Bioethanol (VCE SSCE Chemistry): Revision Notes

Bioethanol

Introduction to bioethanol

Bioethanol is ethanol produced from biological sources, specifically plant material called biomass. While ethanol can be made from crude oil (via ethene), producing it from biomass offers significant environmental advantages. Australia's strong agricultural and forestry sectors provide excellent opportunities for bioethanol production from waste materials, making it an attractive renewable fuel option.

Bioethanol serves as an energy source in:

• Combustion engines
• Fuel cells
• Electricity generators

Fermentation process

Bioethanol is produced from glucose and other sugars through fermentation. Enzymes and microorganisms help speed up the chemical reactions involved. The process must be carried out at 35°C because higher temperatures would destroy the enzymes and microorganisms needed for the reaction.

The main chemical reaction is:

$\text{C}_6\text{H}_{12}\text{O}_6(\text{aq}) \rightarrow 2\text{CH}_3\text{CH}_2\text{OH}(\text{aq}) + 2\text{CO}_2(\text{g})$

This equation shows that one glucose molecule produces two ethanol molecules and two carbon dioxide molecules.

Feedstock sources

Glucose exists in plants both as simple glucose molecules and as part of larger carbohydrate molecules. The bioethanol industry requires abundant carbohydrate sources that can be broken down to release glucose. The three main bioethanol feedstocks in Australia are:

• Sugar cane - high in sucrose (a disaccharide)
• Wheat - high in starch (a polysaccharide)
• Forest waste - high in cellulose (a polysaccharide)

Breaking down carbohydrates to glucose

Different carbohydrate feedstocks require different levels of processing to release glucose. The complexity depends on how the carbohydrate molecules are bonded together:

Production process

The bioethanol production process involves several key stages:

Pre-treatment

The plant material is mixed and blended with water to break down the cellular and plant structures. This creates a pulp mixture. For cellulose-rich materials like forest waste, additional treatment with steam and specialised enzymes is required because the hydrogen bonds holding cellulose molecules together are particularly strong.

Saccharification

During saccharification, enzymes are added to the mixture to break large carbohydrate molecules down into glucose. The specific enzymes used depend on the feedstock:

• Cellulase - breaks down cellulose
• Amylase - breaks down starch

Fermentation

Once glucose is available, enzymes catalyse its conversion into ethanol. This produces a solution containing approximately 10% ethanol by volume ($\%v/v$).

By-products

Distillation

After fermentation produces a solution of approximately $10\%$ ethanol, the ethanol must be separated from water to be useful as a fuel. Distillation achieves this separation by exploiting the different boiling points of the two liquids:

• Ethanol: 79°C
• Water: 100°C

How distillation works

The ethanol solution is heated to boiling and fed into tall distillation columns. Temperature is carefully controlled throughout the column:

• Top of column: Cooler temperature allows ethanol vapour (lower boiling point) to rise and be collected
• Bottom of column: Water (higher boiling point) condenses and falls to the bottom

Energy requirements

Australian bioethanol industry

Australia has three commercial bioethanol manufacturing plants:

All three plants are strategically located in regions where their respective feedstock crops are grown, making it efficient to collect waste materials for fermentation.

Sorghum, used by United Petroleum's Dalby plant, is primarily grown as animal feed for cattle, pigs and poultry. Its high starch content makes it ideal for bioethanol production.

Bioethanol potential and challenges

Food versus fuel debate

Forest waste opportunities

Chemists are investigating less valuable sources of sugars and starches for bioethanol production. Forest waste represents an attractive target because:

• Australia has a large timber industry
• Bark and trimmings have little alternative use
• Abundant cellulose is available

E10 fuel blend

What is E10?

E10 is a fuel blend containing:

• 10% ethanol
• 90% petrol

Australian government regulations limit the proportion of ethanol in petrol to $10\%$. This blend is sold at most Australian service stations, though not all vehicle models are suitable for E10 fuel.

Comparison with petrol

Energy content

The combustion equation for ethanol is:

$\text{C}_2\text{H}_5\text{OH}(\text{l}) + 3\text{O}_2(\text{g}) \rightarrow 2\text{CO}_2(\text{g}) + 3\text{H}_2\text{O}(\text{l})$

where $\Delta H = -1360 \text{ kJ}$

The combustion of $1$ mole of ethanol releases 1360 kJ of energy, equivalent to 29.6 kJ g⁻¹.

Environmental considerations

E10 offers several environmental advantages:

• Renewable fuel source: Can be produced from biomass
• Carbon dioxide absorption: Plants absorb CO₂ as they grow, offsetting the CO₂ released when bioethanol burns, resulting in lower net CO₂ emissions
• Cleaner combustion: Reduces emissions of particulates and nitrogen oxides
• Safer production: Generally safer to produce than petroleum-based fuels

However, petrol has some advantages:

• More widely distributed in Australia
• Produces less CO₂ per kilometre when driving (due to higher energy density)

Future developments: Ethtec case study

Ethtec (Ethanol Technologies) has established a next-generation bioethanol pilot plant near Muswellbrook in the Hunter Valley of New South Wales. Unlike traditional plants that focus on converting one waste source into one product, Ethtec's approach involves:

• Using any available biomass in the district
• Producing a range of useful products

Why Muswellbrook?

Muswellbrook was chosen because both sugar cane and timber are grown in the region, providing access to diverse waste materials from both industries.

Processing forest waste

The forest waste, referred to as lignocellulosic waste, contains high proportions of:

• Cellulose - a glucose polymer
• Lignin - a natural polymer present in plant cells

The process involves:

1. Pulping the forest waste
2. Heating with acid to break down (hydrolyse) cellulose into smaller sugars
3. Removing lignin for alternative uses

Multiple products

Remember!