Cellulosic ethanol technology is among the world’s most widely debated second-generation biofuel technologies. Biofuels made of cellulosic material are made from cellulose found in plants, and some are being developed for “energy” crops rather than to produce food. They include perennial grasses as well as trees like switchesgrass as well as miscanthus . The residues of crop plants, which take the form of leaves and stems constitute a further significant source of biomass from cellulosic sources.
The biggest potential feedstock for the production of ethanol is the lignocellulosic biomass that includes substances like agricultural residues (corn stover straws, crop straws, husks and bagasse) as well as herbaceous plants (alfalfa and switchgrass) and short-rotation woody crops, forest waste, residues from paper production and other materials (municipal as well as industrial).
Bioethanol production using these feedstocks can be an appealing option for the getting rid of these leftovers. Lignocellulosic biomass feedstocks are not likely to hinder food security and are beneficial for both urban and rural regions in terms of security of energy as well as environmental issues, employment opportunities, development of agriculture as well as saving foreign exchange and socioeconomic concerns, among others.
Production of Ethanol
The production of alcohol from lignocellulosic biomass could be accomplished through two distinct methods of processing. These are:
- Biochemical – where enzymes and other microorganisms are employed to convert the hemicellulose and cellulose components of feedstocks into sugars prior to fermentation into ethanol.
- Thermochemical – where pyrolysis/gasification technologies produce a synthesis gas (CO + H2) from which a wide range of long carbon chain biofuels, such as synthetic diesel or aviation fuel, can be reformed.
Lignocellulosic biomass is composed of lignin as well as the polysaccharides cellulose as well as hemicellulose. In comparison to producing ethanol using feedstocks of the first generation, the utilization of the lignocellulosic biomass is more complex since the polysaccharides are more stable, and pentose sugars aren’t readily fermentationable in Saccharomyces cerevisiae.
To transform lignocellulosic biomass into biofuels the polysaccharides first need to be hydrolyzed, or broken down into sugars that can be converted to simple sugars by using enzymes or acid. Biotechnology-based methods are being utilized to address these challenges, for example, the creation new strains from Saccharomyces cerevisiae which can produce pentose sugars, the utilization of yeast strains which naturally convert pentose sugars and the development of enzymes that have the ability to reduce hemicellulose and cellulose into sugars that are simple.
Ethanol generated from the lignocellulosic plant is produced mostly through biochemical pathways. The three main stages involved are pretreatment enzyme hydrolysis, and then fermentation. Biomass is treated to increase the efficiency to enzymes. After pretreatment, biomass is subjected to enzyme hydrolysis to convert polysaccharides to monomer sugars like glucose and the xylose. In the next step, sugars are transformed into ethanol through the use of microorganisms from different species.
The biomass that has been treated can be directly converted into ethanol through the process of simultaneous saccharification and cofermentation (SSCF). Pretreatment is an essential step that improves the process of hydrolysis in biomass. In essence, it alters the chemical and physical properties of biomass and enhances the efficiency and accessibility of enzymes which can lead to changes in the crystallinity as well as extent of polymerization of the cellulose.
The surface area and volume of the pore in pretreated biomass increase, which results in a significant enhancement in the access to enzymes. This process is also helpful in increasing the yield and rate of monomeric sugars through the enzymatic hydrolysis process.
Pretreatment of Lignocellulosic Biomass
Pretreatment techniques can be classified into four types including chemical, physical bio-chemical and physio-chemical. Physical pretreatment techniques employ mechanical comminution or irradiation process to alter only the physical properties of biomass. The physio-chemical method makes use of steam, steam and gas like CO 2 and SO 2, CO 2 as well CO 2.
The chemical process employs acids (H2SO4, organic acids, HCl etc.)) and alkalis (NaOH Na2CO3, Ca(OH)2, NH3 etc). Acid treatment usually exhibits a selectivity to hydrolyze the hemicellulose components, while alkalis are more selective for lignin. The division of biomass components following these procedures helps improve accessibility of enzymes which is crucial to the efficient use of enzymes.
The main cost elements for the production of bioethanol from lignocellulosic biomass is the pretreatment as well as the enzyme hydrolysis process. In reality, these two processes are in some way interconnected in which a well-planned pretreatment strategy will result in a significant reduction in enzyme consumption. Pretreatment can also impact the price of other operations like size reduction prior to the pretreatment.
Thus, optimizing these two critical actions, which together contribute around 70% of total cost of processing is one of the biggest issues in developing the industrialization and commercialization of bioethanol made from 2 2nd second generation feedstocks for biofuel.