2. Pretreatment of lignocellulosic material

Lignocellulosic biomass is abundantly available, relatively low-cost, and is a good feedstock for the production of biofuels due to their compositions (cellulose, hemicellulose, and lignin). The natural microorganisms cannot directly ferment lignocellulosic biomass into biofuels. The pretreatment step is required to overcome the recalcitrance attributed to the structural characteristic of lignocellulosic biomass and hydrolyze the lignocellulose biomass into fermentation sugars. Various pretreatment technologies have been proposed, challenging the complexity of biomass structure and attempting to recover high fermentable sugars. The pretreatment methods must meet the following requirements: (1) increase the sugar production or ability to afterward form sugar by enzymatic hydrolysis, (2) minimize the formation of inhibitors that affect the hydrolysis and fermentation process, (3) avoid the loss of carbohydrates, and (4) be cost-effective. The present section summarizes the performance of various pretreatment technologies, including physical, chemical, physicochemical, and biological processes. Furthermore, the advantages and disadvantages of different pretreatment technologies are also included.

#### 2.1 Physical pretreatment

Physical pretreatment involves an increase in the accessible surface area of lignocellulosic materials to enzymes by breaking down the particle size or disrupting their crystalline structures. The physical pretreatment methods such as chipping, milling, and grinding are applied to pretreat several lignocellulosic materials [15]. Chipping and grinding are used to reduce a huge lignocellulosic material into small pieces. Thus, milling is required to mill lignocellulosic material into fine particles. Among these physical methods, milling can significantly reduce the degree of crystallinity and particle size and consequently improve their enzymatic hydrolysis [16]. The energy requirement for physical pretreatment methods depends on the particle size and the reduction of crystallinity in lignocellulosic material. In fact, the required energy is higher than the theoretical energy content available in the biomass [15]. As aforementioned, these methods cannot be used in an industrial scale process due to its cost.

Microwave irradiation is another physical pretreatment method. It is a heating method which directly applies an electromagnetic field to the molecular structure. Microwaves are nonionizing electromagnetic radiation with the wavelengths ranging from 1 mm to 1 m. The electromagnetic spectrums are located between 300 and 300,000 MHz. The application of microwave pretreatment causes swelling and fragmentation of lignocellulosic biomass. The study of Shahzadi et al. [17] indicates that the use of microwave irradiation can enhance the digestibility of lignocellulosic material. In order to enhance the hydrolysis efficiency, microwave pretreatment assisted with catalysts such as acid and alkaline are applied [18]. The advantages and disadvantages of physical pretreatment method are tabulated in Table 1.

#### 2.2 Chemical pretreatment

Acid, alkaline, ionic liquid, and organic solvent (organosolv) are used as catalysts in the chemical pretreatment methods. Since 1819, acids including sulfuric and hydrochloric are applied to pretreat lignocellulosic materials [19]. After the discovery, various concentrated and diluted acids have been used to pretreat various lignocellulosic materials [20, 21]. The concentrated acid pretreatments can degrade

cellulose and produce a high concentration of inhibitors, such as furfural and 5 hydroxymethylfurfural (5-HMF). In addition, the utilization of concentrated acid causes corrosion of equipment, making the process less attractive [21]. Dilute acid is

Effects Advantages Disadvantages

Control of final particle size, easy handling, less water

Fast heat transfer, short reaction time, energy-

Enzymatic hydrolysis is sometimes not required as the acid itself may hydrolyze the biomass to fermentable

Reduce the absorption of cellulose due to efficient lignin removal, low cost

Working under mild reaction condition, low vapor pressure

Formation of a high purity of

Less water uses, no chemical uses, low environmental

Does not require washing, chemical recovery, or detoxification steps

Low formation of inhibitors, energy-efficient, reduces the absorption by sulfonation of

Selective degradation of lignin, hemicellulose, and cellulose, environmentally

Low formation of by-

High energy consumption

Low penetration of radiation in bulk products, the distribution of microwave power around of material due to nonhomogeneous material

Corrosive and toxic, formation of inhibitors as by-

Generates inhibitors, long residence time required

High cost, complexity of purification and synthesis

High capital cost, need to separate solvent, need washing step

It has a high equipment cost

High water consumption and

Not suitable for lignocellulosic biomass with high lignin

Long pretreatment time, the hydrolysis rate is low

High cost of chemical

energy input

content

recovery

products

consumption

efficient

sugars

lignin

impacts

products

cellulose

friendly

Summary of advantages and disadvantages of each pretreatment methods [22–24].

Pretreatment method

Microwave Swelling and

Acid Lignin cellulose and

Alkaline Lignin and

Ionic liquid Cellulose

Steam explosion Particle size

Physicochemical

Biological Microorganisms and enzymes

Table 1.

107

Organosolv Lignin removal and

Liquid hot water Partial hydrolysis of

SPORL Lignin removal and

AFEX Decreases the

Reduce the particle size and disrupt the crystallinity

DOI: http://dx.doi.org/10.5772/intechopen.85138

Bio-hydrogen and Methane Production from Lignocellulosic Materials

fragmentation of lignocellulosic material

hemicellulose fractionate

hemicellulose removal

precipitation and lignin removal

hemicellulose fractionate

reduction, partial hydrolysis of hemicellulose, lignin removal

hemicellulose, lignin removal

crystallinity and lignin removal

hemicellulose fractionate

hemicellulose, and cellulose degradation

Lignin,

Physical Chipping, grinding, milling

Chemical

