Experimental and Modeling Investigation on the Effect of Intrinsic and

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The experimental and modeling investigation on effect of intrinsic and extrinsic oxygen on biomass tar decomposition Ruizhi Zhang, Shanhui Zhao, and Yong-hao Luo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00989 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 15, 2017

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Energy & Fuels

1 Mass flow controller 2 Check valve 3 Model tar injector 4 Vaporizing chamber 5 Furnace 6 Thermocouple 7 Computer 8 Sampling pipe 9 GC-TCD/FID analyzer 10 GC/MS analyzer

Figure 1. The flow tube reactor system

ACS Paragon Plus Environment

Energy & Fuels

2-methoxyphenol Anisole furfural toluene

100

80

COnversion rate(%)

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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Figure 2. The conversion rate of four tar compounds with temperature


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Figure 3. The product tar of 2-methoxyphenol thermal decomposition(from left to right: 6001000℃)


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

Compound peak area

Energy & Fuels

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500C 600C 700C 800C 900C 1000C

120000

60000

0 C6H6

C7H8

C8H8 C6H5OH C8H6O C7H6O2 C7H8O C7H8O2 C8H10O C10H8 C6H6O2

Tar compounds

Figure 4. The condensation products of 2-methoxyphenol decomposition


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400°C 500°C 600°C 700°C 800°C 900°C 1000°C

300000

Compounds peak area

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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200000

100000

0 C6H6

C7H8 C7H8O C7H6O C6H6O C8H6O C9H8 C7H8O C7H8O C10H8 C12H10 C12H8

Tar compounds

Figure 5. The condensation products of anisole decomposition


Energy & Fuels

60

Yield (% anisole)

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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400C 500C 600C 700C 800C 900C 1000C

Four tar yield

40

20

0 C6H6

C7H8

C6H5OH

C10H8

Tar compounds

Figure 6. Quantitative yield of four tar compounds


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2-methoxyphenol Anisole

Naphthalene yield (mole %)

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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6

3

0 700

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Temperature(C)

Figure 7. Naphthalene yield of two model tar compounds decomposition


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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

(a) 800℃ (b) 900℃ (c) 1000℃ Figure 8. Tar yield of 2-methoxyphenol partial oxidation under different ER and temperature (from left to right: ER=0, 0.1, 0.2, 0.5)


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700C 2-methoxyphenol

Thermal crack ER=0.1 ER=0.2 ER=0.5 ER=1

Compound peak area

80000

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Compound peak area

Compound peak area


8 H 10 C e yd eh ld yla lic sa

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an ur of nz be

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ol en ph

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e en yr

120000

Thermal cracking ER=0.1 ER=0.2 ER=0.5 ER=1

st


e en lu to

e en nz be

l ho ec at C 8 H 10 C O 10 8H C 2 8O 7H C 8O 7H C 2 6O 7H C an ur of nz be H

8

5O 6H C e en yr st

7H

C 6

6H

160000



80000

20000

C

Thermal cracking ER=0.1 ER=0.2 ER=0.5

60000

30000

0


ne le ha

Figure 9. Tar yields of 2-methoxyphenol partial oxidation

t ph na

e en lu to

e en nz be

e en al th ph na de hy de al yl lic sa an ur

of nz be

ol en ph

ne re sty

e en lu to

e en nz be

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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Compound peak area

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120000

80000

ER=0 ER=0.1 ER=0.2 ER=0.5 ER=1

900 partial oxidation-anisole 120000

Compound peak area

100000

60000

40000

90000

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30000

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0

8O H 12

C

8

10

8


8 H 12

--

--

--

H 12

C

H 12

C

8

H 10

C

6

7H

C

6H

C

Figure 10. Tar yields of anisole partial oxidation

C

0

10 H 12

30000

C

60000

8 H 10

90000

C

120000

8 9H

ER=0 ER=0.1 ER=0.2 ER=0.5

1000 partial oxidation-anisole

C

6O 8H

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8 8H

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8 7H

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6 6H

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8O

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H 12

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150000

Compound peak area

Compound peak area

150000

ER=0 ER=0.1 ER=0.2 ER=0.5 ER=1


C

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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800

H2 CO CH4 CO2

600

2-methoxyphenol-900C

H2 CO CH4 CO2

300

Gas yield (mole % anisole)

Gas yield(%/ 2-methoxyphenol)

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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400

200

0

Anisole-900C

200

100

0

ER=0

ER=0.1

ER=0.2

ER=0.5

ER=1

ER=0

ER=0.1

Oxygen amount

ER=0.2

ER=0.5

ER=1

Oxygen amount

Figure 11. The gas yields of 2-methoxyphenol and anisole partial oxidation at 900℃


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120 100

Experiment Model

100

Experiment Model

80

Conversion rate (%)

2-methoxyphenol conversion rate(%)

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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60 40 20 0

50

Furfural decomposition 0

-20 200

400

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Temperature (C)

600

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Temperature

Figure 12. The comparison of experimental and modeling results (left: 2-methoxyphenol, right: furfural)


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12

Experiment Model

Phenol yield

3

Yield rate(%)

9

yield rate (%)

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Experiment Model

Naphthalene

2

1

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Temperature(C)

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Figure 13. The comparison of tar yields of 2-methoxyphenol decomposition calculated (left: phenol yield, right: naphthalene yield)


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60

Experiment Model

200

H2 yield

CO yield 160

40

yield rate (%)

yield rate (%)

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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Experiment Model

40 0

0 700

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Temperature (C)

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Temperature (C)

(a) H2 yield (b) CO yield Figure 14. The comparison of tar yields of 2-methoxyphenol decomposition calculated (left: H2 yield, right: CO yield)


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100

Experiment Model

100

Total tar conversion at 900C Fixed temperature

80

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60

Tar conversion rate(%)

90

conversion rate (%)

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90

Calculation Experiment

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0.0

0.1

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0.3

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0.5

-0.1

0.0

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ER

0.2

0.3

0.4

0.5

0.6

ER

Figure 15. The real tar partial oxidation results comparison between modeling and experiment (Left: isothermal condition; right: adiabatic condition)


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The experimental and modeling investigation on effect of intrinsic and extrinsic oxygen on biomass tar decomposition Ruizhi ZHANG, Shanhui ZHAO, Yonghao LUO*

Institute of Thermal Engineering, School of mechanical engineering, Shanghai Jiao Tong University, Shanghai, 200240, P.R. China Abstract: The heteroatoms, such as oxygen, have great influence on tar decomposition reactivity. In order to investigate the effect of oxygen on biomass tar decomposition, thermal decomposition of three typical oxygenated tar compounds and toluene were conducted under inert and oxidative atmosphere in a flow tube reactor. The key roles of intrinsic oxygen of tar and extrinsic oxygen at controlled atmospheres were taken into consideration. Results show that all three tar compounds were easy to decompose at moderate temperatures accompanied by polymerization reaction. 2-methoxyphenol produced less naphthalene compared with anisole under inert atmosphere. Intrinsic oxygen enhanced the reactivity of tar as well as inhibited the polymerization process. Moderate extrinsic oxygen addition could eliminate tar without sacrifice of combustible gases, such as H2 and CO. RMG (Reaction mechanism generator) method was used to build a complete detailed kinetic model to simulate biomass tar homogenous conversion. The new detailed kinetic model could predict the conversion of model tar compounds as well as real tar mixture. Keyword: biomass tar, homogenous decomposition, oxygen, modeling 1. Introduction Biomass is a kind of alternative energy source that could take advantage of existing fossil energy facilities. Biomass gasification is a promising method that converts biomass into H2 and CO mainly, which could be used in different ways[1, 2]. However, biomass tar produced during


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gasification is an obstacle for commercialization[3-5]. Tar could cause the corrosion, blocking of equipment, energy waste, environment issues and health problems. Tar is fragments of cellulose, hemicellulose and lignin, or the secondary reaction products. Tar composition and properties show significant difference during different stages of gasification. Evans and Milne et al. defined biomass tar as primary, secondary and tertiary tar[6]. Most of biomass pyrolysis tar are oxygenic tar compounds and belong to primary or secondary tar, such as phenols, furfural etc. The primary and secondary tar compounds, are quite different from traditional fuels. The significant difference is the high oxygen content[7, 8]. During gasification, the secondary and tertiary tar could be formed from primary tar compounds, for example, in partial combustion zone of staged gasifier or in the freeboard zone of fluidized bed and also in the flame during combustion process[9-12]. Brandt et al.[13] investigated the thermal cracking of pyrolysis tar from updrift gasifier and found that it could achieve sufficient tar cleaning at least 1250℃ with 0.5s residence time. Tar content as low as 15mg/Nm3 was obtained, which was clean enough for internal combustor. Brandt et al.[14] investigated partial oxidation of pyrolysis volatiles from straw for tar reduction. They found that a considerable tar reduction could be obtained with an excess air ratio of 0.5, which played better result than pure thermal cracking. The corresponding tar reduction rate is 98% compared to no addition of air at same temperature. Different atmospheres bring in different active radicals[15, 16]. Radicals such as H and CH3 are the dominating activating species under inert atmosphere, while H, O, OH and HO2 are dominating under oxidative atmosphere. Milena Nowakowska et al. [17] took anisole as biomass tar model compound to investigate the decomposition process under inert and oxidative atmosphere. The results show that oxygen


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doesn’t react with tar compounds directly. The addition of oxygen inhibits the formation of PAHs, such as naphthalene, which is due to the competing reaction of cyclopentadienyl attacked by O, OH, HO2 etc. All this work has shown that the heteroatoms, such as oxygen, have great influence on tar decomposition reactivity. However, the conversion rule has seldom been compared between different biomass tar compounds and role of oxygen has rarely been discussed. Meanwhile, there is still lack of detailed kinetic model to predict the conversion of biomass primary tar. Most model focuses on the conversion of fossil compounds and light gas compounds. In this work, the decomposition of typical model biomass tar compounds: 2-methoxyphenol, anisole, furfural and toluene under different atmospheres was investigated. The decomposition properties of the aromatic and non-aromatic, oxygenated and non-oxygenated tar compounds are compared. The effect of intrinsic oxygen of tar and extrinsic oxygen at controlled atmospheres on biomass tar reduction were taken into consideration. At last, a new detailed kinetic model was built to predict biomass tar conversion. This work will contribute to the understanding of biomass tar conversion process and model establishment. 2 Material and experiment section 2.1 Material Four kinds of typical primary and secondary model tar compounds were used in this work. The detailed information of model tar samples are as bellow: 2-Methoxyphenol, CAS: 90-05-1, Vendor: Sinopharm Chemical Reagent Co.,Ltd, purity: 99%; Anisole, CAS: 100-66-3, Vendor: Shanghai MERYER CO.,LTD, purity: 98%; Furfural, CAS: 98-01-1, Vendor: Shanghai MERYER CO.,LTD, purity: 99.5%; Toluene, CAS:108-88-3, Vendor:


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Sinopharm Chemical Reagent Co.,Ltd, purity: 99.5%. 2.2 Apparatus and method An flow tube reactor equipped with a tar injector was used to investigate the tar homogenous decomposition. The apparatus is shown in Figure 1 as bellow.

1 Mass flow controller 2 Check valve 3 Model tar injector 4 Vaporizing chamber 5 Furnace 6 Thermocouple 7 Computer 8 Sampling pipe 9 GC-TCD/FID analyzer 10 GC/MS analyzer

Figure 1. The flow tube reactor system The system is mainly composed of gas flow rate controllers and mixer, model tar injector, tar vaporizing chamber, reactor furnace, tar capture and sampling system. The mass flow controllers (CS200-A(SCCM)) was made by Sevenstar company, which were used to control the gas flow rate. The injector pump was manufactured by Baoding Longer Precision Pump Co. Ltd, which is used to control the model tar flow rate. The injector model is LSP01-1A, with a flow rate range of 0.831μl/min-54.155ml/min. The vaporizing chamber temperature was controlled at 300℃ to ensure the vaporization of liquid tar. The tar vapor and gas mixture in chamber was carried to the reactor. The liquid tar feeding rate was kept at 5μl/min. The carrier gas was argon and the residence time was controlled as 1 second. The reactor tube was made of quartz, with a length of 350mm, inner diameter of 8mm. Isopropanol was used to trap tar, and tar samples were analyzed by GC/MS method. Gas compounds were analyzed by GC-TCD method. 2.3 Analysis methods


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The tar samples were analyzed by Agilent 6890N gas chromatography coupled with 5975C mass spectrometry. The GC split ratio was 100:1 and the injector temperature was set at 280℃. An HP-5MS column (30m ×0.25mm, 0.25um) was used at a flow rate of 2.4 mL/min. The column temperature was increased from 45℃ to 180℃ at 5℃/min, and then to 300℃ at 45℃/min, where it was held for 10 min. The sample injection volume was 1 μL. Major tar compounds, including benzene, toluene, phenol, styrene, naphthalene, catechol, 2-Methoxyphenol, anisole and furfural were calibrated. The noncondensable gas compounds were analyzed by a GC 9560 coupled with a TCD (thermal conductivity detector) manufactured by Huaai Corp. The oven temperature was set at 60 °C, and the carrier gas was argon. A TDX-01 packed column was used, with a length of 2 m and an inner diameter of 2 mm. The temperature for the TCD was set at 150 °C. Gas sample was injected into GC with a volume of 500μl. Four gas compounds, H2, CH4, CO and CO2 were calibrated and analyzed. 3. Results and discussion 3.1 The effect of intrinsic oxygen on tar decomposition 3.1.1 Thermal decomposition of model tar compounds The thermal cracking process of four model tar compounds was conducted. Residence time was controlled as 1 second. The conversion results are shown in Figure 2 as bellow.


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2-methoxyphenol Anisole furfural toluene

100

80

COnversion rate(%)

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

Energy & Fuels

60

40

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0

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Temperature(C)

Figure 2. The conversion rate of four tar compounds with temperature It can be seen that 2-methoxyphenol began to decompose at 300℃ and was completely converted at about 600℃. Anisole began to crack at 400℃ and finished at 700℃. Anisole shows weaker reactivity than 2-methoxyphenol at low temperature. It is recognized that the thermal decomposition of methoxybenzene initiated by cleavage of phO-CH3 bond. This may be due to the effect of phenolic hydroxyl group on 2-methoxyphenol promoted the reactivity of phO-CH3 bond. Furfural is typical cellulose/hemicellulose pyrolysis tar, which started to decompose at 500℃ and finished reaction at 800℃. Furfural is more stable than 2-methoxyphenol and anisole. Toluene is typical non-oxygenated tar compounds, which is most stable among four compounds. The results show that the intrinsic oxygen improve tar decomposition reactivity and phO-CH3 is easy to break.


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Figure 3. The product tar of 2-methoxyphenol thermal decomposition(from left to right: 600-1000℃) The products tar solutions of 2-methoxyphenol thermal decomposition at different temperatures are shown in Figure 3, which illustrated tar maturation process at different temperatures. In order to analyze the detailed compounds of tar products, the GC-MS results of 2-methoxyphenol thermal decomposition are shown in Figure 4 as bellow.

Compound peak area

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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500C 600C 700C 800C 900C 1000C

120000

60000

0 C6H6

C7H8

C8H8 C6H5OH C8H6O C7H6O2 C7H8O C7H8O2 C8H10O C10H8 C6H6O2

Tar compounds

Figure 4. The condensation products of 2-methoxyphenol decomposition From the GC-MS results, it can be indicated that the main compounds produced by 2-methoxyphenol cracking are aromatics including benzene, toluene, phenol, salicylaldehyde, methylphenol, benzofuran naphthalene and catechol. The peak area of each compound just shows its variation tendency with temperature. Most compounds show a tendency of first increase then decrease with temperature increasing. Benzene was produced at 500℃ and reached maximum yield at 900℃.Naphthalene shows similar tendency with benzene but at 700℃ it began to appear. At low temperature(lower than 600℃), phenol, salicylaldehyde and methylphenol are the main products. Edwin Dorrestijn et al.[18] used a mixture of 2-methoxyphenol, chlorobenzene and cumene to study the decomposition of 2-methoxyphenol between 680K and 790K. They found major products were 1,2-dihydroxybenzene(catechol), 2-hydroxybenzaldehyde, o-cresol, methane, and carbon monoxide. Two reaction routes were proposed: homolytic and induced decomposition route. The


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homolytic pathway starts from the cleavage of weak phenoxyl-methyl bond and the intermediate (2-hydroxyphenoxyl radicals) abstract a hydrogen atom from cumene to form catechol. The induced decomposition route initiated from the hydrogen abstraction by cumyl radicals. The further reaction leaded to the formation of 2-hydroxybenzaldehyde and phenol directly. However, Adam Michael Scheer[19] thinks that the phenol wasn’t from the unimolecule reaction of de-formaldehyde, but the more was from bimolecular reaction. In this work, pure 2-methoxyphenol was used and no additional hydrogen could be abstracted, so the yield of catechol was inhibited.

The goal of our work is to provide fundamental research and data for building oxygenated tar decomposition model. The quantitative analysis of some main tar compounds was conducted and the results are shown in Table 1 as bellow. Table 1. Tar quantitative analysis Tar yield（mole %/2-methoxyphenol） Compounds

500℃

600℃

700℃

800℃

900℃

1000℃

Benzene

0.21

0.52

2.89

7.40

11.64

6.88

Toluene

0.12

0.25

0.81

1.23

0.42

0.03

Styrene

0.00

0.00

0.60

0.85

0.19

0.00

Phenol

3.87

6.19

10.70

6.59

0.05

0.00

Naphthalene

0.00

0.00

0.20

1.40

1.89

0.52

Catechol

7.19

8.87

3.81

0.00

0.00

0.00

Benzene and naphthalene yields reached a maximum of 11.64% and 1.89% mole per 2-methoxyphenol at 900℃. Toluene and styrene reached maximum yields of 1.23% and 0.85% at 800℃. Phenol yield reached maximum of 10.70% per 2-methoxyphenol at 700℃, which was highest among these six compounds. Catechol is the product of initial decomposition steps, which reached peak yield of 8.87% at 600℃. This result is lower than Dorrestijn’s work[18]. In their work, a mixture of 2-methoxyphenol, chlorobenzene and cumene (mole ratio=6:3:91) was used as feeding material. The initial decomposition of 2-methoxyphenol produced 2-hydroxyphenoxyl


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radicals, which could abstract hydrogen atom from cumene to form catechol. In this work, pure 2-methoxyphenol was used and no additional hydrogen source was included, thus the formation of catechol was inhibited.

3.1.2 Thermal decomposition of anisole Compared with 2-methoxyphenol, anisole has only one oxygen atom attached to the aromatic ring. As we know, the oxidation of aromatic compounds is by way of formation of phenoxyl and then taking off CO to destroy aromatic ring. The structure difference may bring different reaction property. The GC-MS results of tar products of anisole thermal decomposition under different temperature are shown in Figure 5 as bellow. 400°C 500°C 600°C 700°C 800°C 900°C 1000°C

300000

Compounds peak area

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200000

100000

0 C6H6

C7H8 C7H8O C7H6O C6H6O C8H6O C9H8 C7H8O C7H8O C10H8 C12H10 C12H8

Tar compounds

Figure 5. The condensation products of anisole decomposition The aromatic products of anisole thermal decomposition include benzene, toluene, benzaldehyde, phenol, benzofuran, indene, methylphenol, naphthalene, biphenyl and biphenylene, etc. Benzene is a major product, with a peak yield at 900℃. Naphthalene didn’t appear until 700℃ and reached a peak yield at 900℃. Phenol is another important product, with a peak yield at 700℃ and while temperature was higher than 900℃, nearly no phenol was detected. Benzofuran showed similar tendency with phenol. Toluene was detected first at 600℃ and reached peak yield at


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800℃. Milena Nowakowska et al.[17] investigated the pyrolysis and oxidation of anisole in a Perfect stirred reactor and found similar phenomenon. It’s worth noting that at high temperature, only benzene and PAHs were retained. Calibration of major aromatic products was conducted and the results are shown in Figure 6 as bellow. 60

Yield (% anisole)

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

Energy & Fuels

400C 500C 600C 700C 800C 900C 1000C

Four tar yield

40

20

0 C6H6

C7H8

C6H5OH

C10H8

Tar compounds

Figure 6. Quantitative yield of four tar compounds Benzene yield reached maximum of 31.9% mole of anisole at 900℃. Milena Nowakowska et al.[17] reported the benzene peak yield of about 28% mole of anisole at 900℃. Phenol has a peak yield of 61.5% at 700℃, compared with about 50% of Nowakowska’s work. Naphthalene reached peak yield of 7.6% at 900℃, compared with about 10% of Nowakowska’s work. During thermal decomposition of biomass primary and secondary tar at high temperature, tertiary tar compounds, especially PAHs are major products. Naphthalene is typical PAHs compound and is the main product after partial oxidation[9, 10]. The comparison of naphthalene yield between 2-methoxyphenol and anisole thermal decomposition is shown in Figure 7 as bellow.


Energy & Fuels

2-methoxyphenol Anisole

Naphthalene yield (mole %)

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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6

3

0 700

800

900

1000

Temperature(C)

Figure 7. Naphthalene yield of two model tar compounds decomposition It can be indicated that the naphthalene yield of 2-methoxyphenol is obviously lower than anisole under all temperature. At 1000℃, naphthalene yield of 2-methoxyphenol is 0.52% mole, while 7.32% for anisole. This result indicated that the additional phenolic hydroxyl promoted the cracking of aromatic and inhibited the polymerization to form PAHs. Ledesma et al.[20] investigated the thermal decomposition of catechol and found the major decomposition products were CO, acetylene, 1, 3-butadiene, phenol, cyclopentadiene and benzene. CO is the highest yield products and nearly all oxygen atoms were transferred into CO. Catechol could decompose into small species by two steps: the first step is decarbonylation to produce cyclopentadienonyl. The second step is decarbonylation of cyclopentadienonyl to form 1,3-butadiene. Phenol is not able to form cyclopentadienonyl and no 1,3-butadiene was produced in phenol decomposition. It can be indicated that the dual-phenoxyl of 2-methoxyphenol could inhibit the formation of cyclopentadienyl and further inhibit the yield of naphthalene.

3.2 The effect of extrinsic oxygen on tar decomposition


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Energy & Fuels

As illustrated above, intrinsic oxygen could improve the decomposition reactivity and inhibit the formation of PAHs. However, with respect to 2-methoxyphenol, there is still quite lot of PAHs yield during thermal decomposition. Cyclopentadienonyl is a crucial intermediate in aromatic compounds decomposition into small species. Therefore, additional active radicals should be provided to control the reaction path to benefit the formation of cyclopentadienonyl. The addition of oxygen for partial oxidation of tar is a choice. In this section, the effect of extrinsic oxygen on tar homogenous conversion is investigated. The products tar solutions of 2-methoxyphenol partial oxidation at different temperatures are shown in Figure 8. Equivalence ratio(ER) is defined as the ratio of actual oxygen amount to oxygen amount for complete combustion.

(a) 800℃ (b) 900℃ (c) 1000℃ Figure 8. Tar yield of 2-methoxyphenol partial oxidation under different ER and temperature (from left to right: ER=0, 0.1, 0.2, 0.5) The tar maturation process could be illustrated in Figure 8. It can be indicated that tar yield decreased with ER increase at all temperatures. The GC/MS results of tar solution after partial oxidation are shown in Figure 9 as bellow.


Energy & Fuels

Thermal crack ER=0.1 ER=0.2 ER=0.5 ER=1

Compound peak area

80000

60000

40000

100000

20000

60000

40000

20000

0

0

Compound peak area

Compound peak area


8 H 10 C e yd eh ld yla lic sa

0

an ur of nz be

40000

ol en ph

80000

90000

e en yr

120000


st


e en lu to

e en nz be

l ho ec at C 8 H 10 C O 10 8H C 2 8O 7H C 8O 7H C 2 6O 7H C an ur of nz be H

8

5O 6H C e en yr st

7H

C 6

6H

160000



80000

Compound peak area


C

Thermal cracking ER=0.1 ER=0.2 ER=0.5

60000

30000

0

ne le ha

t ph na

e en lu to

e en nz be

e en al th ph na de hy de al yl lic sa an ur

of nz be

ol en ph

ne re sty

e en lu to

e en nz be

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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Figure 9. Tar yields of 2-methoxyphenol partial oxidation Corresponding to Figure 9, almost all tar compounds produced during 2-methoxyphenol decomposition decreased with ER. It can be noticed that at 700℃, both oxygenated and PAHs tar compounds are formed. Oxygen lowers the yields of most compounds but didn’t eliminate them completely, even though at ER=1 case. At 800℃, oxygen shows more significant effect on tar conversion and when ER is up to 0.5, most tar compounds except for benzene, were eliminated completely. At 900℃, benzene, toluene and naphthalene are three major products. Naphthalene was greatly reduced with little oxygen addition and when ER is equal to 1, all tar compounds, including benzene were eliminated. At 1000℃, benzene and naphthalene are two major tar products. When ER>0.1, almost all naphthalene was eliminated. Benzene was converted gradually with oxygen concentration. Extrinsic oxygen could eliminate most tar compounds.


Page 29 of 37

120000

80000

ER=0 ER=0.1 ER=0.2 ER=0.5 ER=1


Compound peak area

100000

Compound peak area

150000

ER=0 ER=0.1 ER=0.2 ER=0.5 ER=1


60000

40000

90000

60000

30000

20000

0

0

ER=0 ER=0.1 ER=0.2 ER=0.5


Compound peak area

8O

8

H 12

C

10

H 12

C

8

H 12

C

8

H 10

C

6O

9H

C

8H

C

150000

8

8H

C 8

6

7H

C

6H

C

8O

8

H 12

H 12

C

C

O

H

10

8

H 12

C

8O

H 10

C

8O

7H

C

8

7H

C

6O

9H

C

8H

C

H

5O

5C

6H

C

8

6H

C

8

8H

C

6

7H

6H

C

C

90000

60000

30000

0 --

--

--

8 H 12

C

10 H 12

C

8

8 H 10

C

6

7H

C

6H

C

Figure 10. Tar yields of anisole partial oxidation The tar yields of anisole partial oxidation are shown in Figure 10. Similar tendency occurred to anisole with the addition of oxygen. It should be noticed that at 1000℃,the naphthalene yield with ER of anisole is quite different with that of 2-methoxyphenol. As mentioned above, a small amount of oxygen(ER=0.1) could achieve great elimination of naphthalene in 2-methoxyphenol, while for anisole, the addition of oxygen could not completely reduce naphthalene at small ER. 800

H2 CO CH4 CO2

600

2-methoxyphenol-900C

H2 CO CH4 CO2

300

Gas yield (mole % anisole)

Gas yield(%/ 2-methoxyphenol)

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

Energy & Fuels

400

200

0

Anisole-900C

200

100

0

ER=0

ER=0.1

ER=0.2

ER=0.5

ER=1

ER=0

ER=0.1

Oxygen amount

ER=0.2

ER=0.5

ER=1

Oxygen amount

Figure 11. The gas yields of 2-methoxyphenol and anisole partial oxidation at 900℃ The gas yields of 2-methoxyphenol and anisole partial oxidation under different oxygen


Energy & Fuels

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

atmospheres at 900℃ are shown in Figure 11. Similar tendency of gas yield occurred to these two model tar compounds. H2 yield slightly increased at ER=0.1 and then decreased at larger ER due to the H2 combustion. But even at ER=1 case, there is quite of H2 not be consumed. The yield of methane is the least of four compounds and with the increasing of oxygen, it decreased and even was eliminated completely at ER=1. Both methane and aromatic tars belong to hydrocarbons and are easy to be reduced by partial oxidation. CO shows the maximum concentration under fuel-rich conditions and reached maximum yield at ER=0.5, of 740% mole per 2-methoxyphenol and 317% mole per anisole separately. CO2 yield just slightly increased at ER

Experimental and Modeling Investigation on the Effect of Intrinsic and

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