Sub- and Supercritical Water Technology for Biofuels



One of the major challenges in utilization of biomass is its high moisture content and variable composition. The conventional thermochemical conversion processes such as pyrolysis and gasification require dry biomass for production of biofuels. Sub- and supercritical water (critical point: 374°C, 22.1°MPa) technology, which can utilize wet biomass, capitalizes on the extraordinary solvent properties of water at elevated temperature for converting biomass to high energy density fuels and functional carbonaceous materials. Here, water acts as reactant as well as reaction medium in performing hydrolysis, depolymerization, dehydration, decarboxylation, and many other chemical reactions. One of the advantages is that the large parasitic energy losses that can consume much of the energy content of the biomass for moisture removal are avoided. In sub- and supercritical water-based processes, water is kept in liquid or supercritical phase by applying pressure greater than the vapor pressure of water. Thus, latent heat required for phase change of water from liquid to vapor phase (2.26 MJ/kg of water) is not needed. For a typical 250°C subcritical water process, the energy requirement to heat water from ambient condition to the reaction temperature is about 1 MJ/kg, equivalent to 6–8% of energy content of dry biomass.


Sugar Hydrolysis Sludge Hydrocarbon Polycyclic Aromatic Hydrocarbon 

1 Introduction

This chapter describes the application and current status of sub- and supercritical water (collectively called hydrothermal) technology for liquid fuels (bioethanol, biocrude), gaseous fuels (methane, hydrogen, synthesis gas), and solid fuels (biochar, other functional carbonaceous materials) production from biomass. Energy security, sustainability, and climate change concerns have led the world to look for renewable and alternative energy resources. The large-scale substitution of petroleum-based fuels and products with renewable sources are needed to minimize the environmental issues [30]. Biomass is the world’s fourth largest energy source, following coal, oil, and natural gas. Biomass is an attractive feedstock for fuel and biomaterials due to three main reasons. First, it is a carbon-neutral renewable resource that could be sustainably developed for the production of bioenergy and biomaterials. Second, it is environmentally benign, as it does not add to the green house gas emission, and possibly reduces NOx and SOx depending on the fossil fuels displaced. However, when combusted in traditional stoves, emission of polycyclic aromatic hydrocarbons, dioxins, furans, and heavy metals is an environmental health concern [89]. Third, it appears to have a significant economic potential given the fluctuating prices of the fossil fuels. Moreover, development of bio-based economy brings the opportunity for rural empowerment and also the energy security, since biomass resource is distributed all over the world.

In recent years, there is significant research interest on nonfood resources or so-called second-generation biofuels from lignocellulosic biomass. Lignocellulose is a generic term for describing the main constituents in most plants, namely cellulose, hemicellulose, and lignin (Fig. 1).
Fig. 1

Typical composition of lignocellulosic biomass

Lignocellulosic biomass, which is the non-starch-based fibrous part of plant material, is a renewable and abundantly available resource that can be considered as a potential feedstock for biofuels. The components of lignocelluloses form 3D polymeric composites to provide structure and rigidity to the plant. The composition of lignocellulosic materials varies with several factors such as type of plant, growth conditions, on the part of plant, and on the age of harvesting [28, 77, 87].

2 Structure and Composition of Lignocellulosic Biomass

Lignocelluloses are derived from wood, grass, agricultural residues, forestry waste, and municipal solid wastes. The major components of lignocellulosic material are cellulose, lignin, and hemicellulose, with others (extractives and ash) also present in small percentage. Cellulose and hemicelluloses are polymers based on different sugars, whereas lignin is an aromatic polymer mainly built of phenylpropanoid precursors. Composition of these polymers within single plant varies with age and stage of growth [90].

Cellulose: Discovered 150 years ago, cellulose is most abundant organic matter on the earth. It is the main structural constituent of plants and algal cell walls. Cellulose in wood is mixed with many polymers such as hemicelluloses and lignin. The fluffy fiber of cotton bolls is the purest naturally occurring form of cellulose. It is an unbranched chain and homopolymer of β-d-glucopyranose units linked together by (1  →  4)-glycosidic bonds with a repeating unit of C6H10O5 strung together by β-glycosidic linkages (Fig. 2).
Fig. 2

Cellulose structure

The β-linked glucose units in cellulose form long linear chains that associate to form nanometer-scale crystalline fibers called elemental fibrils that are highly stable and resistant to chemical attack because of the high degree of hydrogen bonding present between chains of native cellulose. Hemicellulose and lignin cover the cellulose microfibrils which are formed by assembly of elemental fibrils. Hydrogen bonding between cellulose chains makes the polymers more rigid, inhibiting the flexing of the molecules that must occur in the hydrolytic breaking of glycosidic linkages. Hydrolysis can reduce cellulose to a cellobiose repeating unit, C12H22O11, and ultimately to glucose [70, 86, 99].

There are six known polymorphs of cellulose (I, II, III1, IIIII, IVI, and IVII) which can also interconvert. The interconversion of cellulose is shown in Fig. 3.
Fig. 3

Interconversion of polymorphs of cellulose [86]

Cellulose I, also termed as native cellulose, has parallel arrangement of chains and is the only polymorph that occurs naturally. Cellulose II is converted through mercerization or solubilization–regeneration of native or other celluloses. Cellulose II is thermodynamically more stable structure with a low energy crystalline arrangement having an antiparallel arrangement of the strands (two cellulose chains lie antiparallel to one another) and some intersheet hydrogen bonding. Cellulose IIII and IIIII can be obtained from Cellulose I and II, respectively, by treatment with liquid ammonia or some amines, whereas polymorphs IVI and IVII can be obtained from heating cellulose IIII and IIIII, respectively, to 206°C in glycerol [70, 86, 98, 99]. Two decades ago, it was reported that native cellulose (cellulose I) exists as a mixture of two crystalline forms Iα and Iβ, having triclinic and monoclinic unit cells, respectively [2]. Cellulose Iα is thermodynamically less stable, as shown by its conversion to cellulose Iβ by annealing at 260°C. In both crystalline forms, cellobiose is the repeating unit with a strong intra-chain H-bond from 3-OH to the preceding ring O5, whereas the interchain H-bonding and packing of the crystal are slightly different in the two forms [111].

The comparatively lower stability of Iα may provide the site of initial reaction in the microfibril. Cellulose molecules have reducing end groups, as chemical linkage between C1 carbon and the ring oxygen to form the pyranose ring. It is a hemiacetal group that allows the ring to open to form an aldehyde. The other end group is a secondary alcohol and commonly referred as the nonreducing end. The structural unit of cellulose has three hydroxyl groups, one primary and two secondary. These groups undergo chemical modifications (e.g., esterifications and etherifications) during the reactions. But the accessibility of the reactants is limited because of the high degree of crystallinity of the native cellulose [24].

Cellulases enzyme hydrolyze the β-1,4-glycosidic linkages of cellulose. They are divided in two classes and referred as endoglucanases (endo-1,4-β-glucanases, EGs) and cellobiohydrolases (exo-1,4-β-glucanases, CBHs). EGs hydrolyze preferably amorphous region of cellulose and releases new terminal ends. Whereas CBHs act on existing or EGs generated chain ends. Both enzymes can degrade amorphous cellulose, but only CBHs enzyme act efficiently on crystalline cellulose. Thus, CBHs and EGs work synergistically in cellulose hydrolysis and releases cellobiose molecules. β-glucosidases are required to further breakdown the cellobiose to two glucose molecules [90].

Hemicelluloses: Hemicelluloses are important carbohydrate fraction in plants made of mixed polysaccharides and stick to cellulose via hydrogen bonds, to create polysaccharide microfibrils. Together these give a strong rigidity to plant cell walls but the added lignin improves this strength greatly. The molecules are much smaller than the cellulose (degree of polymerization  ≈  102) and are dominated by hydrogen bond from the 3-OH of one sugar to the ring oxygen of preceding sugar. Hemicelluloses are a class of polymers that contain mixed sugars. The monomeric form of mixed sugars can be six carbon sugars such as mannose, galactose, glucose, and 4-O-methyl-d-glucuronic acid and five carbon sugars such as xylose and arabinose depending upon the species. Unlike cellulose, hemicelluloses possess side chains (Fig. 4).
Fig. 4

A segment of hardwood xylan

The side groups include acetic acid, pentoses (β-d-glucose, β-d-mannose, α-d-galactose), hexuronic acids (β-d-glucuronic acid, α-d-4-O-methylglucuronic acid, α-d-galacturonic acid), and deoxyhexoses (α-l-rhamnose α-l-fructose) and are responsible for the solubility of hemicelluloses in water and/or alkali. These side groups stop hemicelluloses molecules to aggregate and make them more susceptible to chemical degradation than cellulose. Within the plant, hemicelluloses are bound to cellulose and lignin component by covalent and noncovalent bonds in the cell wall and are thus fixed in the fiber structure [9, 97]. Xylans, mannans (glucomannans), and galactans are the major categories of hemicelluloses. The xylans have backbone of β-(1  →  4)-glycosidic linked xylose units and some of the xylose molecules have α-(1  →  2)-bonded 4-O-methylglucuronic acids. The xyloses also contain acetyl groups. In softwoods, xylose molecules are connected to arabinose side chains are connected to the xylose molecules by α-(1  →  3)-glycosidic bonds. Corncobs having a high xylan content are highly concentrated in xylans. Hardwoods have mannose and glucose units liked by β-(1  →  4)-glycosidic bonds. Acetyl and galactose groups are connected to glucose–mannose backbone structure in softwoods. A summary of hemicelluloses composition in hardwoods (deciduous trees) and in softwoods (coniferous trees) is given in Table 1.
Table 1

Composition and degree of polymerization (DP) of hemicelluloses [9]

Hardwoods (deciduous trees)

Softwoods (coniferous trees)











Xylose (Xyl), 4-O-methyl-glucuronic acid (Mga), Acetyl gr. (Ac)



Xylose (Xyl), 4-O-methyl-glucuronic acid (Mga), Acetyl gr. (Ac.)




Mannose (Man), Glucose (Glu)


Mannose (Man), Glucose (Glu) Galactose (Gal), Acetyl group (Ac)



Galactose (Gal), Arabinose (Ara), Rhamnose (Rha)



Galactose (Gal), Arabinose (Ara)

Acetyl groups present in hardwood xylans and softwood galactoglucomannans are hydrolyzable by acid at elevated temperatures. The acetic acid released provides the acidity to the reaction media. The presence of uronic acid groups reduces the hydrolysis rate of glycocidic linkages appreciably [9]. Hemicelluloses are biodegraded to monomeric sugars and acetic acid. Xylan is the main carbohydrate found in hemicelluloses and degradation requires the cooperative action of a variety of hydrolytic enzymes. Endo-1,4-β-xylanase generates oligosaccharides from the cleavage of xylan and 1,4-β-xylosidase produces xylose from the xylan oligosaccharides. Hemicellulose biodegradation requires accessory enzymes such as xylan esterases, ferulic and p-coumeric esterases, α-l-arabinofuranosidases, and α-4-O-methyl glucuronidases to work synergistically on wood xylans and mannans [90, 114].

Lignin: Lignin is a complex aromatic polymer. It is synthesized by the generation of free radicals released during peroxidase-mediated dehydrogenation of three phenylpropionic alcohols: trans-p-coumaryl (p-hydroxyphenyl propanol), coniferyl alcohol (guaiacyl propanol), and sinapyl alcohol (syringyl propanol). The structures of these lignin monomers are shown in Fig. 5. The polymerization results in a heterogeneous structure whose basic units are linked by C–C and aryl-ether linkages, with aryl-glycerol β-aryl ether being the predominant structure. Lignin is an amorphous heteropolymer, nonwater soluble, and optically inactive [90].
Fig. 5

Lignin monomers (a) trans-p-coumaryl alcohol, (b) coniferyl alcohol, and (c) sinapyl alcohol (redrawn from Bobleter) [9]

The structural complexity of the lignin and its high molecular weight is the main reason why it is so hard to degrade by enzymes. Lignin content in wood or lignocellulosic varies based on the species type, growing conditions, part of the plant tested, and numerous other factors. Lignin mainly acts as adhesive or binder in wood that provides strength and structure to the cellular composites of the plant and protects against microorganism or chemical attack. It controls the fluid flow, acts as antioxidant by absorbing UV light, and stores energy. When lignin binds, it cross-links with the regular structure of the microfibrils made of cellulose and hemicelluloses [77, 112]. Isolated lignin shows maximum solubility in the solvents such as dioxane, acetone, methyl cellosolve (ethylene glycol monomethyl ether), tetrahydrofuran, dimethyl formaldehyde (DMF), and dimethyl sulfoxide (DMSO).

Carbon–carbon linkages are very resistant to chemical attack, and degradation of lignin is largely limited to the cleavages of ether units at α- and β-positions. Functional groups of lignin follow [3]:
  • Hydrolyzable ether linkages: β-aryl, α-aryl, and α-alkyl ether linkages are the main hydrolyzable ether units in lignin. Lignin may also contain some α-ether linked l to carbohydrate, which is hydrolyzed at relatively lower rates.

  • Phenolic hydroxyl groups: It plays an important role in promoting alkali-catalyzed cleavages of interunitary ether linkages, oxidative degradation of lignin, and in lignin modification reactions.

  • Aliphatic hydroxyl groups: Two major aliphatic hydroxyl groups in lignin are located at the γ- and α-positions of the side chains. Aliphatic hydroxyl group at α-position is a benzyl alcohol, which is very reactive and plays a dominant role in lignin reactions.

  • Uncondensed Units: Units at position C2, C3, C5, and C6 are free or substituted by methyl groups are defined as uncondensed units. Hardwood lignin, which contains high syringical units, has high content of uncondensed units.

  • Unsaturated groups: Lignin contains some unsaturated groups, mainly as coniferyl alcohol and coniferaldehyde end groups.

  • Ester group: Grass lignins contain significant amount of p-coumeric acid and ferulic acid moieties, which are mainly esterified. These functional groups are liable to mild alkali treatment and mainly present at the α-position.

  • Methoxyl groups: These groups are relatively resistant to both acidic and alkaline hydrolysis.

  • Accessibility: Lignins have a very high tendency to form hydrogen bonds like hemicelluloses and cellulose.

Lignin empirical formulae are based on ratios of methoxy groups to phenylpropanoid groups (MeO:C9). The general empirical formula for lignin monomers is C9H10O2 (OCH3) n , where n is the ratio of MeO to C9 groups. Where no experimental ratios have been found, they are estimated as follows: 0.94 for softwoods, 1.18 for grasses, 1.4 for hardwoods. These are averages of the lignin ratios found in the literature. Paper products, which are produced primarily from softwoods, are estimated to have a MeO: C9 ratio of 0.94 (source: National Renewable Energy Laboratory, USA).

Extractives: Extractives are low to moderately high molecular weight compounds, which are soluble in water or organic solvents. They impart color, odor, and taste to the biomass. The composition of extractives varies widely based on the species and class of wood. Some of the important classes of extractives are terpenes, triglycerides, fatty acids, and phenolic compounds [8].

Ash: Ash contains metallic ions of sodium, potassium, calcium, and the corresponding ions of carbonate, phosphate, silicate, sulfate, chloride, etc.

3 Major Conversion Routes for Lignocellulosic Biomass to Biofuels

Lignocellulosic biomass consists of a variety of materials with distinctive physical and chemical characteristics. Typically it is categorized into either woody, herbaceous, or crop residues. Most of these biomass materials are already used, without preliminary conversion, as a fuel for heating purpose and also to produce steam for generating electricity. Direct combustion is best suited to biomass having low contents of moisture and ash. In fact, until the start of twentieth century, biomass and coal were the major sources of fuels and chemicals. The recent energy crisis, fluctuating oil prices, and political factors associated with the import of fossil fuels have brought the focus again on the utilization of abundantly available biomass resources for producing easy-to-handle forms of energy such as gases, liquids, and charcoal [56, 134]. Biomass may be converted to energy by many different processes such as biochemical, thermochemical, and hydrothermal pathways depending on the raw characteristics of the material and the type of energy desired (Fig. 6).
Fig. 6

Major pathways for the conversion of biomass to biofuels

Thermochemical processes depend on the relationship between heat and chemical action as a means of extracting and creating products and energy. Pyrolysis, gasification, and liquefaction which are conducted at a temperature of several hundred degrees Celsius are categorized in thermochemical processes. Pyrolysis is defined as the thermal degradation of biomass in the absence of oxygen to produce condensable vapors, gases, and charcoal; in some instances, a small amount of air may be admitted to promote this endothermic process. The products of pyrolysis can be gas, liquid, and/or solid. In flash pyrolysis, biomass is rapidly heated (e.g., at rates of 100–10,000°C/s) to 400–600°C, while limiting the vapor residence time to less than 2 s [4]. The oil production is maximized at the expense of char and gas. Pyrolysis processes typically use dry and finely ground biomass. Pyrolysis and direct liquefaction processes are sometimes confused with each other, and a simplified comparison of the two follows. Both are thermochemical processes in which feedstock organic compounds are converted into liquid products. In the case of liquefaction, feedstock macromolecules are decomposed into fragments of light molecules in the presence of a suitable catalyst. At the same time, these fragments, which are unstable and reactive, repolymerize into oily compounds having appropriate molecular weights [19]. With pyrolysis, on the other hand, a catalyst is usually unnecessary, and the light decomposed fragments are converted to oily compounds through homogeneous reactions in the gas phase.

In gasification, oxygen-deficient thermal decomposition of organic matter primarily produces synthesis gas. Gasification can be thought of as a combination of pyrolysis and combustion. Gasification has a good potential for near-term commercial application due to the benefits over combustion including more flexibility in terms of energy applications, higher economical and thermodynamic efficiency at smaller scales, and potentially lower environmental impact when combined with gas cleaning and refining technologies. An efficient gasifier decomposes high molecular weight organic compounds released during pyrolysis into low molecular weight noncondensable compounds in a process referred to as tar cracking. Undesirable char that is produced during gasification participates in a series of endothermic reactions at temperatures above 800°C which converts carbon into a gaseous fuel. Typically gaseous products include: CO, H2, and CH4. Fisher–Tropsch synthesis can be used to convert the gaseous products into liquid fuels through the use of catalysts. Gasification and pyrolysis both requires feedstock that contains less than 10% moisture [49, 85].

Biochemical processes takes place at ambient to slightly higher temperature levels using a biological catalyst to bring out the desired chemical transformation. Bioethanol from lignocellulosic biomass is produced mainly via biochemical routes. The biomass is first pretreated by different pretreatment methods (discussed later) for the improving the accessibility of enzymes. After the pretreatment, biomass goes through the enzymatic hydrolysis for conversion of polysaccharides into monomeric sugars such as glucose, xylose, etc. Subsequently, sugars are fermented to ethanol by the use of different microorganisms using the process called simultaneous saccharification and co-fermentation (SSCF) [33].

4 Supercritical Fluid Technology

The unique physicochemical properties of dense supercritical fluids provide an attractive medium for chemical reactions and other processes. Fluids near critical points have solvent power comparable to that of liquids and are much more compressible than dilute gases. The transport properties of such fluids lie intermediate between gas- and liquid-like. Supercritical fluids are attracting much attention in various fields of science and technology. In the last decades, the number of applications has increased continuously in several areas such as:
  • Supercritical water and carbon dioxide as alternative solvents

  • Supercritical fluid extraction and purification

  • Fine particle production by supercritical antisolvent (SAS) and rapid expansion of supercritical solutions (RESS)

  • Supercritical fluid chromatography for analytical applications

  • Supercritical steam cycle technology for power plants

Supercritical fluids can be advantageously exploited in environmentally benign separation and reaction processes, as well as for new kinds of materials processing. Although laboratory scale studies show excellent results, there are relatively few processes in industrial scale. The high pressure processes are generally expensive to design, build, operate, and maintain. Therefore, scaling up from laboratory to industrial scale of such processes can only be successful if clear benefits can be achieved in terms of high efficiency, conversion ratios, product quality, and cost advantages over the conventional processes [25, 104, 128].

4.1 Sub- and Supercritical Water

Water is an ecologically safe and abundantly available solvent in nature. Water has a relatively high critical point (374°C and 22.1 MPa) because of the strong interaction between the molecules due to strong hydrogen bond. Liquid water below the critical point is referred as subcritical water whereas water above the critical point is called supercritical water. Density and dielectric constant of the water medium play major role in solubilizing different compounds. Water at ambient conditions (25°C and 0.1 MPa) is good solvent for electrolytes because of its high dielectric constant 78.5), whereas most organic matters are poorly soluble under these conditions.

As water is heated, the H-bonding starts weakening, allowing dissociation of water into acidic hydronium ions (H3O+) and basic hydroxide ions (OH). Structure of water changes significantly near the critical point because of the breakage of infinite network of hydrogen bonds and water exists as separate clusters with a chain structure [52]. In fact, dielectric constant of water decreases considerably near the critical point, which causes a change in the dynamic viscosity and also increases self-diffusion coefficient of water [71].

Supercritical water has liquid-like density and gas-like transport properties, and behaves very differently than it does at room temperature. For example, it is highly nonpolar, permitting complete solubilization of most organic compounds and oxygen. The resulting single-phase mixture does not have many of the conventional transport limitations that are encountered in multiphase reactors. However, the polar species present, such as inorganic salts, are no longer soluble and start precipitating. The physicochemical properties of water, such as viscosity, ion product, density, and heat capacity, also change dramatically in the supercritical region with only a small change in the temperature or pressure, resulting in a substantial increase in the rates of chemical reactions. It is interesting to see (Fig. 7) that the dielectric behavior of 200°C water is similar to that of ambient methanol, 300°C water is similar to ambient acetone, 370°C water is similar to methylene chloride, and 50°C water is similar to ambient hexane.
Fig. 7

Physical properties of water with temperature at 24 MPa [59]

In addition to the unusual dielectric behavior, transport properties of water are significantly different than the ambient water as presented in Table 2.
Table 2

Comparison of ambient and supercritical water


Ambient water

Supercritical water

Dielectric constant



Solubility of organic compounds

Very low

Fully miscible

Solubility of oxygen

6 ppm

Fully miscible

Solubility of inorganic compounds

Very high


Diffusivity (cm2 s-1)



Viscosity (gcm-1 s-1)



Density (g cm-3)



5 Sub- and Supercritical Water Technology for Biofuels

The use of sub- and supercritical water media also known as hydrothermal media, which can be broadly defined as water-rich phase above 200°C, offers several advantages over the other biofuels production methods [94]. Some of the major benefits are
  • Ability to wet biomass

  • Can use mixed feedstock or waste biomass from other process residues

  • High energy and separation efficiency (since water remains in liquid phase and the phase change is avoided)

  • High throughputs

  • Versatility of chemistry (solid, liquid, and gaseous fuels)

  • Reduced mass transfer resistance

  • Improved selectivity for the desired energy products (methane, hydrogen, liquid fuel) or biochemicals (sugars, furfural, organic acids, etc.)

  • No need to maintain specialized microbial cultures

  • Products are completely sterilized with respect to any pathogens including biotoxins, bacteria, or viruses

  • Processing of postfermentation residues

Since the process is conducted in liquid/water-rich phase, the energy required for phase change of water from liquid to vapor is avoided. This provides an opportunity to reduce the requirement of process heat compared to steam-based processes. As an example, we know that 2.869 MJ/kg of energy is required to convert ambient water from 25°C to steam at 250°C and 0.1 MPa whereas only 0.976 MJ/kg of energy is required to convert ambient water from 25°C to subcritical water at 250°C and 5 MPa. This energy need for heating water to subcritical condition (0.976 MJ/kg) is equivalent to 6–8% of energy contained in dry biomass. Generally, the higher heating value of dry biomass is in the range of 16–19 MJ/kg. This also means that the energy contained in the subcritical water is insufficient to vaporize the water on decompression. Further, it is possible to recover much of the heat from subcritical water. The technology can be applied to produce solid (biochar), liquid (bioethanol, biocrude/bio-oil), and gaseous (methane, hydrogen) fuels from biomass depending on the processing temperature and pressure as shown in Fig. 8.
Fig. 8

Application referenced to pressure–temperature phase diagram of water

The substantial changes in the physical and chemical properties of water in the vicinity of its critical point can be utilized advantageously for converting lignocellulosic biomass to desired biofuels [22, 94]. In fact, reactions in subcritical and supercritical water also provide a novel medium to conduct tunable reactions for the synthesis of specialty chemicals from biomass [76].

In the subcritical region, the ionization constant (K w ) of water increases with temperature and is about three orders of magnitude higher than that of ambient water (Fig. 7) and the dielectric constant (ε) of water drops from 80 to 20 [118].

A low dielectric constant allows subcritical water to dissolve organic compounds, while a high ionization constant allows subcritical water to provide an acidic medium for the hydrolysis reactions. These ionic reactions can be dominant because of the liquid-like properties of subcritical water. Moreover, the physical properties of water, such as viscosity, density, dielectric constant, and ionic product, can be tuned by small changes in pressure and/or temperature in subcritical region [31, 79, 103]. In the supercritical region, density of water drops down to lower value. This means that ionic product of water is much lower and ionic reactions are inhibited because of the low relative dielectric constant of water. The lower density favors free-radical reactions, which may be favorable for gasification [61].

5.1 Reaction Pathways of Cellulose, Hemicelluloses, and Lignin in Hydrothermal Medium

Lignocellulosic biomass is a mixture of cellulose, hemicelluloses, and lignin which are held together by covalent bonding, various intermolecular bridges, and van der Waals forces forming a complex structure. Several studies have been conducted using model compounds such as cellulose, hemicelluloses, and lignin in sub- and supercritical water to establish the reaction pathways of these compounds in hydrothermal medium. The chemistry behind the reactions of the individual components of biomass under hydrothermal conditions is well understood. The generalized individual reaction pathways of the major components of biomass (cellulose, hemicelluloses, and lignin) are discussed in the following section.

5.2 Cellulose Reaction Pathways

Hydrothermal degradation of cellulose is a heterogeneous and pseudo-first-order reaction for which detailed chemistry and mechanism have been proposed earlier [9, 80, 81, 107]. Cellulose reaction in hydrothermal and catalyst free medium mainly proceeds via hydrolysis of glycosidic linkages. The long chain of cellulose starts breaking down in such condition to smaller molecular weight water soluble compounds (oligomers) and further to glucose (monomer). Glucose is water soluble and undergoes rapid degradation in hydrothermal medium at elevated temperature. The key products from the glucose decomposition are shown in Fig. 9. These products are formed mainly via dehydration, isomerization, reverse aldol condensation, and fragmentation reactions.
Fig. 9

Key reaction products from cellulose hydrolysis in catalyst free hydrothermal medium [50, 94, 101, 125]

Hydrolysis of cellulose in supercritical water is very sensitive to residence time. A high residence time enhances the formation of organic acids such as acetic acid, formic acid, and lactic acid. Formation of acids makes the reaction medium more acidic, which is conducive for further degradation of hydrolysis products. Indeed after hydrolysis of cellulose in water at 320°C and 25 MPa for 9.9 s, more than half of the cellulose was converted to organic acids [100]. The solid cellulose-like residues have been inevitably observed because of the rapid change in the polarity of water in going from reaction condition to room temperature. These residues have been reported to have a lower viscosity-average degree of polymerization (DP v ) with no significant change in crystallinity as compared to untreated cellulose [63].

5.3 Hemicelluloses Reaction Pathways

Hemicelluloses are polysaccharides of five carbon sugars such as xylose or six carbon sugars other than glucose. They are usually branched and have much lower degree of polymerization. Branches in the chains do not allow the formation of tightly packed fibrils. Hemicelluloses are not crystalline and easily hydrolyzable to their respective monomers. In fact, about 95% of hemicelluloses were extracted as monomeric sugars using water at 34.5 MPa and 200–230°C in a span of just few minutes [80]. In hydrothermal medium, hemicelluloses are hydrolyzed to sugars, which subsequently degrade into furfural and other degradation compounds. Furfural (2-furaldehyde) is commercially produced from hemicelluloses-derived xylose [94].

5.4 Lignin Reaction Pathways

Lignin is a complex and high molecular weight polymer of phenylpropane derivatives (p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol). The density of hydrothermal medium is found to be a key parameter in lignin decomposition. In hydrothermal reaction medium, most of the hemicelluloses and part of the lignin are solubilized below 200°C. Lignin fragments have high chemical reactivity. Part of these fragments again cross-links and re-condenses to form high molecular weight water insoluble products [9]. Recently, Fang et al. have proposed the reaction pathway (Fig. 10) for lignin in supercritical water [29].
Fig. 10

Reaction pathway of lignin in supercritical water (reproduced from [29])

The reaction steps consist of four phases: oil phase, aqueous phase, gas phase, and a solid residue phase [6, 29]. Their study concluded that lignin can be completely dissolved and undergoes homogeneous hydrolysis and pyrolysis preventing further re-polymerization.

The following sections discuss about application of sub- and supercritical water/hydrothermal medium’s properties for bioethanol, biocrude, biochar, and gaseous fuels production.

6 Subcritical Water/Hydrothermal Pretreatment of Biomass for Bioethanol Production

Lignocellulosic biomass has emerged as a potential renewable biomass resource for the bioethanol production [92, 117]. The concept is to hydrolyze the cellulose and hemicelluloses fraction of biomass (holocellulose) to recover C5 and C6 sugars and then ferment the sugars to bioethanol [93]. The recovered lignin in the process which has relatively higher heating value in the range of 24–26 MJ/kg is typically used for generating steam or providing the process heat. The biochemical pathways which can be realized at a very moderate process conditions using cellulase and accessory enzymes to convert holocelluloses to fermentable sugars are the most promising ones for large-scale bioethanol production. But the efficiency of this technology is limited due to the complex chemical structure of lignocellulose biomass and the inaccessibility of β-glycosidic linkages to cellulase enzymes because of the low surface area and small size of pores in multicomponent structure. Hence, pretreatment is nowadays viewed as a critical step in lignocellulose processing [66].

Pretreatment alters both the structural barrier (removal of lignin and hemicelluloses) and physical barrier (surface area, crystallinity, pore size distribution, degree of polymerization) which help in improving the accessibility of enzyme for hydrolysis [38, 67, 83]. It enhances the rate of production and the yield of monomeric sugars from biomass. Pretreatment is among the most costly step in the bioethanol conversion process as it may account for up to 40% of the processing cost. Moreover, it also affects the cost of upstream and downstream processes [69, 91, 130, 135]. Hence, an efficient, less energy intensive and cost-effective pretreatment method is a necessity for producing ethanol at an economically viable cost. Different pretreatment methods are broadly classified into physical, chemical, physicochemical, and biological processes. The conventional pretreatment, by using acids or alkalis, is associated with the serious economic and environmental constraints due to the heavy use of chemicals and chemical resistant materials [21, 43, 115].

Hydrothermal pretreatment employing subcritical water has attracted much attention because of its suitability as a nontoxic, environmentally benign and inexpensive media for chemical reactions [63, 76]. One of the most important benefits of using water instead of acid as pretreatment media is that there is no need of acid recovery process and related solid disposal and handling cost [21, 51]. Below the critical point, the ionization constant of water increases with temperature and is about three orders of magnitude higher than that of ambient water. Also, the dielectric constant of water decreases with temperature. A low dielectric constant allows liquid water to dissolve organic compounds, while a high ionization constant provides an acidic medium for the hydrolysis of biomass components via the cleavage of ether and ester bonds and favor the hydrolysis of hemicelluloses [31, 65, 79, 103]. The structural alterations due to the removal of hemicelluloses increase the accessibility and enzymatic hydrolysis of cellulose. Enzyme accessibility is increased as a result of the increase in mean pore size of the substrate which enhances the probability of the hydrolysis of glycosidic linkage [42].

Hydrothermal pretreatment is typically conducted in the range of 150–220°C. The temperature range, aiming for the fractionation of hemicelluloses, is decided based on the fact that at temperature below 100°C less/small extent of hydrolytic reaction is observed whereas cellulose hydrolysis and degradation become significant above 210°C [32, 41]. The severity factor (R 0 ) has been used by several researchers to measure the combined effect of temperature and residence time in hot water treatment of biomass processes [1, 88, 96]. The severity index is defined as
$$ {R}_{0}=t\times \mathrm{exp}\left\{\frac{T-100}{14.75}\right\}$$
where t is the residence time in minutes and T is temperature in °C.

Ether bonds of the hemicelluloses are most susceptible to breakage by the hydronium ions. Depending on the operational conditions, hemicelluloses are depolymerized to oligosaccharides and monomers, and the xylose recovery from biomass can be as high as 88–98%. For example, Suryawati et al. have reported 90% removal of hemicelluloses from Kanlow switchgrass at 200°C [116]. Acetic acid is also generated from the splitting of thermally labile acetyl groups of hemicelluloses. In further reactions, the hydronium ions generated from the autoionization of acetic acid also acts as catalyst and promotes the degradation of solubilized sugars. In fact, the formation of hydronium ions from acetic acid is much more than from water [32, 41].

The low pH (< 3) of the medium causes the precipitation of solubilized lignin and also catalyzes the degradation of hemicelluloses. To avoid the formation of inhibitors, the pH should be kept between 4 and 7 during the pretreatment. This pH range minimizes the formation of monosaccharides, and therefore the formations of degradation products that can further catalyze hydrolysis of the cellulosic material during pretreatment [10, 41, 58, 67, 82, 110, 128]. Maintaining the pH near neutral (5–7) helps in avoiding the formation of fermentation inhibitors during the pretreatment. The addition of small amount of K2CO3 increases the glucan digestibility even at low pretreatment temperatures (150–175°C) [66]. Figure 11 shows the SEM images of untreated and hydrothermally pretreated switchgrass in a flow-through reactor where additional pores created after the pretreatment can be seen [62].
Fig. 11

SEM images of untreated and pretreated switchgrass at 150°C in the presence of 0.9 wt.% of K2CO3

In general, the concentrations of solubilized products are lower in hydrothermal pretreatment compared to the steam pretreatment [9]. Since the hot compressed water is used instead of steam, the latent heat of evaporation is saved which makes it easier to apply for a continuous process [57]. Yang and Wyman have reported that flow-through process fractionated more hemicelluloses and lignin from corn stover as compared to batch system under the conditions of similar severity [132]. In a flow-through system, the product is continuously removed from the reactor which reduces the risk on condensation and precipitation of lignin components, making the biomass less digestible. The soluble lignin compounds are very reactive at the pretreatment temperature and if not removed rapidly part of these compounds recondense and precipitate on the biomass [68, 96].

In a subcritical water pretreatment study, the microcrystalline cellulose (MCC) pretreated at 315°C in a continuous flow reactor for about 4 s of residence time showed nearly threefold increase in the initial enzymatic reactivity as compared to the untreated MCC at 3.5 FPU/g of glucan enzyme loading [63]. The percentage crystallinity of MCC slightly increased after the subcritical water pretreatment and remained high (>81%) throughout the treatment range (200–315°C). Increase in percentage crystallinity is generally attributed to the hydrolysis and removal of the amorphous part of cellulose during pretreatment [63]. The DP v of cellulose reduced with the pretreatment temperature and sharp decline was observed in cellulose samples pretreated at 315°C (Fig. 12). The DP v of untreated MCC was 327. It decreased with temperature as expected, but reduced rapidly for treatment above 300°C.
Fig. 12

Degree of polymerization (DP v ) vs. pretreatment temperature for microcrystalline cellulose(MCC) in subcritical water [63]

7 Subcritical Water/Hydrothermal Liquefaction for Biocrude Production

Biocrude is defined as an aqueous carbohydrate solution (oxygenated hydrocarbon) produced from the liquefaction of biomass. Biocrude derived from the direct liquefaction of biomass can be converted to liquid fuel, hydrogen gas, or chemicals. Aqueous phase reforming processes have been successfully utilized for converting the biomass-derived, water-soluble carbohydrates to liquid alkanes and hydrogen [45, 46, 121]. Preliminary studies on the conversion of various biomass types into liquid fuels have indicated that hydrothermal liquefaction can be more attractive than pyrolysis or gasification. In these studies, typically 25% biomass slurry in water is treated at temperatures of 300–350°C and 12–18 MPa pressures for 5–20 min to yield a mixture of liquid, gas (mainly CO2), and water. The liquid is a mixture with a wide molecular weight distribution and consists of various kinds of molecules. A large proportion of the oxygen is removed as carbon dioxide and the resulting biocrude contains only 10–13% oxygen, as compared to 40% in the dried biomass [35]. Hydrothermal upgrading (HTU) process was first developed by Shell, where biomass was subjected to subcritical water at 330°C to produce biocrude. Biocrude was further upgraded to liquid fuels via hydrodeoxygenation process [35]. In a conceptual process scheme, it was shown that each ton (dry basis) of biomass can produce 300 kg (or 95 gal) of liquid fuel.

Karagoez et al. investigated the distribution of products from hydrothermal liquefaction (280°C for 15 min) when wood (sawdust), nonwood biomass (rice husk), and model biomass components (e.g. lignin, cellulose) are used as feedstock. The produced bio-oil was characterized for their differences in the hydrocarbon compositions with respect to feedstock. Cellulose showed the highest conversion among the four samples investigated. Sawdust and rice husk had almost similar conversions. Liquid products were recovered with various solvents (ether, acetone, and ethyl acetate) and analyzed by GC–MS. The oil (ether extract) from the hydrothermal treatment of cellulose consisted of furan derivatives whereas lignin-derived oil contained phenolic compounds. The composition of oils (ether extract) from sawdust and rice husk contained both phenolic compounds and furans; however, phenolic compounds were dominant. Rice-husk-derived oil consists of more benzenediols than sawdust-derived oil. The volatility distribution of oxygenated hydrocarbons was carried out by C-NP gram and it showed that the majority of oxygenated hydrocarbons from sawdust, rice husk, and lignin were distributed at n-C11, whereas they were distributed at n-C8 and n-C10 in cellulose-derived oil. The gaseous products were carbon dioxide, carbon monoxide, methane in sawdust, rice husk, lignin, and cellulose [53, 54].

Liquefaction of biomass in subcritical water proceeds through a series of structural and chemical transformations involving [16]
  • Solvolysis of biomass resulting in micellar-like structure

  • Depolymerization of cellulose, hemicelluloses, and lignin

  • Chemical and thermal decomposition of monomers to smaller molecules

Demirbas has also reviewed the possible mechanism of liquefaction. Organic materials are converted to liquefied products through a series of physical and chemical changes such as solvolysis, depolymerization, dehydration, and decarboxylation. Solvolysis is a type of nucleophilic substitution where the nucleophile is a solvent molecule. This reaction results in micellar-like substructures of the feedstock. Depolymerization reactions lead to smaller molecules. Decarboxylation and dehydration lead to new molecules and the formation of carbon dioxide through splitting off of carboxyl groups [19].

Switchgrass was effectively liquefied to produce biocrude in subcritical water in a flow-through reactor. Biocrude composed of aqueous phase (water-soluble compound) and solid precipitates. The aqueous phase contained oligomers and monomers of five and six carbon sugars, degradation products (5-HMF and furfural), organic acids (lactic, formic, and acetic acid), 2-furancarboxaldehyde, and other phenolic products containing 5–9 carbon atoms. A small amount of potassium carbonate catalyzed the liquefaction and enhanced the decomposition of biomass to water-soluble products. The residual solid contained mainly lignin fractions. Based on the infrared spectroscopy and electron microscopy, it was confirmed that subcritical water treatment lead to a breakdown of lignocellulosic structure. Some of the identified compounds by GC–MS analysis (> 85% of confidence level) in biocrude from switchgrass liquefaction are given in Table 3.
Table 3

Some of the identified compounds in biocrude produced from switchgrass (>85% of confidence level) [65]


GC retention time (min)


Area (%)

Quality (%)




























1,4-Benzenediol, 2-methyl-





Phenol, 2, 6-dimethoxy-





Benzaldehyde, 4-hydroxy-










Homovanillyl alcohol





Benzaldehyde, 4-hydroxy-3,5-dimethoxy-



Hydrolysis of cellulose in hydrothermal medium has been studied extensively. Several research studies have shown that subcritical and supercritical water can be used under a variety of conditions to rapidly (order of seconds) liquefy cellulose to sugar and its degradation products [18, 76, 102]. Hemicelluloses, an amorphous fraction of lignocellulosic have been successfully extracted up to 95% of its fraction as monomeric sugar and sugar products in hydrothermal medium in the range of 200–230°C in a very short reaction time [9, 80]. Low activation energy of lignin causes substantial degradation of lignin in hydrothermal medium at temperature below 200°C. Reaction proceeds through the cleavage of aryl ether linkages, fragmentation, and dissolution [9]. Lignin depolymerization yields low molecular weight fragments having very reactive functional groups such as syringols, guaiacols, catechols, and phenols [29]. The density of water within the hydrothermal medium has been found to be a key parameter in deciding the product pathways [94].

The results of liquefaction studies on model compounds and the actual biomass in hydrothermal medium provides an opportunity for converting biomass to biocrude and other important chemicals. The hydrothermal medium (subcritical water) in the range of 250–350°C regions provides a favorable condition for conducting ionic reactions. In general, hydrothermal liquefaction conditions range from 250 to 380°C, 7–30 MPa with liquid water present, often in presence of alkaline catalyst [94]. Crystalline cellulose was successfully converted to monomers (mainly glucose) and oligomers by hydrolysis in subcritical water in a continuous flow reactor. More than 90% of the cellulose converts to water-soluble products above 330°C. The study showed that a high yield of hydrolysis products can be achieved at comparatively lower temperature (335°C) in subcritical water. For example, up to 66.8% of crystalline cellulose was converted to hydrolysis products at 335°C and 27.6 MPa in merely 4.7 s reaction time. With increase in the reaction time, the hydrolysis products degraded to glycoaldehyde, fructose, 1,3 dihydroxyacetone, anhydroglucose, 5-HMF, and furfural. Yield of glycoaldehyde, a retro-aldol condensation product of glucose, increased with a decrease in the density of supercritical water, and the yield of degradation products, 5-HMF and organic acids, increased with temperature and residence time. In supercritical water conditions, more than 80% of the cellulose converted into the degradation products (oxygenated hydrocarbons) and organic acids [64].

8 Hydrothermal Carbonization for Biochar Production

Biochar is the carbon rich, high energy density solid product resulting from the advanced thermal degradation of organic materials such as wood, manure, agricultural residues, etc. The less fibrous structure and high calorific value of biochar similar to that of coal makes it an excellent candidate for solid fuel. Table 4 compares some of the advantages of biochar as solid fuels over the raw biomass.
Table 4

Comparison of biomass and biochar as solid fuel



High moisture retention

Low moisture retention and easily dried

Low heating value, so high transportation cost

High heating value, so less ($)/MJ cost during transportation

Perishable on storage

Not perishable and can be stored longer

Fibrous and so, difficult material handling

Friable, easier to compact and handle

Poor compatibility with coal for co-firing

Better compatibility with coal for co-firing

Biochar is resistant to decomposition upon land application and has a number of positive effects relating to soil fertility [10]. Pyrolysis and hydrothermal carbonization (HTC) are the two main processes for the production of biochar. Pyrolysis typically utilizes high quality dry biomass for biochar production where air pollution is primary concern during traditional pyrolysis operation due to the emission of volatile compounds. The HTC is an environment friendly and promising process that uses water as solvent. Besides being relatively simple process, HTC has a number of other practical advantages. The HTC process does not require dry biomass and also the final product can be easily filtered from the reaction solution. This way, complicated drying schemes and costly separation procedures are conceptually avoided.

The biomass feedstock typically contains 40–60% oxygen. Therefore, the removal of oxygen from biomass is the major objective for upgrading its energy density during biochar production. This objective can be accomplished by the removal of oxygen by dehydration, which removes oxygen in the form of water, and by decarboxylation, which removes oxygen in the form of carbon dioxide. Thermodynamically, water is fully oxidized compound and has no residual heating value. Therefore, water makes an ideal medium for conducting such reactions. Even in the excess of water, biomass undergoes dehydration reaction at elevated temperature and pressure [94]. Due to the increased ionization of water at elevated temperature, the biomass components undergo depolymerization mainly by hydrolysis reactions. The subcritical water in the temperature region of 180–250°C can effectively be utilized for the production of biochar from biomass [62]. The longer reaction time of the order of several hours is typically needed for the substantial removal of oxygen by dehydration and for the subsequent breakdown of the fibrous structure of biomass. The dehydration of biomass at lower temperature can be accelerated by the addition of small amount of acids.

In nature, coal is formed from plant material undergoing heat and pressure treatment over millions of years. The acceleration of coalification of biomass by a factor of 106–109 in hydrothermal medium under milder process conditions can be a considerable and technically attractive alternative for biochar production [120]. Essentially, all forms of biomass can be converted to biochar. Forest thinning, herbaceous grasses, crop residues, manure, and paper sludge are some of the potentially attractive feedstock. The enhanced transportation and solubilization properties of sub- and supercritical water (hydrothermal medium) play an important role in the transformation of biomass to high energy density fuels and functional materials [44]. Here, water acts as reactant as well as reaction medium which help in performing hydrolysis, depolymerization, dehydration, and decarboxylation reactions. The proton-catalyzed mechanism, direct nucleophilic attack mechanism, hydroxide-ion-catalyzed mechanism, and the radical mechanism play important roles in the conversion of biomass in hydrothermal medium [73, 95]. Under the umbrella of hydrothermal process, the conversion of a variety of biomass to chemicals, including organic monomer, biofuel, hydrogen, and biochar, has been studied widely. Through hydrothermal carbonization, a carbon-rich black solid as insoluble product is obtained from biomass in the range of 180–350°C [105, 120].

The earliest research focused on analyzing the changes in O/C and H/C atomic ratio to understand the chemical transformations taking place during HTC [7]. For example, Marta et al. studied the HTC of three different saccharides (glucose, sucrose, and starch) at temperatures ranging from 170 to 240°C. The result showed that a carbon-rich solid product made up of uniform spherical micrometer-sized particles of diameter 0.4–6 mm range could be synthesized by modifying the reaction conditions. The formation of the carbon-rich solid through the HTC of saccharides was the consequence of dehydration, condensation, polymerization, and aromatization reactions. In a recent study, the same group of researchers used cellulose as starting material and successfully established that highly functionalized carbonaceous materials can be produced by HTC from cellulose in the range of 220–250°C [72]. The formation of this material follows essentially the path of a dehydration process, similar to that previously observed for the hydrothermal transformation of saccharides such as glucose, sucrose, or starch. In another recent study, Titirici et al. compared hydrothermal carbons synthesized from diverse monomeric sugars and their derivatives (5-hydroxymethyl-furfural-1-aldehyde (HMF) and furfural) under hydrothermal conditions at 180°C with respect to their chemical structures. The results showed that type of sugars has an effect on the structure of carbon-rich solids [119].

The traditional method for biochar production from biomass sources is slow pyrolysis, where dry biomass is used for the purpose in the range of 500–800°C. Antal Jr. and coworkers have developed another method for charcoal production which is named as flash carbonization. The process is conducted at elevated pressure by the ignition and control of a flash fire within a packed bed of biomass [122]. Considering the relatively high energy consumption needed in the pyrolysis process, HTC process which is typically conducted in subcritical water below 300°C might be an economical and efficient option for biochar production. The process is particularly important since it can utilize the wet biomass. Switchgrass was converted to biochar by HTC at 200–280°C. Compared with the reaction time and pressure, the temperature plays an important role in the conversion. With increase in temperature, biochar yield decreased but the heating value increased. The HHV of biochar produced at 280°C in just 1 h of HTC was comparable to the bituminous grade coal [62]. In October 2010, AVA-CO2, ZUG, Switzerland & KARLSRUHE launched the world’s first industrial-size HTC plant. With an overall capacity of 14,400 L and an annual capacity to process 8,400 tons of biomass, the HTC reactor demonstrates an impressive way that the experts from AVA-CO2 have succeeded in constructing and operating a plant of industrial size.

9 Carbon-Rich Microspheres from Sugars in Subcritical Water

Carbon-rich microspheres can be formed by the HTC of sugar solution if treated for several minutes to hours at 180–200°C. Titirici et al. had studied the HTC of carbohydrate model compounds such as glucose and xylose. Their study reported the elemental carbon in the microsphere as 64 and 68% from glucose and xylose, respectively [119]. HTC of sugars is a potential alternative to produce uniform carbon-rich microspheres [120]. Kumar et al. studied the HTC of glucose solution to produce the carbon microspheres from soluble organic compounds (Fig. 13). The precipitated solids were globular with their diameter ranging from 0.2 to 2 μm and having a higher heating value (HHV) of 24.8 MJ/kg which is comparable to lignin’s HHV (24–26 MJ/kg) [62]. Glucose in hydrothermal medium at relatively low temperature (180–200°C) range and longer residence time (order of few hours) undergoes mainly dehydration and partial fragmentation (C–C bond breaking) reactions.
Fig. 13

SEM images of carbon microspheres produced via hydrothermal carbonization (HTC) from glucose solution at 200°C for 2 h [66]

The intermediate compounds are mainly furan-like compounds, organic acids, and aldehydes [55]. Furan-like ring compounds may undergo polymerization via aldol condensation to form soluble polymers. Aromatization of soluble polymers takes place under the reaction condition and when the aromatic clusters in aqueous solution reach the critical supersaturation point, they precipitate as carbon-rich microspheres. The process can be a novel tool for recovering the water-soluble oxygenated hydrocarbons as high heating value carbon-rich microspheres. The yield of these microspheres depends on the sugar contents in liquid solution subjected to HTC. Majority of sugar compounds present in the liquid undergoes polycondensation and dehydration processes resulting in carbon-rich microspheres.

10 Supercritical Water/Hydrothermal Gasification

Gasification of biomass into fuel gases, such as synthesis gas or producer gas, is a promising route to produce renewable fuels, which is commonly accomplished via partial oxidation of the feedstock using sub-stoichiometric air or oxygen or by indirect heating with or without steam. Typically, gasification is performed using relatively dry feedstock with moisture <  10 wt.% at 700–1,000°C and near ambient pressures. The synthesis gas or “syngas” can be utilized to produce liquid fuels by Fischer–Tropsch synthesis. Supercritical water gasification is a novel method to process organic matter from biomass [26]. Relatively fast hydrolysis of biomass in supercritical water leads to a rapid degradation of polymeric structure of biomass. The subsequent reactions also are rather fast, which leads to gas formation at relatively lower temperature compared to dry processes [60]. Above the critical point, the lower density of supercritical fluid favors free radical reactions and makes the reaction conditions conducive for the formation of methane and hydrogen gas [61]. There are two types of SCWG operations: (a) low temperature ranging from 350 to 600°C in the presence of metal catalysts, and (b) high temperature ranging from 500 to 750°C without catalyst or using nonmetallic catalysts [74]. For hydrogen formation from biomass, Watanabe et al. [126] used Zirconia (ZrO2) to catalyze the reaction while Elliott et al. [27] and Byrd et al. [11] demonstrated the significant activity of Ru, Rh, and Ni as catalysts. Biomass is gasified to mainly methane and carbon dioxide in the presence of an added heterogeneous catalyst in near critical or supercritical water (350–400°C). At higher temperature in supercritical water, biomass is converted to hydrogen-rich gas without catalyst or with nonmetal catalysts [26]. As discussed earlier, part of lignin and hemicelluloses fraction of biomass undergo solvolysis within few minutes of the exposure to hydrothermal medium. The hydrothermolysis of remaining biomass fractions occurs at somewhat higher temperature. The initial products subsequently undergo a variety of isomerization, dehydration, fragmentation, and condensation reactions that ultimately form gas and tars [74]. There are numerous studies describing supercritical water gasification of cellulosic biomass for methane, hydrogen, or fuel gas production [75, 94]. The Pacific Northwest National Laboratory (PNNL) has extensively studied the catalytic hydrothermal gasification process for several feedstocks in the last 30 years. In a recent feedstock test project, lignin-rich biorefinery residues showed several levels of difficulty such as slow rate of conversion and plugging of feedlines, while algae feedstock were more reliably processed. The gasification was conducted using Ru/C gasification catalyst with a pelletized Raney nickel sulfur scrubbing bed.

The principal advantages of supercritical water for gasification are that process can utilize wet feedstock and can achieve high gasification efficiency at comparatively low temperature (400–700°C). The homogeneous reaction medium with a minimal mass transfer resistance favors decomposition of organic compounds into gases, decreasing formation of tar and char [12]. For example, the highly nonpolar nature of supercritical water permits complete solubilization of most organic compounds. The resulting single-phase mixture does not have many of the conventional transport limitations that are encountered in multiphase reactors. However, the polar species present, such as inorganic salts, are no longer soluble and start precipitating [64]. Due to the different reaction mechanisms, supercritical water exhibits some important inherent advantages over the conventional gasification such as:
  • Energy intensive drying of biomass is avoided

  • The homogeneous reaction medium

  • Easier separation of gaseous products after the reaction

  • Requires lower reaction temperature compared to the conventional gasification

Moreover, fuel gas is produced directly at high pressure, which means a smaller reactor volume and lower energy needed to pressurize the gas in a storage tank. The nonvolatile inorganic constituents of the co-product residue are expected to remain in the aqueous solution. This makes the resulting fuel gas cleaner and less corrosive compared to the conventional dry processes. The presence of alkali salts catalyzes the gasification processes [64]. Due to insolubility of inorganic salts in supercritical water, the alkali salts precipitate out rapidly as fine particles and these in situ-­generated particles provide extra surface area for heterogeneous catalysis [84]. The addition of alkali metal salts (e.g., KHCO3, KOH, Na2CO3, K2CO3) also reduces coke formation and catalyzes the water-gas shift reaction [40]. For example, the addition of KHCO3 leads to an increase in gas formation and a decrease in the amount of carbon monoxide [110]. For example, the use of K2CO3 in the reaction mixture during depolymerization of cellulose in subcritical water substantially enhanced gas formation [64]. SCW has been utilized to gasify coal. For example, Hui [48] obtained gas containing 70% hydrogen from gasification of a 24 wt.% coal-water-slurry with 2 wt.% sodium carboxymethyl cellulose and 1 wt.% K2CO3 at 580°C and 250 bar using a fluidized bed reactor.

11 Microalgae to Biofuels Using Sub- and Supercritical Water Technology

In general, conventional higher land plants are not very efficient in capturing solar energy. Even the fastest growing energy crops can convert solar energy to biomass at a yearly rate of no more than 1 W/m2 [133]. However, the biomass productivity of microalgae, a photosynthetic microorganism, can be 50 times greater than switchgrass [20]. Microalgae grow in marine and freshwater environments. Due to their simple cellular structure and submergence in an aqueous environment where they are in vicinity of water, CO2, and other nutrients, microalgae are generally more efficient in converting solar energy into biomass. Microalgae can be used to produce wide range of second-generation biofuels and bioactive compounds [47, 106]. They offer many potential advantages [14, 106, 133] over conventional biomass sources. Microalgae primarily comprise of varying proportion of proteins, carbohydrates, lipids, and ash. The percentages vary depending upon the species. Table 5 presents the general composition of different microalgae. What really makes algal biomass feasible for biofuels production is the fact that many forms of algae have very high lipid contents. The biomass that is nonlipid provides a high-value co-product such as animal feed or fertilizer that offsets the cost of converting the algae to fuels. Growing algae also removes nitrogen and phosphorus from water and consumes atmospheric CO2.
Table 5

General composition of different algae (% of dry matter) [5]





Chlamydomonas rheinhardii




Chlorella vulgaris




Euglena gracilis




Porphyridium cruentum




Scenedesmus obliquus




Spirulina platensis




11.1 Major Challenges in the Conversion of Microalgae to Biofuels

Microalgae are considered as one of the most promising feedstock for biofuels. The biomass can be used for producing solid (biochar), liquid (biodiesel, liquid hydrocarbons, pyrolysis oil), and gaseous fuels (synthesis gas, methane, hydrogen). However, the attractive target is producing fungible fuels such as gasoline, diesel, and jet fuel ([37]; Rene H. [129]). Some of the major challenges in accomplishing the objective of cost-competitive biofuels production from microalgae are:

Dewatering: It is not uncommon to have less than one gram of algae per liter of water. Therefore, a cost-efficient harvesting and drying process is needed to produce a biomass suitable for oil recovery. Conversion processes that can process wet biomass are highly desirable for reducing the energy intensive dewatering cost.

High Nitrogen Content: Microalgae are rich in proteins (Table 5). The elemental composition (carbon, hydrogen, and oxygen) of microalgae is similar to other cellulosic biomass but differs in nitrogen contents. Apart from water, sunlight, and carbon dioxide, nitrogen and phosphorous are the primary nutrients which are required to grow microalgae. Based on the average elemental composition, microalgae can be represented by a general formula as CH1.7O0.4N0.15P0.0094. The nitrogen content varies between 4 and 8 wt.% of the dry biomass depending upon the physiological state and nutrient limitation condition of microalgae [37, 129]. Due to the high nitrogen content, NOx emission and losses of nitrogen fertilizer are matter of great concern besides the high moisture content if whole microalgae are processed for biofuels [47].

The organically bound nitrogen converts to ammonia in reducing atmosphere and NOx in combustion/oxidizing atmosphere during the biofuels conversion processes. In biogas production, high nitrogen contents lead to the ammonia toxicity during anaerobic digestion process. Also, high nitrogen contents are reported to inhibit the digestion of algal biomass. Similarly, presence of nitrogen in biomass will cause the formation of NOx compounds during gasification process which is conducted in the limited supply of oxygen. NOx is a greenhouse gas and heavily regulated environmental pollutant. Further, gas cleaning also adds to the cost, if synthesis gas is to be used for liquid fuels production via Fischer–Tropsch process. The nitrogen is mainly present as the protein in microalgae. Therefore, it is important to extract high value protein for the sustainable production of biofuels from microalgae. The major emphasis should be on the value addition to the nonfuels components and the recycle of nutrients as much as possible. The approach for producing biofuels from microalgae can be different compared to the conventional cellulosic biomass processes. The high protein contents of microalgae make it a potential candidate for extracting protein [16] and the species which have the high lipid contents are best considered for biodiesel production via transesterification process [15, 23, 108].

Diverse Composition: More than 50,000 species of microalgae are reported to exist [78]. Algal biomass composition varies depending upon the species (Table 5). The challenge involves developing a process that can tolerate the complex compositions found with different species of algae. In order to make biofuels cost competitive, it is important to develop processes for the 100% utilization of algal biomass components.

11.2 Microalgae to Biofuels Conversion Processes

The most attractive target for fuel production from algae is the production of gasoline, diesel, and jet fuel [23, 36]. There are several competing pathways similar to lignocellulosic biomass for converting the biomass to liquid fuel, chemicals, and/or hydrogen.

The anaerobic digestion of microalgae is a process for production of biogas [34] which mainly contains CH4 and CO2 and is an environment friendly clean, cheap, and versatile gaseous fuel [39]. Vergara-Fernandez et al. reported that biogas production levels of 180.4 mL/g-d of biogas can be realized using a two-stage anaerobic digestion process with different strains of algae, with a methane concentration of 65% [124]. Unfortunately, biogas garners less importance than the common transportation fuels. The selling cost per unit energy is considerably lower than gasoline. Because the biogas produced is relatively low in value, the anaerobic process must be simple in design, have low operating cost, and produce gas at high rates.

Pyrolysis processes typically use dry and finely ground biomass. The oxygen content of bio-oils is usually 35–40 wt. %. The presence of oxygen is reason for substantially lower heating value of bio-oils compared to hydrocarbon fuels. Also of concern is the nitrogen content of algae, because this nitrogen tied up mostly as proteins is transformed to nitrogenous species such as pyridines. Removal of such oxygen is possible by decarboxylation, dehydration, and/or catalytic hydrotreating [123]. Therefore, bio-oils are required to be catalytically upgraded to make them fungible fuels. Considering the high moisture content of algae, pyrolysis that requires dried algae may not be an economical option and an inexpensive dewatering or extraction process will have to be developed [23]. In fact, very few studies have been reported on pyrolysis of algae for bio-oil. The bio-oil yield for the microalgae Chlorella protothecoides rose from 5.7 to 55.3% as the pyrolysis temperature rose from 250 to 500°C, and then gradually decreased to 51.8% and was obtained at 600°C [20]. The heating value of bio-oils from algae was in the range of 35 MJ/kg which is relatively higher than that from wood.

Gasification is another transformation pathway for production of liquid fuels from algae. Synthesis gas is produced through the thermal decomposition of organic matter in oxygen-deficient conditions. The presence of sulfur and nitrogen in feedstock can be the cause of impurities in synthesis gas. Sulfur in microalgae is significantly low (< 0.1 wt.% dry basis) and hence may not be a matter of concern. However, high nitrogen contents of microalgae lead to the formation of NH3, NOx, and HCN gases. These impurities should be cleaned before synthesis gas is subjected to FT process for avoiding the catalyst deactivation. It is important to note that the tolerance limits for NH3, NOx, and HCN are reported to be 10 ppmv, 0.2 ppmv, and 10 ppb, respectively [113].

In view of challenges associated with utilizing wet biomass, principally the need for water and nitrogen removal, reactions in sub and supercritical water medium may be an attractive option for extracting valuable bioactive compounds and producing biofuels from microalgae. Figure 14 shows the possibility of producing different kind of biofuels using sub- and supercritical water technologies.
Fig. 14

Different fuels from microalgae using sub- and supercritical water technology

The reactions conditions can be tuned to conduct liquefaction, carbonization, or gasification in water medium based on the product requirement. Microalgae is nonfibrous compared to the lignocellulosic biomass and so feeding algae slurry at high pressure may be less challenging. In another recent study, it was reported that more than 80 wt.% of microalgae (Chlorella vulgaris) could be gasified in supercritical water. The studies were conducted in quartz capillaries under operating temperatures of 400–700°C and reaction times of 1–15 min in the presence of nickel-based catalysts. The dry gas composition from algae gasification in supercritical water was mainly comprised of CO2, CO, CH4, H2, and some C2–C3 compounds [13].

12 Challenges of Hydrothermal Processing

Although in laboratory experiments excellent results have been achieved and the technology possesses many potential benefits over the conventional methods of processing biomass to biofuels or chemicals, there are certain issues, which need to be addressed.
  • Biomass feeding at high pressure: As a “rule of thumb,” the solid loading in excess of 15–20 wt.% is considered economical on commercial point of view. Feeding slurries at high pressure is always challenging especially for the lab scale studies since low capacity slurry pumps are rarely available. Pumping slurry at large scale is less of a problem, where progressive cavity or similar pumps are commercially available.

  • Salt precipitation: Plugging of reactors caused by the precipitation of inorganic salts above supercritical temperature and low density conditions. At room temperature, water is an excellent solvent for most salts. On the other hand, solubility of most salts is very low (typically 1–100 ppm) in supercritical water (low density) and precipitating salts may plug the reactors even at high flow velocities [59]. However, the problem may be used as an opportunity to produce a valuable fertilizer by-product of the process, if managed properly.

  • Corrosion: The halogens, such as sulfur or phosphorous, present in the organic matter are converted to the respective acids, which may cause severe corrosion on the reactor wall under harsh reaction conditions. The corrosion problem can be reduced or avoided by selecting the right material of construction and or a slightly modified reactor concept.

  • Coking and deactivation of heterogeneous catalyst: Some catalyst supports degrade or oxidize in hydrothermal conditions. Decline in catalyst activity is also observed with long period of exposure of catalyst during continuous process [94].

13 Energy Balance of Wet Processes

Most of the biomass feedstock such as forest residues, agricultural residues, food processing wastes, agricultural sludge, etc. contains moisture or large amounts of water. In conventional thermochemical processes, considerable amount of energy is required to evaporate water. In case of microalgae as feedstock, energy required for the dewatering process may account for more than 75% of the total energy consumption. Typical thermal dryers use significantly more energy per kilogram of evaporated water (3.3–3.9 MJ/kg). The drying steps lead to large parasitic energy losses that can consume much of the energy content of the biomass. In sub- and supercritical water processing, water is kept in liquid or supercritical phase by applying pressure greater than the vapor pressure of water at the reaction temperature. Thus, energy (latent heat of vaporization of water 2.26 MJ/kg) required for the phase change of water from liquid to vapor phase is avoided by carrying out the reactions under pressure. The specific energy requirements needed to affect the isobaric expansion from liquid-like to gas-like densities are typically lower than what is needed when boiling occurs under subcritical pressures at an intermediate temperature to form a two-phase mixture [94].

Xu et al. compared the energy balance for wet and dry process for converting microalgae to biofuels [131]. In their study, biodiesel, glycerol, pyrolysis oil, and producer gas were considered as products from conventional dry processes, whereas green diesel, hydrogen, producer gas, and CO2 recycling were considered for the wet/hydrothermal process products. The fossil energy ratio (FER) was selected as performance indicator which was defined as:
$$ \text{FER}=\frac{{\text{HHV}}_{\text{biofuel}}}{{E}_{\text{input}}}$$

HHVbiofuel: Higher heating value of biofuel products.

E input: Fossil energy input.

The FER for dry and wet process was reported as 1.50 and 1.37, respectively. The FER of 1 indicates that the same amount of fossil energy is consumed in the process of converting the fossil energy to a useable fuel. The analysis showed that the drying process in the dry route and the oil extraction process in the wet route consume a significant amount of energy. By coupling waste heat from a nearby power plant to the process, the energy balance can be improved and a potential FER up to 2.38 and 1.82 can be reached for the dry and wet route, respectively. Their study further concluded that the dry route may be more interesting on a short-term basis because of a higher FER, but for the long term, the wet route has more potential because of producing biofuels with a higher value. In the absence of existing commercial operations on wet processes, most of the cost is estimated based on the equipment used in supercritical water oxidation processes, as an example: one wet ton of organic waste with an organic content of 10°wt.% to below 300 US$ [59]. Being a high pressure system, major investment is attributed mainly to the equipment cost. The energy needed for pumping the feed at high pressure is significantly lower compared to the heat input required for heating the reactor at reaction temperature. In December 2010, Ignite Energy Resources, Australia declared that they have developed a supercritical Water (SCW) reactor technology for converting biomass and ancient coal into highly valuable oil and coal products.


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Copyright information

© Springer Science+Business Media New York 2013

Authors and Affiliations

  1. 1.Department of Civil and Environmental EngineeringOld Dominion UniversityNorfolkUSA

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