Alcohols (HSC SSCE Chemistry): Revision Notes

Fuels From Different Sources

Introduction to fuel sources

Fuels play a fundamental role in modern society, enabling transportation, heating, cooking, and electricity generation. These energy sources have made life more convenient and accessible. However, current fuel use presents several significant challenges:

• Heavy dependence on fossil fuels
• Declining availability of fossil fuel reserves
• Environmental pollution and its consequences

Biofuels, such as ethanol and biodiesel, represent a major alternative to fossil fuels. These renewable energy sources can help supplement and eventually replace our current fossil fuel dependence.

Fossil fuels

What are fossil fuels?

Fossil fuels are non-renewable energy sources formed through geological processes spanning millions of years. They develop from the decomposition of ancient plant and animal matter that has been subjected to heat and pressure deep underground.

The main types of fossil fuels include:

• Coal (solid)
• Crude oil/petroleum (liquid)
• Natural gas (gas)

Uses and issues

Fossil fuel combustion releases substantial quantities of energy. This energy is captured and converted for multiple purposes:

• Generating electricity
• Powering vehicles
• Cooking food
• Heating buildings

Biofuels

What are biofuels?

Biofuels are renewable fuels derived from contemporary organic sources rather than ancient fossilised material. They come from renewable resources such as:

• Agricultural crops
• Algae
• Animal waste

Biofuels are classified as renewable because they can be continuously produced from these biological sources. The crops used to make biofuels can be replanted and harvested repeatedly, creating a sustainable fuel cycle.

Biofuels in Australia

In Australia, the two main biofuels are:

1. Ethanol - primarily used as a fuel additive
2. Biodiesel - used in diesel engines

Ethanol as a biofuel

Production methods

Ethanol can be produced through several methods:

• Fermentation (most common for fuel ethanol)
• Industrial hydration of alkenes
• Hydration of haloalkanes

The fermentation process has become increasingly important for fuel production. Since 2007, Australia has more than tripled its bioethanol production through fermentation.

Source materials for bioethanol

Bioethanol is produced from various agricultural crops, including:

• Corn
• Sorghum
• Wheat
• Sugar cane

It can also be produced from biomass such as vegetable waste.

Controversy

Using crops to produce ethanol raises ethical concerns. Agricultural land devoted to fuel production cannot simultaneously grow food. In some regions, this has contributed to:

• Increased food shortages
• Land clearing to create additional crop-growing areas for ethanol production

These issues highlight the complex balance between energy needs and food security.

Properties and performance

Energy content: Ethanol contains approximately 34% less energy per volume than petrol. This means you get less energy from each litre of ethanol compared to petrol.

Octane rating: Despite lower energy content, ethanol has a higher octane rating than petrol. The octane rating measures how much compression a fuel can withstand before igniting spontaneously. Higher octane ratings indicate better fuel performance.

Higher octane fuels are:

• More efficient
• Burn more cleanly
• Produce fewer emissions

Fuel economy: When all properties are considered, E10 fuel delivers only slightly lower fuel economy than pure unleaded petrol. This means vehicles travel similar distances on a tank of E10 compared to regular petrol.

Environmental benefits

Ethanol combustion releases less carbon dioxide than an equivalent amount of petrol. Additionally, we must consider the carbon cycle:

Estimates suggest ethanol combustion produces 20-50% less net $\text{CO}_2$ than fossil fuel combustion. The exact reduction depends on:

• Crop types used
• Production methods employed
• Transportation and processing energy

Biodiesel as a biofuel

Source materials

Biodiesel can be produced from almost any fatty acid source, including oils from:

• Corn
• Palm
• Coconut
• Sunflower
• Peanuts

Most biodiesel currently produced uses waste vegetable oil from restaurants and industrial food producers. Using waste oil:

• Keeps production costs down
• Makes biodiesel financially competitive with fossil fuel diesel
• Reduces waste disposal problems

Understanding fatty acids

Fatty acids are long-chain carboxylic acids that form the building blocks of fats and oils. They have several key characteristics:

• Chain length: Usually contains an even number of carbon atoms (between 4 and 28)
• Carboxylic acid group: Contains a $-\text{COOH}$ group at one end
• Saturation: Can be saturated or unsaturated

Triglycerides structure

Triglycerides are the chemical form of fats and oils found in plants and animals. Each triglyceride molecule contains:

• One glycerol molecule (the backbone)
• Three fatty acid chains (attached to the glycerol)

Glycerol is an alcohol with three hydroxyl ($-\text{OH}$) groups. Its systematic chemical name is 1,2,3-propantriol, reflecting its three-carbon backbone with hydroxyl groups at positions 1, 2, and 3.

The three fatty acid chains in a triglyceride can be:

• All saturated
• All unsaturated
• A mixture of saturated and unsaturated

Diesel vs biodiesel composition

Diesel (petroleum-based) contains a range of alkanes with long hydrocarbon chains containing 8-21 carbon atoms. These are simple hydrocarbons with no functional groups.

Biodiesel is chemically similar to diesel but contains an ester functional group ($-\text{COO}-$). This ester group distinguishes biodiesel from petroleum diesel and gives it slightly different properties.

Production process: Transesterification

Biodiesel is produced through a chemical process called transesterification. This reaction converts triglycerides into esters (biodiesel) and glycerol.

Catalysts in biodiesel production

A catalyst increases the reaction rate without being consumed. Three types of catalysts can be used:

1. Acids
2. Bases
3. Enzymes (biological catalysts)

Properties and performance

Energy content: Biodiesel combustion produces approximately 9% less energy than regular diesel. The exact value varies depending on the fats and oils used in production.

Fuel efficiency: When considering all factors (flash point, viscosity, heating values), there is little practical difference in fuel efficiency between biodiesel and regular diesel. Vehicles travel similar distances on biodiesel compared to petroleum diesel.

Vehicle use and modification

Low percentage blends:

• Biodiesel blends up to 20% can be used without vehicle modification
• No performance or maintenance issues
• Compatible with existing diesel engines

High percentage or pure biodiesel:

• 100% biodiesel requires engine modification
• Once modified, vehicles can run on pure biodiesel without issues

Applications: Biodiesel is increasingly tested and used in:

• Aircraft
• Trains
• Heavy vehicles
• Commercial fleets

Many manufacturers are developing engines specifically designed to accommodate high percentages of biodiesel.

Environmental benefits

Like bioethanol, biodiesel produces less net carbon dioxide than petroleum diesel. The carbon dioxide released during combustion was recently absorbed from the atmosphere through photosynthesis by the plants that produced the oils.

The overall net $\text{CO}_2$ contribution comes primarily from the production and processing steps, not from combustion itself. This makes biodiesel substantially more environmentally friendly than fossil fuel diesel regarding greenhouse gas emissions.

Comparison of fuel types

Understanding the differences between fossil fuels and biofuels helps evaluate their respective advantages and disadvantages. The table below summarises key properties:

Property	Fossil Fuels	Biofuels
Chemical composition	Coal ($\text{C}$) Natural gas (primarily $\text{CH}_4$) Petrol/diesel (alkane chains of varying length)	Ethanol ($\text{C}_2\text{H}_5\text{OH}$) Biodiesel (esters of varying chain length)
Source	Mining	Ethanol – agricultural crops (sugar cane, wheat, etc.) Biodiesel – fats and oils
Enthalpy of combustion ($\text{kJ g}^{-1}$)	Natural gas (53.6) Coal (9.8–27.9) Petrol (48) Crude oil (44.9–46.3) Diesel (42.6)	Biodiesel (37.2) Bioethanol (29.6) Values vary depending on source crops
Emission of $\text{CO}_2$	High net release during both production and use (combustion)	Lower net release during combustion since recently removed from atmosphere through photosynthesis Some release associated with production
Vehicle modification required	None	Modification needed for higher percentages of ethanol or biodiesel
Running costs	No modifications required Fuel more expensive than biofuels but similar to hydrogen	Ethanol – no modification required for up to 10% Fuel cheaper than regular petrol Biodiesel – no modification for up to 20% Modification required for 100% use Fuel cheaper than regular diesel

Key comparison points

Energy content: Fossil fuels generally have higher energy content per unit mass than biofuels. Natural gas has the highest ($53.6 \text{ kJ} \cdot \text{g}^{-1}$), while bioethanol has the lowest ($29.6 \text{ kJ} \cdot \text{g}^{-1}$).

Environmental impact: Biofuels have a significant advantage in net $\text{CO}_2$ emissions. The carbon cycle means that biofuel combustion releases carbon that was recently absorbed, whereas fossil fuels release carbon that has been locked away for millions of years.

Economic considerations: Biofuels are generally cheaper than their fossil fuel counterparts. However, some vehicle modifications may be required for higher blend percentages, which represents an initial investment cost.

Sustainability: Fossil fuels are finite resources that cannot be renewed on human timescales. Biofuels are renewable and can be produced continuously from agricultural sources.

Practical use: Both fuel types can be used effectively. Biofuels at low blend percentages require no vehicle changes, making them easy to adopt. Higher percentages offer greater environmental benefits but need technical modifications.