the char conversion of 80%, the catalyst species move to the surface of the condensed domain, resulting in an instantaneous increase in reactivity of the rest of the char and showing the waving nature of the curve. However, this phenomenon for the CO2 char has totally disappeared, which implies the CO2 can greatly promote the homogeneous deposition of catalyst species throughout the char. It is consistent with the conclusion derived by Borrego et al.6 that CO2 in the devolatilization process was thought to engage in cross-linking reactions and promoted the formation of total pore volume, thereby enhancing the uniform distribution of catalyst species in chars. Table 3 shows kinetic parameters of the four chars in air and oxy-fuel conditions. The results denote that sample rank and devolatilization atmosphere play more important roles on char reactivity, while bulk gas has minor influence on the reactivity of char in combustion process. This phenomenon provides some insight on the significance of CO2 in medium temperatures. Not surprisingly, the activation energy of different chars decreases linearly with decreasing sample rank except the RS char, irrespective of the devolatilization atmospheres and combustion atmospheres. The sample ranks on the basis of the increase in the activation energy show that LNC > XWJ > RS > NCP. The ranking illustrates similar tendencies to the ranking based on the increase in the ratio of ID1/IG and total Raman intensity. Therefore, attempt is made to explore the correlations between the activation energy for different samples and the Raman area ratios aforementioned, together with the total Raman intensity, to reveal the relationship between sample rank and the Raman characterization. 3.7. Correlation between Char Reactivities and Raman Parameters. The Raman spectral parameters have been proved to have close correlations with the degree of char structural order.9,10,30 For example, Sheng9 employed Raman spectroscopy to characterize the microstructure of coal chars

By comparing the chemical structures obtained in N2 and CO2, it is evident that the hydroxyl position in CO2 atmosphere shifts toward higher wavenumbers and shows lower intensity, which suggests that the presence of CO2 will promote the rupture of hydrogen-bonds. The ν(C−O) vibrations (1083 cm−1) in hydroxyl C−OH show a similar tendency as the hydroxyl group with atmosphere changes. Although the amount of CC bonds do not change significantly, the shift to higher wavenumbers (1623 cm−1) of these bands as a result of stretching vibrations likewise indicates the conversion of aromatic CC bonds to olefinic CC bonds. In addition, the peak at 2925 cm−1 caused by asymmetric stretching vibration increases appreciably, manifesting that a variety of methyl or methylene substituents are formed, which eventually causes the increase of the total Raman intensity and the ID1/IG area ratio in CO2 atmosphere for RS char. 3.6. Char Reactivity Evaluation. The reactivity of the char prepared in N2 and CO2 in the fluidized bed are analyzed in the TGA under both air and oxy-fuel conditions. The char reactivities in both conditions from air temperature to 1373 K are shown in Figure 7. The reactivities are higher for the CO2 char than N2 char in both combustion atmospheres. This is consistent with the results of Raman intensity and Raman area ratio characterization for the same char. As for the combustion atmospheres, the char reactivities are slightly higher under the air condition than that under the oxy-fuel condition. Li et al.7 also came to the same conclusion and attributed the lower reactivity in the oxy-fuel condition to the lower diffusivity of O2 in CO2. However, Rathnam et al.46 employed four coals to study the burnout characteristics in air and oxy-fuel conditions. They achieved a contrary conclusion that the coal burnout measured in the DTF at several O2 concentrations revealed significantly higher burnouts for two coals and similar burnouts for the other two coals in oxy-fuel conditions. We deduce the discrepancy is due to the higher combustion temperature (1673 K) and shorter particle residence time (0.62 s) in the DTF in their experiments. For the CO2 char, the higher reactivity implies that the structural disordering with CO2 gasification could increase the combustion reactivity. It is likely to result from the changes in the char structures mentioned, or the dispersion and chemical transformation of catalyst. However, even the latter reason requires changes in char structures as a precondition.47 The most notable feature in Figure 7 is that the TGA profile shows the fluctuated reactivity at conversion levels of 80% for XWJ char generated in N2. The peculiar nature of the combustion reactivity can be explained by the variation of char structures and the heterogeneous distribution of catalyst species during the devolatilization process. Compared to the other three samples, it is worth noting that the parent coal of XWJ char has 1571

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Figure 8. Correlations between the activation energy and the band area ratios of ID1/IG (a and b), IG/IALL (c and d) and ID3/(IG + ID2 + ID3) (e and f) together with total Raman intensity (g and h) of chars prepared in N2 atm (a, c, e, and g) and CO2 atmosphere (b, d, f, and h) (▲ NCP; ▼ RS; ◀ XWJ; ▶ LNP).

generated under various heat treatment conditions, and correlated it with the combustion reactivity measured by thermogravimetric analysis. They found a linear relationship between combustion reactivity and the Raman area ratios,

particularly for the chars generated from dematerialized coals. In this paper, the activation energy is plotted against the Raman characteristic parameter in Figure 8. The reactivities fitted Raman parameters are independent of the combustion 1572

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conditions. The activation energy of the char linearly increases with increasing IG/IALL and decreases with increasing ID1/IG, ID3/(IG + ID2 + ID3) and total Raman area. As described earlier, these correlations are in accordance with the expectation, because lower rank coal generally forms less organized char with highly amorphous structures. In addition, the amorphous structures with aliphatic and O-containing functional groups are also much more abundant in lower rank coal. Therefore, the decrease of amorphous carbon content and substituent group in chars with increasing coal rank lead to the loss of reactivity. Figure 8 shows that the activation energy has a good linear correlation for N2 chars with all the area ratios, which can be regressed with solid lines, while the data points for CO2 chars are perfectly fitted with exponential functions regressed with solid curves. It means the CO2 atmosphere has a more complex effect on the formation of char structure than N2 atmosphere. When only the correlations between the band area ratios of ID3/(IG + ID2 + ID3) and the activation energy are considered, the data are scattered with R of 0.9344 for N2 char and R2 of 0.9695 for CO2 char, respectively, which are quite lower than that of other area ratios for the char derived in the same atmosphere. This conclusion is in agreement with that of Zhu et al.,49 who believed the scattering data for ID3/(IG + ID2 + ID3) are caused by slow transformation of the amorphous carbon into ordered structure.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 86-27-87542417. Fax: 86-27-87545526. E-mail: [email protected] (L.S.); [email protected] (S.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (50806025, 50976038, and 50976043). Also, the partial experimental study was supported by the State Key Development Program for Basic Research of China (2009CB226100). The authors also express their thanks to Huazhong University of Science and Technology Analytical and Testing Center for their help with the measurements of the samples.

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