Gas evolution characteristics during pyrolysis and catalytic pyrolysis of coals by TG–MS and in a high-frequency furnace

Fuel 154 (2015) 222–232

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Gas evolution characteristics during pyrolysis and catalytic pyrolysis of coals by TG–MS and in a high-frequency furnace Lu Ding, Zhijie Zhou ⇑, Qinghua Guo, Shanjun Lin, Guangsuo Yu ⇑ Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Shanghai Engineering Research Center of Coal Gasification, East China University of Science and Technology, Shanghai 200237, PR China

h i g h l i g h t s  A quantitative method of gas analysis was proposed using on-line mass spectrometry.  The yields of light gases in fast pyrolysis were lower than those in slow pyrolysis.  Gas evolution was related to the variation of functional groups in the solid samples.  Heating methods have great effects on the reaction mechanism of coal pyrolysis.

a r t i c l e

i n f o

Article history: Received 9 December 2014 Received in revised form 30 March 2015 Accepted 1 April 2015 Available online 10 April 2015 Keywords: Catalytic pyrolysis Na2CO3 Quadrupole mass spectrometry Gaseous compounds evolution

a b s t r a c t Pyrolysis and catalytic pyrolysis behaviors of two kinds of coals with different ranks were determined by thermogravimetric analyzer–mass spectrometry (TG–MS) and a high-frequency furnace. A quantitative method of on-line gas analysis was proposed using quadrupole mass spectrometry. The effects of pyrolysis temperature (650–800 °C) and Na2CO3 loading amount (0–15 wt.%) on the gaseous compounds evolution were investigated. The results show that loading Na2CO3 on coal was favorable for gas production in pyrolysis process. The yields of light gas species in the high-frequency furnace were lower than those in TG–MS. By comparing the pyrolysis yields of gas and solid in the high-frequency furnace and TG–MS, it was concluded that heating methods had great effects on the reaction mechanism and product distribution of pyrolysis and catalytic pyrolysis of coals. Moreover, the evolution processes of gaseous compounds were related to the presence of different functional groups in the solid samples. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Nowadays, gasification technology is widely used to convert coal into fuel gas for its high efficiency. However, the present industrial gasification process are conducted during harsh conditions (high temperature (>1100 °C) and high pressure), which brings relatively high cost. Catalytic gasification could enhance coal reactivity and reduce the needed reaction temperature for the complete gasification. Therefore, it’s worthwhile to develop catalytic gasification process for lower production costs [1]. Recently, a group of studies were carried out about the effects of added metallic species on the gasification reactivity of coal [2–6]. The added metallic species include sodium salts (Na2CO3 and NaOH) [2,3], potassium salts (K2CO3) [4,5], ferric species

⇑ Corresponding authors. Tel.: +86 21 64252974; fax: +86 21 64251312. E-mail addresses: [email protected] (Z. Zhou), [email protected] (G. Yu). http://dx.doi.org/10.1016/j.fuel.2015.04.003 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.

(FeCl3,FeSO4) [6], etc. All of the results demonstrate that char gasification can be catalyzed by metal salts. In fact, the catalysts not only have effects on the gasification process, but also play important roles in the pyrolysis stage of carbonaceous material. However, there are still rare reports about the effects of added metallic species on the pyrolysis process of coal, especially on the product distributions and structural changes of char [7,8]. As the first step in gasification, coal pyrolysis has a close relationship with coal gasification and combustion. Pyrolysis is not only an effective way for clean use of coal as desulfurized char and tar can be obtained, but also a significant method for high added-value liquid and gaseous products, especially aromatic compounds, can be separated from tar in coal pyrolysis [9]. Compared to non-catalytic pyrolysis, coal pyrolysis with the added catalysts may result in different releasing temperatures and yields of the gaseous products [3,7]. What’s more, the differences in mechanisms of catalytic and non-catalytic pyrolysis of coals must take significant effects on the subsequent gasification reactivity of the

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L. Ding et al. / Fuel 154 (2015) 222–232 Table 1 Properties of tested samples. Samples

SF ZY

Proximate analysis/ad, wt.%

Ultimate analysis/d, wt.%

Ash fusion temperature/°C ⁄

M

V

Cfix

Ash

C

H

N

S

O

4.24 2.58

33.92 7.39

55.82 71.57

6.02 18.46

79.14 76.57

2.32 2.13

1.12 1.10

0.77 0.83

10.36 0.42

DT

ST

HT

FT

1152 1345

1167 1370

1175 1395

1179 1463

Note: M-moisture; V-volatile matter; Cfix-fixed carbon; ad-air dry basis; d-dry basis; O⁄ was calculated by difference.

Table 2 The average composition (vol.%) of the two kinds of calibration gases.

Peak area of the calibration gas,Ai0



Peak area of the heating stage,Ai1

CO

H2

CH4

Ar

Peak area of the constant temperature stage,Ai2

29.3 20.0

38.54 35.52

27.1 24.3

0.53 0.81

4.53 19.37

Temperature profile

produced char [4,5]. Therefore, a clear understanding of the coal catalytic pyrolysis behavior and the gaseous compounds evolution process during devolatilization is important for the whole catalytic gasification process. Although catalytic processes are favorable for more moderate reaction conditions as mentioned above, the catalysts cost often prevents coal catalytic gasification process from being commercialized. Na2CO3 is an effective and inexpensive alkaline that could be used on an once-through basis. Therefore, high pure Na2CO3 (>99.0%) was chosen as the catalyst in the present study. Gas chromatography (GC) is often used for quantitative analysis of pyrolytic gas mixtures [1,3,7]. However, the low time resolution makes it unsuitable for real-time analysis (e.g. to follow a transient) [10]. On the other hand, the response time of the quadrupole mass spectrometry (QMS) or Fourier Transform Infrared Spectroscopy (FTIR) is significantly less than that of GC. Therefore, many researchers studied the pyrolysis behavior and the on-line gaseous compounds evolution process of coal using TG–MS/FTIR recently [11–15]. However, most of previous works just dealt with the qualitative analysis of releasing characteristics of pyrolysis gaseous species. In addition, there is rare literature reported quantitative gas analysis using QMS or FTIR [10]. In this study, a quantitative method of yields of pyrolysis gases in TG–MS was proposed and illustrated in detail. This method was validated and could be used to quantify the yields of light gas species e.g. H2, CH4, and CO2. Coal pyrolysis generally goes through a series of reactions and can be influenced by many factors [16]. Heating rate is the dominant factor affecting the behavior of gas release, tar yield, pyrolysis mechanism and reaction kinetics. Therefore, in the present study, a novel drop tube/fixed bed reactor (fast heating) combined with TG–MS (slow heating) was also employed to investigate the catalytic and non-catalytic pyrolysis processes of two typical coals in China. The main purpose of this study is to explore the catalytic effects of Na2CO3 on the gaseous compounds evolution process during coal pyrolysis. Moreover, the variations of chemical

calibration stage

 

heating stage

constant temperature stage

  

Temperature/ć

CO2

1st calibration gas 2nd calibration gas

Intensity/(A⋅min)

Calibration gas composition (vol.%)

  







 



Time/min Fig. 1. The three main stages in the pyrolysis process of coal samples.

Table 4 The quantitative comparison of pyrolysis gas species by MS and GC for several tests. Yield of gas species, mmol/g coal

H2 obtained by MS H2 obtained by GC CH4 obtained by MS CH4 obtained by GC CO2 obtained by MS CO2 obtained by GC

Pyrolysis temperature, °C 650

700

750

800

1.113 1.151 0.110 0.110 0.939 0.957

2.173 2.254 0.102 0.110 1.046 1.082

3.467 3.575 0.113 0.114 1.147 1.191

3.846 3.900 0.103 0.112 1.130 1.201

structures of the solid samples were analyzed to illustrate the evolution mechanism of gas products. 2. Experimental 2.1. Samples preparation Two different coals and their mixtures with catalysts were evaluated in this study: Shenfu bituminous coal (SF), Zunyi anthracitic coal (ZY), and the coals loaded with Na2CO3 at the loading amount of 5 wt.%, 10 wt.%, and 15 wt.% on a dry basis, respectively. The coals were crushed and sieved the size fraction of 80–120 lm.

Table 3 The peak area of H2 and CO2 in the two kinds of calibration gases for several tests. Number of tests

Area1(H2)/ A min

Area2(H2)/ A min

Area1(H2)/ Area2(H2)

C1(H2)/ C2(H2)

Area1(CO2) / A min

Area2(CO2) / A min

Area1(CO2)/ Area2(CO2)

C1(CO2)/ C2(CO2)

1st test 2nd test 3rd test 4th test

1.146 1.062 1.078 1.094

1.043 1.020 1.002 1.013

1.099 1.041 1.076 1.080

1.115 1.115 1.115 1.115

4.799 4.779 4.770 4.790

3.426 3.422 3.412 3.418

1.401 1.397 1.398 1.401

1.465 1.465 1.465 1.465

Note: The symbol like Area1(H2) and Area2(H2) denote peak area of H2 ionic intensity in the two kinds of calibration gases respectively during the calibration stage. The symbol like C1(H2)/C2(H2) means the ratio of the absolute quantity of H2 in the first kind of calibration gas mixtures to that of H2 in another kind of calibration gas mixtures.

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L. Ding et al. / Fuel 154 (2015) 222–232

Fig. 2. High frequency furnace rapid pyrolysis system.



High temperature zone

Temperature distribution

Temperatureć



0.15Na, respectively. The corresponding ZY-0.1Na pyrolysis char obtained at 800 °C is hereafter referred to as ZY-0.1Na-800P. The designation of SF samples referred to that of ZY samples. 2.2. Pyrolysis experiments in TG–MS







 











Distance above from the crucible/cm Fig. 3. Temperature distribution above the crucible in the reactor.

The characteristic analysis data of coals are summarized in Table 1. The Na-form coals were prepared by incipient wetness impregnation method [1,17]. ZY coal with three different amounts of Na2CO3 additive (5–15 wt.%) were designated as ZY-0.05Na, ZY-0.1Na, ZY-

2.2.1. General operating procedure Pyrolysis experiments of raw carbonaceous materials and their mixture with Na2CO3 were carried out with NETZSCH thermogravimetric analyzer (STA 449 F3) coupled with a MS 403C quadrupole mass spectrometry for on-line gas analysis. In all cases, approximate 15 mg of sample particles were placed in a alumina crucible and heated at 20 °C min1 to the prescribed temperature (650–800 °C) under a continuous argon flow of 30 mL/min. The samples were then stayed at the final temperature with a holding time of 30 min for a complete devolatilization. The response time of the mass spectrometer is less than 2 s. In order to avoid gas condensation, the gas outlet of TGA and the transfer line were all heated to 220 °C. Mass spectrometry identifies the species by using the differences in mass-to-charge ratio (m/z) of ionized atoms or molecules [10]. The gaseous compounds generated during pyrolysis including H2, CH4, H2O, CO2 and CnHm (n P 2) could be easily detected, and their mass-to-charge ratios were set at m/z(2), m/z(16), m/z(18), m/z(44), and m/z(27) separately in this study. While it is difficult to separate CO in the mixture due to

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L. Ding et al. / Fuel 154 (2015) 222–232

1.0

SF-raw-800P  SF-raw-750P SF-raw-700P  SF-raw-650P Temperature profile





 











0.0



400 300

0

10

20

SF-raw-CH 4









Time/min







SF-raw-800P  SF-raw-750P SF-raw-700P  SF-raw-650P Temperature profile



 













Time/min







SF-raw-CnHm

 SF-raw-800P SF-raw-750P SF-raw-700P  SF-raw-650P Temperature profile



 















Time/min







Relative intensity/(A/A)

400 300

0

10

20

30

40

50

60

70

200



800

SF-0.15Na-H2O

700

0.8 SF-0.15Na-800P SF-0.15Na-750P SF-0.15Na-700P SF-0.15Na-650P Temperature profile

0.6 0.4

600 500 400 300

0

10

20

30

40

Time/min

50

60

70

SF-0.15Na-Cn H m







600 SF-0.15Na-800P SF-0.15Na-750P SF-0.15Na-700P 500 SF-0.15Na-650P Temperature profile

0.4

0.0







700

0.2



Relative intensity/(A/A)



200

800

0.6

1.0







70

Time/min



SF-raw-H O 2



60

0.8

0.0



Relative intensity/(A/A)



50

0.2



Temperature/ć



Relative intensity/(A/A)

 



Relative intensity/(A/A)



Temperature/ć

SF-raw-800P SF-raw-750P SF-raw-700P SF-raw-650P Temperature profile

Temperature/ć

Relative intensity/(A/A)







40

1.0 SF-0.15Na-CH4





30

Time/min

Time/min



500

Temperature/ć



0.4

600

Temperature/ć



SF-0.15Na-800P SF-0.15Na-750P SF-0.15Na-700P SF-0.15Na-650P Temperature profile

0.6

0.2





700

0.8

200

 



SF-0.15Na-800P  SF-0.15Na-750P SF-0.15Na-700P  SF-0.15Na-650P Temperature profile





 















Time/min

Fig. 4. Gases evolution curves of SF samples during pyrolysis in TG–MS.









Temperature/ć



800

SF-0.15Na-H 2

Temperature/ć





Relative intensity/(A/A)



SF-raw-H 2

Temperature/ć

Relative intensity/(A/A)



L. Ding et al. / Fuel 154 (2015) 222–232

SF-raw-CO2

 





SF-raw-800P SF-raw-750P



SF-raw-700P



SF-raw-650P Temperature profile



 























Relative intensity/(A/A)



Temperature/ć

Relative intensity/(A/A)



SF-0.15Na-CO2

 

 SF-0.15Na-800P



SF-0.15Na-750P



SF-0.15Na-700P SF-0.15Na-650P



Temperature profile



Temperature/ć

226

 























Time/min

Time/min Fig. 4 (continued)

spectral interference of the fragmentation of the ions produced by CO2 [15]. Therefore, CO was not analyzed in our TG–MS experiments. 2.2.2. Quantitative method of pyrolysis gases in TG–MS The absolute quantity of the pyrolysis gases were calculated from a calibration at operating conditions. In our preliminary experiments, QMS was calibrated at 70 °C using two kinds of calibration gas mixtures with argon as the carrier gas, and the volume amount of the calibration gas is 250 lL for each pyrolysis experiment. The average composition (vol.%) of the two kinds of calibration gases is shown in Table 2. Table 3 shows that the area of the releasing peak intensity of each composition in the same calibration gas mixtures varied within a small range after QMS running for over four days. Moreover, Table 3 also indicates that the absolute quantity of each gas was proportional to the corresponding area of the releasing peak intensity produced in the calibration process. Take the peak intensity of H2 and CO2 in the two kinds of calibration gas mixtures as a representative, the differences between the molar ratio (e.g. C1(H2)/C2(H2)) and peak area ratio (e.g. Area1(H2)/Area2(H2)) were within 7% and 5% for H2 and CO2 separately. The three stages in the pyrolysis process of coal samples are shown in Fig. 1. The main pyrolysis gas compositions, including H2, CH4 and CO2, were calculated with the following formula: Ai þAi 1

ni ¼

Ai

2

 xi0  250  106

0

mcoal  V m

ð1Þ

where ni is the product gas yield of component i (molg1); Ai0 is the area of peak intensity of component i during the calibration stage of the pyrolysis process; Ai1 is the area of peak intensity of component i during the heating stage; Ai2 is the area of peak intensity of component i during the constant temperature stage; xi0 is the volume percent of component i in the calibration gas (%); mcoal is the mass of coal without catalyst (g); Vm is the molar volume of gas (L mol1). In our preliminary tests, pyrolysis of SF raw coal at 650–800 °C were also performed in a fixed bed rector [18], and the pyrolysis gas species were analyzed by Agilent7890A gas chromatograph (GC) to help validate the measurements using QMS. About 10 g samples were used in each experiment, and the temperature in the fixed bed rector was controlled the same as that in TGA. Table 4 shows that yield of a certain gas species in the fixed bed rector was slightly higher than that in TGA. Taking the quantitative

results of TG–MS as the basis, the absolute relative errors between the results of GC and that of QMS was within 3.7%, 9.1%, and 6.2% for H2, CH4, and CO2 separately. A close match was obtained between QMS and GC.

2.3. Pyrolysis experiments in a high-frequency furnace Pyrolysis and catalytic pyrolysis of coal samples were also carried out in an atmospheric pressure high-frequency magnetic field based furnace [19,20]. The schematic diagram of the pyrolysis device is shown in Fig. 2. When the high-frequency power was started, high-frequency alternating magnetic field could be formed in the center of induction coil, and the molybdenum crucible in the high frequency alternating magnetic field can be self-heated rapidly. During each experiment, coal particles flowed gradually to the crucible when the furnace was heated to the target temperature, so rapid pyrolysis could happen at the moment the samples dropped into the molybdenum crucible. Yuan et al. reported that the heating rate of this novel drop tube/fixed bed reactor was higher than 103 °C s1 [19]. The measured temperature distributions above the crucible under the condition of 1200 °C are shown in Fig. 3. It’s obvious that the temperature decreased significantly with the increasing distance above from the crucible, and the high temperature zone was just limited in the small area around the molybdenum crucible. During pyrolysis, the volatile products released from the molybdenum crucible could be rapidly carried out by the carrier gas from the high temperature zone and be quenched, which could suppress the secondary reactions. The pyrolysis temperatures were ranged from 650 °C to 800 °C with an interval of 50 °C. The final temperatures of coal samples were held for 5 min so that most of volatile matters could be removed. The main pyrolysis gas species, including H2, CO, CH4 and CO2,

Table 5 The releasing peak temperatures of H2 for SF samples during pyrolysis at 650–800 °C. Pyrolysis temperature, °C

650 700 750 800

H2 peak temperature, °C SF-raw

SF-0.05Na

SF-0.1Na

SF-0.15Na

650 700 750 780

650 700 700 700

650 680 680 680

650 660 660 660

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L. Ding et al. / Fuel 154 (2015) 222–232

(a) SF-H2

(b) ZY-H 2

(c) SF-CO2

(d) ZY-CO2

(e) SF-CH4

(f) ZY-CH4

Fig. 5. Gaseous yields of SF and ZY samples in TG–MS tests. Note: For SF coal, 1, 2, 3, 4 denote SF-raw-H2-slow-heating, SF-0.05Na-H2-slow-heating, SF-0.1Na-H2-slowheating, and SF-0.15Na-H2-slow-heating respectively. For ZY coal, 1, 2, 3, 4 denote ZY-raw-H2-slow-heating, ZY-0.05Na-H2-slow-heating, ZY-0.1Na-H2-slow-heating, and ZY0.15Na-H2-slow-heating respectively.

were determined using Agilent7890A gas chromatograph. Based on the volumetric percentages of each species from the gas analyser, the yields of H2, CO, CH4 and CO2 were calculated as gas

generation per unit mass of coal in dry basis (g). In calculation, nitrogen was assumed as a tracer gas, and the total volume of nitrogen contained in exhaust gas was assumed to be the same

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L. Ding et al. / Fuel 154 (2015) 222–232

(a) (b)

2

1

3

3. Results and discussion

4

3.1. The gaseous compounds evolution process and pyrolysis gas yield of SF samples in TG–MS

Transmittance / %

(c) (d) (e) (f) (g) (h)

a-SF-0.15Na c-SF-raw o e-SF-raw-800 C o g-SF-0.05Na-800 C 





b-SF-0.1Na o d-SF-raw-650 C o f-SF-0.05Na-650 C o h-SF-0.15Na-650 C 









Wavenumbers/cm-1 Fig. 6. The FTIR spectra of SF samples. Note: 1 denotes aromatic nucleus CH stretching vibration, 2 denotes aromatic C@C bonds or carbonyl with hydrogen bonds, 3 denotes inorganic salt or ACH2 or ACH3, 4 denotes various aromatic CH out-of-plane bending modes.

as the total volume of nitrogen supplied to the reactor. As shown in Table 1, due to both SF and ZY contain nitrogen less than 1.5 percent, it is reasonable to use nitrogen as the tracer gas. Based on these assumptions, the yield of light gas specie was defined by [21]:

Q ni ¼

N2

VN

2

 Vi

 ð2Þ

m  Vm

where ni is the product gas yield of component i (molg1); Q N2 is the flow of N2 (L min1); V N2 is the volume percent of N2 in exit gas (%); Vi is the volume percent of component i in exit gas (%); m is the feed rate of coal sample (g min1); Vm is the molar volume of gas (L mol1). 2.4. Fourier transform infrared spectroscopy (FTIR) analysis The infrared spectra were measured by a Nicolet model 6700 Fourier transform infrared spectrometer. In this study, typical 1 mg samples were mixed and ground with 100 mg KBr and pressed into pellets under 10 MPa for 2 min. The spectra were generated by co-adding 128 scans at a resolution of 4 cm1 and the measured region of the spectra extended from wave numbers of 4000–400 cm1.

Fig. 4 shows that SF bituminous coal with or without Na2CO3 begin to release H2 at nearly 250 °C, which can be attributed to the degradation of heterocyclic compounds rich in hydrogen [11]. The release of H2 became significant at 400 °C, peaking at 700 °C for all the SF samples due to the condensation of aromatic structures and the cracking of heavy hydrocarbons [11,22]. Table 5 shows that the peak temperatures of SF raw coal increase with the pyrolysis temperature, while the peak temperatures of SF samples with a certain loading amount of Na2CO3 remain constant when the pyrolysis temperature was higher than 650 °C. It is interesting to find that the maximum release intensity of H2 turns to lower temperatures with the increase of the catalyst amounts. This means that Na2CO3 could reduce the active energy of hydrogen generation, and thus the reaction rate might be accelerated and reaction temperature were reduced to some extent. Fig. 5 is the quantitative results of pyrolysis gases obtained with the method proposed in Section 2.2.2. It shows that compared to non-catalytic pyrolysis of coal, SF coal pyrolysis with Na2CO3 is indeed more favorable for the production of H2. Moreover, Fig. 5(a) also demonstrates that the release of H2 increases with pyrolysis temperature for SF samples. This can be attributed that the condensation reactions of aromatic structures became more significant in a higher temperature. To illustrate this phenomenon, the chemical functional groups of SF samples were analyzed according to the literature [23]. The FTIR spectra of the samples are shown in Fig. 6. The intensity of the band near 1580 cm1 is attributed to aromatic C@C bonds or carbonyl with hydrogen bonds, and that near 870 cm1 is represented as various aromatic CH out-of-plane bending modes. These two bands could be observed in SF raw samples, and they still exhibited certain intensities when the samples were heated up to 650 °C. However, when the pyrolysis temperature came to 800 °C, the aromatic C@C bonds and aromatic CH out-of-plane bending modes disappeared, which mainly formed H2, CO2, and other hydrocarbons. Fig. 4 also exhibits the release curves of carbon gases and steam. Unlike H2, most of the peak temperatures of these gases are unchanged with pyrolysis temperatures when the dosage of Na2CO3 is kept the same in coal samples. Table 6 shows the characteristic releasing temperatures of carbon gases and steam for SF samples at 650–800 °C. It shows that steam and CnHm are first detected at 100 °C and 270 °C separately, both peaking at 480 °C for SF coal with or without catalyst. Na2CO3 has catalytic effects on the release of CH4 and CO2 during coal pyrolysis, and the peak temperatures of the maximum release intensities for both

Table 6 The characteristic releasing temperatures of carbon gases and steam for SF samples at 650–800 °C. Samples

SF-raw SF450 SF440 SF435

Temperature CH4 initial value (°C)

CH4 peak value (°C)

H2O initial value (°C)

H2O peak value (°C)

CnHm initial value (°C)

CnHm peak value (°C)

CO2 initial value (°C)

CO2 peak value (°C)

200 0.05Na

545 200

100 530

490 100

270 480

480 270

150 480

460 150

0.1Na

200

510

100

480

270

480

150

0.15Na

200

505

100

485

270

480

150

Note: The symbol like CH4 initial value means the initial releasing temperature of CH4; the symbol like CH4 peak value means the temperature value corresponding to the maximum releasing intensity of CH4.

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L. Ding et al. / Fuel 154 (2015) 222–232





Time/min







4

600 ZY-raw-800P ZY-raw-750P ZY-raw-700P 500 ZY-raw-650P Temperature profile

0.6 0.4

400

0.2

10

20

30

40

50

60

70







0.2

40

Time/min

50

60

70

ZY-raw-CO2

600 ZY-0.15Na-800P ZY-0.15Na-750P ZY-0.15Na-700P 500 ZY-0.15Na-650P Temperature profile

0.6 0.4

400

0

10

20

ZY-raw-800P  ZY-raw-750P ZY-raw-700P  ZY-raw-650P Temperature profile

 















Time/min

30

40

50

60

70

200









800

ZY-0.l5Na-H2O

700 600 ZY-0.15Na-800P ZY-0.15Na-750P ZY-0.15Na-700P 500 ZY-0.15Na-650P Temperature profile

0.6 0.4

400 300

0

10

20

30

40

Time/min

50

60

70

ZY-0.15Na-CO 2









Temperature/ ć

300

0.8

0.0

200







800

0.2

300

30

Relative intensity/(A/A)

400

Temperature/ć

500

200

 



Temperature/ć

Relative intensity/(A/A)

600

Temperature profile

0.4



Relative intensity/(A/A)

ZY-raw-800P ZY-raw-750P ZY-raw-700P ZY-raw-650P

0.6

20



700

1.0

700

10



0.8

0.0

200

800

ZY-raw-H 2O

0



Time/min

0.8

0.0



Time/min

ZY-0.15Na-CH 4

Time/min 1.0



0.2

300

0



1.0

700

0.8

 





800

ZY-raw-CH

1.0





Temperature/ć



ZY-0.15Na-800P ZY-0.15Na-750P ZY-0.15Na-700P ZY-0.15Na-650P Temperature profile



ZY-0.15Na-800P ZY-0.15Na-750P ZY-0.15Na-700P ZY-0.15Na-650P Temperature profile





  

















Time/min

Fig. 7. Gases evolution curves of ZY samples during pyrolysis in TG–MS.









Temperature/ ć



Relative intensity/(A/A)















ZY-0.15Na-H 2

Temperature/ć



Relative intensity/(A/A)



Temperature/ć

ZY-raw-800P  ZY-raw-750P ZY-raw-700P  ZY-raw-650P Temperature profile



0.0









Relative intensity/(A/A)



ZY-raw-H 2

Temperature/ć

Relative intensity/(A/A)



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L. Ding et al. / Fuel 154 (2015) 222–232

Table 7 The releasing peak temperatures of H2 for ZY samples during pyrolysis at 650–800 °C. Pyrolysis temperature, °C

650 700 750 800

H2 peak temperature, °C SF-raw

SF-0.05Na

SF-0.1Na

SF-0.15Na

650 700 750 795

650 700 750 795

650 700 750 785

650 700 750 780

gases turn to lower values with the increase of catalyst amounts. Although pyrolysis temperature had few effects on the characteristic releasing temperatures of CH4 and CO2, the yields of CO2 and CH4 increased with pyrolysis temperature. SF raw coal was chosen as representative to study the effect of temperature on the releasing characteristic of CO2. Fig. 4 shows that there is a shoulder peak release at 530 °C for SF raw coal. The shoulder peak is very weak at the pyrolysis temperature of 650 °C, while the peak intensity increases significantly with the increase of pyrolysis temperature from 650 °C to 800 °C. This demonstrates that the mechanism of CO2 formation in the range of 150–530 °C may be different from that of CO2 in the temperature regions higher than 530 °C. Wu et al. found that CO2 was mainly formed from oxygen-bearing heterocycles (i.e., ethers, quinones, and esters) over the temperature of 250–550 °C [11], and the generation of CO2 at 500–800 °C depends on the decomposition of oxocarbon and carbonates [24–26], which was consistent with the FTIR spectra results in Fig. 6. 3.2. Evolution characteristics of gaseous products of ZY samples Fig. 7 shows that initial temperatures for releasing H2 of ZY samples are nearly 590 °C, which are much higher than those of SF samples (250 °C), and that the H2 release peaks of ZY samples are much narrower than those of SF samples. The reason may be that the reaction mechanism of H2 production in the pyrolysis of ZY anthracite coal were more simplified than that of SF bituminous coal because the removable volatile matters of ZY anthracitic coal are fewer than those of SF bituminous coal (as shown in Table 1). Table 7 shows that the peak temperatures of ZY samples increase with the pyrolysis temperature, while the peak temperatures of SF samples with a certain loading amount of Na2CO3 are constant when the pyrolysis temperature is higher than 650 °C. The quantitative results of pyrolysis gases during the pyrolysis of ZY samples are shown in Fig. 5. It shows that compared to noncatalytic pyrolysis of ZY coal, the coal pyrolysis with Na2CO3 was also more favorable for the H2 production, which was the same as SF coal. However, the total gas yields of ZY samples were much lower than those of SF samples when the dosage of Na2CO3 was kept the same in both coal samples. The results of Fig. 5(b) and Table 7 indicate that although catalytic pyrolysis of ZY samples could enhance H2 yield, the characteristic temperature of the maximum release intensity does not decrease with the increase of catalyst amounts. Fig. 7 also shows the release curves of CH4, CO2 and steam, and the CnHm peaks were not shown here due to the significantly low peak intensity of CnHm species for ZY samples. Peak temperatures

of CH4 or steam during the heat treatment of anthracite coal (ZY) are more than 100 °C higher than those of bituminous coal (SF). As shown in Table 6, the peak temperature of CO2 decreases with the increase of amount of the catalyst in the pyrolysis process of SF samples, while Table 8 indicates that this characteristic temperature varies in the opposite trend with the increase of Na2CO3 for the ZY samples. Two main release peaks of CO2 are observed in the pyrolysis of ZY raw coal at the pyrolysis temperatures of 650–800 °C, while only a main peak appears in the catalytic pyrolysis process of ZY samples. As mentioned above, there are two possible mechanisms of CO2 production: one in the range of 250–550 °C, CO2 was mainly formed from oxygen-bearing heterocycles [11,27], and another at 500–800 °C, the generation of CO2 depended upon the decomposition of oxocarbon [25,26]. Compared to ZY raw coal, the reaction of mechanism 2 for ZY coal mixed with Na2CO3 might be significantly accelerated by the catalyst, which resulted in the release peak intensity of CO2 keep up increasing in the temperature range of 500–550 °C and the maximum peak intensity turned to a higher temperature during catalytic pyrolysis of ZY samples.

3.3. Pyrolysis yield of each gas component in the high-frequency furnace The devolatilization experiments of SF bituminous coal and ZY anthracitic coal mixed with Na2CO3 were also studied using a novel drop tube/fixed bed reactor, i.e. a high-frequency furnace. Fig. 8 shows the yields of H2, CH4, CO, and CO2 during pyrolysis and catalytic pyrolysis processes. The yields of both H2 and CO increase significantly with the increase of pyrolysis temperature. H2 yield also exhibits a generally increasing trend with increasing loading amount of Na2CO3 when the pyrolysis temperature was kept constant, and the similar conclusion was also drawn in Popa et al. and Feng et al.’s researches [3,28]. Popa et al. concluded that the added Na+ and the –OH or –COOH groups in coal could react and form –ONa or –COONa, and release H+ as presented below [3],

Naþ þ COOH ! COONaþ þ Hþ

ð3Þ

Naþ þ OH ! ONaþ þ Hþ

ð4Þ

Through this mechanism, Na can promote the release of H2 and thus it is responsible for the difference between the reaction with or without the use of Na2CO3. The production of CO during coal devolatilization could result from several sources, primarily ether linkages, ketone groups, and heterocyclics [29,30], while CO2 was formed from carboxylate groups, aliphatic and aromatic carboxyl at low temperatures. At high temperatures, however, CO2 derived from thermally more stable oxygen-bearing heterocycles, quinones and ether structures [24]. With the use of Na2CO3, CO2 produced in the devolatilization process may react with carbon bonded with Na before the formation of gas products from the surface of solid samples. The extent of char-CO2 increased with increasing temperature. Therefore, CO yield of catalytic pyrolysis increased more significantly than that of non-catalytic pyrolysis when the reaction temperature increased from 650 °C to 800 °C.

Table 8 The characteristic releasing temperatures of carbon gases and steam for ZY samples at 700–800 °C. Samples

ZY-raw ZY-0.05Na ZY-0.1Na ZY-0.15Na

Temperature CH4 initial value (°C)

CH4 peak value (°C)

H2O initial value (°C)

H2O peak value (°C)

CO2 initial value (°C)

CO2 peak value (°C)

200 200 200 200

660 660 670 670

100 100 100 100

650 650 650 650

150 150 150 150

610 690 700 700

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L. Ding et al. / Fuel 154 (2015) 222–232





Absolute yield/(mmol/g)

 

SF-0.05Na-H 2

SF-raw-CO

SF-0.1Na-H 2

SF-0.15Na-H 2

SF-raw-CH 4

SF-0.05Na-CH 4

SF-0.1Na-CH 4



Absolute yield/(mmol/g)

SF-raw-H 2

SF-0.15Na-CH 4

  

SF-0.05Na-CO

SF-0.1Na-CO

SF-0.15Na-CO

SF-raw-CO 2

SF-0.05Na-CO 2

SF-0.1Na-CO 2

SF-0.15Na-CO 2







 























ZY-raw-CO

ZY-0.05Na-H 2

ZY-0.1Na-H 2

ZY-0.15Na-H 2

ZY-raw-CH 4

ZY-0.05Na-CH 4

ZY-0.1Na-CH 4



Absolute yield/(mmol/g)

ZY-raw-H 2

Absolute yield/(mmol/g)



Temperature/ć

Temperature/ć

ZY-0.15Na-CH 4





ZY-0.05Na-CO

ZY-0.1Na-CO

ZY-0.15Na-CO

ZY-raw-CO2



ZY-0.05Na-CO2 ZY-0.15Na-CO2

ZY-0.1Na-CO2

  

  



















Temperature/ć

Temperature/ć

Fig. 8. Variation of the pyrolysis gas products with temperature in high-frequency furnace.

Table 9 Char yields of SF samples using TG–MS and high-frequency furnace. Temperature (°C)

Char yield SF-raw-slow (%)

SF-0.05Na slow (%)

SF-0.1Na slow (%)

SF-0.15Na slow (%)

SF-raw-fast (%)

SF-0.05Na fast (%)

SF-0.1Na fast (%)

SF-0.15Na fast (%)

650 700 750 800

71.34 70.70 69.58 67.64

66.92 63.08 63.01 62.49

60.83 59.74 55.76 55.30

57.71 56.01 52.13 52.24

63.11 61.60 59.96 58.46

62.43 59.23 56.43 53.23

58.45 57.12 54.12 51.97

56.93 55.42 51.69 49.98

Note: SF-raw-slow denotes char yield of SF raw coal under slow heating; SF-raw-fast denotes char yield of SF raw coal under fast heating.

3.4. Comparison of pyrolysis yields of gas and solid in the high-frequency furnace and TG–MS The results demonstrated that, no matter in a slow heating rate or a fast heating rate, H2 was the main gas product for coal samples with or without the use of Na2CO3 in the temperature range of 650–800 °C, and H2 yield increased with the increase of the Na2CO3 loading amount in 0–15 wt.%. The yields of H2, CH4, and CO2 in the high-frequency furnace were lower than the yields of those gas species in TG–MS. The char yields of SF and ZY samples during fast and slow devolatilization process are displayed in Tables 9 and 10. It can be seen that char yields of both rapid heating and slow heating decreased with the increase of pyrolysis temperature and loading amount of the catalyst. Tables 9 and 10 also indicate that for the same coal sample, char yield of fast heating

is lower than that of slowing heating. In other words, fast pyrolysis could enhance the release of volatiles, which is consistent with Cai et al.’s results [31]. Compared to slow heating pyrolysis, rapid pyrolysis owned higher total amounts of volatiles, and lower amounts of light gas species. Gibbins-Matham and Kandiyoti indicated that two competing reactions existed both consuming the ‘‘metaplast’’ phase in the pyrolysis process: tar evaporation and char formation. The higher the heating rate, the less time is available for tar cracking and consequently the more tar and the less methane are produced [32]. In our experiments, the high temperature zone was limited in a small range around the crucible during fast pyrolysis experiments, which are shown in Fig. 3. Therefore, secondary reactions like the decomposition of tar to light gas species could be sharply suppressed as these macromolecular compounds were quickly brought to the low temperature region by

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Table 10 Char yields of ZY samples using TG–MS and high-frequency furnace. Temperature (°C)

650 700 750 800

Char yield ZY-raw-slow

ZY-0.05Na slow

ZY-0.1Na slow

ZY-0.15Na slow

ZY-raw-fast

ZY-0.05Na fast

ZY-0.1Na fast

ZY-0.15Na fast

95.36 94.90 94.80 92.21

93.67 92.95 90.71 89.77

91.70 89.85 88.45 87.47

90.24 88.15 83.80 80.91

91.85 88.43 87.98 87.80

89.55 87.50 86.32 86.00

87.46 86.03 85.78 85.16

84.17 80.50 79.72 78.13

Note: ZY-raw-slow denotes char yield of ZY raw coal under slow heating; ZY-raw-fast denotes char yield of ZY raw coal under fast heating.

carrier gas. Therefore, it could be reasonable to infer that the yields of volatiles with large molecular weights in high-frequency furnace were higher than the yields of those species in TG–MS. The increase in total volatile yield of fast heating was mainly due to the enhanced macromolecular substances yields such as tar production. 4. Conclusions Two coal samples mixed with Na2CO3 were pyrolysis in TG–MS and a high-frequency furnace to study the effects of loading amount of the catalyst and pyrolysis temperature on the release characteristics of gas species. It is found that loading Na2CO3 on coal is favorable for gas production in the pyrolysis process. A detailed method was proposed to quantify the yields of pyrolysis gases in TG–MS. Due to the great differences of volatile matter content and chemical functional groups between SF bituminous coal and ZY anthracite coal, the evolution process of gaseous compounds and gas yields of ZY samples were distinctly different from those of SF samples. The release mechanism of some gas species were verified by the variations of chemical functional groups of coal samples with the increase of pyrolysis temperature. Moreover, the comparisons of pyrolysis yields of gas and solid in the high-frequency furnace and those in TG–MS indicated that heating methods had great effects on the reaction mechanism and product distribution of coal pyrolysis. Therefore, a reactor with a suitable heating method must be carefully considered in order to obtain high added-value gas or liquid products. Acknowledgements This work has been partially supported by the National High Technology Research and Development of China (863 program, 2012AA053101), and the National Natural Science Foundation of China (21376081). References [1] Ding L, Zhou ZJ, Guo QH, Huo W, Yu GS. Catalytic effects of Na2CO3 additive on coal pyrolysis and gasification. Fuel 2015;142:134–44. [2] Xu SQ, Zhou ZJ, Yu GS, Wang FC. Effects of alkaline metal on coal gasification at pyrolysis and gasification phases. Fuel 2011;90:1723–30. [3] Popa T, Fan M, Argyle MD, Slimane RB, Bell DA, Towler BF. Catalytic gasification of a powder river basin coal. Fuel 2013;103:161–70. [4] Kopyscinski J, Lam J, Mims CA, Hill JM. K2CO3 catalyzed steam gasification of ash-free coal. Studying the effect of temperature on carbon conversion and gas production rate using a drop-down reactor. Fuel 2014;128:210–9. [5] Kopyscinski J, Rahman M, Gupta R, Mims CA, Hill JM. K2CO3 catalyzed CO2 gasification of ash-free coal. Interactions of the catalyst with carbon in N2 and CO2 atmosphere. Fuel 2014;117:1181–9. [6] Zhou ZJ, Hu QJ, Liu X, Yu GS, Wang FC. Effect of iron species and calcium hydroxide on high-sulfur petroleum coke CO2 gasification. Energy Fuels 2012;26:1489–95. [7] Monterroso R, Fan MH, Zhang F, Gao Y, Popa T, Argyle MD, Towler B, Sun QY. Effects of an environmentally-friendly, inexpensive composite iron-sodium catalyst on coal gasification. Fuel 2014;116:341–9.

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