Keywords

1 Introduction

Energy Alternatives India (EAI) estimates 450–500 million tons of biomass is produced from various sources in that 31.2% of it is highly utilized for production of bioenergy [1]. Bioenergy is one of the key renewable resources taken from different sources that is awfully heterogeneous. Bioenergy is the most suitable form of alternate energy that is derived from biological substances rich in organic substance – biomasses. For each generation of feedstock, the compositions vary broadly and are tedious to classify as most biomasses are wastes of various biochemical processes and are heterogeneous. The first-generation feedstocks are edible crops like sugarcane, palm oil, and rapeseed that were used for thermochemical conversion [2], whereas second-generation feedstocks are nonedible crops of lignocellulosic biomass like wood, straw, etc., used to avoid the shortage of food compared to first-generation feedstock [3]. In both generations, the plants or crops fix carbon through photosynthesis in the form of carbohydrates. When processed, they leave out carbon dioxide (CO2), and thus the vent gas can be recycled by other plant. In the past few years, the third-generation feedstocks, i.e., algal biomasses, emerges loudly and attract the researchers than the first- and second-generation feedstocks [4]. They can be broadly classified into microalgae and macroalgae based on its size, and like plants, algae store energy in the form of carbon-rich lipids through photosynthesis.

In a crude point, the various classifications of biomass based on their primary source are given in Table 1.

Table 1 Biomass classification based on the source
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The two major categories of biomass that are going to be discussed are lignocellulosic and algal biomass. The lignocellulosic plants belong to the division Spermatophyta of the subkingdom Phanerogame in the kingdom Plantae where the algae belong to the division Thallophyta of the subkingdom Cryptogamae in the kingdom Plantae. There are major differences in the structure and composition of both biomasses. For a terrestrial lignocellulosic plant, the cell wall is made of uniformly arranged layers of cellulose, hemicellulose, lignin, and pectin [5]. It also contains smaller amounts of proteins, soluble extractives, sugars, nitrogenous amino compounds, resins, waxes, oils, chlorophyll, and ash [6]. While the main composition of algae is proteins, carbohydrates, and lipids, their percentage might vary even in a single alga depending upon the conditions of growth and age of the culture, and there are some algal species like Coleochaete thallus which even contains lignin as well in smaller percentages [7].

Cellulose is a polymer of D-glucose subunits linked by α-(1,4)-glycosidic bonds throughout its length that forms the organized fibrous linear structure. These long fibrous polymeric chains are packed into bundles of microfibrils by hydrogen bonds and weak van der Waals bonds. These microfibrils are concealed by hemicelluloses and lignin, thereby adding additional strength to the cell wall. The hemicellulose is a heteropolymer made of short lateral chains and branches of sugars and polysaccharides linked by α-(1,4)-glycosidic bonds like cellulose and occasionally by α-(1,3)-glycosidic bonds. Xylose is the major sugar component, and it also includes arabinose, rhamnose, glucose, galactose, mannose, and other uronic acids like 4-O-methylglucuronic and D-glucuronic, and D-galacturonic acids [8]. Lignin has a complex cross-linked polymeric structure with large molecular weight which mainly constitutes coniferyl alcohol, p-coumaryl alcohol, and sinapyl phenyl propionic alcohols as monomers which are linked together by alkyl-alkyl, alkyl-aryl, and aryl-aryl ether bonds [9]. Extractives include some organic compounds like alkaloids, phenolics, glycosides, saponins, terpenes, pectins, waxes, gums, mucilages, resins, and oils. They act as plant’s energy storage units and also as a defense against insects and microbes that take plants as food. They also contribute to the wood properties such as color, odor, and decay resistance. Ash is the inorganic final residue that remains after complete combustion of the biomass at a high temperature [10]. Proteins are made of one or more peptide chains with various percentages of amino acids, namely, lysine, cysteine, methionine, and tyrosine folded together into a solid or fibrous form. They act as the means of nitrogen storage for the algal cells which decreases as it grows with age [11]. Carbohydrates consist of starches and sugars that are formed as a result of energy absorbed by the chloroplasts and reduction of carbon dioxide into 3-phosphoglycerate in the presence of ribulose-1,5-bisphosphate carboxylase/oxygenase. They act as the structural part of the algal cell wall and provide energy needed for metabolism under dark conditions [12]. Lipids are long carbon chain molecules with carbon numbers 16–24 that consist of polar and neutral constituents of fatty esters, fatty acids, hydrocarbons, and triglycerides. They are formed in the inter-thylakoidal space of the chloroplast and stored inside the cytoplasm which serves as a structural component of the cell membrane. The increased lipid content in algae decreases the specific gravity, making the cells buoyant [13].

The chemical composition of the biomass varies from one plant species to another. The lignocellulosic and algal biomass compositions with their constituents of some of the feedstock are given in Tables 2 and 3, respectively.

Table 2 Lignocellulosic biomass feedstock composition
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Table 3 Algal biomass feedstock composition
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The chemical energy stored in the biomass can be converted into energy and useful products by numerous ways. The two frequently and majorly used pathways are (a) thermochemical processes and (b) biochemical processes. Thermal conversion processes include combustion, gasification, pyrolysis, carbonization, and hydrothermal conversion techniques, while biochemical processes include anaerobic digestion, fermentation, transesterification, and biohydrogen production. Though there are many methods available, this chapter focuses on one of the fastest and cost-effective conversion methods, which is hydrothermal conversion.

2 Hydrothermal Conversion

The biomass is converted into biochar, biocrude, and biogas in the presence of subcritical or supercritical water at high temperatures and pressures. The three phases of water and various points can be identified from the phase diagram as given in Fig. 1. Water at subcritical conditions breaks the long chain macromolecules of the biomass into smaller fragments by acting as a catalyst due to its higher ionic product that favors acids or base-catalyzed reactions [14]. In general, hydrothermal conversion is further divided into three separate processes: (1) hydrothermal carbonization, (2) hydrothermal liquefaction, and (3) hydrothermal gasification depending on the severity of its operating conditions.

Fig. 1
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Phase diagram for water

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2.1 Hydrothermal Carbonization

This is a process in which the biomass is kept suspended in water at elevated operating temperature and pressure range of 180–350°C and 2–10 MPa, where high-potential solid chars of moderate calorific value are obtained. Generally, the process is exothermic since it utilizes the oxygen and hydrogen present in the raw material [15]. The reaction pressure is not actually controlled; rather presence of liquid in biomass helps to promote the pressure up to a maximum of 10 MPa [16]. Though the process is carried over at mild operating conditions, care should be taken in keeping the water under subcritical state. Sometimes the medium becomes alkaline due to the drop in pH, and this is because of the formation of by-products inside the reactor. The main advantage of HTC is handling of high moisture content feedstock; the solid char is highly sterilized (biologically) due to the thermal conversion, and energy incentive product is derived [17]. But still HTC was found to gain less interest among the researchers because of the datum that hydrochar possesses less energy-carrying space for solid products than liquefaction and gasification [18].

2.2 Hydrothermal Liquefaction

Biomass with higher percentage of moisture can be utilized for the liquefaction process. Unlike hydrothermal carbonization, liquefaction is carried out under lower temperature (250–450°C) and high pressure (4–20 MPa) in presence of water or other organic solvents like hexane, methanol, acetic acid, acetone, etc. [19]. At this range the solvent behaves like a compressible fluid which generates high solvation environment, thus making the long chain hydrocarbons dissociate into the shorter chain [20]. A wide variety of raw feedstocks can be utilized for HTL conversion like lignocellulosic biomass, algal biomass, solid wastes that contain organic matters, etc. The primary product from HTL is the biocrude that should be further moved for upgradation. Along with the primary product, other by-products like hydrochar, aqueous, and biogas are produced. HTL finds its own place in attracting the research group because of its integrated characteristics of solvents like high ion product and low dielectric constant [21]. The major advantages of HTL than other thermochemical conversions are the following: effort for preconditioning of feed is not mandatory; it operates at moderate temperature; and heating rates can be slower than pyrolysis [22].

2.3 Hydrothermal Gasification

The other way of depolymerizing biomass is by hydrothermal gasification in which water reacts with biomass to produce gaseous hydrocarbon. Based on the operating conditions, they are classified into supercritical water gasification, near-critical water gasification, and low-temperature aqueous-phase reforming [23]. For low temperature HTG the sugar-like fructose or sugar alcohols like galactitol, sorbitol APR is considered as raw material. For high-temperature SCWG, raw materials like glucose, mannose, and fructose can be considered as good feedstocks [24]. To understand the background of HTG, the behaviors of solvents at high temperature and high pressure should be studied. The operating temperature for gasification ranges between 350 and 700°C which yields the gaseous products like H2, CH4, and CO2 [25]. Unlike other hydrothermal conversions, HTG produces hydrocarbon gas that has high HHV and negligible percentage of tar that is considered as advantage over other hydrothermal processes.

2.4 Conversion Chemistry

It is necessary to understand the behavior and mechanism by which the lignocellulosic and algal biomass degrades in the subcritical and supercritical environment. In case of lignocellulosic biomass, the major constituents present are cellulose, hemicellulose, and lignin, which influence the yield and composition of the products formed. Likewise for algal biomass, the degradation of carbohydrates, proteins, and lipids is to be studied. Due to different structures and properties of the major constituents, the depolymerization reaction pathways are different in the hydrothermal environment as given in Fig. 2.

Fig. 2
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Hydrothermal conversion reaction pathways

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Conversion of Cellulose

Hydrothermal depolymerization of cellulose involves several reaction pathways where linear bulk chain of molecules is fragmented into small fractions by hydrolysis and Lobry de Bruyn-Alberda van Ekenstein transformation. It was also observed that the molecular interaction was different in alkaline and acidic environments during the hydrothermal treatment. The alkaline environments led the products from hydrolysis to further degrade by retro-aldol transformation, followed by rearrangement of aldehydes, hydration, and dehydration into simpler acid and alcohols, while acidic environment resulted in the production of mostly 5-hydroxmethylfurfural (5-HMF) and its acid derivatives [26].

Conversion of Hemicellulose

Hemicelluloses constitute about 20–40% of the total plant biomass, where during high pressure hot compressed water uncatalysed solvolysis on various wood and herbaceous biomass materials showed that 100% hemicellulose was hydrolyzed and selectively decomposed into saccharides at 230°C, 34.5 MPa at 200 reaction time of 2 min [27]. Sasaki et al. [28] experimented on D-xylose, which is a substitute compound for hemicellulose under subcritical and supercritical water extraction, which confirmed the same reaction pathway at lower temperatures. At higher temperatures and pressures of 360–420°C and 25–40 MPa with a time of 0.02–1 s, retro-aldol condensation and dehydration resulting in glycolaldehyde, glyceraldehyde, and dihydroxyacetone followed.

Conversion of Lignin

Lignin consists of p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol units of p-hydroxy-phenylpropanoids held together by C-C or C-O-C bonds. Compared to cellulose and hemicellulose, lignin is relatively resistant to chemical or enzymatic degradation during hydrothermal conversion. In alkaline hydrolysis conditions of lignin, the C-O-C bonds result in phenols and methoxyphenols, where the condensation of these phenolic products resulted in significant amounts of solid residue.

Thus to reduce the solid residue and increase the yield of biocrude, the feedstock chosen must be in such a way that its lignin content is carefully balanced with cellulose and hemicellulose. The hydrothermal conversion of pure lignin at higher temperatures of 350°C, carried out by Wahyudiono et al. [29], resulted in products catechol, phenols, and cresols, which confirms the existence of secondary hydrolysis of methoxy groups. Similar results were found for hydrothermal conversion of Kraft pine resulting in major products as phenol, 4-ethylguaiacol, and methyl dehydroabietate [30].

Conversion of Carbohydrates

The carbohydrates in algae mainly constitute polysaccharides, celluloses, hemicelluloses, and starches. They are hydrolyzed to form simple sugars with glucose as the main product; the glucose is then rapidly converted to fructose which in turn degrades with fragmentation to form carbon-carbon double bond components like aldehydes and phenols which are intermediates, thus converting into gaseous products like H2, CO2, CH4, etc. [31].

Conversion of Proteins

Proteins mostly consist of linear polymeric amino acids that are connected to carboxylic and amine group using C-N bond links. During the hydrothermal conversion process, these bonds get hydrolyzed to produce free amino acids which quickly undergo decarboxylation and deamination to produce hydrocarbons, amines, aldehydes, and acids [32].

Conversion of Lipids

Lipids are mainly comprised of fatty acid triglycerides (TAG) which are nonpolar components with aliphatic characteristics. TAG is hydrolyzed to form fatty acids and glycerol, where glycerol readily gets converted into water-soluble components. Free fatty acids are pretty much stable but partially get converted into hydrocarbons via decarboxylation [33].

3 Hydrothermal Conversion Process Conditions

The hydrothermal conversion processes depend upon various parameters: temperature and heating rate, solvent or hydrothermal media, biomass to solvent loading, residence time, and catalysts. Once the reaction is over, the reaction products are taken out and filtered for the residual water. Then it is washed with extraction solvent to leach out the biocrude from the biochar, where the excess solvent is removed by vacuum evaporation. There are various methods by which the recovery of the products is calculated, and most widely used conventions are given below:

$$ \mathrm{Biocrude}\ \mathrm{yield}\left(\mathrm{wt}\%\right)={W}_{\mathrm{es}}/{W}_{\mathrm{f}}\times 100 $$
(1)
$$ \mathrm{Biogas}\ \mathrm{yield}\left(\mathrm{wt}\%\right)={W}_{\mathrm{c}}-{W}_{\mathrm{rp}}/{W}_{\mathrm{c}}\times 100 $$
(2)
$$ \mathrm{Biochar}\ \mathrm{yield}\left(\mathrm{wt}\%\right)={W}_{\mathrm{r}}/{W}_{\mathrm{f}}\times 100 $$
(3)
$$ \mathrm{Water}\kern0.5em \mathrm{Soluble}\ \mathrm{Products}\ \mathrm{yield}\left(\mathrm{wt}\%\right)=100-\left(\mathrm{Crude}+\mathrm{Gas}+\mathrm{Residue}\right) $$
(4)

where Wf is the weight of the biomass (dry basis); Wes is the weight of products soluble in extraction solvents (ether, acetone, acetone, dichloromethane, etc.); Wc is the charged weight (biomass + hydrothermal media); Wrp is the weight of reaction products; and Wr is the weight of solid residue, i.e., biochar.

Some of the woody and algal feedstock, their possible hydrothermal pathway, type of reactor and its specifications, hydrothermal media used, catalyst, and the extraction solvent used are listed Table 4.

Table 4 Reaction parameters of different woody and algal species
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3.1 Effect of Temperature

Temperature plays an important role in the hydrothermal processing irrespective of biomasses. The temperature for a typical hydrothermal conversion process ranges between 180 and 700°C, and it mainly depends on the type of feedstock and its constituent composition (Table 4). Under such given elevated temperatures, it shows symbiotic effect in the yield of resultant products (biochar, biocrude, and biochar) from the biomass. The increased temperature depolymerizes the long chain complex organic molecule of length 800–10,000 units into chemicals of C15+ to simple gases like H2, CH4, and CO2. Such conditions not only improved the reaction rate but also created variation in reaction mechanisms. Tungal and Shende [34] explained the effect of temperature on pine sawdust under carbonization and liquefaction conditions, where the biocrude yield increased from 24.04 to 30.5% at 1:10 biomass loading and 90 min reaction time [34]. The tar formed during the hydrothermal conversion process is due to the impact of temperature under the subcritical condition, and its formation is extremely lowered under supercritical conditions. The two reasons that controlled this reaction are (1) reduced dielectric constant which led to free-radical reactions to leaving away with gaseous products and (2) under supercritical region tarry material which behaves like a solvent [51]. From studies it is observed that when operating temperature is elevated beyond 500–600°C, the depolymerization reaction occurs resulting in dissociation of complex bonds into simpler gaseous molecules [35]. Figure 3 shows the effect of temperature (225–600°C) on the yield of different hydrothermal products. This effect was also confirmed by a recent study by Alper et.al [38] on hydrothermal liquefaction of spruce wood at 250–300°C; the yield of biocrude increased slightly from 3.9 to 6.6 wt%. But beyond 300–350°C, the yield progressively decreased to 4.9 wt% due to the vaporization of biocrude at this temperature range [38]. Similar phenomena was observed beyond the supercritical condition (>374°C, 219 bar) in the case of algal biomass Posidonia oceanica due to the stimulation of steam-reforming and methanation reactions reducing the liquid and solid residue while increasing the gaseous products [50].

Fig. 3
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Effect of temperature on yield of hydrothermal products

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3.2 Effect of Hydrothermal Media

The hydrothermal media are generally called solvents, and they enhance the stability and solubility of the depolymerized macromolecules. Under subcritical conditions, these solvents act not as molecules but as radicals and induce the ionic reactions [52]. In most of the cases, deionized water is used as solvent, and sometimes Nanopure® water having ionic purity of 18.2 MΩ was used to eliminate any effects caused by the minerals present in tap water [46]. Few other studies [36, 40,41,42,43] revealed that replacing a portion of water with compounds like ethanol, methanol, acetone, and 2-propanol enhances the ionic product of the mixture yielding maximum biocrude and minimum biochar at 50:50 solvent to water ratio as shown in Figs. 4 and 5.

Fig. 4
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Effect of hydrothermal media on biocrude yield

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Fig. 5
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Effect of hydrothermal media on biochar yield

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3.3 Effect of Biomass to Solvent Ratio

The amount of solvent/hydrothermal media present inside the reactor is very crucial for the process. If the quantity is very less, it enhances carbonization, and on excess it promotes gasification of all residual materials irrespective whether it is of lignocellulosic or algal origin. Hence the biomass to solvent ratio has to be maintained carefully. Tungal and Shende [34] explained the effect of biomass to solvent ratio on pine sawdust under liquefaction conditions of 250°C, where the biocrude yield increased from 22.14 to 33.1% with catalyst loading of 5 wt% Ni(NO3)2 and 120 min reaction time [34]. This effect is shown in Fig. 6.

Fig. 6
figure 6

Effect of hydrothermal media on biochar yield

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3.4 Effect of Catalyst

Water acts itself as a catalyst during subcritical or supercritical hydrothermal conditions, and yet the products resulted with mostly oxygenated compounds with low high heating value (HHV). Thus to reduce the high O/H ratio in the resulting biocrude, it is required to add more catalysts into the reacting system. The addition of catalysts improves the gasification efficiency and reduces the solid residue (tar and char) by suppressing the condensation and repolymerization into stable macromolecules. Alkali salts and hydroxides have been frequently used as homogeneous catalysts, whereas Ni, Fe, Al, Pd, Pt, and Ti were used as heterogeneous catalysts, which were utilized to enhance the yield of biocrude.

Homogeneous Catalysts

Homogeneous catalysts improve gasification by accelerating the water-gas shift reaction, thereby increasing the liquid product yields. They raise pH of the mixture to more alkaline, which in turn inhibits the dehydration of the cellulose, hemicellulose, and lignin; thereby increasing the H/C ratio in the biocrude and biochar. Alkali addition suppresses tar and biochar formation during the hydrothermal conversion process. It also prevents higher degree of oxygen removal in the biocrude, and instead of decarboxylation, it might result in the formation of unstable unsaturated compounds that can be easily polymerized into char and tar.

Küçük and Ağırtaş [41] investigated alkali-catalyzed hydrothermal liquefaction of common reed in subcritical water environment, where catalytic solvent (ethanol) + 10% NaOH resulted in high conversion of 57.6% at 290°C [41]. In another study, Khampuang et al. [42] observed that liquefaction performed for 60 min at 340°C with a 1:1 (v/v) ethanol/water with addition of 10 wt% of NaOH resulted in oil yield of 50.2–57.2%; rather with no catalyst it was 49.0%. The oil yield with KOH addition (51.4–57.5%) was slightly higher than that with NaOH (50.5–56.4%) [42]. Similarly, alkali especially potassium in the form of K2CO3 exhibited a progressive effect on yield of biocrude. The hydrothermal treatment of cotton cocoon shell showed a catalytic activity which is as follows: KOH > NaOH > K2CO3 > Na2CO3 [43]. The alkali catalysts weaken the C-C bond, thereby decreasing the activation energy. It also triggers biomass swelling and increase in the surface area exposed, enhancing the retro-aldol cleavage. These alkali catalysts have the tendency to promote water-gas shift reaction during the hydrothermal conversion, thus favoring H2 and CO2 formation from CO. The resulting hydrogen gas in turn acts as a reducing agent, increasing the HHV and quality of the biocrude.

Heterogeneous Catalysts

Heterogeneous catalysts have been mostly used in hydrothermal gasification process and low-temperature applications, where they enhance the quality of the biocrude obtained. de Caprariis et al. [37] investigated that HTL of oak wood with addition of Fe as a catalyst increases the quality and quantity of biocrude. Fe acts as a sacrificial catalyst, which in turn undergoes surface oxidation into Fe2O3, thereby reducing the acids and aldehydes formed during the process. Gasification is crucial to a certain extent for biocrudes containing higher percentages of oxygenated compounds. Few other studies show that by doping oxides like Al2O3 with active substance like KF, Pd, and Pt and using it as a catalyst, the yield of the biocrude increased significantly [38, 46]. However, extensive gasification will reduce the biocrude yield. Some of the other heterogeneous catalysts reported for hydrothermal conversion of lignocellulosic and algal biomass include Ni doped over SiO2, TiO2, and zeolite [44, 48].

4 Biomass to Fuel

The useful products obtained from the hydrothermal processing are biochar, biocrude, and biogas, which have their own significance. These products were always energy intensified compared to their feedstock. This was confirmed by elemental analysis of the feedstock and the resulting products. The biochar obtained is converted into pellets and used for various applications, whereas the biocrude and biogas are further purified to get fine chemicals. The main parameter that is to be noted for any fuel is its heating value. The energy density of products can be compared by calculating the HHV using the united formula by Channiwala and Parikh [59] that can be used for both feedstock and the resulting products [59]:

$$ \mathrm{HHV}\left(\mathrm{MJ}/\mathrm{kg}\right)=0.3491\left(\mathrm{C}\right)+1.1783\left(\mathrm{H}\right)+0.1005\left(\mathrm{S}\right)+0.1034\left(\mathrm{O}\right)+0.0151\left(\mathrm{N}\right)+0.021\left(\mathrm{A}\right) $$
(5)
$$ \mathrm{O}=100-\left(\mathrm{C}+\mathrm{H}+\mathrm{N}+\mathrm{S}\right) $$
(6)
$$ \mathrm{Energy}\ \mathrm{Densification}\left(\mathrm{ED}\right)={\mathrm{HHV}}_{\left(\mathrm{Biochar}\ \mathrm{or}\ \mathrm{Biocrude}\right)}/{\mathrm{HHV}}_{\mathrm{Biomass}} $$
(7)

The elemental composition of the raw material, biochar, and biocrude obtained from the hydrothermal carbonization and liquefaction process is given in Table 5.

Table 5 Energy intensification of woody and algal species
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The high heating value (HHV) of the hydrothermal products compared to the feedstock is increased significantly due to the carbonization. It is observed from Table 5 that the hydrothermal products (biochar and biocrude) have a high energy densification ratio higher than 1, which proves the decarboxylation and deoxygenation reactions were involved during the hydrothermal carbonization and liquefaction. Table 6 shows the elemental composition of reference feedstock. Figure 7 depicts the van Krevelen plot, and it shows that the lignocellulosic feedstock is converted to products similar to that of lignite and liquefaction of algal biomass resulted in biocrude that resembles petroleum crude. The only disadvantage is the high nitrogen content in the biocrude.

Table 6 Elemental composition of reference feedstock
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Fig. 7
figure 7

van Krevelen plot for biomass and hydrothermal products

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Compared to other energy intensification processes, the hydrothermal processes have higher ratios and also low heat of reactions as the subcritical and supercritical water lowers the activation energy. Additionally, the mechanical dewatering or any other preheating is not required, as the feedstock of the process is moist biomass.

Hydrothermal coal is also called biocoal and has ash melting point similar to lignite, although in the case of algal biomass, the minerals in the solid product fuse together to form a sticky dense slurry-like material that changes the behavior of biochar entirely compared to lignocellulosic biochar. The algal biochar and solid residue are hygroscopic, and they are difficult to separate. Materials that are obtained from hydrothermal carbonization also have shown promising properties with varying functionalities, morphologies, and porosities [60]. They have important applications in a variety of modern fields such as electrode materials in superior performance supercapacitors, Na-based batteries, Li-ion batteries, and fuel cells [14].

At supercritical conditions, hydrogen and methane from biomass can also be manufactured from single-step hydrothermal gasification process without the formation of any solid residue:

$$ {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{H}}_2\mathrm{O}\to 6{\mathrm{C}\mathrm{O}}_2+14{\mathrm{H}}_2 $$
(8)
$$ {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\to 3{\mathrm{C}\mathrm{H}}_4+3{\mathrm{C}\mathrm{O}}_2 $$
(9)

Thus, Table 7 shows promising results that both lignocellulosic and algal biomass can be used for hydrogen gas production.

Table 7 Gaseous product composition result of hydrothermal conversion from woody and algal biomass (yield moles/kg biomass)
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5 Biomass to Fine Chemicals

The primary objective of any hydrothermal processing is to derive value-added products (biochar, biocrude, and biogas) in which biocrude on further processing results in a significant amount of fine chemicals. All these years many researchers have confirmed the presence of various fine chemicals in the biocrude through GC-MS analysis. The chemicals identified in biocrude obtained from the hydrothermal conversion of few woody and algal biomass feedstock and its commercial significance are listed in Table 8.

Table 8 Essential chemicals from hydrothermal processing
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6 Conclusion

The hydrothermal conversion of both lignocellulosic and algal biomasses was studied with the help of predefined reaction mechanisms of their respective constituents. This chapter briefly explained about the advanced technologies that are adapted in generation of energy-intensified fuels and value-added fine chemicals. The hydrothermal technologies with biorefinery would be a promising solution for sustainability as it can be implemented with minor modifications to the current industrial infrastructure. In the energy sector, the biocrude in addition to the natural crude can be used as such as the feed to existing refinery equipment, and there is no need for modification or retrofitting. Thus this technology is yet to be explored, and more value-added chemicals can be obtained from the hydrothermal conversion of lignocellulosic and algal biomass, thereby maximizing the yield of products with commercial significance.