Pyrolysis of Lignocellulosic Biomass for Biochemical Production

CHAPTER 11

Pyrolysis of Lignocellulosic Biomass for Biochemical Production Joo-Sik Kim, Gyung-Goo Choi University of Seoul, Seoul, Republic of Korea

Contents 1. Introduction 2. Reaction Pathways for the Formation of Acetic Acid, Furfural, and Phenolic Compounds 2.1 Acetic Acid 2.2 Furfural 2.3 Phenolic Compounds 3. Recent Research on the Production of Acetic Acid, Furfural, and Phenolic Compounds 4. Laboratory Pyrolysis for the Production of Renewable Chemicals 4.1 Laboratory Pyrolysis Process (The Uos Pyrolysis Process) 4.2 Production of Acetic Acid and Furfural via Pyrolysis With ZnCl2 4.3 Phenolic Compounds 5. Staged Pyrolysis of Biomass for Chemical Production 5.1 The Two-Stage Pyrolysis Process (The Uos Two-Stage Pyrolysis Process) 5.2 Experimental Conditions 5.3 Results and Discussion 6. Conclusions and Perspectives References Further Reading

323 325 325 327 330 331 332 332 332 339 342 342 344 344 345 346 348

1. INTRODUCTION There have been significant research efforts to produce renewable energy and chemicals from lignocellulosic biomass to curtail the greenhouse gas (GHG) emission. In particular, lignocellulosic biomass, which is composed mainly of lignin, cellulose, and hemicellulose, has become a focal topic because lignocellulosic biomass does not compete with food and feed. There are several technologies for conversion of lignocellulosic biomass into energy and chemicals. Pyrolysis is one of the technologies that convert lignocellulosic biomass into gas, liquid, and solid products using heat under an inert atmosphere. Depending on the heating rates and/or the residence time of the pyrolysis vapor in the reactor, pyrolysis can be broadly classified into slow and fast pyrolysis. Slow pyrolysis involves thermal cracking of lignocellulosic biomass at low heating rates to produce a high yield of solid product known as biochar (or char). In the fast pyrolysis, a high yield Waste Biorefinery https://doi.org/10.1016/B978-0-444-63992-9.00011-2

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of liquid product can be obtained because a short pyrolysis vapor residence time in the reactor suppresses secondary reactions that promote the formation of gas and biochar [1–3]. The reaction temperature for the pyrolysis of lignocellulosic biomass typically ranges from 500°C to 800°C [4]. The liquid product, known as bio-oil or biocrude, has received increasing attention as a promising alternative fuel. However, the direct use of bio-oils for commercial engines or boilers is limited by its high water content and acidity [5]. Recently, significant research efforts have been made to chemically and physically upgrade bio-oils with considerable progress. However, there are still numbers of technical barriers to overcome, such as catalyst deactivation and enhancing the quality of the upgraded oils. Bio-oil comprises various organic compounds such as anhydrosugars, alcohols, ketones, aldehydes, carboxylic acids, and phenols. Some high value-added chemicals, like levoglucosan, are also present in biooils. The current methods for commercial production of these chemicals consume fossil fuel, thus resulting in depletion of this resource with simultaneous emission of GHGs. Therefore, the production of these chemicals by pyrolysis of lignocellulosic biomass is a promising approach for obtaining renewable chemicals. In this regard, significant focus has been placed on the recovery of chemicals from pyrolysis [6,7]. Among the chemicals mentioned above, there are three representative compounds that have been widely used in industries, that is, acetic acid, furfural, and phenolic compounds. Acetic acid is an important industrial chemical that is widely used in the production of vinyl acetate monomer (VAM), which is an essential chemical in the manufacture of emulsion polymers, resins, and intermediates used in adhesives and coatings. VAM is also widely used in the manufacture of terephthalic acid, which is a monomer for the synthesis of polyethylene terephthalate [8]. The primary industrial method for production of acetic acid is the carbonylation of methanol. Bio-oils derived from lignocellulosic biomass typically contain high amounts of acetic acid, and in particular, the pyrolysis of hemicellulose-rich biomass is favorable for the production of acetic acid. Furfural is considered as a platform for chemicals and fuels, and furfural and its derivatives are widely used in industry as organic solvents and food additives, as well as for the production of rubbers, fibers, resins, flavors, pharmaceuticals, and agrochemicals [9,10]. Furfural is primarily produced via acid-catalyzed hydrolysis of hemicellulose-rich biomass. Pyrolysis of hemicellulose-rich biomass is a prospectively strong candidate for the production of renewable furfural. Phenolic compounds, including simple phenols and phenolic acids, have been widely used in household products and as intermediates for industrial products. For example, phenol is currently used for the production of phenolic resins, epoxy resins, and caprolactam. Phenol is primarily produced by the cumene process, which uses benzene and propylene as feedstocks. The thermal cracking of lignin produces a phenolic-rich bio-oil [11] that can be used as a chemical feedstock for the synthesis of phenolic resin [12]. The chemical compositions of bio-oils derived from different biomass feedstocks differ; the differences may also be induced by the use of different reaction conditions

Pyrolysis of Lignocellulosic Biomass for Biochemical Production

for the pyrolysis operation. In this regard, this chapter provides details of the experimental results obtained with different biomass feedstocks and reaction conditions in pursuit of chemical production through the pyrolysis of lignocellulosic biomass.

2. REACTION PATHWAYS FOR THE FORMATION OF ACETIC ACID, FURFURAL, AND PHENOLIC COMPOUNDS During pyrolysis, the three main components of lignocellulosic biomass, namely, cellulose, hemicellulose, and lignin, are thermally cracked to produce valuable chemicals. Cellulose is a linear, high-molecular-weight polymer consisting of D-glucose monomer units. The basic units are linked to each other by β-1,4-glycosidic bonds. Cellulose imparts mechanical strength to plant cells and generally degrades within the temperature range of 315 400°C [13]. Whereas cellulose is classified as a homopolysaccharide, hemicellulose is a heteropolymer that is referred to as a heteropolysaccharide. Hemicellulose consists of pentosans or hexosans that form various polymers such as xylan, glucan, xyloglucan, and glucomannan. Hemicellulose typically degrades at 220–315°C [13]. Lignin has a three-dimensional complex structure. Hydroxyl- and methoxy-substituted phenyl propanes constitute the amorphous organic polymer. The basic units of lignin are three monolignol precursors, namely, p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. The degradation of lignin takes place over a wide temperature range spanning 150–900°C [13]. Fig. 11.1 shows simple pathways for the formation of the chemicals generated during biomass pyrolysis. Xylan in the figure is the representative polymer of hemicellulose. As can be inferred from Fig. 11.1, the type and amount of chemicals derived from biomass by pyrolysis are strongly dependent on the nature of the biomass.

2.1 Acetic Acid Acetic acid is mainly derived from deacetylation of hemicellulose. The reaction can also be expected in the thermal cracking of cellulose [14]. Therefore, holocellulose is the main source of acetic acid. During pyrolysis, holocellulose is first depolymerized, and the degraded fragments subsequently undergo ring scission, thereby producing acetic acid by deacetylation of the fragments. Lignin is also a raw material for acetic acid production. Depolymerization and cracking reactions of lignin can generate acetic acid as a by-product [15]. Moreover, cellulose can be transformed into levoglucosan during pyrolysis. The pathway involves depolymerization, cyclization, and hydrogen radical abstraction. Levoglucosan formation occurs via the depolymerization step. During this stage, formation of an oxygen radical on the scission points occurs. The monomeric region with radicals is then cyclized to form levoglucosan. The reaction scheme is shown in Fig. 11.2. The generated levoglucosan is then decomposed into acetic acid by secondary cracking reactions such as ring scission and deacetylation (Fig. 11.3).

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Fig. 11.1 Chemical formation pathways during pyrolysis of lignocellulosic biomass. (Reproduced with permission from S. Zhang, G.X. Yang, H. Jiang, W.J. Liu, H.S. Ding, Mass production of chemicals from biomass-derived oil by directly atmospheric distillation coupled with co-pyrolysis, Sci. Rep. 3 (2013) Article number: 1120. Copyright 2013 Macmillan Publishers Limited.)

Pyrolysis of Lignocellulosic Biomass for Biochemical Production

Fig. 11.2 The formation route of levoglucosan from cellulose. (Reproduced with permission from D. Mohan, C.U. Pittman, P. Steele, Pyrolysis of wood/biomass for bio-oil: a critical review, Energy Fuels 20 (2006) 848–889. Copyright 2006 American Chemical Society.)

2.2 Furfural Hemicellulose is the main source of furfural and acetic acid. For example, xylan, the most abundant component of hemicellulose, is transformed into furfural by depolymerization, rearrangement, and dehydration. The reaction pathway for the formation of furfural is summarily depicted in Fig. 11.4. Levoglucosan generated from cellulose degradation can be transformed into furfural during pyrolysis; ring scission and recyclization of levoglucosan lead to furfural formation. The main mechanism of decomposition of levoglucosan to form furfural is also presented in Fig. 11.3 [16], illustrating that cracking of levoglucosan begins with ring scission. The generated linear compounds are subsequently decomposed into lightmolecular-weight compounds. When recyclization occurs at this stage instead of cracking, furfural can be generated. Pyrolysis of hemicellulose-rich biomass using zinc chloride (ZnCl2) is a promising approach for the production of furfural. ZnCl2 can also catalyze cellulose degradation, yielding furfural as a dominant product. In the catalytic degradation of holocellulose, ZnCl2, which plays the role of a Lewis acid, promotes pyrolytic ring scission of holocellulose to produce furfural [7].

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Fig. 11.3 Reaction pathways for secondary decomposition of levoglucosan. (Reproduced with permission from D.K. Shen, S. Gu, The mechanism for thermal decomposition of cellulose and its main products, Bioresour. Technol. 100 (2009) 6496–6504. Copyright 2009 Elsevier.)

Pyrolysis of Lignocellulosic Biomass for Biochemical Production

Fig. 11.4 Proposed reaction pathways of hemicellulose pyrolysis. (Reproduced with permission from P.R. Patwardhan, R.C. Brown, B.H. Shanks, Production distribution from the fast pyrolysis of hemicellulose, ChemSusChem 4 (2011) 636–643. Copyright 2011 John Wiley and Sons.)

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2.3 Phenolic Compounds It is clear that phenolic compounds are the main products of the pyrolysis of lignin because lignin consists of phenyl propanes. Lignin is composed of three basic units, p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, which are linked to each other by various bond combinations. The typical phenylpropane linkages are shown in Fig. 11.5. During pyrolysis, three bonds undergo the bond-breaking process: (1) the CdO bond located on the side chain of the aromatic ring, (2) the aromatic CdO bond, and (3) the side chain CdC bond. However, link cleavage occurs randomly, and the type of phenolic products generated from the pyrolysis of lignin depends strongly on the kind of feed material and pyrolysis temperature. This strong dependence is largely attributed to the fact that lignin from different biomass sources varies in composition and that the lignin degradation temperature range is very wide (150 900°C). If pyrolysis provides enough energy for all the aforementioned bond cleavage reactions, the production of a bio-oil containing various kinds and a large amount of phenolic compounds would be possible.

Fig. 11.5 Typical phenylpropane linkages in lignin. (Reproduced with permission from F.S. Chakar, A.J. Ragauskas, Review of current and future softwood kraft lignin process chemistry, Ind. Crops Products 20 (2004) 131–141. Copyright 2004 Elsevier.)

Pyrolysis of Lignocellulosic Biomass for Biochemical Production

3. RECENT RESEARCH ON THE PRODUCTION OF ACETIC ACID, FURFURAL, AND PHENOLIC COMPOUNDS Representative technologies to produce acetic acid from biomass have been fermentation and hydrothermal treatment, and corn residues and food wastes have been mostly used biomass for the purpose. Those technologies, however, could not report meaningful results from the technological and economic viewpoints. Recently, several trials using the fast pyrolysis have been conducted for the acetic acid production. A study on the fast pyrolysis of cornstalk in an auger reactor, for example, reported a high acetic acid concentration (27.3%) in bio-oil [17]. Meanwhile, catalytic fast pyrolysis for acetic acid production is seldom observed, and considerable works using catalysts have been performed to convert or remove acetic acid in bio-oil to enhance the stability of bio-oil. Because the furfural yield in the noncatalytic pyrolysis of biomass is limited (0.8–2 wt%), most studies focusing on furfural production have used catalysts such as ZnCl2. Branca et al. showed that the primary paths of furfural formation occurred via dehydration of pentosyl and glucosyl residues while performing the pyrolysis of corncobs using ZnCl2 [18]. Wan et al. have also investigated the effects of catalysts (AlCl3, CoCl2, ZnCl2, and MgCl2) on the furfural production, reporting ZnCl2 and MgCl2 were effective in promoting furfural production [19]. In the fast pyrolysis of typical wood feedstocks, a very low content of phenol (up to 3 wt% of the bio-oil) and low contents of phenolic compounds (20 area% of the bio-oil) were produced [20,21]. Hence, much effort has been made to find suitable biomass that could yield high concentrations of phenolic compounds including phenol. Palm-oil residues, which are generated in large quantities after oil extraction at mill especially in Southeast Asia, have gained much attention as sources of phenolic compounds, and some researchers tried to exploit this abundantly available biomass to produce phenolic compounds via fast pyrolysis. For example, Asadullah et al. could produce phenol-rich bio-oils (over 20 area%) via fast pyrolysis of palm kernel shell in a fluidized-bed reactor [22]. Meanwhile, catalytic fast pyrolysis of biomass has been also tested to produce phenolic-rich bio-oils. Most applied catalysts for the purpose were activated carbon, alkaline catalysts such as metal carbonates and hydroxides, and K3PO4 [23]. Lu et al., for example, reported that K3PO4-catalyzed pyrolysis of poplar wood produced phenolic-rich bio-oils. In their research, the maximal peak area% of phenolic compounds in bio-oil was over 60% [24]. Bu et al. also conducted catalytic microwave fast pyrolysis of lignocellulosic biomass using activated carbon and reported that high concentrations of phenol (38.9%) and phenolics (66.9%) could be obtained at 314°C after a series of reactions such as decarboxylation and dehydration [25,26]. Palm-oil residues were also used in the catalytic fast pyrolysis. Salema and Ani, for example, conducted the microwave fast pyrolysis of oil palm empty fruit bunch using a coconut-based activated carbon as a microwave absorber and reported a very high phenol content in bio-oil (85 area%) [27].

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4. LABORATORY PYROLYSIS FOR THE PRODUCTION OF RENEWABLE CHEMICALS 4.1 Laboratory Pyrolysis Process (The UOS Pyrolysis Process) The fast pyrolysis process called “the UOS pyrolysis process” was used in the experiments; the setup is schematically and pictorially depicted in Figs. 11.6 and 11.7. The process setup comprises a feeding system, a fluidized-bed reactor, a charseparating system, and a liquid recovery system. Biomass can be fed into the fluidizedbed reactor through two screw feeders (feeding system). The fluidized-bed reactor, made of 316 SS tube, is indirectly heated using an electric heater. The reactor has a height of 390 mm and an inner diameter of 110 mm. A tuyere-type distributor is generally applied in the reactor. The pyrolysis gas produced in the reactor immediately passes through a char-separating system consisting of a cyclone and a ceramic filter designed to capture particles larger than 10 and 2 μm, respectively. Subsequently, liquid product (bio-oil) is captured in a liquid recovery system consisting of a water-cooled condenser (20°C), an ethanol-cooled condenser ( 20°C), and an impact separator (IS) that captures high-molecular-weight compounds, in series, and an electrostatic precipitator (EP). Generally, as a fluidizing medium, the noncondensable gas product is circulated into the reactor through a preheater using a compressor. The remaining gas product is either sampled using Teflon gasbags or burnt in a flare stack.

4.2 Production of Acetic Acid and Furfural via Pyrolysis With ZnCl2 As mentioned earlier, hemicellulose contributes significantly to the furfural and acetic acid production. Corn residues such as corn stover and corncob are representative lignocellulosic biomass sources with a high hemicellulose content. Pure thermal pyrolysis of corn residues, however, appears not to be attractive for the production of furfural. A study on the noncatalytic pyrolysis of corncob and corn stover reported that bio-oils with furfural contents of  1.2 (corncob) and 0.7 wt% (corn stover) could be obtained [28]. On the other hand, promising results could be obtained when catalysts were applied. In particular, ZnCl2 proved to be the most effective catalyst for furfural production [18,29]. Moreover, ZnCl2 was also effective for the production acetic acid. Lu et al. conducted the pyrolysis of biomass impregnated with ZnCl2 and obtained a high acetic acid yield (4 wt%) [30]. Coproduction of furfural and acetic acid via pyrolysis is very attractive from the economic viewpoint of biomass pyrolysis. Therefore, the ensuing section deals with the pyrolysis of corn residues with ZnCl2 under various reaction conditions and summarizes the experimental results for the production of furfural and acetic acid. 4.2.1 Experimental Conditions Dried corn stover and corncob samples with a particle diameter of 0.25–0.43 and/or 0.43–1 mm were used as the feed materials. The corn stover, consisting mainly of leaves,

Pyrolysis of Lignocellulosic Biomass for Biochemical Production

Fig. 11.6 Diagram of the one-stage pyrolysis process (the UOS pyrolysis process).

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Fig. 11.7 Photo of the one-stage pyrolysis process.

sheaths, and husks, had high contents of volatile matter (76 wt%) and ash (7 wt%). The hemicellulose and cellulose contents of the corn stover were 32 and 34 wt%, respectively. The corncob also had high contents of volatile matter (88 wt%) and ash (8 wt%); these values are higher than those of corncob. The hemicellulose and cellulose contents of the corncob sample were 34 and 28 wt%, respectively. In the experiments with corn residues, 2.6 kg of quartz sand was used as the fluidized-bed material. The specific experimental conditions used for pyrolysis of corn stover (CS) and corncob (CC) are summarized in Tables 11.1 and 11.2, respectively.

Table 11.1 Reaction conditions and mass balance of the corn stover pyrolysis Parameter CS1 CS2 CS3 CS4 CS5 CS6 CS7

Reaction temperature (°C) Flow rate (NL/min) Pretreatment method ZnCl2 (wt%)

CS8

CS9

343

370

430

330

345

346

335

343

333

42 N

44 N

40 N

46 N

42 W

42 W

46 W

42 A

46 A

–

–

–

18.5

–

18.5

18.5

–

18.5

46.8 49.2

46.2 50.8

48.2 52.1

45.5 49.7

55.2 49.3

42.4 50.1

43.5 51.6

58.8 50.8

43.7 48.6

20.6 32.6

20.3 33.5

23.6 28.2

11.7 42.8

17.9 26.9

10.9 46.7

10.7 45.8

11.2 30.0

12.0 44.3

Product distribution (wt%)

Bio-oil Water content in bio-oil Gas Char

A, acid washing; N, no pretreatment; W, water washing.

Table 11.2 Reaction conditions and mass balance of the corncob pyrolysis Parameter CC1 CC2 CC3 CC4

CC5

CC6

CC7

CC8

CC9

CC10

Reaction temperature (°C) Feed rate (g/min) Flow rate (NL/min) Feed size (mm) ZnCl2 (wt%)

358 2 36 0.43–1 20a

353 4 36 0.43–1 20a

355 2 36 0.25–0.43 0

347 4 36 0.25–0.43 10b

345 4 36 0.25–0.43 20b

351 4 36 0.25–0.43 20a

29.8 (68.1) 14.9 55.3

46.6 (70.1) 15.3 38.1

59.0 (57.1) 17.7 23.3

60.0 (63.1) 10.0 30

48.1 (70.4) 10.1 41.8

47.8 (68.8) 11.4 40.8

311 2 38 0.43–1 0

357 2 36 0.43–1 0

411 2 33 0.43–1 0

356 2 36 0.43–1 10a

48.2 (57)

52.7 (57)

17.7 34.1

21.5 25.8

58.3 46.3 (62.4) (56.9) 24.3 16.8 17.4 36.9

Product distribution (wt%)

Bio-oil (water content in bio-oil) Gas Char a

Physical mixing. Wet impregnation.

b

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In the experiments with corn stover, the effects of the reaction temperature (CS1–3), washing method used for the feed material (CS1, 5, and 8 and CS4, 7, and 9), and the ZnCl2 content in the feed material (CS1 and 4, CS5 and 6, and CS8 and 9) were the main targets of investigation. Reduction of the content of alkali and alkaline earth metals (AAEMs) of the feed material was conducted by washing pretreatment via two methods, water and acid washing. ZnCl2 was loaded on the feed material using the wet impregnation method. The ZnCl2 content of corn stover was 18.5 wt%. In the experimental series with corncob, the pyrolysis was performed under the conditions specified for CC1–3 (reaction temperature), CC2 and 7, CC6 and 10 (feed size), CC5 and 6 (feed rate), CC2 and 4–5 (ZnCl2 content), and CC6 and 8–9 (ZnCl2 contacting method with feed material) to investigate the effects of the reaction parameters on the furfural and acetic acid production. Experiments with ZnCl2 were performed by physical mixing with the feed material or by loading on the corncob using the wet impregnation method.

4.2.2 Results and Discussion Tables 11.1 and 11.2 also present the product yields from the pyrolysis of corn stover and corncob, respectively. In the pyrolysis of corn stover, the bio-oil yield (37–59 wt%) increased with increasing temperature. The washing pretreatment led to an increase in the bio-oil yield. In particular, acid washing (bio-oil yield, 59 wt%) was more effective than water washing (bio-oil yield, 53 wt%). Inductively coupled plasma atomic emission spectroscopic (ICP-AES) analysis of the feed materials before and after washing revealed that water- and acid-washing reduced the AAEMs in corn stover, and acid-washing, in particular, considerably removed Ca, Mg, and Na from the biomass. AAEMs in biomass are known to catalyze secondary reactions to promote formation of gas, water, and char products [31,32]. The use of ZnCl2 led to a decrease in the bio-oil yield and an increase in the char yield. Most of the ZnCl2 remained in the char after pyrolysis. The catalyst is known to favor char-forming reactions [33]. The mass balance for corncob shown in Table 11.2 revealed that the yields of bio-oil from the noncatalytic pyrolysis (CC1–3) were in the range of 48–58 wt% with high water contents of 57 wt%. A higher reaction temperature, a smaller feed size, and a higher feed rate increased the bio-oil yield. This result was mainly due to more active thermal decomposition of the feed material at the higher temperature, better conversion of feed material with a small particle size, and reduced secondary cracking reactions at short residence time, caused by the higher feed rate. The aforementioned trends were confirmed by other studies [34,35]. The water content of bio-oil increased with an increase of the catalyst loading because ZnCl2 acts as a dehydrating agent [30]. Tables 11.3 and 11.4 present (i) the analytic results obtained with GCs using the relative response factors (RRFs) of the bio-oil components, which were obtained by using the effective carbon number (ECN) method; (ii) the furfural and (or) acetic acid

Table 11.3 Main compositions of bio-oils from the corn stover pyrolysis GC analysis using RRF (wt%) Compounds

CS1

CS2

CS3

CS6

Acetic acid 1-Hydroxy-2-propanone 1-Hydroxy-2-butanone Furfural Furfuryl alcohol 2(5H)-Furanone Phenol Methylphenol o-Guaiacol 4-Ethylphenol Levoglucosan

32.9 15.5 1.8 2.4 2.0 – 0.6 – 1.2 – –

36.3 10.5 2.1 2.3 1.4 0.2 0.6 0.2 1.0 0.4 –

44.9 8.4 2.2 1.8 1.6 0.4 0.6 0.2 0.8 0.2 –

13.8 – 0.3 38.3 – 1.3 0.5 – 0.3 0.3 25.7

CS1

CS2

CS3

CS4

CS5

CS6

CS7

CS8

CS9

0.2

0.4

10.8

3.4

27.1

26.2

2.7

24.9

0.1

0.2

4.9

1.9

11.5

11.4

1.6

10.9

External standard method (wt%)

Furfural

1.3

Furfural yield (wt%)

Furfural

0.6

RRF, relative response factor.

Table 11.4 Main compositions of bio-oils from the corncob pyrolysis GC analysis using RRF (wt%) Compounds

CC1

CC2

CC3

1-Propanol Acetic acid Hydroxyacetone 1-Hydroxy-2-butanone Methyl acetate Furfural Furfuryl alcohol 2(5H)-Furanone 1,2-Cyclopentanedione Phenol 4-Methylbenzaldehyde p-Vinylguaiacol Levoglucosan

1.0 33.1 9.6 3.2 0.6 3.5 4.4 0.9 3.0 0.5 2.7 3.3 1.9

1.2 33.4 7.4 1.2 2.7 6.4 3.1 1.1 3.0 0.5 2.1 2.6 3.0

1.3 38.5 7.9 0.9 2.0 5.5 1.7 0.9 2.1 0.4 1.7 2.4 4.3

CC1

CC2

CC3

CC4

CC5

CC6

CC7

CC8

CC9

CC10

External standard method (wt%)

Acetic acid Furfural

9.5

11.8

17.0

22.5

44.0

16.7

13.4

9.2

17.5

15.5

0.4

1.5

1.0

3.0

6.7

5.6

1.0

5.7

17.0

8.4

4.6

6.2

9.9

10.4

13.1

7.8

7.9

5.5

8.4

7.4

0.2

0.8

0.6

1.4

2.0

2.6

0.6

3.4

8.2

4.0

Compound yield (wt%)

Acetic acid Furfural

RRF, relative response factor.

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concentration in the bio-oils, obtained by using the external standard method; and (iii) the furfural and (or) acetic acid yields from the pyrolysis of corn stover and corncob, respectively. The bio-oils produced from the noncatalytic pyrolysis of corn stover (CS1–3) consisted mainly of acetic acid, 1-hydroxy-2-propanone, 1-hydroxy-2-butanone, and furfural. The acetic acid content was in the range of 17–22 wt%. A higher temperature within the evaluated temperature range had a positive effect on the acetic acid production. The furfural content in the bio-oils from CS1–3 was only 1 wt% (the external standard method). The pyrolysis of corn stover with ZnCl2 significantly increased the production of furfural and levoglucosan, while the production of other compounds decreased. Such a high furfural yield appeared to result from the high catalytic activity of ZnCl2 as a Lewis acid in the depolymerization and dehydration reactions to form furfural along with levoglucosan [36]. ZnCl2 also reportedly prohibits the devolatilization of lignin [30]. ZnCl2 can also promote the formation of D-glucose from cellulose. Coordinate bonding between the glycosidic oxygen of cellulose and Zn in the hydrated form can easily break the glycosidic bond of cellulose. The D-glucose thus generated can be transformed into furfural and 5-hydroxymethylfurfural by dehydration [37]. In the experiment where the water-washed corn stover was pyrolyzed, a bio-oil containing the highest furfural content (27 wt%; the external standard method) was produced. The resulting furfural yield was 11.5 wt%. Acid treatment was less effective than water treatment because the former removed hemicellulose and amorphous cellulose in the feed material along with the AAEMs. Furfural should be separated from bio-oil for practical application. Extraction, which is a simple and the most cost-effective approach, appeared to be suitable for this separation. Toluene was used as the extraction solvent. The highest recovery ratio (82%) was obtained at a bio-oil/toluene ratio of 1:4. GC analysis via the external standard method revealed that the extract from a 1:1 mixture of bio-oil and toluene consisted mainly of acetic acid (11 wt%) and furfural (89 wt%). Extraction with toluene appeared to be very attractive because it achieved high separation efficiency without any complex procedures. The bio-oil from corncob had a similar composition to that from corn stover. The concentration of furfural in the bio-oils obtained from the noncatalytic pyrolysis was low, in the range of 0.4–1.5 wt% (the external standard method). However, application of ZnCl2 clearly increased the furfural concentration up to 17 wt%, corresponding to a yield of 8 wt%. A larger amount of ZnCl2 positively influenced the furfural production, and impregnation with ZnCl2 produced better results than physical mixing with corncob. A higher feed rate positively impacted the furfural production, and a smaller feed size substantially increased the furfural concentration. A smaller feed size better facilitates devolatilization of the feed material and the contact between the feed material and ZnCl2 than a larger feed size. In the pyrolysis of corncob, variation of the acetic acid production according to various reaction parameters was also observed. The acetic acid concentration of bio-oils was 9–44 wt% (the external standard

Pyrolysis of Lignocellulosic Biomass for Biochemical Production

method), corresponding to acetic acid yields of 5–13 wt%. Prior studies reported acetic acid concentrations of 0.5–12 wt% in bio-oils obtained from energy crops and typical wood [38] and 27.3% acetic acid concentration in the bio-oil from cornstalk [17]. The high acetic acid concentration obtained in this study appeared to result from the use of a relatively low pyrolysis temperature range compared with the typical pyrolysis temperature (500°C), which suppressed decomposition of the lignin fraction of corncob, and from the use of ZnCl2 at high loading (up to 18. 5 wt%), which promotes the production of acetic acid. A high temperature, a low feed rate, a smaller feed size, a high amount of ZnCl2, and physical mixing of ZnCl2 positively affected the acetic acid production. It was interesting and at the same time difficult to explain why physical mixing, which would result in loose contact between corncob and ZnCl2, was better than the impregnation method. Lu et al. reported that the acetic acid yield increased with a ZnCl2 loading of up to 4 wt% but decreased with a further increase of the ZnCl2 loading [30].

4.3 Phenolic Compounds Phenolic compounds can also be derived from the degradation of lignin. Phenolic resins are mainly synthesized by the reaction between phenolic compounds and formaldehydes and are used in a wide variety of applications. Many attempts have been made to replace fossil phenols used for the synthesis of phenolic resins with phenolic-rich biooils or pyrolytic lignin that is thermal degradation products of lignin in the synthesis of phenolic resins [12,39]. A whole bio-oil was expected to be directly useful for the synthesis of phenolic resins because it contains aldehydes and phenolic compounds. However, the application of whole bio-oil or pyrolytic lignin as a feedstock for the production of phenolic resins resulted in lower mechanical strength than that of the commercial resin, mainly because of the low reactivity of the phenolic compounds in bio-oil. To achieve adequate properties as a commercial resin, phenolic compounds should have empty ortho- and parasites to react with aldehydes. However, for the phenolic compounds in bio-oil, the positions are occupied by other groups such as methoxy and carbonyls [12]. Therefore, numerous attempts have been made to efficiently remove these functional groups from the phenols or to find a biomass that contains a high amount of lignin that degrades into phenols with fewer functional groups at these positions. Palm-oil residues such as empty punch bundles and palm kernel shell (PKS) turned out to have a high amount of lignin that generated a high amount of phenol during pyrolysis. We previously studied the pyrolysis of PKS and found that PKS is a promising biomass source for the renewable phenol production [35]. This section places emphasis on the production of bio-oil rich in phenolic compounds by using PKS as the feed material at different reaction temperatures. Finally, the compressive strength of plywood panels manufactured using a whole bio-oil obtained from PKS as a substitute for commercial phenol resins is discussed.

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Table 11.5 Reaction conditions and mass balance of the PKS pyrolysis Parameter P1 P2 P3

P4

Reaction temperature (°C) Flow rate (NL/min) Feed amount (g) Operation time (min)

479 30 300 30

503 29 300 30

555 27 300 30

505 29 1000 100

52.4 (34.2) 24.5 23.1

53.4 (39.1) 25.3 21.3

50.9 (48.0) 27.8 21.3

51.7 (44.7) 26.7 21.6

Product distribution (wt%)

Bio-oil (water content in bio-oil) Gas Char

4.3.1 Experimental Conditions PKS with a diameter of 1–2 mm and high lignin content (47 wt%) was used in the experiment. The cellulose and hemicellulose contents were 30 and 21 wt%, respectively. The experimental conditions are summarized in Table 11.5. The reaction temperature range was 480–560°C, and a feed rate of 10 g/min was used for all runs. For experiments P1–3, the feed amount and the reaction time were 300 g and 30 min, respectively, while 1000 g and 100 min were used for P4. 4.3.2 Results and Discussion Table 11.5 shows that the yields of bio-oils obtained within the reaction temperature range were 51–53 wt%, and the reaction temperature of 503°C produced the maximum bio-oil yield. The bio-oil yield obtained in P4 decreased compared with that from P2 for which the reaction conditions were similar to those of P4. The char produced in P4 accumulated in the reactor at longer reaction times, and ash from the char appeared to influence the product yield. Secondary cracking of the pyrolysis vapor by the ash components enhanced the product gas generation. The water content of the bio-oils ranged from 34 to 48 wt%. In P4, the ash components appeared to promote water formation by enhanced condensation reactions. Table 11.6 presents the compositions of the bio-oils produced from PKS. The GC results obtained using the RRF method showed that the bio-oils consisted mainly of phenolic compounds (33–42 wt%) and acids (35–44 wt%). Phenol, cresols, and 1,2-benzenediol were the main components of the phenolic compounds, and the main acids were acetic acid (20–34 wt%) and hydroxybenzoic acid. With an increase of the reaction temperature up to 503°C, the contents of phenolic compounds increased due to enhanced lignin cracking. However, further increasing the reaction temperature to 555°C promoted gas formation via secondary cracking of phenolic compounds. The increased amount of ash led to a decrease in the concentrations of phenolic compounds from 42 (P2) to 38 wt% (P4). The most interesting result was the high phenol content in the bio-oil. Phenol constituted almost half of the phenolic compounds.

Pyrolysis of Lignocellulosic Biomass for Biochemical Production

Table 11.6 Main compositions of bio-oils from the PKS pyrolysis Compounds P1 P2

P3

P4

37.3 29.3 1.4 2.8 39.5 20.8 3.1 4.3 0.0 0.1 0.1 0.1 9.8 0.5 4.0 0.5

38.1 28.1 3.5 2.8 37.8 19.4 2.4 2.9 0.8 0.1 0.2 1.1 12.8 0.6 4.5 0.1

GC analysis using RRF (wt%)

Acids Acetic acid Hydroxybenzoic acids Dodecanoic acid Phenolics Phenol Cresols 1,2-Benzenediol o-Guaiacol Vanillin Ethylguaiacol Syringol Other compounds Butyrolactone Furfural Furfuryl alcohol

43.6 33.7 4.2 2.2 33.2 15.6 1.6 2.9 1.4 0.6 0.4 1.5 12.1 0.9 4.3 0.8 P1

35.3 19.9 3.5 5.3 41.6 20.7 2.8 3.0 0.9 1.4 0.8 0.8 11.4 0.4 4.4 0.7 P2

P3

P4

7.4

8.1

7.3

7.5

3.9

4.3

3.7

3.9

External standard method (wt%)

Phenol Compound yield (wt%)

Phenol

The external standard method revealed that the phenol content in the bio-oils was in the range of 7–8 wt%, which was by far higher than the phenol content of bio-oils from the pyrolysis of woody biomass [20]. The phenol yield obtained in the experiment was 4 wt%. This high phenol production is mainly due to the high lignin content of PKS (47 wt%) compared with that of typical woody biomass (15–40 wt%) [40]. The lignin composition would also influence the phenol yield. If the lignin has high contents of monomeric units with empty ortho- and parasites, the phenol production increases. Due to the high content of phenolic compounds (especially phenol), a whole bio-oil was expected to be directly used for the production of phenolic resins. To explore the possibility of using a whole bio-oil obtained from PKS as an alternative phenol in the production of phenolic resin, plywood panels were manufactured using both a commercial phenolic resin and a whole bio-oil from the pyrolysis of PKS. The specific gravity and compressive strength were used as analytic parameters. With an increase of the bio-oil content in the manufacturing of plywood panels, the compressive strength of the plywood panels deteriorated. Fig. 11.8 shows the panels prepared in order of increasing bio-oil content.

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Fig. 11.8 Photo of panels manufactured using bio-oil.

For preparation of the panels, the samples were first semidried at 110°C for 15 min and subsequently compressed at 130°C for 40 min with a pressure of 80 kgf/cm2. The use of 25 wt% bio-oil (panel 2) in the manufacturing gave rise to a specific gravity and compressive strength of the panel of 0.99 and 120.3 MPa, respectively, which were largely similar to those of the panel made only with commercial resole resin. When the ratio of bio-oil/commercial phenolic resin exceeded 50:50 (panel 3), adhesion between the sheets of plywood panel was no longer effective. As a result, the maximum substitution ratio for bio-oil in the manufacturing of plywood panels was 25 wt%. A further increase of the bio-oil content used in the manufacturing may be possible if a catalyst can be used during resin formation or if heating is applied during the mixing or panel production process.

5. STAGED PYROLYSIS OF BIOMASS FOR CHEMICAL PRODUCTION Although the temperature ranges for thermal degradation of cellulose, hemicellulose, and lignin cannot be distinctly divided, such a distinction would make it possible to produce bio-oils rich in hemicellulose, cellulose, or lignin degradation components. This can be accomplished by using the staged pyrolysis concept that has at least two different pyrolysis units in series. This section discusses the newly developed two-stage pyrolyzer and the experimental results obtained using the pyrolyzer and PKS.

5.1 The Two-Stage Pyrolysis Process (The UOS Two-Stage Pyrolysis Process) The setup for the two-stage pyrolysis process is similar to that of the one-stage pyrolysis process presented in Fig. 11.6, except that an auger reactor is located in front of the fluidized-bed reactor. The auger reactor made of a 310S tube has an inner diameter of 28 mm and length of 700 mm. A product recovery system is positioned after the auger reactor, similar to the case of the one-stage pyrolysis process. The product recovery system consists of a water condenser, an IS, and an EP. The product gas escaping the EP is burnt in a flare stock. Fig. 11.9 presents a schematic of the two-stage pyrolysis process. The main task for developing the two-stage process was to provide separate pyrolysis conditions for the two reactors. For this purpose, a gas inlet was positioned near the boundary between the auger reactor and the fluidized-bed reactor to prevent mixing of the gases from both reactors. During pyrolysis in the two-stage process, a part of the product gas is

Pyrolysis of Lignocellulosic Biomass for Biochemical Production

Fig. 11.9 Diagram of the two-stage pyrolysis process (the UOS two-stage pyrolysis process).

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introduced into the gas inlet, which both facilitates escape of the pyrolysis gas from the auger reactor and prevents any flow from the fluidized-bed reactor into the auger reactor. Nitrogen gas supplied from the silo during pyrolysis also facilitates smooth feeding and, at the same time, pushes out the pyrolysis gas generated in the auger reactor.

5.2 Experimental Conditions The feed material used was PKS. The reaction conditions are summarized in Table 11.7. Before the two-stage experiment, one-stage pyrolysis was conducted at reaction temperatures of 350°C (O1) and 510°C (O2). T1, the two-stage experiment, was performed at the auger reactor temperature of 380°C and at the fluidized-bed reactor temperature of 520°C. The auger reactor was expected to decompose most holocellulose of PKS, and the fluidized-bed reactor was expected to decompose a large portion of lignin. The feed rate of PKS was 5.5 g/min.

5.3 Results and Discussion Table 11.7 also shows the mass balances of the experiments. The bio-oil yields from the one-stage pyrolysis were in the range of 53–60 wt%. The produced bio-oils contained 48–56 wt% water. In the two-stage pyrolysis, the preceding degradation of PKS in the auger reactor caused a decrease of the total bio-oil yield (50 wt%), and the ratio of the bio-oil yield from the auger reactor to that from the fluidized-bed reactor was 1:3. The bio-oil from the auger reactor contained 63 wt% water, while that from the fluidizedbed reactor contained 35 wt% water. Table 11.8 presents the main compositions of the bio-oil obtained from the two-stage pyrolysis. Interestingly, the compositions of the bio-oils from the auger reactor and fluidizedbed reactor differed significantly. The bio-oil from the auger reactor, consisting mainly of acetic acid, methyl acetate, and furfural, had fewer chemical components than that from the one-stage pyrolysis (O1). In particular, the acetic acid content of the bio-oil (T1) was Table 11.7 Reaction conditions and mass balance of the two-stage pyrolysis One stage Parameters

O1

O2

Two stage T1

Reaction temperature (°C)

– 353 (FBR) – 43

– 512 (FBR) – 32

384 (AR) 517 (FBR) 2 29

52.9 9.4 37.7

60.2 11.5 28.3

50.2 (25/75) 19.9 29.9

Solid residence time in AR (min) Flow rate of fluidizing gas (Nl/min) Product distribution (wt%)

Bio-oil (AR/FBR) Gas Char

AR, auger reactor; AR/FBR, weight ratio of oils obtained from AR and FBR; FBR, fluidized-bed reactor.

Pyrolysis of Lignocellulosic Biomass for Biochemical Production

Table 11.8 Main compositions of bio-oils from the two-stage pyrolysis Compounds O1 O2 T1 (AR)

T1 (FBR)

GC analysis using RRF (wt%)

Glycolic acid Methyl acetate Acetic acid Acetol Furfural Phenol o-Cresol o-Guaiacol o-Benzenediol Vanillin 4-Hydroxybenzoic acid Lauric acid

3.07 3.78 30.27 – 2.23 6.08 0.15 1.09 0.37 0.21 4.19 1.95

2.46 1.69 29.59 4.51 2.66 6.42 0.86 0.86 1.07 0.23 2.55 0.58

8.33 5.2 61.75 2.94 5.96 2.4 – – – – – –

– 1.26 44.95 9.11 1.56 9.57 0.47 0.83 1.5 0.18 4.88 0.91

1.99 9.92

2.43 9.97

0.12 3.23

2.47 12.14

Compound yield (wt%)

Phenol Acetic acid

very high with a value of 62 wt% (GC results obtained using the RRF method). Clearly, the main compounds obtained from the auger reactor were the typical degradation products of hemicellulose and cellulose. In contrast, the bio-oil from the fluidizedbed reactor consisted mainly of the degradation products of lignin and the portion of holocellulose that were not degraded in the auger reactor. The phenol content of the bio-oil was 9.6 wt% (GC results obtained using the RRF method), making its yield 2.5 wt%. Apart from phenol, other phenolic compounds were abundant in the biooil from the fluidized-bed reactor. Based on the high content of reactive phenol, the bio-oil from the fluidized-bed reactor appeared to be a good alternative to fossil phenols for the synthesis of phenol resin. In summary, the two-stage pyrolysis process continuously produced two different bio-oils in one operation. Each type of bio-oil had fewer chemical components than typical bio-oils (over 400 compounds) due to enrichment with specific chemicals. Therefore, it appears possible to directly employ the two biooils from the two-stage process, for example, in the synthesis of calcium magnesium acetate deicer (the bio-oil from the auger reactor) or in phenol resin synthesis (the bio-oil from the fluidized-bed reactor) without any separation of the chemicals from bio-oil.

6. CONCLUSIONS AND PERSPECTIVES In this chapter, we considered the possibility of chemical production from the pyrolysis of lignocellulosic biomass. Corn residues appear to be a very attractive biomass source for

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the production of furfural and acetic acid, while PKS appears attractive for the production of phenolic compounds. The maximum concentrations of furfural and acetic acid in the bio-oils from the pyrolysis of corn residues were 27 and 44 wt% (the external standard method), respectively. Toluene extraction appeared to be a promising method for the recovery of furfural from bio-oils. The highest furfural recovery ratio (82%) was achieved at a bio-oil/toluene ratio of 1:4. Palm-oil residues, like palm kernel shell, contain a high amount of lignin. During pyrolysis, these residues degrade into simple phenols and pyrolytic lignin. Herein, pyrolysis of PKS was found to produce bio-oils with high concentrations of phenolic compounds. The bio-oils had a high phenol content (7 8 wt%); this phenol would be very reactive toward formaldehyde in the synthesis of phenolformaldehyde resins. A whole bio-oil from the pyrolysis of PKS was used for the manufacture of plywood panels as a substitute for commercial phenol resin. This experiment demonstrated that bio-oil could replace commercial resin up to 25 wt% without any significant deterioration of the compressive strength of the plywood. Meanwhile, the staged pyrolysis process is very attractive for the production of chemicals, producing bio-oils with two different compositions in one operation. In an experiment conducted using a two-stage pyrolysis process called “the UOS two-stage pyrolysis process,” an acid-rich bio-oil and a bio-oil enriched with phenolic compounds were simultaneously produced. Because the coproduction of different bio-oils in a process could preclude costly separation procedures for recovery of chemicals, this technique is prospectively highly favorable for commercial biomass pyrolysis. In summary, the biochemical production via fast pyrolysis of lignocellulosic biomass is a promising route for producing value-added chemicals and at the same time for environmental protection. In order for the technology to be implemented at commercial scales, however, feedstocks that can profitably produce aimed chemicals should be easily available. Along with this, an innovative process that can add economic feasibility to the technology should be developed.

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FURTHER READING [41] X.S. Zhang, G.X. Yang, H. Jiang, W.J. Liu, H.S. Ding, Mass production of chemicals from biomassderived oil by directly atmospheric distillation coupled with co-pyrolysis, Sci. Rep. 3 (2013). Article number: 1120. [42] D.K. Shen, S. Gu, The mechanism for thermal decomposition of cellulose and its main products, Bioresour. Technol. 100 (2009) 6496–6504. [43] P.R. Patwardhan, R.C. Brown, B.H. Shanks, Production distribution from the fast pyrolysis of hemicellulose, ChemSusChem 4 (2011) 636–643. [44] F.S. Chakar, A.J. Ragauskas, Review of current and future softwood kraft lignin process chemistry, Ind. Crop Prod. 20 (2004) 131–141.