https://doi.org/10.15255/KUI.2014.032
Published: Kem. Ind. 65 (1-2) (2016) 25−38
Paper reference number: KUI-32/2014
Paper type: Review
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New Trends in the Ethanol Production as a Biofuel

M. Ivančić Šantek, E. Miškulin, S. Beluhan and B. Šantek

Abstract

Rapidly growing fossil energy consumption, due to population and industrial growth, has caused increasing greenhouse-gas emissions, growing energy dependency and supply insecurity. Bioethanol has become an attractive alternative biofuel because of its environmental benefits and the fact that it is made from renewable resources. Ethanol is widely used as transport fuel, pure hydrous ethanol or anhydrous ethanol in mixtures with gasoline (Fig. 1). Bioethanol is produced from carbohydrates such as sugar, starch and cellulose by fermentation with yeast S. cerevisiae or other microorganisms. Thereupon, ethanol is separated and purifyed by distillation-rectification-dehydration to meet fuel specifications. Currently, conventional crops such as corn or sugarcane are the main feedstock for bioethanol production. Bioethanol production from the sucrose-containing feedstock is simpler compared to the starchy materials and the lignocellulosic biomass due to an additional step – feedstock hydrolysis. The process of ethanol production from starchy materials includes the hydrolysis of starch to glucose using α-amylase (1,4-α-D-glucan-4-glucanohydrolase) and glucoamylase (1,4-α-D-glucanglucohydrolase). Finally, the glucose is fermented to ethanol by yeast cells. Enzymatic hydrolysis of starch and fermentation of glucose can be carried out in different process configurations, such as separate hydrolysis and fermentation (SHF), and simultaneous saccharification and fermentation (SSF, Fig. 2). In consolidated bioprocessing (CBP), the conversion of starch into ethanol is performed in one step without added enzymes. This process configuration has potential to lower the cost of biomass processing due to elimination of operating and capital costs associated with dedicated enzyme production. Current bioethanol production from corn and sugarcane is unable to meet the global demand for bioethanol, due to their primary value as livestock feed and human food. The lignocellulosic biomass such as agricultural wastes (corn stover, crop straws, husks and bagasse), herbaceous crops (switchgrass), woody crops, forestry residue, waste paper and other wastes (municipal and industrial) is favourable feedstock for bioethanol production. The major advantages of lignocellulosic biomass are its renewable and ubiquitous nature and its noncompetitiveness with food crops. Ethanol production from lignocellulosic feedstock is complex and comprises two steps prior to fermentation: biomass pretreatment (breaking down the structure of the lignocellulosic matrix) and cellulose hydrolysis (enzymatic hydrolysis of cellulose to glucose). The lignocellulosic hydrolysate contains not only hexoses, but also pentoses that are not assimilated by yeast S. cerevisiae. Furthermore, the lignocellulosic hydrolysate contains a broad range of compounds that inhibit the yeast’s cells. The composition of the inhibitors depends on the type of the lignocellulosic material, and the chemistry and nature of the pretreatment process. The pretreated cellulose can be enzymatically hydrolysed either prior or simultaneously with glucose fermentation (Fig. 3). The four main steps involved in the process of lignocellulosic bioethanol production (pretreatment, cellulose hydrolysis, hexoses and pentoses fermentation) can be arranged in various process configurations, including separate hydrolysis and fermentation (SHF), simultaneous saccharification and fermentation (SSF), simultaneous saccharification and co-fermentation (SSCF) and consolidated bioprocessing (CBP, Fig. 3). Specific strains of bacteria and yeasts have been developed to ferment sugars released from lignocellulosic biomass and hydrolysed cellulose, through a selection of new strains and genetic engineering of traditional strains.

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Keywords

bioethanol, fermentation, lignocellulose, integrated bioprocess systems