Characteristics of Soot from Rapid Pyrolysis of Coal and Petroleum Coke

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Characteristics of soot from rapid pyrolysis of coal and petroleum coke Zhike Gai, Rong Zhang, and Jicheng Bi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03366 • Publication Date (Web): 21 Feb 2017 Downloaded from http://pubs.acs.org on February 22, 2017

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Characteristics of soot from rapid pyrolysis of coal and petroleum coke Zhike Gaia, b, Rong Zhanga,*, Jicheng Bia

a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Shanxi, 030001, PR China;

b

University of Chinese Academy of Sciences, Beijing, 100049, PR China.

ABSTRACT: Rapid pyrolysis of several types of solid carbonaceous materials (lignite, bituminous coal and petroleum coke) was conducted in a drop-tube furnace at 1300 °C under atmospheric pressure. Soot properties, including yield, particle size, microstructure and reactivity, were studied, with focus on the key factors that influence soot gasification reactivity. Results show that soot yield of bituminous coal is up to 15-20 wt%, which is nearly half of the volatile matter, while those of lignite and petroleum cokes are merely several percent of the raw materials. The reactivity of soot is lower than that of coal char and higher than petcoke char. Among the several soots, lignite soot is most active, while petcoke soot displays the lowest reactivity. Catalytic effect of mineral content is excluded since no alkali metal and alkali earth metal were detected in soot samples. Reactivity difference is independent of particle size but closely related to the microstructure. The 1 ACS Paragon Plus Environment

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soot particles consist of primary spherical or nearly spherical nanometer particles with a recognizable central nucleus of about 5-15 nm. Concentrically arranged graphene layers form an onion-like structure. Soot formed from lignite has more bent graphene layers and higher amorphous carbon content due to lattice defects, coincident with its high gasification reactivity.

KEYWORDS: soot; rapid pyrolysis; gasification reactivity; drop-tube furnace

2 ACS Paragon Plus Environment

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1. INTRODUCTION Entrained-flow gasification technology has been developed rapidly and used widely in recent years, especially in China, due to feedstock flexibility, high conversion, and high throughput1. When coal particles are injected into a gasifier, the initial reaction is pyrolysis which produces gases, char and tar. Tar, mainly consisting of polycyclic aromatic hydrocarbons (PAHs), has a propensity for soot formation at elevated temperatures above 1000 °C in fuel-rich regions2. Primary soot particle is a nanoscale solid carbon with spherical or nearly spherical shape. The presence of soot in a furnace flame promotes radiation and the efficiency of heat transfer from the flame3. After formed, the majority of soot particles are oxidized in the flame front. However, a part of soot particles may break through the flame due to the turbulent nature of the flame4. Soot particles, one part of residual carbon in fly ash or slag, may cause carbon waste and reduce energy efficiency. Moreover, soot is a principal class of particulate air pollution.5 Fundamental understanding of soot formation is necessary for the better design and modelling of coal gasification systems. The relationship between soot properties and fuel origin is needed to study. Most of studies on soot formation from coal focused on soot physical properties, such as yield, particle/agglomerate size, optical properties, as well as the reactivity. Research on fuel type effect on soot formation has also been carried out. It has been recognized that nearly half of the volatile matter of bituminous coal converts to soot under inert atmosphere at high temperatures.6 The general soot formation trend of coal is as follows: high volatile bituminous > medium volatile bituminous > lignite > anthracite.7 Zeng and coworkers compared the primary tar yield with the yield of soot plus tar during pyrolysis.


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It was found that tar derived from higher rank coal tended to form soot while that from low-rank coal was readily to crack.8 With the development of gasification technology, the consumption of other types of solid carbonaceous materials, such as lignite9 and petroleum coke10, is increasing. Extended research is needed to study the effect of fuel type on soot properties and to provide suggestions for the usage of these materials. Moreover, gasification reactivity of soot is expected to be rather low.2 The conversion of volatiles to soot may slow total carbon burnout. It is necessary to explore how to retard soot formation and to enhance soot gasification. It is known that gasification reactivity is affected by fuel identity and formation conditions11-15. Qin et al.16 and Trubetskaya et al.12 studied soot formation from biomass and found that the reactivity of soot formed at high temperatures was higher than those formed at low temperatures. Since the potassium content was high in the soot produced at a higher temperature, they confirmed that the higher reactivity was related to the presence of potassium. Researches on other sources of soot particles, such as spark discharge soot, diesel soot, and commercial carbon black, indicated that soot oxidation reactivity depended on initial microstructures11,13-15, including the length and curvature of basic structural units, surface functionalization, and the ratio of sp2/sp3 hybridized carbon. Miura and coworkers2 carried out rapid pyrolysis studies of bituminous coals in a pressurized drop tube furnace and found the gasification reactivity of soot was expected to be 1/20-1/6 of the corresponding char. Unfortunately, the exact nature for the reactivity difference of soot from solid carbonaceous materials is still unclear. In the present work, three different types of fuels, such as lignite, bituminous coal and petroleum coke, were selected to carry out rapid pyrolysis in a drop-tube furnace in


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nitrogen atmosphere. A convenient method was applied to quantify soot based on the particle size distribution characteristics of soot and char. Electron microscopes, as well as Raman spectroscopy, were used to characterize the microstructure of soot particles. Soot reactivity to carbon dioxide was measured in a thermobalance, and the key factors controlling the reactivity were clarified. At last, the effect of fuel origin on soot reactivity was discussed. 2. EXPERIMENTAL SECTION 2.1. Raw materials Proximate and ultimate analysis of five solid carbonaceous materials are listed in Table 1. Xiaolongtan (XLT) lignite is from Yunnan province. Buliangou (BLG) and Xichagou (XCG) bituminous coal are from Inner Mongolia and Shaanxi province, respectively. Jinshan (JS) petcoke and Yanshan (YS) petcoke are from Shanghai petrochemical Co. Ltd. and SINOPEC Beijing Yanshan Company, respectively. Materials were crushed and sieved to 45-75 µm. Small particles were dislodged by liquid-suspension gravity separation17. All the materials have a narrow and similar particle size distribution as shown in Fig.1. The results are offset for clarity. No small particles (diameter less than 30 µm) were detected, which was a key factor in the following soot quantification by the particle size distribution analysis. Samples were dried at 105 °C for 2 h under vacuum prior to pyrolysis.


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Table 1. Proximate and ultimate analysis of the test samples. Proximate Analysis /wt%

Ultimate Analysis/wt%

Ad

VMdaf

FCdaf

Cdaf

Hdaf

Ndaf

Sdaf

Rank XLT

lignite

10.22

51.78

48.22

66.64

4.57

1.97

1.34

BLG

bituminous coal

14.91

36.39

63.61

77.79

4.60

1.38

0.73

XCG

bituminous coal

2.23

37.75

62.25

79.75

5.48

1.25

0.18

YS

petcoke

0.16

7.11

92.89

92.53

3.10

1.39

0.87

JS

petcoke

0.32

9.64

90.36

90.79

4.15

1.63

3.12

10%

Note: d, dry; daf, dry and ash-free; A, ash; VM, volatile matter; FC, fixed carbon.

Volume fraction (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.01

XLT XCG BLG YS JS

0.1

1

10

Particle diameter (µm)

100

1000

Figure 1. Particle size distribution of raw materials after liquid-suspension gravity separation. 2.2. Pyrolysis tests The rapid pyrolysis tests were carried out in a drop-tube furnace (DTF) under atmospheric pressure. The schematic construction of the experimental setup is shown in Fig.2. The reactor tube was made of alumina (40 mm i.d. ×1270 mm length) and heated by four MoSi2 heating elements. The maximum attainable gas temperature in this furnace was 1650 °C. Furnace wall temperature was fixed at 1300 °C in the study, and the length


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of isothermal reaction zone (temperature range from 1250 °C to 1350 °C) was about 0.5 m. When the furnace was heated to the setting temperature, coal particles were continuously fed by a screw feeder with the primary gas (N2; 0.3 L/min STP) through the water-cooled injection probe. A preheated secondary gas (N2; 2.0 L/min STP) entered the reactor through the annular space between the injection probe and reactor tube. Solid products were mainly collected in the holding tank. Particles entrained in the gas were captured by the downstream filter. The filter was a front end closed cylinder and made of three-layer qualitative filter paper as shown in Fig.2. It has been proved to capture nearly all the submicron and nanoscale particles. The pyrolytic gases were dried by silica gel and analyzed by gas chromatograph. In the DTF facility, particle residence time was assumed to be equal to the gas residence time because the estimated free-fall velocity of the particle under 100 µm was much lower than the gas flow. The gas residence time was determined by the sum flow rate of carried gas and pyrolytic gas. Since the difference of volatile matter content among different raw material was noticeable, the feeding rate of raw material was varied to maintain the similar gas residence time to exclude the effect of residence time on soot properties. Based on the preliminary experiments, feeding rate of XLT lignite was chosen to be 1.22 g/min, while those of XCG bituminous coal and YS petcoke were 2.0 g/min and 2.60 g/min respectively. The gas residence time was estimated to be about 2 s. Small portion of pyrolytic carbon deposited on the constant temperature zone of the reactor. It is classified as one part of soot.18 It was difficult to collect. The amount of this part of soot was measured by burning with air at the end of each run. The yield of tar was determined by extraction of solid products using tetrahydrofuran. Carbon content of tar


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was determined by the carbon difference between the solid product before and after extraction using an elemental analyzer (Vario EL).

Figure 2. Schematic diagram of DTF equipment. (1) N2 gas cylinder; (2) Flowmeter; (3) Preheater; (4) Screw feeder; (5) Reactor; (6) Sampling probe; (7) Holding tank; (8) Filter; (9) Drier; (10) Gas sampler. 2.3. Gasification reactivity Gasification

reactivity

studies

of

solid

products

were

performed

by

temperature-programmed reaction (TPR) using a thermogravimetric analyzer. About 5 mg sample was placed in a platinum crucibe and heated at a rate of 5 °C /min in pure CO2 (60 mL/min). Conversion was calculated by the following equation:

X = (W0 − Wt ) /(W0 − Wf ) Where X is the conversion; W0 and Wf are the initial and final mass of the solid products (mg); Wt is the instantaneous mass during gasification (mg). The temperatures at which


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the weight loss ceased were selected to be as the reactivity index to compare the reactivity of solid products. 2.4. Characterization of solid particles The morphology and composition of solid products were characterized using a scanning

electron

energy-dispersive

microscopy X-ray

(SEM,

JEOL

JSM-7001F)

spectroscopy

(EDS,

BRUKER

equipped

with

an

QUANTAX200).

A

high-resolution transmission electron microscopy (HRTEM, JEOL JSM-2010) operated at 200 kV was used for the in-depth microstructure analysis of soot. Gatan image software (version 3.9) was applied to carry out lattice fringe analysis. Particle size distribution (PSD) was measured by light scattering using a laser diffraction particle size analyzer (Malvern, MS-2000). Water was used as dispersion medium. Sodium lignosulphonate (60 mg/L) was added as surfactant to prevent soot aggregation. The concentration of surfactant was rather low that the effect on PSD was negligible. Raman spectra were recorded on a LabRam HR800 (RM, Jobin Yvon) using laser wavelength of 514 nm in the range of 600-2000 cm-1(Stokes shift). The laser spot diameter on soot surface was 40 µm. Since the diameters of soot particles stay in the range of 50-300 nm, several hundred particles could be probed. XPS Peak 4.1 software was used to conduct curve fitting. Fitting results were regarded as convergent when the reduced χ2 values stay between 1 and 3, and representative when relative error between χ2 values was less than 0.5%. 3. RESULTS AND DISCUSSION 3.1. Soot quantification method


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After running continuously for 20 minutes, solid products were sufficiently collected from four parts of DTF equipment: the filter, holding tank, inner wall of the reactor bottom, and the sampling probe. As confirmed by SEM and EDS results showing in Fig.S1 and Table S1 included in the Supporting Information, particles collected from the filter are soot particles, and those trapped in the holding tank are char particles. While both of particles deposited on the bottom of the reactor inside wall and sampling probe are mixtures of soot and char as shown in Fig.S2. It was difficult to separate soot from char. Some methods were used to measure the amount of soot. Miura et al.2 used a thermogravimetry to estimate the amount of soot in the mixture according to the reactivity difference between soot and char. Particle size distribution (PSD) analysis technology was used to quantify soot by Umemoto et al.19. The laser diffraction intensity increased monotonically with the char volume fraction. The volume basis fraction was converted to weight basis ratio assuming that the density of char and soot were identical. In the present study, this method was modified by considering the density difference of char and soot, and the method was verified using synthetic mixtures. Fig.3a shows the PSD of char and soot from different raw materials in the form of volume fraction. Char particles are larger than 30 µm, and show similar PSD to the raw materials (Fig. 1), indicating that almost no char fragments formed during pyrolysis. Soot agglomerates from different materials are all smaller than 30 µm. As seen in Fig.3b, PSD of the mixture of char and soot from sampling probe and reactor wall have two groups of peaks: at small diameter (< 30 µm), and at large diameter (> 30 µm). According to the PSD features, cut-point diameter of 30 µm is used to separate soot. Thus, the volume fraction of the particles less than 30 µm is equal to the volume fraction of soot in the


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mixture of soot and char. Mass fraction of soot was calculated based on the volume fraction and the density. In the case of the laser diffraction type particle size analyzer, particle size is presented as an equivalent-volume sphere diameter20. Therefore, char apparent density determined by a mercury porosimeter was used in calculation. The apparent density of char particles ranges from 0.85 g/cm3 to 1.36 g/cm3, due to the differences in ash content and porosity. Park et al.21 found that the inherent density of soot formed at high temperatures was 1.77±0.07 g/cm3 independent of particle size. Thus, the density of soot was selected to be 1.80 g/cm3. On the one hand, soot is assumed to be non-porous for the low porosity.12On the other hand, most of the soot was dispersed as single particle in the water from the results of PSD in the form of number fraction as shown in Fig.S3 in the Supporting Information. In order to check the validation of the method, candle lampblack was used to represent soot and mixed with char samples in different proportions. Fig.4 shows the particle size distributions of lampblack and the samples prepared by mixing lampblack and XCG char. The synthetic mixtures have a similar PSD to those of mixtures obtained in the present study. Fig.5 shows the correlation between the calculated and the actual fraction of lampblack in the synthetic mixtures. The calculated values are consistent with the actual values. Thus, the PSD analysis technology was used to quantify soot and char in the following section.


0.01

0.1

XLT soot XCG soot BLG soot YS soot JS soot

1

a 5%

XLT char XCGchar BLG char YS char JS char

Volume fraction(%)

Volume fraction(%)

10

100

1000

0.01

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XLT sampling probe XCG sampling probe BLG sampling probe YS sampling probe YS reactor bottom JS sampling probe JS reactor bottom

XLT reactor bottom XCG reactor bottom BLG reactor bottom

0.1

10


1

100

b

1000


Figure 3. PSD of the char and soot particles (a) and the mixture of char and soot from

Volume fraction (%)

5%

sampling probe and reactor bottom (b).

0.01

Lampblack Lampblack- XCG char(9:1) Lampblack- XCG char(7:3) Lampblack- XCG char(5:5) Lampblack- XCG char(3:7) Lampblack- XCG char(1:9) XCG char

0.1

1

10

100

1000


Figure 4. PSD of the synthetic mixtures of lampblack and XCG char. Measured lampblack fraction(wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

15%

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100 Mixture of lampblack and XCG char Mixture of lampblack and JS char Mixture of lampblack and YS char

80 60 40 20 0

0

20

40

60

80

Lampblack in synthetic mixture(wt%)


100

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Figure 5. The measured and actual lampblack fraction in the synthetic mixtures. 3.2. Soot yield Fig.6 shows cumulative product yields of different raw materials obtained at 1300 °C, as well as the carbon distribution in volatile matters (soot+tar+gas). Mass and carbon balances are close to 99±2%. The weight loss, equal to the weight difference between raw material and char yield of dry ash-free basis, is in the following order: lignite > bituminous coal > petcoke. While soot formation tendency is as follows: bituminous coal > petcoke > lignite. The results indicate that soot yield is influenced by not only the amount but also the composition of volatile matter. The experimentally observed order of sooting tendency is consistent with the literature.6-8 In the case of bituminous coals, 19.66% of XCG coal is converted to soot and the amount is 15.21% for BLG. Nearly half of raw material volatile matter determined by proximate analysis (PVM) is converted into soot. The high soot yield of bituminous coal is similar to those reported by other researchers.2,3,6-8,22 Nenniger et al.7 and Wornat et al.22reported that about 20% of Pittsburgh bituminous coal was converted to soot at around 1300 °C in inert conditions. Furthermore, about 60-67% of the converted carbon is soot in the volatile component. During bituminous coal pyrolysis, primary tar accounts for 50-70% of weight loss.23,24 The bituminous coal tar is mainly composed of larger and more stable PAHs, which are readily to form soot nuclei and add to soot surface.8 Therefore, the majority of bituminous tar is converted to soot during secondary reactions. The soot yield of XLT lignite is only 4.47%, even though PVM is as high as 46.49%. Only 16% of the volatile carbon is converted to soot, while up to 79% is present in CO. The primary tar yield of XLT lignite may account for about 20-40% of weight loss23,24. 13 ACS Paragon Plus Environment

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The empirical value of CO yield is estimated to be around 7% based on the oxygen content of the raw coal23. However, the yield of soot plus tar is 5.2%, while CO yield is nearly half of the raw coal in the present study. Soot yield accounts for only 11-22% of the primary XLT lignite tar, suggesting that the tar formed from a low rank coal has a low sooting propensity. The low soot yield of lignite is consistent with the results of Zeng et al.8 They reported that lignite tar was susceptible to cracking due to large aliphatic attachment portion, high substitution and functional groups. In addition, lignite produces relatively large amount of oxidizing gases (H2O and CO2) during primary pyrolysis. Reactions of CO2 and H2O with hydrocarbon radicals may also contribute to CO formation, which is consistent with the low yields of light hydrocarbons and oxidizing gases in pyrolysis gas. Furthermore, gasification reactions between soot and oxidizing gases are also excluded in this study. The following HRTEM studies show that no gasification reactions take place. Soot yield of petroleum coke is merely 1-2.6%, and the volatile carbon that converted to soot is around 24-38%. The low soot yield is closely related to the low volatile matter content of petcokes. The ratio of carbon in volatile to that in raw material is very low (5-7%) compared with those of coal (30-40%), resulting in low concentration of soot precursors. Collision efficiency of soot precursors and surface growth are depressed, which might retard soot formation. Furthermore, petcokes are regarded to be composed of polynuclear aromatic structures with few alkyl substituents.25 Some non-completely carbonized structures, such as unreacted pitch and mesophase spheres, were detected in the petcokes.26 These components might be difficult to vaporize to form soot in the gas phases.


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100

Cumulative yield(wt% daf coal)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

Tar+C1-C3 H2

H2O+CO2

60

CO

40

Soot

20

Char

0

a

XLT

BLG

XCG

YS

100

Cumulative carbon fraction(%)

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80 60

CO2 CO

40 20 Soot

0

JS

b

Tar C1-C3

XLT

BLG

XCG

YS

JS

Figure 6. Cumulative product yields (a) and cumulative carbon fraction in the volatile component (b) of fuels pyrolysis at 1300 °C. 3.3. Soot gasification reactivity Fig.7 shows the temperature-programmed reaction (TPR) curves of soot and char. Char particles display great gasification reactivity difference. It is well known that char reactivity depends on coal type, texture properties, degree of graphitization and mineral content. As expected, lignite char is most active, which may be related to the low rank and the catalytic effect of alkali earth metal in the raw coal ash (CaO: 35.95 wt%, Fe2O3: 13.97 wt%). Petroleum coke char displays the lowest reactivity determined by the morphology and optical texture of the petroleum coke chars27. In addition, the reactivity of soot from coal (lignite and bituminous) is lower than that of corresponding char. The results indicate that the formation of soot may slow down the carbon burnout rate and soot may act as one part of the stubborn carbon in the entrained-flow gasifier residues. High temperature and/or long residence time might be needed to enhance soot gasification efficiency to reduce soot content in the outlet of entrained-flow gasifiers. However, the reactivity of soot from petroleum coke is higher


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than that of corresponding char. The low reactivity of petcoke chars has been confirmed by some studies27,28. The present results suggest that soot formation has almost no impact on the low petcoke reactivity. The gasification reactivity order of carbonaceous materials is coincident with previous research. Miura and coworkers2 found that the gasification activation energies of the soots were higher than those of the bituminous chars. The gasification rates of soots were 1/20-1/6 of those of the corresponding coal chars at 1200°C. Huang et al.28 studied the properties of soot formed from anthracene oil and petroleum ether. They found that the gasification reactivity presented the following order: coal char > soot > petcoke char, and attributed it to the surface area difference. Even though soot is formed from the secondary reactions of primary pyrolysis products, the difference in soot gasification reactivity is clearly observed. The sequence of soot reactivity is: lignite soot > bituminous soot > petcoke soot. Soot reactivity can be affected by a number of factors, such as alkali content, microstructure and particle size. Qin et al.16 found that the wood soot produced at a higher temperature(1400 oC) is more reactive than the soot produced at a lower temperature(1000 oC), and attributed it to the relatively high content of potassium. In the present study, EDS spectra of soot from coal and petcoke show no signals of alkali metal and alkaline earth metal as shown in Table S1 in the Supporting Information. In addition, the ultimate analysis of different soot samples shown in Table 2 shows that soot particles are nearly composed of carbon element, more than 97% in all cases with very tiny or none of mineral content. It suggests that catalytic effect of mineral on soot reactivity can be excluded. Therefore, it can be deduced that soot reactivity difference is closely related to the structure. Table 2 also shows C/H atomic ratio of soot samples. The values of C/H ratio of all soot samples exceed 100, and


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that of JS petcoke soot is around 200. The C/H ratios are larger than those of soot samples formed at short residence time (about 100 ms) which range from 2 to 1029. High C/H ratio in the present study indicates substantial graphitization to loss hydrogen due to long residence time at high temperature. Hydrogen content of solid has been suggested to be related to the available of active sites and to its reactivity30. However, the hydrogen content of soot samples obtained in the present study is rather low and the differences are not significant. Therefore, the exact reason for the difference in carbon active sites content is studied by the microstructure analysis of different soot samples. 1.0 YS char

0.8 XLT char

1-X (-)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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JS char

XLT soot

0.6 0.4

XCG char

YS soot

XCG soot

JS soot

BLG soot BLG char

0.2 0.0 600

700

800

900

1000

1100

1200

1300

o

Temperature( C)

Figure 7. TPR curves of soot and char (5 °C /min, 60 mL/min CO2).


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Table 2. Ultimate analysis and C/H atomic ratio of soot samples.

C

H

N

S

C/H atomic ratio

XLT soot

97.49

0.07

0.16

1.37

116.06

BLG soot

98.50

0.06

0.29

0.66

136.81

XCG soot

99.72

0.06

0.14

0.15

138.50

YS soot

97.57

0.05

0.17

0.77

162.62

JS soot

97.61

0.04

0.33

1.96

203.35

Ultimate Analysis /wt%, dry

3.4. Microstructure analysis of soot High resolution transmission electron microscopy (HRTEM) has enabled the detailed observation of soot particle microstructures. Fig.8a-c shows the low magnification TEM images of different soot samples. Soot consists of primary spherical or nearly spherical nanometer particles agglomerated in chain-like secondary structure. Some primary soot particles connect each other by the inner carbon layers as pointed by the arrow in Fig.8f. Furthermore, the surfaces of primary soot particles seem smooth, without tough and irregular outside surfaces and/or some hollow structure, indicating that gasification between soot and oxidized gases can be neglected. Soot particles size is also affected by fuel type. The particle size of XCG soot is larger than those of XLT soot and YS soot. A majority of the XCG soot and XLT soot particles stay in the range of 100-300 nm and 50-100 nm, respectively. The particle size of YS soot seems two times smaller than XLT soot. Soot particle size corresponds well with the soot yield. The higher soot yield is, the larger particle size displays. However, it’s hard to build relationship between soot particle size and gasification reactivity. In other words, 18 ACS Paragon Plus Environment

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primary soot particle size does not account for the difference of soot gasification reactivity, which confirms previous results of Trubetskaya et al.12 and is contrary to the common observation that the reactivity increases with decreasing the particle size. The reason might be related to the nanoscale feature of soot particles. The HRTEM images of soot (Fig.8d-f) show a recognizable central nucleus of about 5-15 nm. Inter planar distances of graphenes are about 0.35-0.36 nm, which is slightly larger than that of pure graphite (0.33 nm), indicating that the graphitization extent of soot samples is quite low. Concentrically arranged graphene layers, roughly parallel and equidistant, form an onion-like structure. The outer part of soot particle shows more ordered structure. The microstructure of coal and petcoke soot is similar to that of carbon blacks and diesel soot 11. a

b

c

d

e

f

Figure 8. TEM images of soot samples at different magnifications.(a), (d) XLT soot; (b), (e) XCG soot; (c), (f) YS soot.

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In order to get detail nanostructure information of the carbon lamella, Gatan image software was applied to carry out lattice fringe analysis of HRTME image. The basic steps are: (1) spatial filtering to accentuate and perfect the crystalline image; (2) thresholding to form a two-phase image; (3) skeletonization and post-processing.31 These steps effectively converted the lattice fringes to discrete line segment. Fig.9 shows that some crystalline domains with up to 6-8 parallel stacked graphene layers exist in soot particles, but the structure is turbostratic for the randomly stacked layer planes32. Some amorphous domains also exist, and they are mainly composed of polycyclic aromatic compounds, which are short and in irregular of onion-like arrangements. Particle analysis was also carried out to give the statistics of the lattice fringes. Table 3 lists the structure parameters by HRTEM image analysis of soot particles. Here curvature is defined as the ratio between the length (being the end to end distance of the projected graphene) and the fiber length of the graphene, describing the degree of distortion due to defects such as five-membered rings in the graphitic layers11. The value of curvature is defined to be one in the case of perfect order and zero for complete disorder. The relatively large variance in curvature in the case of the XLT soot displays the non-uniform shape of the graphenes. The order of curvature is as follows: XLT soot < XCG soot < YS soot, suggesting that soot from lignite, to some extent, has more crystalline defects than that from bituminous coal. Petcoke soots have the highest degree of graphitization, which corresponds well with the highest value of C/H ratio and the lowest reactivity.


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120

XCG soot

100

Count

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80 60 40 20 0 0.4

0.5

0.6

0.7

0.8

0.9

1.0

Curvature(%)

Figure 9. Basic steps of lattice fringe analysis of HRTEM images of XCG soot: Image selection, threshold analysis, particle processing, and statistical analysis. Table 3. Structure parameters by HRTEM image analysis Curvature

Fiber length/nm

MV

SD

MV

SD

XLT soot

0.704

0.093

1.602

0.776

XCG soot

0.711

0.084

1.906

0.899

YS soot

0.727

0.068

1.745

0.926

Note: MV, Mean value; SD, Standard deviation Soot is regarded as disordered materials comprising crystalline and amorphous domains. Raman spectroscopy is promising to study this structure because it is sensitive not only to crystal structures but also to molecular organization (nanostructure). The first-order Raman spectra of different soots have two overlapping peaks with intensity maxima near 1580 cm-1 and 1350 cm-1 as shown in Fig.10a. The first-order Raman spectra in the range of 600-2000 cm-1 were curve-fitted with five bands to give detailed structure


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information32. Exemplary curve fitting result of XLT soot, which includes one Gaussian-shaped D3 band and four Lorentzian-shaped, G, D1, D2, and D4 bands, is demonstrated in Fig.10b. G band (~1580 cm-1) corresponds to ideal graphitic lattice stretching mode with E2g symmetry.32,33 D1 band (~1360 cm-1) has been suggested to arise from graphene layer edges (A1g symmetry) or heteroatoms in the matrix.33,34 D2 band (~1620 cm-1) has been attributed to surface graphene layers.35,36 D3 band (~1500 cm-1), signal intensity between the two main peaks, originates from the amorphous carbon fraction.37,38 D4 band (~1200 cm-1) has been associated with vibration mode of disordered graphite lattices (A1g symmetry), polyenes and ionic impurities.39,40 In the present study, D1 band full width at half maximum (FWHM, cm-1) was calculated predominantly, as well as the relative intensity of D3 band (ID3/ (ID3+ID2+IG)). They are pertinent to the relative abundance and structural order of graphite-like and amorphous carbon38. Previous studies have reported that increasing ID3/ (ID3+ID2+IG) indicated the increase of amorphous carbon content, while increasing D1 band FWHM was expected with increasing ratio of edge to basal plane sites or decreasing in the crystallite dimension32. The Raman spectral parameters as well as the reactivity index are shown in Fig.11. For XLT soot, the FWHM of D1 band lies at 161 cm-1, with D3 band relative intensity 0.294. D1 FWHM of JS soot, however, lies at 149 cm-1, and D3 band relative intensity is 0.268. This progressive decrease of D1 FWHM and D3 band relative intensity indicates an increase of structure order and a decrease of amorphous carbon content. Soot formed from lignite has highest value of D1 band width and D3 band relative intensity, suggesting that lignite soot particles comprise more amorphous carbon and/or small crystallite, coincident with its high gasification reactivity.


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a JS soot

YS soot BLG soot

XCG soot

Observed spectrum and curve fit

XLT soot

b

D1

Intensity(a.u.)

Intensity(a.u.)

G D3 D4 D2

XLT soot

600 800 1000 1200 1400 1600 1800 2000 2200 -1

600

800

1000

Raman shift(cm )

1200

1400

1600 -1

1800

2000

Raman shift(cm )

Figure 10. Raman spectra of different soot particles (a) and curve fitting for the Raman spectra of XLT soot (b). 1500

0.30

Reactivity index 160

1300

150

1200 140

1100

0.28

ID3/(ID3+ID2+IG)

o

-1

ID3/(ID3+ID2+IG)

D1 FWHM(cm )

D1 FWHM

1400

Reactivity index( C)

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0.26

0.24

1000

XLT soot XCG soot BLG soot YS soot

JS soot

130

0.22

Material

Figure 11. Raman spectra parameters and reactivity index of soot samples. 3.5. Effect of fuel type on soot properties Obviously, the origin of raw material has a strong influence on soot properties. Bituminous coal produces more soot and larger primary soot particles than lignite and petcoke. Soot formed from lignite is characterized as more crystal defects and higher amorphous carbons content, while soot from petcoke shows more ordered structure. As soot is formed from the secondary reactions of primary volatile matters, the


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rank-dependence of sooting propensity is determined by gas-phase species concentrations and the rates of key reactions. From a mechanistic point of view, three stages are involved in the formation of primary soot particles: a) particle inception; b) surface growth; and c) coagulation.41 Surface growth leads to an increase in the amount of soot and is responsible for at least 90% of the ultimate soot yield.42 Soot yield and primary particle size are closely related to soot precursor properties and the formation condition. The crack tendency of lignite tar and the reactions between H2O/CO2 and hydrocarbon radicals lead to the low soot yield and small primary particle size. On the contrary, bituminous coal tar is mainly composed of larger and more stable PAHs, resulting in higher soot yield and bigger primary particle size. The low soot yield of petcoke was due to the low concentrations of soot precursors. Numerous experimental and modeling studies have suggested that both tar and light hydrocarbons participate in the surface growth of soot.29,43 Zhang et al.43 devised a simple model to describe the secondary reactions of coal volatiles. Results showed that about 5-10% of bituminous soot yield was attributed to the addition of hydrocarbons at 1600 K, while light hydrocarbons accounted for nearly 40% of soot yield for lignite. Compared to bituminous coal, more light hydrocarbons incorporated into soot formation for lignite, which mainly by hydrogen-abstraction-C2H2-addition (HACA) mechanism.44Applying the HACA mechanism to a graphene armchair edge of a finite size leads to the formation of zigzag edges and five-member rings. The competition between migration of five-member rings along the zigzag edge and nucleation of six-member rings was a key mechanistic feature dictating growth rate and morphology. The incorporation of five-member rings creates portions of edge. These portions are unable to grow.44 The


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results of HRTEM image analysis indicate that lignite soot has the shortest average fiber length of graphene, suggesting the presence of five-member rings. Moreover, growing layers become significantly curved due to more frequent inclusion of five-member rings. On the contrary, during soot formation from bituminous coal, more aromatic PAHs incorporate into surface growth, forming planar graphene layers with larger lateral extension. As mentioned above, neither alkali metal nor particle size has a strong influence on the difference in soot reactivity. The gasification reactivity of soot is controlled by microstructure. It’s postulated that as more light hydrocarbons incorporated into soot formation, and with the presence of five-membered carbon rings structure, soot becomes more reactive towards CO2. 4. CONCLUSIONS When solid carbonaceous materials are injected into an entrained-flow gasifier, the initial reaction is rapid pyrolysis, then soot is formed from the secondary reactions of volatile matters. Fuel type has a strong influence on soot properties. Soot yield of bituminous coal is about 15-20 wt% of the raw coal, while those of lignite and petroleum cokes are merely several percent. The reactivity of soot from coal is lower than that of coal char, indicating that soot may act as one part of stubborn carbon in the entrained-flow gasifier residues. The reactivity of soot with CO2 from different fuel type shows a great difference, which is controlled by microstructure rather than alkali metal and particle size. Soot from lignite pyrolysis is most active and characterized as more lattice defects and higher amorphous carbon content, resulting from the addition of more light hydrocarbons to soot surface during soot growth. During bituminous coal pyrolysis,


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PAHs are the main species for soot growth, forming planar graphene layers with larger lateral extension. Soot from petroleum coke is characterized as lowest gasification reactivity and most ordered structure. AUTHOR INFORMATION Corresponding author *Tel.: +86 3514151519. E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from the National Basic Research Program of China (2010CB227003). SUPPORTING INFORMATION AVAILABLE Supporting Information: SEM images of samples from raw coal, holding tank and filter(Figure S1); Results of EDS referred to SEM micrographs reported in Figure S1(Table S1); SEM images of samples from sampling probe and reactor bottom(Figure S2); PSD of soot particles in the form of number fraction(Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES 1. Minchener, A. J. Fuel 2005, 84, 2222-2235. 2. Miura, K.; Nakagawa, H.; Nakai, S.-I.; Kajitani, S. Chemical Engineering Science 2004, 59, 5261-5268. 3. Brown, A. L.; Fletcher, T. H. Energy & Fuels 1998, 12, 745-757.


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Characteristics of Soot from Rapid Pyrolysis of Coal and Petroleum Coke

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