Second Generation Bioethanol Process Technology

 Overview: Second Generation Bioethanol Process Technology

- Atul Choudhari.

Biofuels provide an attractive and sustainable energy option as they are considered to be clean fuels

having ‘very low to no sulphur’ content. Biofuels therefore help in creating a positive environmental impact.

Bioethanol is one such fuel that can be blended with gasoline. Typically and conventionally, bioethanol is

extracted from sugars. Bioethanol can be produced at industrial scale through the sugar fermentation route.

The bioethanol technology is termed as either first generation (1G) or second generation (2G) depending

upon the origin of such sugars. While the 1G bioethanol technology uses starch as a source of sugar, the

2G bioethanol technology uses cellulose and hemicelluloses as a source of sugar. Starch required in 1G

technology can be found in various feed stocks such as cereals (wheat, corn, sorghum barley, etc) and

sugarcane. Feedstock such as wheat straw, corn, wood, agricultural residues or municipal solid waste are

typically lingo-cellulosic materials and are used as a source of bioethanol in 2G technology. As compared

to 1G technology, which use grain as feedstock, the 2G technologies use crop residues, a waste that

otherwise would be of no value. This article provides an overview of the 2G bioethanol technology.

Advantages of Bioethanol:

There are number of advantages of using bioethanol as fuel. Some of the benefits are as below.

 Reduced dependency on crude oil imports: For oil importing countries like India, The major driving

force is to reduce their dependency on fossil fuels. It benefits energy security as it can reduce

crude oil by using domestically produced energy sources. Countries like India, having a limited

access to crude oil resources, can grow crops for energy use and gain some economic freedom.

 Cleaner environment: Due to the fact that the exhausts from the automobile engines using

bioethanol blended gasoline is more cleaner in nature, the second major benefit of using

bioethanol is its ability to reduce the overall carbon footprint and their use help in reduction of

greenhouse gas (GHG) emissions. It will also reduce the GHG emissions by reducing the of

agriculture residues burning.

 Renewable energy source: Bioethanol is produced using plant materials such as corn, sugarcane,

crop residues, etc. Since, all these are crops can be grown; bioethanol fuel is a renewable energy

source.

 Financial benefit for farmers: Agricultural residues and wastes which otherwise are burnt by

farmers can be utilized for producing bioethanol.

Raw material components:

As stated earlier, the 2G bioethanol technology uses ligno-cellulosic biomass as a feedstock.

Lignocellulosic biomass is mainly composed of plant cell walls. It essentially contains three major

components viz. Lignin, Cellulose, and Hemicelluloses. Cellulose and Hemicelluloses are the structural

carbohydrates while lignin is heterogeneous phenolic polymer.

Cellulose is a polysaccharide made up of linear glucan chains held together by intra molecular hydrogen

bonds and by intermolecular Van-der Waals forces. In order to obtain glucose, the crystalline cellulose

must be subjected to some preliminary chemical or mechanical degradation.

Hemicellulose consists of short, highly branched chains of sugars. Hemicelluloses are highly amorphous

and branched structures. It contains pentoses, hemicelluloses chains. Compared to cellulose, the

hemicelluloses can easily be broken down to form their simple monomeric sugars. The exact sugar

composition of hemicelluloses can vary depending on the type of plant.

Lignin is a non-sugar-based polymer. Lignin is not a suitable component for microbial fermentation process.

It inhibits microbial growth and fermentation. However, lignin can be used as energy source as it yields.

more energy when burned, and thus can be utilized for combined heat and power production in the

bioethanol process.

Process Description:

The bioethanol process is carried out in following four major steps.

1. Pre treatment: Physical or chemical pre-treatment of the fibers to expose the cellulose so as to

reduce its crystallinity.

2. Hydrolysis : Cellulose polymer is hydrolysed with enzymes or acids, to convert it into simple

(glucose) sugars

3. Fermentation: Microbial fermentation of simple sugars to form ethanol.

4. Distillation and dehydration to produce 99.5% vol. fuel grade ethanol.

Pretreatment:

Due to the presence of lignin in ‘Lignocellulosic “materials, and compared to the accessibility of sucrose in

sugar cane and starch in grains, cellulose and hemicelluloses are not easily and readily available for

saccharification and fermentation. A “pre-treatment” step is hence required to facilitate conversion of

cellulose and hemicelluloses to fermentable sugars.

The pre-treatment process converts hemicellulose carbohydrates into soluble sugars (like glucose, xylose,

etc.) by hydrolysis reactions in which acetyl groups in the hemicellulose are liberated in the form of acetic

acid. Biomass feedstock is chemically treated by disrupting cell wall structures in the pre-treatment step

which facilitates downstream enzymatic hydrolysis. This section is also termed as ‘Delignification’ section

as the pre-treatment drives some lignin into solution. This step reduces cellulose crystallinity and chain

length. Process parameters such as residence time, temperature, and catalyst loading affects the pre

treatment process. The pre treated biomass is sent to the hydrolysis reactor.

Hydrolysis:

Hydrolysis process is used to convert hemicellulose and cellulose content of lignocellulosic biomass into

fermentable monomeric sugars. This process can be carried out by two different routes. These routes are

Acid hydrolysis and Enzymatic hydrolysis.

In acid hydrolysis process, mineral acids such as HCl, H2SO4, HNO3, or HF are widely used for hydrolysing

lignocellulosic biomass. In enzymatic hydrolysis, Cellulose is converted to glucose using cellulase

enzymes. Enzymatic hydrolysis process is also termed as ‘Enzymatic Saccharification’ process. A

cellulase enzyme is prepared from mixture of enzymes (catalytic proteins) which work together to break

down cellulose fibers into glucose monomers.

For higher conversion and it’s suitability to the lower grade of metallurgy, enzymatic hydrolysis route is

preferred over the acid hydrolysis route.

The glucose and other sugars obtained from hydrolysis of hemicelluloses are co-fermented to form ethanol

in the next step.

Fermentation:

Fermentation process step is similar to the 1G ethanol technology. In this step, the hexoses and pentoses

are converted into ethanol by employing variety of micro organisms, such as yeast, bacteria, fungi, etc.

Depending on how the enzymatic hydrolysis and fermentation steps are integrated, the technology can

follow either of following route.

 Separate Hydrolysis and Fermentation

 Separate Hydrolysis and Co-fermentation

 Simultaneous Saccharification and Co-fermentation

Indian Scenario:

The practice of blending ethanol in gasoline was started in India in 2001. Government of India, in 2003,

mandated blending of 5% ethanol with gasoline in 9 States and 4 Union Territories. This was subsequently

continued on an all-India basis in November 2006 (in 20 States and 8 Union Territories except a few North

East states and Jammu & Kashmir). Indian Oil companies were asked to increase the ethanol blending

target to 15% by Ministry of Petroleum and Natural Gas, on 1 September, 2015 and achieve this blend in

as many states as possible.

At 10% blending, the projected ethanol demand is ~5500 million liters per anum by year 2021-2022. It

would certainly require significant investments in near future. Presently, First few plants to produce

bioethanol using 2G technologies are under consideration and are at various stages of planning and

design.

Future Technology:

Third generation (3G) bioethanol technology is based on ethanol production from Microalgae. Currently,

Microalgae is gaining increased attention as it is an alternative renewable source of biomass which can be

used for 3G bioethanol production. The increased interest to use microalgae is also attributed to the fact

that it can be produced all year along and does not require any pesticides or herbicides. It can be produced

in sea water or brackish water and thus do not compete with agricultural land. Comparatively, microalgae

have potential to reduce freshwater consumption as it requires less water than terrestrial crops.

Another technology to produce bioethanol from the CO2 emission sources (Iron and steel producers) is also

recently commercialized.

Concluding Remarks:

Bio-ethanol is considered as an important renewable fuel. The Indian economy is growing at a rate of

approximately 7% to 7.5% resulting in the increased demand for energy. Bioethanol presents a sustainable

source of energy. 2G Bioethanol technologies are being implemented in India.

TCE is associated with one of the first 2G bioethanol plants being installed in India and have first hand

experience of commercializing such technology.