: Bio-oil Production through Hydrothermal Liquefaction (HTL) of Biomass: Recent Developments and Future Prospects

Leichang Cao Daniel C. W. Tsang Shicheng Zhang

Introduction

Since the Industrial Revolution of the 18th century, fossil fuels have become the means of accessing energy for human society, which currently account for nearly 85% of the total energy consumed worldwide (Jahangiri et al., 2018). The high level of development and utilization of fossil fuels has promoted technological progress and social development. However, traditional fossil fuels damage the environment and cause a large amount of greenhouse gas emissions in the process of their production and use.

In addition, the reserves of these fuels, such as coal, oil, and natural gas, are very limited and cannot be used sustainably by everyone (Posmanik et al., 2018). Faced with the increasing exhaustion of fossil energy and the deterioration of the ecological environment caused by its combustion, the development of renewable and sustainable new energy sources has become an urgent need for human survival and development.

New energy sources mainly include wind energy, water energy, nuclear energy, solar energy, and biomass energy (Duongbia et al., 2019; Si et al., 2019). Amongst these, biomass has advantages, such as the diversity of its raw material, its high energy reserves, and its lack of additional carbon emissions. Biomass is considered as the new energy with the highest potential to replace traditional fossil fuels. In addition, it is the only resource of renewable energy that can be converted into liquid fuel (Bi et al., 2017; Yang et al., 2018). Compared with fossil energy, biomass energy has a wide range of sources, and is clean, storable, and can realize the carbon cycle, so it has received widespread attention (Han et al., 2020; Sunphorka et al., 2015).

Biomass mainly includes agricultural and forestry waste, energy crops, algae, and organic waste. According to statistics, the total biomass produced by green plants worldwide through photosynthesis is as much as 220 billion tons, but less than 1 % of it is used as energy (Li et al., 2019). Generally, biomass energy conversion and utilization technologies can be roughly divided into three categories (Figure 10.1) (Chang et al., 2015; Mishra and Mohanty, 2020): first, biomass is directly combusted to obtain heat or is used for power generation; second, biomass is subjected to HTL, direct pyrolysis, gasification, and transesterification (from extracted oil) to obtain bio-based liquid fuels, chemicals, and combustible gases; third, biomass is subjected to anaerobic fermentation to obtain biogas, or by combining enzymolysis and fermentation to obtain bioethanol.

HTL has broad research prospects in the production of bio-oil from biomass. It is favored by scientific researchers and has become one of the hot topics of research (Lin et al., 2017; Mishra and Mohanty, 2020). The HTL of biomass uses water or green organic solvent to liquefy biomass to produce a value-added product under a certain temperature (200-400°C), pressure (5-25 MPa), and catalyst. The products are mainly bio-oils, with co-products such as coke, water-soluble substances, and gases (Wang et al., 2018). HTL has advantages unmatched by other biomass treatment technologies, but there are still many problems that need to be solved urgently. At present, research on the preparation of bio-oil from the HTL of biomass is mainly at the laboratory stage (Hayward et al., 2015; Wang et al., 2019b).

The main problems are: (1) the temperature and pressure are high during the reaction and the choice of parameters basically depends on experience with a certain blindness (Sun et al., 2020); (2) the composition of bio-oil is complex and the quality is low as a drop-in liquid fuel, the N content of some bio-oils (e.g. from algae) is high, and it is difficult to upgrade and refine bio-oils (Prajitno et al., 2018); (3) the solubility of coke, tar, and solid residues formed in the reaction process in water is very small, which can easily cause deposition and block the equipment (Hirano et al., 2020).

FIGURE 10.1 Conversion routes for biomass and its main products.

In response to the above issues, this chapter first briefly introduces the hydrothermal degradation pathway of different components of various biomasses and the main factors influencing bio-oil yield from the HTL of biomass. The catalytic upgrading of bio-oil is comprehensively reviewed and analyzed in a following section.

Comparison of HTL and Other Technologies for Biofuel Production

Biofuels from Various Biomasses

Among the renewable energy sources, biofuels derived from biomass are the most favorable ones for sustainable development. The reasons are: (1) biomass has the advantages of huge reserves and extensive distribution which ensure a supply for biofuel production (Bi et al.. 2018); (2) biofuel has a high energy density and is comparable to petroleum (Alma et al., 2016; Bunt et al., 2018) (Table 10.1); (3) biofuel can realize zero carbon emissions since it is derived from biomass (Eboibi et al., 2015); (4) some high value-added chemicals can be generated from biofuels as an alternative for petroleum biorefinery (Wang et al., 2019a). In the past decade, biofuels such as ethanol and biodiesel have been successfully developed and used. As an alternative fuel for fossil fuels, it can be blended with gasoline or diesel for engines. Because of the great advantages of biofuels, they have been

TABLE 10.1

The Calorific Values of Various Biofuels

TABLE 10.2

rapidly developed and utilized in recent years, and their development has gone through three or four generations (Gu et al., 2020; Kumar and Pant, 2015; Subagyono et al., 2015) (Table 10.2).

Merits of HTL for Biomass Conversion

There are various methods of biomass energy utilization: direct combustion, biochemical conversion, and thermochemical conversion (Gu et al., 2020; Remon et al., 2019). The technologies have different features (Table 10.3). At present, the main methods are biological and thermochemical conversion. The biological method is to use the activity of certain specific enzymes or bacteria to destroy polymers in biomasses and transform cellulose, hemicellulose, lignin, sugars, lipids, and proteins into bioethanol or biogas (Tsegaye et al., 2020). Thermochemical methods mainly include direct pyrolysis and HTL to produce bio-oil (Kumar and Pant, 2015; Remon et al.. 2019).

Compared with other biomass conversion technologies, HTL has the following obvious advantages. First, it does not require drying the raw materials in advance, which significantly reduces operating costs for the waste with high moisture content. Second, the reaction conditions are relatively mild. A pressurized hot solvent is used

TABLE 10.3

Features of Different Biomass Utilization Methods

as the reaction medium and reaction reagent, and bio-oil can even be produced without adding other chemicals. The whole process is environmentally friendly. Third, the HTL process is less corrosive to equipment.

HTL of Biomass

Major Components of Biomass and Their Decomposition Routes in the HTL Process

Their Decomposition Routes in the HTL Process

Cellulose

The temperature ranges of the main components in lignocellulose are: hemicellulose (0'-200°C), cellulose (2OO'-3OO°C), and lignin (250-340 °C) (Cao et al., 2016). Cellulose is made up of glucose through 0-1.4 glycosides. When the temperature rises, 0-1,4 glycoside bonds will gradually break and glucose monomers will be formed. Glucose monomers will further transform some oligomers under hydrothermal conditions (Li et al., 2015). The main components of the degradation products are furfural, ketones, and phenol compounds (Cao et al., 2017; Usman et al., 2019).

Hemicellulose

Hemicellulose is the second most abundant polysaccharide in nature after cellulose, accounting for 14-50% of the dry weight of lignocellulosic biomass (Baloch et al., 2018). Hemicellulose includes a variety of structural units, including five carbons (xylose and arabinose) and six monosaccharides (mannose, galactose, and glucose).

The most abundant hemicellulose in hardwood and many agricultural residues is ‘xylan’ (mainly consisting of xylose) (Cao et al., 2018b). HTL is one of the ideal methods for treating hemicellulose. Under appropriate conditions, it can produce soluble products such as monosaccharides and degradation products of monosaccharides, such as furfural or hydroxymethylfurfural, and carboxylic acid.

Lignin

Lignin is the most abundant aromatic hydrocarbon polymer, which is polymerized from phenolic compounds (Cao et al., 20l8a-b; Younas et al., 2017). The precursors of these phenolic compounds are three aromatic alcohols, namely coumarol, coniferyl alcohol, and glucosinolate. More than two-thirds of these monomers are connected through ether bonds (C-O-C). and the rest are connected through C-C bonds. The three aromatic components in the polymer are called p-hydroxyphenyl (H), guaiacyl (G), and syringal (S). The structure of lignin indicates that it can be used as a source of valuable chemicals, especially phenolic compounds (Cao et al., 2018c). Lignin is usually combined with hemicellulose, which is known as the skeleton structure of plant cell walls. It is not only physically bonded, but also covalently bonded. In hydrothermal treatment, the cross-linked structures of lignin and lignin-hemicellulose can be degraded, partially depolymerized, and severely degraded so that their structures are rearranged (Cao et al., 2017). The hydrothermal products are monocyclic phenolic compounds, including phenol, 2-methoxy-phenol, 4-methyl-l,2-ben-zenediol, 1,2-benzenediol, 3-methyl-l, and 2-benzenediol (Lyckeskog et al., 2017).

Protein

Protein is the main source of nitrogen heterocyclic compounds in bio-oils (Gu et al., 2020; Teri et al., 2014). At temperatures of 0-100°C, proteins undergo hydrolysis reactions to form various amino acids. As the temperature rises, various reactions occur in the generated amino acids. The decarboxylation and the deamination are the two main reactions and, between 100 and 200°C, carboxyl functional groups in some amino acids are decarboxylated to form amine compounds, in which carboxyl groups are released as gas-phase products, such as CO, (Teri et al., 2014). Another part of the amino acid is formed into organic acid through a deamination reaction, in which the amino group is released in the form of NH,. In addition, the amino acid may undergo a Maillard reaction’ with a carbohydrate hydrolysate such as reducing sugars to form a variety of heterocyclic nitrogen oxides, including pyrrole, pyrrolidone, pyridine, and imidazole, and finally to form a nitrogen-rich polymerized substance which is called melanin (Feng et al., 2018).

Lipids

A lipid is the component closest to bio-oil in biomass, and the conversion efficiency of it into bio-oil is also the highest (Chen et al.. 2015; Gu et al., 2020). These make it an ideal component for preparing bio-oil. The HTL reaction process of lipids has been deduced and lipids have been hydrolyzed to produce glycerol and long-chain fatty acids in the temperature range 0-100°C (Qiu et al., 2019). As the temperature rises, some fatty acids are converted into long-chain hydrocarbons. In addition, the hydrolysis intermediates of fatty acids also undergo decarboxylation reactions and polymerization reactions to produce alkane and olefin hydrocarbons.

Carbohydrates

Although a carbohydrate is ideal feedstock for fine chemical production through acidic hydrolysis with various liquid or solid catalysts, it is one of the least easily ones to be converted into bio-oil among the major components of biomass (Cao et al., 2019). Within the temperatures 0-100°C. carbohydrates are hydrolyzed to produce reducing and non-reducing sugars. As the temperature rises, the reducing sugar and non-reducing sugar molecular bonds are broken and repolymerized to form epoxy compounds. During the hydrothermal process, some sugars are converted into the oil phase, water phase, and gas phase, but most of the sugar compounds are converted into the solid phase. Therefore, it is wise to use the two-step liquefaction method that first extracts the sugar compounds in the biomass (mainly algae) and then prepares the bio-oil without degrading its quality.

Effect of HTL Parameters

Biomass Feedstock Composition

The specific compositions of different biomasses are quite different, and even under the same reaction conditions, the yield and composition of the obtained liquid products will vary greatly. It is found that biomass with a higher cellulose or carbohydrate content is easier to liquefy, and lignin has the greatest effect on the composition and quality of liquefied bio-oils.

Huang et al. (2013) studied the liquefaction reaction process of three different biomasses (straw', microalgae, and sludge), and found that sludge liquefaction had the highest bio-oil yield, reaching 39.5%, which was higher than those of straw (21.1%) and microalgae (34.5%). In addition, the sludge bio-oil had the highest calorific value (36.14 MJ/kg). The conversion rate of microalgae was the highest and reached 79.7%. Feng et al. (2014) liquefied the bark of white pine, spruce, and birch at 300°C and an initial N, pressure of 2.0 MPa for 15 mins using ethanol and water as the reaction solvent. They investigated the effect of ash content in biomass on biooil yield and composition. The results showed that w'hen the three kinds of biomass bark (white pine, spruce, white birch) were pre-deashed, their conversion rate and bio-oil yield decreased, w hile when the basic catalysts of K₂CO₃ and Ca(OH), were added, the yield of the liquefied bio-oils increased and the composition changed, indicating that the ash substances (compounds containing K and Ca) play a catalytic role in the liquefaction process (Feng et al., 2014).

Reaction Temperature

The reaction temperature is the most important factor in biomass HTL, which affects its conversion, the yield of liquefied products, the distribution of products, and the calorific value of products (Lin et al., 2017; Wang et al., 2019b). As the reaction temperature rises, the chemical bonds in the macromolecular compounds of the biomass will be broken, and the depolymerization reaction will occur with the free radical concentration increase. The probability of repolymerization of the decomposed small fragments will also increase, which will be beneficial to the production of bio-oil.

When the temperature approaches or exceeds the critical point of water, the higher the temperature, the more severe the secondary reaction will be, resulting in the decomposition reaction of the intermediate products to produce gas, and the polycondensation reaction to produce solid residues, leading to a decrease in the yield of bio-oil. Zhu et al. (2015) studied the HTL process of barley straw in the temperature range 280-400°C. The results showed that low temperature was beneficial to the production of bio-oil and that its yield was the highest (34.9%) at 300°C, though when the reaction temperature further increased to 400°C, the yield was reduced to 19.9%. This decrease in bio-oil yield was due to the polycondensation reaction of intermediates at high temperature to produce solid residues (Zhu et al., 2015).

Reaction Time

The reaction time refers to a period after the liquefaction reaction rises to a set reaction temperature and is maintained at that temperature, and which does not include heating and cooling time. Many researchers have studied the effect of the reaction time on the HTL of biomass and found that the yield of bio-oil is closely related to the length of the reaction time (Pourkarimi et al., 2019; Xu and Savage, 2015). In the process of biomass HTL, usually a short reaction time is conducive to the production of a large amount of bio-oil, but a too short period of time will make the reaction incomplete (Chan et al., 2015). In addition, if the reaction time is too long, it will cause the polymerization of the intermediate products and generate solid residues, which will reduce the yield of bio-oil. Therefore, it is very important to choose a suitable reaction time. Generally, the favorable reaction time ranges from 30 to 120 min for bio-oil production.

Reaction Solvents

The reaction solvent is an important factor that affects the liquefaction reaction process. It dissolves the biomass raw materials and stabilizes the free radicals generated by the reaction. It can inhibit the polymerization of intermediate products and provide active hydrogen. During the HTL of biomass, the solvents used in biomass liquefaction are mainly methanol (Han et al., 2020), ethanol (Jogi et al., 2019), water (Posmanik et al., 2017), isopropanol (Wang et al., 2019a), glycerol (Kosmela et al., 2017), ethyl acetate (Tsubaki et al., 2019), oxygenates (Pedersen and Rosendahl, 2015), hydrocarbons (Kaur et al., 2019), and other organic solvents (Wang et al., 2020). Among these solvents, water is the cheapest reaction solvent and is non-toxic and does not cause any pollution to the environment. But the use of water as a solvent often requires severer conditions, and compared to bio-oil produced in ethanol and other organic solvents the yield in water is low (Jogi et al., 2019).

Methanol is a toxic organic solvent that can be harmful to humans, though it is rarely used. As we all know, ethanol is the most commonly used organic solvent in the liquefaction of biomass. It is non-toxic and can be recycled. Compared with water, ethanol dissolves more easily high molecular weight cellulose, hemicellulose, and lignin derivatives due to its low dielectric constant. In recent years, many researchers have found that there is a synergy between ethanol and water in the HTL of biomass, which not only affects the biomass conversion rate, the bio-oil yield, but also the composition of bio-oil (Han et al., 2020; Yang et al., 2018).

Reaction Pressure

Reaction pressure is another factor in the liquefaction of biomass. By keeping the reaction pressure above the critical pressure of the solvent medium, the rate of biomass hydrolysis and dissolution can be controlled. At the same time, high pressure will increase the density of the solvent, and high-density media can effectively penetrate the molecules of biomass components, thereby strengthening the decomposition of biomass molecules (Gu et al., 2020; Subagyono et al., 2015). However, once supercritical liquefaction conditions are reached, the effect of pressure on liquid oil yield and gas yield is negligible, because in the supercritical region, pressure has very little effect on the performance of the solvent medium.