Coal rapid pyrolysis in a transport bed under steam-containing syngas atmosphere relevant to the integrated fluidized bed gasification

Fuel 176 (2016) 200–208

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Coal rapid pyrolysis in a transport bed under steam-containing syngas atmosphere relevant to the integrated fluidized bed gasification Zhaohui Chen a,b, Yong Shi c, Dengguo Lai a,b, Shiqiu Gao a,⇑, Zhen Shi a,b, Yong Tian a,b, Guangwen Xu a,⇑ a

State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China c Anhui University of Technology, Ma’anshan 243002, Anhui Province, China b

h i g h l i g h t s
A transport bed was used to study the effect of steam-containing syngas on coal rapid pyrolysis.
Steam-containing syngas combined the advantages of steam and syngas on tar production.
Steam-containing syngas atmosphere was beneficial to CH4 production.

a r t i c l e

i n f o

Article history: Received 21 November 2015 Received in revised form 25 February 2016 Accepted 26 February 2016 Available online 5 March 2016 Keywords: Pyrolysis Coal Transport bed Steam Syngas Tar

a b s t r a c t An integrated fluidized bed (IFB) consisting of an upper transport bed section and a bottom fluidized bed section was adopted to investigate the transport bed coal pyrolysis by varying its reaction temperature and reaction atmosphere adjusted to simulate steam-containing syngas produced by the bottom fluidized bed char gasification. Steam and syngas, in comparison with N2, as the reaction atmosphere little affected the tar yield below 600 °C but significantly decreased it for the former and increased it for the latter at rather higher temperatures. The presence of H2 in the syngas increased tar yield significantly because it could suppress polymerization and condensation reactions through providing H as radical stabilizer and hydrogenation agent. In the steam-containing syngas atmosphere, the tar yield obtained from transport bed rapid pyrolysis increased rapidly with raising temperature to a peak value of 10.5 wt.% (daf) at 600 °C, about 1.1 wt.% higher than the Gray–King assay yield, and then decreased due to the excessive secondary reactions. Analyzing tar composition further showed that steam-containing syngas combined their respective advantages that syngas improved the yields of both light and heavy tars while steam reduced the heavy tar yield, especially at temperatures above 600 °C. The steam-containing syngas atmosphere also promoted CH4 production in comparison with syngas atmosphere below 700 °C. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction The ever increasing demand of natural gas in China as well as the limited domestic supply provides a strong incentive to produce synthesis natural gas (SNG) from coal, especially low-rank coal [1]. Many SNG projects have actually been planned or in demonstration in China [2,3]. The Lurgi moving bed gasifier has been widely adopted for SNG production because its producer gas contains high-content CH4 [4]. This gasifier also produces a considerable amount of tar which is highly demanded by actual industries due to the limited availability of crude oil in China [5,6]. Because Lurgi gasifiers adopts only lump coal above 6 mm, there is a great need ⇑ Corresponding authors. Tel.: +86 10 82544885; fax: +86 10 82629912. E-mail addresses: [email protected] (S. Gao), [email protected] (G. Xu). http://dx.doi.org/10.1016/j.fuel.2016.02.082 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.

to treat the more abundant powder coal [7], such as below 10 mm, via a Lurgi-type gasifier. Accordingly, an integrated fluidized bed (IFB) consisting of an upper transport bed section and a bottom fluidized bed section has been tested to process low-rank powder coal and co-produce tar and CH4-rich syngas [8]. This IFB process was motivated to treat powder coal via the Lurgi-type process, which incorporates coal pyrolysis into char gasification. As schematically shown in Fig. 1, coal is fed into the upper transport bed section to carry out fast pyrolysis, and the formed char is circulated into the bottom fluidized bed section for gasification. The produced gas from char gasification by oxygen-steam agent, which contains syngas and unreacted steam, supplies both sensible heat and reaction atmosphere for the coal pyrolysis in the upper transport bed section. Our previous study has verified the technical feasibility of the IFB

Z. Chen et al. / Fuel 176 (2016) 200–208

Char Ash

H2O&O2

gasifier

Heat Exchange

Pyrolyzer

Tar & Syngas

201

ucts. For this, the effects on the yield and quality of tar and production of CH4 in the transport bed section of IFB are investigated with respect to the pyrolysis atmospheres composed of N2, steam, simulated syngas, steam-containing simulated syngas and the major gas components containing in the syngas. 2. Experimental

Coal

Steam-containing syngas Fig. 1. Principle illustration of the integrated fluidized bed gasification process.

process for treating powder coal and producing CH4-rich syngas [8]. At the appropriate operating conditions, the CH4 content reached about 11.2 vol.%, about six times higher than 2.0 vol.% for the usual fluidized bed gasification and close to 12.0 vol.% for the Luigi gasifier. This study focuses on the coal pyrolysis in the upper transport bed section which is subject to the interaction between coal and hot steam-containing syngas atmosphere from the bottom char gasification. In the Lurgi gasifer, the coal pyrolysis occurs with slow heating, as a result of its feeding at the low temperature top of gasifier and using the large coal particle sizes (lump coal). In the IFB reactor, the powder coal feeding is at the hightemperature bottom of the transport bed pyrolysis section to cause thus rapid heating of coal particles that can be around 1000 °C/s [9]. Understanding transport bed coal pyrolysis can also deliver knowledge of coal rapid pyrolysis. Wen and Dutta [10] reported that at temperatures below 600 °C the rapid heating leads to high yield of total volatiles and high ratio of liquid to gas. Above 600 °C the secondary reactions of tar become significant, and increasing the final temperature with a particular heating rate is beneficial to total volatiles yield but decreases the product ratio of liquid to gas. In the IFB, the syngas from bottom gasification provides heat and also atmosphere for coal pyrolysis. Many studies have shown the influences of dry-syngas, coke-oven gas and pyrolysis gas on yield and quality of pyrolysis-generated tar in fixed bed and fluidized bed reactors [11–14], but there is almost no study on the transported bed rapid pyrolysis in steamcontaining syngas (in situ wet syngas) atmosphere. Jüntgen [15] discussed the effect of H2 on the evolution of tar during coal pyrolysis. When replacing inert gas with H2, the pyrolysis causes additional formation of CH4 and tar at the expense of coke. It is said radical groups are stabilized by H2 to block radical recombination reactions and coke formation. Partial hydrogenation of polynuclear aromatics would also occur, while subsequent tar hydrocracking including demethylation, deoxygenation, hydroxylation and elimination of functional groups may lower the tar production to yield more CH4 and light tar. Depending on operation conditions and reactor type, hydrogen would have different effects on tar production from coal pyrolysis [16–25]. The presence of CO in the atmosphere promotes tar production due to its inhibition on secondary reactions occurring to phenols [20,26]. Carbon dioxide (CO2) can participate in reforming reactions of volatiles to affect tar formation and in char gasification to enhance production of no-condensable gases [20,21,27]. Steam in the atmosphere likely penetrates micro-pores in coal to facilitate desorption of volatile matter and recovery of tar species [28]. This is why steam is reported to enhance tar yield by its inhibiting secondary reactions at low temperature [13,28,29]. On the other hand, high temperature definitely enhances reforming reactions of nascent tar to cause tar loss [30], especially at high steam concentration [31]. The intention of this study is thus to explore the interaction of rapid coal pyrolysis and steam-containing syngas atmosphere to obtain the potential high yields of pyrolysis liquid and gas prod-

Fig. 2 shows a schematic diagram of the experimental apparatus. The integrated fluidized bed (IFB) combines a transport bed pyrolysis section on the top and a fluidized bed gasification section on the bottom. In this study, only the upper transport bed with inner diameter of 40 mm and height of 6.0 m was employed as a coal pyrolyzer and the bottom fluidized bed packed with Al2O3 ball with diameter of 2–3 mm was used as a gas preheater. Coal feeding inlet is at 300 mm above the joint flange of the transport bed bottom. The gas supplying system was consisted of cylinders for N2, H2, CO, and CO2, a gas preheater, a water pump and a steam generator for generating steam, and a gas mixer (also as the second gas preheater). When all the parts or sections of the IFB reached their desired temperatures, a gas stream mixing all components necessary was preheated in the bottom gas preheater and fed into the bottom of the IFB. The tested pyrolysis atmospheres are denoted as atmospheres of steam, simulated syngas and steam-containing simulated syngas. Their compositions are (in volume) 15% H2O + 85% N2, 40% H2 + 30% CO + 15% CO2 + 15% N2 and 40% H2 + 30% CO + 15% CO2 + 15% H2O, respectively. Other atmospheres are denoted by the names of individual gas components, like H2, CO, CO2 refer to the compositions of 50% N2 + 50% individual gas (H2, CO and CO2). The tested coal in this study is a kind of subbituminous coal from Inner Mongolia, China. Table 1 shows the results of proximate and ultimate analyses for the tested coal. The coal contains relatively high volatile matter (VM) and ash contents. The tar yield of Gray–King assay is 9.4 wt.% (daf). The coal sample was crushed to a size range of 0.3–0.4 mm and then dried in an atmospheric oven of 105 °C for 2 h before each test. The terminal velocity for coal particle with the sizes of 0.3–0.4 mm is about 1.7–2.6 m/s in the temperature range from 500 °C to 800 °C. Coal particles were fed continuously via a screw feeder with a feeding rate of 1.2 kg/h. For each test, coal feeding was continued for about 1.0 h to ensure every parameter was at the desired stable state. The coal particles were conveyed pneumatically into the transport bed and heated by hot carrier gas to carry out rapid pyrolysis under various reaction conditions. The superficial gas velocity is about 3.1–4.1 m/s corresponding to a carrier gas flow of 80 L/min for guaranteeing coal particles to be entrained in the transport bed. The residence time of hot gases as well as coal particles is about 2 s. Volatiles released from coal together with char were conveyed by hot carrier gas out of the reactor. The char was separated from gas via a cyclone into a receiver. A metal filter was used to further separate the fine char. The quench cooler and the condenser allowed the majority of tar and steam to condense. The cotton filter was used for the removal of the remnants of fine particles. Most of the gas was scrubbed by a large amount of acetone to collect residual tar and then ventilated into the air, but a part of gas was scrubbed by several acetone traps to further remove residual tar for gas analysis. The non-condensable gas dried by a silica gel column including N2, H2, CH4, CO, CO2, C2–C3 was metered by a wet gas flowmeter and sampled in every 4 min using gas bag. Then, the producer gas was analyzed by a micro GC (Agilent 3000A). Fig. 3 shows the outlet gas compositions as measured against time at 800 °C under syngas atmosphere. The gas compositions of N2, H2, CH4, CO, CO2, C2–C3 maintained a stable value during 1 h experiment. The average gas composition was calculated by arithmetic average method.

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16 13 14

19 18

12 23 15 11 17 20

17

21

10

26

9

25 23 24

22 8

7

N2

6 ` `

4 1

3 2

5

N2

H2

CO

CO2

Fig. 2. Schematic diagram of the integrated fluidized bed reactor system. (1) Gas cylinders; (2) Gas preheater; (3) Water tank; (4) Plunger pump; (5) Steam generator; (6) Gas mixer and preheater; (7) Ball valve; (8) Fluidized bed as a preheater; (9) Feed position; (10) Screw feeder; (11) Transport bed; (12) Electric furnace; (13) Safety valve; (14) Cyclone; (15) Char receiver; (16) Metal filter; (17) Tar collector; (18) Quench cooler; (19) Condenser; (20) Pneumatic control back pressure valve; (21) Cotton filter; (22) Icewater bath; (23) Acetone trap; (24) Wet gas meter; (25) Gas bag; (26) Micro GC.

Table 1 Proximate and ultimate analyses for the tested coal. Proximate analysis (wt.%) Ad 19.56

VMdaf 36.62

Ultimate analysis (wt.%, daf) FCdaf 63.38

C 77.55

H 4.58

Oa 15.49

Gray–King assay yield (wt.%, daf) N 1.36

S 1.02

Tar 9.4

d – Dry basis; daf – Dry and ash free basis. a By difference.

The entire pipeline, cooler and condenser were washed several times by acetone until the color of acetone was almost not changed. The tar collected from two tar collectors was also mixed with the acetone washing liquid. All the obtained acetone washing liquid was first filtered twice using buchner funnel (3–4 lm) to remove the dust completely, and then treated by anhydrous magnesium sulfate to remove the water. After further filtration, the dehydrated acetone washing liquid was evaporated in a vacuum rotary evaporator to remove acetone to recover the dissolved tar. The same experiment was repeated twice under atmospheres of N2, H2 and steam-containing syngas to exam the uncertainty of tar yield, as shown in Table 2. The absolute error of tar yields in two tests is less than 0.1 wt.% (daf). This is because that in each test

Fig. 3. The outlet gas compositions as measured against time.

about 1.2 kg coal was pyrolyzed to produce a relative large amount of tar which led to small error of measurement. The dehydrated tar sample was analyzed twice by a simulated distillation chromatograph (Agilent 7890A) to characterize the fractional distribution of tar. The components with a boiling point below 360 °C were

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Z. Chen et al. / Fuel 176 (2016) 200–208 Table 2 Repeatability of tar yield. Atmosphere

Temperature (°C)

Test 1 (wt.%, daf)

Test 2 (wt.%, daf)

Average value (wt.%, daf)

Absolute error (wt.%, daf)

N2 H2 Steam-containing syngas

700 700 800

7.54 9.14 6.55

7.67 9.06 6.38

7.61 9.10 6.47

0.07 0.04 0.09

Table 3 A comparison of product distribution between slow and rapid pyrolysis. Products yield (wt.%, daf)

Slow pyrolysisa

Rapid pyrolysisb

Tar Gas Char Volatiles

8.1 9.8 70.5 29.5

10.2 7.8 68.9 31.1

In two tests, coal particle with the sizes of 0.3–0.4 mm was used. a Heating rate: 0.2 °C/s. b Heating rate: about 1000 °C/s.

defined as the light tar fraction, while those above 360 °C as the heavy tar fraction. The tar yield (Ytar, wt.%, daf) was calculated via dividing the weight of dehydrated tar by the weight of feed coal on dry and ash free basis (daf) via Eq. (1),

Y tar ðdaf Þ ¼

W tar W coal ð1  Acoal Þ

ð1Þ

where Wtar and Wcoal are the weight of collected tar and feed coal on dry basis (kg), respectively. Acoal is the ash content in coal on dry basis (wt.%). The volatile yield (Yvolatile, wt.%, daf) was calculated via ash tracer method, which assumed that all the ash in the coal remains in the char, according to the following equations [32,33],

W ash ¼ W char  Achar ¼ W coal  Acoal ;

ð2Þ

W char Acoal ¼ ; W coal Achar

ð3Þ

Y char ðdÞ ¼

W char ð1  Achar Þ Acoal ð1  Achar Þ Y char ðdaf Þ ¼ ¼ ; W coal ð1  Acoal Þ Achar ð1  Acoal Þ Y v olatile ðdaf Þ ¼ 1  Y char ðdaf Þ ¼

1  ðAcoal =Achar Þ ; 1  Acoal

3.2. Pyrolysis in steam atmosphere

ð4Þ

ð5Þ

where Wash and Wchar are the weights of ash and produced char on dry basis (kg), respectively. Achar is the ash contents in char on dry basis (wt.%). Ychar represents the yield of char on dry basis (wt.%). Ychar(daf) and Yvolatile(daf) represent the yields of char and volatile on dry and ash free basis (wt.%). The CH4 yield (Y CH4 , L/kg-coaldaf) was obtained by N2 balance via Eq. (6),

Y CH4 ðdaf Þ ¼

C CH4 ðV N2 =C N2 Þ ; W coal ð1  Acoal Þ

bed, respectively. A rapid heating rate produced more volatile and tar and less gas and char than that at slow heating rate. It was beneficial to the tar production at high heating rate. The sum of tar and gas yields is less than the volatile yield due to the generation of pyrolysis water. Sathle et al. [34] found a linear increase in the tar yield from brown coal with the logarithm of heating rate under conditions of atmospheric pressure and maximum pyrolysis temperature of 600 °C, while the gas yield decreased as heating rate rose. Similar results were also reported by Ladner and Gibbins [35,36]. Hayashi et al. [37] discussed the link between rapid heating and high tar yield from following two aspects. During the rapid pyrolysis, the higher internal pressure or concentration gradients inside coal particles due to the rapid release of more volatile precursors can enhance driving force of their diffusion and advection so that increasing heating rate can reduce the residence time of volatile precursors in coal particle, and prevent them from cracking and repolymerization inside the particle to generate more gas and char. On the other hand, the higher heating rate leads to the greater rate of broken bridge reaction and intensify the depolymerization reaction rate. Morgan and Kandiyoti [38] also thought that the difference in tar yields between rapid and slow heating may be explained by the formation of more char via recombination reactions at slow heating and internally released hydrogen during rapid pyrolysis works on obstructing rapid char forming free-radical recombination reactions. Thus, rapid pyrolysis of coal in the transport bed can obtain high tar yield, which meets the requirements of the integrated fluidized bed gasification process in term of co-production of tar and syngas.

ð6Þ

where V N2 is the volume of nitrogen fed into the reactor (L). C N2 and C CH4 are the contents of N2 and CH4 in gas (vol.%), respectively. 3. Results and discussion 3.1. Distinction of transport bed pyrolysis Table 3 makes a comparison of the product distribution between slow and rapid coal pyrolysis at the final temperature of 600 °C in the reactor of Gray–King assay and in the transport

Coal rapid pyrolysis in the transport bed under N2 and steam atmospheres was carried out to clarify the effect of steam on the tar yield and quality and the volatiles yield. Fig. 4 shows the effect of steam on the yields of tar and volatiles and the ratio of tar to volatiles in a temperature range from 500 °C to 800 °C. As shown in Fig. 4, the tar yield increased rapidly with raising pyrolysis temperature to a peak value at 600 °C and then declined, while the volatiles yield increased monotonously and the ratio of tar to volatile decreased regardless of atmosphere. The higher pyrolysis temperature resulted in a lower tar yield due to its excessive secondary reaction in the gas phase [39]. But at low temperature of 500 °C, coal pyrolysis failed to be completed and caused thus the low yields of tar and volatiles because of short residence time of coal particles in the transport bed. Fig. 4(a) shows that adding 15 vol. % steam into N2 atmosphere had a little positive effect on tar yield below 600 °C because steam could retard the secondary reaction of tar via blocking its contact with hot char surface [28], but caused a significant decline of tar yield with further increasing the temperature due to the reforming reaction at high temperature. Hayashi et al. [30] also found adding steam into N2 atmosphere significantly decreased the tar yield via the steam reforming reaction at high temperatures in a drop-tube reactor, which is like the transport bed reactor in this study. Compared to pure N2 atmosphere, adding steam promoted the release of volatiles due to its gasification reaction with coal or char to some extent, and thus led to the

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Fig. 4. The yields of tar and volatiles and the ratio of tar to volatiles under N2 and steam atmospheres varying with the pyrolysis temperatures.

Fig. 5. Variation of the light and heavy tar yields under N2 and steam atmospheres at different pyrolysis temperatures.

ratio of tar to volatiles decreased with the temperature increasing as shown in Fig. 4(b). The maximum tar yields obtained at 600 °C were about 10.2 wt.% and 10.4 wt. % under atmospheres of N2 and steam, about 0.8–1.0 wt.% higher than Gray–King assay tar yield (Table 1). Fig. 5 presents the variation of light and heavy components in the tar with increasing the pyrolysis temperature from 600 °C to 800 °C. With increasing the pyrolysis temperature, both the yields of light and heavy tar decreased under N2 and steam atmospheres. At 600 °C, changing atmosphere from N2 to steam has a little effect on light and heavy tar yields. Increasing the temperature from 600 °C to 800 °C caused the decrease of the light and heavy tar yields under N2 and steam atmospheres, while the heavy tar yield decreased more obviously compared with that of light tar under steam atmosphere. Steam could promote the cracking of heavy tar to form light tar by steam reforming reactions. Therefore, the decrease in light tar appeared to be less than that of heavy tar. Song et al. [40,41] also observed that larger aromatic rings were easier to be destructed than smaller ones in the gas phase.

3.3. Pyrolysis in syngas atmosphere Fig. 6 illustrates the effect of syngas on the yields of tar and volatiles and the ratio of tar to volatiles under syngas and N2 atmospheres from 500 °C to 800 °C. As shown in Fig. 6(a), with the replacement of N2 by syngas as the reaction atmosphere, the tar yield changed slightly below 600 °C, while syngas could greatly inhibit the decrease of the tar yield above 600 °C. Corresponding to the pyrolysis temperature of 700 °C and 800 °C, the tar yield increased pronouncedly from 7.6 wt.% and 5.6 wt.% under N2 atmosphere to 9.5 wt.% and 8.2 wt.% under syngas atmosphere, respectively. The tar yield of 10.3 wt.% at 600 °C was obtained under

Fig. 6. The yields of tar and volatiles and the ratio of tar to volatiles under N2 and syngas atmospheres varying with pyrolysis temperatures.

Z. Chen et al. / Fuel 176 (2016) 200–208

205

syngas atmosphere similar to that of N2 and steam. Below 600 °C the secondary reaction is not obvious yet as reported in previous studies [42,43] and syngas mainly play a role on enhancing stabilization of free radicals. At high temperatures above 600 °C, the tar yield not only depends on the secondary reaction of tar governed by its decomposition under inert atmosphere [30], but also on the reactions of tar with syngas. On the other hand, high temperature could promote secondary reactions of volatiles via dealkylation, dehydroxylation, etc. to form more gas similar to syngas components [44,45], and thus the secondary reaction was also suppressed by syngas in this way. As shown in Fig. 6(b), the total volatiles yield decreased under atmosphere of syngas in comparison with that of N2, indicating that syngas inhibited the volatiles release because pyrolysis gas components were similar to syngas. The ratio of tar to volatiles is notably greater in atmosphere of syngas than that in N2 especially above temperatures of 700 °C. This also proves that syngas can inhibit tar thermal cracking at high temperature. The above results represent a synergetic effect of different components in syngas on the tar yield. The effect of different gas components in syngas on the tar yield was further investigated at 700 °C to clarify the role of each gas component in syngas, as shown in Fig. 7. The results show that H2 was mainly responsible for the increase of tar yield and adding CO into N2 atmosphere also raised the tar yield. Whereas, the presence of CO2 in N2 led to a slight reduction of the tar yield. The coal pyrolysis process consists of a very complex set of reactions involving the formation of radicals via rupture of weak bonds, their stabilization via polymerization and condensation reactions to form volatiles and char and subsequent secondary gas phase reactions of volatiles because of extended exposure to high temperatures [46]. In the syngas, H2 could inhibit the polymerization and condensation reactions via providing active H to stabilize free radicals to block their charforming reactions and lead to a partial hydrogenation of aromatics especially for the substances of heavy molecular weight [15]. Nitsch et al. [47] found that H2 could reduce the carbon deposited on the reactor wall during conversion of phenol-based tar in the gas phase because hydrogen is a byproduct of recombination and condensation and a high H2 partial pressure has inhibiting effect on the formation of larger polycyclic aromatic hydrocarbons and coke. Jüntgen [15] also reported condensation of the substances with high molecular weight would yield the coke along with elimination of H2 but in the presence of H2 a partial hydrogenation of aromatics with subsequent hydrocracking occurred to promote the conversion of heavy oil or macromolecules into small molecules. CO would react with pyrolysis water to produce high active H that has the same effect of H2 [48]. On the other hand, CO as a

main product from the decomposition of oxygen functional groups of tar can retard the deoxidation reaction of tar [30]. CO2 as a mild oxidant maybe involve in volatiles reforming reactions to affect the evolution of tar [49]. Thus, the slight effect of CO2 on tar yield might be attributed to the reforming reaction because CO2 facilitates the cracking of aromatic ring and rupture of hydroxyl and alkyl substituents [49]. Fig. 8 shows variation of the light and heavy tar yields under N2 and syngas atmospheres at different pyrolysis temperatures. As shown in Fig. 8, both the light and heavy tar yields decreased with increasing the temperature under syngas atmosphere as same as N2 and steam shown in Fig. 5. This is attributed to the secondary cracking reactions of tar, especially for heavy tar because it with larger molecular weight was easily convert into coke at high temperature. Larger aromatic rings in the tar served as cross-linking sites of higher coordination number than smaller one systems, which easily led to the formation of char at high temperature [50]. At 600 °C, the light tar yield increased but the heavy tar decreased with changing atmosphere from N2 to syngas. While at 700 °C and 800 °C, however, syngas not only promoted the increase of light tar yield, but also increased the heavy tar yield. Like H2 atmosphere, H2-rich syngas enhanced the heavy tar upgrading because the occurrence of partial hydrogenation led to the conversion of tar from high molecular weight to low molecular weight. On the other hand, H2-rich syngas could inhibit the conversion of large aromatics into coke so as to preserve them to cause the increase of heavy tar yield.

Fig. 7. Tar yield varying with the atmosphere of each gas component in syngas at the pyrolysis temperature of 700 °C.

Fig. 8. Variation of the light and heavy tar yields under N2 and syngas atmospheres at different pyrolysis temperatures.

3.4. Pyrolysis in steam-containing syngas atmosphere In the IFB gasification process, the bottom fluidized bed char gasification provided directly steam-containing syngas atmosphere for the upper transport bed pyrolysis. Fig. 9 shows the

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comparison of the tar and volatiles yields and the ratio of tar to volatiles under steam, syngas and steam-containing syngas atmospheres from 500 °C to 800 °C. Compared to syngas and steam atmospheres, steam-containing syngas simulating the real atmosphere from the bottom fluidized bed char gasification has no obvious effect on tar yield below 600 °C, while the tar yield was obviously different under different atmospheres above 600 °C. It can be observed that the yields of tar and volatiles and the ratio of tar to volatiles lies between these tar yields obtained from syngas and steam atmospheres. This implies that adding syngas into steam inhibited the reforming reactions of tar and the steam gasification of coal, and also manifests that the positive effect of syngas on the production of tar was retarded by steam. Under steamcontaining syngas atmosphere simulating the real working condition in the transport bed, the tar yield reached a peak value of 10.5 wt.% (daf) at 600 °C, about 1.1 wt.% more than the Gray–King assay yield. Fig. 10 shows the comparison of the light and heavy tar yields under steam, syngas and steam-containing syngas atmosphere at different pyrolysis temperatures. Mixing 15 vol.% steam in syngas atmosphere brought about a reduction of light tar yield, while the heavy tar yield was first enhanced at 600 °C and then decreased above 600 °C. Fidalgon et al. [13] also analyzed the coal tar obtained at 700 °C and 3 MPa from a small fixed bed by the same method using simulated distillation GC and found that the presence of steam in syngas (H2/CO) increased heavy fractions and led to higher yields of tars with slightly broader and heavier molecular weight distribution which further observed by size exclusion chromatography. A significant decrease for heavy tar yield above 600 °C under steam-containing atmosphere was attributed to steam reforming reactions as discussions in Section 3.2. It is

Fig. 9. Comparison of the yields of tar and volatiles and the ratio of tar to volatiles under steam, syngas and steam-containing syngas atmospheres at different pyrolysis temperatures.

Fig. 10. Comparison of the light and heavy tar yields under steam, syngas and steam-containing syngas atmospheres at different pyrolysis temperatures.

obvious that steam-containing syngas combines the advantages of syngas and steam that syngas improves the tar yield and quality, while steam enables heavy tar cracking at high temperatures. In addition, one of the main objectives of adding the upper pyrolysis section in the IFB gasification process is to increase the concentration of CH4 in the producer gas through the application of possible hydropyrolysis under steam-containing syngas atmosphere from the bottom gasification section. Hence, the variation of CH4 yield under different atmospheres was also investigated in this study. As shown in Fig. 11, with the temperature increasing CH4 yield increased monotonously under all of the atmospheres. Introducing steam into N2 atmosphere first promoted the CH4 formation below 700 °C and then reduced the CH4 production at 800 °C due to the occurrence of CH4 reforming reaction with H2O. Replacing N2 by syngas as the reaction atmosphere gives rise to a significant increase in CH4 yield in the temperature range from 500 °C to 800 °C. Steam-containing syngas enabled markedly the higher CH4 yield with raising the temperature to 700 °C in comparison with the case without steam while further raising the temperature to 800 °C led to an opposite result. This is because steam contained in N2 or syngas atmosphere will react with CH4 at high temperature, for example 800 °C. Enough H2 in syngas promoted the shedding of CH3 and CH2 groups and their stabilization to form CH4 as well as the occurrence of hydrogasification with char at high temperature [51]. CO and CO2 in syngas also enhance the fracture of CH3 and CH2 groups to intensify the formation of CH4 to a certain degree [49,52]. Overall, compared with inert atmosphere the application of steam-containing syngas in the transport bed pyrolysis realized actually the aim of raising the CH4 yield, especially at temperatures of 700 °C and 800 °C, changing the pyrolysis atmosphere from inert to steam-containing syngas increased the CH4 yield by 12–13 L/kg-coaldaf.

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Fig. 11. Influence of atmospheres on the CH4 yield at different pyrolysis temperatures.

4. Conclusion The effect of steam-containing syngas on coal rapid pyrolysis was investigated in a bench scale transport bed with a continuous coal feed rate of 1.2 kg/h. Adding steam or syngas into N2 atmosphere had a little positive effect on tar yield below 600 °C. Further raising the temperature above 600 °C resulted in a significant increase of tar yield under syngas atmosphere while an obvious decrease of tar yield under steam atmosphere in comparison with N2 atmosphere. The presence of H2 in the syngas inhibited the polymerization and condensation reactions via providing active H to stabilize free radicals and led to a partial hydrogenation of aromatics and thus increased the tar yield. Under steam-containing syngas atmosphere, the tar yield reached a peak value of 10.5 wt. % (daf) at 600 °C, about 1.1 wt.% more than the Gray–King assay yield. Compared with N2 atmosphere, syngas improved CH4 formation significantly while steam only promoted CH4 production below 700 °C. The higher CH4 yield was obtained under syngas with steam atmosphere in comparison with the case without steam below 700 °C. Results of tar analysis showed that in the steam-containing syngas atmosphere syngas promoted the yields of both light and heavy tar while steam atmosphere can reduce the heavy tar yield above 600 °C. All of these experimental results verified that it is beneficial to the production of tar and CH4-rich syngas by the application of steam-containing syngas atmosphere on coal rapid pyrolysis in the transport bed. Acknowledgments The authors wish thank the financial supports from the International Science & Technology Cooperation Program of China (2013DFG60060), the National Natural Science Foundation of China (21376250 and U1302273) and the Strategic Priority Research Program of Chinese Academy of Sciences (XDA07010100). References [1] Huo JW, Yang DG, Xia FQ, Tang H, Zhang WB. Feasibility analysis and policy recommendations for the development of the coal based SNG industry in Xinjiang. Energy Policy 2013;61:3–11. [2] Ding YJ, Han WJ, Chai QH, Yang SH, Shen W. Coal-based synthetic natural gas (SNG): a solution to China’s energy security. Energy Policy 2013;55:445–53. [3] Xu J, Yang Y, Li YW. Recent development in converting coal to clean fuels in China. Fuel 2015;152:122–30. [4] Bai YH, Wang P, Yan LJ, Liu CL, Li F, Xie KC. Effects of CO2 on gas evolution and char structure formation during lump coal pyrolysis at elevated pressures. J Anal Appl Pyrolysis 2013;104:202–9.

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