Hydrogen donor solvents in liquefaction of biomass: A review

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Hydrogen donor solvents in liquefaction of biomass: A review ⁎

Khairuddin Md Isaa,b, , Tuan Amran Tuan Abdullahc, Umi Fazara Md Alia a b c

School of Environmental Engineering, Universiti Malaysia Perlis (UniMAP), Kompleks Pusat Pengajian Jejawi 3, 02600 Arau, Perlis, Malaysia Centre of Excellence for Biomass Utilisation, Universiti Malaysia Perlis, Malaysia Centre of Hydrogen Energy, Institute of Future Energy, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia

A R T I C L E I N F O

A BS T RAC T

Keywords: Alcohols Water Hydrogen donor solvents Biomass Liquefaction

The environmental impact of global warming, caused by greenhouse gases has also fuelled the needs to utilise biomass, as its energy utilisation creates less environmental pollution and fewer health risks than fossil fuel combustion. Liquefaction of biomass using hydrogen donor solvents is a promising route to obtain clean biofuel using various solvents at moderate to high temperature (250–460 °C) and pressure (150–320 bar). Solvents such as sub-and supercritical water, alcohol, decalin, glycerol and tetralin can be used as potential hydrogen donor to enhance liquid oil yield with a reduced of oxygen content. Supercritical water with its excellent transport properties as well as hydrogen donor capability leads to hydrothermal decomposition of biomass and enhancing various compounds depending upon operating parameters. The selection of alcohol as a solvent related to the action of hydrogen donor and to its alkylating ability. The hydrogen donor solvents provide an alternative to hydrogen gas as a reducing gas. The advantage of using hydrogen donor solvent is to stabilise the free radical in the biomass liquefaction and yielding a higher product conversion. Compared with non-hydrogen donor solvents, hydrogen donor solvents such as tetralin and decalin show significant improvement not only in conversion and product distribution to liquid but also on the quality of bio-oil (oxygen content) due to the improvement of hydrogenation and hydrocracking reactions with inhibition of polycondensation. The advantage of hydrogen donor solvents over the molecular hydrogen due to a lower strength bonding of C-H as compared to H-H bond. A review on performances of water, alcohols and other hydrogen donor solvents in liquefaction of biomass has been made. The yield of hydrogen donated in the reaction has also been reported.

1. Introduction The biomass energy is produced from wood and wood wastes (64%), followed by municipal solid waste (24%), agricultural waste (5%) and landfill gases (5%) [1]. The utilisation of biomass has become a significant topic recently due to the need to find an alternative to reduce dependency fossil fuels reserves. Fossil fuel resources are finite and non-renewable, catalysing the efforts of employing biomass as a source for the production of renewable energy. Environmental pollution and global energy crisis, caused by the massive use of conventional fossil fuels, have triggered to a move towards sustainable, clean energies and cost-effective energy sources with less pollution and also overcome the gradual depletion of traditional fossil energies [2]. In the production of biomass energy, biochemical and thermochemical conversion are widely employed in a small scale in the laboratory or large scale in the industry [3–7]. Bio-chemical conversion comprises two common processes, digestion (production of biogas, a mixture of mainly methane and carbon dioxide) and fermentation (production of

⁎

ethanol) [8]. Thermochemical conversion is the application of thermal to convert the chemical and physical properties of biomass including direct combustion, pyrolysis, gasification, liquefaction and torrefaction. Direct combustion of biomass generates heat and combusted gas of carbon dioxide and water vapor with tar and ash are also produced. However, the major drawbacks are the requirement of high efficiency of combustor due to shapes and form of biomass, as well as the total carbon return to the atmosphere. Liquefaction is a process of producing liquid products in which the feedstock macromolecule compounds are decomposed into fragments of light molecules in the presence of suitable catalyst. Meanwhile, gasification is the first step in indirect liquefaction in which the gasifying coal/biomass is partial oxidised to produce syngas, and direct pyrolysis is a conversion process of organic compounds in the inert environment to produce liquid, char and gas products [9,10]. The pyrolysis processes (slow or fast) can be carried out depending on the types of products (liquid, solid or gas) required. Most processes that convert biomass to liquid fuels begin with pyrolysis, followed by

Corresponding author at: School of Environmental Engineering, Universiti Malaysia Perlis (UniMAP), Kompleks Pusat Pengajian Jejawi 3, 02600 Arau, Perlis, Malaysia. E-mail address: [email protected] (K.M. Isa).

http://dx.doi.org/10.1016/j.rser.2017.04.006 Received 18 October 2016; Received in revised form 8 March 2017; Accepted 8 April 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.

Please cite this article as: Md Isa, K., Renewable and Sustainable Energy Reviews (2017), http://dx.doi.org/10.1016/j.rser.2017.04.006

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region is still as polar as acetone although the dipole moment decreases with increasing temperature [35]. This review is an effort to elucidate the role of hydrogen donor solvents (alcohols, sub-and supercritical water, tetralin and etc) in liquefaction of biomass, and reporting the product conversion as well as the produced bio-oil yield and also the oxygen content of the produced oil. Some comparison with coal related study also included to give a better insight. The mechanisms of hydrogen donor solvents such as tetralin, cyclohexane and glycerol in liquefaction of biomass have also been discussed and reviewed.

catalytic upgrading (deoxygenation or hydrotreating) of the resulting biocrude liquids [11]. Deoxygenation (DO) process capable to upgrade oxygenates oil into a higher quality of hydrocarbon based biofuel via decarboxylation/decarbonylation under hydrogen-free atmosphere and produced CO2/CO as by products. The pyrolysis of various biomass such as beech bark, miscanthus, pine sawdust, sludge, empty fruit bunch, maize, and rice husk have been performed by numerous researchers to produce a liquid yield and chemicals [12–19]. Bio-oil is composed of a complex mixture of oxygenated that provide both the potential and challenge for utilisation. The kinematic viscosity of bio-oil varies from as low as 11 mm2/s to as high as 115 mm2/s at 40 °C depending on the nature of the feedstock, temperature of the pyrolysis process, thermal degradation degree and catalytic cracking, the water content of the bio-oil, the amount of light ends that have collected, and the pyrolysis process used [20]. The liquid is highly oxygenated, approximating the elemental composition of the feedstock [21]. The oxygen content of bio-oils is usually 35–40%, distributed in more than 300 compounds depending on the resource of biomass and the severity of the pyrolytic processes [22]. Furthermore, bio-oil with high water content derived from the original moisture in the feedstock will lower the heating value and affect the product quality. Pyrolysis oil is very corrosive as a result of high acidity with average pH values of 2–3 [5]. There are a few methods which can be performed to improve the quality of bio-oil, such as hydrodeoxygenation, catalytic cracking, emulsification and steam reforming. Hydrothermal liquefaction (HTL), also known as hydrous pyrolysis, is a very flexible technology as far as the type of feedstock is concerned, as a wide variety of bio-based and waste feedstocks have been tested [23]. Bio-oil produced via liquefaction has a reduced oxygen content (10–18%) compared to the parent material (ca. 40%) [24]. Chan et al. reported that bio-oil yields from liquefaction of empty fruit bunch, palm mesocarp fibre and palm kernel shell generally increased when temperature was increased from 330 (sub-critical water) to 390 °C (supercritical water) [25]. The rate of decomposition and cracking of lignocellulosic components from the matrix structure of biomass was enhanced as the temperature was increased, thus leading to increase in formation of bio-oil components. The effects of various solvents, including phenol, ethylene glycol (EG) ethylene carbonate (EC) and supercritical ethanol have been investigated in the liquefaction of biomass [26–29]. Zeb et al. reported that the biomass to solvent ratio (BS) was increased from 0.10 to 0.17 by changing the amount of ethanol, the bio-oil yield decreased significantly from 79.5 to 37.8 wt%, and this was attributed to the combined effect of BS ratio and reaction pressure on the liquefaction reaction [29]. Supercritical fluid extraction is very important to be used in the conversion of biomass. It has been employed to improve bio-oil yield and quality, with much higher calorific values [30]. Durak and Aysu reported that in supercritical liquefaction conditions, acetone was more effective than ethanol and isopropanol in both non-catalytic and catalytic runs at 295 °C with the yields of bio-oil were 21.71% and 25.79% in non-catalytic and catalytic (Ferric chloride) runs respectively [31]. Water has received extensive attention because it is clearly an inexpensive, generates strong hydrogen bonding and high polarity, and easy to recycle reaction medium in liquefaction works [32]. Water can simultaneously act as both a reactant and a catalyst. Hydrothermal liquefaction using subcritical water (200–370 °C and 4–20 MPa) sufficient to keep the water in a liquid state, and going close to the critical point, water has several properties such as low viscosity and high solubility of organic substances, that means it can serve as an excellent medium for fast, homogenous and efficient reactions [33,34]. Both the rate of hydrolysis as well as phase partitioning and solubility of components can be controlled under sub-and supercritical water conditions so that potentially more favourable pathways to gases and liquid biofuels may be realised. The advantage of water in the critical

2. Advantages of biomass Biomass, unlike coal has a relatively high hydrogen-to-carbon ratio. Pure cellulose (C6H10O5) has a H/C ratio of about 1.7 compared with 0.8 for a typical bituminous coal [36]. The use of biomass can lessen the dependency on the limited fossil fuels and there is the advantage of reduced net carbon dioxide emissions [37–39]. Biomass has the advantage of fixing the carbon dioxide balance in the atmosphere by photosynthesis process. Zero net emission of carbon dioxide (CO2) can be achieved because CO2 released from biomass will be reused into the plants by photosynthesis quantitatively [22]. It was reported that by burning large portions of carbon (from fossil fuels) per year we have released (and are continuing to release) enormous quantities of CO2 within a very short time of about 200 years [40]. CO2 emissions on g/ kW h electricity generation bases are the lowest in the case of biomass (17−27) compared to coal (955), oil (818) and gas (446) [39]. The utilisation of biomass will be beneficial to the environment and society through sustainable energy (renewable biomass), CO2 neutral fuel, reduction in gases like NOx/SOx due to less sulphur and nitrogen contents present in biomass, and its abundant availability in all regions of the world [41–44]. As such, renewable energy is of growing importance in satisfying environmental concerns over fossil fuel usage. 3. Thermochemical conversions Under the umbrella of thermochemical conversions, biofuels and chemicals can be produced via gasification, pyrolysis and liquefaction. However direct processes (liquefaction and pyrolysis), which have easier methods and are less time consuming, have recently been found to be suitable. The effect of hydrogen donor solvents in liquefaction of biomass will be reviewed in this article. 3.1. Historical background the way towards biomass hydrothermal liquefaction A number of reports have been published on various aspects of hydrothermal biomass processing over the years. Furthermore, the International Energy Agency conducted the meetings, that bring together researchers and engineers working on thermochemical of biomass conversion, have been held every 3–4 years since 1981; the published proceedings of these conferences offer plenty of information on hydrothermal biomass production [34]. A variety of processes have been researched and patented, initially developed for coal, peat, and wood sludge liquefaction in the 1970's. Early works by Gupta et al. and Chin and Engle in improving the liquid quality have catalysed the research on thermochemical potential as reported by Vasilakos and Austgen [36]. As early as the first oil crisis in the first half of 1970s, direct liquefaction gained attention as one potential pathway from crude oil and was envisaged as a ‘suitable substitute’ for fossil fuels. The difficulties of these early approaches were mainly attributed to the high process prices and the lack of basic scientific understanding of the process, which led to the failure of early works built during that period. However, numerous investigations world-wide led to in a much better understanding of the process. Recent developments and commercialization have targeted waste biomass as a feedstock. 2

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The Industrial processing knowledge in the field of hydrothermal processing comes from two application areas: (1) supercritical-water oxidation (SCWO), and (2) supercritical-water power generation cycles. SCWO has been successfully used to detoxify and remediate variety of organic and biological wastes [34]. It was developed in the early 1980s based on research at MIT by Modell and co-workers in the late 1970s, and has been installed at both pilot and demonstration scales commercially in the U.S., Europe, and Asia [34]. The previous researches on coal liquefaction also greatly influenced the move towards biomass liquefaction. The effects of tetralin and alcohol solvents have been investigated in Kansk-Achinsk brown coal liquefaction, with tetralin was reported to produce the highest coal conversion [45]. Phenols (polar compound) have also been reported and considered as beneficial for the liquefaction of coal [46]. The history of coal liquefaction can be traced back to the by-product from coke production in the UK and Germany in the 1840s [47]. Those byproducts had been normally used as solvents, wood preservatives and fuels. By using high temperatures and pressures, coal could be dissolved in solvents to produce high boiling point liquids without hydrogen and catalysts present. The process, which was known as the Pott-Broche or I G Farben process, was patented in 1913 and commercialised in the early 1920s [48]. Research performed at the Coal Research Establishment in the late 1960s and early 1970s investigated the use of anthracene oil and a range of polycyclic aromatic hydrocarbons to produce a pitch-like material in high yield [49]. However, researchers then realised that the use of hydrogen donor solvents had the advantage of giving higher overall conversions and producing heavy liquids more amendable to upgrading to distillate fuels via hydrocracking. Miscanthus and scotch pine have been liquefied using tetralin as a hydrogen donor solvent at 410 °C [50].

effects of hydrogen donor solvents (such as tetralin, decalin and glycerol, water and alcohols) in liquefaction of biomass are reviewed in Sections 3.2.1, 3.2.2 and 3.2.3. 3.2.1. The effect of alcohols The effects of alcohols on the thermochemical liquefaction of biomass or coal have been studied broadly such as the use of supercritical alcohol for the conversion of low rank coal into liquid oil [61]. Aliphatic alcohols have been shown and reported to be effective solvents for coal liquefaction [45]. The action of alcohol can be relate to the hydrogen donor and to its alkylating ability [45,61]. Solvolysis has been performed using a number solvents and revealed that ethylene glycol had the highest efficiency in terms of the conversion yield in empty fruit bunch liquefaction [56]. Solvolysis on selected lignins have been performed at 380 °C using formic acid and ethanol and iso-propanol as co-solvents [62]. The authors reported high yields of an organic or “oil” phase that has suitable chemical properties for use as a blending component in motor fuels. The advantages of the supercritical alcohol has been summarised by Brand and Kim [63] and Brand et al. [64] as follows: 1. Better solubility: The electric constant of alcohol is much lower than that of water, providing a better solvent for biocrude at both ambient and supercritical conditions. 2. Easier product separation: The liquid product in supercritical alcohol liquefaction can yield a single liquid phase whereby biocrude can be produced by simple alcohol drying, as compared to liquid products in sub-and supercritical water which split up into water soluble and insoluble fraction. 3. Lower corrosivity: Corrosion is not a significant factor using alcohol in relative to subcritical water. 4. The hydrogen donation advantage at the liquefaction condition 5. Higher bio-crude yield

3.2. Direct liquefaction Direct liquefaction is seen as a promising biofuel/chemicals production technology which can take advantage of biomass and coal as the feedstock. Liquefaction is known as a better method for biomass thermochemical conversion for wet feedstocks, unlike pyrolysis where the biomass has to be dried before use. More importantly, employing water as a reactant and as the reaction medium in the biomass conversion process will eliminate the costly drying process of wet feedstocks, which make it a promising reactant for biomass liquefaction [51]. With this process, a variety of wet biomass can be utilised. The liquefaction process on microalgae (an example of an aquatic plant) has been investigated for an alternative fuel [52]. The liquefaction process has many advantages such as: (1) the chosen solvent could dilute the concentration of the products and prevent the cross-linked reactions between hydrocarbon and aromatic compounds generating tar compounds, and (2) relative low reaction temperature (less energy consumption) in comparison with other thermochemical processes (pyrolysis and gasification) [53]. Another interesting advantage of employing the liquefaction process is the ability to reduce the oxygen content of the products, which is quite promising when compared to the fast pyrolysis route. The oxygen content for biomass varies ranging from 35 to 46 wt% [5,54,55]. Solvolysis liquefaction has been employed by researchers’ worldwide for biomass and coal conversion [45,56]. Solvolysis is chosen because it can be carried out at a lower temperature compared to pyrolysis. Hydrous pyrolysis (part of solvolysis) is a hydrothermal experiment which employs the heating of samples in a closed vessel with water as reaction medium [57–59]. A number of experiments involving hydrous pyrolysis for biomass and coal conversion have been performed [50,60]. Hydrous pyrolysis conducted by Lewan, showed that heating organic-rich rocks at 330 °C for 72 h in the presence of liquid water resulted in the generation and expulsion of a petroleum like oil that was absent in olefins, similar to natural crude oils [58]. The

The hydrogen produced from sub-and supercritical ethanol serves as a deoxygenation agent to remove oxygen by forming water, as a hydrogenolysis agent to depolymerize biomass, as well as a radical quenching agent to prevent char formation. The mechanism of ethanol as a hydrogen donor solvent has been explained by Huang and Yuang as shown in Fig. 1. However Huang and Yuan suggested the further research should be carried out to understand more on the specific interaction between biomass and ethanol. Baggase and cotton stalk liquefactions have been conducted using

Fig. 1. The possible mechanisms of ethanol as a hydrogen donor- reproduced from Huang and Yuan [65].

3

4

[72] [73] [74] [56] [75] NR NR NR 75% (O+G) 40% Liquid 57% 64% 52% 96% 51%

26.50% 63% NR NR 74.2% 88–91% 58% 56%

Pragmite australis Typha latifolia Reed canary grass Oil palm empty fruit bunch Ferula orientalis L

NR-not reported.

0.250 L Autoclave 316 Stainless steel 75 ml Autoclave, 316 stainless steel 75 ml autoclave, 316 stainless steel 150 ml autoclave with a turbine stirrer 75 ml cylincrical autoclave, stainless steel

NaOH NaOH, Na2CO3 NaOH, Na2CO3 Nil ZnCl2, NaOH, Na2CO3

523–723 K, 65 MPa, 20 min 473–623 K, 2–10 MPa, 20–60 min 523–563 K, 8.1 MPa, 90 min 533 K, 553 K and 573 K, supercritical fluids pressures, 75 min 523 K and 563 K, supercritical fluids pressures, 90 min 518, 538 and 558 K, supercritical fluid pressures, 90 min 530, 550, 570 K, supercritical fluid pressures, 75 min 548 K, 30 min, pressures were not reported 513–633 K Nil Iron-based NaOH ZnCl2 200 ml cylindrical autoclave Microreactor 16 ml, 316 stainless steel 0.250 L Autoclave 316 Stainless steel 100 ml Autoclave 316 Stainless steel

Ethanol Ethanol Methanol Methanol, ethanol and acetone Methanol and Ethanol Ethanol, methanol, 2-butanol 2-butanol, methanol, ethanol Ethylene glycol, ethanol 2-Propanol, methanol, ethanol Pine wood Jack pine powder Sunflower stalk Verbascum stalk

Reactor Solvent Biomass

Table 1 Product conversion under liquefaction of biomass with alcohol solvents.

Catalyst

Operating parameters

Highest conversion

Oil yield

Researchers

polyethyleneglycol (PEG)/glycerine with a ratio of 4/1 (solvent/raw material) [66]. The authors reported ~19% residual content for baggase and ~22% for cotton stalks at 150 °C for 1 h, by using PEG alone and sulphuric acid as a catalyst. Replacing 10% of PEG with glycerine as well as increasing the sulphuric acid concentration, decreased the amount of residue for both baggase and cotton stalks to less than 10%. However, the authors did not report the liquid yield in their study. Liquefaction of pinewood has been conducted at 250 °C, 300 °C, 350 °C and 400 °C and 450 °C with ethanol as a solvent [67]. When the pressure in the autoclave was raised to 1 MPa with argon gas and it was maintained for 20 min, the highest conversion of 74.2% was obtained at 450 °C. The highest oil of 26.5% was obtained at 400 °C. All samples were conducted using 10 g pinewood vs 60 g solvent. Liquefaction of microalgae in sub- and supercritical ethanol has been performed to investigate biomass conversion [28]. The authors reported the bio-oil yield decreased slightly with the temperature increasing from 280 °C to 320 °C, and increased continuously with the reaction temperature increasing from 320 °C to 380 °C, with the maximum of 43% of bio-oil obtained at 380 °C. The highest conversion of 82% was obtained at 380 °C. The reaction pressures for all run were in range of 6.9–11.4 MPa. A solid to liquid ratio of 0.05 g/ml was used for these runs. However, they did not mention the duration time for these runs. Huang et al. reported the effect of the solvent (ethanol) filling ratio, showed that the yield of bio-oil increased with increasing solvent filling ratio [28]. The bio-oil was increased to 45.34% from 37.14% when the solvent filling ratio was increased from 10% to 30% at T=633 K. Huang and co-authors reported the highest product conversion of 90% was obtained with NaOH as a catalyst at T=633 K. Elemental analysis showed the oxygen content for the produced bio-oil was in range of 10– 15%. However a slightly lower of carbon content was observed for the produced bio-oil which gave ~65–69.43%. Yuan et al. reported a different trend using methanol and ethanol in liquefaction of microalgae at 653 K [52]. A higher product conversion of 82% was obtained using methanol, while ethanol recorded 80%. The properties of bio-oil were reported, showing a greater reduction of oxygen content from 39.19 (oxygen content of microalgae) to a range of 10–12%. However a different trend of the produced bio-oil under catalytic supercritical liquefaction of Syrian mesquite was obtained [68]. Under supercritical methanol condition with 10% of NaOH as a catalyst, the lowest oxygen content produced was ~36% which greater than work by Yuan et al. which using microalgae as a feedstock. Brand et al. [63] conducted liquefaction of cellulose from temperature 265–350 °C using supercritical ethanol, and found the lowest oxygen content for the produced bio-oil obtained at 350 °C which had 20.28% and recorded the heating value of 31.38 MJ/kg. Brand reported the CO, CO2 and H2 yields were increased at 350 °C. This is in agreement with the findings reported that the highest conversion obtained at 350 °C where the free radicals have been stabilised at this temperature by hydrogen consumed in cellulose conversion. The reaction was influenced mainly by pyrolytic cleavage was influenced mainly by pyrolytic cleavage rather than hydrolytic cleavage [63], hence contributed the highest gas yield (CO, CO2 and H2) at 350 °C. Table 1 tabulated the previous works in alcohol liquefaction of biomass. It is observed a lower product conversion was obtained using ethanol, methanol, 2-butanol and 2-propanol ~ 50–74% in temperature range of 523–723 K. However, liquefaction of Jack pine wood with ethanol and iron-based catalyst in temperature range of 473–623 K produced a higher product conversion of 88–91%. It is found ethylene glycol a good solvent to produce a higher conversion ~96% as compared to works with ethanol and methanol. It was reported that the ethanol can provide active hydrogen as a hydrogen donor, has lower critical temperature and pressure, and can react with acidic components in the bio-oil to obtain esters [65]. All the studies listed in Table 1 did not report the oxygen content of the produced oils.

[67] [69] [70] [71]

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Hydroaromatic structures are visualised as ‘ideal’ for liquefying coal/biomass due to their ability to readily transfer hydrogen to free radical fragments. Their structures in heterocyclic compounds are more reactive than homocyclic in term of dehydrogenation; the order can be explained as follows: tetrahydroquinoline > tetralin > indoline [78]. Tetralin has been shown to undergo thermal dehydrogenation to naphthalene and rearrangement to methylindan in either the absence or presence of free radical acceptors [78,79]. Tetralin has been used as a hydrogen donor reagent to relate various high-temperature tars with regard to their hydrogen acceptance [78]. Pajak and co-workers have carried out experiments, not exceeding 340 °C to avoid the decomposition of tetralin. Furthermore, they have made a comparison of naphthalene formation as given in Table 2. Hydrogen transfer to coal has been measured by the naphthalene formation for the reaction of Ziemowit coal and Pokoj coal with tetralin at 300 °C. The result shows by prolong the reaction time, the higher the naphthalene formation although it was influenced by pressure applied as well. However, the net effect is that tetralin/naphthalene are not a true measure of “transferred hydrogen” [79]. Pajak and co-workers have also suggested decalin to be used at high temperature as it was not disproportionate below 420 °C. Two routes of tetralin conversion to form naphthalene and 1methylindan are shown in Fig. 2. The conversion of tetralin to form isomer, 1-methylindan during rearrangement is known as the loss of hydrogen, however, the tetralin conversion into naphthalene gives more free radical hydrogen and readily to be donated in the liquefaction of biomass. Ability of a hydrogen donor solvent to release hydrogen defines its hydrogenation power. Deng et al. conducted a blank run using tetralin at 460 °C and reported that tetralin was stable at that condition with > 90% of tetralin recovered with 6.5% formed as 1-methylindan and 1.4% as naphthalene [80]. Deng also suggested that the remaining tetralin concentration can be recycled multiple times at 410 °C for 1 h. Kuznetsov et al. performed coal liquefaction at 380 °C with 6 g dry coal sample and 120 g solvent for 30 min [45]. The autoclave had been pressurised to 3 MPa with hydrogen previously before being heated to a pre-set temperature. The authors reported 51% (DAF) conversion obtained by using 100% tetralin. They performed the coal liquefaction using binary solvents with 83% of tetralin and 17% of ethanol and found the conversion increased to 79.2% (DAF). However, the liquid yield was not determined in their report. Vasilakos and Austgen conducted liquefaction of cellulose with tetralin as the hydrogen donor solvent at 400 °C, 500 psig total initial pressures (Argon) for 60 min duration [36]. An amount of 30 g cellulose and 300 g tetralin were used in this experiment. They reported nearly 100% conversion obtained with 51.9% liquid yield produced. The authors reported 1% hydrogen consumption for this experiment. Glycerol can be used as a solvent and hydrogen donor solvent and reported by Wolfson et al. [81]. Dehydrogenation of glycerol resulted in the formation of dihydroxyacetone. Wolfson and co-workers also highlighted the important of base as co-catalysts. Hart et al. studied

Table 2 Naphthalene formation (wt%) in the reaction of Ziemowit coal-(a) and Pokoj coal-(b) with tetralin at 300 °C. Time (h)

2 5 18

5 (Mpa)

30 (MPa)

50 (MPa)

A

B

A

B

A

B

3.8 5.5 9.6

1.4 3.4 3.7

3.7 6.2 9.4

2 3.3 3.6

4.1 6.6 9.4

2 3.3 3.5

Most of the researches using alcohols focused on the product yield. Eventhough they always highlighted the importance of hydrogen donor to stabilise the free radicals and getting the higher product conversion, they did not specifically study the mechanisms of hydrogen donor and its role in liquefaction of biomass using alcohols. Therefore, there is a room for improvement in this study (referring to the alcohols) to explore to get a better insight. 3.2.2. The effect of hydrogen-donor solvents (other than water and alcohols) Solvent extraction of biomass can be described as hydrogenation with the use of solvents, but not molecular hydrogen. A number of studies have been conducted previously using molecular hydrogen as the reducing agent [45,61]. Low pressure molecular hydrogen has been utilised primarily in the deoxygenation of the product oil to yield additional water [36]. Liquefaction employing hydrogen donor solvents in which the destructive hydrogenation of wood lignin with cyclohexanol as the hydrogen-donor solvent was described [36]. The idea was to suggest a minimum degree of processing with the hope of obtaining a high quality liquid yield. Two mechanisms were thought to be involved: 1. Biomass must be heated up to promote the bonding-cleavage. 2. Hydrogen atoms act to prevent the repolymerization as a result of biomass decomposition. Many solvents can function as hydrogen donors as long as they have mobile carbon-hydrogen bonds with the ability to donate hydrogen to the unstable biomass fragments. In the liquefaction stage, the hydrogen donated from the solvent was more efficiently employed than the gaseous hydrogen to produce oil product [76]. The liquefaction of brown coal using methanol, ethanol, isopropanol and tetralin as the solvents, has been investigated, with the highest coal conversion observed with tetralin [45]. Aliphatic alcohols have been shown to be effective solvents for liquefaction with hydrogen donor capability [77]. Coal liquefaction by employing the binary tetralin-alcohol mixtures gave the maximum conversion and product yields. However, the increase of alcohol in the mixtures gradually decreased the product yields [45]. The product yields were found to be influenced by the following; a) coal hydrogenation by a hydrogen donor, and b) alkylation of the aromatic coal fragments by alcohols [45]. Tetralin is a good model to elucidate an effective hydrogen-donor capability. It can dehydrogenate under liquefaction conditions, thus enhancing its solvation capabilities. In the case of biomass liquefaction, the hydrogen transfer from the hydrogen-donor solvents stabilises the free radical of fragmented biomass [50]. The free-radical mechanism takes place when the heated-biomass cleaves into free-radical and seeks stabilisation depending on the energy requirements. Furthermore, hydrogen donor solvents can provide a dual-function to donate hydrogen for fragments stabilisation and assisting thermal cleavage [45]. It was reported that the partially hydrogenated aromatic compounds play an important role in stabilising the coal-derived fragments through hydrogen transfer, and the solvent has ability to carry hydrogen to solvate or to swell the coal structure [45]. In the case of promoting thermal cleavage, the presence of hydrogen is essential in inhibiting charring process [50]. When there is a reduction of donor hydrogen the biomass fragments recombine to form char.

Fig. 2. Routes of tetralin conversion to form naphthalene and 1-methylindan-reproduced from Deng et al. [80].

5

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to form naphthalene was in range of 1.3–2.5% [50,80]. The lowest oxygen content of ~6% was obtained under liquefaction of scotch pine with tetralin at 410 °C [50,80]. However, with lignin as a feedstock, a slightly higher oxygen content ~10% was obtained under the same condition. Isa and co-workers made a comparison between using supercritical water and tetralin in liquefaction of miscanthus and found that eventhough with a lower mass ratio of 1:2.5 (biomass to tetralin) still yielding high product conversion ~90% and low oxygen content ~12% for the produced bio-oil [6]. Meryemoglu et al. conducted liquefaction of kenaf from 250 to 350 °C using a 100 ml of batch reactor with tetralin as a solvent. Catalytic liquefaction with Ru/C under hydrogen atmosphere gave the highest oil+gas yield of 78% at 300 °C. At high pressure and temperature, it is likely formation of free radicals due to cleavage of some bonds, such as C–O–C or C–C, in the biomass chemical structure. These radicals can react with hydrogen to produce stable and smaller products or recombine with each other to form polymeric compounds having high molecular weights [88]. However, Meryermoglu did not explain why they used tetralin under the hydrogen atmosphere to liquefy the kenaf because the effect of tetralin as a hydrogen donor solvent would not be seen clearly with that condition.

Fig. 3. Two parallel reactions occurring are simplified by the reaction scheme involving cracking of cyclohexane to light hydrocarbons and dehydrogenation to benzenereproduced from Hart et al. [91].

the effect of cyclohexane as hydrogen donor in ultradispersed catalytic upgrading of heavy oil [82]. The release of hydrogen and the formation C1–C5 due to ring opening contributed to the increased pressure observed upon the addition of cyclohexane. The increased pressure increases the partial pressure of the liberated hydrogen to favour hydrocracking and hydrogenation reactions. When the cyclohexane to oil mass ratio was increased, the system pressure increased as well and it was in agreement with the mechanism of hydrogen donor as reported by Hart et al. [82]. It was found that increasing the cyclohexane to oil ratio from 0.01 to 0.08 g g−1 not only suppressed coke yield from 4.4% to 2.6% respectively, but also increased the yield of upgraded oil from 87% to 89%. The two parallel reactions occurring are simplified by the reaction scheme shown in Fig. 3 involving cracking of cyclohexane to light hydrocarbons and dehydrogenation to benzene [82]. Schuchardt and Marangoni Borges [83] studied liquefaction of hydrolytic eucalyptus lignin in hydrogen and non-hydrogen donor solvents with ferrocene and ferrocene with the presence of sulphur or carbon disulphide. It was found that in donor solvents, the hydrogen was transferred primarily from the solvent itself rather than from the initial pressurised hydrogen gas and the catalyst had less effect on enhancing oil yields with the hydrogen donor solvents. However, for non-hydrogen donor solvents, the high initial hydrogen pressure and catalyst present were two key factors to increase the heavy oil yields. Deng et al. [80] investigated a novel thermolytic liquid solvent extraction (LSE) process under various conditions of hydrogen donor solvents, temperature, and biomass species. Compared to currently available commercial pyrolysis approaches, this process using tetralin as a solvent is shown to be capable of producing high quality bio-oil with low oxygen contents (ca. 5.9%) at high overall conversions of up to 87 and 92 (%) dry and ash free basis (DAF) from Scotch pine and miscanthus, respectively. Compared with non-hydrogen donor solvents, hydrogen donor solvents displayed significant enhancement not only on conversion and product distribution but also on the quality of bio-oil due to the improvement of hydrogenation and hydrocracking reactions with inhibition of polycondensation. These abilities were also much higher than gaseous hydrogen due to a lower strength bonding of C–H as compared to H–H bond. The bio-oil product contained more hydrocarbons but less esters and alkenes by using hydrogen donor solvents [84,85]. A similar result was also reported by Afifi et al. [86], who used different ratios of guaiacol and tetralin mixture solvents and found that in the absence of hydrogen donor solvent, products were mainly oligomeric and polymeric species. However, using excess of tetralin, the products were favourable to form catechol and o-cresol. Many researchers investigated the effect of ethanol, methanol, cyclohexane, glycerol, decalin and tetralin as hydrogen donor solvents on product conversion [28,52,78,81,82,87]. However, they did not report the amount of hydrogen been donated during product conversion. Deng et al. however reported the percentage of hydrogen donated

3.2.3. The effect of water Numerous studies have been conducted employing hydrogen gas as a reducing agent and hydrogen donor solvents to improve the liquid quality [45,89]. However, the process discussed above faces an uphill task due to its cost. Thus, water has been utilised as a ‘cheap’ solvent. In hydrothermal liquefaction water simultaneously acts as reactant and catalyst, and this makes the process significantly different from pyrolysis [33]. The presence of water as the reactant leads to hydrolysis reactions and the polymeric structure's degradation for the biomass occurs quickly. Furthermore, water's characteristics as a good solvent at high temperature are interesting to investigate. At conditions close to the critical point, water has several interesting properties. Among them are low viscosity with high solubility of organic substances, which make subcritical water an excellent medium for homogenous, quick and efficient reactions [33,39]. Subcritical water exhibits interesting properties to hydrolyze biomass. Toor et al. reported the dielectric constant falls from 78 Fm−1 at 25 °C and 0.1 MPa to 14.07 Fm−1 at 350 °C and 20 MPa [33]. When temperature increases and dielectric constant decreases, it indicates water molecules change from very polar to fairly nonpolar [90]. The dissociation of water increases rapidly with the increase of temperature. Water splits into H+ and OH- ions in hydrolysis or dissociation. Water molecules dissociation constant at 300 °C is about 500 times higher than that of 25 °C at atmospheric pressure, resulting in the increasing rate of both acid and base-catalysed reactions in water, far beyond the natural acceleration [90]. This gives a significant increase in the solubility of hydrophobic organic compound such as free fatty acids. While many biomass compounds such as cellulose and lignin, are not water soluble at ambient temperature conditions, most are readily solubilized in supercritical water or sub-critical water [35]. These soluble components are subject to hydrolytic attack, engendering fragmentation of biomacromolecules. Water, both in its dissociated and native form, can act as a catalyst in hydrolysis and other reactions. Supercritical water behaves very much like a polar organic solvent. It was reported when going supercritical (T > 374 °C, P > 22.1 MPa), the values of density (0.2 to 0.7 g/cm3), dielectric constant and ionic product of water decrease, and the supercritical water acts as a non-polar solvent of high diffusivity and excellent transport properties [91]. In this region, density varies very rapidly with small changes in temperature at constant pressure. Among the fundamental knowledge necessary for a process involving near critical or supercritical water are the physical-chemical properties of water and its solvent power for organic substances, salts, and gases. Some very useful information regarding supercritical water has been reviewed and reported [35]. They are as follows: 6

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39%

[105]

1. P, v, T and transport properties are that of a supercritical gas; density is relatively high, up to liquid-like densities, but varies strongly with slight changes in pressure and temperature. 2. Viscosity is of the order of a normal gas and the diffusion coefficient is at least one order of magnitude higher than of a liquid. 3. Solubility of water for gases is high in the critical region. 4. At near critical and supercritical conditions water and gases like O2, N2, NH3, CO, CO2 are completely miscible. 5. The solvent power of water decreases for inorganic compounds in the critical region. Organic compounds, on the other hand, are readily dissolved by water in the near critical and supercritical region up to total miscibility. A range of other hydrothermal conversion processes exist, one of which is supercritical water oxidation. Supercritical water oxidation (SCWO) employs temperatures above the critical temperature of water (374 °C) and oxidative conditions to produce thermal energy and aiming CO2 rich gas phase. The SCWO process was developed for the destruction of industrial waste materials, such as sewage sludge and toxic effluents. The production of CO2 by utilising SCWO has been demonstrated [92]. However there are problem associated with SCWO, one of them is salt precipitation, which occurs extensively at such high temperatures. Another hydrothermal process is supercritical water gasification (SCWG). The SCWG is conducted to gasify biomass to CO2, H2, and CH4 under supercritical, but not oxidative conditions. The process temperature is heated up to 500 °C and heterogeneous catalysts are required for products selectivity. Hydrothermal processing can be carried out in a continuum of temperatures and pressure regimes to produce the desired liquid, solid and gaseous products. By utilising mild conditions (250–350 °C, 40– 165 bar), biomacromolecules hydrolyze and react to yield a viscous biocrude oil. At near-critical temperatures up to about 500 °C, effective reforming and gasification generally requires catalytic enhancement to achieve reasonable rates and selectivity. Hydrothermal liquefaction is an environmental friendly technology. The hydrothermal oxidation converts heteroatoms present in biomass into harmless by-products. The hydrothermal oxidation of biomass at temperatures 300–350 °C has been conducted and found a large proportion of oxygen is removed as CO2 [93]. High-temperature water (HTW), defined broadly as water above 200 °C, has attracted increasing attention as a medium for chemical synthesis, materials synthesis, waste destruction, plastics recycling, coal liquefaction and biomass processing [94]. Liu et al. performed hydrolysis of pinewood by using water at 250 °C, 300 °C, 350 °C and 400 °C and 450 °C [67]. With the pressure in the autoclave raised to 1 MPa with argon gas and maintained for 20 min, the highest conversion of 69.4% was obtained at 450 °C. The highest oil of 18.6% was obtained at 300 °C. All runs used 10 g pinewood vs 60 g water. Song et al. conducted liquefaction of corn stalk with 3 ml/min of water as a solvent at a pressure of 25 MPa, the temperature heated up to 410 °C at a constant heating rate (10 K/min) in semi-continuous apparatus [54]. The authors reported 95.4% (wt% dry) conversion obtained with water (without catalyst) and 33.4% of bio-oil produced. Water with 1.0 wt% Na2CO3 yielded 95.4% conversion and significantly increased bio-oil to 47.2%. Co-liquefaction of coal and cellulose in supercritical water has been performed at 400 °C and 25 MPa [95]. An amount of 0.5 g (cellulose +coal) was placed in the reactor and water was delivered by an HPLC pump at 4 g/min into the reactor. They reported the yield of the water soluble product, H/C and O/C increased by co-liquefaction. However, the authors did not report the product conversion and bio-oil yield in their study. Thermochemical liquefaction of Indonesian biomass residues has been conducted [96]. Eighteen kinds of biomass residues were used as samples and heated to 300 °C for 1 h with an operating pressure was 10 MPa. The authors used 10 g sample, 100 ml of distilled water, and

Nil

8.8 ml Inconel batch reactor

603–663 K, 1 h, 25–35 MPa

NR

[104] NR

Empty fruit bunchPalm mesocarp fibre, palm kernel shell

Rice stalk

Sub- and supercritical water Sub- and supercritical water Sub- and supercritical water Oil palm biomass

Nil

500 ml batch autoclave

523–648 K

~81–85%

[25] 39% 8.8 ml Inconel batch reactor

603–663 K, 1 h, 25–35 MPa

NR

[100] 40% Subcritical water Birchwood sawdust

KOH,K2CO3, colemanite Nil

Subcritical water

Algae

Algae

NaCO3

100 ml batch reactor (high pressure equip comp, Erie, PA) 100 ml Parr reactor 4950 micro bench top

573 K, 30 min, 8.5 MPa

~86–88%

[98] 49% NR

[97] 44% Nil

4.1 ml mini batch reactor, 316 stainless steel

NR

[6] 33.00% ~90%

683–733 K, 1 h, sub-and supercritical water pressures 623 K, 10 min, 30 min, 45 min and 60 min, 16.5 MPa 523–623 K, 75 min, 6.7 MPa Sub- and supercritical water Subcritical water Miscanthus

Nil

75 ml Parr reactor 4740 stainless steel

Conversion Solvent Biomass

Table 3 Recent publication (2014–2015) for liquefaction hydrothermal of biomass.

Catalyst

Reactor

Operating parameters

Oil yield

Researcher

K.M. Isa et al.

7

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K.M. Isa et al.

Table 4 A comparison for the performances of the hydrogen donor solvents in liquefaction of biomass on oxygen content of the produced bio-oil. Sample

Reaction Conditions

Catalyst

Solvent

Oxygen content (%)

Reseachers

Bio-oil from pre-treated baggase

T=350 °C P=not reported Tubular reactor 25 ml, 1 h

KOH

Water

14.50

[106]

Bio-oil from mischantus

Biomass to liquid ratio (1:15), 1 h T=410 °C, P= 320 bar, Parr reactor 75 ml

Supercritical water

14

[6]

Bio-oil from mischantus

Biomass to liquid ratio (1:2.5), 1 h T=410 °C, P= not reported, Parr reactor 75 ml

Tetralin

12

[80]

Bio-oil from rice husk

S/L ratio 15%, Batch stainless steel reactor T=240–360 °C, P=4.1–9.7 MPa, 20 min

Ethanol

29.63

[107]

Bio-oil from rice straw

Batch stainless steel reactor T=360 °C, P=4.1–9.7 MPa, 20 min

Ethanol

16.1

[108]

Bio-oil from baggase

S/L ratio (1/10), 500 ml autoclave stainless steel T=250 °C, P=not reported, 15 min

Sub-critical water

23.26

[109]

NaOH

MgMnO2

between the pine sawdust and cellulose may be due to the recalcitrant nature of wood which limits the access of water in order to hydrolyze the cellulosic polymers. Another suggestion by them was the lower conversion of the pine sawdust liquefaction could be condensation reactions of cellulose and lignin derivatives during the liquefaction. However, they did not try to relate the effect of low hydrogen donor into the system to stabilise the free radicals, hence recombination has led to an increase of char yield. Isa et al. investigated miscanthus liquefaction by using sub- and supercritical water as a hydrogen donor solvent and reported ~0.5% hydrogen was donated in the product conversion [6]. It was found that high conversions (ca. 90 wt%) can be achieved with high bio-oil yield (ca. 39 wt%) and low oxygen contents (ca. 12–16 wt%) by only increase high water ratio in the system at temperature above 400 °C. A comparison for the performances of the hydrogen donor solvents in liquefaction of biomass on oxygen content of the produced bio-oil is given in Table 4.

0.5 g of sodium carbonate as the catalyst, charged in the autoclave. Oil yield was found in the range of 23–36% (wt% dry) while biomass conversion was 84–95 (wt% dry). Table 3 tabulates the recent work in year 2014–2015, investigating the liquefaction hydrothermal of biomass under sub- and supercritical water conditions. The highest conversion of 90% obtained with miscanthus as a feedstock under supercritical water above 673 K [6]. Higher oil yields were recovered ~44–49% using algae as a feedstock [97,98]. Researchers from University technology Petronas conducted hydrothermal liquefaction oil palm biomass with the highest oil yield recovery of 39% [25]. It is observed the latest trend of hydrothermal liquefaction is using algae as a feedstock. Shakya et al. reported the advantages of algae over terrestrial biomass feedstock are high biomass productivity [98]. Many researchers conducted the hydrothermal liquefaction of biomass under sub- and supercritical water and reported the product conversion and oil recovery [25,97–100]. However, they did not report the generated water yield, therefore the estimation of the light oil yield (always assumed product loss) cannot be made. Isa et al. reported the generated water yield of ~28%, and therefore the light oil can be estimated at ~12% under supercritical water condition at 410 °C for 1 h [101]. Co-liquefaction of microalgae and rice husk was conducted by Gai et al. [102]. Gai and co-workers reported the major compounds identified in bio-crude from hydrothermal liquefaction at 300 °C were cyclic oxygenates (20.62%), followed by esters, ketones and alcohols (17.19%). However they did not report the oxygen reduction for the bio-crude as a result of experiment in subcritical water. Toor et al. [32] reported the lowest oxygen content of ~19% with heating value of 38 MJ/kg was obtained with sample name of O-2.2, however they did not specify the actual condition for that in hydrothermal liquefaction of Spirulina and Nannochloropsis salina under subcritical and supercritical water conditions. The lowest oxygen content of 12% obtained at 460 °C under subcritical water with miscanthus as a feedstock [6]. Isa and co-workers also reported the oxygen content for the produced bio-oil under supercritical water at 410 °C was 14%. Brand et al. studied the effect of heating rate on biomass conversion using subcritical with a 160 ml batch reactor at 250–350 °C [103]. They reported the significant effect on biomass conversion and influenced by the higher heating rate was only can be seen above 280 °C. However, the biocrude yield obtained in their work was only 27% at 350 °C. Brand and co-workers compared the data of their work with Kamio et al. which reported the higher cellulose conversion was obtained with slower heating rate at the final temperature of 170– 280 °C [103]. They suggested the different liquefaction behaviour

4. Conclusions 1. Works with hydrogen donor solvents have attracted a number of researchers in investigating the potential of producing bio-fuel as a renewable energy. 2. Various biomasses have been investigated as feedstock, aiming to obtain a higher product conversion, a higher bio-oil yield with low oxygen content using hydrogen donor solvents. 3. Many solvents can function as hydrogen donors as long as they have mobile carbon-hydrogen bonds with the ability to donate hydrogen to the unstable biomass fragments. The mechanisms of hydrogen donor from tetralin, glycerol, cyclohexane, and supercritical water in the liquefaction of biomass have been reported by previous researchers. 4. The hydrogen produced from hydrogen donor solvent serves as a deoxygenation agent to remove oxygen by forming water, as a hydrogenolysis agent to depolymerize biomass, as well as a radical quenching agent to prevent char formation. 5. Compared with non-hydrogen donor solvents, hydrogen donor solvents such as tetralin and decalin show significant improvement not only in conversion and product distribution to liquid but also on the quality of bio-oil (oxygen content) due to the improvement of hydrogenation and hydrocracking reactions with inhibition of polycondensation. 6. The advantage of hydrogen donor solvents over molecular hydrogen due to a lower strength bonding of C–H as compared to the H–H bond. Approximately 1.3–2.5% of hydrogen was donated using tetralin in biomass liquefaction, however a much lower of 0.5% was donated using supercritical water as a solvent. 8

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