**2. Liquid hot-water (LHW) pretreatment**

#### **2.1. Description**

This hydrothermal process involves cooking of biomass in liquid water at high temperatures (150–240°C) and short times (≤ 50 min). Pretreatment causes pressurized water to rupture and penetrate the cell structure, resulting in fractionation of biomass into two product streams liquid hydroxylate containing hemicelluloses sugars, minerals, and degradation products such as furfural and acetic acid, and a solid fraction comprising most of the cellulose and lignin and some residual hemicellulose. Pretreated solid substrates have increased surface area and pore volume, and consist of separated individual cellulose fibers, with large particles of repolymerized lignin on the surfaces of the cellulose matrix [5]. The harshness of the process is described by a severity factor (*Ro* ) that allows for the determination of combined effects of temperature and reaction time on sugar yields and degradation products. It is expressed as: *Ro* <sup>=</sup> *<sup>t</sup>* <sup>×</sup> *exp*[(*<sup>T</sup>* <sup>−</sup> 100)/14.75], where *t* = reaction time (min), and *T* = temperature (°C) [6].

Generally, the carbohydrate content of pretreated substrates increases with temperature until a maximum temperature is reached where further temperature increase result in substantial degradation. Thus, high temperatures greater than 230°C disrupt pretreated particles and reduce the surface area and pore volume, which in turn limit enzymatic digestibility [5].

Based on the work of Mosier et al. [2], it is observed that at reaction conditions of 200–230°C and ≤ 15 min, biomass dissolution ranged from 40 to 60%, comprising 4–22, 35–60, and 100% of cellulose, lignin, and hemicellulose, respectively. Much of the hemicellulose dissolve into poly- and oligosaccharides even at high severities which is beneficial since the minimization of monomer formation reduces the chance of further degradation into aldehydes (2-furaldehyde, 5-hydroxymethylfurfural, etc.), which are known inhibitors of enzymatic hydrolysis. However, oligomers also possess inhibitory tendencies to cellulase activity [7]. It has been established that digestibility of pretreated substrates depended more on partial removal of hemicellulose and relocalization of lignin than the modification of crystallinity and rupture of the cell wall [5, 8].

#### **2.2. Applications**

for a material or mix of feedstocks is influenced by factors that include carbohydrate preservation and digestibility, sugar and ethanol yields, energy consumption, equipment and solvent costs, lignin removal and quality, formation of degradation products from sugars and lignin,

Pretreatment may be categorized as physical (e.g., size reduction, autoclaving, irradiation, popping, ultrasonication, steaming and extrusion), chemical (use of acids, alkali, solvents, etc.), biological (white-rot fungi, brown rot fungi, etc.), and physico-chemical which combine both physical and chemical processes to ensure the digestibility of the lignocellulosic material. Physical pretreatment usually demands high-energy consumption (e.g., side reduction) and is undertaken before chemical or biological pretreatment. While chemical methods offer benefits such as efficient fractionation of biomass and good sugar yields during enzymatic hydrolysis, their environmental impacts are higher than physical and biological methods due to biomass degradation into enzyme-inhibiting compounds, corrosion of reactors, solvent

The well-known physico-chemical pretreatment include liquid-hot water (LHW), steam explosion (SE), ammonium fiber explosion (AFEX), soaking in aqueous ammonia (SAA) and irradiation-chemical method. This chapter reviews and present novel findings as well as process innovations in physico-chemical processing of recalcitrant biomass to sugars and ethanol.

This hydrothermal process involves cooking of biomass in liquid water at high temperatures (150–240°C) and short times (≤ 50 min). Pretreatment causes pressurized water to rupture and penetrate the cell structure, resulting in fractionation of biomass into two product streams liquid hydroxylate containing hemicelluloses sugars, minerals, and degradation products such as furfural and acetic acid, and a solid fraction comprising most of the cellulose and lignin and some residual hemicellulose. Pretreated solid substrates have increased surface area and pore volume, and consist of separated individual cellulose fibers, with large particles of repolymerized lignin on the surfaces of the cellulose matrix [5]. The harshness of the process

temperature and reaction time on sugar yields and degradation products. It is expressed as:

Generally, the carbohydrate content of pretreated substrates increases with temperature until a maximum temperature is reached where further temperature increase result in substantial degradation. Thus, high temperatures greater than 230°C disrupt pretreated particles and reduce the surface area and pore volume, which in turn limit enzymatic digestibility [5].

Based on the work of Mosier et al. [2], it is observed that at reaction conditions of 200–230°C and ≤ 15 min, biomass dissolution ranged from 40 to 60%, comprising 4–22, 35–60, and 100% of cellulose, lignin, and hemicellulose, respectively. Much of the hemicellulose dissolve into

*Ro* <sup>=</sup> *<sup>t</sup>* <sup>×</sup> *exp*[(*<sup>T</sup>* <sup>−</sup> 100)/14.75], where *t* = reaction time (min), and *T* = temperature (°C) [6].

) that allows for the determination of combined effects of

waste production, and water usage.

42 Fuel Ethanol Production from Sugarcane

recycling issues, and generation of waste [3, 4].

**2. Liquid hot-water (LHW) pretreatment**

is described by a severity factor (*Ro*

**2.1. Description**

LHW pretreatment has been applied to pretreat various feedstocks including agricultural residues, woods and industrial waste. Archambault-Leger et al. [9] applied both batch and flowthrough pretreatment to corn stover, bagasse, and poplar and observed higher hemicellulose recovery, removal of non-carbohydrate carbon, and glucan conversion under simultaneous saccharification and fermentation (SSF) in the flowthrough reactor. Some authors have included additives to positively influence pretreatment outcomes. For example, by adding AlCl<sup>3</sup> to pretreatment solution of microcrystalline cellulose, low concentration of degradation products and high glucose yields were obtained [10]. Optimum glucose yield of 80% was also observed when solid carbon dioxide was used in hydrothermal fractionation of *Eucalyptus* [11]. In another study, an alcohol-water mixture was used to overcome challenges due to deposition of lignin particles on pretreated materials, and thus achieved increased pore volume and higher sugar yields [12].

Combinations of LHW and other methods have also been employed to overcome inherent drawbacks and to improve sugar yields. Low degradation products and higher sugar yields—xylose (91.62%) and glucose (88.12%)—was observed when LHW treatment (180°C, 20 min) of *Eucalyptus* was followed by wet disk milling before saccharification [13]. Alkalineassisted LHW treatment of rice straw was found to improve glucose recovery and yield under enzymatic hydrolysis, caused by increased removal of hemicellulose and lignin [14].

At demonstration and industrial scale, one notable application of LWH is in the Integrated Biomass Utilization System (IBUS) platform where biomass is converted into ethanol, C5 molasses, and lignin pellets using uncatalyzed steam in an energy efficient manner underscored by high dry matter content in all process routes [15]. It was developed as a three-stage, pilot-scale process for treating wheat straw—by soaking at 80°C for 20 min, hemicellulose recovery at 170–180°C for 7.5–15 min, and cellulose hydrolysis at 195°C for 3 min. Under these conditions both ethanol production and lignin recovery for power production are maximized [16]. High glucose yield required the avoidance of water addition to the third stage while high hemicellulose yield (83%) required water addition. It was improved by Petersen and team [17] who used a two-stage procedure to achieve high cellulose recovery (over 90%) on wheat straw: soaking of biomass at 80°C for 5–10 min followed by pretreating at temperatures and residence times ranging from 185 to 198°C and 6–12 min respectively. The optimum pretreatment temperature was observed at 195°C at which cellulose and hemicellulose recovery reached 93–94 and 70% respectively at lower water/biomass ratio compared to the threestage process. However, the two-step process was found to present economic challenges in the recovery of C5 sugars after the first pretreatment in a commercial-scale plant, prompting Inbicon to settle for a simpler, one-stage treatment processs [15]. Currently, the Inbicon demonstration plant, which is based in Kalundborg (Denmark), processes about 4 tonnes straw/h and at yields greater than 198 L ethanol/tonne of wheat straw.

water—occurs to form acetic and other acids which enhance further fractionation of hemicellulose [24], and trigger the release of carbonium ions from benzyl alcohol structures in lignin which cause the breakdown of some of the β-O-4 structures in lignin leading to reduced molecular weight [25]. Simultaneously, condensation reactions may take place in the presence of electron-rich carbon atoms, resulting in lignin repolymerization [25, 26], with the composi-

Emerging Physico-Chemical Methods for Biomass Pretreatment

http://dx.doi.org/10.5772/intechopen.79649

45

The process is affected by temperature, reaction time, material size, moisture content and efficient mixing of biomass. The explosion mechanism and time which are independent of the severity factor are also known to affect yields [28]. Increasing reaction time and temperature decreases the degree of polymerization of cellulose [29]. Though severe conditions contribute to reduction in crystallinity and increase in moisture retention, they do not necessarily lead to increased hydrolysis rates due to possibility of thermal degradation of cellulose. Similarly, xylose recovery is reduced for longer pretreatment times due to formation of degradation products. Further, severe conditions increase the intensity of repolymerization and condensation reactions from byproducts of lignin, hemicellulose, and extractives leading to increased molecular weights of lignin [30]. This development reduces substrate amenability to enzymatic hydrolysis caused by the covering of cellulose surface with the repolymerised lignin-like materials (pseudo-lignin). The problem of lignin repolymerization was overcome by Li et al. [31] who used a carbonium ion scavenger (2-napththol) to achieve solubilize lignin, resulting in improved recovery (91%) as against 51% for steam pretreated aspen wood without the additive.

SE has been applied in combination with additives and pretreatment methods to improve yields and overall process economics. The major variations include the use of acids and bases

In this process, SE is undertaken after the biomass is soaked with dilute acid or impregnated

(5–100°C). It favors solubilization of hemicelluloses into monomer units, making substrates more reactive while improving enzymatic hydrolysis of cellulose. Compared to dilute acid,

of different sizes and shapes in the outer region of the cell wall of pretreated substrates, with

A major positive attribute about ACSE is that most glucan and lignin are untouched and remain in solid form after pretreatment [34] though lignin presence hinders enzymatic hydrolysis [35]. Nonetheless, high sugar yields are generally obtained. Yields obtained by some

impregnates biomass substrates better and more uniformly but requires harsher condi-


at low or atmospheric pressures for 0.5–25 h depending on the temperature


, CO2 is


has a lower solubility compared to SO2

tion affected by pretreatment severity [27].

**3.3. Acid-catalyzed steam explosion (ACSE)**

tions to remove hemicellulose [32]. Both SO2

under similar conditions [33]. Though CO2

highly available, less toxic and corrosive, and thus safer to apply.

the effect more noticeable in SO2

investigators are given in **Table 1**.

**3.2. Applications**

as catalysts.

with SO2

SO2

or CO2

#### **2.3. Positive attributes and drawbacks**

LHW offers improved digestibility of cellulose by enzymes due to the solubilization of hemicelluloses and avoidance of inhibitors. Compared to steam explosion, LHW gives lower concentrations of solubilized hemicellulose and lignin products due to higher water input as well as higher pentosan recovery. Generally, catalysts/chemicals are avoided resulting in no/ low neutralization demands and byproduct/precipitate generation, with additional benefits such as reduced risk of reactor corrosion and explosion. Reactor cost is lower compared to methods such as AFEX [18]. The effect of particle size reduction on hydrolysis is low, thus, large biomass flowrates can be handled effectively.

There are however drawbacks in LHW related to hemicellulose fractionation into large fractions of oligomers, and xylose yields are generally low, which affect sugar and ethanol yields. There is a risk of sugar degradation into byproducts such as carboxylic acids and furans at severe conditions [19, 20]. A major cost involved in LHW pertains to high energy used to generate saturated liquid water. Consequently, solid loadings are restricted to about 20% [21].
