**6.1 Hydrothermal carbonization (HTC)**

When biomass feedstock in water is heated at temperatures below 200°C in a sealed vessel at autogenous pressure, mostly solids (hydrochar) are formed in a process known as HTC. The residence time of HTC varies from minutes up to several hours. Hemicellulose and cellulose decomposition temperature in subcritical water is usually around 160°C and 180 to 200°C, respectively, while lignin decomposes above 220°C. HTC converts biomass into three distinct product fractions: solid residue (hydrochar), bio-oil mixed with water in liquid fraction (aqueous solution), and a small volume of gases (consisting mainly of CO2). HTC aims to maximize the hydrochar yield. The three factors (type of biomass, pH, and maximum temperature used) primarily influence the product distribution and characteristics. The other factors, such as solids concentration (in biomass water mixture) and reaction time, have a relatively smaller influence. The overall extent of hydrochar formation from glucose is negligible below 160°C and yield is maximum at 200°C. Hydrochar formation is reduced with the increase in temperature above 200°C as a result of gasification reactions converting part of the hydrochar formed into volatile compounds. Process conditions and the type of biomass feed are the two factors that influence energy requirements and final product composition. Hydrochar has high hydrophobic and homogeneous properties and can be easily separated from the liquid fraction. Dried hydrochar pellets can be produced from the separated solid fraction, which can be used for energy production. The liquid fraction can be used to recover mono sugars. The gas fraction has less CO and CO2 and is less harmful. The hydrochar has carbon content similar to lignite and the yield of hydrochar varies from 35% up to 80% [2, 4, 46–48].

The HTC reduces both the oxygen and hydrogen content of the biomass through dehydration and decarboxylation. During HTC, hemicelluloses and cellulose are hydrolyzed to oligomers/monomers, whereas lignin mostly remains unchanged. The reaction mechanism of the HTC process mainly involves dehydration, decarboxylation, and polymerization. Dehydration is favored at temperatures less than 300°C. The hydroxymethylfurfural (HMF) generated from hexose (D-fructose and D-glucose) and furfural generated from pentoses (D-xylose) are well-known dehydration products of sugars. The hydrothermal process under acidic conditions allows the effective conversion of D-glucose to HMF. D-glucose first isomerizes to D-fructose and then undergoes dehydration to form HMF. The HMF, in turn, decomposes

into levulinic acid, formic acid, and soluble polymeric carbonaceous material with increasing residence time [49, 50].

Hydrochar has a higher energy content than the feedstock used and lower O/C and H/C ratios than the feedstock. Hydrochar has higher H/C ratios than biochar specifications. HTC is a high-energy-consuming process. Solar energy appears to be an attractive renewable energy source to combine with HTC. HTC can combine with other processes to produce hydrochar with characteristics (morphology, porosity, conductivity, H/C ratio, O/C ratio, energy content, elemental composition, etc.) suitable for applications in many fields such as solid fuel in power generation, soil amendment, adsorbent in water purification and carbon capture. Hydrochar can be further processed to use as carbon electrodes or nanocomposites. HTC process was initially used for the degradation of organic materials, production of liquid and gaseous fuels, and production of basic chemicals. In recent years, the technology gained research interest to produce solid hydrochar and as a technique to synthesize nano- and microsize carbon particles [46, 47, 51, 52].

The hydrochar produced by HTC directly from carbohydrates or biomass lacks porosity. Only a tiny porosity is developed even after further carbonization at a higher temperature. This is due to hydrochar being pre-carbonized material produced under autogenic pressures and temperatures between 160 and 200°C. For most industrial applications such as adsorption or catalysis, the high surface area and porosity of hydrochar are essential. This would ensure efficient transport and diffusion throughout the material. Different techniques have been developed to improve the porosity of hydrochar [47]. Some of the advantages of the HTC process include low carbonization temperatures, can be synthesized in the aqueous phase (no drying is required), and inexpensive process. Hydrochar obtained from HTC has the following properties: (a) uniform spherical micro-sized particles; (b) oxygenated functional groups at the surface (OH, C=O, COOH groups); (c) controlled porosity can be easily introduced using activation procedures, thermal treatments, etc.; (d) easily controlled surface chemistry and electronic properties via additional thermal treatment; (e) special physicochemical properties can be obtained by adding other components (such as inorganic nanoparticles) to biomass [49, 50].

#### **6.2 Hydrothermal liquefaction (HTL)**

At temperatures between 200 and 350°C and pressures of 5–20 MPa, biomass is primarily converted to a liquid fraction (aqueous soluble) in a process known as HTL. Leading reactions in HTL are considered to be free radical and ionic reactions. At ambient conditions, the dielectric constant (a measure of hydrogen bonding) of water is about 80 F/m. It decreases rapidly with increasing temperature, at 250°C and 5 MPa dielectric constant is about 27 F/m and at 350°C and 25 MPa about 14 F/m. Due to decreasing dielectric constant (number of hydrogen bonds), water displays less polar behavior. An increase in temperature increases the dissociation of water. The ionic product of water (pKw) at 25°C is 14 and decreases to 11 at 250°C. With increasing temperature, mass transfer is enhanced because of accelerated mass-transfer-limited chemical reactions resulting from a decrease in the viscosity of water [4, 53, 54].

The primary conversion of biomass during the HTL comprises three pathways; depolymerization, decomposition, and recombination. Higher MW biomass is depolymerized and decomposed into smaller MW compounds. These compounds are highly reactive and recombined (repolymerized) to form bio-oil, gaseous and solid

#### *Advances in Bioenergy Production Using Fast Pyrolysis and Hydrothermal Processing DOI: http://dx.doi.org/10.5772/intechopen.105185*

products. The parameters such as temperature and pressure are important for the depolymerization of long-chain polymer structures to shorter-chain hydrocarbons. The decomposition step involves three steps: dehydration (loss of water molecule), decarboxylation (loss of CO2 molecule), and deamination (removal of amino acid content). The dehydration and decarboxylation steps remove oxygen from the biomass in the form of H2O and CO2, respectively. Macromolecules of biomass are hydrolyzed to form polar monomers and oligomers. Subcritical water at HTL temperatures and pressure breaks down the hydrogen bonds of the cellulose structure to form sugar monomers. It is rapidly degraded by different reactions (such as isomerization, hydrolysis, dehydration, reverse aldol defragmentation, rearrangement, and recombination) into a series of products. Most of the degradation products such as polar organic molecules, furfurals, phenols, glycolaldehyde, and organic acids are highly soluble in water. Recombination and repolymerization of light MW compounds occur due to the unavailability of the hydrogen compound or excess oxygen [53, 55, 56].

During HTL of lignin, hydrolysis and splitting of the ether and C-C bond, demethoxylation, alkylation, and condensation reactions occur. Competition occurs between these main reactions. The gaseous, liquid, and solid yield of HTL of biomass depends on several parameters, including biomass feedstock, temperature, heating rate, residence time, pressure, mass ratio of water/biomass, and catalyst. The main product of HTL is the liquid fraction (bio-oil). The temperature and pressure directly (activation energy, reaction equilibria) and indirectly (solvent properties) impact the reaction. During HTL, the major components of biomass cellulose, hemicellulose, and lignin behave differently. In general, biomass with high cellulose and hemicellulose produces higher bio-oil yields. Higher bio-oil yields have been reported from hardwood samples (cherry) than softwood (cypress). Softwood contains higher lignin than hardwood, hence, lower bio-oil yield [54, 57]. Other studies have also shown that both temperature and lignin contents of wood had a marked effect on bio-oil yield. Bio-oil production was maximum for wood with low lignin contents [58, 59]. Subcritical water in HTL acts as a heat transfer medium to overcome the heat transfer limitations. As a result, biomass particle size has negligible to minimal effects on HTL. Excessive size reduction of biomass feedstock is not required [54, 58].

Usually, the effect of temperature on the bio-oil yield is synergetic due to the increased fragmentation of biomass at higher temperatures. Depolymerization occurs when the temperature is sufficient for bond dissociation. The competition among hydrolysis, fragmentation, and repolymerization reactions describes the role of temperature during the HTL process. Depolymerization is a dominant reaction during the initial stages of HTL. Repolymerization becomes active at later stages of HTL, leading to the formation of hydrochar. Intermediate temperatures usually produce higher bio-oil yields [54, 58]. The increase in HTL temperature not only enhances the reaction rates but also changes the reaction mechanisms. Hence, lower temperatures favor ionic reactions; higher temperatures promote the formation of radicals by homolytic bond breakage. Radical reactions usually lead to a diverse product spectrum and finally to gas formation [54, 60, 61]. Various authors have observed increased bio-oil yields with increasing temperature during the HTL process. Different authors have proposed various optimum temperatures for a variety of biomasses. It can be assumed that the temperature range of 280–350°C would be suitable for the decomposition of biomass under both subcritical and supercritical conditions. Final HTL temperature varies with the type of biomass [54, 58].

The temperature gradients during the heating of biomass are important for the sequence and extent of chemical reactions. Due to the better dissolution and stabilization of fragmented compounds in subcritical water, the effect of heating rates on the product distributions in HTL is minimal compared to pyrolysis. Because of secondary reactions, slow heating rates typically tend to yield solid fraction (hydrochar). Secondary reactions are also dominant at very high heating rates and yield more gases. Furthermore, bio-oil yield is not significantly affected by large variations in high heating rates. Moderate heating rates may be suitable to overcome heat transfer limitations leading to extensive fragmentation and minimal secondary reactions. Many researchers have investigated the effect of residence times on product distribution during the HTL process. Duration of reaction time may characterize the product compositions and the overall biomass conversion. Short residence times are usually preferred during the HTL of biomass. Longer residence times can decompose preasphaltenes and asphaltene into lighter products enhancing the yield of bio-oil and gases. It is essential to inhibit the decomposition of lighter products to obtain a high liquid oil yield. Generally, bio-oil yield attains a maximum before decreasing for extended residence times, whereas gas yield and biomass conversion increase continuously until reaching saturation [54–56, 58].

Pressure helps maintain single-phase media for HTL to avoid large heat inputs required for phase change. Two-phase systems need a large heat supply to maintain the temperature of the system. Pressure increases solvent density and a high-density medium penetrates effectively into molecules of biomass components resulting in improved decomposition and extraction. Many investigations have been performed to study the influence of different solvents (such as subcritical and supercritical alcohols) on the liquefaction yield of lignocellulosic biomass. Critical temperatures and pressures of alcohols are lower than in water and significantly milder reaction conditions could be used. Alcohols are expected to dissolve relatively high MW products derived from cellulose, hemicelluloses, and lignin due to their lower dielectric constants than water. Ethanol and methanol have been widely employed for biomass liquefaction. The mass ratio of biomass/water is considered a vital parameter for the HTL process. Different authors investigated the effect of water density on HTL yield. All solvolytic conversions are benefitted from the dilution of reactants, intermediates, and products during the reaction. This dilution minimizes cross-reactions and produces a more distinct product spectrum. Higher substrate concentrations inevitably lead to cross-reactions leading to undesirable polymerization of the reaction products. Such processes have been observed for HTL of biomasses, promoting the formation of solid fractions. Catalysts are important in the HTL of biomass. A range of homogeneous catalysts (such as mineral acids, organic acids, and bases) and heterogeneous catalysts (such as zirconium dioxide, anatase, and other materials) has been proposed to tailor the reaction toward a specific product and enhance the reaction rates [54–56, 62].
