**4. Hydrothermal (HT) processing**

Biomass with a moisture content of more than 30 wt.% needs to be dried before being suitable for some thermochemical processes such as pyrolysis, combustion, and gasification. Drying is a highly energy-intensive process and requires a large amount of energy. As a result, for biomass with high moisture content, the heat of moisture evaporation is higher than the heat available from biomass, becoming a net energy consumption. Wet biomass, typically with 70 wt.% or more water, can be converted using HT processing without energy-intensive drying [1, 5, 8].

HT processing applies heat and pressure in subcritical or supercritical water in a closed reactor. The biomass is surrounded by water during the reaction, and the presence of water speeds up biomass conversion. HT reactions are supported by water, which plays an active role as a solvent, reactant, and even a catalyst or catalyst precursor in the hydrolysis reactions. Due to the high ionic product in subcritical conditions, water shows both acidic and basic properties and behaves as a catalyst. As a result, the addition of acid or base catalyst can be avoided. Water behaves as a catalyst precursor due to relatively high concentrations of hydronium (H3O+ ) and hydroxide (OH− ) − ions resulting from the dissociation of water. In HT processing, water is used as a reacting medium. It is cheap, environmentally friendly, nontoxic, inherently present in wet biomass, and a better alternative to corrosive chemicals and toxic solvents. In HT processing, the reaction pressure is usually not controlled, but the temperature maintains the autogenic pressure corresponding to the saturation vapor pressure of water. It is an attractive process to convert wet biomass into three distinct product fractions, solid (HC), liquid (bio-oil/water), and a mixture of non-condensable gases. All three product fractions (solid, liquid, and gaseous) are formed at all temperatures in HT processing. However, reaction temperature (and pressure) determines the product distribution. The reaction shifts from solid products at low temperatures through liquid products at medium temperatures to gaseous products at high temperatures. The advantages of HT processing include the elimination of energy-intensive drying, high conversion efficiency, and relatively low operating temperature. Depending on the operating conditions, HT processes can be classified into three processes as hydrothermal carbonization (HTC), hydrothermal liquefaction (HTL), and hydrothermal gasification (HTG) [1, 5, 8, 21–23].

The physicochemical properties of water are strongly affected by pressure and temperature. Water exhibits gas-like behavior at or around the critical point with low density, low viscosity, high compressibility, and high diffusivity. Some physical and chemical properties of water at various conditions are shown in **Table 1**. Above the critical point, a single homogeneous fluid phase exists, and the density, viscosity, compressibility, and diffusivity of water are very sensitive to pressure and temperature changes. This unique property of water is a result of hydrogen bonding. When water is heated, hydrogen bonding in the water molecules becomes weak, allowing the dissociation of water into H3O+ and OH− ions. The characteristics of water change


#### **Table 1.**

*Physical and chemical properties of water at various conditions [24–27].*

from a polar solvent at ambient conditions to a nonpolar solvent at supercritical conditions. At ambient conditions, organic compounds, and gases are poorly miscible in water. But the high dielectric constant (about 80) makes water a good solvent for salts. The dielectric constant decreases rapidly with increasing temperature to about 27 (at 250°C and 5 MPa) and about 14 (at 350°C and 20 MPa). Under HT conditions, water displays less polar behavior due to decreasing dielectric constant and the miscibility of organic compounds is improved, which opens new reaction pathways. Temperature and pressure can be controlled to adjust reaction rates. Above the critical point, organic compounds are completely miscible. Below the critical point, organic compounds are miscible. When HT processing products are cooled down to ambient conditions, water, and organic compounds are separated again [23, 26, 28–30].

#### **4.1 Hydrothermal carbonization (HTC)**

HTC is carried out in compressed water at temperatures between 180 and 260°C under autogenous pressures (up to 4 MPa), and biomass feedstock in water is converted mostly to solids (HC). The residence time of HTC varies from minutes up to several hours. HTC aims to maximize the yield of HC. In addition, HTC produces bio-oil mixed with water in the liquid fraction (aqueous solution) and a small volume of gases (consisting of H2, CH4, and CO2). The product distribution and characteristics primarily depend on the process conditions and the type of biomass feed. The reaction temperature remains the main process parameter even though both reaction time and temperature influence the physicochemical characteristics of products. Solids concentration has a relatively smaller influence. The characteristics of HC are significantly affected by the chemical composition of the feedstock. The liquid fraction contains valuable chemical compounds, including organic acids (acetic acid, formic acid, lactic acid, levulinic acid, and propionic acid), furan compounds

#### *Hydrothermal Conversion of Lignocellulosic Biomass to Hydrochar: Production… DOI: http://dx.doi.org/10.5772/intechopen.112591*

(furfural, furfuryl alcohol, and 5-hydroxymethylfurfural (5-HMF)), esters, phenolic compounds, lignin fragments, amino acids, sugars, and some nutrients (nitrogen and phosphorus) that are formed *via* the biomass polymers degradation. Hemicellulose degradation temperature in subcritical water is usually around 180°C and cellulose decomposition in subcritical water usually starts above 200°C, while lignin decomposes above 220°C [5, 8, 21, 31–34].

During HT decomposition, the physical structure of biomass is altered through a series of reaction mechanisms such as hydrolysis, dehydration, decarboxylation, aromatization, polymerization, and condensation. These reactions do not run sequentially; instead, they show simultaneous and interconnected reaction paths and the mechanisms depend on the feedstock type. Hydrolysis has a lower activation energy than most decomposition reactions, which initiates biomass HT degradation [35–37]. Under HTC conditions, marked changes in water properties (such as dielectric constant, ionic product, and polarity) zcatalyze the hydrolysis of biomass. These property changes unlock different reaction pathways that are not possible in other conversion conditions such as pyrolysis, liquid water, or steam at atmospheric pressures. The dissociation of water into H3O+ ions facilitates hydrolysis reactions. As a result of property changes, under HTC conditions, water acts as a reagent for various reactions and as a solvent for a wide range of organic biomass compounds. In hydrolysis, biomass polymers (such as hemicellulose, cellulose, extractives, and lignin) react with water and break ester and ether (mainly β-1-4 glycosidic) bonds creating oligosaccharides and fragments of lignin that enter the liquid phase. Consequently, complete disintegration of the physical structure of biomass may occur. Rapid degradation and depolymerization of hemicellulose and cellulose chains following hydrolysis produces a wide range of fragments, including water-soluble oligomers (cellobiose, celotriose, cellotetraose, cellopentaose, cellohexaose, arabinose, etc.) and monomers (glucose and fructose). Hydrolysis of lignin fragments gives rise to phenolic compounds. Most extractables possess good water solubility and are eluted simultaneously [35–37].

Subsequently, two important reactions (dehydration and decarboxylation) occur simultaneously, reducing both the oxygen and hydrogen content of the biomass. The extractables increase dehydration and decarboxylation, leading to a condensation reaction. The reaction mechanism of the HTC process mainly involves dehydration, decarboxylation, and polymerization. Dehydration is favored at low temperatures (< 300°C) [38, 39]. The soluble hydrolysis products undergo further dehydration, which removes water from the biomass by eliminating hydroxyl (-OH) groups. It also releases more water into the reaction medium and carbonizes biomass by lowering the H/C and O/C ratios. Decarboxylation removes CO2 from the biomass, eliminating carboxyl (-COOH) groups. Condensation and polymerization convert these compounds to larger molecules, which undergo further aromatization. The net result for LCB is highly aromatized fragments combining phenolic structures derived from the lignin dehydration with aromatization resulting from the carbonization of carbohydrates. The fragments undergo condensation and polymerization to form humic acid and bitumen-like material and partly precipitate to form HC [23, 40].

The main reason for a significant decrease in oxygen content could be the elimination of carboxyl groups, mainly from hemicellulose, cellulose, and extractives. Aromatization occurs because of dehydration and decarboxylation. In the process, hydroxyl and carboxyl groups in biomass are replaced by C=O and C=C bonds. Aromatic carbon structures formed are considered the building block of HC. The furfural compounds generated undergo hydrolysis, which further breaks them down into acids, aldehydes, and phenols. Due to the formation of organic acids such as acetic acid, lactic acid, formic acid, propionic acid, and levulinic acid pH of biomass and process water decrease as HTC proceeds, which further promotes hydrolysis and dehydration of small-chain polymers and monomers into much smaller fragments. The dehydration process of eliminating OH groups is sometimes called chemical dehydration. During the HTC of biomass, physical dehydration also occurs in addition to chemical dehydration. In physical dehydration, the reduction in the hydrophilic nature of biomass expels water out of the biomass matrix [35, 36].

The dehydration and decarboxylation of hydrolyzed products (fragments and extractable) lead to the formation of intermediate compounds. The 5-HMF generated from C6 sugars (D-fructose and D-glucose) and furfural generated from C5 sugars (D-xylose) are well-known dehydration intermediate compounds of sugars. The HTC process under acidic conditions allows the effective conversion of D-glucose to 5-HMF. D-glucose first isomerizes to D-fructose, and then undergoes dehydration to form 5-HMF. The 5-HMF (C6H6O3) is C6 heterocyclic aldehyde containing aldehyde and alcohol functional groups. It has formyl (-CHO) and hydroxy methyl (-CH2-OH) functional groups attached to 2 and 5 positions, respectively. The 5-HMF, one of the "top 10 biomass derived value-added chemicals" defined by the US Department of Energy, is a versatile platform chemical for a wide range of fuel and chemical products, including 2,5-furan dicarboxylic acid (FDCA), 2,5 dimethylfuran (DMF), 2 methyl furan, levulinic acid, formic acid, furfuryl alcohol, adipic acid, caprolactam, and maleic acid. The furfural (C5H4O2) is C5 heterocyclic aldehyde containing aldehyde (-CHO) and two olefins (-CH=CH-) functional groups. Furfural can undergo various chemical reactions due to its aldehyde and olefin functional groups are considered a promising platform chemical that can be used to produce a range of chemical products, including DMF, tetrahydrofuran (THF), furan, furfuryl alcohol, 2-methyl furan, and levulinic acid [1, 35, 38, 39, 41, 42].

There are two pathways to HC formation: solid-to-solid conversion (primary char) and polymerization of aqueous phase intermediate compounds (secondary char). The carbon-rich intermediate compounds (such as furfural and 5- HMF) resulting from hydrolysis and dehydration undergo condensation, polymerization, and aromatization to produce bio-oil. After successive polymerization and aromatization, bio-oil converts into a solid product with or without auto nucleation to form secondary char [37]. A high concentration of HMF favors secondary char formation. The typical HTC temperatures are too low to convert lignin completely, and lignin forms primary char *via* solid-to-solid conversion. Sequential hydrolysis, dehydration, and isomerization producing furfurals and cleavage reactions yielding intermediate organic acids are thought to have resulted in secondary char. These dissolved intermediates can lead to the precipitation of the furfurals as a secondary organic phase, which polymerizes as microspheres. Both primary and secondary chars are called HC when they are formed by HTC, despite their different chemical structures [34, 43].

The solid fraction of the HTC is in the form of slurry mixed with an aqueous fraction, and it has to pass through a series of steps such as mechanical dewatering (compressing), filtering, and drying before it can be used as a fuel. HC has high hydrophobic and homogeneous properties and can be easily separated from the liquid fraction. HTC process removes part of oxygen from biomass during decarboxylation and dehydration. Consequently, by mechanical dewatering, the moisture content of solid fraction can be reduced to 50% (only 70–75% moisture content can be achieved in mechanical dewatering of wet biomass). This reduces

*Hydrothermal Conversion of Lignocellulosic Biomass to Hydrochar: Production… DOI: http://dx.doi.org/10.5772/intechopen.112591*

the energy and time required for drying. Dried HC (moisture content <5%) pellets can be produced from the separated solid fraction, which can be used for energy production [21, 44].

Initially, the liquid fraction was considered a waste, hence higher costs for wastewater treatment and disposal. Recently, it has been considered a potential added resource of the HTC process as it contains various valuable organic and inorganic compounds, including organic acids, furan compounds, esters, phenolic compounds, lignin fragments, amino acids, sugars, and some nutrients. Mono sugars and chemicals can be recovered from liquid fraction. The gas fraction contains less CO and CO2 and is less harmful [5, 8, 21, 31–34].

The inorganic elements (Ca, Mg, P, K, Na, S, and Fe) present in LCB remain in ash as oxide forms (CaO, MgO, P2O5, K2O, Na2O, SO3, and Fe2O3) after combustion. Even though the ash content of LCB is usually minimal, these oxides can cause severe agglomeration, fouling, clinker formation, and corrosion during combustion, pyrolysis, and gasification. Inorganic compounds are very stable and may remain unchanged under HTC conditions, but the degradation of biomass polymers might release inorganics from the solid structure; hence some of the ash-forming elements can leach out from the biomass to the aqueous solution reducing the overall ash content of the HC. In addition, the HTC process can convert organic chlorine to inorganic chlorine reducing chlorine content in HC, which reduces the potential for corrosion and dioxin formation in combustion [21, 22].

Some of the advantages of the HTC process include (a) fewer emissions, low carbonization temperatures, and lower energy consumption than pyrolysis; (b) a greater variety of wet and dry feedstocks can be processed in an aqueous medium (no drying is required); (c) higher solid yields; (d) final product with lower ash content; and (e) inexpensive process. Feedstock material and processing conditions significantly influence the performance and properties of the HC. HC obtained from HTC has the following properties: (a) versatile properties make it suitable for a wide range of applications in the energy and environmental sectors, (b) oxygenated functional groups at the surface (OH, C=O, and COOH groups), (c) controlled porosity can be easily introduced using activation procedures, thermal treatments, etc., (d) uniform spherical micro-sized particles, (e) easily controlled surface chemistry and electronic properties *via* additional thermal treatment, and (f) specific physicochemical properties tailored for particular applications can be obtained by adding other components (such as inorganic nanoparticles) to biomass [1, 37–39].

HC is more energy dense, hydrophobic, and easily friable and has a lower H/C ratio and O/C ratio than the feedstock used. H/C and O/C ratios of selected biomasses and HC are given in **Table 2**. This is a result of reducing the oxygen and hydrogen content of the biomass through dehydration and decarboxylation, disrupting colloidal structures, and reducing the hydrophilic functional groups. A complex series of reactions, reducing hydrophilic functional groups, take place in HT medium, including the removal of hydroxyl groups through dehydration, removal of carboxyl and carbonyl groups through decarboxylation, and cleavage of many ester and ether bonds through hydrolysis [23, 40]. Specific characteristics, including H/C ratio, O/C ratio, elemental composition, porosity, conductivity, morphology, energy content, etc., are needed for applications in many fields such as solid fuel in soil amendment, adsorbent in water purification, power generation, and carbon capture. The characteristics of HC can be modified by combining HTC with other processes. The initial use for the HTC process was organic materials degradation, liquid and gaseous fuels,


#### **Table 2.**

*Properties of biomass and hydrochar.*

and basic chemical production. In recent years, the technology gained research interest to produce solid HC and as a technique to synthesize nano- and micro-size carbon particles [1, 31, 32, 48].

The HC produced by HTC directly from LCB lacks porosity. Only a small porosity is developed after further carbonization at a higher temperature. This is due to HC being pre-carbonized material produced under autogenic pressures and temperatures between 160 and 200°C. The high surface area and porosity of HC are essential for most industrial applications, such as adsorption or catalysis. This would ensure efficient transport and diffusion throughout the material. Different techniques have been developed to improve porosity [1, 32].

#### **4.2 Hydrothermal liquefaction (HTL)**

HTL is carried out at temperatures between 260 and 350°C under autogenous pressures of 5–20 MPa (subcritical); biomass in compressed water is primarily converted to an aqueous fraction (bio-oil, bio-crude). Bio-oil is a complex mixture of organic compounds containing acids, alcohols, aldehydes, ketones, furans, sugars, amines, amides, esters, ethers, phenols, etc. Free radical and ionic reactions

#### *Hydrothermal Conversion of Lignocellulosic Biomass to Hydrochar: Production… DOI: http://dx.doi.org/10.5772/intechopen.112591*

are considered to be leading reactions in HTL. The dissociation of water increases with an increase in temperature. The ionic product of water (Kw) 10−14 mol2 /L2 at 25°C increases to 10−11 mol2 /L2 at 300°C. At HTL conditions (with elevated temperature), mass transfer is improved because of accelerated mass-transfer-limited chemical reactions resulting from the lower viscosity of water. In HTL, water plays an important role as reaction media, helps in the dispersion of biomass, provides hydrogen, and stabilizes radicals. Water inhibits the polymerization of intermediate products, improving the quality and production of bio-oil but at the expense of HC yield [5, 26, 49–51].

The primary conversion of biomass during the HTL comprises three basic reaction mechanisms: depolymerization, decomposition, and recombination. Longchain biomass polymers are depolymerized and decomposed into shorter-chain compounds. These short-chain compounds are highly reactive and may recombine (repolymerize) to form liquid, gaseous, and solid products. Some bio-oil components could originate from biomass feedstocks, while others could be converted through hydrolysis, depolymerization, decomposition, and recombination of reactive fragments. The important parameters for the depolymerization of long-chain polymer structures to shorter-chain hydrocarbons are temperature and pressure. Depolymerization liquefies macromolecules of biomass by breaking down their physical and chemical components and characteristics. Higher temperatures of the HTL process and desirable physicochemical properties of water help the depolymerization process to overcome the resistance of biomass. Water acts as a catalyst at HTL conditions. The three steps involved in decomposition are dehydration (loss of H2O molecule), decarboxylation (loss of CO2 molecule), and deamination (removal of amino acid content). The two mechanisms, dehydration and decarboxylation, facilitate the removal of oxygen from the biomass in the form of H2O (eliminating the hydroxyl group) and CO2 (eliminating the carboxyl group), respectively. Biomass macromolecules are hydrolyzed to form monomers and oligomers. When HTL occurs at lower temperatures, biomass undergoes hydrolysis to convert biopolymers into soluble intermediates such as 5-HMF, furfural, amino acids, and fatty acids. The hydrolysis of polysaccharides and proteins begins around 190°C. When temperature increases, subcritical water at HTL conditions breaks down the hydrogen bonds of the cellulose structure to form sugar monomers, which are rapidly degraded by different reactions, including isomerization, dehydration, hydrolysis, reverse aldol defragmentation, rearrangement, and recombination to reactive intermediates. The degradation products, such as furfurals, phenols, organic molecules, glycoaldehydes, and organic acids, are highly soluble in water. Recombination and repolymerization of light MW compounds to yield higher MW compounds occur due to excess oxygen or unavailability of hydrogen molecules [26, 52–55].

Higher temperatures of the HTL process provide the necessary activation energy to decompose lignin in the presence of water. In addition, desirable physicochemical properties, such as low dielectric constant, high ionic product, weak hydrogen bond, and high diffusivity at HTL conditions, catalyze the hydrolysis of biomass breaking down the hydrogen bonds of the cellulose structure to form sugar monomers *via* free radical reactions [56]. Hydrolysis and alkylation reactions rapidly convert lignin fragments into phenolic compounds, including catechols, phenols, guaiacols, etc. The phenolic compounds are then polymerized to form solid residues, which take place at a slower rate than hydrolysis reactions. Unconverted lignin forms char by solid-tosolid conversion [51]. Lignin fragmentation depends on the operating conditions and

reactor design and/or is further converted into H2, CO, CO2, CH4, C2H6, and other low MW fragments. HTL process relies on the cleavage of ether or C–C bonds in lignin to form low MW fragments. Competition reactions hydrolysis and cleavage of the ether and C-C bond, demethoxylation, alkylation, and condensation occur during HTL of lignin for phenolic production [50].

The increase in temperature reduces the HC yield. At high temperatures, biomass polymers are degraded, and solid-to-solid conversion necessary for the formation of the HC does not occur or is reduced. The heating rate has been shown to affect the production of HC. Fast heating rates have resulted in higher bio-oil yield and lower HC yield; however, slow heating rates have resulted in a lower bio-oil and higher HC yield. The fast-heating rate significantly increased the reaction temperature, thereby inhibiting the formation of the char [51]. Bio-oil (liquid fraction) is the main product of HTL. The product distribution (gaseous, liquid, and solid) depends on the operating parameters of HTL, including biomass feedstock, temperature, heating rate, residence time, pressure, biomass-to-water ratio (B/W), particle size, and catalyst. The HTL process is endothermic at low temperatures but exothermic at higher temperatures. The advantages of HTL compared to pyrolysis are low operating temperature, high energy efficiency, low gases yield, and low tar yield [52].

The HTL temperature and pressure could impact the reaction directly (activation energy and reaction equilibria) and indirectly (solvent properties). During HTL, cellulose, hemicellulose, and lignin behave differently. Generally, biomass with high cellulose and hemicellulose tends to yield more bio-oil. But there are conflicting views on this. In addition to the three main components, extractives may also affect the yields of bio-oil and HC. Higher bio-oil yields have been obtained from hardwood samples than from softwood. Hardwood contains less lignin than softwood, hence, higher bio-oil yield [49, 50, 57]. Both temperature and lignin contents of wood were shown to have a marked effect on bio-oil yield. Maximum bio-oil production was obtained from wood with low lignin contents [58, 59].

Biomass particle size has negligible to minimal effects on the HTL process as sub/ near supercritical water in HTL acts as a good heat transfer medium to overcome the heat transfer limitations, which makes the particle size of biomass a secondary parameter. Excessive biomass feedstock size reduction is not needed. Particle sizes between 4 to 10 mm have been reported as more suitable for the HTL process, which may vary with the operating conditions. No stipulated criteria for particle size selection are available to achieve maximum bio-oil yield during HTL [49, 55, 58].

Although many process parameters can influence the results of the HTL process, generally, it is accepted that temperature is a dominant factor that can alter the yield and properties of bio-oil. It is considered the key process parameter because it has the highest impact, as well as the parameter that can be directly controlled. A temperature range of 260–350°C would be viable for HTL of LCB [49, 52, 55, 58, 60]. Due to the increased fragmentation of biomass at higher temperatures, the effect of temperature on the bio-oil yield is synergetic. Biomass depolymerization occurs when the temperature is sufficiently large enough to overcome the activation energy of bond dissociation. The competing reactions hydrolysis, fragmentation, and repolymerization define the role of temperature during the HTL process. The dominant reaction during the initial stages of HTL is depolymerization. At later stages of HTL, repolymerization becomes active, leading to the formation of HC [49, 58]. Higher HTL temperatures enhance the reaction rates, as well as change the reaction mechanisms. Ionic reactions are favored at lower temperatures; the formation of radicals by homolytic bond breakage is promoted at higher temperatures. A highly

#### *Hydrothermal Conversion of Lignocellulosic Biomass to Hydrochar: Production… DOI: http://dx.doi.org/10.5772/intechopen.112591*

diverse product spectrum usually results in free radical reactions, leading finally to gas formation [49, 61, 62]. Various authors have observed increased bio-oil yields with increasing HTL temperature. Due to reduced bio-oil yield at the maximum operating temperature, various optimum temperatures have been proposed by different authors for various biomasses. The optimum bio-oil yield is considered to occur in a specific temperature range which depends on the properties of biomass feedstock. The reduction of bio-oil yield at high temperatures is due to hydrolysis and repolymerization reactions. It can be assumed that the temperature range of 280–350°C would be suitable for the decomposition of biomass under HTL conditions. Intermediate temperatures usually produce higher bio-oil yields. The peak of bio-oil yield is usually around 300°C with an optimum range of 280–320°C have been reported. The final optimum HTL temperature varies with the biomass type. Temperature increase beyond optimum results in lower bio-oil yield and higher HC yield [49, 52, 55, 58, 60].

The heating rate is still a contentious parameter. Some researchers believe that the bio-oil yield and conversion rate mainly depend on the final temperature; the contribution of the heating rate is negligible. The perception of other researchers is that the heating rate is an important parameter in enhancing the bio-oil yield. The effect of heating rates on the product distributions in HTL is minimal compared to pyrolysis due to the better dissolution and stabilization of fragments in subcritical water. The undesirable breakdown of organic compounds and excessive polymerization of intermediates are reduced at faster heating rates. A positive correlation between heating rates and bio-oil yields has been obtained. Slow heating rates tend to yield more HC due to the increased secondary reactions. Secondary reactions are dominant at very high heating rates and yield more gases. In another study, it was found that the effect of the heating rate depends on the solvent. The heating rate is an important parameter in subcritical water but an optional parameter in supercritical alcohol. Moderate heating rates may be suitable to overcome heat transfer limitations, leading to extensive fragmentation and minimal secondary reactions [49, 52, 53, 58, 60].

The effect of residence time (reaction time) on product distribution during the HTL process has been examined by several researchers. Residence time has been observed to affect biomass conversion, bio-oil yield, and residue yield. Sufficient residence times are required to get high biomass conversions and bio-oil yield. The residence time may characterize the overall biomass conversion and the product compositions. For short residence time, bio-oil yield is lower due to incomplete biomass conversion. Alternatively, if the residence time is too long, the bio-oil may be decomposed and repolymerized, resulting in higher gaseous and solid fractions. A high repolymerization is not conducive to the yield and quality of bio-oil as degradation and repolymerization reactions convert some liquid organic fractions into other compounds. It is essential to inhibit the decomposition of intermediates into lighter products to obtain a higher bio-oil yield. Retention time can be regulated to control further cracking and repolymerization of intermediates. Improved yield and properties of bio-oil have been obtained with increasing residence times. However, a decrease in bio-oil yield has been observed at extended residence times (beyond the threshold limit). The type of biomass, its composition, type of catalysts, and operating conditions define the threshold limit of residence time. Two optimum residence times for maximum bio-oil yield have been reported depending on reaction temperature (120 min at 280°C and 5 min at 375°C), suggesting that shorter residence times are needed at higher reaction temperatures to reduce repolymerization of liquid fraction. In contrast, longer residence times can improve biomass conversion and biooil yield at lower reaction temperatures. The products obtained at different residence

times of the same temperature can vary. Therefore, it is important to consider all factors and obtain the optimal residence time and temperature. Generally, bio-oil yield is maximum at optimum residence time, whereas gas yield and biomass conversion continue to increase. Residence time is a key parameter in HTL, which strongly affect the bio-oil yield [49, 50, 52–55, 58, 60].

In the HTL process, pressure is an important parameter for biomass decomposition. It alters the bio-oil yield and supports a single-phase system. HTL uses pressure to maintain a single-phase system and avoid large heat inputs required for phase change. A large heat supply is needed to maintain the temperature of two-phase systems. Pressure increases solvent density and penetrates effectively into larger molecules of biomass, resulting in improved disintegration into smaller fragments. Pressure, once reached supercritical condition, has no significant effect on bio-oil or gas yield. Higher pressures increase the density of the water; it may also cause the local solvent density to increase. This lead to a cage effect around the C–C bonds, which impedes the free radical reactions, inhibits the cleavage of C–C bonds, and ends up in low fragmentations, leading to a decrease in bio-oil yield. Within a specific range, pressure is positively correlated with bio-oil yield, but increasing the pressure above the upper limit shows no noticeable effect [24, 49, 52, 53, 55].

The influence of different solvents (such as subcritical and supercritical alcohols) on the bio-oil yield of LCB has been investigated. Alcohols have lower critical pressures and temperatures than water and significantly milder HTL reaction conditions could be used. Dielectric constants of alcohols are lower than water; hence, relatively high MW intermediates derived from cellulose, hemicelluloses, and lignin are expected to dissolve. Widely employed alcohols for biomass liquefaction have been ethanol and methanol. The biomass/water mass ratio is considered a vital parameter for the HTL process. Different authors have investigated the effect of water density on HTL bio-oil yield. The dilution of reactants, intermediates, and products during the reaction benefit all solvolytic conversions. Cross-reactions are minimized by dilutions and produce a more distinct product spectrum. Cross-reactions due to higher substrate concentrations inevitably lead to undesirable polymerization of the reaction products. Such processes have been observed for the HTL of biomasses. High biomass concentrations were shown to promote HC formation [24, 49, 52, 53, 55].

The lower oxygen and moisture content of HTL bio-oil promotes the higher calorific value and stability. The HTL bio-oil has lower H/C and O/C ratios, indicating a higher energy density than pyrolysis bio-oil. The decrease in the O/C ratio is higher than the H/C ratio indicating that more oxygen is removed as CO2 or CO during HTL. As a result, HTL produces bio-oil with low oxygen content and improved quality [60].

#### **4.3 Hydrothermal gasification (HTG)**

At temperatures between 400 and 600°C and pressures of 23–45 MPa, biomass is primarily converted to a gas fraction (a mixture of non-condensable gases, including H2, CO, CH4, and CO2) in a process known as HTG. It can produce syngas enriched with H2. The biomass polymers decompose in supercritical water (SCW) above the critical point of water (374°C and 22.1 MPa) with enhanced conversion efficiency. The conversion rate of HTG is typically higher than 80%. The higher reaction temperatures of HTG cause reactions to progress faster, achieving complete decomposition of biomass, a distinctive feature of HTG compared to HTC and HTL. HTC and HTL often produce undesirable by-products that are occasionally dissolved in liquid fraction, and one of the problems of HTC and HTL is the difficulty in by-products

*Hydrothermal Conversion of Lignocellulosic Biomass to Hydrochar: Production… DOI: http://dx.doi.org/10.5772/intechopen.112591*

posttreatment. Due to high conversion, only a small amount of organic compounds remain in the liquid fraction of HTG. Consequently, posttreatment of liquid fraction is not required or easily carried out [5, 8, 63].

When water changes from ambient conditions (25°C and 0.1 MPa) to supercritical conditions (400°C and 25 MPa), the properties of water change significantly and exhibit lower density, viscosity, ionic product, and dielectric constant. The dielectric constant at 400°C and 25 MPa is about six. Because of the decrease in the number of hydrogen bonds, water begins to display the behavior of a nonpolar solvent that can completely dissolve many organic compounds, hydrocarbons, and gases (such as CO2, CH4, H2, and N2). Due to the absence of phase boundaries, many rapid homogeneous reactions involving organic compounds occur at supercritical conditions. Inorganic polar compounds, usually soluble in subcritical water, are insoluble in SCW and easily separated from the reaction products. SCW has low density, low viscosity and exhibits gas-like properties. SCW combines both the dissolution of the liquid phase and the diffusion of the gaseous phase. The advantages of using SCW as the reaction medium in HTG include high diffusion coefficient, enhanced mass transfer, increased solvation properties, single-phase reaction environment in the reactor, and complete miscibility with different organics and gases, and product gas (syngas) does not have tar. It has a high heating value and prevents poisoning of catalyst (if used) and coke formation [6, 26, 49, 63, 64].

Conventional gasification is ineffective for wet biomasses (moisture content >80%), but HTG can be effectively employed when biomass is wet. In conventional gasification, syngas is partially diluted with nitrogen due to partial oxidation using air and contain tar. HTG converts biomass in the presence of water; hence, no dilution of syngas occurs with nitrogen. Syngas from HTG does not contain tars; if produced, they remain in the liquid fraction. HTG produces a high amount of H2 and a very small amount of CO and char compared to conventional gasification. At low reaction temperatures or when the B/W is too high, HTG encounters a problem with the formation of tarry material. It can cause low gasification efficiency, reactor plugging, or the process water turns dark. At high heating rates, tarry material yield is decreased. This can be explained by assuming ionic reaction during tarry material production and free radical reaction during gasification [5, 8, 63].

During the HTG process, the biomass polymers cellulose and hemicellulose are converted into simple sugars (glucose, fructose, xylose, etc.). Some of the glucose is isomerized to fructose. Fructose produces 5-HMF and furan, which are then converted to alcohols, ketones, and organic acids. Hydrolysis products of small molecules in water are consequently gasified to produce H2-rich syngas. Simultaneously, glucose and fructose are converted into glyceraldehyde, dihydroxyacetone, etc. Additionally, highly polymerized oligomers produced from the gasified small molecules and some intermediates eventually become liquid products. Phenols that are formed during the reaction are considered to be the final obstacle to complete biomass gasification. Inverse aldol condensation reaction converts xylose (a decomposition product of hemicellulose) to glyceraldehyde and methyl formate, producing propionic acid and acetic acid. Propionic acid and acetic acid are eventually gasified into small molecule gases (H2 and CO). Propanoic acid may decompose in a second pathway and be gasified to ethane and CO2. Simultaneously, dehydration of xylose produces furfural, which is decomposed in three proposed pathways. In the first pathway, furfural can be converted into water-soluble humic substances, which are gasified to CO and H2, or in the second pathway, methyl cyclopentenolone, which is gasified to CO, CH4, and H2, or third pathway, gasified directly to CO, H2, CH4, and CO2 [63, 65–67].

At low temperatures (< 500°C), biomass tends to produce low concentrations of H2 in the gaseous fraction along with oil-based liquid faction. The liquid fraction contains a wide range of products, including acids, phenols, aldehydes, and furfurals. Ions (H<sup>+</sup> and OH<sup>−</sup> ) from SCW ionization support the cleavage of ring compounds to form simple molecules. At low-temperature HTG of biomass, ions are stable and ionic reactions dominate. In contrast, at high temperatures (> 500°C), free radicals are more stable than ions, and free radical mechanisms dominate in HTG of biomass. The gasification temperature and reactants influence free radical concentration. The most difficult biomass component to gasify is lignin. The efficient conversion of lignin in SCW is particularly important for the efficiency of the gasification process. In SCW, Lignin primarily dissociates into phenolics; phenolics decompose into gases [65, 66].

The operating conditions, including biomass characteristics, temperature, pressure, residence time, B/W, and catalyst, strongly influence the performance of HTG. The key parameters that affect the H2 yield of the HTG process include temperature, pressure, B/W, and residence time. Short residence times are expected to degrade biomass successfully as the rate of hydrolysis and decomposition is relatively fast in the HTG process. For efficient destruction of biomass, residence time optimization is required. A large heat supply is needed to maintain the temperature of two-phase systems. HTG uses pressure to maintain a single-phase system and avoid large heat inputs required for phase change. By maintaining pressure higher than the supercritical pressure, the rate of hydrolysis and biomass dissolution can be controlled, which may enhance favorable reaction pathways for bio-oil or gas yield. In the supercritical region, pressure has a minimal effect on the properties of water. As a result, the effect of pressure on bio-oil or gas yield is minor or negligible [49, 58, 64, 68].
