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30 Feasibility and Economics of Biobutanol from Lignocellulosic and Starchy Residues

been made by the researchers in recent year for the bioconversion of lignocellu-

losic biomasses into biobutanol in an economic way. One of the major cost intensive

steps in biobutanol production involves cost of feedstock and biomass pretreatment.

The former can be tackled by selecting feedstock which is waste biomass generated

from industrial and agricultural activities. As reported in literature the worldwide

availability of plant biomass reaches 200 billion tons/year, of which lignocellulosic

resource constitute 90% of world biomass [35].

Extensive research is carried out to reduce the cost of pretreatment and achieve

high degree of hydrolysis. Pretreatment protocols necessitate the efficient utiliza-

tion of biomass for the conversion to fuel, wherein lignocellulose biomass is broken

down into its monomeric sugars. The microorganism used in ABE fermentation uti-

lizes the released sugar producing biobutanol. The main aim of the pretreatment

is to disrupt the crystalline structure of the cellulose by breaking the lignin barrier

and ease acids or alkali to access and hydrolyze the cellulose and hemicellulose.

Pretreatment for lignocellulosic residues is carried out in mild condition to reduce

the formation of fermentation inhibitory compounds. Formation of inhibitory com-

pounds in turn makes the process more expensive by incorporating detoxification

step to increase the efficiency and yield of butanol.

Physical, chemical, or biological pre-treatments are thoroughly explored process

for butanol production from lignocellulosic biomass. However, the methods of

pretreatment and optimum pretreatment condition are decided by the type and

nature of lignocellulose biomass. The major factors affecting the degree of hydrol-

ysis are cellulose crystallinity, available surface area and composition of lignin and

hemicelluloses [36]. Different pretreatment processes have been investigated for

lignocellulose biomass, which includes physical pretreatment (grinding, milling),

chemical pretreatment (dilute alkali, acid, organic solvents, etc.), physico-chemical

pretreatment (steam explosion, autohydrolysis, hydrothermolysis) and biological

pretreatment. Although, there are many pretreatment steps discussed in the liter-

ature, the success of any pretreatment method depends upon energy requirement,

consumption of chemicals and formation of fermentation inhibitors. Therefore,

exploration of new feedstock for biofuel production demands a systematic study of

the pretreatment method and conditions that are required.

The process of ABE fermentation is described in Figure 30.3. After pretreatment,

the biomass releases fermentable sugars which are used as a substrate for ABE fer-

mentation by Clostridia sp. For example Clostridium acetobutylicum, Clostridium

saccharobutylicum, Clostridium beijerinckii, etc. [37] undergoes metabolic shift pro-

ducing a significant amount of acetic acid and butyric acid thus producing acetone,

butanol, and ethanol respectively. Clostridia sp. has the vast ability to ferment vari-

ety of sugar monomers such as fructose, glucose, starch, dextrins, mannose, lactose,

and sucrose. It also partially ferment sugars such as xylose, galactose, mannitol, raf-

finose, and arabinose [38]. The production of ABE by Clostridia species follows an

intracellular pathway. The products of this intracellular pathway are categorized into

(i) organic acids (butyric acid, acetic acid, and lactic acid), (ii) solvents (acetone,

butanol, and ethanol), and (iii) gases (CO2 and H2). The first stage of ABE fermen-

tation begins with exponential growth phase called acidogenic phase wherein, each