Thermochemical behaviors, kinetics and gas emission analyses during co-pyrolysis of walnut shell and coal

Thermochimica Acta 673 (2019) 26–33

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Thermochemical behaviors, kinetics and gas emission analyses during copyrolysis of walnut shell and coal

T

Fusheng Yanga,b, , Anning Zhoua,b, Wei Zhaoa,b, Zhiyuan Yanga,b, Huajing Lia ⁎

a b

College of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, 710054, Xi’an, Shaanxi, China Key Laboratory of Coal Resources Exploration and Comprehensive Utilization, Ministry of Land and Resources, 710021, Xi’an, Shaanxi, China

ARTICLE INFO

ABSTRACT

Keywords: Walnut shell Coal Kinetics Synergistic Acetic acid

Thermogravimetry coupled with mass spectrometry measurements of individual coal (SF), individual walnut shell (WS), and their blends were performed. Interaction between the coal and the WS, activation energy, and reaction mechanism, were determined. Evolution profiles of volatile products before 1173 K were also elaborated. It was shown that incorporation of the coal into the WS, enhanced apparent activation energies of the blends in 533–613 K; synergistic reactions happened in approximately 573–773 K, for the blends excluding SF/ WS (1:1), bringing about improved emissions of acetic acid; mass ratio of the coal to the WS at 1:2, was more favourable to the improvement, maximum relative intensity of emission being 3.11 × 10−10 A/g; carbonyl and carboxyl containing species, from pyrolysis of the WS and the coal in 573–673 K, contributed to the improvements, through radicals stabilization, deacetylation and isomerization. A possible copyrolysis synergy mechanism, relevant to gases emission, was proposed based on experimental results.

1. Introduction Walnut shell (WS) is a very important renewable resource. The annual output in China, has reached 1.1 million tonnes in 2016, accounting for 50% of the world's total level. Conversion of walnut shell, into bio-oils or intermediate compounds for chemical industry, has attracted considerable attention. Co-pyrolysis and catalytic co-pyrolysis, which are initial procedures during thermal conversion, were employed as promising techniques to obtain specific products rich in value-added chemicals [1]. Previous investigations have been performed mainly on WS influence on co-pyrolysis characteristics. Zhu et al [2] revealed that average activation energy of blends from WS and bio-oil distillation residue, gradually increased with increase in walnut shell proportion. The lowest average activation energy of blend sample (217.87 kJ mol−1 by FWO method and 199.79 kJ mol−1 by KAS method) was obtained through adding 25% WS. Özsin et al [3] investigated thermochemical behaviors and interactions during co-pyrolysis of WS and polystyrene (PS). Iso-conversional methods were adopted for apparent activation energy acquisition. Gas evolution analyses were performed qualitatively, without describing relationship between ion current intensity and WS content. Similarly, Uzun et al [4] studied co-pyrolytic behavior and kinetics of WS and polyolefin. According to Arrhenius method, average activation energies of WS, polyethylene (PP) and low density ⁎

polyethylene (LDPE), were obtained as 69.32, 295.65 and 254.55 kJ mol−1, respectively. Average activation energies of WS, PP and LDPE, were determined by using Coats-Redfern method as 101.58, 333.53 and 316.77 kJ mol−1, respectively. Zhang et al [5] found that bituminous coal and WS exhibited an additive effect, with respect to char yield and devolatilization rate, during their co-pyrolysis step. Obviously, significant deviations in activation energies appeared, to the same pyrolysis processes, due to different calculation methods, indicating inappropriate mathematical models and reaction mechanisms. In addition, some relative fundamental theories on co-pyrolytic characteristics are still controversial issues, such as additive effect or synergistic effect. Vuthaluru [6] found that pyrolytic characteristics of blends, from coal and wood waste or wheat straw, followed those of parent fuels in an additive manner. Meesri et al [7] suggested that yield of major pyrolysis products (volatiles and char) is linearly proportional to percentage of biomass and coal in the mixture. Wan et al [8] reported lack of synergistic effects between coal and straw particles. A synergistic effect, in terms of higher gas yield, lower tar and char yields, was detected in coal and rice straw or wood co-pyrolysis by Krerkkaiwan [9]. Li [10] claimed that H/C molar ratio, heat transfer properties of rice straw, saw dust, microcrystalline cellulose, lignin affect the interaction between biomass and Shenfu bituminous coal. Synergistic effect of gas distribution between wheat straw and coal was described by Wu et al [11].

Corresponding author at: College of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, 710054, Xi’an, Shaanxi, China. E-mail address: [email protected] (F. Yang).

https://doi.org/10.1016/j.tca.2019.01.004 Received 30 September 2018; Received in revised form 27 December 2018; Accepted 6 January 2019 Available online 07 January 2019 0040-6031/ © 2019 Elsevier B.V. All rights reserved.

Thermochimica Acta 673 (2019) 26–33

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Nomenclature A Ea R T ad daf f(ɑ) g(ɑ)

TG DTG MS WS

Pre-exponential factor, s−1 Apparent activation energy, kJ·mol−1 Universal gas constant, J·mol−1·K−1 Temperature, K Air-dried basis Dried and ash-free basis Mechanism function for differential form Mechanism function for integral form

Thermogravimetry The first derivative thermogravimetry Mass spectrometry Walnut shell

Greek symbols α β

Conversion degree, % Heating rate, K·min−1

province (China) and coal from Shenfu (SF) coal field were selected in this investigation. Proximate and ultimate analyses of the samples were listed in Table 1, according to Chinese national standards, such as GB/T 30732-2014 Proximate analysis of coal-Instrumental method, GB/T 476-2008 Determination of carbon and hydrogen in coal, GB/T 192272008 Determination of nitrogen in coal, GB/T 214-2007 Determination of total sulfur in coal; oxygen content was obtained by difference. The coal and the WS were blended in different mass ratios (1:5, 1:4, 1:3, 1:2 and 1:1, respectively). For example, SF/WS(1:5) is the symbol for blend from one portion of SF coal and five portion of WS. All of the blends, together with the coal and the WS were ground into fine powders then sieved by a 200 mesh screen. As can be seen in Table 1, WS presents higher concentrations of hydrogen and oxygen, lower carbon content compared to SF coal, resulting in higher H/C and O/C atomic ratio.

On the other hand, acetic acid, which can be obtained from biomass pyrolysis [12], as vital value-added chemicals, is widely used in bioadditives preparation [13], anticancer agents development [14], orderinducing agent and pore-forming agent [15], coordination modulation [16], desulfurization [17], transient electronic application [18], hydrophilic quality and biocompatibility improvement [19], vapour absorption refrigeration [20], superhydrophobic surface formation [21]. However, preparation of acetic acid from WS pyrolysis still remains insufficient. Co-pyrolysis of WS and coal, especially catalytic co-pyrolysis [22], is regarded as attractive technique, to produce high valueadded liquid [23] and gaseous products [24], which is also meaningful in alleviating environmental issues caused by carbon dioxide emission for direct combustion. Meanwhile, different opinions on synergistic effect illustrate that, kinetic modeling of the co-pyrolysis processes is still challenging and not well understood. Actually, thermochemical processes such as devolatilization, cracking and interaction are usually in competition with mass transfer [25]. Investigations into thermochemical behaviors, kinetics and gas emission, during the co-pyrolysis of WS and coal, can obtain more significant information with respect to the heat and mass transfer; are prospective to reveal a widely acceptable pyrolysis mechanism, assist in scaling up and optimization of WS large-scale treatment, such as coking, co-gasification and co-combustion. Unfortunately, few attentions have been paid to kinetic parameters, interaction mechanism, as well as gaseous products distribution, during co-pyrolysis of WS with addition of coal. This work aimed at following aspects: firtsly, gaining a detailed understanding of controlling steps during copyrolysis of WS with coal incorporation, based on thermogravimetric characteristics, interaction mechanism, and kinetics investigations; secondly, achieving suitable blending ratios for target gaseous products, by means of thermogravimetry coupled with mass spectrometry (TG/MS) technology; as a result, providing possible reference for gradient pyrolysis of WS at commercial scale, and subsequent stepwise recovery of value-added products. So objectives of this study conform to Thermochimica Acta, which is devoted to all aspects of thermoanalytical methods and their application, from fundamental research to practical application, such as thermal properties and behavior of materials, as well as kinetics of thermally stimulated processes.

2.2. Thermogravimetry and mass spectrometry (TG/MS) experiments TG/MS measurements were performed using a Mettler-Toledo TGA/ DSC1 simultaneous thermal analyzer coupled with a quadrupole mass spectrometer (Pfeiffer ThermoStar GSD 320). Gaseous products from the thermal analyzer were coupled to the mass spectrometer through a quartz capillary heated to 393 K. High purity nitrogen with 20 mL min−1 flow rate was used as purge gas, about 10 mg sample was put in an alumina crucible, then heated from room temperature to 1173 K at heating rate of 10 K min−1 in each experiment. 2.3. Reaction kinetics and mechanism of pyrolysis In order to avoid above-mentioned deviation in activation energy, arising from inappropriate mechanism assumptions and calculation methods, pyrolysis kinetics was calculated based on Achar differential equation and Coats-Redfern integral equation simultaneously [26,27] in this investigation. Achar differential equation:

In [

d / dT A ] = In f( )

Ea RT

(1)

Coats-Redfern integral equation:

2. Experimental

In [

2.1. Materials

g( ) AR ] = In ( ) T2 Ea

Ea RT

(2)

where ɑ is degree of conversion (%), Ea is apparent activation energy (kJ∙mol−1), A is Arrhenius pre-exponential factor, T is temperature (K),

WS from national walnut seed base in Shangluo city of Shaanxi Table 1 Proximate and ultimate analyses of the samples. Proximate analysis (wt %, ad)

WS SF

Ultimate analysis (wt %, daf)

Molar ratio

Moisture

Ash

Volatile matter

Fixed carbon

S

C

H

N

O

H/C

O/C

8.16 9.37

0.70 6.43

81.66 33.18

9.48 51.02

– 0.28

48.47 66.64

6.23 5.86

0.18 0.77

45.12 26.45

1.54 1.06

0.70 0.30

27

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R is molar gas constant (8.314 J∙mol−1 K−1), β is heating rate (K∙min−1). f(ɑ) and g(ɑ) refers to mechanism functions for differential and integral forms, respectively. So plots of ln[g(α)/T2] versus 1/T and ln[dα/(dT∙f(α))] against 1/T give straight lines, combined with mechanism functions of solid degradation listed in Table 2, Ea can be calculated from the slopes. If activation energies calculated from the differential and integral equations come near, and R-Squares (COD) are better, probable mechanism functions can be determined [28,29].

1

, roughly equaled to those of blends SF/WS (1:2) and SF/WS (1:4) at 623 K, but was faster than those of the individual WS and other blends, accounting for synergistic reactions occurred in approximately 573–773 K. The synergistic reactions might originate from hydrogen donating [36,37] to the coal, by the cellulose and hemicellulose-derived volatile from the WS. In presence of the hydrogen donors, from the WS pyrolysis before 773 K, radical species from the coal were more easily stabilized into light volatile, attributing to increases in the mass losses of the blends, such as SF/WS (1:3) and SF/WS (1:4). Unfortunately, similar trends were not observed, during whole pyrolysis for SF/WS (1:1); mass ratio of the coal to the WS 1:1 gave a sharp deviation in experimental and theoretical mass losses, indicating insufficiency of hydrogen donors to the coal; as the result, large quantity of radical species from the coal, might consequently undergo coupling with radical species from the WS, forming compounds as precursors of char, at the same time, further decomposition of radical species into light volatile were terminated, producing its improvement in residual mass. On the other hand, co-pyrolysis phenomena exceeding 873 K, were similar for all of the blends, with relatively higher experimental residual masses, compared with corresponding theoretical ones. The fundamental cause lay in strengthened termination of the radical species, and condensation of the char precursors. Because increasing trend of free radicals up to 873 K could be well depressed, with further raising temperature, free radicals even disappeared [38]. Lignin in the WS should also complete its decomposition prior to 773 K [34].

3. Results and discussion 3.1. Pyrolysis analysis of the WS and the coal Mass loss and derivative thermogravimetric (DTG) characteristics, of individual WS and individual coal, in nitrogen at heating rate 10 K min−1, were presented in Fig. 1. As can be seen in Fig. 1, initial mass loss stages, due to moisture evaporation [3], were all observed between room temperature and around 423 K, regardless of the sample properties. Dominant mass loss for the coal, located in 523–1073 K, which was comprised of two stages, compared to one stage for the WS; the former stage, ranging from 523 K to 933 K, might be attributed to devolatilization of carbonaceous structures, mainly phenolic compounds containing aromatic ring, hydroxyl group and alkyl groups [30], with release of carbon dioxide [31] and relatively high molecular weight species, peak mass loss rate being 1.25 × 10−4 s-1 for heating rate 10 K min-1; the latter stage, namely 933–1073 K, was mainly caused by aromatic rings condensation and minerals decomposition, such as decomposition of calcium carbonate, which agreed with investigations by Ulloa et al [32] and Tian et al [33]. Ulloa et al [32] attributed volatiles loss over 923 K to condensation reactions of aromatic rings and thermal decomposition of carbonates; similarly, Tian et al [33] attributed DTG peak at 950 K to carbonate decompostion, with carbon dioxide emission, peak temperature at 1011 K was assigned to condensation of aromatic rings. Dominant mass loss for the WS, was observed between 473 K and 653 K, as shown in Fig. 1(b), which might result from degradation of hemicellulose, cellulose and lignin, with a peak mass loss rate of 9.30 × 10−4 s-1. Because hemicellulose decomposes at temperatures of 473–633 K; cellulose degradation occurs at 513–623 K; lignin decomposition takes place at 553–773 K [34]. The following process over 663 K, was designated as lignin decomposition, with mass loss rates 1.03 × 10-4 to 1.54 × 10−4 s-1, which were equivalent to those of the coal devolatilization in the same temperature range. Therefore, partially overlapped decomposition ranges, together with the equivalent devolatilization rates, may facilitate interaction among the intermediate products, which stem from decomposition of the coal and the WS.

3.3. Kinetics of pyrolysis Mass loss data obtained in the TG measurements, were used to calculate activation energies, based on Eqs. (1) and (2). A group of best linear regressions with similar slopes, for plots of ln[g(α)/T2] versus 1/T and ln[dα/(dT∙f(α))] against 1/T in the same temperature range, were chose to determine kinetics of the samples. Kinetics results and corresponding temperature ranges, of individual SF, individual WS and their blends at different ratios, were given in Table 3. It can be concluded that R2 coefficients of determination (COD) between 0.9907 and 0.9995, were achieved for all the regressions. Hence, the above kinetics mechanisms should be properly proposed. Generally, activation energies of the blends varied with fraction ratios of the coal to the WS. Relatively higher activation energy in 533–613 K, Table 2 Commonly used functions of solid degradation. Mechanism

Random nucleation and nuclei growth Two-dimensional A2 Three-dimensional A3 Diffusion One-dimensional D1 Two-dimensional D2 Three-dimensional (Jander D3 equation) Three-dimensional (G-B D4 equation) Order reaction First-order F1 Second-order F2 Third-order F3 Exponential nucleation Power law, n = 1/2 P2 Power law, n = 1/3 P3 Power law, n = 1/4 P4 Phase boundary controlled reaction One-dimensional R1 movement Contracting area R2 Contracting volume R3

3.2. Co-pyrolysis analysis of the WS and coal Thermal decomposition characteristics of blends, from the coal and the WS, at 10 K min−1 in nitrogen, were plotted as Fig. 2. To clarify potential synergistic effect between the coal and the WS, theoretical mass loss was defined as follows [35]: MLtheoretical=X1*ML1+X2*ML2

Symbol

(3)

where MLtheoretical was calculated mass loss of the blend, X1 and ML1 were mass fraction and experimental mass loss of the WS, respectively. X2 and ML2 were mass fraction and experimental mass loss of the coal, respectively. It can be found that blends, including SF/WS (1:5), SF/WS (1:4), SF/ WS (1:3) and SF/WS (1:2), maintained little less residues than corresponding theoretical results, from approximately 573 K to 773 K. Moreover, peak mass loss rate of blend SF/WS (1:3) was 1.01 × 10−3 s28

f(α)

g(α)

2(1-α)[-ln(1-α)]1/2 3(1-α)[-ln(1-α)]2/3

[-ln(1-α)]1/2 [-ln(1-α)]1/3

1/(2α) [-ln(1-α)]−1 3/2(1-α)2/3 [1-(1-α)1/3]−1 3/2[(1-α)−1/3-1]-1

α2 α+(1-α)ln(1-α) [1-(1-α)1/3]2

1-α (1-α)2 (1-α)3

-ln(1-α) (1-α)−1 -1 [(1-α)−2-1]/2

2α1/2 3α2/3 4α3/4

α1/2 α1/3 α1/4

1

α

2(1-α)1/2 3(1-α)2/3

1-(1-α)1/2 1-(1-α)1/3

1-2α/3-(1-α)2/3

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Fig. 1. Pyrolysis curves of the individual coal and WS at heating rate β = 10 K∙min−1.

was acquired for blend SF/WS (1:3), compared to those of other blends and individual WS, implying significant interaction occurred in this period, which agreed with the TG and DTG results. Meanwhile, activation energies of the blends, such as SF/WS (1:5), SF/WS (1:4), SF/WS (1:3) and SF/WS (1:2), averaged 98.15 kJ mol−1, were much greater than those of SF/WS (1:1) and individual WS, revealing that interaction formation may be ascribable to adequate hydrogen donating from the WS to a considerable extent. Reaction mechanisms of co-pyrolysis appeared three-dimensional diffusion, in comparison with second-order reaction for individual WS pyrolysis, rather than first-order reation stated by many previous investigations. Biagini et al [39] and Aboyade et al [40] have also noticed poorer fits to experimental data for first-order behaviour simulations and predictions. Pyrolysis of individual WS, occurred at 473–653 K, was rapid with a sharp decrease in mass (see Fig. 1), illustrating large quantities of fragments formation, from hemicellulose and cellulose in a short time; thus, collision between fragment radicals in 533–613 K was control step, suggesting a second-order reaction model. During further

Table 3 Kinetics results of individual SF, individual WS and their blends at different ratios. Sample

Temperature range (K)

Ea (kJ∙mol−1)

R2 (COD)

Mechanism symbol

SF WS

713-803 533-613 713-803 533-613 533-613 533-613 533-613 533-613

47.40 59.94 10.18 93.90 97.58 102.36 98.75 75.82

0.9995 0.9983 0.9994 0.9981 0.9907 0.9979 0.9975 0.9992

D3 F2 F3 D3 D3 D3 D3 D3

SF/WS SF/WS SF/WS SF/WS SF/WS

(1:5) (1:4) (1:3) (1:2) (1:1)

pyrolysis of the WS in 713–803 K, recombination of fragments into benzene substitutes was stated [32], involving hydrogen radical, methyl radical and methoxyphenolic fragment, resulting in a third-order reaction. Copyrolysis of blends SF/WS (1:5), SF/WS (1:4), SF/WS (1:3),

Fig. 2. Thermal decomposition curves of blends from the coal and the WS in nitrogen. 29

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SF/WS (1:2) and SF/WS (1:1), were all in accordance with three-dimensional diffusion mechanism, because coal devolatilization, by and large, was just initiated [41] in range of 533–613 K, pores left after volatile emission were relatively scarce; fragments diffusion in the coal particles, was bound to encounter resistance. Therefore, three-dimensional diffusions of fragments in the blends and coal were both control steps, revealing three-dimensional diffusion mechanisms. In addition, according to basic kinetics concepts, variations in slopes for curves of ln[g(α)/T2] versus 1/T, illustrating decreases (Fig. 3a) or increases (Fig. 3b) in activation energy values, suggest that controlling process shifts from one mechanism to another [42] as shown in Fig. 3. It was described by Fig. 3 that all the blends, along with the individual WS, underwent multi-step consecutive reaction during pyrolysis in 533–803 K (Fig. 3c); ensuring hydrogen transfer, from intermediates of the WS pyrolysis to radicals from the coal, in the blends. On the contrary, competitive processes were prominent and controlling for the individual coal in 533–803 K.

600–1100 K; correspondingly, emission of acetic acid, which is produced more from hemicellulose, due to its abundant acetyl groups [45,46], was highly strengthened during pyrolysis of the blends (Fig. 4c), such as SF/WS (1:5), SF/WS (1:4), SF/WS (1:3) and SF/WS (1:2); during SF/WS (1:2) pyrolysis, maximum relative intensity of acetic acid emission, appeared at 3.11 × 10-10 A/g, which was much higher than those of individual WS (7.89 × 10-11 A/g) and SF (4.15 × 10-11 A/g). Hence, carbonyl/carboxyl containing groups in the blends, were believed to transformed into acetic acid [47], by means of radical stabilization [48], deacetylation [49] and isomerization [50], rather than decarboxylation [31,42] into carbon dioxide, which could also be confirmed by decline in furan (Fig. 5a), methylfuran (Fig. 5b) and furfural (Fig. 5c). Furan compounds, mainly derived from the ring scission of cellulose and hemicellulose, are potential to be upgraded to other value-added chemicals. Variation in content of the coal played negligible role in furan compounds emission. The decrease of furan and methylfuran also indicated the absence of decarbonylation [51] due to the coal incorporation.

3.4. Evolved gases analysis during pyrolysis

3.5. Synergy mechanism analysis

The evolution of volatile matters, form the coal, the WS and their blends, were simultaneously monitored, by quadruple mass spectrometry during thermogravimetric measurements. Characteristic ion intensities of the volatile, with mass to charge ratios (m/z) 18, 44, 60, 68, 82 and 96, assigned to vapor (H2O), carbon dioxide (CO2), acetic acid (C2H4O2), furan (C4H4O), methylfuran (C5H6O) and furfural (C5H4O2), respectively, were discussed in this investigation. Evolution profiles of the most significant ionized fragments, associated with above mentioned volatile, were recorded in Figs. 4 and 5. Vapor releases can be easily classified into two major stages (Fig. 4a). The first evolution peaks were obtained at approximately 363 K, owing to evaporation of absorbed water; the second peaks ranged in 513–713 K, arsing from dehydration of various oxygen containing groups, mainly hydroxyl groups [29] in the WS. Carbon dioxide was mainly released in 483–973 K for the WS pyrolysis (Fig. 4b), caused by degradation of carbonyl and carboxyl groups [43] in hemicellulose and cellulose, with appearance of broadened intervals in the evolution curves. Increase in content of carbon dioxide above 973 K might originate from breakage of substitutes on aromatic rings [44], more thermally stable ether structures and oxygen-containing heterocycles [41]. Furthermore, peak release of the WS at about 610 K was much abundant than that at 683 K or so, illustrating most of the carbonyl/carboxyl containing structures were destroyed prior to 610 K, which was demonstrated by higher activation energy 59.94 kJ mol−1 in 533–613 K, compared to 10.18 kJ mol−1 in 713–803 K, presented in Table 3. In case of the blends, additions of the coal were not beneficial to carbon dioxide emissions, which was inconsistent with carbon dioxide fluctuation of individual coal pyrolysis in proximately

Generally, synergistic effect relevant to volatile emission, for the blends excluding SF/WS (1:1), were in evidence during co-pyrolysis in 573–773 K. First of all, it could be attributed to higher H/C and O/C atomic ratios of the WS, which were about 1.5 and 2.3 times higher than those of the coal, respectively (see Table 1). Consequently, large quantity of hydrogen and hydroxyl radicals, were created, which acted as hydrogen donor species, promoting cracking of aromatic structures in the coal [52,53], into light volatile matters. As a result, secondary reactions between intermediates from the WS and the coal, such as condensation, polymerization and cross-linking, were remarkably suppressed, reducing residue formation [32,54]. It can be seen that temperature played a crucial role in the synergy. At temperature lower than 573 K, nearly only the WS was devolatilized, coal devolatilization occurred until 573 K, in the periphery of mobile phase of macromolecular structure [32]; hereafter, free radicals from the coal were produced during liberation of volatile fragments, free radical concentrations in the coal increased sharply between 673–773 K [38], due to thermal cracking; bond-making reactions also proceeded simultaneously [55]. Subsequently, radicals extracted hydrogen and stable fragments from surroundings, in presence of sufficient hydrogen and hydroxyl radicals from the WS pyrolysis, bringing about more volatile emission; otherwise, underwent polymerization. On the other hand, alkali metal, usually present in WS and coal, such as potassium, was conceived as a key to reactivity [56]. For example, Trendewicz et al [57] reported potassium catalysis in char formation; Di et al [58] stated an increased charring from potassium; Guo

Fig. 3. Variation of controlling mechanism by activation energy values. 30

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Fig. 4. Curves of evolution of the ionized fragments with m/z 18 (a), 44 (b) and 60 (c).

et al [59] found that production of carbon dioxide was enhanced by presence of potassium; Zhang et al [60] indicated an increased yield of furans, because of potassium impregnation into WS. Nevertheless, enhancement in char formation, or carbon dioxide evolution, or furans yield, was all not noticed in synergy of the blends. Hence, synergistic effect, with respect to component interaction and gas emission, was unlikely initiated by catalysis of inherent alkali metal in the blends. In addition, mass ratio of the coal to the WS, was another vital factor to synergy. Sufficient radical formation, convenient radical diffusion, as well as highly-effective radical collision, jointly contributed to the synergistic effect. For example, for blend SF(1:1), insufficient hydrogen and hydroxyl radicals, relative to large amounts of macromolecular structures of coal, were created. Meanwhile, owing to low content of the WS, abundant pores were hardly formed, which ensured radicals diffusivity [3,61], together with transport of mobile phase of macromolecular structure [32,62]. Thus, no synergistic effect were observed; release of volatile from the WS pyrolysis, were also subject to interference due to the coal incorporation, giving rise to a higher residual mass than theoretical one (see Fig. 2f). With increased content of the WS, sufficient hydrogen and hydroxyl radical formation came true; synergistic effect in terms of gaseous products emission were observed, during dominant pyrolysis stages of the blends, such as SF/WS (1:2), SF/WS (1:3), SF/WS (1:4) and SF/WS (1:5). It is noteworthy that the synergy began to weaken for SF/WS (1:5); it was attributed to accumulation and obstruction of exessive WS in SF/WS (1:5), leading to decrease in active sites, along with impeded highly-effective collisions between radicals from the WS and radicals from the coal. Shi et al [63] also declared that diffusion of volatile within pores of solid particles may be inhibited by mass transfer resistance, which delays the volatiles

release, allowing more volatile reaction in the pores. As an intermediate, mainly from the WS decomposition, phenolic hydroxyl groups began to decompose at 573 K, with breakage of aromatic rings, generating carboxyl groups [64]. Because inherent carboxyl groups present in the parent feedstocks, decomposed into carbon dioxide and water at about 473 K [64]; acetic acid was obtained prior to 533 K by hemicellulose deacetylation [34], improved emission of acetic acid between 573–673 K due to synergy, should mostly derive from stabilization [48], between carboxyl radicals from the WS decomposition and methyl radicals from the coal decomposition; strengthened deacetylation [49] and isomerization [50] of lignin, should also devote to the improvement, owing to smaller mass transfer and diffusion resistance. Above mentioned hydrogen transfer to the coal, predominantly in hemicellulose and cellulose pyrolysis stages, led to a less formation of residues on surface of the particles, which contributed to the smaller resistance in mass transfer and diffusion. 4. Conclusions In order to facilitate commercial scale application of WS for valueadded target chemicals, thermogravimetry coupled with mass spectrometry (TG/MS) measurements in nitrogen of individual coal, individual WS, and their blends were performed. Synergistic effect, reaction mechanism, activation energy, gaseous products emission were described. Dominant mass losses for the WS and the coal were observed in 473–653 K and 573–1073 K, respectively. Synergistic reactions, with respect to gaseous products emission, were only detectable in approximately 573–773 K, for blends SF/WS (1:5), SF/WS (1:4), SF/WS (1:3) and SF/WS (1:2). The synergy could be attributed to hydrogen

Fig. 5. Curves of evolution of the ionized fragments with m/z 68 (a), 82 (b) and 96 (c). 31

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transfer, from hydrogen and hydroxyl radicals formed in the WS pyrolysis to the coal, bringing about improved emissions of acetic acid and suppressed residue formation. Acetic acid generation was intensified, mainly by means of termination between carboxyl radicals from the WS decomposition and methyl radicals from the coal decomposition, partly due to deacetylation and isomerization of lignin. Incorporation of the coal into the WS, at mass ratio of 1:2, facilitated higher yield of acetic acid in 473–673 K co-pyrolysis. Suitable temperature, sufficient hydrogen/hydroxyl radical formation, convenient radical diffusion, as well as highly-effective radical collision were considered responsible for the synergy in the blends pyrolysis.

[19]

[20]

[21]

[22]

Acknowledgements

[23]

The authors are grateful for the supports from Key Laboratory of Coal Resources Exploration and Comprehensive Utilization, Ministry of Land and Resources of China (Grant No. KF2018-8), Key Industry Innovation Chain Project of Shaanxi Province (Grant No. 2017DZCXLGY-10-01-02), National Natural Science Foundation of China (Grant No. 41772166).

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