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How is Alcohol Prepared from Biomass?

Komal B. Patil Mar 3, 2020
The rising price of fossil fuels coupled together with their depleting and non-renewable nature has generated a need to form a new renewable source of energy. One such viable option is the production of alcohol as fuel from biomass.

Dual Identity

Ethanol and bioethanol are chemically the same, but are termed differently on the basis of their source. Ethanol is derived from fossil fuels, whereas bioethanol is obtained from processing biomass.
Bioethanol is favored over conventional fuel sources, due to its renewable nature. However the cost of production of biofuels at present is quite higher than that of producing fossil fuels. To overcome this, many institutions are researching on ways to produce biofuels at a cheaper cost and a higher quantity.
Recently, there has been a major development in this direction, with the use of algae to produce alcohol fuel.

The alcohol obtained after fermenting biomass is called bioethanol. The prefix "bio-" implies the fact that it has been produced from biological sources.
These sources include agricultural feedstock and even common crops like sugarcane, potato, and corn. The sugars present in these crops are fermented by strains of yeast to produce bioethanol. Alternatively, cellulosic biomass can also be used. Based on the varied sources of bioethanol, it is categorized into different categories.

Classification of Bioethanol

  • 1st Generation: It refers to the conventionally produced ethanol from crops rich in sugar or starch, that are also used as food crops.
  • 2nd Generation: This category of fuel is produced from crops that are not used for human consumption. It may include agricultural feedstock and other such plants. It is sometimes also referred to as "advanced bioethanol".
  • 3rd Generation: It refers to any biofuel that is derived from processing algae. It is categorized separately due to its unique method of production, and also due to the possibility of it overcoming the drawbacks of fuels produced by other methods.

Ethanol Production from Biomass

General Outline

The first step in this process is to sterilize the biomass, in order to prevent bacterial and fungal growth. Once this is done, it is subjected to pre-treatment, which makes the sugars and starch in the biomass readily available for fermentation. Hydrolysis may be carried out if there is an abundance of complex sugars, to break them down into simple sugars.
Having accomplished this, the lysate is then introduced to the anaerobic yeast strains (like Saccharomyces cerevisiae) along with a few other nutrients and allowed to undergo fermentation.
When the fermentation process is complete, i.e. the sugars have been completely exhausted, the fermented product is filtered, and the liquid obtained is subjected to distillation. This yields a hydrous form of ethanol, that contains 95.5% ethanol and 4.5% water.
To remove the water content and to obtain pure ethanol, this hydrated form is dehydrated to produce anhydrous, pure ethanol. This is usually carried out with the help of a molecular sieve like ZEOCHEM Z3-03 or a desiccant like calcium oxide.
To make this ethanol unfit for human consumption, it is often mixed with trace amounts of toxic substances like methanol.

Ethanol from Disaccharides

The most common disaccharide used for this purpose is sucrose, which is obtained from plants like sugarcane, sugar beet, and sorghum. 60% of the ethanol produced worldwide is obtained from the utilization of this sugar.
It is a complex sugar that can be broken down by the yeast cells, into its components of glucose and fructose due to the action of the enzyme invertase.

C12H22O11 (Sucrose) + H2O ➜ C6H12O6 (Glucose) + C6H12O6 (Fructose)
The glucose and fructose sugars are then converted into ethanol and carbon dioxide by the action of another enzyme called zymase.

C6H12O6 (Glucose/Fructose) ➜ 2 CH3CH2OH (Ethanol) + CO2
The alcohol thus produced is then distilled and dehydrated to obtain pure anhydrous ethanol. Theoretically, one tonne of a simple sugar like glucose or fructose, when fermented, would yield almost 1126.56 pounds of ethanol.

Ethanol from Starch

The starch obtained is derived from plant sources such as corn, potatoes, and grains. It naturally occurs in the form of chains of α-glucose monomers. In order to be fermented, these chains have to be broken down into separate monomers of simple sugar.
This is achieved by the enzyme glucoamylase, which hydrolyzes and digests the polymeric structure of starch to yield monomeric form of glucose.

n(C6H10O5) (Starch) + n(H2O) ➜ n(C6H12O6) (Glucose)
The glucose obtained is an isomer of itself called dextrose or D-glucose. This glucose monomer is then subjected to fermentation, followed by distillation and dehydration. The process followed is the same as that used for disachharides.
While using grains, they are first dried, ground into powder, and then hydrolyzed. The lysate is used for ethanol production, while the rest is discarded. Alternatively, the grains could also be wet-milled, in order to get rid of all the non-required protein, germ, and fiber of the grains.

Ethanol from Lignocellulose

Lignocellulose refers to the structural component of plant matter, that mainly comprises complexes of polymerized cellulose, hemicellulose, and lignin. It is derived from saw dust, wood chips, dried plant matter, grasses, etc.
Due to the presence of lignin, the lignocellulosic ingredients have to undergo a pre-treatment, that involves the process of delignification. This process separates the lignin from the cellulose and hemicellulose, and can be carried out via a combination of acid hydrolysis, steam explosion, or alkali treatment.
The most commonly administered pretreatment includes steam explosion, dilute acid hydrolysis, and an enzymatic hydrolysis.

For steam explosion, the raw materials are introduced to high pressure saturated steam (0.69-4.83 Mpa) at a high temperature of 160-260 °C for a few minutes.
Subsequently, the pressure is dropped suddenly, till it equals the atmospheric pressure. This sudden change in pressure causes the substrate to explode, and the hemicellulose to be solubilized to a certain extent.
For the dilute acid hydrolysis, the exploded substrate is treated with a 0.5 - 1% solution of sulfuric acid and then heated at 160 - 190 °C for 10 min. This leads to the hydrolysis of the hemicellulosic components into simple sugars such as xylose, mannose, and galactose. Other byproducts are removed and the acids are neutralized till a pH of 10 is reached.
The cellulosic parts are then hydrolyzed enzymatically with the help of cellulases such as endoglucanases, exoglucanases, and β-glucosidases. These enzymes are isolated from fungi like Trichoderma, Aspergillus, Schisophyllum, and Penicillium. These enzymes breakdown the cellulosic structures, and convert them into glucose.
This mixture of glucose and other simple sugars is then used for fermentation, which is carried out by yeast as well as Zymomonas mobilis. This bacteria is used in addition to yeast due to its ability to break down sugars like xylose. The fermented product is then distilled and dehydrated to obtain pure ethanol.
The commercial production of ethanol as a fuel source is believed to present a solution to the problem of global warming caused by increase in greenhouse gases. It is hypothesized that the use of this alternative fuel will drastically reduce emission of greenhouse gases, particulate matter, along with other such toxic substances.
But the major obstacle with this idea is the extremely high cost of production. Scientists believe that they may be able to bring down the cost by using an alternative such as algae, which can be easily cultivated and maintained.