Study on Char Surface Active Sites and Their Relationship to

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Study on Char Surface Active Sites and Their Relationship to Gasification Reactivity Kai Xu, Song Hu,* Sheng Su,* Chaofen Xu, Lushi Sun, Chao Shuai, Long Jiang, and Jun Xiang State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China ABSTRACT: Char gasification reactivity was considered to be proportional to the number of active sites in the char. Therefore, in this study, char surface active sites (including carbon active sites and catalytic active sites) were first measured with the help of the chemisorption process of CO2 at 300 °C, using a thermogravimetric apparatus. It was found that strong chemisorption (Cstr) and weak chemisorption (Cwea) of CO2, which relate to the presence of active inorganic components and organic matter of char, respectively, existed in this reaction procedure. A higher pyrolysis temperature and slower heating rate induced a decrease of both Cstr and Cwea. Then, char structure evolution was systematically investigated with multi-techniques, such as N2 adsorption isotherm, elemental composition, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and energydispersive spectrometry (EDS) analysis, and attempts were made to correlate the measured structure parameters with CO2 chemisorption parameters. The results showed that a higher pyrolysis temperature and slower heating rate improved coal char C/ H and C/O mole ratios, promoted the growth of the char crystalline structure and significant loss of functional groups, reduced the active mineral matters in the char, and consequently, resulted in a decrease of char surface active sites. Moreover, for both slow chars (SC) and rapid chars (RC), the activation energy decreased linearly with the increase of CO2 chemisorption parameters: Cstr, Cwea, and Cstr + Cwea. Hence, CO2 chemisorption parameters were better than the Brunauer−Emmett−Teller (BET) surface area to correlate with char reactivity. to happen.6−10 However, in the practical gasifiers, such as the widely used entrained flow gasifiers, the operating temperatures are much higher; for example, the gasification temperatures in Texaco and Shell gasifiers are up to 1400 °C.11 Higher temperatures may modify intraparticle pore structure and decrease the active surface area, resulting in better crystallites of the chars, deformation or fusion of ash, and consequently, a different behavior and mechanism of char gasification from those at lower temperatures.12−14 High-temperature fixed-bed reactor and drop-tube furnace have been used successfully in the past for gasification reactivity tests and in the preparation of char under reasonably well-controlled conditions. Liu et al.5 made slow-pyrolysis char and fast-pyrolysis char in a fixed-bed reactor and drop-tube furnace, over the temperature range of 1100−1400 °C. They concluded that the heating rate of coal has a significant effect on the gasification characteristics. Zhu et al.15 used a horizontal tube furnace to pyrolyze lignite and its demineralized sample over the temperature range of 500−1400 °C with a heating rate of 10 K/min. This work showed that, for the chars prepared from demineralized lignite, the decrease of reactivity with the elevated treatment temperature was attributed to the structural changes under heat treatment. While for the chars generated from raw lignite, the decrease of reactivity resulted from not only the evolution of char structure but also the change in the catalytic effect of mineral matter. A higher treatment temperature resulted in the transformation of mineral matter, such as the evaporation of active elements.

1. INTRODUCTION Coal is likely to remain an important source of energy in any conceivable future energy scenario. In the longer term, the prospects for coal will depend upon whether renewables can achieve the required level of power production to replace coal and whether coal can be used at higher efficiencies and with closer to zero emissions of sensitive compounds, particularly CO2.1 As a clean coal technology, coal gasification technology is widely used in the world and can achieve higher efficiencies than pulverized-fired technology. Coal gasification is a complex thermochemical process, which consists of pyrolysis (release of volatile) and conversion of residue char (char gasification).2 The overall process is dominated by char gasification because of its slower reaction speed. However, pyrolysis considered as a thermal deactivation process plays a key role in the subsequent char gasification reactivity. During char formation, many changes occur, including loss of functional groups on the carbon surface, ordering of the carbon microstructure to become more graphitic, and a decrease of the inorganic matter catalytic role under heat treatment, which had been recognized to be the reasons responsible for the decrease of reactivity of coal char to reactant gases.3,4 The changes of resulting chars may differ from each other varying with the treatment conditions, such as temperature, heating rate, pressure, residence time, particle diameter, coal rank, etc.2,5 Therefore, understanding the gasification behavior of char particles prepared under different pyrolysis conditions, especially on the aspect of reactivity and active sites, is of great importance. There now have been abundant works studied on coal or char gasification at the temperatures lower than 1000 °C, where the chemical kinetics controlled gasification reaction is thought © 2012 American Chemical Society

Received: September 6, 2012 Revised: December 11, 2012 Published: December 11, 2012 118

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Table 1. Total Analyses of Huainan Coal Properties proximate analysis (wt %)

ultimate analysis (wt %)

moisture (ad) ash (ad) volatile matter (ad) fixed carbon (ad)

a

1.39 18.06 35.50 45.05 chemical composition of ashes (wt %)

C (daf) H (daf) N (daf) S (daf)

84.12 6.07 0.44 1.54

SiO2

Al2O3

Fe2O3

CaO

MgO

TiO2

Na2O

K2O

SO3

P2O5

43.84

26.59

13.67

9.65

2.53 1.46 macerals (wt %)

0.38

0.62

1.01

0.25

vitrinite

inertinite

exinite

62.3

22.2 ash fusion temperature (°C)a

15.5

DT

ST

FT

1250

1510

1600

DT, deformation temperature; ST, soften temperature; FT, flow temperature.

Table 2. Proximate and Ultimate Analysis Data of the Char Samples proximate analysis (wt %, d)a sample

a

A

V

ultimate analysis (wt %, d) FC

SC950 SC1200 SC1400

26.14 26.35 26.55

1.68 1.42 1.08

72.18 72.23 72.37

RC950 RC1200 RC1400

24.29 26.22 26.97

13.24 6.42 5.09

62.47 67.36 67.94

C Slow-Pyrolysis Char 71.25 71.55 72.68 Rapid-Pyrolysis Char 65.99 67.52 68.62

mole ratio (mol/mol)

H

Ob

C/H

C/O

0.23 0.13 0.10

1.69 1.55 0.38

25.82 45.87 60.57

56.21 61.55 255.02

2.30 1.10 0.65

5.22 3.07 1.91

2.39 5.12 8.80

16.86 29.32 47.90

A, ash; V, volatile matter; FC, fixed carbon. bBy difference.

Kajitani et al.16 employed a drop-tube furnace to measure the rate of char reacting with CO2 over the temperature range of 1100−1500 °C. This work has shown that the gasification reaction most likely occurs in regime II at temperatures higher than 1300 °C, when the surface gasification reactions and the gas diffusion through pores of char particles happen simultaneously. Wall et al.17 summarized the formation of char’s structure of bituminous coals in the literature and inferred that faster heating rates caused more micropores and mesopores and created greater internal surface area. Previous work usually correlated char gasification reactivity with its physicochemical structure, such as pore volume, carbon crystalline structure, bonding functional groups, etc. However, the underpinning parameter, which determines reactivity of char gasification, is the concentration of active sites in the char particles. Studies18,19 have shown that char reactivity was considered in some cases to be proportional to the number of active sites in the char. Therefore, the measurement of char surface active sites and its relationship with char reactivity must also be understood in more detail. In this work, the Huainan lignite (HN) was first pyrolyzed at different heating rates and temperatures in a muffle and a smallscale drop-tube reactor, and then the char surface active sites (including carbon active sites and catalytic active sites) were measured with the help of the chemisorption process of CO2 at 300 °C, using a thermogravimetric apparatus. After that, the char particles were systematically analyzed by N2 adsorption isotherm, elemental analyzer, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and energy-dispersive

spectrometry (EDS). Attempts were made to seek a relationship between char structure evolution and surface active sites. Furthermore, a direct correlation was made between CO2 chemisorption parameters with char gasification reactivity.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. A typical Chinese coal, HN, was used as the raw material, for different reactors and experimental techniques. The particle size fractions were between 106 and 150 μm. The total analyses of Huainan coal properties are given in Table 1. It can be seen that HN contains high volatiles and low fixed carbon and the ash fusion point appears at about 1250 °C. SiO2 and Al2O3 are the main species in the ash, being as high as 70%, whereas Fe2O3, CaO, MgO, Na2O, and K2O account for only less than 27% of all ash components. Chars were prepared under a wide range of experiment temperatures and by two different kinds of reactors that exhibited complete different heating rates. The slow chars were prepared in a conventional horizontal-tube furnace with a low heating rate and under an inert atmosphere of high-purity nitrogen (99.999%). In the runs, 1 ± 0.05 g of the coal particles was placed in a ceramic container located in the center of the quartz tube and the temperature of the bed was raised to 950, 1200, and 1400 °C, at a heating rate of 6 K/min and then kept isothermal for 20 min to complete pyrolysis. After that, the furnace was cooled under N2 protection before the solid char residues were taken out. The rapid chars were prepared in a small-scale drop-tube furnace, and specific preparation of the experimental procedure has been described in detail elsewhere.11 Briefly, the reactor was an alumina tube with an inner diameter of 50 mm and a length of 1500 mm and was heated using an external electrical furnace, which can provide a 1600 °C temperature in the center of the reactor. The top and end of the reactor were equipped with a star-type feeder and a char sample collector, respectively. In the runs, 500 g of coal samples 119

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was put into the hopper, and the reactor was heated to a prescribed temperature (950, 1200, and 1400 °C in each run) in high-purity nitrogen and atmospheric pressure. Then, coal samples were pneumatically transported into the reactor from the hopper by nitrogen carrier gas with a feeding rate of 5 g/min. As a normal droptube furnace, the coal samples was heated rapidly at a ratio well in excess of 103 °C/s, and the residence time was estimated below 2 s. In this work, the slow chars and rapid chars were called SC and RC, respectively; for example, RC950 means rapid char pyrolyzed under the temperature of 950 °C. Proximate and ultimate analysis data of the char samples are shown in Table 2. 2.2. Characterizations of Char Particles. The main organic components (C, H, O, N, and S) of the solid chars were analyzed using an elemental analyzer (VarioEL II CNHS/O, Germany). The surface alkali/alkali earth metal content of solid chars was investigated by an energy-dispersive X-ray spectrometer (Quanta 200, FEI, The Netherlands). During the tests, the samples were prepared to the finest thickness and then proper magnification times were selected to make sure that all of the samples were in the range of scanning. The crystalline components of solid chars were determined by XRD (X’Pert Pro, PANalytical B.V., The Netherlands; 40 mA; 40 kV; Cu Kα radiation, λ = 1.5406 × 10−10 m). The X-ray patterns were recorded in the scan range 2θ = 10−90°. The surface property (surface area, pore distribution, and volume) measurement of solid chars was fulfilled using an accelerated surface area porosimetry instrument (ASAP 2020, Micromeritics Instrument, Inc., Norcross, GA), following the Brunauer−Emmett−Teller (BET) method. It was detected using liquid N2, and the isothermal adsorption temperature was set at −196 °C. The surface chemistry structures of solid chars were provided by FTIR spectroscopy (VERTEX 70, Bruker, Germany). The samples were first powdered in an agate mortar and then mixed with KBr at an approximate ratio of 1:100 to prepare transparent wafers. The mixture of coal char and KBr powder was dried in an oven at 105 °C overnight. The pellets for FTIR spectroscopy were made under exactly the same conditions, including the sample weight, diameter of pellet, pelleting time, and pelleting pressure. The IR spectra were obtained using the spectrometer with a 4 cm−1 resolution and 32 scans between 4000 and 400 cm−1. The FTIR spectra were curve-fitted using Origin7.5/Peak Fitting Module. First, selected regions were baseline-linearized using an interactive procedure of the program by connecting the left and right points of the interval with a straight line. After the baseline adjustment, the selected zone was then deconvoluted into several Gaussian-type bands. During deconvolution, band positions were fixed (deviation in the central wavenumbers observed was ±1−10 cm−1 depending upon the kind of sample), while the bandwidths were restricted to certain maximum limits according to the bandwidths of possible structures. For each sample, the KBr disc samples were prepared in triplicate, and the peak areas obtained from the triplicate FTIR spectra were within ±4% error range. The mean value was applied in the study. 2.3. CO2 Chemisorption. CO2 chemisorption of char samples was carried out in a thermogravimetric apparatus following the experimental procedures reported by Molina et al.20 In each run, about 15 mg of the char sample was first heated in high-purity argon (99.999%) flow at 20 K/min to 850 °C for 10 min, to remove any oxygen that could have been adsorbed during char sample preparation. It was then cooled to 300 °C in argon flow, and the CO2 chemisorption experiment was initiated. CO2 chemisorption was conducted by reswitching argon to CO2 and soaking for 30 min, and the weight gain was analyzed as a function of the reaction time. Finally, CO2 was switched to argon, and the sample was outgassed at atmospheric pressure for 30 min to remove any weakly chemisorbed CO2 molecules. The flow rates of argon and CO2 for the above procedures were always 100 mL/min. A typical CO2 chemisorption curve as a function of time is shown in Figure 1, normalized with the net weight of the char after preheated at 850 °C in argon. According to Figure 1, two sets of chemisorption data were obtained in this experimental procedure. The first set was strong chemisorption of CO2 at 300 °C, which was still absorbed on the char surface after the

Figure 1. Typical CO2 chemisorption curve of SC950. experiment, denoted by Cstr. The second set was weak chemisorption, which desorbed from the char surface during the outgassing stage, denoted by Cwea. To avoid random error, measurement experiments were repeated 3 times at each condition. 2.4. Char Gasification. The gasification reactivity of the chars was measured by a thermogravimetric apparatus (NETZSCH STA 409C, Germany) using a non-isothermal method. A resulting char sample (HN SC950−1400 and RC950−1400) of 20.0 ± 0.5 mg for each run was placed and spread uniformly in a corundum crucible located in the thermal balance, and then high-purity nitrogen (99.999%) set as the carrier gas was purged into the reactor at a flow rate of 100 mL/min. After sweeping the furnace for about 5 min, the furnace was then heated to 950 °C from the ambient temperature at a heating rate of 50 K/min. Then, the gas stream was switched to CO2, keeping the flow rate constant and controlling the heating rate at 5, 10, and 20 K/min, until the temperature reached 1300 °C, and then kept isothermal to make sure that the sample was gasified completely. The activation energy of gasification was calculated by the Ozawa method21 using at least three different heating rates. The formula is as follows:

log β = − 0.4567E /(RTm) − 2.315 + log(AE /R ) − log G(xm) (1) where β is the heating rate of char gasification (K/min), E is the activation energy (kJ/mol), R is the universal gas constant (8.314 J mol−1 K−1), Tm is the absolute temperature corresponding to the maximum weight loss, G(x) equals ∫ x0 dx/g(x) [where dx/dt = A exp(−ΔE/RT)g(x)], and xm is the conversion of the convertible part of the sample corresponding to Tm. Generally, there exists a linear relationship between log β and 1/Tm; thus, the activation energy can be obtained from the slope of the line.

3. RESULTS AND DISCUSSION 3.1. Char Gasification Reactivity. As an example, the gasification profiles of char samples pyrolyzed under 950 °C in the above two different reactors are plotted in Figure 2 in DTG (wt %/min), in which three different heating rates 5, 10, and 20 K/min were considered. It can be seen from Figure 2 that the maximum weight loss rate DTGmax increased with an elevated heating rate during gasification, indicating higher reactivity at a faster heating rate. To compare the gasification characteristics of char samples prepared at different pyrolysis conditions, some typical parameters are chosen to represent gasification reactivity of char samples and listed in Figure 3. The general trends can be found that Tmax (which represent the temperature corresponding to DTGmax) of both kinds of chars increased with the pyrolysis temperature, while DTGmax had a declining trend (not obvious for RC). When chars generated at the same pyrolysis temperature are compared, Tmax of SC is higher than 120

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Figure 2. DTG curves of char gasification.

Figure 3. Typical parameters of char gasification: (a) Tmax and (b) DTGmax.

that of RC, which is in contrast to DTGmax evolution characteristics. Obviously, the higher Tmax means the more difficulty reacting at a certain temperature. DTGmax can somewhat also reflect the reactivity of char samples. The bigger the DTGmax, the faster the reaction.1 The variation of char gasification activation energy with the pyrolysis condition is presented in Figure 4. From Figure 4, we can see that the activation energy of both types of chars increased with the pyrolysis temperature, identical to the alternation of Tmax. For RC, the activation energies of chars pyrolyzed under elevated temperatures show little changes and vary between 212 and 243 kJ/mol. While for SC, the variation is more pronounced, from 192 to 310 kJ/mol. Zhu et al.15 investigated the variation in activation energy of the char prepared from a lignite over the temperature range of 500− 1400 °C with a heating rate of 10 K/min and found that the activation energies increased with the treatment temperatures, with the values falling in the range of 162−230 kJ/mol when temperatures similar to the current study were considered. They contributed to the increase of activation energies with the decrease of char reactivity. As we all know, the char sample with higher activation energy is generally less reactive at lower temperatures; therefore, the increase of activation energy indicated that reactions are apt to happen under higher temperatures, and consequently, a higher Tmax could be obtained. This reasonably explained why Tmax increased with the elevated pyrolysis temperature, as discussed above. In

Figure 4. Variations of gasification activation energy with the BET surface area of char samples.

addition, the relative remarkable variation of activation energy for SC was consistent with the big difference of Tmax shown in Figure 3. According to the above explanation, the results in Figures 3 and 4 indicated that an elevated pyrolysis temperature and a slow-pyrolysis heating rate made char reactivity decline and the 121

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Table 3. CO2 Chemisorption Data of Char Particles sample

RC950

RC1200

RC1400

SC950

SC1200

SC1400

Cstr (mg of CO2/g of char) Cwea (mg of CO2/g of char)

1.025 0.824

0.831 0.583

0.521 0.349

0.489 0.177

0.354 0.147

0.135 0.098

by Molina et al.,20 they stated that the Cwea value of different chars was almost keep constant; therefore, Cwea does not explain the difference in the reactivity between different chars, while Cstr seems to be a reasonable parameter in correlating with the reactivity. For the different phenomenon, a decrease of Cwea observed in this study may be ascribed to the fact that chars prepared under variation pyrolysis conditions were chosen to compare to the gasification reactivity in our work, while chars chosen in Molina’s study were all pyrolyzed in the same condition. What they focused on were just the different catalyst concentrations. Given that the Cwea value was related to the presence of organic matter (carbon active sites) of char, to observe whether char structuring could be a cause of this decrease of Cwea, here below elemental composition, XRD and FTIR analyses were taken to interpret this phenomenon. The C/H and C/O mole ratios of chars are shown in Table 2. It can be seen from the table that C/H and C/O mole ratios of coal chars increased with the elevated pyrolysis temperature, regardless of SC or RC. Moreover, at the same temperature, C/ H and C/O mole ratios of SC were always higher than that of RC. It indicated that a higher pyrolysis temperature and lower heating rate may improve the coal char C/H and C/O mole ratios, implying that a more ordered crystalline structure was created. This can be validated from XRD analysis of these char samples (not shown here), from which we found that the major peak (002) corresponding to the stacking height (Lc) of the crystalline structure and the other minor peak (100) corresponding to the radial spread (La) dimension in char crystallites became sharper and more intense with the increase of the pyrolysis temperature and residence time. The reason for this phenomenon is that a higher temperature and prolonged residence time caused by a slower heating rate enhance the devolatilization degree and a large part of basic radicals, i.e., −CH2, −OH, and −R−CH2, formed during the pyrolysis process are believed to be active and easy to vaporize as aliphatics, water, and tars (see eqs 3−5). In addition, at higher temperatures, polynuclear aromatic compounds still diffuse slowly and start to condense with the elimination of H2 (see eqs 6 and 7).1,15,17 The elements H and O contained in the volatiles were liberated out of char samples, resulting in relatively higher C/H and C/O mole ratios, and consequently, more graphitic structures were constructed.

activation energy increase with the treatment temperature. The reasons for this phenomenon will be discussed below in detail. 3.2. Correlation between Gasification Reactivity and Structure of Char. Previous extensive studies focused on the pore structure evolution during pyrolysis and concluded that pores in char samples with different diameters may have dissimilarly inhibition to the mass and heating transfer during the gasification process; thus, it played a significant role in char gasification reactivity.17,22 The variations of gasification activation energy with the BET surface area of char samples are listed in Figure 4. It seemed that there was no good relationship between the char surface area and gasification reactivity; that is, evolution of the char pore structure had not obvious direct correlation with char reactivity. Miura et al.18 stated in their study that the pore structure was not the dominant parameter determining the char gasification rate. On the other hand, most studies demonstrated that char gasification reactivity was determined by the active site concentration on the char surface, which is related to the active surface area (ASA).23,24 Then, the ASA was measured in this study based on chemisorption of CO2 at a temperature of around 300 °C, which is higher than the critical temperature of CO2 (31 °C). Therefore, all of the adsorption data are considered as chemisorption of CO2, definitely different from that of the specific surface area measured by physical adsorption of absorbates CO2 or N2. As mentioned in the typical CO2 chemisorption curve (Figure 1) shown above, there exist strong chemisorption (Cstr) and weak chemisorption (Cwea) of CO2 in this experimental procedure. Molina et al. 20 compared results of CO 2 chemisorption between chars prepared from raw coal, demineralized coal, and catalyst-loaded coal. For the demineralized coal, they found that the total CO2 chemisorption was similar to weak CO2 chemisorption; therefore, they came to a conclusion that Cstr and Cwea were related to the presence of the active inorganic components (catalytic active sites) and organic matter (carbon active sites) of the char, respectively. Generally, the polynuclear aromatic ring systems of char are inert during char conversion. The edge carbon atoms, carbon atoms bonded to heteroatoms, and nascent sites attached to the aromatic clusters are chemically unstable and thereby regarded as carbon active sites. Catalytic active sites represent catalytic activity of minerals (such as salts of K, Ca, Na, Mg, and Fe) in the char. The catalyst type, amount, and its dispersion decide the concentration of catalytic active sites. However, during pyrolysis, the evolution of the char structure, such as ordering of the carbon microstructure and decrease of the inorganic matter catalytic role, may dramatically affect the concentration of such active sites, and then, it is still indispensable to investigate the relationship between char structure evolution and the ultimate concentration of active sites on the surface of char. 3.2.1. Char Structure Evolution and Its Influence on Carbon Active Sites. The detailed chemisorption data of char samples in the current study are shown in Table 3. It can be seen that a higher pyrolysis temperature and slower heating rate induced a decrease of both Cstr and Cwea. However, in the work

cracking:

saturation:

R−CH 2−R′ → R−R′ + −CH 2

(2)

−CH 2 + 2H′ → CH4

(3)

−OH + H′ → H 2O

(4)

tar production:

−R−CH 2 + H′ → R−CH3

condensation reactions: R−H + H−R′ → R−R′ (coke) + H 2 R−OH + H−R′ → R−R′ (coke) + H 2O

(5)

(6) (7)

The values of Cwea are correlated with C/H and C/O mole ratios shown in Figure 5. It can be seen that, accompanying with the increase of C/H and C/O mole ratios, Cwea showed a 122

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cm−1), and C−H out-of-plane bending in olefinic and aromatic structures (≈795 and 550 cm−1). To understand changes in surface chemistry of chars prepared under different pyrolysis conditions, the relative proportions of functional groups in different chars must be compared in terms of the corresponding peak areas. However, the amounts of sample used in each analysis were not constant, and the intensities of FTIR signals may also flux, resulting in difficulty for evaluation. Therefore, researchers15,25,26 inferred ratios of the peak areas with respect to a reference peak to make a comparison. In this work, according to the advice by Sharma et al.,26 the band at 1590 cm−1 representing the aromatic nuclear −CC− stretching vibration was taken as the reference band and ratios of the peak areas of various functional groups relative to the reference band were calculated. When we accumulated ratios of the peak areas of these four main functional groups [O−H stretch vibrations, CH3 asymmetric and symmetric stretching vibrations, alkyl chain structure of CH2 and CH3 deformation vibrations, and ether-type structures (−O−) and C−O stretching vibrations], the resulting total peak area ratios under different pyrolysis conditions are correlated with Cwea, presented in Figure 7. As

Figure 5. Correlations between the CO2 chemisorption parameter Cwea of the chars and C/H and C/O mole ratios.

declining trend during the char samples. On the whole, for both RC and SC, a linear correlation could be found (except for the C/O mole ratio of RC). These showed that the growth of the char crystalline structure could cause a decline of Cwea, thus reducing carbon active sites of chars. FTIR analysis is a useful method for comparing qualitatively either vibrating absorption spectra of chars or relative intensities of the respective bands. Typical FTIR spectra of resulting char sample SC950 and raw coal are plotted in Figure 6. A correction was made to eliminate an increasingly upward

Figure 7. Correlations between the CO2 chemisorption parameter Cwea of the chars and accumulated total peak area ratios of four main functional groups.

we can see, the total peak area ratios of chars decreased as the final pyrolysis temperature and residence time increased, indicating that severe treatment conditions may cause a significant loss of functional groups. With the decrease of total peak area ratios, there is also a decrease of the CO2 chemisorption parameter Cwea. To some extent, we can see that an approximate linear correlation was found among RC and SC. This showed that the loss of functional groups ascribed to structure evolution also had a significant influence on carbon active sites. From the elemental composition, XRD, and FTIR analyses above, it can be concluded that a higher pyrolysis temperature and slower heating rate improved coal char C/H and C/O mole ratios, promoted the growth of the char crystalline structure and significant loss of functional groups, and hence, strongly reduced the carbon active sites. When structure evolution was compared to char gasification reactivity analyzed in section 3.1, it reasonably explained a decrease of char gasification reactivity by char chemical structure evolution to diminish the carbon active site concentration. Therefore, we

Figure 6. FTIR spectra of the raw coal and SC950.

drift in the baseline at high wave numbers. It can be seen from Figure 6 that the intensity of all bands present in the raw coal declined more or less after the pyrolysis process. Some of them even disappeared in the spectra of SC950. The functional groups still contained in the char samples are generally as follows: O−H stretch vibrations in hydroxyl groups (≈3430 cm−1), CH3 asymmetric and symmetric stretching vibrations (≈2920 and 2850 cm−1), aromatic nuclear −CC− stretching vibration in alcohol, phenol, and carboxylic acid (≈1590 cm−1), alkyl chain structure of CH2 and CH3 deformation vibrations (≈1434 cm−1), Si−O−Si and Si−O− vibration or ether-type structures (−O−) and C−O stretching vibrations (≈1050 123

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Table 4. Percentages of Elements in Samples Analyzed from EDS Results (wt %) sample

C

Si

Al

Na

K

Mg

Ca

Fe

total

alkali/alkali earth/Fe

RC950 RC1200 RC1400 SC950 SC1200 SC1400

92.41 93.73 90.21 91.76 96.01 97.70

2.17 2.41 4.60 3.67 1.78 0.92

1.87 2.38 4.27 3.35 1.44 0.90

0.14 0.29 0.17 0.22 0.03 0.05

0.11 0.07 0.14 0.19 0.06 0.07

0.08 0.11 0.15 0.10 0.14 0.07

2.66 0.44 0.22 0.29 0.28 0.21

0.57 0.57 0.26 0.42 0.25 0.07

100 100 100 100 100 100

3.56 1.48 0.94 1.22 0.76 0.47

Figure 8. Correlations between CO2 chemisorption parameters of the chars and char gasification activation energy: (a) char RC and (b) char SC. R = correlation coefficient.

around the ash fusion temperature of 1100 °C when he studied the gasification of low-melting coal at high temperatures. Thus, summarizing first, one main reason for the declining catalytic effect is caused by aggregation of mineral matter at high temperatures. Vaporization of inorganic matter under high temperatures may be another reason responsible for the decrease of active mineral matters. Zhu et al.15 found in their study that, when the pyrolysis temperature was higher than 1273 K, the char yield of the demineralized lignite only decreased slightly with the temperature increase, while that of the raw lignite decreased considerably. They interpreted this as vaporization of inorganic matter at an elevated temperature for the raw lignite. Third, carbon crystallite growth leading to a decrease in the amount of edge carbon atoms in char samples may have a side effect on the catalytic capability. This reduces the available functional groups for catalytic species bonding and then results in a poor dispersion of catalytic species.30 3.2.3. Correlations between CO2 Chemisorption Parameters and Char Gasification Reactivity. CO2 chemisorption parameters of the chars are correlated with gasification activation energy analyzed in section 3.1 in Figure 8. For a more comprehensive comparison, not only Cstr and Cwea but also Cstr + Cwea were chosen for analysis. It can be seen that, for both RC and SC, the gasification activation energy decreased linearly with the increase of CO2 chemisorption parameters: Cstr, Cwea, and Cstr + Cwea. Obviously, CO2 chemisorption parameters (Cstr caused by active inorganic components on char and Cwea caused by organic matter on char) were better than the BET surface area to correlate with char reactivity. Moreover, it is should be noted that Cstr and Cwea in Figure 8a are numerically close to each other when the same char was considered, confirming the almost equal importance of inorganic and organic matter effects on char RC in the current study.

can conclude that the growth of the char crystalline structure accompanied with the loss of functional groups is the main reason accounting for char deactivation. 3.2.2. Char Structure Evolution and Its Influence on Catalytic Active Sites. Table 3 also showed that a higher pyrolysis temperature and slower heating rate induced a decrease of Cstr. Because of Cstr related to the presence of active inorganic components of the char, the fact that Cstr of char samples declined with a higher pyrolysis temperature and slower heating rate seemed to have some relationship with the active mineral matters remaining in the char. It is well-known that K, Na, Ca, Mg, and Fe species in coals may exhibit catalytic activity for char gasification, whereas Si and Al generally act as retardants.27,28 The main metal element compositions of chars were analyzed with EDS, listed in Table 4. Generally, as we can see, Ca and Fe had a relatively higher concentration among the active mineral matters, exhibiting catalytic potential. Just like the evolution characteristics of Cstr, the results in Table 4 showed that the total amount of active mineral matters (alkali, alkali earth metal, and Fe) also declined with a higher pyrolysis temperature and slower heating rate. While the above behaviors are expected, it is clear known that alkali and alkaline earth metal compounds react with aluminaand silica-bearing minerals to form stable and water-insoluble aluminosilicates, such as kaliophilite and kalsilite (KAlSiO4), that exhibit no catalytic activity.29 This happens dramatically at severe pyrolysis conditions. Moreover, Solano et al.24 revealed that CaO, as a catalyst, rapidly sintered in CO2 at 800 °C, corresponded to a sharp decrease in the CaO surface area with the reaction time. When the pyrolysis temperature is high enough, exceeding the ash melting point, ash fusion should happen. An enhanced heating temperature could make the ashes adhere to each other to agglomerate into larger spherical granules. Wu et al.11 also observed ash fusion phenomenon 124

dx.doi.org/10.1021/ef301455x | Energy Fuels 2013, 27, 118−125

Energy & Fuels

Article

(15) Zhu, X. L.; Sheng, C. D. Energy Fuels 2010, 24, 152−159. (16) Kajitani, S.; Suzuki, N.; Ashizawa, M.; Hara, S. Fuel 2006, 85, 163−169. (17) Yu, J. L.; Lucas, J. A.; Wall, T. F. Prog. Energy Combust. 2007, 33, 135−170. (18) Miura, K.; Hashimoto, K.; Silveston, P. L. Fuel 1989, 68, 1461− 1475. (19) Meijer, R.; Weeda, M.; Kapteijn, F.; Moulijn, J. A. Carbon 1991, 29, 929−941. (20) Molina, A.; Montoya, A.; Mondragón, F. Fuel 1999, 78, 971− 977. (21) Ozawa, T. J. Therm. Anal. Calorim. 1970, 2, 301−324. (22) Hurt, R. H.; Sarofim, A. F.; Longwell, J. P. Fuel 1991, 70, 1079− 1082. (23) Zhang, Y.; Ashizawa, M.; Kajitani, S.; Miura, K. Fuel 2008, 87, 475−481. (24) Linares-Solano, A.; Almela-Alarcón, M.; Salinas-Martínez De Lecea, C. J. Catal. 1990, 125, 401−410. (25) Ibarra, J.; Moliner, R.; Bonet, A. J. Fuel 1994, 73, 918−924. (26) Sharma, R. K.; Wooten, J. B.; Baliga, V. L.; Lin, X.; Geoffrey Chan, W.; Hajaligol, M. R. Fuel 2004, 83, 1469−1482. (27) Zhang, Y.; Ashizawa, M.; Kajitani, S. Fuel 2008, 87, 475−481. (28) Bai, J.; Li, W.; Li, C. Z.; Bai, Z. Q.; Li, B. Q. J. Fuel Chem. Technol. 2009, 37, 134−138. (29) Wood, B. J.; Sanciera, K. M. Catal. Rev. 1984, 26, 233−279. (30) Zhang, S. Y.; Lu, J. F.; Zhang, J. S.; Yue, G. X. Energy Fuels 2008, 22, 3213−3221.

4. CONCLUSION Char particles of HN prepared at different temperatures were systematically analyzed with multi-techniques to study their physical−chemical structure and active site characteristics. The follow conclusions were drawn from the present work: The char pore structure determined by physical adsorption of absorbate N2 had no obvious direct correlation with char reactivity. Strong chemisorption (Cstr) and weak chemisorption (Cwea) of CO2, which relate to the presence of active inorganic components and organic matter of char, respectively, existed in the CO2 chemisorption procedure. A higher pyrolysis temperature and slower heating rate induced a decrease of both Cstr and Cwea. This is because a higher pyrolysis temperature and slower heating rate improved coal char C/H and C/O mole ratios, promoted the growth of the char crystalline structure and significant loss of functional groups, reduced the active mineral matters in the char, and consequently, resulted in a decrease of char surface active sites. For both RC and SC, the gasification activation energy decreased linearly with the increase of CO2 chemisorption parameters: Cstr, Cwea, and Cstr + Cwea. Moreover, CO2 chemisorption parameters were better than the BET surface area to correlate with char reactivity.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 86-27-87542417, ext. 8209. Fax: 86-27-87545526. E-mail: [email protected] (S.H.); susheng_2003@ 163.com (S.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the National Natural Science Foundation of China (21176098, 51176062, and 51021065) and the National Basic Research Program of China (973 Program, 2010CB227003). The authors also express their thanks to the Analytical and Testing Center of the Huazhong University of Science and Technology for help with the measurements of the samples.

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