Polycyclic Aromatic Hydrocarbon Formation from the Pyrolysis

Sep 17, 2014 - Energy Research Institute, University of Leeds, Leeds LS2 9JT, United ... [email protected]., *E-mail: [email protected]...
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Polycyclic Aromatic Hydrocarbon Formation from the Pyrolysis/ Gasification of Lignin at Different Reaction Conditions Hui Zhou,†,‡ Chunfei Wu,*,‡ Jude A. Onwudili,‡ Aihong Meng,† Yanguo Zhang,*,† and Paul T. Williams*,‡ †

Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, People’s Republic of China ‡ Energy Research Institute, University of Leeds, Leeds LS2 9JT, United Kingdom ABSTRACT: The effect of reaction conditions, temperature, heating rate, and reaction atmosphere on the formation of polycyclic aromatic hydrocarbons (PAHs) from lignin was studied in a fixed-bed reactor. When the temperature was increased from 500 to 900 °C, most PAHs increased with temperature, except 1-methynaphthalene and 2-methynaphthalene, which decreased slightly when the temperature was increased from 800 to 900 °C. With the increase of the temperature, the percentage of 2-ring PAHs decreased and the percentage of 3- and 4-ring PAHs increased. The increase in the total PAH with the temperature could be fitted by a quadratic function. The PAH generation from slow pyrolysis of lignin was much lower than that from fast pyrolysis. In comparison of the PAH generation in different reaction atmospheres, experiments in N2 produced the most PAHs, followed by the reaction in air and CO2. During the pyrolysis/gasification of lignin, it is suggested that there were two kinds of secondary reactions, dehydroxylation and demethoxylation, and they might occur at the same time. Then, PAHs could be formed from secondary reactions of derivatives of benzene, which increased with the increase of the temperature. Slow pyrolysis generated less PAHs because of the limitation of secondary reactions. With the addition of air or CO2, derivatives of benzene and phenol could be oxidized; thus, less PAHs were generated. effect.15 The United States Environmental Protection Agency (U.S. EPA) listed 16 kinds of PAHs as priority pollutants, i.e., naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, dibenzo[a,h]anthracene, benzo[g,h,i]perylene, and indeno[1,2,3-cd]pyrene.16 It was estimated that the total global atmospheric emission of these 16 PAHs in 2004 was 520 Gg/ year, with biofuel (56.7%), wildfire (17.0%), and consumer product usage (6.9%) as the major sources.17 It has been reported that pyrolysis of lignin produced more kinds and a larger amount of PAHs than the pyrolysis of cellulose and hemicellulose.18,19 Our previous research showed that, during the pyrolysis process, lignin produced naphthalene, methylnaphthalene, acenaphthylene, fluorene, phenanthrene, pyrene, and fluoranthene, while cellulose and xylan only produced naphthalene, methylnaphthalene, and acenaphthylene.18 During the process of pyrolysis/gasification of lignocellulosic biomass, the reaction conditions have been reported to have an influence on PAH formation. For example, Font et al.20 studied the decomposition of kraft lignin in an air atmosphere in a horizontal laboratory furnace; naphthalene, acenaphthylene, phenanthrene, fluoranthene, and pyrene were identified at 800−1100 °C; in addition, the PAH formation was increased with the increase of the reaction temperature in the air atmosphere, probably as a consequence of the pyrosynthesis

1. INTRODUCTION Renewable energy is of growing importance in satisfying environmental concerns over fossil fuel usage.1 Biomass, including wood, energy crops, and agricultural waste, is one of the most promising sources of renewable energy. Thermal conversion processes, which include incineration, pyrolysis, and gasification, are considered as an important pathway of the utilization of biomass. The products of pyrolysis and gasification (oil, char, and syngas) allow for a broad range of applications, e.g., electricity and heat production from combustion of syngas and chemicals and transportation fuels extracted from liquid pyrolysis oils.2,3 Therefore, the pyrolysis and gasification of biomass are generating increasing interest.4 Lignin is one of the three main components of biomass, together with cellulose and hemicellulose.5 Lignin is primarily a structural material to add strength and rigidity to cell walls and constitutes between 15 and 40 wt % of the dry matter of woody plants.6,7 In addition, lignin is an important waste of the pulping process of the paper industry.8 Therefore, there is much work concentrated on lignin pyrolysis and gasification. Lignin is an aromatic polymer consisting of phenylpropane units linked through ether and condensed types of linkages.9 There are three main monomers in the lignin structure, i.e., p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol.10,11 One of the challenges for the development of thermal processing of lignocellulosic biomass is the production of polycyclic aromatic hydrocarbons (PAHs).12 PAHs are a group of semivolatile, persistent, organic chemicals that are ubiquitous in the environment.13 Many PAHs have toxic, mutagenic, and carcinogenic properties.14 They are lipid-soluble and can be easily absorbed by mammalian bodies with a bioaccumulative © 2014 American Chemical Society

Received: June 20, 2014 Revised: September 12, 2014 Published: September 17, 2014 6371

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Table 1. Proximate and Ultimate Analyses of Lignin proximate analysisa (wt %)

a

ultimate analysisb (wt %)

sample

ash

volatile

fixed carbon

C

H

O

N

S

HHVa (MJ/kg)

lignin

24.38

56.44

19.17

53.53

5.47

33.47

0.08

7.46

15.69

Dry basis. bDry and ash-free basis.

reactions.20 Dieguez-Alonso et al.21 have reported that 2-ring aromatic compounds were the main PAHs present in the pyrolysis vapor of pine wood and beech wood in an inert atmosphere using a fixed-bed reactor, while an increase of 3and 4-ring aromatic compounds was observed with an increase of the retention time of pyrolysis vapors.21 Thomas et al.22 performed pyrolysis and fuel-rich oxidation experiments in a laminar-flow reactor, using the model fuel catechol of lignin. At temperatures of >850 °C, catechol conversion was complete and the increases in oxygen brought about marked decreases in the yields of virtually all hydrocarbon products, because destruction reactions dominated.22 The formation of PAHs from lignin is probably a multi-step process. Lignin is converted first into primary products, such as tar and char, and then PAHs are formed through secondary reactions.23,24 Research by Asmadi et al.25,26 showed that the thermal decomposition of lignin began by cleavage of the weak α−ether and β−ether bonds of the lignin structure, which released guaiacyl- and syringyl-type aromatics. These aromatics further reacted to produce catechols and pyrogallols, which could decompose to form PAHs.25,26 Dorrestijn et al.10 have also reported that, between 200 and 400 °C, linkages between the lignin units were cleaved (α−O−4 ether bond was the weakest). At higher temperatures, secondary decomposition of pyrolysis vapors took place and resulted in, for example, the conversion of guaiacols into catechols. By combination of carbon-centered radicals, strong carbon−carbon bonds were formed to generate tar compounds.10 At present, there are few reports relating a systematic study of PAH formation during lignin pyrolysis/gasification under different reaction conditions. In this work, several process conditions, such as temperature, heating rate, and reaction atmosphere, were investigated using a fixed-bed reactor. The formation of PAHs during lignin pyrolysis/gasification was analyzed using gas chromatography/mass spectrometry (GC/ MS) and is presented and discussed; in addition, gas and hydrogen yields and molecular mass distribution of the pyrolysis oil are also presented.

Figure 1. Schematic diagram of the pyrolysis reaction system. gasification conditions, because of the mass (1 g) of the sample introduced into a small hot reaction zone. The flow rate of inlet gas (N2, air, or CO2) was 100 mL min−1, and the residence time of volatiles was about 2.6 s. For the fast pyrolysis/gasification, the reactor was initially heated to the set point. Once the temperature had stabilized, the lignin sample (1 g) was inserted into the hot zone of the reactor and rapidly reacted. For the slow pyrolysis, the lignin sample was placed in the reactor at room temperature and then the reactor was heated at a constant heating rate (10 °C min−1). The reactor was kept at the final reaction temperature for a further 30 min. The products from the pyrolysis/gasification experiments were cooled using air- and dry-ice-cooled condensers, thereby collecting the condensed tar. The non-condensed gases were collected using a Tedlar gas sample bag and further analyzed off-line using packed column gas chromatography (GC). An additional 20 min was allowed to collect the non-condensed gases to ensure complete collection of gas products. All of the experiments were repeated to ensure the reliability of the results. 2.3. Product Analysis and Characterization. 2.3.1. GC. Noncondensed gases collected in the Tedlar gas sample bag were analyzed off-line by packed column GC. H2, CO, and N2 were analyzed with a Varian 3380 gas chromatograph on a 60−80-mesh molecular sieve column with argon carrier gas, while CO2 was analyzed by another Varian 3380 gas chromatograph on a Hysep 80−100-mesh column with argon carrier gas. C1−C4 hydrocarbons were analyzed using a third Varian 3380 gas chromatograph with a flame ionization detector, with an 80−100-mesh Hysep column and nitrogen carrier gas. From the known N2 mass, calculated from the flow rate and collection time, the volume and mass of other gases can be calculated. 2.3.2. Characterization of PAHs. After each experiment, the condenser and tar connecting sections were weighed to obtain the mass of tar (weight difference of the condensation system). The tar products from the condensers were washed by ethyl acetate. The water content in the liquid mixture was eliminated by filtering through a column of anhydrous sodium sulfate. Finally, the liquid, which contained PAHs, were analyzed using a Varian CP-3800 gas chromatograph coupled with a Varian Saturn 2200 mass spectrometer fitted with a 30 m × 0.25 μm DB-5 equivalent column. A total of 2 μL of the extracted tar sample was injected into the gas chromatograph injector port at a temperature of 290 °C; the oven program temperature was at 50 °C for 6 min, then ramped to 210 °C at 5 °C min−1, held for 1 min, and ramped at 8 °C min−1 to 300 °C (total analysis time of 61 min). The transfer line was at 280 °C, with the manifold at 120 °C, and the ion-trap temperature was held at 220 °C. The ion trap was initially switched off for 7 min to allow for the elution of the solvent prior to data acquisition to safeguard the life of the trap. The PAH compounds present in the tars were quantified by an internal standard method with 2-hydoxyacetophenone as the internal standard (IS). GC/MS was calibrated by standard PAHs supplied by Sigma-Aldrich, Ltd.; thus, PAHs could be quantitatively determined.

2. EXPERIMENTAL SECTION 2.1. Materials. The lignin (dealkaline) used in this research was obtained from Tokyo Chemical Industry Co., Ltd. The sample was in granular form with a size of less than 150 μm. Before the experiments, lignin was dried at 105 °C to obtain a dry basis sample. The results of the proximate and ultimate analyses of lignin are shown in Table 1. As shown in Table 1, lignin has considerable ash (24.38 wt %) and fixed carbon (19.17 wt %) contents. The sulfur content of lignin is also quite high, which has also been reported by Yu et al.19 The higher heating value (HHV) of lignin is slightly lower than that of biomass.5 2.2. Pyrolysis/Gasification Process. Pyrolysis/gasification of lignin was carried out with a fixed-bed reactor system (Figure 1). The reaction system was composed of a reactor, tar collection system, and gas collection stages. The pyrolysis reactor tube was 40 cm long and 1 cm diameter, constructed of stainless steel. For the pyrolysis experiments, the carrier gas was N2; for the gasification experiments, the inlet gas was air or CO2. For the case of air, the atmosphere inside the reactor would consist of sub-stoichiometric, partial combustion/ 6372

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The analysis reported 10 of the 2−4-ring PAHs from U.S. EPA priority list and also 2 naphthalene derivatives (1-methylnaphthalene and 2methylnaphthalene). The GC peaks of benzo[a]anthracene and chrysene could not be separated clearly by the GC system used; therefore, the concentration of these two compounds was reported together. 2.3.3. Size-Exclusion Chromatography (SEC). SEC is a nondestructive technique, in which the largest molecules are eluted first.27 The equipment used to obtain information on the molecular mass distribution was a PerkinElmer Modular, Series 225. A solution of tetrahydrofuran (THF) was used as the mobile phase during the analysis. The samples were prepared by dissolving a portion of the tar in THF in an approximate concentration of 0.2%. The column used was a Polymer Laboratories 5 μm SEC column and was calibrated using polystyrene samples of low polydispersity in the molecular weight range of 800−860 000 and also included single-ring and PAH standard samples for low-molecular-mass calibration. The detector used was a PerkinElmer Series 200a refractive index detector.

3.1.2. Gas Analysis in Relation to the Temperature. The gas concentrations from the pyrolysis of lignin at different temperatures are shown in Figure 3. With the increase of the

3. RESULTS AND DISCUSSION 3.1. Influence of the Temperature on Pyrolysis of Lignin under a N2 Atmosphere. 3.1.1. Mass Balance in Relation to the Temperature. The mass balance for the fast pyrolysis of lignin in a N2 atmosphere from 500 to 900 °C is shown in Figure 2. The mass balance was calculated as the mass

Figure 3. Gas release from the fast pyrolysis of lignin in N2 at different temperatures.

temperature from 500 to 900 °C, H2 production increased greatly. CO production also increased significantly with the increase of the temperature, and the CO2 yield increased slightly with the increase of the temperature. Because of the presence of −CH3 in the lignin structure,30 lignin pyrolysis produced a certain amount of CH4, which also increased with the increase of the reaction temperature. It has been reported that CH4 formation is probably related to the demethylation reaction.9 The amount of C2−C4 hydrocarbons was quite low and increased slightly with the increase of the pyrolysis temperature. The increased yield of the main gas components (Figure 3) is consistent with the increase of the total gas yield (Figure 2), when the pyrolysis temperature was increased from 500 to 900 °C. 3.1.3. Molecular Mass Distribution of Tar in Relation to the Temperature. The molecular mass distribution of tar was analyzed by SEC. The SEC chromatograms are shown as a linear function of the molecular mass (Figure 4). As shown in Figure 4, the molecular mass distribution from 100 to 250 g mol−1 is reported. The molecular mass was mainly distributed in the range of 120−180 g mol−1, with the peak at 140−160 g mol−1. This peak might be due to naphthalene, which is very common in the tar derived from biomass.25 The molecular mass of the derived tar increased with the increase of the temperature. In particular, when the temperature was increased from 800 to 900 °C, the fraction of lower molecular mass decreased and the fraction of higher molecular mass increased considerably. The increased fraction of compounds with a higher molecular mass might be due to enhanced secondary reactions at higher reaction temperatures.24 3.1.4. PAH Analysis in Tar Products in Relation to the Temperature. The PAHs present in the tar derived from the pyrolysis process were analyzed quantitatively by GC/MS. As shown in Figure 5a, the naphthalene yield increased with the increase of the temperature, particularly from 700 to 900 °C. When the temperature was increased from 500 to 800 °C, the yields of 1-methynaphthalene and 2-methynaphthalene increased and then slightly decreased with the further increase of

Figure 2. Mass balance for the fast pyrolysis of lignin in N2 at different temperatures.

of outputs (tar, gas, and residue) divided by the mass of inputs (lignin sample). The gas yield was obtained by the mass of noncondensed gases (calculated from the GC analysis) divided by the sample mass (1 g). The residue fraction was calculated by the mass of residue after pyrolysis in the reactor divided by the sample mass. The tar yield was calculated as the mass of the collected tar from the condensers divided by the sample mass. As shown in Figure 2, the gas yield increased with increasing the temperature from 500 to 900 °C. In contrast, the tar and residue yields decreased with the increase of the temperature. From our previous research of thermogravimetric analysis of lignin, lignin had a mass loss from 200 to 1000 °C,5 which was consistent with the residue results of this research. The reduction of tar has also been reported in the range of 700−850 °C in a circulating fluidized-bed gasifier.28 The phenomena can be suggested to be due to the following reasons: when the temperature increased, the secondary reaction (i.e., tar cracking) prevailed, which led to tar decomposition.29 6373

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threne, and anthracene increased considerably. The same scenario was found for the 4-ring PAHs, because fluoranthene, pyrene, benzo[a]anthracene, and chrysene could only be detected when the pyrolysis temperature was higher than 700 °C. Figure 5d shows the total 2−4-ring PAH generated from the pyrolysis of lignin at different temperatures. The total PAHs increased with the increase of the temperature, and the trend could be fitted by a quadratic function, which suggests that, at higher temperatures, the amount of PAHs increased more significantly. Similar results have also been reported by Sharma and Hajaligol.24 They studied the pyrolysis of lignin in a twozone reactor. Zone 1 was constant at a temperature of 600 °C, and when the temperature of zone 2 was increased from 700 to 920 °C, the yields of most PAHs increased. Ledesma et al.31 pyrolyzed catechol (a model compound of lignin) using a tubular-flow reactor. When the reactor temperature was increased from 500 to 900 °C, the yield of PAHs increased significantly.31 The change of PAH percentage formed in the tar in terms of the number of aromatic rings is shown in Figure 6. With the increase of the temperature, the percentage of 2-ring PAHs decreased, and in contrast, the percentage of 3- and 4-ring PAHs increased. The results were consistent with those shown in Figure 4.

Figure 4. Size-exclusion chromatogram of the fast pyrolysis tar in N2 at different temperatures.

the temperature from 800 to 900 °C, perhaps because of demethylation reactions. The 3-ring PAHs could hardly be detected when the reaction temperature was below 700 °C. Nevertheless, when the temperature was increased from 700 to 900 °C, the amounts of acenaphthylene, fluorene, phenan-

Figure 5. PAH formation from the fast pyrolysis of lignin in N2 at different temperatures. 6374

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ratio in an entrained-flow reactor.33 The tar yield might reach a maximum with an increase of the air flow;34 the participation of O2 might increase the tar yield because of the formation of oxygenated organic compounds; while these compounds might be further oxidized to CO2 or CO with the further increase of air. It should also be noted the conditions of the reaction system used 1 g of lignin in a small volume of reactor (1 cm diameter tube), creating sub-stoichiometric conditions. Particularly, gasification in CO2 released the highest amount of gases, which might be due to the strong reaction between CO2 and char. 3.2.2. Gas Analysis with Different Heating Rates and Reaction Atmospheres. The gas production at different reaction conditions is shown in Figure 8. Slow pyrolysis of

Figure 6. Percentage of PAHs with a different number of rings.

3.2. Influence of the Heating Rate and Reaction Atmosphere. To investigate the influence of the heating rate on PAH formation, the results of fast and slow pyrolysis (10 °C min−1) were compared. In addition, experiments with different reaction atmospheres, including N2, air, and CO2, were also compared in terms of the product yield and PAH formation. The temperature of the experiments was 800 °C, and the flow rate was kept at 100 mL min−1. 3.2.1. Mass Distribution with Different Heating Rates and Reaction Atmospheres. The mass distribution obtained from experiments at different reaction conditions is shown in Figure 7. Because the gasification agent (air or CO2) participated in Figure 8. Gas release from lignin at different heating rates and atmospheres at 800 °C.

lignin under N2 generated less CO, CO2, and CH4 compared to fast pyrolysis. Gasification in air produced less H2 and more CO 2 and CO compared to pyrolysis in N 2 . The CO 2 production from fast CO2 gasification of lignin is not shown in Figure 8. Gasification in CO2 produced less H2 and far more CO compared to pyrolysis in N2. The large amount of CO was derived from the reaction of CO2 and char.35 Produced CO contributed to the highest gas yield of lignin gasification with CO2 (Figure 7). 3.2.3. Molecular Mass Distribution of Tar with Different Heating Rates and Reaction Atmospheres. The size-exclusion chromatogram, which provided the molecular mass distribution of the tar, is shown in Figure 9. It seems that the mass fraction of compounds with molecular mass between 140 and 160 g mol−1 is much higher for the slow pyrolysis (21 wt %) compared to the fast pyrolysis of lignin under the N2 atmosphere (13 wt %), indicating that the slow pyrolysis of lignin generated smaller molecules. This indicated that the mass percentage of 2-ring PAH (with molecular mass between 140 and 160 g mol−1) from the slow pyrolysis of lignin might be higher than that from fast pyrolysis. The tar produced from the gasification of lignin in air also had a slightly lower molecular mass than that in N2. The gasification in CO2 had the lowest molecular mass compared to that in N2 and air. 3.2.4. PAHs in Tar Products with Different Heating Rates and Reaction Atmospheres. The PAHs produced from the pyrolysis/gasification of lignin at different heating rates and in relation to the reaction atmosphere are shown in Figure 10. The results of 2-, 3-, and 4-ring PAHs are shown in panels a, b,

Figure 7. Mass distribution of lignin at different heating rates and atmospheres at 800 °C.

the reactions, more than 100 wt % gas yield was obtained for both air and CO2 gasification. In comparison to fast pyrolysis, slow pyrolysis generated more tar and less gas yield, while the residue yield was similar. In comparison to fast pyrolysis in N2, gasification of lignin under air and CO2 generated lower yields of residue, because of the reaction of char with air and CO2. Accordingly, gasification in air or CO2 generated more gas and tar compared to that in N2. The influence of an excess air ratio on the tar yield was complicated. As reported by Gordillo and Annamalai, at 800 °C, with the increase of air, the tar yield was increased,32 while Yu et al.33 have reported different results, where the tar yield decreased with the increase of the excess air 6375

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generation in different reaction atmospheres, the presence of oxidant, e.g., air or CO2, significantly reduced the yield of 2-ring PAHs. In comparison to air gasification, CO2 gasification produced relatively less 2-ring PAHs. In terms of the formation of 3-ring PAHs, as shown in Figure10b, the slow pyrolysis of lignin under a N2 atmosphere generated the lowest amount. In addition, there was a considerable decrease of acenaphthylene, acenaphthene, and fluorene from gasification of the lignin in air and CO2 compared to that in a N2 atmosphere at a similar heating rate, while the amounts of phenanthrene (∼45 μg g−1 of lignin) and anthracene (∼7 μg g−1 of lignin) were similar for fast pyrolysis/gasification under different atmospheres. The 4-ring PAHs were barely detected in the slow pyrolysis of lignin in N2 or for the fast gasification of lignin in CO2. Only a certain amount of pyrene (6 μg g−1 of lignin) was identified from fast gasification in air, as shown in Figure 10c. The results of experiments in N2 and air were consistent with reports from Font et al.,20 where pyrolysis/gasification of lignin was carried out at 850 °C, and they reported that a lower PAH yield was obtained in air compared to a N2 atmosphere. The percentage of 2-, 3-, and 4-ring PAHs in terms of the total weight of detected PAHs (PAH percentage) in the tar from each experiment is shown in Figure 10d. In comparison to fast pyrolysis, slow pyrolysis generated a higher percentage of 2ring PAHs and a lower percentage of 3- and 4-ring PAHs,

Figure 9. Size-exclusion chromatogram of the tar from pyrolysis/ gasification at different heating rates and atmospheres at 800 °C.

and c of Figure 10, respectively. It seems that much lower 2ring PAHs were obtained from the slow pyrolysis process compared to the fast pyrolysis conditions. For example, the naphthalene yield reduced from approximately 144 μg g−1 of lignin to less than 5 μg g−1 of lignin when the process was changed from fast to slow pyrolysis. In comparison of PAH

Figure 10. PAH formation from the pyrolysis of lignin at different heating rates and atmospheres at 800 °C (a, 2-ring PAHs; b, 3-ring PAHs; c, 4ring PAHs; and d, PAH percentage of different rings). 6376

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Figure 11. Identification of the derivatives of benzene and phenol from the pyrolysis of lignin at different temperatures.

Figure 12. Mechanism of PAHs from pyrolysis/gasification of lignin.

anisole were generated because of demethylation and demethoxylation reactions. When the temperature was increased to 700 °C, guaiacol and anisole could not be found, which indicated that demethoxylation reactions were complete. At 900 °C, xylenol and cresol could not be identified, which might be due to dehydroxylation reactions. The results agree with the report by Asmadi et al.,26 where syringols and guaiacol decreased when the reaction temperature was increased from 400 to 600 °C. In their work, catechols and pyrogallols were not found, perhaps because they are quite reactive.26 According to Figure 11, there were three kinds of secondary reactions, dehydroxylation, demethoxylation (including OCH3 homolysis), and demethylation, and these secondary reactions might occur at the same time. The mechanisms of PAH formation from pyrolysis/gasification of lignin are summarized in Figure 12, while the process of demethylation was not expressed to simplify the discussion of the mechanism. Monomers (such as guaiacols and syringols with alkyls) were generated initially from the lignin pyrolysis. Then, through the process of dehydroxylation and demethoxylation, derivatives of benzenes were formed and the process of demethylation occurred simultaneously. As shown in Figure 11, the methoxy group could only be identified when the temperature was lower than 700 °C, indicating that demethoxylation of benzene derivatives was complete below 700 °C. More hydroxyl and methoxy compounds were found at lower temperatures, while more benzenes were found at higher temperatures. With the increase of the temperature, the secondary reactions (dehydroxylation, demethoxylation, and demethylation) would be promoted. It was suggested that PAHs could be formed from

which is consistent with the mass distribution using SEC analysis (Figure 9). 3.3. Discussion of PAH Formation from Pyrolysis/ Gasification of Lignin. In our research, abundant phenol, ocresol, p-cresol, m-cresol, guaiacol, 3,5-xylenol, 3,4-xylenol, 2,5xylenol, and 3-ethyl-5-methylphenol were detected in the product tar from the pyrolysis/gasification of lignin. These hydrocarbon compounds are produced from the thermal decomposition of lignin through cleavage of weak α−ether and β−ether bonds.25,26 In addition, Asmadi et al. also proposed that some of these compounds, e.g., cresols and xylenols, are the precursors of PAHs.25,26 As a result of the limitations placed on the PAH analysis methodology because of the presence of the ethyl acetate solvent used in the analytical procedure, C5−C6 hydrocarbons (eluted in the first few minutes of the analysis) were not analyzed using the GC/MS system to protect the GC/MS detector. However, the level of C5−C6 hydrocarbons can be estimated; for example, Font et al.20 reported that the amount of benzene was 580 μg g−1 of lignin at 850 °C pyrolysis. The derivatives of benzene and phenol in the tar produced from lignin pyrolysis at different temperatures are shown in Figure 11. Phenol, a stable hydrocarbon,9 was present in the tar at all pyrolysis temperatures. It has been reported that phenol formation was enhanced with the increase of the temperature from 400 to 800 °C using pyrolysis−gas chromatography/mass spectrometry (Py−GC/MS).36 At 500 °C, ethyguaiacol, dimethoxytoluene, methylguaiacol, and xylene were identified. When the temperature was increased to 600 °C, these four compounds disappeared, while alkylbenzenes, guaiacol, and 6377

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Figure 13. Formation of naphthalene from benzene.

the hydrogen abstraction acetylene addition (HACA) process from benzenes, as shown in Figure 13,37,38 which would also be promoted at higher temperatures.39 Therefore, with the increase of the temperature, PAH formation was enhanced (Figure 5). In addition, methylnaphthalenes were reduced at 900 °C because of demethylation reactions. Pyrolysis of lignin did not generate many gases, which indicated that PAHs were probably not formed from the condensation of alkenes. For the slow pyrolysis of lignin, monomers of lignin (guaiacols and syringols) were generated at a very low temperature (300−350 °C).25 In comparison to fast pyrolysis, slow pyrolysis generated more tar and less gas (Figure 7), because the secondary reactions were limited for a larger proportion of tar released at lower temperatures in the case of the slow heating rate. A higher temperature is required for dehydroxylation/demethoxylation of these compounds to generate PAH precursors. Therefore, during slow pyrolysis under a N2 atmosphere, a much lower yield of PAHs was obtained in comparison to the fast pyrolysis under N2. This is supported by the analysis of tar collected from the slow pyrolysis experiment, where abundant methyguaiacol, dimethoxytoluene, ethylguaiacol, and syringol were identified, while the amount of derivatives of benzene (PAH precursors) and PAHs was low. In the presence of oxidants, e.g., air or CO2, the PAH precursors shown in Figure 12 could be oxidized; thus, less PAHs were generated (Figure 10). Additionally, few derivatives of phenol and benzene (PAH precursors) could be detected in the tar derived from lignin gasification in the presence of air or CO2.

increasing the temperature could be fitted by a quadratic function. In comparison to fast pyrolysis, slow pyrolysis generated more tar and less gas, and gasification in air or CO2 generated less residue and more gas and tar. The PAH generation from slow pyrolysis of lignin was much lower than that from fast pyrolysis. In comparison of the PAH generation in relation to the reaction atmosphere, experiments in N2 produced the most PAHs, followed by PAHs produced in air and CO2. Large amounts of derivatives of benzene and phenol were detected in the tar derived from lignin pyrolysis. PAHs could be formed from secondary reactions of derivatives of benzene, which could be enhanced with the increase of the pyrolysis temperature. The slow pyrolysis in this work generated less PAHs than for fast pyrolysis because of the limitation of secondary reactions. With the addition of air or CO2, derivatives of benzene and phenol (PAH precursors) would be oxidized; thus, less PAHs were generated.

4. CONCLUSION The pyrolysis/gasification of lignin at different reaction conditions was investigated in a fixed-bed reactor. When the temperature was increased from 500 to 900 °C, tar and residue yields from lignin pyrolysis decreased and non-condensed gases increased. Most PAHs increased with the temperature, except 1-methynaphthalene and 2-methynaphthalene, which decreased slightly when the temperature increased from 800 to 900 °C. With the increase of the temperature, the percentage of 2-ring PAHs decreased and the percentage of 3- and 4-ring PAHs increased. The trend of the increase of total PAHs with

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the National Basic Research Program of China (973 Program, 2011CB201502) is gratefully acknowledged. REFERENCES

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