Production of bioethanol from sugarcane bagasse: Status and perspectives
Introduction
For large-scale biological production of fuel ethanol, it is desirable to use cheaper and more abundant substrates. When producing ethanol from maize (made up from starch chains) or sugarcane (in the form of either cane juice or molasses) the raw material constitutes about 40–70% of the production cost (Sendelius, 2005, Quintero et al., 2008). By using waste products from forestry, agriculture and industry, the costs of the feedstocks may be reduced. Lignocellulose (complex polymer made up from three carbohydrates: cellulose hemicelluloses and lignin) is considered as an attractive feedstock for the production of fuel ethanol, because of its availability in large quantities at low cost (Cardona and Sánchez, 2007, Cheng et al., 2008) and for reducing competition with food but not necessarily with feed. Today the production cost of ethanol from lignocellulose is still too high, which is the major reason why ethanol from this feedstock has not made its breakthrough yet.
Many lignocellulosic materials have been tested for bioethanol production as was reviewed by Sánchez and Cardona (2008). In general, prospective lignocellulosic materials for fuel ethanol production can be divided into six main groups: crop residues (cane bagasse, corn stover, wheat straw, rice straw, rice hulls, barley straw, sweet sorghum bagasse, olive stones and pulp), hardwood (aspen and poplar), softwood (pine and spruce), cellulose wastes (newsprint, waste office paper and recycled paper sludge), herbaceous biomass (alfalfa hay, switchgrass, reed canary grass, coastal Bermudagrass and thimothy grass), and municipal solid wastes (MSW). Numerous studies for developing large-scale production of ethanol from lignocellulosics have been carried out in the world. However, the main limiting factor is the higher degree of complexity inherent to the processing of this feedstock. This is related to the nature and composition of lignocellulosic biomass (which contain up to 75% of cellulose and hemicelluloses). Cellulose and hemicelluloses should be broken down into fermentable sugars in order to be converted into ethanol or other valuable products (xylans, xylitol, hydrogen and enzymes). But this degradation process is complicated, energy-consuming and non-completely developed (Sánchez and Cardona, 2008). With the advent of modern genetics and other tools the cost of producing sugars from these recalcitrant fractions and converting them into products like ethanol can be significantly reduced in the future.
Several reviews have been published on the theme of fuel ethanol production especially from lignocellulosic biomass (Lin and Tanaka, 2006, Cardona and Sánchez, 2007, Sánchez and Cardona, 2008). Lignocellulosic materials from different crop residues have been used for conversion to ethanol. One of the major lignocellulosic materials found in great quantities to be considered, especially in tropical countries, is sugarcane bagasse (SCB), the fibrous residue obtained after extracting the juice from sugar cane (Saccharum officinarum) in the sugar production process (Martín et al., 2007a).
SCB is produced in large quantities by the sugar and alcohol industries in Brazil (Martínez et al., 2003, Hernández-Salas et al., 2009), India (Martínez et al., 2003, Chandel et al., 2007), Cuba (Martínez et al., 2003), China (Martínez et al., 2003, Cheng et al., 2008), México (Hernández-Salas et al., 2009), Indonesia (Restuti and Michaelowa, 2007) and Colombia (Quintero et al., 2008). In general, 1 ton of sugarcane generates 280 kg of bagasse, and 5.4 × 108 dry tons of sugarcane are processed annually throughout the world (Cerqueira et al., 2007). About 50% of this residue is used in distillery plants as a source of energy (Pandey et al., 2000); the remainder is stockpiled. Therefore, because of the importance of SCB as an industrial waste, there is great interest in developing methods for the biological production of fuel and chemicals that offer economic, environmental, and strategic advantages (Adsul et al., 2004).
In the approximately 80 sugarcane producing countries there is a potential to make better use of the SCB. Subjected to improved energy efficiency, sugar producers could supply energy either as co-generated electricity, or as fuel ethanol through cellulose hydrolysis followed by fermentation (Botha and Blottnitz, 2006). The most common use for SCB is the energy production by combustion (Ramjeawon, 2008). In addition, SCB can be used also to produce chemical compounds such as furfural or hydroxymethylfurfural (Almazán et al., 2001), paper paste (Pattra et al., 2008) or ethanol (Laser et al., 2002). The use of SCB in chemistry and biotechnology has been reviewed elsewhere (e.g. Pandey et al., 2000).
As raw material, SCB should be analyzed from composition, structure and surface properties. SCB is primarily composed of lignin (20–30%), cellulose (40–45%) and hemicelluloses (30–35%) (Peng et al., 2009). Because of its lower ash content, 1.9% (Li et al., 2002), bagasse offers numerous advantages compared with other agro-based residues such as paddy straw, 16% (Goh et al., 2009), rice straw, 14.5% (Guo et al., 2009) and wheat straw, 9.2% (Zhao and Bai, 2009). Work on structure and surface characterization of SCB has not been done extensively, but some works can be found (Zhao et al., 2007, Quintero and Cardona, 2009). In a previous work (Quintero and Cardona, 2009) SCB was obtained from a small sugarcane juice factory and milled for its structural analysis. Obtained fibers had smooth surface layers and characteristic elongations with lengths over 200 μm (this was obtained from SEM micrographs with in a JEOL JSM-5910LV microscope). XRD analysis (Rigaku MiniFlex II unit with CuKα used at 30 kV and 15 mA, diffraction angle ranged from 35° to 2° with a scan speed of 5°/min) showed that crust and marrow bagasse exhibit different structures and crystallinity. Crust bagasse presents two diffraction peaks at 2θ values of 18.04° and 21.9°, while marrow bagasse presents only a peak at 21.86°, characteristic of the cellulose structures. It is important to note, that most of the developments in SCB transformation to sugars and ethanol have the common scientific basis with other lignocellulosic materials, due to the fact that there are not considerable qualitative differences in composition and structure.
Overall fuel ethanol production from SCB includes five main steps: biomass pretreatment, cellulose hydrolysis, fermentation of hexoses, separation and effluent treatment (see Fig. 1). Furthermore, detoxification and fermentation of pentoses released during the pretreatment step can be carried out. Solid fraction from pretreatment contains the cellulose which is later hydrolisated, and liquid fraction contains the hemicellulose hydrolysate. Once cellulose hydrolysis is completed, the resulting hydrolysate is fermented and converted into ethanol. This process is called separate hydrolysis and fermentation (SHF). SHF is one of the configurations that have been tested more extensively. Pentose fermentation, when it is carried out, is accomplished in an independent unit. The need of separate fermentations is due to the fact that pentose utilizing microorganisms ferment pentoses and hexoses slower than microorganisms that only assimilate hexoses. Moreover, these microorganisms are more sensitive to the inhibitors and to the produced ethanol. For this reason, the hemicellulose hydrolysate resulting from pretreatment should be detoxified. If the fermentation of the hemicelluloses and cellulose hydrolysates is carried out in a separate way, less liquid volumes of hydrolysate have to be detoxified. The ideal organism for the production of ethanol would be the one which can utilize pentose and hexose sugars generated by lignocellulose hydrolysis (Chandel et al., 2007).
Present paper deals with the uses, pretreatment and biological transformation of SCB into added value products, emphasizing on fuel ethanol production. Potential uses of lignocellulosic biomass depend on its composition and in some extend of its availability. Moreover, the required pretreatment is a function of the structure complexity. Main pretreatment methods for SCB are presented. Potential applications of bagasse hydrolysate and the detoxification methods are discussed. Finally, some modeling and stability aspects are considered. Separation and purification, and effluent treatment technologies are not discussed in this paper, because, these technologies are well established for other types of raw material. Additionally effluent product and wastes are similar, despite the highly variations in raw material composition.
Section snippets
Pretreatment methods
Lignocellulosic materials do not contain monosaccharides readily available for bioconversion. Instead they contain polysaccharides, such as cellulose and hemicelluloses, which have to be hydrolyzed, by means of acids or enzymes, to fermentable sugars. Enzymatic hydrolysis is a promising way for obtaining sugars from lignocellulosic materials, but the low enzymatic accessibility of the native cellulose is a key problem for biomass-to-ethanol processes. Cellulose in plants is closely associated
Cellulose hydrolysis
Cellulose obtained from pretreatment should be degraded into glucose (saccharification) using acids or enzymes. In the former case, concentrated or dilute acids can be used. If dilute acids (H2SO4 and HCl) are employed, temperatures of 200–240 °C at 1.5% acid concentrations are required to hydrolyze the crystalline cellulose, but the degradation of glucose into HMF and other non-desired products is unavoidable under these conditions. One variant of the acid hydrolysis is the use of extremely low
Detoxification
During pretreatment of lignocellulosics, in addition to the sugars, aliphatic acids (acetic, formic and levulinic acid), furan derivatives furfural and HMF, and phenolic compounds are formed. The existence of these substances is more probably when acid and/or high-temperatures are used. These compounds are known to affect ethanol fermentation performance. Furfural could be generated as a degradation product from pentoses. It was found that furfural contents increase with the concentration of
Production technologies
The configuration employed for fermenting biomass hydrolysates involves a sequential process where the hydrolysis of cellulose and the fermentation are carried out in different units (Sánchez and Cardona, 2008). This configuration is known as separate hydrolysis and fermentation (SHF). When this sequential process is employed, solid fraction of pretreated lignocellulosic material undergoes hydrolysis (saccharification). This fraction contains the cellulose in a form accessible to acids or
Energy cogeneration
Energy cogeneration is well established process in sugar industry, due to the high quantity of SCB available, which is composed of 50% fibre, 48% moisture and 2% sugars. It is normally burnt to generate steam and electricity to meet the energy requirements of the cane sugar factory. The bagasse has a gross calorific value of 19.25 MJ/kg at zero moisture and 9.95 MJ/kg at 48% moisture. The net calorific value of bagasse at 48% moisture is around 8 MJ/kg. The fact that the sugar cane plant provides
Xylanases and cellulases production
High cost commercial xylanases and cellulases used in the saccharification step for SCB transformation to ethanol can be produced from the same bagasse. Many microorganisms, including filamentous fungi, yeasts and bacteria, have been cultivated in media containing SCB or its hydrolysate. The use of SCB as low cost raw material for xylanase production by Bacillus circulans D1 in submerged fermentation has been investigated (Bocchini et al., 2005). The microorganism was cultivated in a mineral
Mathematical modeling
The modeling of the hydrolysis of a polysaccharide is very complicate. Multiple factors related to the lignocellulosic material (size, particle shape, structure, accessibility of proton to heterocyclic ether bond, etc.) and to the reaction medium (type of acid, concentration, temperature, time, etc.) affect the hydrolysis. The solution of compromise between the complexity of a rigorous model and the search of equations modeling the empirical data in a simple and satisfactory way have conducted
Stability of fermentation systems based on SCB
In general, more efficient pretreatment technologies, detoxification methods and the construction of microorganism strains capable to ferment lignocellulosic materials have different advances during last years. However, other restrictions of the fermentation processes related to original microorganism have not been passed. One of them is the existence of nonlinear phenomena such as multiplicity and oscillation. The complexity of stability can be increased as a result of the inhibition problems
Perspectives, challenges and conclusions
An increased use of biofuels would contribute to sustainable development by reducing greenhouse-gas emissions and the use of non-renewable resources. In recent years it has been suggested that, instead of traditional feedstocks, cellulosic biomass (cellulose and hemicellulose), including SCB could be used as an ideally inexpensive and abundantly available source of sugar for fermentation into transportation fuel ethanol. The efficiency of biomass conversion to ethanol depends upon the ability
Acknowledgements
The authors express their acknowledgments to the National University of Colombia at Manizales for funding different research projects in fuel ethanol production and lignocellulosics exploitation.
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