A review on delignification of lignocellulosic biomass for enhancement of ethanol production potential
Introduction
Conversion of abundant lignocellulosic biomass to ethanol as a transportation fuel presents an important opportunity to improve energy security, reduce the trade deficit, reduce greenhouse gas emission and improve price stability [1]. In last few decades several ways of utilizing biomass and associated waste for energy production in different forms e.g., biogas, bio-diesel, pyrolytic bio-oil, etc. have been envisaged thoroughly by researchers worldwide [2], [3], [4], [5], [6]. It becomes imperative to explain in beginning why lignocellulosic biomass should be converted to ethanol in comparison to other biofuels such as bio-diesel, pyrolytic oil, bio-gas and many others. This can be explained simply because of these main reasons delignification of lignocellulosic biomass becomes so lucrative and important for ethanol production because of its abundance and low cost. Also low oil content in lignocellulosic biomass makes it useless for bio-diesel production. Along with this other technologies e.g., biogas can mainly be used for electricity or thermal energy and not as a vehicle fuel. The designing and operation of a biomass gasification plant involves a number of factors and most of these are critical that may increase the chances of malfunctioning of the plant [5]. Another main energy conversion technology is biodiesel; it is produced from vegetable oils and animal fats through the process of trans-esterification. Mostly, it is obtained from Pongamia, Jatropha and other crops such as mustard, jojoba, flax, sunflower, palm oil, coconut etc. Several researchers have used crops such as rubber seed [7], jatropha [8], [9], mahua [10], tobacco seed [11], castor [12], Eruca sativa [13] and pongamia [14]. Major lignocellulosic biomass does not contain oils which are essential for the production of biodiesel and some crops such as mustard which contain oils are consumed as animal feed. Moreover, the viscosity of neat vegetable oil (range of 28–40 mm2/s) is high due to which its direct use has led to diesel engine problems such as deposits formation and injector coking arising from poor atomization [15]. In India, the recent studies have reported that the present economics of molasses-based ethanol production is not in the favor of commercial blending of ethanol in petrol. The study has indicated that if the government is targeting to bring into effect 10% blending by the year 2016–2017, as planned in the National Biofuel Policy, production of approximately 736.5 million t of sugarcane with area coverage of 10.5 million ha would be required [16] which is not feasible solution and will require huge investment. Moreover, it would be highly unsustainable to extend the sugarcane area beyond a certain limit, as the sugarcane is a highly water-intensive crop with water requirement of 20,000–30,000 m3/ha/crop [16]. Therefore, it is necessary to find out an alternative way for the production of ethanol. Lignocellulosic substances such as cereal straws are available in large quantities and can be easily fermented to produce ethanol, which can be used either as a motor fuel in pure form or as a blending component in gasoline. Otherwise, farmers burnt these straws openly for clearing the field that led to the air pollution and emission of greenhouse gases. [17].
There are so many lignocellulosics agricultural waste available for ethanol production such as sugarcane baggase, rice hull, timber species, willow, salix, switch grass, softwood, rice straw, wheat straw etc. (Fig. 1.).
In agricultural dominating countries like India the crop residue and waste have great potential for ethanol production. Table 1 shows total crop residue production and their availability for ethanol production in India.
One can see there is significant increase in the availability of crop residue for ethanol production; it may be due to increased agricultural productivity and mechanization of agricultural industry. Different crops have different potential for ethanol yield. Table 2 shows ethanol yield for different crop residue. The cost of production of ethanol depends upon the procurement cost of raw material, processing, transportation from the site to the industry, ethanol production cost, market price and policies. Table 3 shows the procurement prices for major agro-residues in India.
In India, seasonal availability of agricultural crop residues depend on different types of crops (Table 4) e.g., maize stalk have availability period from August till the end of December whereas cotton stalk have availability in January and March. This type of information is very important to ensure the raw material availability throughout the year for the ethanol industry.
Lignocellulose consists primarily of plant cell wall materials; it is a complicated natural composite with three main biopolymers—cellulose, hemicellulose, and lignin [27], [28]. Ash and other minor compounds are also present in the lignocellulosic biomass. It is a heterogeneous complex of carbohydrate polymers and lignin [29] and contains 55–75% carbohydrates by dry weight. Lignocellulosics do not contain simple sugars that can be readily converted to ethanol. Beside this, they contain polysaccharides such as cellulose and hemi-cellulose which need to be converted to the monosaccharide. Cellulose and hemicellulose are closely associated with lignin thus prevent the access to the hydrolytic agents unless the lignin is modified or removed by chemical and/or biological methods [30]. Cellulose is a major structural component of cell walls and it provides mechanical strength and chemical stability to plants. Cellulose is the β-1,4-polyacetal of cellobiose (4-O-β-d-glucopyranosyl-d-glucose) and since the cellobiose consists of two molecules of glucose, it is considered as a polymer of glucose. The specific structure of cellulose helps in the ordering of the polymer chains into a tightly packed, highly crystalline structure that is water insoluble and resistant to depolymerization [31]. Hemicellulose is the another carbohydrate component present in the lignocellulosic biomass that is composed of 5- and 6-carbon sugars and has a random, branched and amorphous structure with little strength. Both the cellulose and hemicellulose fractions are polymers of sugars and thereby a potential source of fermentable sugars that can be easily processed into different products [32], [33], [34], [35]. Though there are many problems in producing ethanol from crop residues, presence of lignin is one of major constraint. Lignin is a highly stable biopolymer built from three cross-linked phenylpropane (C6–C3) units of p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol [36] which are bonded together with over two-thirds being ether bonds (C–O–C) and the rest C–C bonds [37]. Lignin can be hydrolyzed via cleavage of the ether bonds that are catalyzed by [H+] and [OH−] or water molecules [38]. Ash content is generally composed of minerals such as silicon, aluminum, calcium, magnesium and sodium. Other minor compounds present in the lignocellulosic biomass are extractives that include resins, fats and fatty acids, phenolics, phytosterols, salts and minerals. Composition of some lignocellulosic biomass is given in Table 5.
Lignocellulose bio-refineries via biological conversion generally have three main steps: (1) lignocellulose pretreatment, which converts the recalcitrant lignocelluloses structure to reactive cellulosic intermediates; (2) enzymatic cellulose hydrolysis, by which cellulases hydrolyze reactive intermediates to fermentable sugars (e.g., glucose and xylose); and (3) fermentation, which produces cellulosic ethanol or other bio-based chemicals (e.g., lactic acid, succinic acid) [52], [53], [54], [55].
Fractionation of Lignocellulosic biomass is shown in Fig. 2. Fractionation contains two main parts recalcitrant and biodegradable. Opening up the recalcitrant structures would enable their biodegradability [56]. In which recalcitrant part requires pretreatments. There are many reasons [57], [58] for using pretreatment and defined that an efficient method should have some major features summarized below;
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no production or very limited amount of production of sugar;
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production of very limited amount or no, for lignin;
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high digestibility of the cellulose;
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high recovery of all carbohydrates;
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liquid fraction must have high solid and liberated sugar concentration;
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need a very little demand of energy and
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should be economic and cost effective.
The process of pretreatment is one of very daunting steps in whole process of ethanol production. Flow diagram of enzymatic ethanol production process [59] is given in Fig. 3. Authors [59] mentioned that pretreatment large impact on all other steps in the process.
Section snippets
Pretreatment of ligno-cellulosic substrates
The goals of the pretreatment are to decompose the polymeric components of the ligno cellulosic and form monomer sugars thus enhance enzymatic conversion of the cellulose fraction that increase the digestibility of the material for microbial and enzymatic bioconversion [60] and obtain a higher ethanol yield. Therefore, pretreatment is essential for the removal of lignin, reduction of cellulose crystallinity and increased porosity of the material. An effective pretreatment should produce
Pretreatment methods
Pretreatment is one of the most expensive steps in biological conversion of cellulosic biomass. Pretreatment methods may be chemical, physical or hybrid in nature and mostly energy intensive. Some of the important pretreatment methods will be discussed in this section followed by appraisal of pretreatment methods.
Appraisal for pretreatment methods
It is being reported how pretreatment [59] has a very significant impact on other steps of ethanol production. Author [59] also mentioned formation of inhibitory substances during the pretreatment process. The variety of biomass also affects production of toxic compounds after pretreatment [59]. However in most of the pretreatment methods mainly furans, organic acids, aldehydes, ketones and aromatic compounds from lignin are toxic compounds which are generated. Pretreatment has large impact on
Effect of delignification on physico-chemical structure of agri residues
Major physico-chemical changes that occur due to different types of pretreatments are: change in surface area, pore volume, biomass crystallinity, melting and relocation of lignin and degree of cellulose polymerization.
Crystallinity can affect the enzymatic saccharification of glucan. Different thermo chemical pretreatments can change cellulose crystalline structures by disrupting or destroying inter- or intra-hydrogen bonding of cellulose chains. Low pH pretreatments can enhance biomass
Effect of pre-treatment on ethanol production
The ethanol production after the pretreatment depends on percentage of sugar recovery, type of simple sugar (pentose or hexose) and production of inhibitors. Table 9 shows the effect of some treatments on the ethanol recovery from the different substrates. It is quite evident that same types of pretreatment have significant difference on various types of crops e.g., using H2SO4 on sugarcane leaf litter (3.35 g/l) and Wheat straw (19 g/l). The ethanol recovery mainly depends on type of crops
Uses of lignin
However as discussed above lignin does affect the production of ethanol and need to be removed for better production efficiency but it had several uses reported in literature. Some of the major uses are discussed below [163], [164]:
Lignin can be used as a low grade fuel by the direct combustion and it cannot be upgraded to oil, gas or recovered as chemicals [67]. Supercritical water can used a weak polar solvent to dissolve and hydrolyze lignin for potential production of phenolic chemicals or
Conclusions
The accessible surface area is the key determinant for the extent of delignification. Delignification with single method is not very effective for the further utilization of substrate for the ethanol production. The focus of the research should be on the development of combination of the delignification method on the basis of the lignin and cellulose profile of different potential raw material for the ethanol production. Energy auditing and the feasibility study should also be done along with
Acknowledgment
The authors are thankful to Prof. Guido Zacchi for his valuable time. Indian authors Dr. Renu Singh, Sapna Tiwari and Monika Srivastava are grateful to Science and Engineering Research Board, Department of Science and Technology, Government of India, for providing funding during the course of the study.
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