Interaction between coal and distillation residues of coal tar during co-pyrolysis

Fuel Processing Technology 138 (2015) 221–227

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Interaction between coal and distillation residues of coal tar during co-pyrolysis Dexiang Zhang a,⁎, Shengchun Wang a,b, Xiaolong Ma a, Yingqi Tian a a b

Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, 383 box, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China College of Chemical Engineering, North China University of Science and Technology, No. 46 Xinhua West Road, Tangshan 063009, PR China

a r t i c l e

i n f o

Article history: Received 18 January 2015 Received in revised form 29 May 2015 Accepted 1 June 2015 Available online xxxx Keywords: Coal pyrolysis Coal tar distillation residue Co-pyrolysis Light oil Interaction mechanism

a b s t r a c t Co-pyrolysis of Huainan coal (HN) and its distillation residue of the primary coal tar (DRPT) was carried out in a tubular furnace. The experimental results show that the pyrolysis tar yields are 3.74%–1.48% (relatively) higher than the calculated values from blending ratios of DRPT:HN = 1:100 to 5:100, and decrease by 0.95% (relatively) at DRPT:HN = 10:100. The n-hexane-soluble fraction (nHS) in pyrolysis tar is 5.22%, 6.30%, 6.81% and 6.63% higher than the corresponding calculated values at proportion of DRPT:HN = 1:100, 3:100, 5:100 and 10:100, respectively. The pyrolysis residue (PR) yields are negatively affected by adding DRPT to HN. To further investigate the interaction mechanism between HN and DRPT during pyrolysis, thermogravimetry (TG), gas chromatography (GC), hot stage microscopy and electron spin resonance (ESR) techniques were applied to analyze the behavior of their co-pyrolysis. It is found that there are significant interactions between HN and DRPT in co-pyrolysis and the pyrolysis tar and light oil yields are evidently increased by blending DRPT. © 2015 Elsevier B.V. All rights reserved.

1. Introduction As a fundamental energy source, coal has played an important role in Chinese economic growth. According to statistics, about 75% of Chinese total energy is from coal [1]. The provision of cheap, clean and reliable energy supplies will be a long-term research and development challenge to China with the aspiration of improving living standards for an ever-increasing population. Thus the clean coal application becomes the challenge for sustainable economy development. Pyrolysis is an old yet still continuously evolving process technology, which is well recognized as a simple and effective method for clean conversion of coal [2,3]. During the past several decades after the first oil crisis in the 1970s, a great amount of research works about coal pyrolysis have been done [2,4–13]. In recent years, co-pyrolysis of coal and biomass [14–17] has been extensively studied according to the inexpensive and widely available advantages of biomass. However, the relatively dispersed distribution and poor quality of pyrolysis products due to the higher O/C atomic ratio have severely hampered the large scale harness of the biomass resources in China. Moreover, the interests in co-pyrolysis have also been focused on coal and various industrial wastes, such as waste plastic [18–23], petroleum residue [24,25], lube oil wastes [23,26,27], refuse [28,29], waste tire rubber [30], ferment residue [31], and printed circuit board [32]. Among these research works, most of them are concerned on not the yield and ⁎ Corresponding author. E-mail addresses: [email protected] (D. Zhang), [email protected] (S. Wang).

http://dx.doi.org/10.1016/j.fuproc.2015.06.002 0378-3820/© 2015 Elsevier B.V. All rights reserved.

quality of pyrolysis oil/gas but the waste disposal and environmental protection. As to co-pyrolysis of coal and coal tar or distillation residue of coal tar (coal tar pitch), studies on coal tar or coal tar pitch as a kind of binder from the byproducts of coking plant aimed to improve the quality of formed or metallurgical coke were widely reported [27, 33–38]. Special attention has also been given to the physical and chemical characteristics of the binder and coal. For example, Collin et al. [39,40] investigated the coking properties and coke qualities of poor coking coal or coal blends by co-coking with coal tar pitch or modified pitch from co-thermolysis of plastic wastes and coal tar pitch. Fernández et al. [41] studied the interaction of two different ranks of coal with reactive additives (coal tar pitch, benzol distillation residue, tire crumbs) during co-carbonization in order to understand their effect on the coal plastic stage. Apart from the above-mentioned research works, there are few available literatures caring about the yield and quality of the pyrolysis tar. Low temperature flash pyrolysis coal tar is a kind of primary tar derived from high-volatile bituminous coal pyrolysis by circulating fluidized bed and has the characteristics of relatively larger molecular weight and higher solid content. Therefore the high-value and low-cost utilization of this primary tar, especially the residues after distillation of the low-boiling fraction, has become the urgent task in China [42]. Furthermore, the current pyrolysis technology has the disadvantages of low productivity as well as poor coal tar quality. The main objective of this work was to improve the quality and yield of pyrolysis tar by means of co-pyrolysis of a high-volatile bituminous coal and the distillation residues of primary tar (DRPT) derived from low-

222

D. Zhang et al. / Fuel Processing Technology 138 (2015) 221–227

temperature flash pyrolysis of the coal from the same mine, as well as investigate the interaction mechanism between coal and its DRPT during co-processing. The co-pyrolysis of the feedstock will provide a practicable method of optimizing the product distribution and establish basis for the application of the multiple-stage conversion furnace pyrolysis process by circulating coal tar residue. 2. Experimental 2.1. Materials and preparation Huainan coal (HN, from Anhui province, China), a high volatile bituminous coal, was pulverized to less than 75 μm and sealed through preservation by N 2 . DRPT was obtained by distilling the low-temperature flash pyrolysis coal tar at 543 K for 8.0 h and pulverizing to less than 75 μm and sealed through preservation at a temperature of 253 K. The primary tar was generated from HN pyrolysis at 873 K in a 75 t/h circulating fluidized bed polygeneration system constructed by Zhejiang University and Huainan Mining Industry (Group) Co., Ltd. [43]. Proximate analysis of HN and DRPT was performed following the international standard ISO 589:2008 for moisture, ISO 1171:2010 for ash and ISO 562:2010 for volatile matter contents. Ultimate analysis of the two kinds of feedstocks was performed according to the international standard ISO 625:1996. The proximate and ultimate analysis results of HN and DRPT are given in Table 1. 2.2. Instrumentation and procedure The crushed powders of HN and DRPT were premixed in a dry way at different blending proportions of DRPT:HN = 1:100, 3:100, 5:100 and 10:100 by weight of air dry basis (ad) sample at room temperature. Pyrolysis experiments were carried out in a tubular furnace (see Fig. 1). About 50 g of sample was compressed in the quartz boat and loaded into the middle of the quartz tube placed in the tubular furnace under argon atmosphere at 100 dm3/min of volume flow. The sample was heated to final temperature of 873 K at a rate of 5 K/min and kept for 30 min. The pyrolysis volatiles were cooled by ice water and a helically coiled glass tube was placed in a semiconductor cold trap at 233 K. The pyrolysis residue was accurately weighed by an analytical balance (± 0.0001 g) after cooling to room temperature in a dryer. The pyrolysis residue (PR, wt.%, dry and ash free basis, daf) yield was calculated as formula (1): PR ¼

mc −m0 ð1−0:01Mad Þ  Ad  100 m0  ð1−0:01Mad Þð1−0:01Ad Þ

ð1Þ

where mc is the mass of PR, g; Mad is the moisture content of the sample, wt.%, ad; Ad is the ash content of the sample, wt.%, dry basis (d) and m0 is the initial mass the sample, wt.%, ad. The pyrolysis coal tar (CT, wt.%, daf) yield was calculated as formula (2): CT ¼

ð2Þ

where mL is the mass of the recovered liquid products, g and mw is

HN DRPT a

m0 −mc −mL  100: 100 100 m0   100−Mad 100−Ad

ð3Þ

Based on our previous work, the n-hexane-soluble fraction (nHS) is mainly alkane and alkene [45], and nHS is defined as the light fraction of the pyrolysis coal tar. In this study, nHS (wt.%) was obtained as the soluble fraction dissolved by 200 dm3 n-hexane under a constant temperature of 308 K stirring for 24 h. nHS yield was calculated according to formula (4): nHS ¼

ðmL −mw Þ−nHI  100 ðmL −mw Þ

ð4Þ

where nHI is the mass of n-hexane-insoluble fraction, g. The calculated values of gas, CT, PR and nHS are linear addition of the corresponding values of HN and DRPT pyrolysis alone, and the specific formula is just as below: X ¼

a  X1 þ b  X2 aþb

ð5Þ

where X⁎ is the theoretical calculated value of CT, PR, gas or nHS, wt.%, daf; X1 and X2 are the corresponding values of DRPT and HN, respectively, wt.%, daf; and DRPT:HN = a:b (daf). For example, when DRPT:HN = 1:100 (ad), the calculated CT yield is just as follows. X1 = 56.56, X2 = 16.90, a = 1 ∗ (1 − 0.01 ∗ 8.51) = 0.91 g, and 8.51 (see Table 1) is the ash content (dry basis) of DRPT; b = 100 − 2.30 − (1 − 0.01 ∗ 2.30) ∗ 26.34 = 71.97 g, where 2.30 and 26.34 (see Table 1) are the moisture content (air dry basis) and ash content (dry basis) of HN; thus C T⁎ = (0.91 ∗ 56.56 + 71.97 ∗ 16.90)/(0.91 + 71.97) = 17.39 (%). The results obtained in the pyrolysis process were mean values of two experimental results, and the relative errors were within 2%. 2.3. Thermogravimetric analysis HN and DRPT (ca. 5 mg) samples were placed in an alumina crucible (3 mm tall × 5 mm inner diameter) and thermogravimetric (TG) analysis was carried out in a thermogravimetric apparatus (SETARAM LABSYS, France; software: SETSOFT 2000) at a heating rate of 10 K/min from ambient temperature to 1173 K with a nitrogen flow of 60 dm3/min. 2.4. Gas chromatography analysis The operating parameters of gas chromatography (GC) are listed in Table 2.

About 20 mg of sample was compressed to wafer with a dimension of Φ7.5 mm × 2.0 mm before being placed in the crucible. The type of microscope is Axio Imager A2m (Zeiss, Germany). The heating procedure was as follows: 298 K→723 K ð40 K= minÞ→873 K ð5 K= minÞ→1073 K ð10 K= minÞ:

Table 1 Proximate and ultimate analysis of samples. Proximate analysis/%

gas ¼

2.5. Observation through hot stage microscope

mL −mw  100 100 100 m0   100−Mad 100−Ad

Sample

the mass of the recovered total water, g, which was measured by the Karl Fischer method [44]. The pyrolysis gas (wt.%, daf) yield was calculated as formula (3):

Ultimate analysis/%, daf

Mad

Ad

Vdaf

FCdaf

C

H

N

S

Oa

2.30 ~0

26.34 8.51

42.12 78.01

57.88 21.99

83.12 88.72

6.07 6.63

1.54 1.39

0.66 0.43

8.61 2.83

By difference; ad: air dry basis; d: dry basis; daf: dry and ash free basis.

2.6. Electron spin resonance tests The free radicals remaining in the pyrolysis residue were measured by using electron spin resonance (ESR). Based on our previous works [46,47], the standard curve method was established to apply to the

D. Zhang et al. / Fuel Processing Technology 138 (2015) 221–227

3

2

4

12

• • • • • • • • • • • • • • •• 5

223

• • • • • • • • • • • • • • ••

Ar 1

6

7

8

9

10 11

14 13

16

17

15

Fig. 1. Schematic diagram of the pyrolysis equipment. 1. Gas cylinder; 2. Relief valve; 3. Valve; 4. Mass flow meter; 5. Thermocouple; 6. Temperature control; 7. Quartz pipe; 8. Tubular furnace; 9. Heating wire; 10. Quartz boat; 11. Plain bend; 12. Receiver flask; 13. Ice water tank; 14. Glass Graham condenser; 15. Cold trap; 16. Triple valve; 17. Gas bag.

ESR technique for quantitative analysis of the free radicals. All ESR spectra were obtained from a Bruker EMX-8/2.7 (Germany) model ESR spectrometer employing 100 kHz modulation.

3. Results and discussion 3.1. Results of pyrolysis experiment 3.1.1. CT and gas yields The yield of pyrolysis coal tar and gas at different DRPT blending ratios was shown in Fig. 2. It was visually apparent in Fig. 2 that obvious increases in both CT and gas yield were achieved by adding DRPT to HN. As shown in Fig. 2, CT yields at DRPT ratios of 1:100, 3:100, 5:100 and 10:100 were 1.12%, 1.64%, 2.63% and 4.00%(wt.%, daf) higher than that of HN respectively. Meanwhile, the corresponding gas yields were 1.85%, 2.18%, 2.44% and 2.54% higher than that of HN respectively. Comparison of CT and gas yields of experimental values with the linear addition (calculated) ones was presented in Fig. 3. As shown in Fig. 3, gas yields were 1.84%–2.45% higher than the calculated values with the addition of DRPT to HN. CT yields are 3.74% to 1.48% (relatively) higher than the calculated ones at DRPT blending ratios from 1 to 5 per 100, but 0.95% lower than the calculated value at a DRPT blending ratio of 10 per 100. It is noteworthy that the synergistic effects on CT yield at lower DRPT mixing ratios, especially, the interaction strength reached the peak while DRPT:HN = 1:100 and gradually diminished with the increasing blend ratios of DRPT (see Fig. 3).

3.1.2. nHS content in pyrolysis tar The comparison of improvements on nHS at different DRPT ratios was shown in Fig. 4. It was shown in Fig. 4 that both the experimental value and the calculated value of nHS content in pyrolysis tar were all higher than that of HN and increased with the increase of DRPT proportion. As shown in Fig. 4, the content of nHS in pyrolysis tar at proportions of DRPT/HN = 1:100, 3:100, 5:100 and 10:100 was 5.69%, 7.82%, 9.18% and 10.87% higher than that of HN, respectively. Moreover, with the increase of DRPT content in the mixture, nHS content in pyrolysis tar was 5.22%, 6.30%, 6.81% and 6.63% higher than the corresponding calculated value at the same DRPT proportion. Conspicuously, data in Fig. 4 showed a strong expression of a significant synergistic effect on the improvement of pyrolysis tar quality by co-pyrolysis of HN and DRPT.

3.1.3. Variation in pyrolysis gas species The variation of gas species at different DRPT ratios was shown in Fig. 5 and the changes of Ci/Σ(C1–C4) in hydrocarbon gas (i: 1–4, carbon number; Σ(C1–C4): total hydrocarbon (C1–C4) gas) were shown in Fig. 6. As presented in Fig. 5, the relative contents of CO and CO2 in pyrolysis gas were all decreased with the increasing ratio of DRPT, which could be easily explained by the data provided in Table 1 as the O/C atomic ratio of DRPT (0.024) is lower than that of HN (0.078). There was a slight decrease of CO as well as an increase of H2 content in gas with the increasing DRPT blending ratio (see Fig. 5). Conversely, as it was shown in Fig. 5, there was an obvious decrease of CO2 and increase of Σ(C1–C4) content in gas with the increasing DRPT proportion. It could also be seen in Fig. 6 that no significant differences in the relative proportion of C3/Σ(C1–C4) and C4/Σ(C1–C4) were observed with the increase of the DRPT blending ratio. As to the relative proportion of C1 and C2 in Σ(C1–C4), C1/Σ(C1–C4) decreased but C2/Σ(C1–C4) increased with the increasing proportion of DRPT.

3.2. Analysis of interaction between HN and DRPT 3.2.1. TG analysis of HN and DRPT TG and DTG curves of HN and DRPT were plotted in Fig. 7. The characteristic parameters were listed as follows: Ti (initial decomposition temperature); Tf (final temperature); Tmax (temperature of maximum rate of decomposition) and ΔT (reaction interval, Tf − Ti). The characteristic parameters calculated by system software were also given in Table 3. As presented in Fig. 7 and Table 3, ΔT length of DRPT was 245.5 K, which was more than 2.2 times than that of HN, and there was a huge overlap between HN and DRPT, thus the reaction temperature interval of HN would be sharply increased by adding DRPT during the pyrolysis process. Consequently, the temperature range and time of the successive stage of devolatilization would be prolonged largely due to mixing DRPT to HN. Therefore the pyrolysis fragments generated during the co-pyrolysis process would undergo more serious secondary pyrolysis than that of HN pyrolysis alone and thus caused to increase gas yield. Otherwise, due to undergo more severe cracking reactions during the period of secondary pyrolysis, the larger size of pyrolysis fragments might form to medium molecular size of CT, resulting in the increase of CT yield synergistically. Merely, the degree of the interaction for the formation of CT was milder than that for Gas, which could account for the relatively weaker synergistic effects on CT yield displayed in Fig. 3. Similar interactions have simultaneously been reacted on nHS formation

Table 2 Parameters of gas chromatography. Instrument

GC126

Detected gas Injection temperature Detector temperature Column Column dimensions Column temperature Carrier gas

H2 100 °C 120 °C TDX-01 2 m × 3 mm 40 °C N2

CO, CO2, CH4 100 °C 120 °C TDX-01 2 m × 3 mm 40 °C (6 min); 40 °C→100 °C (20 °C/min); 100 °C (10 min) He

C2–C4 150 °C 150 °C HT-PLOT 50 m × 0.33 mm × 25 μm 40 °C N2

D. Zhang et al. / Fuel Processing Technology 138 (2015) 221–227

HN DRPT/HN=1/100 DRPT/HN=3/100 DRPT/HN=5/100 DRPT/HN=10/100 DRPT

Yield/ wt%, daf coal

56

20

16

12

90 calculated value experimental value

88 86

nHS in tar/ wt%

224

84 82 80 78 76

8

gas

CT

0

Fig. 2. CT and gas yield at different DRPT ratios.

3 1 5 -1 Mass of DRPT/ g.(100g air dry coal)

10

Fig. 4. Comparison of improvements on nHS at different DRPT ratios.

(see Fig. 4). The larger ΔT of HN by adding DRPT may contribute to the increasing yield of nHS and enhance the synergistic effect on nHS as shown in Fig. 4. The enlarged ΔT could prompt the coal particles to be well infiltrated by improving the thermoplastic properties during pyrolysis of DRPT/HN. As described in the literature, the plastic stage existing in the period of pyrolysis plays an important role on the improvement of volatile products due to its higher activity and hydrogen transfer ability [41,48,49]. The thermoplastic properties of DRPT/HN should be highly modified due to the plenty of hydrogenrich free radical fragments (H• or CH3• etc.) existing during the plastic range on account of the relatively higher H/C atomic ratio of HN (0.88) and DRPT (0.90) (see Table 1). Due to the existence of the modified plasticity, the medium molecular fragments sufficiently contact with the hydrogen-rich free radical fragments with light molecular weight. They are further stabilized to form products with moderate molecular weight, leading to increase CT yield and the content of nHS in the tar (Figs. 3–4). 3.2.2. Direct observation of pyrolysis behavior by in-situ hot stage microscope The in-situ hot stage microscope images of HN and DRPT/HN = 5:100 at room temperature and the temperature of appearing visible fused plastic were shown in Fig. 8. The corresponding temperatures of appearing visible fused plastic (elliptic regions plotted by a dotted line) were 753 K (Fig. 8b) and 731 K (Fig. 8d) for HN and DRPT/HN = 5:100, respectively. The corresponding temperature of DRPT/HN was

22 K lower than that of HN, which implied that the thermal plastic appeared earlier in DRPT/HN than in HN during the pyrolysis process. Due to DRPT addition, the temperature difference was caused by the tiny drop existing in DRPT/HN mixture rather than in HN at the early stage, which might reduce the energy barrier of appearing thermoplastic fluid phase in HN to some extent during the pyrolysis process. The softening temperature of DRPT was 358 K, which was much lower than Ti of HN (685.1 K, see Table 3). As a result, HN particles would be infiltrated by the fused pitch in DRPT and thus hinder the escape of oxygenic permanent gas products from the particle at the early stage during pyrolysis. The tendency of relative content reduction of CO2 and CO in gas (Fig. 5) supported the above-described elucidation. The enhancement of CT with the increase of the DRPT ratio, especially when DRPT/HN is less than 5% as observed in Figs. 2–3, is considered to be a combination of two factors. Firstly, the increased proportion of DRPT to HN will directly enhance CT yield by vaporizing the inherent tar in DRPT. Secondly, the synergistic increase of CT yield could be attributed to the improvement on the thermoplastic properties of higher activity and hydrogen transfer capacity during the plastic range, which has a positive effect on upgrading the yield and quality of pyrolysis tar.

3.2.3. Analysis of free radical concentration of PR Comparison of calculated and experimental values of PR yield was displayed in Fig. 9. Apparently, the inverse relationship was that the

60 58 56

experimental value of CT calculated value of CT

20 19 18

experimental value of gas calculated value of gas

17 13 12 11 10

Relative content/ vol%

Yield/ wt%, daf basis coal

21

Σ ( C1-C4)

H2

20 15

CO2

10 CO

0

2 4 6 8 10 -1 Mass of DRPT/g.(100g air dry basis coal)

Fig. 3. Comparison of improvements on CT and gas at different DRPT ratios.

5 -1

0

1 2 3 4 5 6 7 8 9 -1 Mass of DRPT/g.(100g air dry coal) Fig. 5. Composition of gas at different DRPT ratios.

10 11

Relative proportion/ %

D. Zhang et al. / Fuel Processing Technology 138 (2015) 221–227

82 81 80 79 14

Table 3 Characteristic parameters of HN and DRPT based on TG analysis.

C1/Σ (C1-C4)

C2/Σ (C1-C4)

12

Species

Ti/K

Tf/K

Tmax/K

ΔT/K

HN DRPT

685.1 524.8

796.0 770.3

721.6 667.0

110.9 245.5

Ti: initial decomposition temperature; Tf: final temperature; Tmax: temperature of maximum rate of degradation; ΔT: reaction interval, Tf − Ti.

10 8

C3/Σ (C1-C4)

6 4

liable to be quenched by the macromolecular free radicals from coal pyrolysis [51], there are more relatively small-sized free radical fragments from DRPT pyrolysis coupled with the macromolecular free radicals in PR with the increasing proportion of DRPT and therefore lead to the reduction of free radicals in PR. This is in agreement with the fact that the higher DRPT blending ratio has a negative effect on CT yield.

C4/Σ (C1-C4)

2 0 -1

225

0

1 2 3 4 5 6 7 8 9 -1 Mass of DRPT/ g.(100g air dry coal)

10 11

Fig. 6. Relative proportion of C1–C4 in Σ(C1–C4).

(1) CT yield was increased by 3.74%–1.48% (relatively) synergistically at DRPT blending ratios of 1–5 per 100 HN, and the maximum improvement of CT yield was obtained at DRPT:HN = 1:100, but the synergistic effects on CT yield were gradually decreased subsequently as DRPT increased. The light oil fraction in pyrolysis

HN DRPT

100 80 60 40 20 0

4. Conclusion

a 400

600

800

Temperature/ K

1000

1200

HN DRPT

1 dm/dt/ (%/min)

percentage of residual wt/ %

experimental PR yields were markedly lower than the corresponding calculated values with the increasing proportion of DRPT, which is compatible with the tendency of synergistic enhancement of CT and gas yield of pyrolysis. It is conspicuous that the decrement of PR has been found in either liquid or gaseous products, and partly increased CT yield and nHS content in pyrolysis tar (Figs. 3–4). To further investigate the interaction mechanism of the co-pyrolysis of HN and DRPT, the concentration of free radical fragments in PR was detected by ESR technology. Since the bulk of reactive free radicals are short lived [50], which are mainly scattered in pyrolysis gas and liquid phases and difficult to be detected by the instrument, detection of the free radicals in PR is feasible because of their relative larger molecular size as well as longer surviving period. Comparison of concentration of free radicals in PR at different DRPT proportions was given in Fig. 10. It is obvious that the concentration of free radicals in PR from HN was lower than that of from DRPT/HN in Fig. 10. The data in Fig. 10 gave two kinds of information. First, the level of the concentration of free radicals remaining in PR of DRPT/HN is apparently higher than that of HN. Meanwhile, the greater the number of free radicals remaining in PR of DRPT/HN, the more active free radicals may be generated from co-pyrolysis of DRPT/HN. The reactive activity of DRPT/HN during pyrolysis should be much higher than that of HN pyrolysis alone. Secondly, the maximal value of the free radicals in PR of DRPT/HN appeared in DRPT blending ratio about 5:100. From the experimental results obtained so far (Figs. 9–10), there are more of free radical fragments contacting with the medium-sized to slightly large-sized free radicals and crosslinking to larger macromolecular network of char when DRPT proportion is excess during co-pyrolysis of DRPT and HN. Since the small-sized free radical fragments are more

3.2.4. Analysis of variation on composition of pyrolysis gas As presented in Figs. 5–6, the contents of H2 and CH4 were by far higher than other gaseous species in pyrolysis gas, which indicated that vast quantities of H• as well as CH3• should be generated during the pyrolysis process. The variation of H2 and CH4 content in pyrolysis gas could reveal some interaction mechanism during co-pyrolysis of HN and DRPT. Due to the obvious reduction of methane relative content in Σ(C1–C4) with the growing DRPT ratio as shown in Fig. 6, there were more CH3• fragments to be stabilized and formed to lighter tar fraction, and consequently facilitated nHS yield and the quality of CT. As to the hydrogen content in gas, the growing tendency was not evident with the increase of DRPT blending ratios as presented in Fig. 5, which was related to two reasons. Notwithstanding a great many of H• were abstracted by pyrolysis fragments during the early stage of pyrolysis, meanwhile, the resolidification of the macromolecular networks generated plenty of H• to form to H2 at the late stage of pyrolysis, and therefore caused the inconspicuous increase of hydrogen content in gas. According to the above-described data, the strong interactions exist between HN and DRPT and enhance the pyrolysis activity by adding DRPT to HN.

0 -1 -2 -3

b 400

Fig. 7. TG, DTG curves of HN and DRPT (a: TG; b: DTG).

600 800 1000 Temperature/K

1200

226

D. Zhang et al. / Fuel Processing Technology 138 (2015) 221–227

a

c

HN, T=298K

DRPT/HN=5/100, T=298K

b HN, T=753K

d

DRPT/HN=5/100, T=731K

Fig. 8. Photomicrograph of HN and DRPT/HN = 5/100 with increasing temperature recorded by the in-situ hot stage microscope.

tar (nHS) was 5.22%, 6.30%, 6.81% and 6.63% higher than the corresponding calculated values at proportions of DRPT = 1, 3, 5 and 10 pre 100 HN. (2) The improvement on CT yield and quality is attributed to the number of generated hydrogen-rich free radicals during the pyrolysis process, which is verified by the variation of methane content in Σ(C1–C4) of gas. (3) The strong interactions exist between HN and DRPT during co-pyrolysis and lead to the improvement on reactive activity and hydrogen-donating capacity of thermoplastic. These are demonstrated by the enlarged reactive temperature interval, decreased temperature of appearing visible fused plastic as well as increased free radical concentration in PR of DRPT/HN, thus the yields of CT, nHS and gas can be enhanced significantly.

Nomenclature ash content of dry base coal Ad CT pyrolysis coal tar DRPT distillation residue of the primary coal tar DTG derivative thermogravimetry ESR electron spin resonance Fixed carbon content of dry and ash-free base coal, wt.% FCdaf GC gas chromatography HN Huainan bituminous coal moisture content of air dried coal, wt.% Mad nHI n-hexane-insoluble fraction nHS n-hexane-soluble fraction PR pyrolysis residue TG thermogravimetry

69

1.95 experimental value of PR calulated value of PR

67

1.90

Ng/(10 spins/g)

66

19

Yield/ wt%,daf basis coal

68

65 64

1.85

1.80

63 62 0

2 4 6 8 10 -1 Mass of DRPT/g.(100 air dry basis coal)

Fig. 9. Comparison of experimental and calculated PR yield at different DRPT ratios.

1.75

0

2 4 6 8 10 -1 Mass of DRPT/g.(100g air dry basis coal)

Fig. 10. Free radical concentration of PR at different DRPT ratios.

D. Zhang et al. / Fuel Processing Technology 138 (2015) 221–227

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