Cellulosic Ethanol – The future of biofuel

G S Krishnan, Regional President, Novozymes India

The term “biofuels” normally refers to either bioethanol or biodiesel. Bioethanol is made with the help of enzymes and subsequent fermentation, and is based on crops or other biomass.

Biodiesel is made from vegetable oils. Other types of biofuels such as bio-butanol are in the process of development.The conversion processes of all types of source stocks into biofuel is a complex process. Biofuels can be classified into two types :

db00162Starch biofuels are produced from crops such as corn, wheat, barley, rye, sorghum, and cassava. The technology is in place and all commercial production of bioethanol is currently from starch. Crops are essential for food and feed production, but they also contain elements that can be used for energy and for other commodities. There are many methods to produce biofuels and, done the right way, it is possible to produce sustainable energy while ensuring food and feed production to meet the global needs.

Cellulosic biofuels are produced from feedstocks containing cellulosic biomass – such as the stalks, leaves, and husks of corn plants, wood chips, or sawdust – but the process is a bit more complicated. Unlike agricultural products like corn and wheat grain that are also used for fuel production, biomass contains a lot of lignocellulosic fibers. These fibers make processing biomass more difficult than its starch-based counterparts – but not impossible. Cellulosic biofuels may also be produced from energy crops such as switchgrass, or municipal waste.

Lignocellulose is a matrix mainly composed of cellulose, hemicellulose, and lignin. The cellulose and hemicellulose can be converted with enzymes into fermentable sugars. Lignin, or its residues from the pretreatment processes, can result in inhibition that slow down the enzymatic reactions as well as the alcohol fermentation process. Cellulose consists of long chains of glucose molecules, not unlike starch, but connected by a different type of chemical bond that is more resistant to hydrolysis. The structure of cellulose makes it difficult to degrade, which is (in part) the purpose of pre-treating substrates prior to enzymatic hydrolysis. The purpose of pretreatment is to loosen up this structure, providing access to the cellulose and allowing the enzymes to break down the cellulose into its component sugars.

Enzymes help in biofuel production

For many years, enzymes have been providing a big helping hand in streamlining biofuel production. Enzymes are biological catalysts in the form of proteins that catalyze chemical reactions in the cells of living organisms. They have evolved - along with cells - under the conditions found on Earth, and satisfy the metabolic requirements of an extensive range of cells. Enzymes optimize the conversion of grains such as corn, barley, wheat and rye into fuel ethanol, plus enzymes that assist the conversion of biomass for cellulosic ethanol.

Making fuel from biomass
To make ethanol out of biomass feedstock, the sugar components that are hidden in the substrate must first be liberated.

Biomass is composed of three major fractions: cellulose, hemicellulose, and lignin. Cellulose and hemicellulose contain sugars in polymeric form that can be converted by enzymes into monomers for subsequent fermentation. But the hard, wood-like lignin component of the plant protects the fibers against microbial and enzymatic attack by preventing the cellulose and hemicellulose from reacting with water and swelling. Hence, the use of biomass as a raw material requires disruption of the lignin so that the cellulose and hemicellulose fractions become accessible for enzymatic hydrolysis. Typically, an initial physical or chemical pretreatment is applied to open the fiber structure, followed by the use of enzymes to liberate the C6 from the cellulose and in some cases C5 sugars from the hemicellulose fractions. Afterwards, the sugars can be fermented and further processed into ethanol.

The need for biofuels
High oil prices and energy security are propelling governments and consumers alike to demand ethanol as fuel for cars. Due to increasing demand for fuel ethanol, the market for enzymes used to produce the fuel is also growing rapidly. Not only is bioethanol a renewable resource; it also burns cleaner than gasoline and produces fewer harmful greenhouse gases.

Biofuels are currently the only currently available option for large-scale reduction of CO2 emissions from transportation. Use of ethanol in transportation helped reduce US greenhouse gas emissions by approximately 20 million tons in 2008 alone - equivalent to taking more than 5 million cars off the road.

Among the biofuel sector, there are distinct advantages of encouraging Cellulosic biofuel, which offers a number of benefits to the society. Cellulosic ethanol can be produced from a vast selection of readily available feedstock resources –corn cobs, wheat straw, sugar cane bagasse, and woody biomass. It will reduce CO2 emissions by more than 90% over conventional petroleum-based fuels

Moving ahead in the fuel ethanol industry means that we must not only optimize the ‘first-generation,’ starch-based processes used today; we must also prepare for the next generation of biofuels based on cellulosic biomass


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