**2. Lignocellulosic biomass for biorefinery applications**

Lignocellulosic biomass typically refers to plant materials composed primarily of cellulose, hemicellulose and lignin. This type of biomass usually includes forest materials, agricultural residues, wood processing residues and non-edible plant materials usually referred to as energy crops (**Table 1**). In the context of biofuel production, lignocellulosic biomass are referred to as second generation biomass which is used to differentiate them from sugar and starch based biomass (1st generation biomass) and algal biomass (3rd generation biomass). They are typically composed of 40–50% cellulose, 25–30% hemicellulose and 15–20% lignin [30]. The effective use of these three primary components would significantly determine the economic viability of cellulosic ethanol production.

Cellulose refers to the linear polymer made up of glucose monomer units bonded together by β-1,4 glycosidic bonds. Hemicellulose refers to branched heteropolymers of xylose, glucose, galactose, mannose, arabinose and some uronic acids. Lignin is primarily made up of three major phenolic components, namely p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol [20]. The ratio of these


#### **Table 1.**

*Composition of typical lignocellulosic biomass used in biorefinery applications.*

*Integrated Biorefinery Approach to Lignocellulosic and Algal Biomass Fermentation Processes DOI: http://dx.doi.org/10.5772/intechopen.97590*

components varies between various plant tissues as shown in **Table 1**. The cellulose units are packed into microfibrils which are attached to each other by hemicelluloses and amorphous polymers of different sugars as well as other polymers such as pectin covered by lignin. The units of individual microfibrils in crystalline cellulose are packed so tightly that neither enzymes nor water molecules can enter the complex framework [20]. This high molecular weight and ordered tertiary structure of natural cellulose makes it insoluble in water. However, some parts of the microfibrils have a less ordered, non-crystalline structure referred to as amorphous regions [31]. The crystalline regions of cellulose are more resistant to biodegradation than the amorphous parts while cellulose with low degree of polymerisation will be more susceptible to cellulolytic enzymes. The composition of typical lignocellulosic biomass that have been considered for various biorefinery applications are presented in **Table 1**.

## **3. Algal biomass for biorefinery applications**

Marine biomass accounts for over 50% of primary biomass produced globally but has been the least harnessed for various applications [32]. It is mainly grouped into two, namely macroalgae (commonly known as seaweeds) and microalgae. However, cyanobacteria is conventionally regarded as a form of algae often called blue-green algae [33]. Both groups have been used in the production of various biofuels. Microalgae has been explored predominantly as substrate for bio-oils and biodiesel while macroalgae has been used mainly in bioethanol and biogas production [32].

Marine algae are plant-like multicellular organisms that live attached to hard substrata such as rocks in coastal areas [34]. Their basic structure consists of a thallus, which forms the body of the organism and a holdfast, a structure on its base which allows it to be attached to hard surfaces such as rocks near the shoreline of coastal areas. Brown seaweeds are the largest in size, growing up to 4 m in length for some species. Green and red seaweeds are smaller ranging from a few centimetres in some species to a meter in others [35]. According to the FAO [36], 8.2 and 15.8 million tons of brown and red seaweed respectively were produced in the year 2013. This was valued at USD 1.3 billion and 4.1 billion for the brown and red seaweeds respectively. For the green seaweed 14,800 tons valued at USD 15.7 million was produced globally in the year 2013 [36]. The enormous difference in the production values of the brown and red from the green seaweed can be attributed to the valuable hydrocolloids such as alginate, carrageenan and agar found only in the red and brown seaweeds.

The structural differences found between land-based plants and algae gives algal biomass an advantage of a higher yield per hectare. In comparison to landbased plants, seaweeds have an average yield per hectare per year of 730,000 kg while sugarcane, sugar beet, maize and wheat have 68,260; 47,070; 4,815 and 2,800 kg respectively [37]. The high yields from macroalgae in general is attributed to the low energy required in the formation of its supporting tissue during growth. Seaweeds can also absorb nutrients across its entire surface and can be cultivated three dimensionally in water [37].

Seaweeds are composed of carbohydrates, proteins, lipids and minerals which ranges from 30 to 60%, 10–40%, 0.2–3% and 10–40%, respectively [38]. Besides their unique and varying composition, seaweeds have been grouped into three, based on their pigmentation. They are rhodophyceae (red seaweeds), phaeophyceae (brown seaweeds) and chlorophyceae (green seaweeds) based on their pigments r-phycoerythrin, chlorophyll and xanthophyll, respectively [39].



### *Integrated Biorefinery Approach to Lignocellulosic and Algal Biomass Fermentation Processes DOI: http://dx.doi.org/10.5772/intechopen.97590*

**Table 2.**

*Composition of typical algal biomass used in biorefinery applications.*

Algal biomass composition has been found to vary based on several factors such as the season, availability of nutrients, water salinity and availability of sunlight (**Table 2**) [55]. The algal component of primary importance to bioethanol production is the carbohydrates (polysaccharides), since they currently form the only fraction that can be fermented to ethanol. Generally, some algae are composed of large fractions of complex sulphated polysaccharides which are uniquely different in each group serving as their cellular storage and structural support tissue [56]. The composition of typical algal biomass that have been considered for various biorefinery applications are presented in **Table 2**.

## **4. Processes for bioethanol production**

The conversion of cellulosic and algal biomass to bioethanol usually involves four major processes excluding biomass selection. They include biomass pretreatment, hydrolysis of pretreated biomass, fermentation of biomass hydrolysates and ethanol recovery from the fermentation broth using distillation and dehydration processes [46]. The various efficiencies of each process will influence the final ethanol yield therefore each process condition and catalyst used is carefully selected and in most cases optimised to maximise the process efficiencies.

One of the most influential processes in bioethanol production from cellulosic and algal biomass is pretreatment. This process is used to render biomass susceptible to further breakdown by separating the cellulose, hemicellulose and lignin fractions. The selection of an efficient and cost effective biomass pretreatment method has been a major hurdle in cellulosic bioethanol production and its commercialisation for several decades. Different pretreatment mechanisms have been developed with varying degrees of efficiency [20]. All these methods have been developed with a common aim of finding a good balance between efficiency, cost, environmental effects and energy use. So far, all the methods developed have come with intrinsic advantages and disadvantages. Some common disadvantages experienced include: degradation of sugars, formation of inhibitors, high energy requirements, catalyst requirements, difficulties in catalyst recovery, challenges in waste treatment and high overall costs [20]. One or more these drawbacks are experienced in the various pretreatment processes currently developed. Nonetheless, a careful comparison and risk analysis could be used to distinguish and select one from the other. The biomass specificity for particular pretreatments could be explored to see the variations in the interactions between various cellulosic and algal biomass and various pretreatment methods as a solution.

The hydrolysis process in bioethanol production is one of the most limiting stages in the entire production process since it is the stage where the sugars to be converted to ethanol is obtained. Hydrolysis simply refers to cleavage or division through the addition of water molecules. In the context of complex sugars (polysaccharides), it involves the use of a water molecule by a catalyst to break the glycosidic linkages within their polymeric form (di-, tri-, oligo- or polysaccharide) to their monomeric form (monosaccharides or reducing sugars). During the cleavage of sugars, a hydrogen atom (H+ ) is gained by one part of the polymeric structure whiles the other gains a hydroxyl group (OH− ). Thus, the separation continues until all polymeric units are reduced to their individual monomeric form [46].

The hydrolysis of cellulosic biomass for bioethanol production involves the breakdown of polymeric units such as cellulose and hemicellulose whiles the hydrolysis of algal biomass (particularly macroalgae) involves the breakdown of polymeric units such as laminarin, ulvan, alginate, carrageenan, mannitol, agar and cellulose. The simple sugars (monosaccharides) recovered from both agal and

#### *Integrated Biorefinery Approach to Lignocellulosic and Algal Biomass Fermentation Processes DOI: http://dx.doi.org/10.5772/intechopen.97590*

cellulosic biomass include glucose, galactose, rhamnose, mannose, fucose, xylose and arabinose for fermentation to ethanol [57]. The common methods that have been used in cellulosic and algal biomass hydrolysis includes dilute acid thermal [58], dilute alkaline thermal [59], enzymatic [3] and thermal [58] hydrolysis. All other hydrolysis methods are usually derivatives of these and are usually broadly grouped under physical, chemical, thermal and biological hydrolysis. Two or more of these methods are often combined to improve the efficiency of monomeric sugar recovery.

Enzymatic hydrolysis, particularly the use of cellulases in both cellulosic and algal biomass hydrolysis, has been promoted extensively over all other forms of hydrolysis. This is because enzymes are considered more environmentally friendly in their application and generate no inhibitors as is the case with chemical catalysts. Three major cellulase activity systems have been identified to be involved in cellulosic hydrolysis. The enzymes involved in these systems include endoglucanases, exoglucanases (cellodextrinases) and β-glucosidases [60]. Cellulase synthesis is predominant among fungi such as *Trichoderma reesei*, *Aspergillus niger*, and *Humicola insolens*; and bacteria such as *Bacillus subtilis*, *Streptomyces drodowiczi*, and *Bacillus pumilusand* [20]. Studies in enzymatic hydrolysis have focused on process optimization, improving cellulase activities, optimisation of reaction conditions, enzyme-to-substrate ratios and enzyme recovery and reuse strategies. The ideal final enzyme or enzyme cocktail should have high hydrolytic efficiencies on the preferred biomass, operate at mildly acidic or alkaline pH, be resilient to process stresses and be cost-effective [30].

The fermentation process in bioethanol production is the stage within which the reducing sugars obtained after hydrolysis are converted to ethanol by an organism. This process is always dependent on the overall ethanol production pathway selected. Currently, the ethanol pathways that have been used in cellulosic and algal biomass processing include: separate hydrolysis and fermentation (SHF), simultaneous saccharification and fermentation (SSF), simultaneous saccharification and co-fermentation (SSCF) and consolidated biomass processing (CBP) [46]. SHF is the most common and most well-developed approach which allows the use of the optimal conditions for both the hydrolysis and fermentation processes [61]. It offers the flexibility of choosing various hydrolysis processes, a feature which cannot be found in the use of the SSF approach. The SSF process involves the co-application of the enzyme for saccharification and the organism for fermentation to the pretreated biomass in the reactor under similar conditions of operation. This process is considered more cost-effective than SHF but comparisons on its process efficiency relative to SHF is currently inconclusive [46].
