Bio-oil derived from palm empty fruit bunches: Fast pyrolysis, liquefaction and future prospects

Biomass and Bioenergy 119 (2018) 263–276

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Review

Bio-oil derived from palm empty fruit bunches: Fast pyrolysis, liquefaction and future prospects

T

Siu Hua Chang Faculty of Chemical Engineering, Universiti Teknologi MARA (UiTM), 13500 Permatang Pauh, Penang, Malaysia

ARTICLE INFO

ABSTRACT

Keywords: Bio-oil Palm empty fruit bunches Optimum production conditions Characteristics Upgrading Fuel applications

Bio-oil is a potential biofuel for fossil fuel substitution due to its great versatility in feedstock and environmental benefits. In particular, bio-oil derived from palm empty fruit bunches (PEFB) has drawn considerable research attention in recent years owing to the abundant supply of PEFB and the emergence of motivation to turn waste into wealth for environmental sustainability. Therefore, this paper aims to provide a state-of-the-art review on the bio-oil derived from PEFB. A particular emphasis is placed on the optimum production conditions of PEFBderived bio-oil by various fast pyrolysis and liquefaction processes, as well as on the characteristics of PEFBderived bio-oil in terms of their physicochemical properties, major chemical components and their compositions. A comparison of physicochemical properties of PEFB-derived bio-oil with those of other biomass-derived bio-oil and petroleum fuel oil is also outlined. Upgrading of PEFB-derived bio-oil by different methods along with its fuel applications and future prospects are also discussed.

1. Introduction Energy is essential for life on Earth and plays a crucial role as a cornerstone in driving the global socio-economic development. It is primarily derived from fossil fuels such as crude oil, coal, and natural gas, which account for the lion's share of the total world energy consumption of over 80% [1], due to their relatively simple and low-cost conversion processes. However, fossil fuel reserves are finite, and with the escalating world population and rapid industrialization in developing countries, it is only a matter of time before they are diminished. Some projections say that there may only be as few as 35, 107 and 37 years of crude oil, coal and natural gas [2], respectively, left in the world, while others estimate them to be last in 40, 200 and 70 years [3], respectively. Meanwhile, the excessive burning of fossil fuels from various anthropogenic activities also contributes to global warming due to the emissions of noxious greenhouse gases which may lead to potentially catastrophic changes in climate, environment, biodiversity and public health [4]. Therefore, environmentally benign and sustainable renewable energy sources such as wind, sunlight, waves, and biomass become vital and have been proactively sought after by numerous researchers over the past few decades [5,6]. Of all the renewable energy sources, biomass has been a subject of intense interest among researchers owing to its abundant supply, low cost, biodegradability and carbon neutrality [7]. In general, biomass is any inedible organic matter or waste derived from flora and fauna that

contains solar energy such as wood waste, agro-industrial waste, animal waste, food waste and a whole host of other materials [8]. It can be used directly as a solid fuel or converted into a liquid or gaseous fuel by means of thermochemical or biochemical processes. The thermochemical conversion processes include pyrolysis, liquefaction, and gasification, where the former two convert biomass into bio-oil (also known as biocrude, pyrolysis or pyrolytic oil) as the main product [9] while the latter generates predominantly syngas (a mixture of carbon monoxide, carbon dioxide, hydrogen, and methane) [10]. The biochemical conversion processes, on the other hand, convert biomass into bioethanol [11] and biogas [12] by fermentation and anaerobic digestion, respectively. Among these biofuels, bio-oil, a complex liquid mixture resulting from the thermal degradation of biomass building blocks (cellulose, hemicellulose and lignin), is one of the most technically promising alternatives to fossil fuels. A diverse range of biomass, for example birch sawdust [13], rice straw [14], soap stock [15], waste tire [16], sewage sludge [17], swine manure [18], waste potato starch [19] and algae [20], have been utilized to produce bio-oil, mainly by fast pyrolysis or liquefaction [8]. By converting a huge amount of biomass into bio-oil, a clean and safe environment, good public health and wellbeing, as well as energy independence and economic growth could be accomplished when waste is managed effectively. These advantages, coupled with the environmental benefits of bio-oil of being renewable, biodegradable and significantly low in greenhouse gas emission, have rendered bio-oil substantially cleaner than fossil fuels.

E-mail address: [email protected]. https://doi.org/10.1016/j.biombioe.2018.09.033 Received 27 April 2018; Received in revised form 20 September 2018; Accepted 28 September 2018 0961-9534/ © 2018 Elsevier Ltd. All rights reserved.

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List of abbreviations BOaq BOmix BOorg CFP CSL CSPL FAME H:C

HHV HSHFO LFO LSHFO NCFP NCSL NCSPL O:C PEFB TAN

Aqueous-phase bio-oil Mixture of BOorg and BOaq Organic-phase bio-oil Catalytic fast pyrolysis Catalytic subcritical liquefaction Catalytic supercritical liquefaction Fatty acid methyl esters Hydrogen to carbon

Nevertheless, bio-oil is a low-quality fuel due to some of its deleterious physicochemical properties such as high viscosity, high water content, high oxygen content, high acidity, high instability and low heating value which have hindered its direct application as a fuel [8]. To increase the quality of bio-oil, several upgrading methods such as hydrotreating, hydrocracking, solvent addition, fuel blending or emulsification, esterification, supercritical fluid treatment and steam reforming have been employed [8,21]. In spite of their efficiencies in laboratory-scale studies, none of these upgrading methods have been utilized in large-scale applications due to some of their unresolved technological challenges which have led to their low energy and cost efficiencies [8]. As yet, the application of bio-oil has been restricted largely to substitute petroleum fuel oil, either partially or completely, in stationary devices such as boilers [22], turbines [23], furnaces [24] and diesel engines [25], which are known for their fuel flexibility and high tolerance to low-grade fuels, for electricity generation at power plants. Although relatively rare and still in their infancy, conversion of bio-oil into biochemicals such as olefins [26–28], aromatics [27] and fatty acids [29], as well as into transportation biofuels such as biodiesel [17], green diesel [30], bio-gasoline [31] and bio-jet fuel [32,33], have also been reported. Palm empty fruit bunches (PEFB), a type of palm biomass generated abundantly from the rapidly growing oil palm industry in Southeast Asia, particularly Malaysia and Indonesia, has drawn a great deal of research attention in recent years following the emergence of motivation to turn waste into wealth for environmental sustainability [34–37]. In Malaysia, for instance, the total PEFB generated in 2017 was approximated to 19.92 million tonnes [38] based on a conservative estimate of the one-to-one ratio of the amount of crude palm oil produced to that of PEFB generated in that year [39]. This prodigious amount of PEFB poses a dire environmental hazard to wildlife and human health if it is not handled appropriately. One of the lucrative methods to mitigate the immense amount of PEFB is utilizing it as a renewable energy feedstock to produce biofuels like bio-oil [9]. In fact, a review of PEFB as bio-oil feedstock centering around such topics as the fundamental characteristics of PEFB, conversion processes of PEFB into bio-oil, and the properties of PEFB-derived bio-oil in comparison with those of petroleum fuel oil had been documented [9]. Nevertheless, detailed discussions on the production conditions, characteristics, upgrading, and fuel applications of PEFB-derived bio-oil were lacking or insufficient. In light of the aforementioned, the present work aims to provide a state-of-the-art review on the bio-oil derived from PEFB. A particular emphasis is placed on the optimum production conditions of PEFB-derived bio-oil by various fast pyrolysis and liquefaction processes, as well as on the characteristics of PEFB-derived bio-oil in terms of their physicochemical properties, major chemical components and their compositions. A comparison of physicochemical properties of PEFB-derived bio-oil with those of other biomass-derived bio-oil and petroleum fuel oil is also outlined. Upgrading of PEFB-derived bio-oil by different methods along with its fuel applications and future prospects are also discussed.

Higher heating value High sulfur heavy fuel oil Light fuel oil Low sulfur heavy fuel oil vc Non-catalytic fast pyrolysis Non-catalytic subcritical liquefaction Non-catalytic supercritical liquefaction Oxygen to carbon Palm empty fruit bunches Total acid number

2. Optimum production conditions of bio-oil derived from PEFB The bio-oil derived from PEFB is mostly produced by fast pyrolysis and liquefaction processes [9]. In general, fast pyrolysis is characterized by relatively higher temperature but lower pressure and shorter residence time than liquefaction. It requires drying of feedstock down to approximately 10 wt% moisture content or less [40] which is, however, not needed in liquefaction. Fast pyrolysis undergoes a gas-phase reaction while liquefaction occurs in a liquid medium or solvent under either subcritical or supercritical condition [8]. The solvent used in liquefaction could be water (popularly known as hydrothermal liquefaction), a pure or a mixture of organic solvents, or a mixture of water and organic solvents [41,42]. Both fast pyrolysis and liquefaction are carried out with or without a catalyst, usually under an inert gas (N2, He or Ar) atmosphere [43,44], but sometimes under a CO2 [45], steam [46] or vacuum [47] atmosphere for fast pyrolysis and a H2 or CO atmosphere for liquefaction [43]. Although bio-oil can be produced by both fast pyrolysis and liquefaction as the main product along with biochar and syngas as side products, its optimum production conditions are totally different between these processes and, in most cases, vary from one biomass feedstock to another in both processes [48,49]. Table 1 summarizes the optimum production conditions of bio-oil derived from PEFB via fast pyrolysis and liquefaction obtained from the literature. All of the PEFB used were unwashed with water prior to their use unless stated otherwise. It was found that fast pyrolysis of PEFB was largely carried out in either a fluidized- or fixed-bed reactor while liquefaction of PEFB in an autoclave reactor regardless of whether the process was catalytic or not. On the whole, the non-catalytic process was more widely used than the catalytic counterpart in either fast pyrolysis or liquefaction of PEFB and a majority of them produced biooil as the main product. As shown in Table 1, the highest bio-oil yields of non-catalytic fast pyrolysis (NCFP) and non-catalytic subcritical or supercritical liquefaction (NCSL or NCSPL) of PEFB were attained at 61.34% and 56.20%, respectively, while those of their catalytic counterparts, namely, catalytic fast pyrolysis (CFP) and catalytic subcritical or supercritical liquefaction (CSL or CSPL), were achieved at 44.10% and 68.00%, respectively. Both of the former non-catalytic processes utilized water-washed PEFB as feedstock but produced bio-oil at totally different conditions: temperature of 500 °C vs. 350 °C, pressure of 1 bar vs. 291 bar and residence time of ∼1 s vs. 3600 s (Table 1). The upside of washed PEFB over the unwashed PEFB was that the former had much lower ash content, particularly potassium content, than the latter, which in turn suppressed the unfavorable secondary reactions for water, syngas and biochar yields and increased the bio-oil yield [50]. Table 1 also reveals that the bio-oil yields achieved by NCFP of PEFB were often higher than those by CFP of PEFB but those accomplished by NCSL or NCSPL of PEFB were lower than those by CSL or CSPL of PEFB (Table 1). The former could be deduced from the catalytic cracking of bio-oil into water and syngas that led to the diminution of bio-oil yield in CFP of PEFB [51,52], while the latter was due to the enhanced degradation of PEFB by catalysts in CSL or CSPL of PEFB [53].

264

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Nevertheless, more research on CFP and CSL or CSPL of PEFB is needed to further verify these findings considering their limited study relative to their non-catalytic counterparts. As for now, several catalysts that have been determined to achieve maximum yields of bio-oil are 20 wt% of boric oxide (B2O3), 5 wt% of HY and 5 wt% of HZSM-5 zeolite catalysts for CFP of PEFB, as well as 6 wt% of potassium carbonate (K2CO3) and 1 wt% of cerium (IV) oxide (CeO2) for CSL and CSPL of PEFB, respectively (Table 1). Other optimum production conditions of PEFBderived bio-oil via NCFP or CFP include temperature of 300–600 °C, pressure of 1 bar, particle size of < 1 mm, heating rate of < 100 °C/min and operating atmosphere of inert gas (N2 or Ar) or vacuum, whereas those via NCSL, NCSPL, CSL or CSPL include temperature of 270–400 °C, pressure of 20–291 bar, particle size of < 1 mm, operating atmosphere of inert gas (N2), solvent of water, ethylene glycol, methanol or water-ethanol mixture, and PEFB to solvent ratio of 1:2 to 1:17 (Table 1).

brown, high energy density and polar organic liquid with a smoky odor, while the latter is a non-viscous, light to dark amber-yellow, low energy density and polar aqueous liquid [70]. Therefore, a separation process like liquid-liquid extraction [59,69] or fractional distillation [55] is normally carried out right after the bio-oil production process to separate BOorg from BOaq prior to the physicochemical characterization of each of the phases. Sometimes, characterization of a mixture of BOorg and BOaq, i.e. BOmix, right after the production process without any prior phase separation is also conducted [60]. In the following sections, the characteristics of bio-oil derived from PEFB in the forms of BOorg, BOaq and/or BOmix are reviewed in terms of their physicochemical properties, major chemical components and their compositions. 3.1. Physicochemical properties Table 2 shows the physicochemical properties of bio-oil derived from PEFB and other biomass along with those of petroleum fuel oil obtained from the literature. The physicochemical properties studied include elemental composition via ultimate analysis, hydrogen to carbon (H:C) and oxygen to carbon (O:C) molar ratios, water, solid and ash contents, higher heating value (HHV), density, viscosity, pH and total acid number (TAN) which are crucial in governing the quality of a fuel. In general, the elemental composition, H:C and O:C molar ratios, water content and density of a fuel affect its HHV which reflects its

3. Characteristics of bio-oil derived from PEFB Regardless of the production processes and conditions used, bio-oil derived from PEFB is usually an inhomogeneous liquid consisting of two phases: an organic-phase bio-oil (BOorg) which is dominated by aromatic and oxygenated organic compounds, and an aqueous-phase bio-oil (BOaq) which is mainly water [70]. The former is a viscous, dark

Table 1 Optimum production conditions of bio-oil derived from PEFB via fast pyrolysis and liquefaction. Production process

Reactor type Optimum production conditions

Product yields (wt%)

o

o

Catalyst (type; wt%)

T ( C) P (bar) Dp (mm) t (s)

Q ( C/min)

Op. atm. Solvent type PEFB: Solvent (g/g)

Bio-oil Biochar Syngas Ref.

Fast pyrolysis NCFP NCFPa NCFP NCFP NCFP NCFP NCFP NCFP NCFP NCFP NCFP NCFP CFP CFP CFPd

FBR FBR FBR FBR FBR FBR FXR FXR FXR FXR AUR FXR FXR FXR FXR

– – – – – – – – – – – – B2O3; 20 HY; 5 HZSM-5; 5

500 500 450 487 500 500 600 540 500 550 530 400 400 500 300

1 1 1 – 1 – – 1 1 <1 1 – – – –

< 0.5 < 0.5 < 0.5 ∼0.7 0.2 < 0.1 0.8 < 0.5 0.3 < 0.5 < 0.5 < 0.5 < 0.5 < 0.5 < 0.5

∼1 ∼1 1 2 – – 5400 31 10800 3600 450b – – – –

– – – – – 100 – 30 – 30 – 66 66 20 –

N2 N2 N2 N2 N2 Ar – N2 N2 Vacuum N2 N2 N2 N2 N2

– – – – – – – – – – – – – – –

– – – – – – – – – – – – – – –

34.71 61.34 55.10 53.97 27.00 42.00 36.49 48.40 30.00 55.00 54.80 23.00c 23.00c 44.10 30.00

24.52 10.76 23.90 28.90 50.00 22.00 18.51 29.60 – 25.00 24.90 30.00 38.00 – 42.00

22.31 14.70 18.60 17.10 23.00 36.00 45.00 17.80 – 20.00 20.30 42.00 30.00 – 28.00

[50] [50] [54] [55] [56] [57] [58] [59] [60] [61] [46] [62] [62] [63] [64]

Liquefaction NCSL NCSL NCSPL NCSPL NCSPLa NCSPLa CSL CSPL

ACR ACR ACR ACR ACR ACR ACR ACR

– – – – – – K2CO3; 6 CeO2; 1

275 350 400 390 350 350 270 390

– – – 250 291 291 20 250

0.8 0.2 0.4 < 0.7 – – 0.5–1.0 < 0.7

3600 18000 18000 3600 3600 1800 1200 3600

– – – – – – – –

N2 N2 N2 – – – N2 –

EG H2O MeOH H2O H2O H2OeEtOHg H2O H2O

1:2 1:8 1:17 1:10 1:8 1:8 1:5 1:10

78.00e 17.00f 19.60f 37.39 56.20 42.00 68.00 1.44h

3.60 64.00 70.02 – 11.50 8.00 27.60 –

– – – – – – – –

[42] [65] [66] [67] [68] [68] [53] [69]

T: temperature; P: pressure; Dp: particle size; t: residence time; Q: heating rate; Op. atm: operating atmosphere; NCFP: non-catalytic fast pyrolysis; CFP: catalytic fast pyrolysis; NCSL: non-catalytic subcritical liquefaction; NCSPL: non-catalytic supercritical liquefaction; CSL: catalytic subcritical liquefaction; CSPL: catalytic supercritical liquefaction; FBR: fluidized-bed reactor; FXR: fixed-bed reactor; AUR: Auger reactor; ACR: autoclave reactor; B2O3: boric oxide; CeO2: cerium(IV) oxide; K2CO3: potassium carbonate; HY, HZSM: zeolite catalysts; EG: ethylene glycol; N2: nitrogen gas; Ar: Argon gas; EG: ethylene glycol; H2O: water; EtOH: ethanol; MeOH: methanol; All of the PEFB used were unwashed with water prior to their use unless stated otherwise. a PEFB used was prewashed with water. b Screw rate in unit of rpm. c Aqueous-phase bio-oil increased by 4 wt% and organic-phase bio-oil reduced by 4 wt% with CFP compared to the corresponding NCFP. d PEFB used was pretreated consecutively with NaOH and H2O2. e Inclusive of syngas yield. f Organic-phase bio-oil yield only. g At 3:1 ratio. h Relative bio-oil yield with catalyst to that without catalyst. 265

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energy content and determines the amount of energy released during combustion, solid and ash contents of a fuel affect its tendency for carbon deposit formation which causes slagging and fouling in a combustion system, viscosity of a fuel affects its atomization quality and delivery rate in a combustion system, and pH and TAN of a fuel influence its acidity and corrosiveness which are imperative in determining the suitable construction materials for a combustion system [70,71]. As can be seen in Table 2, three types of bio-oil had been characterized: BOorg, BOaq, and BOmix, each of which was produced by either fast pyrolysis (NCFP or CFP) or liquefaction (NCSL, NCSPL, CSL or CSPL) in the absence or presence of a catalyst. All of the PEFB used were

unwashed with water prior to their use unless stated otherwise. For NCFP of unwashed PEFB, the BOorg produced normally contained somewhat higher carbon (58.65–69.35 wt%) and hydrogen (7.02–9.61 wt%) but lower oxygen (20.02–30.14 wt%) and nitrogen (0.74–2.74 wt%) than those (50.36–59.98 wt% of carbon, 5.08–8.81 wt % of hydrogen, 33.72–37.21 wt% of oxygen and 0.01–4.45 wt% nitrogen) of BOmix (Table 2). According to an empirical correlation proposed by Demirbas et al. [72], the carbon and hydrogen contents of a lignocellulosic biofuel are directly proportional to its HHV while the oxygen and nitrogen contents are inversely proportional to it. This is in good agreement with the HHVs of both BOorg and BOmix where the HHV

Table 2 Physicochemical properties of bio-oil derived from PEFB and other biomass along with those of petroleum fuel oil. Conversion Process or Type

Ultimate analysis (wt%a)

C Bio-oil derived from PEFB NCFP – BOorg 58.65 NCFP – BOorg – NCFP – BOorg 69.35 NCFPd – BOorg 41.86 NCFP – BOaq 13.83 NCFP – BOaq 11.47 NCFP – BOaq – NCFP – BOmix 53.75 NCFP – BOmix 50.36 NCFP – BOmix 59.98 CFP – BOmix 25.33 CFP – BOmix – NCSPL – BOorg 75.28 Sv: Ethanol NCSL – BOorg 70.80 Sv: Toluene NCSL – BOorg 40.81 Sv: Ethylene glycol 39.93 NCSPL – BOorg Sv: Acetone 10.82 NCSL – BOorg Sv: Water Bio-oil derived from other biomass NCFP – BOorg 62.6 B: Microalgae 65.2 NCFP – BOorg B: Jatropha seed waste NCFP – BOorg 49.3 B: Peanut shell NCSPL – BOorg 78 B: Duckweed Sv: Hexane/Petroleum ether 70 NCSPL - BOorg B: Duckweed Sv: Isopropanol Petroleum fuel oil Diesel 86.52 LFO 86 LSHFO 87.3 HSHFO 85.6

• • • • • • • • • • • • • • • • • • • • • •

b

H:Cc

H

O

N

S

7.02 – 9.61 7.82 11.4 9.64 – 8.81 7.83 5.08 9.32 – 9.57

30.14 – 20.02 33.94 74.56 78.83 – 36.04 37.21 33.72 64.59 – 14.73

2.74 – 0.74 0.10 0.14 0.04 – 1.40 4.45 0.01 0.32 – 0.42

< 0.1 – – – – 0.02 – – 0.16 < 0.1 0.45 – –

1.44 – 1.66 2.24 9.89 10.09 – 1.97 1.87 1.02 4.42 – 1.53

8.68

20.17

0.41

–

9.50

49.56

0.13

4.13

55.70

6.64

O:Cc

Water

Solids

Ash

HHV

Density

Viscosity

@ 25 °C

@ 25 °C

3

pH

TAN

Ref.

(wt%)

(wt%)

(wt%)

(MJ/kg)

(g/cm )

(cP)

(mg KOH/g)

0.39 – 0.22 0.61 4.04 5.15 – 0.50 0.55 0.42 1.91 – 0.15

– 1.61 7.9 21.68 64.01 65 72.27 18.21 – 24.30 50 – –

– – – – – – – – <2 13.5 – – –

– – – – – – – 0.37 – 2.43 – – –

24.90 34.91 36.06 20.32 – – – 21.17 21.62 19.80 10.43 – 29.42

– 0.958 – 1.206 – – 0.972 0.99 1.15–1.2 – 1.07e 0.832g –

– – – 46.31 – – – – – – 45.47f 1.664f –

– – – 2.70 – – 3.94 3 2.9 – 2.69 3.4 –

110 – – – – – – 67.75 – – – – –

[55] [59] [50,70] [50,70] [50,70] [56] [59] [57] [60] [71] [63] [64] [42]

1.47

0.21

–

–

–

27.6

–

–

–

–

[42]

–

2.79

0.91

–

–

–

19.67

–

–

–

–

[42]

0.24

–

1.24

1.05

–

–

–

15.82

–

–

–

–

[42]

82.54

0.65

–

7.36

5.72

–

–

–

9.31

–

–

–

–

[42]

8.77

22.5

8.8

< 0.1

1.68

0.27

–

–

–

29.6

1.09

–

–

162.6

[73]

8.49

18.8

6.5

0.14

1.56

0.22

–

–

–

31.2

1.08

–

–

112.5

[73]

8.1

39.7

2.9

–

1.97

0.60

51.3

–

–

22.5

–

–

3.1

–

[52]

10

6

5

0.8

1.54

0.06

–

–

–

40

–

–

–

–

[74]

9

14

7

0.5

1.54

0.15

–

–

–

34

–

–

–

–

[74]

13.30 13.6 12.19 10.3

0.03 0 0.17 0.6

0.04 0.2 0.06 0.6

0.11 < 0.2 0.28 2.5

1.84 1.90 1.68 1.44

0 0 0 0.01

– 0.025 – 0.1

– – – <1

– 0.01 – 0.03

45.8 43.18 44.7 42.89

0.853h 0.89h 0.939h 0.95h

2.601i 2.6–6.68f – 333.45i

7 7 – –

– – 0.451 –

[75] [76] [55] [76]

NCFP: non-catalytic fast pyrolysis; CFP: catalytic fast pyrolysis; NCSL: non-catalytic subcritical liquefaction; NCSPL: non-catalytic supercritical liquefaction; BOorg: organic-phase bio-oil; BOaq: aqueous-phase bio-oil; BOmix: mixture of organic- and aqueous-phase bio-oil; HHV: higher heating value; TAN: total acid number; C: carbon; H: hydrogen; O: oxygen; N: nitrogen; S: sulfur; Sv: solvent; B: biomass; LFO: light fuel oil; LSHFO: low sulfur heavy fuel oil: HSHFO: high sulfur heavy fuel oil; All of the PEFB used were unwashed with water prior to their use unless stated otherwise. a On dry weight basis. b Calculated by difference. c Molar ratio. d PEFB used was prewashed with water. e At 20 °C. f At 40 °C. g At 30 °C. h At 15 °C. i At 50 °C. 266

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(24.90–36.06 MJ/kg) of BOorg was relatively higher than that (19.80–21.62 MJ/kg) of BOmix owing to its larger carbon and hydrogen and smaller oxygen and nitrogen contents (Table 2). In addition, the substantially lower water content of BOorg (1.61–7.9 wt%) than that (18.21–24.30 wt%) of BOmix also led to its higher HHV, even though the H:C molar ratio (1.44–1.66) and density (0.958 g/cm3) of BOorg were marginally lower than those (H:C of 1.02–1.97; density of 0.99–1.2 g/ cm3) of BOmix (Table 2). The HHV of BOaq was, however, not reported in the literature due to its extremely high water content (64.01–72.27 wt%) (Table 2). This high water content of BOaq had given rise to its significantly lower carbon (11.47–13.83 wt%) and nitrogen (0.04–0.14 wt%) contents but higher hydrogen (9.64–11.4 wt%) and oxygen (74.56–78.83 wt%) contents than those of either BOorg or BOmix (Table 2). Therefore, it is fair to say that BOorg is the most favorable phase of bio-oil for fuel application as compared to BOmix and BOaq on account of its highest HHV (24.90–36.06 MJ/kg (Table 2)) which is also considerably greater than that (17.02–19.35 MJ/kg [9]) of PEFB fibers. For NCFP of washed PEFB, on the other hand, the BOorg produced was found to possess a significantly lower HHV (20.32 MJ/ kg) than its unwashed counterpart (36.06 MJ/kg) (Table 2). This could be explained by its considerably greater oxygen content (33.94 wt% vs. 20.02 wt%), larger O:C molar ratio (0.61 vs. 0.22) and substantially higher water content (21.68 wt% vs. 7.9 wt%) as a result of a large amount of water used during PEFB washing. Meanwhile, the BOmix produced from NCFP of unwashed PEFB was found to exhibit a considerably greater HHV (19.80–21.62 MJ/kg) than that (10.43 MJ/kg) produced from CFP of unwashed PEFB (Table 2). This could be deduced from the substantially smaller water content of the former (18.21–24.30 wt%) than that (50 wt%) of the latter following the enhanced deoxygenation and dehydrogenation reactions by catalysts which generated a large amount of water at the expense of organic oil in the latter [63]. To sum up, between NCFP of unwashed and washed PEFB, the former produced bio-oil with a higher HHV (Table 2) but a lower yield (Table 1) than the latter, whereas between NCFP and CFP of unwashed PEFB, the former produced bio-oil with not only a higher HHV (Table 2) but also a greater yield (Table 1) than the latter. For liquefaction of PEFB, the BOorg produced from NCSPL of PEFB with ethanol as solvent was found to achieve the highest HHV (29.42 MJ/kg), followed by BOorg from NCSL of PEFB with toluene as solvent (27.6 MJ/kg), BOorg from NCSL of PEFB with ethylene as solvent (19.67 MJ/kg), BOorg from NCSPL of PEFB with acetone as solvent (15.82 MJ/kg) and, finally, BOorg from NCSL of PEFB with water as solvent (9.31 MJ/kg) (Table 2). Among these BOorg, only those produced by NCSPL of PEFB with ethanol as solvent and by NCSL of PEFB with toluene as solvent had HHVs, i.e. 29.42 MJ/kg and 27 MJ/kg, respectively, that were on par with those produced by NCFP of PEFB (unwashed), i.e. 24.90–36.06 MJ/kg (Table 2). In spite of their compatible HHVs, it is interesting to note that the carbon (70.80–75.28 wt %) and oxygen (14.73–20.17 wt%) contents of the BOorg produced by NCSL or NCSPL of PEFB were somehow higher and lower than those (58.65–69.35 wt% of carbon and 20.02–30.14 wt% of oxygen) of the BOorg produced by NCFP of PEFB, which resulted in a lower O:C molar ratio (0.15–0.21) of the former than that (0.22–0.39) of the latter. This suggests that liquefaction of PEFB has a tendency to produce bio-oil with a lower O:C molar ratio, which is a desirable property for highquality fuels [57], than fast pyrolysis of PEFB. Next, the HHVs of bio-oil derived from PEFB were compared with those of other biomass-derived bio-oil and petroleum fuel oil. It was found that the HHVs of BOorg produced from NCFP of PEFB (unwashed) (24.90–36.06 MJ/kg) were quite compatible with those of BOorg produced from NCFP of other biomass such as microalgae (29.6 MJ/kg), Jatropha seed waste (31.2 MJ/kg) and peanut shell (22.5 MJ/kg). Nevertheless, the HHV of BOorg produced from NCSPL of PEFB with ethanol as solvent (29.42 MJ/kg) was found to be relatively lower than those of BOorg produced from NCSPL of duckweed with a mixture of hexane and petroleum ether as solvent (40 MJ/kg) and with

isopropanol as solvent (34 MJ/kg) (Table 2). All of the HHVs obtained for bio-oil derived from both PEFB and other biomass were well below 40 MJ/kg which were somewhat lower than those of petroleum fuel oil that had HHVs well above 42 MJ/kg (Table 2). The greater HHVs of petroleum fuel oil were attributed to their considerably higher carbon (85.6–87.3 wt%) and hydrogen (10.3–13.6 wt%) contents, tremendously lower oxygen (0–0.6 wt%) and nitrogen (0.04–0.6 wt%) contents, significantly lower O:C (0–0.01) molar ratio, as well as their substantially lower water content (0.025–0.1) than those of bio-oil (Table 2). Moreover, bio-oil also contained more solids (< 2–13.5 wt%) and ash (0.37–2.43 wt%) but less sulfur (0.02–0.8 wt%), as well as exhibited slightly higher density (0.832–1.206 g/cm3 @ 25–30 °C), lower pH (2.70–3.94) and higher TAN (67.75–162.6 mg KOH/g) than different types of petroleum fuel oil (< 1 wt% of solids, 0.01–0.03 wt% of ash, 0.11–2.5 wt% of sulfur, density of 0.853–0.95 g/cm3 @ 15 °C, pH of 7 and TAN of 0.451 mg KOH/g) (Table 2). The viscosity of bio-oil (1.664–46.31 cP @ 25–40 °C), on the other hand, varied from slightly below those of diesel and light fuel oil (LFO) (2.60–6.68 cP @ 40–50 °C) to somewhere between those of diesel and LFO and that of high sulfur heavy fuel oil (HSHFO) (333.45 cP @ 50 °C) (Table 2). This implies that bio-oil is more prone to carbon deposit formation, more acidic and corrosive than petroleum fuel oil, but emits less sulfur oxides during combustion and has a viscosity that falls within the range of those of petroleum fuel oil. It should also be highlighted that only about 1.2 kg of the bio-oil derived from PEFB is required to generate the same amount of energy as 1 kg of petroleum fuel oil based on the highest HHV of 36.06 MJ/kg for the PEFB-derived bio-oil and the lowest HHV of 42.89 MJ/kg for the petroleum fuel oil as given in Table 2. 3.2. Major chemical components and their compositions Bio-oil is a mixture of multiple polar and non-polar compounds with a vast range of molecular weights (72–1927 g/mol [77]) resulting from depolymerization and fragmentation of cellulose, hemicellulose, and lignin in lignocellulosic biomass. Its chemical composition is a function of feedstock and production process, among other parameters [78]. Table 3 shows the major chemical components and their compositions (measured in % peak area by GC-MS) in the BOorg derived from PEFB which was produced by either fast pyrolysis (NCFP or CFP) or liquefaction (NCSL, NCSPL, CSL or CSPL) in the absence or presence of a catalyst. All of the PEFB used were untreated with chemicals prior to their use unless stated otherwise. As can be seen in Table 3, the BOorg consists predominantly of aromatic and oxygenated compounds, in addition to some alkanes and alkenes, regardless of the types of production processes employed. The aromatic compounds are made up primarily of simple phenols and their derivatives, along with other aromatic compounds like benzene, xylene, pyridine, and furan derivatives, while the oxygenated compounds are composed of carboxylic acids, esters, ketones, aldehydes, and alcohols. In general, the aromatic compounds are degraded from lignin by an assortment of reactions such as the dehydration reaction of hydroxyl groups in the alkyl chains of phenylpropanes, i.e. the building blocks of lignin, followed by the cleavage of inter-aromatic bonds in lignin [64], as well as the hydrolysis reaction of ether bonds in lignin [79]. The oxygenated compounds, alkanes, and alkenes, on the other hand, are generated from the decomposition of cellulose and hemicellulose through a series of hydrolysis, decarboxylation, dehydration, fermentation and/or isomerization reactions [53]. Among these chemical components, aromatic compounds, alkanes and alkenes are the desirable ones for high-quality biooil owing to their high carbon and hydrogen contents that bring beneficial effects to the HHV [42] and octane rating [80] of bio-oil, while oxygenated compounds are the undesirable ones due to their high oxygen contents that cause detrimental effects on the quality of bio-oil [81]. The excessive amount of oxygenated compounds also leads to thermal and storage instability [82], high viscosity, as well as high acidity of bio-oil particularly in the presence of a large amount of 267

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S.H. Chang

carboxylic acids [83]. The latter would give rise to a number of corrosion issues for the construction materials of bio-oil production, processing, transport and storage systems. Table 3 reveals that the BOorg produced from NCFP of PEFB contained a considerably lower amount of aromatic compounds than its CFP counterpart (40.75% vs. ∼65%). This implies that catalysts enhanced the conversion of lignin into aromatic compounds during fast pyrolysis of PEFB. To boost the amount of aromatic compounds in the BOorg produced by NCFP, pretreated PEFB by consecutive treatment of NaOH and H2O2 was used as feedstock which virtually doubled its amount of aromatic compounds, mostly of phenol derivatives, from 40.75% to ∼70% (Table 3). This remarkable enhancement in the amount of aromatic compounds was attributed to the complete breakdown of lignin in pretreated PEFB through the synergistic reactions of lignin with NaOH in largely hydrolyzing chlorolignin and with H2O2 in further oxidizing the lignin structure [64]. When this pretreated PEFB

was subjected to CFP, the amount of aromatic compounds in BOorg was further increased to ∼80% and ∼90% by using microporous HZSM-5 and mesoporous Al-MCM-41 zeolite catalysts, respectively, but was remained unchanged at ∼70% with microporous HY zeolite catalyst (Table 3). The excellent performance of Al-MCM-41 was due to its high porosity, stemming from its largest particle size of 19.5 nm as compared to 14.3 nm for HZSM-5 and 14.5 nm for HY, which increased the accessibility of molecules to its active sites and improved its catalytic activity [64]. For oxygenated compounds, alkanes, and alkenes, on the other hand, their compositions in BOorg produced by both NCFP and CFP of PEFB, with and without pretreatment, were found to differ from each other by up to 10% only (Table 3). This indicates the relatively smaller effects of catalysts and PEFB pretreatments on the conversion of cellulose and hemicellulose into oxygenated compounds, alkanes and alkenes compared to their effects on the lignin conversion into aromatic compounds. Nonetheless, it is interesting to note that CFP of pretreated

Table 3 Major chemical components and their compositions in BOorg derived from PEFB. Conversion Process

% Peak area measured by GC-MS

Ref.

Aromatic compounds

Oxygenated compounds

Phenols

Phenol derivatives

Othersa

Carboxylic acids

Esters

Ketones & aldehydes

Alcohols

Alkanes & alkenes

18.10 – –

15.27 ∼70 ∼65

7.38 – –

<5 ∼5 <5

– <5 <5

9.35 ∼10 ∼10

<5 ∼10 ∼10

– ∼10 –

[57] [64] [64]

b

–

∼80

–

∼10

<5

∼10

∼10

∼10

[64]

b

–

∼90

–

0

<5

∼10

<5

∼10

[64]

b

–

∼70

–

∼5

<5

∼10

∼10

∼10

[64]

11.39

42.7

–

–

6.76

<5

5.63

20.77

[42]

19.22

63.83

<5

–

7.07

–

–

–

[42]

–

–

–

–

35.92

52.26

11.83

–

[42]

–

–

–

<5

<5

–

95.25

<5

[42]

59.68

5.27

–

16.31

–

–

–

8.73

[42]

–

–

–

–

26.44

–

–

–

[53]

60.08

–

–

–

39.92

–

–

–

[53]

18.4

–

–

–

81.6

–

–

–

[53]

6

–

–

–

86.45

–

–

–

[53]

<5

<5

86.13

<5

–

<5

–

–

[69]

31.52

5.85

12.56

12.92

–

37.17

–

–

[69]

–

43.03

12.09

11.36

–

33.51

–

–

[69]

26.68

12.9

17.22

8.14

–

35.98

–

–

[69]

• NCFP • NCFP •Cat:CFPHZSM-5 •Cat:CFPHZSM-5 •Cat:CFPAl-MCM-41 •Cat:CFPHY •Sv:NCSL Ethanol •Sv:NCSL Toluene •Sv:NCSL Acetone •Sv:NCSL Ethylene glycol •Sv:NCSL Water •Sv:NCSL Water •Cat:CSLK CO b

2

Sv: Water

•Cat:CSLKOH

Sv: Water

3

•Cat:CSLNaOH Sv: Water

•C:CSPL La O 2

3

S: Water

•Cat:CSPLCeO

2

Sv: Water

•Cat:CSPLMnO

Sv: Water

•Cat:CSPLCaO

Sv: Water

NCFP: Non-catalytic fast pyrolysis; CFP: Catalytic fast pyrolysis; NCSL: Non-catalytic subcritical liquefaction; CSL: Catalytic subcritical liquefaction; CSPL: Catalytic supercritical liquefaction; Cat: Catalyst; Sv: Solvent; HZSM-5 and HY: Microporous zeolites; Al-MCM-41: Mesoporous alumininosilicate zeolite; K2CO3: Potassium carbonate; KOH: Potassium hydroxide; NaOH: Sodium hydroxide; La2O3: Lanthanum oxide; CeO2: Cerium(IV) oxide; MnO: Manganese(II) oxide; CaO: Calcium oxide; All of the PEFB used were untreated with chemicals prior to their use unless stated otherwise. a Benzene, xylene, pyridine and furan derivatives. b PEFB used was pretreated consecutively with NaOH and H2O2. 268

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S.H. Chang

PEFB with Al-MCM-41 catalyst was able to completely remove carboxylic acids from BOorg while increasing the amounts of alkanes and alkenes by 10% in comparison with both NCFP and CFP of untreated PEFB (Table 3). This suggests the importance of catalysts and PEFB pretreatments in producing high-quality bio-oil. For NCSL or NCSPL of PEFB with different solvents (ethanol, toluene, acetone, ethylene glycol and water), the major components formed in the BOorg produced varied from one solvent to another (Table 3). As shown in Table 3, it seems that aromatic compounds, mainly phenols and their derivatives, were the major components of BOorg when ethanol (11.39% of phenols and 42.7% of phenol derivatives) and toluene (19.22% of phenols and 63.83% of phenol derivatives) were used as solvents, but esters (35.92%), ketones and aldehydes (52.26%), as well as alcohols (95.25%), were the major components when acetone and ethylene glycol were used as solvents, respectively. This denotes the greater efficiencies of ethanol and toluene than acetone and ethylene glycol in lignin degradation for producing aromatic compounds in BOorg. Nevertheless, there is a discrepancy in the major components of BOorg produced from NCSL of PEFB with water as solvent between Fan et al.’s [42] and Akhtar et al.’s [53] works, where Fan et al. [42] claimed that aromatic compounds, or more precisely, phenols (59.68%) were the major components while Akhtar et al. [53] asserted that oxygenated compounds, or more specifically, esters (26.44%) were the major components. This discrepancy may be deduced from the much longer residence time of 60 min used by Fan et al. [42], as opposed to just 20 min by Akhtar et al. [53], which prompted a more complete degradation of the complex and highly cross-linked structures of lignin to produce aromatic compounds. Akhtar et al. [53], however, managed to boost the negligible amount of phenols and 26.44% of esters in BOorg produced from NCSL of PEFB to 60.08% of phenols and 39.92% of esters, 18.4% of phenols and 81.6% of esters, as well as 6% of phenols and 86.45% of esters by catalyzing the hydrothermal liquefaction process with different alkaline-based catalysts, namely, potassium carbonate (K2CO3), potassium hydroxide (KOH) and sodium hydroxide (NaOH), respectively. This suggests that catalysts played an important role in converting lignin into aromatic compounds during liquefaction similar to the case for fast pyrolysis. Since the esters found in BOorg derived from PEFB comprised mostly of fatty acid methyl esters (FAME) [53], which represent the bulk components of biodiesel that has HHV of 39.4–45.2 MJ/kg [84], the presence of a large amount of FAME in BOorg implies that it could be used as biodiesel. Meanwhile, CSPL of PEFB with various metal oxide catalysts and water as a solvent were found to produce BOorg with over 50% of aromatic compounds (Table 3). The major aromatic compounds produced from these hydrothermal liquefaction processes were other aromatic compounds (86.13%) with lanthanum oxide (La2O3) catalyst, phenols (31.52%) and other aromatic compounds (12.56%) with cerium (IV) oxide (CeO2) catalyst, phenol derivatives (43.03%) and other aromatic compounds (12.09%) with manganese oxide (MnO) catalyst, as well as phenols (26.68%), phenol derivatives (12.9%) and other aromatic compounds (17.22%) with calcium oxide (CaO) catalyst (Table 3). The latter three metal oxide catalysts were also found to produce BOorg with substantial amounts of ketones and aldehydes (33.51–37.17%) (Table 3). Although carboxylic acids were not the major components in the BOorg derived from PEFB, their amounts could reach as high as 16.31% (Table 3) which brought about their low pH of 2.7 and high TAN of 110 mg KOH/g (Table 2). In addition, the large amounts of a wide variety of aromatic and oxygenated compounds in the BOorg also led to its high density of 0.958–1.206 g/cm3 @ 25 °C and viscosity of 46.31 cP @ 25 °C (Table 2). These unfavorable properties of BOorg have prohibited its direct fuel application in various combustion engines, among other factors.

4. Upgrading of bio-oil derived from PEFB In spite of the superior environmental benefits of bio-oil like carbon neutrality, negligible sulfur oxides emission and over 50% lower nitrogen oxides emission than fossil fuels [8], the direct application of bio-oil as an engine fuel is often restricted by its inferior physicochemical properties such as high water, oxygen, acid, solid and ash contents to that of fossil fuels (Table 2), as well as its poor thermal and storage stability [82]. Therefore, bio-oil has to be upgraded by methods such as hydrotreating, hydrocracking, solvent addition, fuel blending or emulsification, esterification, supercritical treatment, catalytic cracking and different reforming processes before it could perform on par with, if not better than, the fossil fuels [8,85]. These upgrading methods may vary from one to another, but they generally possess similar aims, that is, to enhance the physicochemical properties and stability of bio-oil or to convert bio-oil into high-octane fuels like bio-gasoline, methane- and hydrogen-rich syngas that possess favorable physicochemical properties as engine fuels [8,85]. The fundamental features, working principles, as well as merits and demerits of each of these upgrading methods for biooil are well documented in the literature [8,85]. Table 4 shows the various upgrading methods that have been used on the bio-oil derived from PEFB along with their upgrading objectives, conditions, and outcomes. The upgrading methods used include solvent addition, fuel blending or emulsification, esterification, supercritical treatment, catalytic cracking, catalytic hydrocracking, steam reforming, autothermal reforming and partial oxidation. Among these upgrading methods, solvent addition is the simplest one which improves the physicochemical properties and stability of bio-oil by simply adding a polar solvent to it [8]. Polar solvents are preferable to nonpolar ones in this method since they could readily dissolve the large amount of polar compounds in bio-oil based on the ‘like dissolves like’ aphorism [86]. Yii et al. [87] added different polar solvents, namely, acetone, ethyl acetate and ethanol to the bio-oil derived from PEFB and studied its stability under accelerated aging at 80 °C for seven days. They discovered that ethanol was the most effective solvent in stabilizing the bio-oil, followed by ethyl acetate and acetone, and no phase separation was observed in the bio-oil added with ethanol compared to those added with ethyl acetate and acetone throughout the accelerated aging process. Although considerable improvements in the viscosity and heating value of bio-oil could be achieved with solvent addition, the quality of the upgraded bio-oil by this method is often not as good as those upgraded by other methods [8]. Fuel blending or emulsification is another simple upgrading method which enhances the physicochemical properties of bio-oil by blending it with an additional fuel in the presence of an emulsifier [78]. Emulsification is indispensable when blending the highly polar bio-oil with non-polar fossil fuels like diesel. Khor et al. [75] blended the bio-oil derived from PEFB with diesel in different proportions (5–75 wt% of bio-oil in diesel) with the aid of 5 wt% of Hypermer B246SF as emulsifier and succeeded in enhancing the physicochemical properties of bio-oil such as carbon, hydrogen, oxygen and ash contents, flash and pour points, HHV, pH, density and viscosity after the blending. Nevertheless, fuel blending suffers from a few drawbacks such as high consumption of pricey fossil fuel to meet the fuel standards, expensive emulsifier required and antagonistic side reactions of an emulsifier with the compounds in bio-oil that deteriorate the thermal and storage stability of the bio-oil blends [78]. Esterification, which improves the physicochemical properties of bio-oil by reacting it with an alcohol in the presence of an acid catalyst under mild conditions via reactive distillation [8], is another viable upgrading method for bio-oil due to its abundant reactive oxygenated compounds like carboxylic acid, ketones, and aldehydes (Table 3). During esterification, the reactive oxygenated compounds in bio-oil are

269

270

To improve the physicochemical properties of bio-oil in supercritical ethanol without using a catalyst and external hydrogen

To improve the physicochemical properties of bio-oil by reacting it with an alcohol in the presence of an acid catalyst via reactive distillation

Esterification

Supercritical treatment

To improve the physicochemical properties of bio-oil by blending it with diesel in different proportions

Fuel blending or emulsification

To improve the physicochemical properties of bio-oil in different supercritical alcohols without using a catalyst and external hydrogen

To improve the physicochemical properties and stability of bio-oil by adding different solvents to it

Solvent addition

Supercritical treatment

Objective

Upgrading method

Table 4 Upgrading methods that have been used on bio-oil derived from PEFB.

feedstock used was the heavy-fraction bio-oil • Bio-oil with concentration of 9.1 wt% 400 °C • Temperature: 347–378 bar • Pressure: • Reaction time: 30 min

and isopropyl alcohol, respectively

• Reaction time: 30 min

4

3

contents exceeding 80 wt% and HHV about 40 MJ/kg

of bio-oil increased tremendously from being too low to • HHV 27 MJ/kg after the upgrade of bio-oil decreased from 0.87 to 0.73 g/cm after the • Density upgrade content of bio-oil decreased from 30 to 10 wt% after the • Water upgrade aldehyde and ketone contents of bio-oil decreased after the • Acid, upgrade alkane, alkene and aromatic compound contents of bio-oil • Ester, increased after the upgrade of bio-oil increased significantly from 12.5 to 29.9, 30.5 and • HHV 28.9 MJ/kg after treatment in supercritical methanol, ethanol and

respectively

change of water content of bio-oil added with acetone, • Percentages ethyl acetate and ethanol were 6.63, 4.10 and 0.75%, respectively was the most effective solvent, followed by ethyl acetate • Ethanol and acetone phase separation was observed for bio-oil added with ethanol • No after seven days of accelerated aging at 80 °C and hydrogen contents, flash and pour points, pH and HHV • Carbon of bio-oil-diesel blends were higher than those of pure bio-oil and oxygen contents, density, viscosity and corrosiveness of • Ash bio-oil-diesel blends were lower than those of pure bio-oil blends with 5–25 wt% bio-oil exhibited similar • Bio-oil-diesel physicochemical properties to neat diesel, with their carbon

change of viscosity of bio-oil added with acetone, • Percentages ethyl acetate and ethanol were 79.09, 64.60 and 38.31%,

Outcomes

[90]

[89]

[88]

[75]

[87]

Ref.

(continued on next page)

of bio-oil increased considerably from 24.3 to 34.1 MJ/kg • HHV after the upgrade content of bio-oil decreased substantially from 14.0 to • Water 1.6 wt% after the upgrade of bio-oil decreased remarkably from 69.4 to 4.8 mg KOH/g • TAN after the upgrade content of bio-oil decreased remarkably from 9.9 to 0.0 wt% • Ash after the upgrade

bio-oil obtained and relatively cheap price of methanol

methanol treatment was the most favorable method • Supercritical in view of its highest energy efficiency, lowest TAN of upgraded

the different supercritical treatments studied

methanol treatment exhibited the highest energy • Supercritical efficiency since it consumed the lowest amount of alcohol among

and isopropyl alcohol, respectively

and isopropyl alcohol, respectively

of bio-oil decreased remarkably from 92.2 to 4.0, 10.8 and • TAN 53.4 mg KOH/g after treatment in supercritical methanol, ethanol

feedstock used was the light-fraction bio-oil • Bio-oil with concentration of 9.1 wt% isopropyl alcohol, respectively 400 °C • Temperature: 347–366 bar, 259–284 bar and 225–272 bar Water content of bio-oil decreased substantially from 23.7 to 4.4, • Pressure: • for supercritical treatment with methanol, ethanol 3.8 and 6.0 wt% after treatment in supercritical methanol, ethanol,

2

feedstock used was derived from pretreated • Bio-oil PEFB with NaOH 60 °C • Temperature: 1.6 wt% H SO • Catalyst: • Bio-oil to n-butanol volume ratio of 1:1 was used

concentration was varied from 5 to 75 wt% in • Bio-oil different bio-oil-diesel blends • Emulsifier: 5 wt% Hypermer B246SF

types: Acetone, ethyl acetate and ethanol • Solvent • Solvent concentration: 10 wt%

Conditions

S.H. Chang

Biomass and Bioenergy 119 (2018) 263–276

Objective

To convert bio-oil into high-quality volatile organic liquids by cracking it under mild pressure with the aid of a catalyst but without using external hydrogen

To convert bio-oil into high-quality volatile organic liquids by cracking it under high pressure with the aid of a catalyst and external hydrogen

To convert bio-oil into methane-rich syngas by reacting it with steam at low temperature

To convert bio-oil into hydrogen-rich syngas by different reforming processes, i.e. steam reforming, autothermal reforming and partial oxidation

Upgrading method

Catalytic cracking

Catalytic hydrocracking

Steam reforming

Steam reforming, autothermal reforming, and partial oxidation

Table 4 (continued)

respectively

to carbon molar ratio used in partial oxidation • Oxygen reactor was 3:10 was carried out based on the actual bio-oil • Simulation composition by Aspen Plus

program

700 °C for steam and autothermal • Temperature: reformers and 850 °C for partial oxidation reactor 1 bar • Pressure: to carbon molar ratio used in steam reformer • Steam was 2:1 to carbon and oxygen to carbon molar ratios • Steam used in autothermal reformer were 1:1 and 1:10,

and boiling point with those of commercial gasoline Syngas consisting of 44.5 vol% methane, 42.7 vol% carbon dioxide and 12.7% hydrogen was produced

309 °C • Temperature: 1 bar • Pressure: •Steam to carbon molar ratio of 3:1 was carried out based on the actual bio-oil • Simulation composition by Chemical Equilibrium and Application

oxidation reactor (34%)

consisting of 67 vol% hydrogen and 4.15 vol% carbon • Syngas monoxide was produced by steam reformer consisting of 64 vol% hydrogen and 2.82 vol% carbon • Syngas monoxide was produced by autothermal reformer consisting of 61 vol% hydrogen and 2.58 vol% carbon • Syngas monoxide was produced by partial oxidation reactor highest reformer efficiency was achieved by steam reformer • The (66%), followed by autothermal reformer (51%) and partial

liquid and bio-gasoline yields of 91.20% and 46.67%, • Organic respectively, were obtained produced contained mainly saturated hydrocarbons • Bio-gasoline and alkylbenzenes produced showed compatible physicochemical • Bio-gasoline properties such as carbon, hydrogen and oxygen contents, density

7 wt%

over silica-alumina catalyst was increased from 1 to 7 wt%

deposit yield decreased from 10.22 to 2.29% when Ni • Carbon concentration over silica-alumina catalyst was increased from 1 to

of bio-oil increased from 28.36 to 34.40–36.41 MJ/kg after • HHV the upgrade and bio-kerosene yields increased from 15.41 to • Bio-gasoline 17.52% and 11.55–15.09%, respectively, when Ni concentration

Outcomes

400 °C • Temperature: Not given • Pressure: time: 15 min • Reaction • Catalyst: 14 wt% ZSM-5 zeolite

500 °C • Temperature: 1 bar • Pressure: • Catalyst: 0.24 wt% Ni/silica-alumina

Conditions

[101]

[99]

[95]

[94]

Ref.

S.H. Chang

Biomass and Bioenergy 119 (2018) 263–276

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deoxygenation reactions like dehydration, decarboxylation and decarbonylation for producing gases and water, and polymerization reaction for coke formation [85,93]. Sunarno et al. [94] upgraded the biooil derived from PEFB by catalytic cracking with nickel-doped silicaalumina catalyst and observed substantial increments in the HHV, biogasoline and bio-kerosene yields, as well as a significant reduction in the coke yield, with increasing nickel doping concentrations on the catalyst. This suggests the important role of nickel as a catalyst support in promoting the cracking, deoxygenation, hydrogenation and aromatization reactions, as well as in suppressing the polymerization reaction, during the catalytic cracking of bio-oil. On the other hand, Hew et al. [95] employed a catalytic hydrocracking method using ZSM5 zeolite catalyst to upgrade the PEFB-derived bio-oil and succeeded in producing high yields of volatile organic liquids, particularly bio-gasoline, from the bio-oil. They also claimed that the bio-gasoline produced showed compatible physicochemical properties such as carbon, hydrogen and oxygen contents, density and boiling point with those of commercial gasoline. However, field testing of the bio-gasoline on real engines is needed to examine its actual quality against the commercial gasoline. Reforming is also a potential upgrading method for bio-oil. It converts bio-oil into syngas by reacting it with steam (known as steam reforming) mainly, but also with oxygen (known as partial oxidation) and a mixture of steam and oxygen (known as autothermal reforming) in the presence of a catalyst at 300–950 °C under atmospheric pressure [96]. While most of the focus of bio-oil reforming in previous research was on the production of hydrogen-rich syngas [97,98], production of methane-rich syngas through this avenue had also been reported [99]. The latter was synthesized through the Sabatier or methanation reaction where carbon monoxide, carbon dioxide and hydrogen generated from the preceding reforming reactions reacted with each other to form methane and water at 200–500 °C and 10–30 bar [100]. Both hydrogen and methane are important energy sources of modern urban society in which hydrogen is largely used to produce electricity through fuel cells [101] while methane, a major component of natural gas, is consumed dominantly in commercial and residential buildings, as well as in power plants [99]. Yun et al. [99] simulated the methanation of bio-oil derived from PEFB through a low-temperature steam reforming process by using the Chemical Equilibrium and Application program. They obtained methane-rich syngas consisting of 44.5 vol% of methane under the optimum operating condition. Authayanun et al. [101], on the other hand, simulated the hydrogen production of bio-oil derived from PEFB through different reforming processes, namely, steam reforming, autothermal reforming and partial oxidation by using the Aspen Plus program. All of the reforming processes studied produced syngas with hydrogen as the major component (61–67 vol%) under their respective optimum operating conditions and the highest reformer efficiency was

converted into esters and acetals, which in turn reduce the acidity and viscosity while increasing the heating value and stability of bio-oil [8]. Amin et al. [88] employed esterification to upgrade the bio-oil derived from NaOH-pretreated PEFB by reacting it with n-butanol in the presence of a catalyst of sulfuric acid via reactive distillation and achieved a considerable enhancement in the HHV, density and water content of bio-oil after esterification. However, a large amount of volatile and toxic n-butanol (50 vol%) was required for this method. Another attractive upgrading method for bio-oil is supercritical treatment. This method takes advantage of the exceptional transport properties and reactivity of supercritical fluids to improve the physicochemical properties of bio-oil by removing its high oxygen content, enhancing the cracking of pyrolytic lignin into smaller molecules and suppressing the re-polymerization reaction during storage, with or without using a catalyst and external hydrogen [89]. Jo et al. [89] treated the light fraction of the bio-oil derived from PEFB in different supercritical alcohols, i.e. methanol, ethanol and isopropyl alcohol, without using a catalyst and external hydrogen. They observed a significant enhancement in the HHV, water content and TAN of bio-oil after the treatment, with supercritical methanol treatment being the most favorable one owing to its highest energy efficiency, lowest TAN of the upgraded bio-oil obtained and the relatively cheap price of methanol used compared to both supercritical ethanol and isopropyl alcohol treatments. Instead of treating the light fraction of PEFB-derived bio-oil, Prajitno et al. [90] treated the heavy fraction of PEFB-derived bio-oil in supercritical ethanol without using a catalyst and external hydrogen. They obtained similar encouraging findings on the physicochemical properties of the upgraded bio-oil as Jo et al. [89]. In spite of the promising performance of supercritical treatment on bio-oil, the high operating cost associated with the extreme operating conditions (pressure of 225–378 bar (Table 4) have put a damper on the commercialization and industrial scale-up of this method. Catalytic cracking and hydrocracking are other alternative upgrading methods for bio-oil which involve cracking of bio-oil into highquality volatile organic liquids like bio-gasoline and bio-kerosene, on top of other side products like syngas, water, and coke (carbon deposit), with the aid of a catalyst. As oppose to catalytic hydrocracking that requires high pressure (7–138 bar [8]) processing and external hydrogen, catalytic cracking is carried out under a mild pressure condition (1–3 bar [91,92]) without using external but in-situ hydrogen generated from the reforming reaction of oxygenated compounds with the existing water in bio-oil [92]. The mild operating condition without an external hydrogen source for catalytic cracking makes this upgrading method more economically appealing than catalytic hydrocracking. The main reactions occurred during both catalytic cracking and hydrocracking of bio-oil include dehydration, cracking, reforming, hydrogenation and aromatization reactions for the production of volatile organic liquids, Table 5 Bio-oil specifications for different low-grade fuel stationary systems. Properties

Water (wt%) Solids (wt%) Ash (wt%) S (wt%) Density @ 20 °C (g/cm3) Viscosity @ 40 °C (cP) HHV (MJ/kg) Flash point (oC) Pour point (oC) a b c

Burnera [104] Grade G

Grade D

Boilerb [105]

Diesel engineb [105]

Gas turbineb [105]

< 30 < 2.5 < 0.25 < 0.05 1.1–1.3 < 150 > 15 > 45 < -9

< 30 < 0.25 < 0.15 < 0.05 1.1–1.3 < 150 > 15 > 45 < -9

< 27 <1 < 0.1 – – – > 19 – –

< 27 <1 < 0.1 – – – > 18 – –

< 25 <1 < 0.05 – – – > 15c – –

Standard specifications based on ASTM D7544. Preliminary specifications based on end-user feedback and past research. Lower heating value (LHV). 272

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found to be attained by steam reformer (66%), followed by autothermal reformer (51%) and partial oxidation reactor (34%). Nevertheless, these findings were obtained from simulation-based research and further validation by experiments in actual process systems is necessary.

produce other products like solid fuel [115], biogas [116], biochar [117], biocompost [118] and activated carbon [119]. Therefore, continued and unremitting efforts to overcome these challenges are crucial to improve the competence of PEFB-derived bio-oil as an engine fuel. These include enhancing and standardizing the quality of bio-oil produced, developing economically feasible technology, strengthening local expertise and widening their experience in this rather new area, creating deliberate and purposeful marketing plans, allocating subsidies and incentives to the bio-oil industry, as well as optimizing the supply chains of PEFB. In addition, implementation and administration of prudent and strategic government schemes, policies and mandates to boost the sustainability growth of palm oil industry should also be duly planned and legislated. For instance, the government of Malaysia launched the Malaysian Sustainable Palm Oil (MSPO) certification scheme in 2015 to safeguard the sustainable development in palm oil cultivation and supply chains as a voluntary measure for palm oil producers and will make it obligatory across the board by the end of 2019 [120]. In fact, the MSPO scheme is resembling the Indonesian Sustainable Palm Oil (ISPO) scheme introduced by the Indonesian government in 2011 and the Roundtable on Sustainable Palm Oil (RSPO) scheme founded by multi-stakeholders way back in 2004 [120]. Ultimately, full cooperation and contribution of all parties, namely, the local governments, private sectors, and communities, to boost the biooil industry, both to improve the global energy security and to achieve the global environmental sustainability, are reliable and decisive strategies for the race to the finish line.

5. Fuel applications and future prospects Bio-oil is always in the spotlight among a wide array of biofuels due to its promising prospects for high-efficiency energy production compared to the classical biofuels like black liquor and hog fuel [102]. However, owing to the deficiencies in fuel quality of bio-oil, even after the upgrade [8], fuel applications of bio-oil have been restricted to replacing heating oil and diesel fuel in various low-grade fuel stationary systems such as burners, boilers, diesel engines and gas turbines for heat and power generation at power stations [102,103]. The bio-oil specifications for different low-grade fuel stationary systems are tabulated in Table 5. Of late, numerous researchers had investigated the combustion characteristics and exhaust emissions of various stationary systems, particularly single-cylinder diesel engines, that run on bio-oil which was either pure [106] or blended with other fuels like diesel [107], biodiesel [108] and ethanol [109]. The bio-oil used was derived from a diverse range of biomass materials such as tire waste [106,110], wood waste [109,111], Mahua seed [107] and Jatropha curcas shell [112]. In most cases, the bio-oil was upgraded before use and the engines were modified to adapt for the acidity and poor ignition characteristics of bio-oil, as well as for its propensity to repolymerize and to form soot [111]. Among the major modifications required for a bio-oil powered diesel engine include the fuel pump, linings and the injection system [111]. Although most of the researchers claimed to obtain positive results of smooth running in engines powered by bio-oil, various issues associated to the poor quality of bio-oil such as extended combustion duration due to the prolonged ignition delay and carbon deposition on fuel injectors, piston heads and intake valves caused by the high solid and ash content of bio-oil had also been reported [103]. The latter resulted in an incomplete combustion of bio-oil in diesel engines, which in turn increased their emissions of carbon monoxide, total hydrocarbons, soot and particulate matters compared to those of the conventional diesel engines [106,108,111]. The nitrogen oxides emission of bio-oil powered diesel engines, however, could be higher or lower than the conventional diesel engines depending on the nitrogen content of bio-oil relative to that of diesel fuel used [106,111]. To accomplish a complete combustion of bio-oil in a diesel engine, several techniques such as increasing the compression ratio of engine [111], preheating the intake air so as to raise the temperature of the compressed air in the cylinder [111], increasing the cetane number of bio-oil by adding a cetane improver to it [113], and introducing a pilot fuel to initiate the ignition of bio-oil [106] could be employed. To date, applications of PEFB-derived bio-oil in diesel engines and other stationary systems are limited to simulation research using Aspen Plus [101] and computational fluid dynamics [90] programs only, and empirical studies in this area, as well as in its applications as a transportation fuel, have not been reported yet. Moreover, despite the rapid growth of commercial-scale bio-oil production plants all over the world such as Dynamotive and Ensyn from Canada, New Earth from the United Kingdom, Genting Bio-oil from Malaysia, BTG-BTL from the Netherlands and Fortum from Finland [114], only that operated by Genting Bio-oil in Malaysia is using PEFB as the biomass feedstock. The major challenges responsible for the slow development of PEFB-derived bio-oil and its production plants in the world's top palm oil producing countries like Malaysia and Indonesia include, among others, poor and non-standard quality of the bio-oil produced, lack of cost-effective technology, inadequate local experts and capacity to manufacture, operate and handle the production plants and equipment, uncompetitive and ineffective marketing plans, financial obstacles, poor management of PEFB supply chains, as well as the growing competitions for PEFB to

6. Conclusions Bio-oil derived from PEFB is produced largely by fast pyrolysis in a fluidized- or fixed-bed reactor and by liquefaction in an autoclave reactor, both of which are carried out mostly under inert gas atmospheres in the absence or presence of a catalyst. Its optimum production conditions via fast pyrolysis include temperature of 300–600 °C, pressure of 1 bar, particle size of < 1 mm and heating rate of < 100 °C/min, whereas those via liquefaction include temperature of 270–400 °C, pressure of 20–291 bar, particle size of < 1 mm, solvent of water, ethylene glycol, methanol or water-ethanol mixture, and PEFB to solvent ratio of 1:2 to 1:17. The optimal catalysts for maximum bio-oil yield are boric oxide, HY and HZSM-5 zeolites for catalytic fast pyrolysis of PEFB, as well as potassium carbonate and cerium (IV) oxide for catalytic liquefaction of PEFB. PEFB-derived bio-oil has lower heating value and pH, as well as higher solid and ash contents, density and total acid number, but a lower sulfur content, than petroleum fuel oil. Hence, bio-oil is more susceptible to carbon deposit formation, more acidic and corrosive than petroleum fuel oil, but emits less sulfur oxides during combustion. It contains mainly aromatic (phenol and its derivatives, benzene, xylene, pyridine and furan derivatives) and oxygenated (carboxylic acids, esters, ketones, aldehydes, and alcohols) compounds, in addition to some alkanes and alkenes, regardless of the production processes involved. Its chemical compositions are influenced significantly by the catalysts, PEFB pretreatments, and/or solvents used in both fast pyrolysis and liquefaction processes. Although PEFB-derived bio-oil has been upgraded by methods like solvent addition, fuel blending or emulsification, esterification, supercritical treatment, catalytic cracking, catalytic hydrocracking, steam reforming, autothermal reforming and partial oxidation, each of these methods suffers from its own inherent drawbacks and more research work is required for further advancement. The deficiencies in fuel quality of bio-oil have restricted its applications to replacing heating oil and diesel fuel in various low-grade fuel stationary systems for heat and power generation at power stations. However, applications of PEFB-derived bio-oil in stationary systems are limited to simulation research only and empirical studies in this area, as well as in its applications as a transportation fuel, have not been reported yet. This limited fuel applications of PEFB-derived bio-oil give 273

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the assurance of the many obstacles that have to be tackled before a pronounced progression in this field could be foreseen in the future.

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