Article pubs.acs.org/EF
Thermal Behavior and Char Structure Evolution of Bituminous Coal Blends with Edible Fungi Residue during Co-Pyrolysis Zhiqiang Wu, Shuzhong Wang,* Jun Zhao, Lin Chen, and Haiyu Meng Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic of China S Supporting Information *
ABSTRACT: Co-pyrolysis of coal and lignocellulosic biomass has the potential to mitigate the emission of greenhouse gases from an energy supply. Successful application of this technology requires proper investigation on the influence of coal and lignocellulosic biomass mixing on thermal behavior and product characteristics. Therefore, in this study, thermal behavior of a kind of Chinese bituminous coal blended with edible fungi residue (EFR) was evaluated through nonisothermal thermogravimertic analysis. Raman spectroscopy and scanning electron microscopy with energy dispersive spectroscopy techniques were applied to determine the char structure evolution. The results revealed that the EFR promoted thermal decomposition of the bituminous coal and synergy effect on char yield was observed. The activation energy distribution calculated via an isoconversional method showed nonadditivity performance, which may be caused by the catalytic effects of alkali and alkaline earth metals and the char structure evolution. The Raman spectrum results indicated that the Raman intensity of the co-pyrolysis char increased with the EFR ratio, which can be due to the combined effect of the O-containing groups and nonproportional effects of alkali and alkaline earth metallic species. The area ratio of the G (graphite) band to all the bands (AG/ Aall) and that of the valley between D (disordered) and G bands to the D band (AGR/AD and A(GR+VL+VR)/AD) had been found useful in evaluating the evolution of the char structure. An increase in AG/Aall seemed to suggest the increasing aromatization of the chars. The increase in AGR/AD and A(GR+VL+VR)/AD implied the generation of more smaller (3−5 rings) aromatic ring structures and the elimination of lager (no less than 6 rings) aromatic ring systems in the char samples as the EFR ratio increasing.
1. INTRODUCTION Co-thermochemical conversion of coal and lignocellulosic biomass for energy generation is an attractive solution for feedstock diversity and climate change mitigation. It also has great potential for the utilization of coal in an environmentally friendly way and for the exploitation of lignocellulosic biomass on a commercial scale.1−3 Four typical technologies are developed for the co-thermochemical conversion of coal and lignocellulosic biomass, i.e., co-pyrolysis, co-combustion, cogasification and co-liquefaction. As a promising technology for clean energy solution, co-pyrolysis can produce more fuels and chemicals than other co-thermochemical technologies, which are widely used in different industrial applications, such as power generation and chemical synthesis.4−7 Besides, copyrolysis of coal and lignocellulosic biomass significantly affects the quality, availability and composition of the components of reactions during co-combustion and co-gasification.7−10 Therefore, it is essential to investigate the co-pyrolysis behavior of coal and lignocellulosic biomass, which can improve the performance of co-thermochemical conversion technologies. As a prerequisite for predicting co-pyrolysis performance, thermal behavior is one of the most important co-pyrolysis characteristics. Much attention has been devoted to the thermal behavior of co-pyrolysis of different rank coals blend with a variety of lignocellulosic biomass, such as rice straw,1,2 switch grass,8 hazelnut shell,10 corn and sugarcane residues3 from agricultural residues, pine sawdust7 and pine chips11 from forestry residues. However, to our best of knowledge, there is © 2014 American Chemical Society
no report on thermal behavior of co-pyrolysis of coal blends with edible fungi residue (EFR), which is a lignocellulosic biomass with remarkable calorific value and high yield in China. Approximately 6.5 million tons of EFR is generated after edible fungi harvests in China annually.12 Generally, the heating value of EFR is not less than 10 MJ·kg−1, which is comparable with the heating value of coal.12 The volatile content in EFR is significantly greater than that in coal.12,13 Besides, co-pyrolysis of coal and EFR is not limited by seasonal availability, which is a big obstacle for other lignocellulosic biomass to further applying co-pyrolysis into industries. To explore the copyrolysis of coal blends with EFR as a valuable and alternative energy source, there is an unmet need to study thermal behaviors during co-pyrolysis of EFR and coal. To optimize the efficiency of co-pyrolysis, much attention has also been focused on another co-pyrolysis characteristic, i.e., the evolution and yield of products. Previous studies have been devoted to the evolution and the yield of gaseous and liquid products, as these products can be easily transported and have wide applications in industries.4,7,8 However, less attention has been focused on the characteristics or structure evolution of solid products, i.e., co-pyrolysis char. Only a few researchers have reported that the addition of lignocellulosic biomass improved the physical characteristics (e.g., pore structure and Received: November 15, 2013 Revised: February 28, 2014 Published: February 28, 2014 1792
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2.2. Experimental Methods. 2.2.1. Pyrolysis Experiment. To evaluate the pyrolytic behavior of the raw samples, a type of WCT-2C thermogravimetric analyzer (TGA) manufactured by Beijing Optical Instrument Factory was employed. In each pyrolysis experiment, approximately 10 mg of the raw samples was loaded into an Al2O3 ceramic crucible and heated from ambient temperature to 1223 K with various heating rates (10, 20, 40 K·min−1). On the basis of previous literature,1,3 a N2 flow rate of 60 mL·min−1 was chosen in this study to avoid the influence on the weight measurement and heat transfer limitation. Each raw sample and mixture was tested three times within the experimental error of less than ±3% in weight loss measurement. McDonald et al.19 and Asadullah et al.20 reported that the low heating rates (≤373 K·min−1) have little influence on char structure evolution. Thus, in this study, the char samples formed under 20 K·min−1 during this process were collected for further analysis. For the sake of quantifying the performance of volatile matters release, the devolatilization index (Di)21 was defined:
specific surface area) of co-pyrolysis char, which promoted the conversation of the co-pyrolysis char.2,14 Furthermore, another important characteristics of co-pyrolysis char, i.e., evolution of chemical structure, remains unclear. Previous studies have found that the chemical characteristics of char made from coal or lignocellulosic biomass individually have significant influence on the product’s distribution and reactivity of the char in the subsequent processes of gasification or combustion.14,20−23 Nevertheless, during co-pyrolysis of coal and lignocellulosic biomass, evolution of a co-pyrolysis char structure will be more complicated than that of individual coal or lignocellulosic biomass char due to the effects of volatiles−chars interactions.15 In addition, conversion of the co-pyrolysis char is also a ratedetermining step that may control the overall co-combustion or co-gasification processes.9,15−18 Therefore, it is vital to indicate the co-pyrolysis char structure evolution for obtaining a further understanding of the effect of EFR addition on the structure variation of co-pyrolysis char and promote the process efficiency. The aim of this paper was to examine the thermal behavior and char structure evolution during co-pyrolysis of lignocellulosic biomass and coal. Thermal behavior was evaluated using a thermogravimetric analyzer. Activation energy was calculated through the Flynn−Wall−Ozawa (FWO) and the Kissinger− Akahira−Sunose (KAS) methods. The structural features of the char samples were determined by Raman spectroscopy and scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS) analyses. The effects of the EFR ratio on the char yield and char structure evolution were investigated. This study provided a better understanding of the thermal behavior between coal and EFR during co-pyrolysis and also, the co-pyrolysis char structure evolution on a structural level.
Di = R max /TinTmax ΔT1/2
where Rmax is the maximum decomposition rate, Tin is the initial devolatilization temperature, Tmax is the maximum mass loss temperature. Tin and Tmax are obtained from the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves according to previous references.22,23 ΔT1/2 is the temperature interval when Rd/ Rmax equals 1/2. Rd is the decomposition rate, which is defined as follows:
R d = − dmt /dt
2.1. Materials. The EFR was collected from an edible fungi (Flammulina velutipes) production base in Southern Guangzhou, China. The main compositions of the EFR were straw-based substrate and casing layer used during edible fungi production. The air-dried EFR and bituminous coal were milled and sieved to particles with size less than 74 μm. Mixtures with the EFR mass proportion of 25%, 50% and 75% were produced. The proximate and ultimate analyses for the EFR and coal are listed in Table 1. The volatile content of EFR was twice more than that of the coal, which was consistent with the result published.13
Table 1. Proximate and Ultimate Analyses of the Raw Samples coal
(2)
where mt is the mass of the raw sample at time t. Rmax and Rd can be obtained from the derivative thermogravimetric (DTG) curves. 2.2.2. Char Characterization. For proper understanding the characteristics of co-pyrolysis char, crystalline and morphology structures of the char samples were examined by Raman spectroscopy and SEM with EDS, respectively. Raman spectroscopy has been widely employed to investigate the microcrystalline structure of carbonous materials. The spectral data of the char from Raman spectroscopy can be used to analyze the size and transformation of aromatic rings in the char samples.16,18 The LabRAM HR800 Raman microspectroscopy system installed with an excitation laser of 514 nm was applied to investigate the crystalline structure changes of the char samples. The laser power was 20 mW, and the spectra were from 800 to 2000 cm−1. Each char sample was smeared on an object slide and was analyzed on a random fraction for three times. The spectra obtained from the firstorder Raman spectra was curve-fitted using the LabSpec 5.0 spectroscopy software with 10 Gaussian bands.24 The averaged spectrum of each char sample was used to derive spectral parameters with standard deviations. Surface morphology and elementary composition of char samples were obtained using a JEOL JSM-6390 SEM with EDS. The accelerating voltage was 15 kV and the applied current was 20 nA. 2.3. Kinetics Model. Typical kinetic analysis of the thermal decomposition of carbonous materials under nonisothermal conditions is usually written as follows:
2. MATERIALS AND METHODS
proximate analysis (wt %, ad) moisture, M 4.18 ash, A 15.38 volatile, V 30.56 fixed carbon, FC 49.88 ultimate analysis (wt %, daf) carbon, C 79.31 hydrogen, H 4.72 nitrogen, N 1.03 sulfur, St 1.3 oxygen, Oc 13.38 high-heating value (MJ·kg−1, ad) 25.44
(1)
EFR
⎛ E ⎞ dα A ⎟ = f (α)exp⎜− ⎝ RT ⎠ dT β
11.03 12.44 60.37 16.16
(3)
where α is the conversion degree of the raw sample, T is the temperature, A is the pre-exponential factor, β is the heating rate, f(α) is the reaction model, E is the activation energy and R is the universal gas constant. The conversation α and heating rate β can be obtained by the following:
51.47 2.87 1.69 0.03 43.93 16.71
α=
(m0 − mt ) (m0 − m∞)
(4)
where m0 and m∞ are the initial and final mass of the raw sample.
ad, air-dried; daf, dry ash-free; t, total content; c, calculated by difference.
β= 1793
dT dt
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It is well-known that the isoconversional approach can easily estimate the value of E without considering the reaction model or f(α). Isoconversional methods only require a group of experimental data from multiple heating rates, and deal the calculation of E as a function of conversation or α. Therefore, these methods have been widely employed in recent research. And in this paper, the Kissinger− Akahira−Sunose (KAS)25 and Flynn−Wall−Ozawa (FWO)23,26 methods were employed to obtain the activation energy for their relatively higher dependability. The integral expression of eq 3 is shown as follows: G(α) =
∫0
α
⎛ E ⎞ A T ⎟dT exp⎜− ⎝ RT ⎠ β Tin AE u − e−u = du βR ∞ u 2 u − AE ∞ e = du βR u u 2 AE = · P(u) βR =
Figure 1. Pyrolysis characteristics of the coal, EFR and their mixtures in TGA with the β = 20 K·min−1: (a) TG curves and (b) DTG curves.
dα f (α)
∫
agaric peaks were obtained between 993 and 1024 K. Generally, decomposition of the coal began around 643 K and had a continuous mass loss. Several characteristic parameters of the pyrolysis process with the β of 20 K·min−1 are listed in Table 2. The initial devolatilization temperature (Tin) of the coal in Table 2 was higher than that of the EFR and the mixtures. This was mainly due to the relatively weak ether bonds linked the macromolecular structure (primarily cellulose, hemicellulose and lignin) of lignocellulosic biomass, and the bond energy was 380−420 kJ·mol−1. In contrast, the molecular structure (primarily polyaromatic hydrocarbon) of coal was linked by CC aromatic bonds and the bond energy was about 1000 kJ· mol−1, which were more resistive to the heat.7,11 Except for the moisture removal, the peak of the bituminous coal’s DTG curve was obtained at 734 K with an intensity of 1.39 mg·min−1. Two peaks of the of EFR’s DTG curve were observed with the intensities of 4.37 mg·min−1 and 0.89 mg·min−1, at 619 and 1021 K, respectively. Addition of the EFR into coal with a proportion of 25% made the intensity of the first peak decrease to 1.25 mg·min−1 at 605 K. When the proportion of the EFR was 50% and 75%, the intensity of the first peak increased to 1.93 mg·min−1 at 622 K and 3.78 mg·min−1 at 621 K, respectively. These peaks can be related to thermal decomposition of the hemicelluloses, cellulose and lignin.28 A certain extent of variation in the intensity of the first peak can be detected for the mixtures, but the variation tendency was not significant. Therefore, it can not lead to the conclusion just from the peaks of DTG curves that the interaction did exist during the copyrolysis process. Furthermore, the devolatilization index (Di) was put forward to evaluate the performance of volatile matters release. It can be found from eq 1 that the higher Di meant an easier releasing of the volatile. The Di of the raw samples were also listed in Table 2. Table 2 indicated that Di of the coal was 1.80, which agreed with the results of other researchers.21,29 While Di of the mixtures with 25%, 50% and 75% EFR increased to 2.87, 4.93 and 9.66, respectively. The Di of mixtures in Table 2 increased with mass ratio of the EFR, indicating the addition of EFR may promote the release of the volatiles. These may be due to the high content of volatile matter in EFR, which affect the Di of the blends. Given the effect of EFR addition on the pyrolysis degree of coal, the dimensionless devolatilization index (D) was defined to quantify the pyrolysis degree of the mixture.
∫ ∫
T = Tin + βt
(6) (7)
where u = E/RT. P(u) is the temperature integral and without analytical solution. With appropriate assumption and simplification, the rational approximation of P(u) are proposed with these methods applied in this paper. For the KAS method, P(u) can be approximated with integrating by parts and assuming u ≫ 1 as follows:25 P(u) =
e −u u2
(8)
By taking the logarithm of eq 6 and using eq 8, the KAS equation can be obtained:
⎛ β⎞ ⎡ AR ⎤ E ln⎜ 2 ⎟ = ln⎢ ⎥− ⎣ EG(α) ⎦ RTα ⎝ Tα ⎠
(9)
where Tα is the temperature when a raw sample under conversion degree of α. According to eq 9, the plot of ln (β/Tα2) versus 1/Tα under a given conversion (α) should be a straight line. Thus, the value of E is calculated from the slope of straight line. As for the FWO method, P(u) is approximated as follows:28 P(u) = 0.0048e−1.0516u
(10)
By combining eqs 6 and 10, the FWO method can be derived:
⎡ AE ⎤ E ln β = ln⎢0.0048 ⎥ − 1.0516 ⎣ RG(α) ⎦ RTα
(11)
Similarly, according to eq 11, the plot of lnβ versus 1/Tα under a constant α at several β can be obtained by linear regression, and then the E at various α could be determined.
3. RESULTS AND DISCUSSION 3.1. Pyrolysis Characteristics. Figure 1 illustrates the TG and DTG curves of coal, EFR and their mixtures under 20 K· min−1. It can be seen from Figure 1 that the evolutions of the TG and DTG curves of the raw samples were similar except those of the coal. The pyrolysis process of the mixtures and EFR can be divided into three phases without considering the evaporation of moisture. The first phase was from 473 to 673 K with the loss of light volatile compounds.14,27 Most of the volatile matter was released in the second phase from the end of the first phase to 873 K. The first two phases depended much on mass ratio of the EFR. The temperature range for the third phase was from 873 to 1223 K, in which more visible
D=
(Di − Di0) (Di1 − Di0)
(12)
where Di is the devolatilization index of the mixture, Di0 is the devolatilization index without EFR, Di1 is the devolatilization index of the EFR. The results of D under various heating rates 1794
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Table 2. Pyrolysis Parameters of the Raw Samples with β = 20 K·min−1 Tin (K) T1 (K) R1 (mg·min−1) T2 (K) R2 (mg·min−1) T3 (K) R3 (mg·min−1) Tmax (K) Rmax (mg·min−1) ΔT1/2(K) Di (10−8mg·min−1·K−3)
parameters
coal
25% EFR
50% EFR
75% EFR
EFR
initial devolatilization temperature temperature of the first DTG peak decomposition rate of the first DTG peak temperatures of the second DTG peak decomposition rate of the second DTG peak temperatures of the third DTG peak decomposition rate of the third DTG peak temperature of maximum decomposition rate maximum decomposition rate temperature interval when Rd/Rmax = 1/2 devolatilization index of the sample
643 734 1.39
500 605 1.25 724 0.95 1024 0.69 605 1.25 144 2.87
524 622 1.93 720 0.87 993 0.48 622 1.93 120 4.93
548 621 3.78
543 619 4.37
1013 0.63 621 3.78 115 9.66
1021 0.89 619 4.37 118 11.02
734 1.39 164 1.80
were also shown in Figure S1 (Supporting Information). Figure S1 (Supporting Information) illustrates that D of the mixtures was not linear with the EFR ratio, which indicated that the addition of EFR affected the D of mixtures nonlinearly. The improved pyrolysis behaviors of mixtures maybe due to the synergistic effects. In addition, the synergy effects during the co-pyrolysis process of the bituminous coal and EFR can be evaluated from the deviation of experimental and calculated values of the TG curves. The ΔW was defined to investigate the degree of the synergistic effects on the thermal decomposition: ΔW = Wexperimental − Wcalculated
(13)
where ΔW is the relative mass loss deviation (%), which represented the synergistic effects degree. Wexperimental is the experimental value from the TG curve of the mixture. Wcalculated is calculated as the sum of the TG curves of each individual component, which can be obtained by the following: Wcalculated = X CWC + XEWE
(14)
where XC and XE are the mass fraction of the coal and EFR in the mixture, and WC and WE are the weight losses from the TG curves of coal and EFR under the same condition of the mixture individually. The variation trends of Wexperimental and Wcalculated with various EFR ratios when β equals 20 K·min−1 were presented in Figure 2a−c, respectively. The Wexperimental was lower than the Wcalculated at all EFR ratios, indicating that the synergistic effect between coal and EFR may exist, and this effect accelerated the thermal decomposition. The disparity between the experimental and calculated values can be explained as the pyrolysis process progressed. As shown in Figure 2d, the deviation between the experimental and calculated TG curves was greater than −3% with the EFR ratio of 25%. While with the EFR ratios were 50% and 75%, the ΔW was less than −3% when the temperature was above 623 and 798 K, respectively. In addition, all the deviation curves of the Wexperimental and Wcalculated values showed a slight decrease at temperature between 948 and 1023 K and then kept increasing until the final pyrolysis temperature. Before a pyrolysis temperature of 948 K was achieved, the EFR in the mixture went through the first two stages of thermal degradation and left some residues, which may stay in the gap between coal and EFR particle and gather on the coal surface. Part of the residues may transform into carbonaceous deposits through several polymerization and condensation reactions and plug the pores of coal. With the temperature increasing to about 948 K, the third stage decomposition of EFR occurred. The DTG curve of EFR showed visible agaric peaks during the aforementioned
Figure 2. TG curves comparison between the experimental and calculated value from the mixtures: (a) 25% EFR with β = 20 K·min−1, (b) 50% EFR with β = 20 K·min−1, (c) 75% EFR with β = 20 K·min−1; (d) ΔW versus with β = 20 K·min−1, (e) 50% EFR with β = 10 K· min−1, (f) 50% EFR with β = 40 K·min−1.
temperature range (948−1023 K), as shown in Figure 1b. The releasing of volatile matters generated from EFR may be hampered as the gap was blocked. However, with the rising of the temperature, the inner pressure of the particle increased and the residues of the EFR may flow, which was beneficial to the diffusion of volatile. The present results were consistent with the results achieved by other researchers.2,22 Figure 2 compared the trends of the experimental and calculated mass loss versus temperature under the EFR ratio of 50% and β of 10 and 40 K·min−1. As can be seen from Figure 2b,e,f, the Wexperimental was lower than the Wcalculated under different heating rates, which meant that the synergistic effect between coal and EFR still existed. In addition, the heating rates affected the peak value of DTG curves. It can be seen from Figure S2 (Supporting Information) that the Rmax increased with increased heating rates. This may be due to 1795
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the resistance to mass transfer inside the coal and EFR particles under low heating rates. The driving force of mass transfer in the particles may be enhanced with increased heating rates, then overcame these resistances and led to a higher decomposition rate.3,6 Remarkable synergy was found in the char yields under the 50% EFR, which was much lower (i.e., 9%) than the calculated values based on individual raw samples. Other authors also found the similar synergistic effect on char yields during copyrolysis of the coal and lignocellulosic biomass blends.10,30 However, the exact mechanism of the synergetic effect between coal and lignocellulosic biomass during co-pyrolysis was still not very clear.30 It was reported that the hydrogen radicals from lignocellulosic biomass, aliphatic fraction from coal and inorganic matter from both coal and biomass chars may cause the increasing of volatiles or synergy.7,9,31−33 The analysis of Raman spectra (see section 3.3) for co-pyrolysis char has shown that production of smaller aromatic ring structures became more dominant with increased EFR ratios. This phenomenon indicated that the cracking of the aromatic ring structure of coal was promoted and a lot of −CH2−, CH3−,− OH and −O− radicals groups were formed. These radicals can recombine to form volatiles or char through polymerization. Furthermore, extra hydrogen radicals during the pyrolysis significantly promoted the generation of tars and hydrocarbons.9 The hydrogen radicals from EFR during this process preferred the formation of volatiles instead of char by preventing the repolymerisation and cross-linking reaction, which resulted in a decrease in the char yield.7,8,17 On the other hand, the alkali and alkaline earth metallic species (AAEM) catalyzed the demethoxylation reactions. This resulted in an increase of the methoxyphenols that would form char through repolymerise reaction under usual conditions.6 However, the methoxyphenols were thought to occur secondary reactions which produced volatiles instead through detecting the aliphatic compounds from the volatile components of lignocellulosic biomass and coal blends.3,6 Thus, the formation of char was inhibited, while the generation of volatile material was more favored. Moreover, other researchers reported that the aliphatic fraction of coal was responsible for the synergy in the copyrolysis reactions of coal and biomass.5,33 The bituminous coal samples were rich in aliphatic segments, which could be contributed to the synergy effect in this paper. 3.2. Kinetics Analysis. The values of E under β of 10, 20 and 40 K·min−1 calculated from eqs 9 and 11 according to the FWO and KAS methods together with the correlation coefficient are presented in Table 3. As can be seen in Table 3 and Figure 3, the correlation coefficients R2 of the parameters calculated with the selected value of α (0.20 ≤ α ≤ 0.80) were within a narrow interval, from 0.9090 to 0.9985, and most of them were larger than 0.9600. It meant that the data was fitted with satisfaction. The deviations of the E values between the FWO and KAS methods were within 5% in most cases, which properly indicated that the results were acceptable and reasonable. Table 3 presented that the value of E was not constant at different α from these two methods. The E of coal and EFR decreased with increasing α, which was consistent with results obtained previous research.23 The average E values of the mixtures were 124.94, 117.67 and 184.50 kJ·mol−1 for 25% EFR, 50% EFR and 75% EFR calculated by the FWO method, and 118.46, 111.70 and 182.59 kJ·mol−1 calculated by the KAS method. The average E value reach the minimum (117.67 kJ·mol−1 by the FWO method and 111.70 kJ·mol−1 by
Table 3. Kinetics Parameters of the Raw Samples Calculated by FWO and KAS Methods FWO method materials coal
25% EFR
50% EFR
75% EFR
EFR
KAS method
α
E (kJ·mol−1)
R2
E (kJ·mol−1)
R2
0.20 0.30 0.40 0.50 0.60 0.70 0.80 average 0.20 0.30 0.40 0.50 0.60 0.70 0.80 average 0.20 0.30 0.40 0.50 0.60 0.70 0.80 average 0.20 0.30 0.40 0.50 0.60 0.70 0.80 average 0.20 0.30 0.40 0.50 0.60 0.70 0.80 average
378.18 338.24 264.77 266.03 251.25 176.00 224.91 271.35 126.41 137.31 125.15 93.76 116.02 133.09 142.82 124.94 93.62 120.27 129.29 142.76 146.74 86.78 104.25 117.67 199.92 183.55 208.41 250.51 193.75 102.69 152.68 184.50 167.28 205.71 190.07 247.25 104.89 92.08 74.03 154.47
0.9138 0.9488 0.9990 0.9806 0.9314 0.9533 0.9712
386.04 343.32 265.50 266.16 249.67 169.20 219.42 271.34 122.97 133.69 119.86 85.90 108.43 124.96 133.41 118.46 88.88 116.23 125.28 138.71 141.80 77.22 93.78 111.70 200.84 183.12 209.11 252.75 192.14 95.00 145.19 182.59 166.23 206.18 189.39 190.45 67.83 64.12 63.69 135.41
0.9090 0.9452 0.9989 0.9893 0.9237 0.9443 0.9666
0.9950 0.9955 0.9814 0.9771 0.9990 0.9978 0.9956 0.9824 0.9561 0.9763 0.9938 0.9723 0.9981 0.9961 0.9694 0.9915 0.9955 0.9948 0.9810 0.9985 0.9928 0.9912 0.9597 0.9611 0.9668 0.9942 0.9718 0.9815
0.9943 0.9949 0.9779 0.9706 0.9989 0.9973 0.9946 0.9782 0.9482 0.9721 0.9928 0.9672 0.9976 0.9943 0.9915 0.9872 0.9951 0.9830 0.9786 0.9980 0.9914 0.9901 0.9558 0.9658 0.9639 0.9968 0.9805 0.9714
the KAS method) under 50% EFR. In addition, pyrolysis characteristics of the mixtures showed that the synergy in char yields was more remarkable under 50% EFR. Therefore, 50% EFR may be an optimal mass ratio for the EFR during the copyrolysis. As shown in Figure 4, the average E values of the mixture first decreased and then increased slightly with the increasing of the EFR ratio. The dotted lines in Figure 4 were the predicted value of the average E based on the weighted average of the coal and EFR. The average E values of the mixture pyrolysis were less than the predicted value which can be calculated by weighed average method according to the E values of individual samples and mass ratio except the 75% EFR. A nonadditivity performance of the average E values of the mixture was observed, which may due to the interaction of the coal with EFR. The lower E value of the biomass may be due to the higher content of K in the ash, which increases the reaction rate 1796
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rings).15,16,18,35 As indicated in Figure 5, the Raman spectra of the co-pyrolysis char at 75% EFR prepared with β of 20 K·
Figure 5. Raman spectra fitted with 10 bands (75% EFR char prepared at β = 20 K·min−1). Gray line represents the raw spectra, black line represents the fitted spectra and short dot dash lines represent ten typical bands described in Table 4. Figure 3. Linear correlation between lnβ − 1/T and ln (β/T2) − 1/T with various α values: (a) lnβ versus 1/T of the coal, (b) ln (β/T2) versus 1/T of the coal, (c) lnβ versus 1/T of the 25% EFR and (d) ln (β/T2) versus 1/T of the 25% EFR (solid lines represent linear fitting corresponding to various α values).
min−1 was deconvoluted into 10 Gaussian bands. The height or area ratio between different bands was employed to characterize the char structure evolution.17,35,36 Thus, two parameters were investigated in this paper to analyze the structure of char samples as follows: (1) AG/Aall, the area ratio of the G band to all the bands, and this parameter was used to describe the aromatization degree of the char samples;16 (2) AGR/AD and A(GR+VL+VR)/AD, area ratio of the valley between the D (disordered) and G bands to the D band, and these band area ratios represented the contribution of smaller (mainly 3−5 fused rings) to the larger (mainly no less than 6 fused rings) aromatics in the char samples.18 The Raman spectra of the char samples were compared in Figure 6. All of the char samples showed two broad peaks at ∼1300 and ∼1590 cm−1, as shown in Figure 6. The 75% EFR char had the maximum intensity of the G band, while the EFR char had the minimum of that. For the 75% EFR char, the graphitization degree of the char sample was higher than those of the 25% EFR char and 50% EFR char. The increasing intensity of the D and G bands indicated that the aromatization extent increased with the EFR ratio during the co-pyrolysis. The Raman scattering ability, the light absorptivity and oxygencontaining functional groups of char have significant effects on the Raman intensity.18,20 The oxygen-containing structures had a high Raman scattering ability, which had a positive effect on the increments of the Raman intensity.18 In this research, the oxygen content of EFR was 43.93 wt %, whereas that of coal was 13.38 wt %. As the EFR ratio increased, the abundance of oxygen in the mixture increased. This phenomenon may explain the reason for the increasing intensity of the G and D
Figure 4. Variation of the obtained average values of E against EFR ratio.
by decreasing the activation energy via the catalytic effects on pyrolysis.3 3.3. Char Characteristics. 3.3.1. Crystalline Structure of the Char Samples. The Summary of typical bands distribution of the char samples is shown in Table 4. Just like most of carbonaceous materials, coal and biomass char showed two typical characteristics peaks appearing at ∼1300 and ∼1590 cm−1, which correspond with the D band (disordered/defect) and G band (graphite), respectively.19,34,35 The overlap between the G and D bands can be described as VR (∼1380 cm−1), VL (∼1465 cm−1) and GR (∼1540 cm−1) bands, which mainly represented small aromatic rings systems (3−5
Table 4. Summary of Typical Bands Distributions of the Char Samples15,16,37 band name
band position (cm−1)
GL G GR VL VR D SL S
1700 1590 1540 1465 1380 1300 1230 1185
SR R
1060 960−800
bond type
description carbonyl group CO mainly quadrant breathing of aromatic ring, graphite E2g2, alkene CC small aromatics rings with 3−5 rings, amorphous carbon structures methylene or methyl group, aromatic rings semicircle breathing, amorphous carbon structures methyl group, aromatic rings semicircle breathing, amorphous carbon structures mainly CC between aromatic rings and aromatics with not less than 6 rings mainly aryl−alkyl ether, para-aromatics mainly links CaromaticCalkyl, aromatic (aliphatic) ethers, CC on hydroaromatic rings, hexagonal diamond carbon sp3, CH on aromatic rings CH on aromatic rings, benzene ring mainly CC on alkanes and cyclic alkanes, CH on aromatic rings 1797
sp2 sp2 sp2 sp2, sp2, sp2 sp2, sp2,
sp3 sp3 sp3 sp3
sp2 sp2, sp3
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Figure 8. Variation of the AGR/AD area intensity ratios against the EFR ratio and Di for char samples prepared at β = 20 K·min−1: (a) AGR/AD versus the EFR ratio, (b) AGR/AD versus Di. The AGR/AD data are presented as means ± standard deviation (n = 3).
Figure 6. Raman spectra of the char samples prepared at β = 20 K· min−1.
bands, because the oxygen containing groups conduce to the increment of the Raman intensity. Furthermore, several researchers have found that the existence of alkali and alkaline earth metallic species had a passive effect on the Raman intensity.36,37 Especially Na and Ca species presenting as carboxylates may decrease the Raman intensity.32 During the co-pyrolysis, these Na and Ca species enhanced the reactions of bond breaking and blocked the release of the larger aromatic ring molecules, which also contributed a liberation of the oxygen-containing groups into volatiles. In addition, Li et al.37 reported that the contents of AAEM species did not affect the block effect proportionally. The undermentioned EDS results also indicated that K and Ca in the char samples were predominant with the addition of EFR. Clearly, an increase in oxygen content in the mixture and the nonproportional effect of the AAEM species may cause this phenomenon. The height or area ratios between different curve-fitted bands described in Table 4 were employed to characterize the evolution of the char structure in this paper. As displayed in Figure 7a, the AG/Aall showed an increases with the increment of the EFR mass ratio. In other words, the extent of the aromatization may grow with the EFR ratio during the copyrolysis process. Furthermore, as shown in Figures 8a and 9a, AGR/AD and A(GR+VL+VR)/AD increased nonlinearly as the EFR ratio increased from 25% to 75%, and a square relationship between all the intensity ratios and the EFR ratios was found. McDonald et al.19 found that the IV/IG (height intensity ratio of the VL and VR bands to the G band) was in exponential relationship with the heat treatment temperature for biomass
Figure 9. Variation of the A(GR+VL+VR)/AD area intensity ratios against the EFR ratio and Di for char samples prepared at β = 20 K·min−1: (a) A(GR+VL+VR)/AD versus the EFR ratio, (b) A(GR+VL+VR)/AD versus Di. The A(GR+VL+VR)/AD data are presented as means ± standard deviation (n = 3).
char samples. In this paper, AG/Aall, AGR/AD and A(GR+VL+VR)/ AD area intensity ratios showed the square relationship with the EFR ratio for all of the char samples. This can be attributed to increase of AAEM species (K) in the mixture, which promoted the breakdown of larger aromatic ring structures into smaller aromatic ring systems.31 In this research, the extent of the smaller ring growth reaction had been promoted with increase of the EFR ratio in the mixture. With the increasing of the EFR ratio, the extent volatile-char interaction between the EFR and the coal was becoming strong, which resulted in more accumulation of volatiles in the char. The accumulated volatiles were mainly small aromatic compounds grafted with alkyl groups. Not all of them polymerized during the char formation process, so it induced a large quantity of small aromatic ring systems in char sample. The char samples with a higher concentration of small (3−5 rings) aromatic ring systems had higher reactivity in reactions such as gasification and combustion.19,20 In addition, the values of Di correlated with intensity ratios of AG/Aall, AGR/AD and A(GR+VL+VR)/AD are shown in Figures 7b, 8b and 9b. It can be seen that, with the
Figure 7. Variation of the AG/Aall area intensity ratios against the EFR ratio and Di for char samples prepared at β = 20 K·min−1: (a) AG/Aall versus the EFR ratio, (b) AG/Aall versus Di. The AG/Aall data are presented as means ± standard deviation (n = 3). 1798
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Figure 10. SEM images and EDS of char samples prepared at β = 20 K·min−1: (a) coal char, (b) 25% EFR char, (c) 50% EFR char, (d) 75% EFR char, (e) EFR char and (f) EDS chemical compositions of the char samples.
increment of Di, all the intensity ratios first showed an increasing trend and then declined. These showed that the degree of volatile releasing during co-pyrolysis process also affected the char structure evolution. 3.3.2. Morphological Characteristics of the Char Samples. The morphological characteristics and qualitative chemical constituent of the char samples were investigated through SEM/EDS. A comparison of the surface morphologies of different char samples is presented in Figure 10a−e. It can be seen from these SEM images that the surface of the coal char was smoother than those of the mixtures and EFR. With increments of the EFR ratio, char samples became more homogeneous with the shape transition from needle-like to lamellate particles. This transformation was likely due to the more release of volatile during the EFR pyrolysis. At the same time, the agglomerations seem to start to occur with increases to the the EFR ratio, which suggested that a certain extent of plastic deformation might take place.27 The results of EDS analysis in Figure 10f showed that the coal char had high contents of C and Fe, while the species of O, K and Ca were more abundant in the co-pyrolysis char and EFR char. In addition, the variation of E values was probably related to the char structure evolution as well as the differences in AAEM concentration.27 It has been well-known that the changing of solid structure during pyrolysis was related to the bond
dissociation and polymerization, which caused by the aromatization of the carbonaceous structures via softening and melting.17,35,38 During the pyrolysis of lignocellulosic biomass, it seemed to have the opportunity to transform an amorphous solid or soften, accompanying the changes in chemical structures via cyclic aliphatic units transforming to aromatic units. Thus, in this study, the addition of the EFR produced more aromatic ring systems in the co-pyrolysis char samples, resulting in the increasing of G band intensity and AG/ Aall. Furthermore, the AGR/AD and A (GR+VL+VR)/AD ratios reached a maximum at about 75% EFR, and generated much smaller aromatic ring (3−5 rings) systems. Thus, the decreased in the value of E may be caused by the transformation of cyclic aliphatic units to aromatic units from EFR with the increasing of its mass ratio.38 In other words, high concentrations of the aromatic ring structures in char sample may correspond with lower activation energy.28 However, the E value of 75% EFR was the highest among the mixture at 184.50 kJ·mol−1 calculated by the FWO method with higher content of aromatic ring systems. This indicated that the variation of E values cannot be explained by only the structure evolution of co-pyrolysis char caused by EFR. It was also essential to take account the influence of the AAEM. It is generally known that the AAEM species catalyzed the pyrolysis process.7,9 The AAEM compositions of the co-pyrolysis char samples analyzed 1799
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University, Dongxia Ouyang from Zhong Tai Creative Holdings for their valuable suggestions.
with EDS were 3.76, 9.65 and 8.72 wt % for 25%, 50% and 75% EFR char, respectively. As shown in Figure 4, the average E value of 50% was the lowest activation energy at 117.67 kJ· mol−1 compared with 25% and 75% EFR of 124.94 and 184.50 kJ·mol−1 calculated via the FWO method, respectively. Thus, the higher AAEM concentration in the 50% EFR char sample may exhibit catalytic potential during the co-pyrolysis process, resulting in the lower value of E.
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4. CONCLUSIONS The thermal behavior and char structure evolution during copyrolysis of coal and lignocellulosic biomass were explored. The results indicated that the addition of edible fungi residue had a significant influence on thermal behavior of the co-pyrolysis and evolution of char structure. Synergistic effect was observed during the pyrolysis process with lower char yields than the calculated value, especially when the EFR proportion was 50%. Nonadditivity performance of the activation energy values was found. The characterization of the char samples from copyrolysis with Raman spectroscopy revealed that addition of lignocellulosic biomass during coal pyrolysis played a crucial role in char structure evolution. An increase in the Raman intensity was a combine result of the increase of oxygen content in the mixture and the nonproportional effect of alkali and alkaline earth metallic species. The Raman spectra area ratio of the G (graphite) band to all the bands and that of the valley between D (disordered) and G bands to the D band increased nonlinearly with the EFR ratio. Analysis based on the changing of these parameters could have profound applications in estimating the char structure evolution and evaluating the addition of biomass during the co-pyrolysis process. In addition to the investigation on co-pyrolysis char for further development of the facility and the process of co-conversation of coal and lignocellulosic biomass, the association between reactivity and Raman spectra characteristics of the co-pyrolysis char and the effect of lignocellulosic biomass model compounds on the performance of co-pyrolysis process will be investigated in future studies. Our results could be helpful to exploit the potential of coal and lignocellulosic biomass as an effective way to reduce the carbon footprint.
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ASSOCIATED CONTENT
S Supporting Information *
Variation of the dimensionless devolatilization index (D) versus EFR ratio (Figure S1) and DTG curves of 50% EFR under different heating rates (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.
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ABBREVIATIONS DTG = derivative thermogravimetric EDS = energy dispersive spectrometry SEM = scanning electron microscopy TGA = thermogravimetric analyzer
AUTHOR INFORMATION
Corresponding Author
*S. Wang. Tel: +86-29-82665157. Fax: +86-29-82668708. Email:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Low-carbon Development Special Fund of Guangdong Province, China. The authors are thankful Zhihui Wu, Caijian Zeng from GuangDong ShunDe Academy of Xi’an Jiaotong University for their support of this work. Z.Q.W is also grateful to Zhengyuan Luo, Yang Guo, Donghai Xu and Qiang Xu from Xi’an Jiaotong 1800
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