1. Introduction

Figure 1. Thermochemical process conceptual diagram and outline of the article.

Figure 1. Thermochemical process conceptual diagram and outline of the article.

2. Bio-Crude Properties

Table 1. Bio-crude produced from various feedstock and their properties.

2.2. Chemical Properties

2.2.1. Oxygen Content

Liquefaction bio-crudes have significant oxygen content resulting from the depolymerisation of biomass components (i.e., cellulose, hemicellulose and lignin). These oxygenated compounds take the form of organic acids, alcohols, ketones, aldehydes, sugars, furans, phenols, guaiacols, syringols, and other oxygenates [13]. In crude oil refining, oxygen is removed to prevent poisoning of catalysts in the reforming process [55]. Studies correlating oxygen content to fuel properties, engine operation and performance have been done on biodiesel. Lower CO emissions [56] and PM [57] have been observed for relatively highly oxygenated fuels such as biodiesel.

2.2.2. Nitrogen Content

Nitrogen in fuel may interact with degradation products and form solid deposits [58]. Nitrogen content is not regulated by diesel or biodiesel standards, although in crude oil refining, nitrogen content is reduced through hydrotreatment to minimise catalyst deactivation and improve diesel stability [55].

Bio-crude from HTL of lignocellulosic materials usually has low levels of nitrogen with a maximum of 2% [14,16,21,22]. Higher levels of nitrogen have been reported for bio-crudes produced from garbage, wastewater sludge, and algae (up to 10%) due to the protein content of the feedstock [25,27,30,31,32].

2.2.3. Sulphur Content

The sulphur content of fuel is a regulated quantity as burning sulphur in fuel produces sulphur oxides [55] and sulphate particles that contribute to PM emissions [59]. Moreover, sulphur can cause increased cylinder wear and deposit formation [59]. ASTM D975 [58] and D6751 [60] limits sulphur content in diesel and biodiesel, respectively, to 15 ppm.

Lignocellulosic materials and algae have very minimal sulphur content. Bio-crude has been produced with only 0.1–1.3 wt % sulphur [22,31,33,35]. Biochar, on the other hand, has a higher sulphur content [13,61], which may mean reactions in liquefaction favour sulphur binding into compounds in the solid fraction.

2.2.4. Chemical Composition

Diesel is mainly composed of alkanes, alkenes and aromatics [62], while biodiesel is more oxygenated, comprised of fatty acid methyl/ethyl esters [63]. HTL bio-crude, on the other hand, is a complex mixture of oxygenated organic chemicals [13,64], aliphatics, sugars, oligomers, nitrogenous aliphatics, and nitrogenous aromatics [65].Table 2 shows the main chemical groups for bio-crude.

The chemical composition of bio-crudes is usually determined through gas chromatography-mass spectrometry (GC-MS). However, the vast amount of components and high complexity of the bio-crude prevent effective chromatographic separation, resulting in broad background signals [66]. More recent studies have used nuclear magnetic resonance (NMR) spectroscopy [66] and Fourier transform ion cyclotron resonance-mass spectrometry (FTICR-MS) to perform analyses with higher resolution and accuracy [67].

Table 2. Groups of chemicals of hydrothermal liquefaction bio-crude.

Table 2. Groups of chemicals of hydrothermal liquefaction bio-crude.

Main Components	Area% * Range	References
Phenolics	6%–65%	[14,20,39]
Esters	2%–44%	[14,27,39]
Aromatics and heterocyclics	6%–35%	[14,39]
Aldehydes	0%–18%	[14,20]
Carboxylic acids	2%–40%	[20,27,35]
Ketones	0%–38%	[20,27,35,40]
Alkanes	9%–13%	[35,40]
Nitrogenates	12%–23%	[35,40]

Note:*. Area % from gas chromatography-mass spectrometry results.

The effects of varying compositions on the physical properties of diesel and biodiesel have been studied, while for HTL bio-crudes these relationships have not been elucidated.Table 3 shows the properties of various groups in diesel and their effect on fuel properties. In biodiesels, chain length and unsaturation of fatty acids are usually correlated to properties. Increasing chain length increases cetane number (an indication of ignition quality;Section 3), heating value and viscosity, while increasing unsaturation in fatty acids decreases viscosity and cetane number, but increases density and volumetric heating value [68]. Although these relationships are for diesel and biodiesel they provide an idea of the potential effects chemical composition may have on the physical properties of HTL bio-crude.

Table 3. Properties of various chemical groups and their effect on diesel properties [62].

Table 3. Properties of various chemical groups and their effect on diesel properties [62].

Group	Ignition Quality	Heating Value	Density
n-Alkanes	Good	Low	Low
Isoalkanes	Low	Low	Low
Alkenes	Low	Low	Low
Cycloalkanes	Moderate	Moderate	Moderate
Aromatics	Poor	High	High

3. Upgrading Processes

3.1. Separation

Products of HTL are usually a gas phase portion, a liquid oily fraction, a liquid aqueous fraction and solid residue. Studies on HTL and pyrolysis of biomass use a number of physical separation methods, mainly done as part of the work-up to segregate product fractions for analysis. These methods can realistically carry out separations to isolate high-value products or facilitate further processing to produce fuel and high-value products. Removal of water content in the oil fraction is specifically important for suitability for upgrading processes, as water may cause catalyst inactivity [72].

3.1.1. Solvent Extraction

Addition of a solvent to the two-phase product can enhance separation and extraction. The liquid product can be decanted to separate aqueous and oil portions. This crude separation results in an oil fraction with a moisture content of around 5% [12]. The choice of solvent will primarily be based on its immiscibility with water to facilitate separation, and its efficiency to extract the organic components and maximise yield. Selection of an appropriate solvent can be done through a number of methods. One such method is the use of the Robbins’ chart of solute-solvent interactions, which describes effect of functional groups on solubility based on hydrogen bonding and electron donor-acceptor interactions [73]. Use of this chart will be beneficial when targeting extraction of chemicals with specific functional groups. However, due to the complexity of bio-crude as a mixture, many researchers have investigated various chemicals to determine the most appropriate solvent. The most efficient solvent is somewhat dependent on the composition of the bio-crude and hence, the original feedstock.

Figure 2. Block diagram of upgrading processes discussed in this section.

Figure 2. Block diagram of upgrading processes discussed in this section.

Solvent polarity is a key parameter for consideration in choosing appropriate solvents. With the abundance of polar compounds in bio-crude and bio-oil, polar solvents are often more appropriate for extraction. This was confirmed by Garcia-Perezet al. [74] by performing successive extractions of bagasse pyrolysis bio-oil using solvents of increasing polarity. The solvents used were, in this order, pentane, benzene, dichloromethane, ethyl acetate, and methanol. The fractions that were extracted in ethyl acetate and dichloromethane were the two largest, owing to the high polarity of the bio-oil components. Another fractionation method, performed by Chumet al. [75] used ethyl acetate to separate a phenol fraction from pyrolysis bio-oil.

In liquefaction studies, oils for product characterisation are usually extracted using polar solvents such as acetone [14,16,17,20,31], tetrahydrofuran (THF) [22], ethanol [27], chloroform [76], or dichloromethane (DCM) [25,29,30,32,33,40,77].

On the other hand, microalgae bio-crude contains significant amounts of alkanes [35,40], which means non-polar solvents may be more effective. Valdezet al. [78] studied the use of nonpolar solvents hexadecane, decane, hexane, and cyclohexane, and polar solvents methoxycyclopentane, dichloromethane, and chloroform to separate bio-crude from products of algae liquefaction. Higher yield was obtained from nonpolar solvents due to the similarity of the long carbon chain solvents to chemicals in bio-crude, as confirmed by GCMS analysis. On the other hand, polar solvents extracted oil with higher carbon content, with the researchers posing that these solvents recovered carbon-rich compounds akin to resins and asphaltenes.Table 4 shows the oil fraction yields of HTL of different feedstock at the optimum condition reported, without use of catalysts or pretreatment, where the bio-crude was extracted from the liquid product with a solvent.

Table 4. Yields of solvent extraction of HTL bio-crude from the liquid fraction using polar and non-polar solvents.

Table 4. Yields of solvent extraction of HTL bio-crude from the liquid fraction using polar and non-polar solvents.

Feedstock	Solvent	Yield	References
Acacia mangium wood	Acetone	32%	[21]
Ailanthus wood	Acetone	29%	[17]
Bagasse	Acetone	59%	[14]
	Acetone	31%	[21]
Bagasse pith	Acetone	30%	[21]
Banana stem	Acetone	21%	[21]
Beech wood	Acetone	28%	[16]
	Acetone	34%	[17]
Botryococcus braunii microalgae	Dichloromethane	58%a	[32]
Cattle manure	Dichloromethane	49%b,c	[29]
Chaetomorpha linum macroalgae	Dichloromethane	17%	[33]
Chlorella microalgae	Dichloromethane	42%	[77]
Cladophora coelothrix macroalgae	Dichloromethane	20%	[33]
Cladophora vagabunda macroalgae	Dichloromethane	28%	[33]
Coconut husk	Acetone	28%	[21]
Coconut shell	Acetone	34%	[21]
Corn stalk	Acetone	28%	[21]
Corncob	Acetone	76%	[17]
Cypress wood	Diethyl ether	15%	[18]
Derbesia tenuissima macroalgae	Dichloromethane	33%	[33]
Dunaliella tertiolecta microalgae	Dichloromethane	37%	[30]
Dunaliella tertiolecta cake	Chloroform	~22%	[76]
Garbage	Dichloromethane	~22%	[25]
Hazelnut seedcoat	Acetone	22%	[17]
Hazelnut shell	Acetone	22%	[16]
	Acetone	28%	[17]
Kenaf	Acetone	28%	[21]
Metroxylon sp. petioles	Acetone	23%	[21]
Metroxylon sp. stem	Acetone	29%	[21]
Nannochloropsis salina microalgae	Acetone	46%	[31]
Nannochloropsis sp. microalgae	Chloroform	35%	[78]
	Dichloromethane	30%	[78]
	Methoxycyclopentane	32%	[78]
	Hexane	32%	[78]
	Hexadecane	38%	[78]
	Decane	39%	[78]
	Cyclohexane	34%	[78]
Oedogonium sp. macroalgae	Dichloromethane	36%	[33]
Oil-palm empty fruit bunch	Acetone	33%	[21]
Oil-palm fruit press fiber	Diethyl ether	19%	[23]
Oil-palm husk	Acetone	27%	[21]
Oil-palm petioles	Acetone	23%	[21]
Oil-palm shell	Acetone	36%	[21]
Olive husk	Acetone	23%	[17]
Pine bark	Acetone	21%	[17]
Pineapple leaf	Acetone	24%	[21]
Rice husk	Diethyl ether	1.7%	[20]
	Acetone	29%	[21]
Rice straw	Acetone	23%	[21]
	Tetrahydrofuran	40%	[22]
Rubber tree	Acetone	31%	[21]
Scenedesmus sp. microalgae	Dichloromethane	34%	[41]
	Hexane	31%	[41]
Sewage sludge	Ethanol	~55%	[27]
Spirulina platensis	Acetone	38%	[31]
Spruce wood	Acetone	26%	[16]
	Acetone	26%	[17]
Tea waste	Acetone	23%	[16]
Ulva ohnoi macroalgae	Dichloromethane	30%	[33]
Wheat straw	Acetone	40%	[17]

Note: a. Organic content basis; no run without catalyst; b. Volatile content basis; c. NaOH was used as catalyst.

There are very few studies that have used more than one solvent to fractionate the oil product. Karagözet al. [20] separated the liquid product of liquefaction of sawdust and rice husk with diethyl ether and ethyl acetate. The solid fraction was also washed with acetone to obtain adhering oils. The largest oil fraction was obtained from the acetone wash, containing mostly phenolic compounds. The fraction obtained from extraction with diethyl ether has the second highest yield and is also comprised of mostly phenolics.

3.1.2. Distillation

Another separation process that can be applied is distillation. Its ubiquity in petroleum refining makes it a likely candidate for industrial scale bio-crude fractionation. There are various methods of distillation, and process selection depends on physical and chemical characteristics of the feed and the range of its components. Fractional distillation separates components using differences in boiling points. Vacuum distillation operates at reduced pressures, lowering the boiling point and enabling separation of components in less severe temperatures, preventing cracking or decomposition of components. Steam distillation uses steam to lower the partial pressure of the mixture, reducing the boiling point of components. Molecular distillation employs pressures below 1 Pa to separate components without the pressure exerted by the gaseous phase [51], thus the separation relies on differences of mean free paths of each component [79]. A typical continuous industrial scale distillation set-up is shown inFigure 3.

Distillation has been used in studies that characterise pyrolysis bio-oils. The temperatures used ranged from 100–250 °C for atmospheric distillation and 80–230 °C for vacuum distillation [79,80,81,82]. The ranges can be attributed to the varying tendency of bio-oil for cracking and polymerisation [83] when it reaches a certain temperature. Vacuum and molecular distillation allow for separation at lower temperatures to minimise thermal degradation. Removal of moisture is also a key result of distillation, to a resulting moisture content of 0.49% to 6.46% in middle and heavy fractions [79,80,81].

Figure 3. Continuous Binary Fractional Distillation [84].

Figure 3. Continuous Binary Fractional Distillation [84].

Fractional distillation of corn stover pyrolysis bio-oil was performed by Capunitan and Capareda [80] at both atmospheric and reduced pressures. The bio-oil was separated into three fractions. In atmospheric distillation, 84% of the bio-oil was recovered and at 500 mbar, 73% was recovered. Dramatic reduction of moisture was reported for the middle and heavy fractions, as well as a reduction of total acid number of the heavy fraction. These results were attributed to separation of water to the light fraction and of acidic components to the middle fraction.

Vacuum distillation and two stages of molecular distillation of pyrolysis bio-oil in series were conducted by Guoet al. [79]. Vacuum distillation was performed only as a pre-treatment step to remove water and light hydrocarbons. The product of vacuum distillation was fed into the first molecular distillation at 1600 Pa and the process yielded 26% bio-oil. The product of the first molecular distillation was fed into the second molecular distillation at 340 Pa and yielded 23%. It was observed that viscosity of the light fractions was less than the original bio-oil since lower molecular weight compounds were separated into these fractions. Furthermore, the separations conducted and a detailed analysis of chemicals in each fraction using GCMS led the researchers to conclude that acids and ketones are easier to separate, compared with aldehydes and phenols, while diphenols and sugars cannot be removed by distillation.

Steam distillation was used by Murwanashyakaet al. [82] to obtain a phenol-rich fraction and separate syringol from wood pyrolysis bio-oil at 105 °C. While this study was geared towards isolation and purification of valuable chemicals, this might also be a pathway to remove unwanted compounds or concentrate compounds with heteroatoms for more focused processing.

At this time, there are limited studies on distillation of HTL products before hydrotreatment, since separation is not necessary to analyse products.

3.2. Hydrogenation

As discussed in the previous section, hydrogen and oxygen content is directly correlated to the bio-crude’s heating value. A way to improve heating value is to increase hydrogen content and remove oxygen content [13]. Hydrogenation is a process used in petroleum refining to increase saturation of hydrocarbons and remove sulphur, nitrogen, and oxygen. This is done to prevent catalyst deactivation in further processing, to minimise coking, and to improve fuel characteristics [55].

Another issue is the unstable nature of pyrolysis and liquefaction products due to polymerisation or degradation of components. Adjayeet al. [61] studied the stability of wood HTL bio-oil and observed an increase in viscosity and the amount of residue collected after distillation. Jenaet al. [35] observed an increase of 73% in viscosity of algae HTL bio-crude over 90 days. This suggests reactions of aldehydes and organic acids and an increase in the amount of higher molecular weight compounds due to polymerisation and condensation [85]. These potential reactions in bio-crude can be addressed by hydrogenation. Several hydrogenation methods and processes that have been investigated are discussed in this section. Typical reactions are shown inFigure 4.

Figure 4. Typical reactions in hydrogenation and cracking processes [86].

Figure 4. Typical reactions in hydrogenation and cracking processes [86].

3.2.1. Hydrogen-Donor Solvents

Addition of a hydrogen-donor solvent improved stability of pyrolysis and liquefaction oils and prepared the bio-oil for further upgrading processes. Studies in catalytic hydrodeoxygenation (HDO) observed thermal decomposition, production of coke and decrease in catalyst activity [87,88] when processing pyrolysis bio-oils.

Solvents such as tetralin, methanol, and ethanol [85] have been used to arrest free radical polymerisation. Adjayeet al. [61] added tetralin to wood HTL bio-crude and observed stable viscosities over 31 days. It was also important to note that the amount of oil decreased and the amount of residue increased in samples without tetralin, compared to a stable composition in samples treated with tetralin. Rezzoug and Capart [89] added tetralin to wood liquefaction bio-crude prior to catalytic hydrogenation and obtained increasing light fractions with increasing tetralin/oil ratio. Diebold and Czernik [90] studied ethyl acetate, ethanol, acetone, methanol, a mixture of methyl isobutyl ketone and a mixture of methanol and acetone to wood pyrolysis bio-oil and observed the effect of each additive over accelerated aging at 90 °C. Of all the solvents, the sample with 10% methanol had the slowest aging rate, measured as change in viscosity over time. It was further inferred that the low aging rate was caused by molecular dilution and formation of intermediate products that hinder polymerisation.

3.2.2. Mild Hydrogenation

Mild hydrogenation prior to a more severe hydrogenation process was proposed in several studies of pyrolysis bio-oils due to reports of coking during the severe HDO step [85]. This initial step uses catalysts similar to a typical hydrogenation process, but with less severe conditions. Processes are run with temperatures below 300 °C, and lower hydrogen pressure is required. This process aims to stabilise the bio-oil and reduce reactive oxygen sites that can produce char [72]. In several studies, mild hydrogenation resulted in higher thermal stability and reduced char formation [91] due to hydrogenation of aldehydes and ketones in the initial step [72]. It can then be inferred that lower coking has decreased catalyst inactivity and resulted in a higher oil yield [87].

A review of mild hydrogenation studies by Diebold [85] revealed reduction in oxygen content but an increase in viscosity of bio-oil. Furthermore, a study by Elliot and Baker [91] observed that HTL bio-oil can be directly subjected to HDO, unlike pyrolysis bio-oils which need to undergo low temperature treatment. This posits that mild hydrogenation prior to a more severe hydrogenation is an option for HTL bio-crude that has properties similar to pyrolysis bio-oil, such as high oxygen content and high amounts of carbonyl-containing groups.

3.2.3. Hydrodeoxygenation (HDO) with Metal Catalysts

Hydrodeoxygenation, which consists of hydrogenation and oxygen-removal processes, is done to improve the properties of the HTL product and bring it as close as possible to petroleum fuels or biodiesel. HDO of bio-crude involves high temperature, high hydrogen pressure, and the use of a catalyst to provide the right conditions for the hydrogenation process to proceed. Oxygenated components mentioned inSection 2 are the target chemicals in this process. Ease of oxygen removal depends on bonding of the heteroatom and steric effects [86]. A study of HDO of model compounds in a cobalt-molybdenum (CoMo) catalyst proposed a low-temperature reactivity ranking of the various categories of components in pyrolysis bio-oil. It was concluded that ketones have high reactivity, followed by carboxylic acids, phenols and furans. It was also postulated that saturation of double bonds and HDO of alcohols and ethers will occur at a lower temperature than what is required for a ketone [92]. These conclusions were based on observed activation energies, temperature of identical conversions and hydrogen consumption of the HDO of the model chemicals, which are shown inTable 5. A study by Laurent and Delmon [93] also observed a similar trend in activity of groups in HDO of pyrolysis bio-oil. They further observed that decarboxylation occurs alongside the HDO of carboxylic groups, and carbon is being converted to CO₂.

Table 5. Activation energies, iso-reactive temperature and hydrogen consumption of hydrodeoxygenation of model compounds with a CoMo catalyst, presented by Grangeet al. [92].

Table 5. Activation energies, iso-reactive temperature and hydrogen consumption of hydrodeoxygenation of model compounds with a CoMo catalyst, presented by Grangeet al. [92].

Chemical/Group	Activation Energy (KJ/mol)	Iso-Reactive Temperature (°C)	Hydrogen Consumption
Ketone	50	203	2 H2/group
Carboxylic acid	109	283	3 H2/group
Methoxyphenol	113	301	~6 H2/molecule
4-Methylphenol	141	340	~4 H2/molecule
2-Ethylphenol	150	367	~4 H2/molecule
Dibenzofuran	143	417	~8 H2/molecule

The use of CoMo and nickel catalysts is common for hydroprocessing in oil refineries. The selection of appropriate catalysts depended on the properties of crude oil, including metal, nitrogen, and sulphur content [94]. Therefore, application of oil refining processes and selection of appropriate catalysts for the hydrotreatment of bio-crude will have different considerations, some of which have been investigated in various studies.

An early study by the Pacific Northwest National Laboratory [91] investigated catalytic hydrotreatment of HTL bio-crude by using model chemicals, bio-crude distillates, and whole bio-crude. The study concluded that sulphided CoMo and Ni catalysts are the most effective for HDO since these two catalysts have high specificity, compared to other metallic catalysts tested. Furthermore, when sulphided CoMo was used, there was less saturation of aromatic compounds, which was desired for producing fuel similar to gasoline.

A study of 12 catalysts and eight hydrogen donors in hydrotreatment of wood HTL bio-crude was conducted by Grlicet al. [95]. It was observed that use of sulphided NiMo/Al₂O₃ catalysts resulted in the highest yield, lowest viscosity and high gross calorific value. The highest gross calorific value was obtained using an oxide form of the NiMo/Al₂O₃ catalyst. Of the solvents, use of tetralin contributed to a high product calorific value and low amounts of residue. It should be noted, however, that products of HDO with oxided NiMo/Al₂O₃ had very high viscosity, with oils sticking to reactor parts. In another study of the same group [96], effects of process conditions were observed in upgrading wood HTL bio-crude over a NiMo/Al₂O₃ catalyst. Temperature was the most influential factor: an increase in the heating value with an increase in the reaction temperature.

Several HDO studies observed that products from processes that use sulphided catalysts have some amount of sulphur [91,97]. To prevent adding sulphur into the HDO product, the use of noble metal catalysts can be an alternative to the more conventional catalysts already discussed. Noble metals such as platinum and rhenium were used in reforming processes to increase the octane number of fuels. However, the presence of sulphur in the feed poisons the catalyst [55]. On the other hand, use of noble metals to upgrade bio-crude is possible because of its low sulphur. A study using Ru/C and Pd/C in hydrotreatment of corn stover bio-oil observed 25.5% deoxygenation and a product H/C ratio of 1.47 at 300 °C using Ru/C [98].

Metal catalysts containing Rh, Rh-Co, Ni and Ni-Cu and using SiO₂, Al₂O₃, ZiO₂, CeO₂, and CeO₂-ZrO₂ supports were investigated by Yakolev,et al. [99]. Anisole and biodiesel (esters) were used as model reactants. In this study, it was concluded that the Ni-Cu catalysts were most effective in HDO, having a degree of deoxygenation of 60%–100%. Using CeO₂ as support achieved 100% HDO with Ni–Cu and 94.6% HDO with Rh, while using ZrO₂ achieved 60% HDO with Ni–Cu and 90.8% HDO with Rh. The oxide forms provided adequate oxygen vacancy on the support surface, allowing more oxygen removal.

Subcritical water with metallic catalysts in HDO was also studied. Zhang,et al [100] upgraded duckweed HTL bio-crude in subcritical water at 350 °C using Ru/C, Pd/C, Pt/C, Pt/γ-Al₂O₃, Pt/C-sulfide, Rh/γ-Al₂O₃, activated carbon, MoS₂, Mo₂C, Co–Mo/γ-Al₂O₃, and zeolite. Since the duckweed bio-crude had small amounts of sulphur and nitrogen, removal of sulphur and nitrogen was also considered in upgrading. The researchers observed that Ru/C had activity for HDO, desulphurisation, and denitrogenation, resulting to a product with the lowest sulphur, highest hydrocarbon content, and highest heating value. Conversely, Pt/C was observed to have the best HDO performance.

Another pathway for hydrogenation and reduction of carbonyl groups into methylene groups is the use of zero valent metals such as Fe, Zn, Al and Mg. Liuet al. [101] used Zn to upgrade pyrolysis bio-oil at ambient temperature and pressure. The experiment resulted in a minor change in % C, % H and % O; however it was observed through NMR spectroscopy that alcohols and ethers (C–O) increased, while ketones, aldehydes and carboxyls (C=O) decreased. GCMS results also confirmed a reduction in carboxylic acids and aldehydes.

3.3. Catalytic Cracking

While hydrodeoxygenation aims to remove oxygen atoms from the bio-crude, cracking processes aim to produce lighter products with improved properties. Thermal cracking, which was used in the early 1900s to produce gasoline from gas oil [55], is not considered to be a viable alternative for cracking bio-crudes. The highly-oxygenated bio-crudes have high coking potential [102]. Catalytic cracking, on the other hand, has better selectivity and can be performed with milder conditions, decreasing production of undesirable side-products like gases and coke [102]. Using hydrogen in cracking processes is termed hydrocracking, which is a hydrogen addition process with more severe conditions, compared to hydrotreatment [55]. Catalysts used in cracking are natural clay materials, synthetic amorphous silica-alumina, and synthetic crystalline zeolites [55]. One example of a synthetic zeolite is the ZSM-5, shown inFigure 5. The catalytic cracking process carries out dehydration, decarbonylation, dehydrogenation, hydrogenation, and hydrogen-transfer reactions [103]. There have been several studies on cracking pyrolysis bio-oils with zeolites [104,105] but only a few for HTL bio-crudes [106,107,108,109].

A continuous, downflow, fixed bed reactor with HZSM-5 at 330–410 °C and atmospheric pressure was used by Adjaye and Bakhshi [106] to upgrade wood powder HTL bio-crude. This process used cracking catalysts to upgrade bio-oil without the use of H₂. The study compared results of using HZSM-5, H-mordenite, H-Y, silicalite, and silica-alumina. Volatile “organic distillate” fraction yields ranged from 40%–67%, with optimum yield obtained from using H-mordenite and H-Y at 370 °C. However, the organic fraction still contained 10.7%–36.5% oxygenated compounds. Furthermore, 4.4%–20.5% weight of coke was produced. The researchers also studied reactions of bio-crude model compounds with the HZSM-5 catalyst. The study observed minimal coke formation and increase in cracking of non-volatile components in low feed concentration and low temperature. However, low conversion was also observed [107,108]. Furthermore, cracking, deoxygenation, aromatisation and polymerisation were proposed as the main reactions with this catalyst, which is similar to the proposal of Cormaet al. [103].

Figure 5. Microporous molecular structure of ZSM-5.

Figure 5. Microporous molecular structure of ZSM-5.

Thermal cracking and catalytic cracking were compared by Gevert and Otterstedt [109] using alpha alumina, EKZ-4 (containing rare earth zeolite Y), and EKZ-2 (commercial equilibrium catalyst). Better yield was achieved at 500 °C for thermal cracking, compared to a higher temperature, owing to the sensitivity to thermal cracking of components of the hydroprocessed bio-crude. Catalytic cracking achieved better liquid product yield when compared to thermal cracking at the same temperature. Furthermore, it was observed that lower catalyst to oil ratios resulted in better oil yield and lower gas and coke production.

3.4. Esterification

Adding alcohols to bio-crude is another method for changing chemical composition and improving physical properties. The added alcohol reacts with the organic acids to form esters, similar to chemicals that comprise biodiesel. The esterification reaction is shown inFigure 6. Using ethanol to upgrade pyrolysis bio-oils has been investigated as an alternative to hydrogen. Zhanget al. [110] reacted rice husk pyrolysis bio-oil with ethanol over solid acid 40SiO₂/TiO₂-SO₄²⁻ and solid base 30K₂CO₃/Al₂O₃-NaOH catalysts. It was observed that esterified bio-oils had vast improvements in viscosity, density and calorific value. The amount of esters increased 20-fold with the acidic catalyst, while also producing acetals.

Figure 6. Esterification reaction.

Figure 6. Esterification reaction.

Use of these alcohols in the supercritical regime has also been considered, exploiting superior fluid properties, the same rationale for hydrothermal liquefaction. Penget al. [111] compared the subcritical and supercritical upgrading of rice husk pyrolysis bio-oil with ethanol and HZSM-5 and reported the supercritical process as being more effective. The researchers observed that residue after vacuum distillation of unprocessed bio-oil and upgraded product was reduced from 38% to 15%.

The use of supercritical ethanol with Pt/C, Pd/C, Ru/C and Ru/HZSM catalysts to upgrade rice husk pyrolysis bio-oil was studied by Chenet al. [112]. The heating value of the bio-oil increased from 21.45 to 30 MJ/kg. It was also observed that the relative amount of desired products was achieved when Ru/C was used. The amount of acids and methyl esters decreased, while ethyl acetate increased. The amount of phenols also decreased, while cyclic ketones and alcohols increased.

4. Challenges and Future Research Prospects

4.1. Economic Considerations

Production of second-generation fuels uses low-cost feedstock but incurs significant capital costs. For liquefaction, a study estimated that using the current state of technology (SOT), the minimum selling price of woody biomass HTL fuel is US$ 4.44/ gallon gasoline equivalent (GGE) [117]. This is assuming a two-stage hydrotreatment process. In 2050, it is projected that the cost of production will be around US$ 3.03–3.79/GGE (0.80–1.00 per litre), depending on development of more cost-effective technology and process improvements [9]. In the same techno-economic study, Zhuet al. [117] obtained the goal case minimum selling price at US$ 2.52/GGE, assuming less organics lost to the water phase and a hydrocracking unit added to a single-step hydrotreatment process. The HTL and upgrading units comprise 61% of total installed costs for the SOT case and 49% in the goal case. Other costs such as those related to consumption of hydrogen and catalysts, waste treatment and disposal also differ according to efficiency of processes. Adding a hydrocracker for the goal case and increasing product yield eliminates costs associated with selling the heavy oil produced in HTL as a by-product, and offsets capital costs in installing hydrocracking equipment.

In another techno-economic study by another group led by Zhu [118], the minimum fuel selling price of products of HTL of lipid-extracted algae (LEA) was between US$ 2.07–7.11/GGE. The study included an HTL process, a hydrotreatment unit, a hydrocracking unit, and three separation columns. The highest capital cost was determined to be the hydrotreatment process, which was 39% of the total installed cost. The feedstock price affects price sensitivity the most, followed by product yield and upgrading equipment cost.

In these studies, it can be observed clearly that upgrading processes impact immensely on product price. As technologies improve, product yield and quality improves, translating to better revenues, offsetting installation costs. Moreover, cheaper and more efficient technologies can also reduce installation and operating costs. Lastly, hydrogen requirements and waste treatment and disposal costs may also be reduced.

Table 6. Upgrading processes and their effect on physical and chemical properties. Direct influence of processes to bio-crude property towards standard values.

Table 6. Upgrading processes and their effect on physical and chemical properties. Direct influence of processes to bio-crude property towards standard values.

Upgrading Process	Upgrading Mechanism	Viscosity	Density	Heating Value	O-Content	N-Content	S-Content	Chemical Composition	References
Solvent Extraction	Separation from water; increasing organic yield.			√	√			√	[14,16,17,20,21,22,25,27,29,30,31,32,33,35,40,76,77,78]
Distillation	Removal of water; separation of light from heavy components.			√	√			√	[79,80,81,82,112,113]
Addition of hydrogen-donor solvents	Provision of hydrogen in liquid phase for stability.	√	√	√	√			√	[61,85,89,90]
Mild hydrogenation	Provision of hydrogen in gas, hydrogenation reaction in mild conditions.			√				√	[72,85,91,92]
HDO/HDN/HDS	Hydrogenation in severe conditions to remove heteroatoms.			√	√	√	√	√	[86,92,93,94,95,96,97,98,99,100,101,115]
Cracking	Cleavage of high molecular weight compounds.	√	√	√	√			√	[102,103,104,105,106,107,108,109,115]
Esterification	Conversion of organic acids to esters.	√		√				√	[110,111,112,113,114,115]

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