Lignocellulosic biomass to bioethanol, a comprehensive review with a focus on pretreatment
Introduction
Recent economic developments in many countries all around the globe have heightened the need for alternative energy resources due to the well-documented drawbacks of fossil fuels: (1) their finite supply (2) greenhouse gasses emission and global warming and (3) increasing price and unexpected fluctuations. All these weaknesses have strengthened the interest in alternatives, renewable, sustainable, and economically viable fuel such as bioethanol. Bioethanol can be either mixed with gasoline or used as a sole fuel using dedicated engines; moreover, it has higher heat of vaporization and octane number compared to gasoline [1]. Ethanol is already blended with gasoline and supports by vehicle manufacturers have resulted in vehicles that can use up to an 85% ethanol–15% gasoline mixture [2]. In fact, gasoline can use bioethanol as an oxygenated fuel to increase its oxygen content, causing better hydrocarbon oxidation and diminishing greenhouse gasses [3].
In the first generation bioethanol production, expensive starch and sugar derived from sugar cane and maize are employed as feedstock but in the second generation process, lignocellulosic materials, which are cheap, abundant and renewable, are used [4]. Besides, lignocellulosic materials do not negatively affect the human food supply chain by eliminating the food in favor of bioethanol production [5]. Lignocelluloses are composed of cellulose, hemicelluloses and lignin (Fig. 1) in an intricate structure, which is recalcitrant to decomposition. One of the best strategies to convert such biomass into sugars is enzymatic saccharification due to its low energy requirement and less pollution caused; but, the major problem is the low accessibility of cellulose because of rigid association of cellulose with lignin [6]. This leads to difficulties within the conversion process; therefore, breaking down lignin seal in order to make cellulose more accessible to enzymatic hydrolysis for conversion is one main aim of pretreatment (Fig. 2). In other words, pretreatment is the crucial and costly unit process in converting lignocellulosic materials into fuels [7].
A suitable pretreatment procedure involves (1) disrupting hydrogen bonds in crystalline cellulose, (2) breaking down coross-linked matrix of hemicelluloses and lignin, and finally, (3) raising the porosity and surface area of cellulose for subsequent enzymatic hydrolysis [8], [9]. There are several pretreatment methods including, physical pretreatment (grinding and milling, microwave and extrusion), chemical pretreatment (alkali, acid, organosolv, ozonolysis and ionic liquid), physico-chemical pretreatment (steam explosion, liquid hot water, ammonia fiber explosion, wet oxidation and CO2 explosion) and biological pretreatment. On the other hand, regrdless of the pretreatment method used, some inhibitory compounds are produced during the process, which have negative effects on microbial activity in the hydrolysis step. Inhibitors are classified into three major groups: (1) weak acids such as levulinic, acetic and formic acids, (2) furan derivatives such as HMF (5-hydroxy-2-methyll furfural) and furfural as well as (3) phenolic compounds [10].
The purpose of this paper is to review different pretreatment methods for bioethanol production and to offer an in-depth discussion on the benefits and drawbacks of each while striving to present and highlight several combined pretreatment methods. Moreover, the crucial role of genetic and metabolic engineering in facilitating pretreatment and hydrolysis processes and consequently in economical production of ethanol from lignocellulosic wastes has been also discussed.
Section snippets
Cellulose
Cellulose (C6H10O5)x, the main constituent of lignocellulosic biomass, is a polysaccharide that consists of a linear chain of D-glucose linked by β-(1,4)-glycosidic bonds to each other. The Cellulose strains are associated together to make cellulose fibrils. Cellulose fibers are linked by a number of intra- and intermolecular hydrogen bonds [12]. Therefore, cellulose is insoluble in water and most organic solvents [13].
Hemicelluloses
Hemicelluloses (C5H8O4)m, located in secondary cell walls, are heterogeneous
Influence of lignocellulosic biomass composition and structure on cellulose hydrolysis and bioconversion
As mentioned earlier, lignocellulosic biomass composition plays a very crucial role in the performance and efficiency of both pretreatment and biodegradation stages. Table 1 presents the compositions of several suitable lignocellulosic biomass for bioethanol production [8], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. Utilization of cellulose in native form, not only consumes large amount of enzyme but also results in low cellulose enzymatic digestibility yield (<20%).
Physical pretreatment
The objective of physical pretreatment such as milling, grinding, chipping, freezing, radiation is to increase surface area and reduce particle size of lignocellulosic materials [29]. Moreover, it leads to decrease degree of polymerization and decrystallization of feedstock. Combination of physical pretreatments and other pretreatment is usually used.
Combination of pretreatment methods
As mentioned earlier various pretreatment methods have some drawbacks limiting their applications. Combined pretreatment methods have been recently considered as a promising approach to overcome this challenge, by increasing efficiency of sugar production, decreasing formation of inhibitors and shortening process time. These would collectively result in higher bioethanol yield and more economical process.
Future prospective of pretreatment; genetic manipulation of energy crops
It has been estimated that about 18–20% of the total projected cost for biological production of lignocellulosic ethanol can be attributed to pretreatment, more than for any other single steps [121], [122]. Genetic and metabolic engineering could also play a crucial role in facilitating pretreatment and hydrolysis processes and consequently in economical production of ethanol from lignocellulosic wastes. Currently, different omicses tools (functional genomics, metagenomics, transcriptomics,
Conclusion
Lignocellulosic biomass as an available and cheap source is gaining popularity as a source of fermentable sugars for liquid fuel production. One of the most expensive steps of bioethanol production from such biomass is pretreatment followed by enzymatic treatment. Extensive research has been carried out in order to increase fermentable carbohydrate recovery, decrease inhibitors produced from sugar degradation during pretreatment process, diminish utilization of chemical materials and energy
Acknowledgment
The authors would like to thank Dr. Mohammad A. Nikbakht for his comments on the manuscript.
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