torrefied rice

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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Co-pyrolysis Behavior and Char Structure Evolution of Raw/ Torrefied Rice Straw and Coal Blends Qing He,† Qinghua Guo,† Lu Ding,*,§ Yan Gong,† Juntao Wei,† and Guangsuo Yu*,†,‡

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†

Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science and Technology, Shanghai 200237, PR China ‡ State Key Laboratory of High-Efficiency Coal Utilization and Green Chemical Engineering, College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, PR China § Division of Energy Science, Department of Engineering Sciences and Mathematics, Luleå University of Technology, Luleå 97187, Sweden ABSTRACT: Combination of biomass and coal for energy production is conducive to the sustainable development of society and a clean-energy future. This study investigates co-pyrolysis behavior of raw/torrefied rice straw and coal blends. Mild torrefaction (250 °C) and severe torrefaction (300 °C) were taken into consideration. Samples of five mixing ratios were tested by thermogravimetric analyzer, and the resulting chars were characterized by Raman spectroscopy and SEM−EDS. The results show that co-pyrolysis had little effect on char yields. Decomposition rate curves showed two distinct peaks for raw/mildtorrefied rice straw and coal blends, and the reaction rate was enhanced below 380 °C. However, only one peak appeared for severely torrefied rice straw blended with coal. During co-pyrolysis, the secondary pyrolysis of coal around 700 °C was inhibited, and the graphitization degree of biomass char increased, while the crystalline structure of coal char was poorly organized. The activation energy of mixtures also changed in different pyrolysis stages. coal at lower temperatures.10,11 It is reported that the hydrogen transfer reaction, the catalytic effects of alkali and alkaline earth metals (AAEM), and heat transfer were important factors in the synergy of co-pyrolysis.7 Moreover, co-pyrolysis is a complex process involving the mutual reactions between coal and different components of biomass. Li et al. compared the copyrolysis characteristic of coal with microcrystalline cellulose and lignin mixtures and concluded that H/C molar ratio, pyrolysis temperature range, and thermal conductivity would affect the interaction.12 Wu et al. found that the addition of cellulose increased the graphitization degrees of co-pyrolysis char, whereas the hemicellulose and lignin had the opposite effect.13 Lu et al. reported that the change of activation energy at different stages of co-pyrolysis was related to the content of cellulose and lignin.14 Torrefaction changes the components of biomass and may lead to different interactions during co-pyrolysis. Therefore, in order to research the synergy of co-pyrolysis more comprehensively, the influence of raw/torrefied biomass and coal on the whole co-pyrolysis process should be studied. In addition, thermogravimetric analysis (TGA) is an important way to study thermal behavior, which can record the pyrolysis reaction process in a controlled manner and better characterize the reaction kinetics. And it also facilitated the comparison between different reactant systems by other researchers using TGA.3,7,9−14 In this study, rice straw, a typical agricultural waste in China, and Shenfu bituminous coal were used, and two torrefaction

1. INTRODUCTION Economic development has stimulated energy consumption. Clean and efficient use of energy has been extensively studied over the past decades.1,2 Pyrolysis, gasification, and combustion are the main applied methods in thermal transformation of carbonaceous materials.2 Meanwhile, pyrolysis is considered as the initial stage of gasification and combustion and has significant effect on the reaction rate.1,3 The utilization of coal has led to an alarming increase in CO2, NOx, and SOx emissions.4 Biomass, as an environmentally friendly energy source, has many advantages, such as renewability, a huge reserve, carbon neutrality, and less sulfur than coal.5 Combination of biomass and coal for energy production is conducive to the diversity of raw materials and reduction of the greenhouse effect. A large amount of agricultural residues are generated in China every year.6 However, undesirable properties such as low calorific value, high moisture content, uneven properties, and difficulty in collection and transportation have limited their utilization.7 It is suggested that the pretreatment under 200−300 °C in an inert atmosphere, known as torrefaction, is a promising method for biomass upgrading and can solve these problems.8 Torrefied biomass has gained increasing attention from both industry and research institutions. As for co-pyrolysis, the pyrolysis rate and temperature of biomass and coal can be significantly different. When two fuels are mixed, debate exists over whether or not the co-pyrolysis synergy occurs. Some researchers believe that the interaction is very slight, since the pyrolysis characteristics of the mixture correspond to the weighted average of the individual materials.3,9 However, Aboyade et al. and Yangali et al. pointed out that adding biomass to coal promoted devolatilization of © XXXX American Chemical Society

Received: October 4, 2018 Revised: November 24, 2018 Published: November 30, 2018 A

DOI: 10.1021/acs.energyfuels.8b03469 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels temperatures were selected. Thermal behavior was evaluated through non-isothermal TGA. The synergistic effects were reflected by comparing the experimental values with the calculated values, including char yield, decomposition rate, and activation energy. In order to evaluate the influence on char structure evolution, the char structure characteristics were further investigated by Raman spectroscopy and scanning electron microscopy (SEM) equipped with an energy-dispersive X-ray spectroscopy (EDS). This study provided a better understanding of the effect of torrefaction on co-pyrolysis behavior and the char structure evolution on a structural level.

Table 1. Proximate and Ultimate Analyses of Samples SF

RS

RST250

RST300

57.28 24.74 17.98

37.27 38.22 24.51

39.93 4.17 35.82 1.85 0.26

43.46 3.38 26.26 2.14 0.25

proximate analysis (wt %, dry basis) volatiles 30.28 68.21 fixed carbon 62.26 15.77 ash 7.46 16.02 ultimate analysis (wt %, dry basis) C 70.95 34.35 H 3.96 6.05 O 16.11 41.91 N 0.94 1.47 S 0.58 0.21

2. MATERIALS AND METHOD 2.1. Samples. Rice straw (RS) and Shenfu bituminous coal (SF) were used in this paper. The samples were crushed and sieved to a particle size of 95∼125 μm. The RS was then torrefied using a fixed-bed reactor. The schematic diagram is shown in Figure 1. RS (5 (±0.1) g)

maintain an inert atmosphere at a flow rate of 50 mL/min. Each test was repeated three times for the reproducibility of accurate data. The weight loss data obtained from TGA was converted into conversion x, which is defined by eq 1 w − wt x(t ) = 0 w0 − wf (1) where w0, wf, wt refer to initial, final, and instantaneous weights during the reaction, respectively. The rate of reacted material with respect to time dx/dt was calculated to analyze the pyrolysis characteristics and kinetics. 2.3. Property Test of Char. The Raman spectroscopy is very suitable for obtaining information about the chemical structures of chars.17,18 The resulting chars were detected by a Thermofisher DXR Raman spectrometer with a laser wavelength of 455 nm. The wavenumber range of 800∼2000 cm−1 was recorded to cover the first-order bands. For each sample, 10 points were randomly selected for recording spectra. The surface morphology and element composition of samples were tested by a Hitachi SU-1510 scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectroscopy (EDS). 2.4. Pyrolysis Kinetics. The rate of pyrolysis reaction is usually considered as a function of temperature and conversion. Combined with the Arrhenius equation and the first-order reaction, the reaction rate equation can be expressed as i E y dx = A expjjj− a zzz(1 − x) dt k RT {

Figure 1. Schematic diagram of a fixed-bed reactor (1 = N2 cylinder; 2 = temperature controlling system; 3 = fixed-bed reactor; 4 = sample basket; 5 = condensing unit; 6 = washing unit; 7 = filter unit; 8 = cumulative flow meter; 9 = water-cooling jacket; 10 = magnet.).

(2) −1

where A is the pre-exponential factor (min ), Ea is the apparent activation energy (J/mol), R is the gas constant (8.3145 J/mol·K), and T is the temperature (K). For the constant heating rate during pyrolysis, β = dT/dt, eq 2 can be transformed into

was loaded into the basket in each run. Carrier gas (high-purity N2, ≥ 99.999%) was continuously blown into the reactor at 50 mL/min to maintain an inert atmosphere. Two torrefaction temperatures of 250 and 300 °C were selected, representing mild torrefaction and severe torrefaction. Torrefied rice straw samples at 250 and 300 °C were denoted as RST200 and RST350. Biomass/coal blends were mixed in different ratios; the biomass blending ratios (BBR) were 0, 25, 50, 75, and 100% (wt %, dry basis), respectively. The proximate and ultimate analyses of samples are listed in Table 1 (National Standards of PRC, GB/T 212-2008 and GB/T 31391-2015, respectively). The results of RS are similar to those reported previously (60−79% volatiles, 4−16% fixed carbon, and 9−23% ash).4,15,16 With torrefaction temperature increasing, the carbon content of biomass gradually increased, whereas the oxygen and hydrogen content decreased sharply. In addition, the ash content of biomass after torrefaction also increased. Table 2 shows the ash chemical compositions of RS and SF (GB/T 1574-2007 for coal ash and GB/ T 30725-2014 for biomass ash, respectively). It can be seen that RS contains more AAEM than SF coal. 2.2. Thermogravimetric Analysis (TGA). The pyrolysis experiments were carried out via thermogravimetric analysis (NETZSCH STA2500). The sample (8.0 mg) was taken in an aluminum crucible (Al2O3) and heated from room temperature to 900 °C with a heating rate of 20 °C/min. N2 with a purity higher than 99.999% was used to

A exp(− Ea /RT ) dx = dT (1 − x) β

(3)

Using the integral method based on the Coats-Redfern (CR) equation,9,19,20 the approximate integration of eq 3 gives ÅÄÅ ÑÉ ÅÄÅ − ln(1 − x) ÑÉÑ 2RT yzÑÑÑ ÅÅ AR ij E Å ÑÑ Å Ñ j z = − lnÅÅÅ ln 1 Å ÑÑÑ − Ñ j z 2 Å ÅÅÇ Ñ Å Ñ E {ÑÖ RT ÑÖ T (4) ÅÇ βE k The plot of ln[−ln(1− x)/T2] versus 1/T should be a straight line with a high correlation coefficient of linear regression analysis. Accordingly, the kinetic parameters can be obtained from the slope and intercept of the regression line.

3. RESULTS AND DISCUSSION 3.1. Characteristics of Individual Pyrolysis. The variation of dx/dt with temperature for individual samples at a heating rate of 20 °C/min is shown in Figure 2. It was observed that the RS pyrolysis mainly appeared in the range of 200∼500 °C, and there were two shoulder peaks. The left shoulder peak (200∼270 °C) and the main peak (270∼380 °C), called the B


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Energy & Fuels Table 2. Ash Composition of RS and SF chemical composition/% samples

SiO2

Al2O3

CaO

MgO

Na2O

K2O

Fe2O3

P2O5

RS SF

53.7 29.03

1.02 17.33

4.33 19.2

15.99 6.63

1.05 1.23

9.25 0.78

0.74 6.73

2.09 0.07

this stage, the weakest macromolecular structures such as aliphatic and hydroaromatic structures began to decompose.25 And the pyrolysis reaction entered the second stage, as the pyrolysis temperature continued to rise. Another peak appeared around 700 °C. This was mainly due to the condensation reactions of the aromatic rings.26 3.2. Characteristics and Synergistic Effect of Copyrolysis. 3.2.1. Effect of Co-pyrolysis on Char Yield. The char yield from TGA plays an important role in accounting for synergistic effects from blending fuels.27 In this study, the char yield was defined as the solid residue at 900 °C. And the calculated values were obtained from the mass ratio of individual materials. The effect of BBRs on char yields of RS/SF, RST250/ SF, and RST300/SF are demonstrated in Figure 3. The comparison of experimental and calculated values showed that the relative error was within 3%, illustrating that the char yield was independent of BBRs whether the RS was torrefied or not. It was thus concluded that the synergistic effect of co-pyrolysis on decreasing char yield or promoting the total amount of volatiles was slight. Similar results were reported for different co-pyrolysis processes, such as raw/torrefied wood blending with anthracite coal, energy grass blending with lignite blends, risk husks blending with anthracite coal, and nut shells blending with bituminous coal.14,27−29 Char yield may represent a global synergy effect but cannot reflect the specific interactions of the co-pyrolysis process. Therefore, the decomposition rate of copyrolysis is further discussed. 3.2.2. Effect of Co-pyrolysis on Decomposition Rate. To investigate the detailed interactions between biomass and coal during co-pyrolysis, the theoretical decomposition rate curves of the blends were calculated based on the curves of their parent materials in an additive manner.14 Thus

Figure 2. Decomposition rate curves for individual material (20 °C/ min).

Figure 3. Comparison of experimental and calculated char yield with different BBRs.

(dx /dt )cal = y1(dx /dt )biomass + y2 (dx /dt )coal

active pyrolysis zone, were attributed to the breakdown of hemicellulose and cellulose.21 And the tailing peak at 380∼500 °C was the decomposition of lignin, forming the right shoulder.22 Compared with RS, the left shoulder of the RST250 curve was smoothed, the right shoulder was enhanced, and the maximum decomposition rate increased. Smoothing of the left shoulder (circular area in Figure 2) may be a result of the removal of hemicellulose. The increase in the right shoulder (rectangular area in Figure 2) and the maximum decomposition rate can be primarily attributed to the accumulation of cellulose. Similar conclusions were reached in previous work by the authors using camphorwood.23 After severe torrefaction at 300 °C, the maximum decomposition rate dropped considerably, and the peak position shift back to about 450 °C. Severe torrefaction results in complete degradation of hemicellulose and cellulose, reducing the rate of decomposition.22,24 Moreover, the increase of the number of C−C bonds leads to the increase of pyrolysis temperature.23 According to the above analysis of the individual pyrolysis curve, it can be concluded that the amount of torrefied biomass cellulose and lignin were increased, and hemicellulose was decreased. As for coal pyrolysis, the pyrolysis reaction started around 302 °C and reached the maximum weight loss rate around 446 °C. At

(5)

where (dx/dt)coal and (dx/dt)biomass are the decomposition rate of the individual fuels, and y1 and y2 are the proportions of biomass and coal in the blends. The experimental and the calculated curves are shown in Figure 4. Figure 4a shows the co-pyrolysis characteristics of the RS/SF mixture. The first peak appeared in 220−380 °C, and the experimental decomposition rate of the mixture was higher than the calculated rate, demonstrating that the synergistic effect occurred in co-pyrolysis. The second peak appeared in 380−500 °C, and the experimental value was lower than the calculated value, showing some inhibition effects. The secondary pyrolysis reaction of coal almost disappeared at around 700 °C, suggesting that the interaction of co-pyrolysis occurred. Figure 4b shows the pyrolysis characteristics of the RST250/SF mixture, and they are similar to those of RS/SF, but the inhibition of the second peak was attenuated. Figure 4c shows that only one peak appears in the RST300/SF mixture. The experimental and calculated curves matched in most of the range, except the higher temperatures of coal secondary pyrolysis. In general, with the increase of the biomass torrefaction temperature, two peaks of the decomposition curves turn into one peak during coC


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Energy & Fuels

Figure 4. Comparisons of experimental and calculated decomposition rate curves of (a) RS/SF, (b) RST250/SF, and (c) RST300/SF.

Figure 5. Apparent activation energy as a function of mass fraction (a) RS/SF, (b) RST250/SF, and (c) RST300/SF (20 °C/min).

pyrolysis. When two peaks exist, the first one is usually promoted, while the second one is inhibited.

The co-pyrolysis of RS/SF, RST250/SF, and RST300/SF showed different interactions. The decomposition rate of RS/SF D


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Energy & Fuels

Figure 6. SEM images of SF char after (a) individual pyrolysis and (b) co-pyrolysis (RS/SF as 50:50 mixtures as an example).

Table 3. EDS Analysis of SF Chara element atomic % samples

SF

RS/SF (50:50)

position

C

N

O

Na

Mg

Al

Si

K

Ca

Fe

ash 1 ash 2 coal matrix 1 coal matrix 2 ash 1 ash 2 coal matrix 1 coal matrix 2

83.9 88.4 90.3 88.2 68.0 59.7 84.2 84.3

2.33 3.95 2.10 5.49 7.87 3.79 4.33 5.32

12.0 6.31 6.22 5.18 21.4 27.8 10.0 9.19

0.63 0.55 0.60 0.57 0.93 0.53 0.68 0.53

0.20 0.27 0.19 0.19 0.62 0.39 0.26 0.18

0.45 0.25 0.21 0.15 0.15 0.28 0.13 0.11

0.42 0.14 0.15 0.09 0.42 6.17 0.10 0.13

0.04 0.04 0.05 0.07 0.45 1.00 0.18 0.19

0.07 0.08 0.10 0.05 0.15 0.17 0.08 0.06

0.05 0.06 0.05 0.07 0.05 0.18 0.06 0.03

a

Ash: circular areas; coal matrix: rectangular areas.

releasing.31 Consequently, pyrolysis of coal was inhibited in the mixtures around the temperature of second peak. However, the content of hydrogen and volatiles in RST300 was much lower than that of RS or RST250 (Table 1). The initial pyrolysis temperature of biomass increased with the increase of torrefation temperature. Even though the pyrolysis reaction between RST300 and coal occurred almost simultaneously, the decomposition rate did not deviate from the weighted average in most temperature ranges. 3.2.3. Effect of Co-pyrolysis on Activation Energy. The activation energy of mixture pyrolysis was calculated by the segmentation model fitting method. The Ea of the main pyrolysis regime was calculated according to the peak position of DTG curves, where the first-order reaction was adopted. According to Vyazovkin, the uncertainty of activation energy came from experiment and computational methods.32 In this study, the R2 of the fitting curve was above 0.94, and most of them were around 0.98, reflecting the fact that the pyrolysis process was well correlated. And according to the repeated experiment, the Ea with the error bar of each regime is shown in Figure 5. In summary, the uncertainty of activation energy in this paper was acceptable. Figure 5a shows the Ea of the RS/SF mixture as a function of biomass fraction. As the biomass percentage increased, the Ea increased in the low-temperature regime (220−380 °C). Meanwhile, in the higher-temperature regime (380−500 °C), the Ea showed the opposite trend. The change in the Ea of the RST250/SF mixture was similar to that of the RS/SF mixture, as shown in Figure 5b. This is a frequently encountered pattern in the biomass/coal co-pyrolysis literature.11 Figure 5c shows the

Figure 7. Typical Raman spectra of char (SF char as an example).

and RST250/SF were promoted near the temperature of the first peak (200∼380 °C) but were inhibited around the temperature of the second peak (380∼500 °C). In hard coals, small molecules are distilled and diffused below 350 °C, which may lead to the formation of CH2·, CH3·, OH·, and O· radicals. And they are repolymerized to form char or volatiles components.10 The presence of hydrogen from lignocellulosic biomass prevents recombination reactions and favors the formation of volatiles accelerating the total reaction rate in lower temperatures. But the cohesiveness of bituminous coal is strong. And large amounts of soft residues generated from RS and RST250 would cohere to the coal surface before coal initial pyrolysis, which may congest the pores of coal particles.30 In addition, the swell effects cannot be neglected and would block the channel of volatiles E


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Figure 8. Effects of co-pyrolysis on char structure evolution at different BBRs.

corrected Raman curve is fitted with one Gaussian peak (D3) and four Lorentz peaks (G, D1, D2, and D4), which represented typical carbon structures possibly existing in samples.34 The typical Raman spectrum of the resulting char and band distributions was shown in Figure 7. Ix is the area of band x, where x is G, D1, D2, D3, and D4. Therefore, the IG/IAll band area ratio could evaluate the content of the large aromatic ring and the order of carbon.34 In addition, the ID3/(IG + ID2 + ID3) band area ratio is broadly related to the amorphous carbon content.35 The evolution of the graphitization of biomass char is shown in Figure 8a. The IG/IAll ratio of RS was 0.0986, while the value of RST300 was 0.0797. The IG/IAll ratio decreased, indicating that torrefaction led to a lower graphitization degree of pyrolysis char. Moreover, the IG/IAll ratio of the biomass char changed after the addition of SF coal for co-pyrolysis. As the biomass mass ratios decreased from 100 to 25%, the IG/IAll ratio gradually increased, indicating that various forms of structural defects were eliminated, and the surface of char was more ordered. Figure 8d shows the evolution of the amorphous carbon structure on the surface of SF coal char. As the coal mass ratios decreased, the value of ID3/(IG + ID2 + ID3) showed an increasing trend, suggesting that more amorphous carbon appeared on the SF char. The ID3/(IG + ID2 + ID3) ratio of biomass and the IG/IAll ratio of SF char was changed correspondingly, as shown in Figure 8b,c. The result of the char structure evolution was affected by the reaction especially in the late stage of pyrolysis, because the SEM−EDS and Raman analysis was ex situ. It has been suggested that AAEM could promote the decomposition of large aromatic rings and induce an increase in the small aromatic ring systems.1,30 Moreover, not all of these structures polymerized during the char formation process.36 During the co-pyrolysis process, the decrease in concentrations of small aromatic rings of

Ea of the RST300/SF mixture at 300−600 °C, and the experimental values were in good agreement with the calculated values. RS contains more C−O and C−H bands, and the Ea is lower.23 RST250 is removed as part of O and H, leading to higher Ea. The reduction in the Ea of RST300 is because of the decomposition of cellulose and the increase in lignin content.14 Cellulosic pyrolysis converts cyclic aliphatic units into aromatic structures, making solids amorphous or soft with decreasing of Ea.33 RS and RST250 contain more cellulose. The formation of the amorphous structure of cellulose exerts an effect during copyrolysis with bituminous coal. The Ea is higher than the prediction in the low-temperature regime and is lower in the high-temperature regime, indicating that the coal devolatilizes at lower temperature with the incorporation of biomass into the mixture.11 3.3. Char Structure Evolution. The coal char was magnified 750 times to observe its surface characteristics, as shown in Figure 6. The circular area represented the ash supported on the surface of the coal char, and the rectangular area represented the carbon crystal structure of the coal char surface. Unlike the individual pyrolysis char, different shapes of ash can be observed on the SF char surface after co-pyrolysis, as shown in Figure 6b. In addition to the appearance of granular coal ash, the long strips of fly ash also appeared on the surface of SF char in mixtures and are characteristic of rice straw ash. It can be confirmed that the pyrolysis of the coal/biomass mixture will load a certain amount of rice straw ash onto the surface of SF coal char. And this rice straw ash contains more AAEM than coal ash, especially K content, which is about 10−20 times of coal ash, as shown in Table 3. All char samples were tested by Raman spectroscopy to analyze the order of the char matrix for both coal and biomass. In order to determine the exact spectral parameters, the baseline F


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(4) Hu, J.; Si, Y.; Yang, H.; Shao, J.; Wang, X.; Lei, T.; Agblevor, F. A.; Chen, H. Influence of volatiles-char interactions between coal and biomass on the volatiles released, resulting char structure and reactivity during co-pyrolysis. Energy Convers. Manage. 2017, 152, 229−238. (5) Wang, Y.; Wu, Q.; Dai, L.; Zeng, Z.; Liu, Y.; Ruan, R.; Fu, G.; Yu, Z.; Jiang, L. Co-pyrolysis of wet torrefied bamboo sawdust and soapstock. J. Anal. Appl. Pyrolysis 2018, 132, 211−216. (6) Chen, J.; Wang, Y.; Lang, X.; Ren, X.; Fan, S. Evaluation of agricultural residues pyrolysis under non-isothermal conditions: Thermal behaviors, kinetics, and thermodynamics. Bioresour. Technol. 2017, 241, 340−348. (7) He, Y.; Chang, C.; Li, P.; Han, X.; Li, H.; Fang, S.; Chen, J.; Ma, X. Thermal decomposition and kinetics of coal and fermented cornstalk using thermogravimetric analysis. Bioresour. Technol. 2018, 259, 294− 303. (8) Peduzzi, E.; Boissonnet, G.; Haarlemmer, G.; Dupont, C.; Maréchal, F. Torrefaction modelling for lignocellulosic biomass conversion processes. Energy 2014, 70, 58−67. (9) Masnadi, M. S.; Habibi, R.; Kopyscinski, J.; Hill, J. M.; Bi, X.; Lim, C. J.; Ellis, N.; Grace, J. R. Fuel characterization and co-pyrolysis kinetics of biomass and fossil fuels. Fuel 2014, 117, 1204−1214. (10) Aboyade, A. O.; Görgens, J. F.; Carrier, M.; Meyer, E. L.; Knoetze, J. H. Thermogravimetric study of the pyrolysis characteristics and kinetics of coal blends with corn and sugarcane residues. Fuel Process. Technol. 2013, 106, 310−320. (11) Yangali, P.; Celaya, A. M.; Goldfarb, J. L. Co-pyrolysis reaction rates and activation energies of West Virginia coal and cherry pit blends. J. Anal. Appl. Pyrolysis 2014, 108, 203−211. (12) Li, S.; Chen, X.; Liu, A.; Wang, L.; Yu, G. Co-pyrolysis characteristic of biomass and bituminous coal. Bioresour. Technol. 2015, 179, 414−420. (13) Wu, Z.; Yang, W.; Chen, L.; Meng, H.; Zhao, J.; Wang, S. Morphology and microstructure of co-pyrolysis char from bituminous coal blended with lignocellulosic biomass: Effects of cellulose, hemicellulose and lignin. Appl. Therm. Eng. 2017, 116, 24−32. (14) Lu, K. M.; Lee, W.-J.; Chen, W.-H.; Lin, T.-C. Thermogravimetric analysis and kinetics of co-pyrolysis of raw/torrefied wood and coal blends. Appl. Energy 2013, 105, 57−65. (15) Nam, H.; Capareda, S. C.; Ashwath, N.; Kongkasawan, J. Experimental investigation of pyrolysis of rice straw using bench-scale auger, batch and fluidized bed reactors. Energy 2015, 93, 2384−2394. (16) Park, J.; Lee, Y.; Ryu, C.; Park, Y. K. Slow pyrolysis of rice straw: analysis of products properties, carbon and energy yields. Bioresour. Technol. 2014, 155, 63−70. (17) Guizani, C.; Haddad, K.; Limousy, L.; Jeguirim, M. New insights on the structural evolution of biomass char upon pyrolysis as revealed by the Raman spectroscopy and elemental analysis. Carbon 2017, 119, 519−521. (18) Zhu, H.; Wang, X.; Wang, F.; Yu, G. In Situ Study on K2CO3Catalyzed CO2 Gasification of Coal Char: Interactions and Char Structure Evolution. Energy Fuels 2018, 32 (2), 1320−1327. (19) Zhou, L.; Wang, Y.; Huang, Q.; Cai, J. Thermogravimetric characteristics and kinetic of plastic and biomass blends co-pyrolysis. Fuel Process. Technol. 2006, 87 (11), 963−969. (20) Gil, M. V.; Casal, D.; Pevida, C.; Pis, J. J.; Rubiera, F. Thermal behaviour and kinetics of coal/biomass blends during co-combustion. Bioresour. Technol. 2010, 101 (14), 5601−8. (21) Mansaray, K. G.; Ghaly, A. E. Thermal degradation of rice husks in nitrogen atmosphere. Bioresour. Technol. 1998, 65 (1−2), 13−20. (22) Eseltine, D.; Thanapal, S. S.; Annamalai, K.; Ranjan, D. Torrefaction of woody biomass (Juniper and Mesquite) using inert and non-inert gases. Fuel 2013, 113, 379−388. (23) Cao, L.; Yuan, X.; Jiang, L.; Li, C.; Xiao, Z.; Huang, Z.; Chen, X.; Zeng, G.; Li, H. Thermogravimetric characteristics and kinetics analysis of oil cake and torrefied biomass blends. Fuel 2016, 175, 129−136. (24) Chen, W. H.; Kuo, P. C. Torrefaction and co-torrefaction characterization of hemicellulose, cellulose and lignin as well as torrefaction of some basic constituents in biomass. Energy 2011, 36 (2), 803−811.

biomass char and the increase in amorphous carbon of SF coal char imply that AAEM migrate from biomass chars to coal chars. The DTG curves showed that the secondary pyrolysis of coal was attenuated in the late stage of pyrolysis, which is related to the condensation of aromatic rings.26 It could be further inferred that these structures are not susceptible to condensation. In addition, the ash content of the torrefied rice straw is higher than that of the raw material, which may explain the decrease of graphitization for biomass individual pyrolysis after torrefication. According to the analysis of the char structural evolution, the char surface of the mixture of severe torrefied RS and coal was more disordered under the same mixing ratio conditions, which means that this mixture may be more suitable for the combustion and gasification process.

4. CONCLUSIONS Raw/torrified rice straw and SF coal were used in this work to study the co-pyrolysis characteristics and char structure evolution. The main conclusions are as follows (1) Torrefaction affects the physicochemical properties and pyrolysis characteristics of rice straw. The C/H and C/O atomic ratios of biomass increase after torrefaction. The initial reaction temperature also increases. (2) Co-pyrolysis of raw/mild torrefied RS blending with SF coal has some synergistic effect. The activation is higher than predicted in low temperatures. However, copyrolysis of severe torrefied RS blending with SF coal has no obvious synergistic effect in the main pyrolysis temperature range. (3) Despite the lack of a synergistic effect on char yield, the secondary pyrolysis of coal around 700 °C is inhibited with the addition of biomass during the co-pyrolysis process. And the order of biomass char is promoted, while the amount of amorphous carbon of coal char is increased. The synergistic effect may be related to the accumulation of biomass ash on the surface of coal char.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-21-64252974; Fax: +86-21-64251312; E-mail: [email protected] (L.D.) *Tel.: +86-21-64252974; Fax: +86-21-64251312; E-mail: [email protected] (G.Y.) ORCID

Guangsuo Yu: 0000-0003-4085-9736 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key R&D Program of China (2017YFB0602601). REFERENCES

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