Reactivity 1.3.3 — Fossil Fuels: Coal, Crude Oil, and Natural Gas

What Are Fossil Fuels?

Fossil fuels are non-renewable energy sources formed over millions of years from the remains of ancient plants and marine organisms. Due to intense heat and pressure underground, these biological materials were transformed into energy-rich substances such as coal, crude oil (petroleum), and natural gas. They are hydrocarbons—compounds primarily made of carbon and hydrogen—and are used extensively for electricity generation, transportation, and industrial processes.

Types of Fossil Fuels

1. Coal

• Solid fossil fuel composed primarily of carbon with varying amounts of hydrogen, sulfur, oxygen, and nitrogen.
• Formed from the remains of terrestrial plants in swampy regions subjected to high pressure and low oxygen conditions.
• Often classified by carbon content: lignite, bituminous coal, anthracite (from least to most carbon and energy content).

2. Crude Oil

• Liquid fossil fuel made of complex hydrocarbons of varying chain lengths.
• Formed from the remains of marine plankton buried under sediment layers and subjected to high pressure and temperature.
• Crude oil is refined via fractional distillation to produce useful fuels like gasoline, diesel, kerosene, and lubricants.

3. Natural Gas

• Gaseous fossil fuel composed mainly of methane (CH₄), with small amounts of ethane, propane, and butane.
• Usually found near crude oil deposits or as an independent source in porous rock formations.
• Burns more cleanly than other fossil fuels and is widely used for electricity, heating, and as a chemical feedstock.

Advantages of Fossil Fuels

• High Energy Density: Fossil fuels release a large amount of energy per unit mass, making them very efficient for energy production.
• Reliable and Consistent: Unlike solar or wind, fossil fuels provide constant, controllable energy regardless of weather or time of day.
• Well-Established Infrastructure: Global energy systems, including power stations, transport networks, and vehicles, are designed to use fossil fuels.
• Economic Importance: Fossil fuel industries support millions of jobs and contribute significantly to national economies.
• Versatility: They can be used for electricity generation, transport fuels, heating, and as raw materials in petrochemical industries.

Disadvantages of Fossil Fuels

• Non-Renewable: Fossil fuels take millions of years to form, and current consumption rates are depleting known reserves.
• Air Pollution: Combustion releases pollutants such as sulfur dioxide (SO₂), nitrogen oxides (NO_x), particulates, and carbon monoxide (CO).
• Greenhouse Gas Emissions: Burning fossil fuels emits large amounts of carbon dioxide (CO₂), a major contributor to global warming and climate change.
• Environmental Damage: Oil spills, coal mining, and natural gas extraction (e.g., fracking) can damage ecosystems, contaminate water sources, and lead to habitat loss.
• Health Risks: Air pollution from fossil fuel use is linked to respiratory diseases, cardiovascular conditions, and premature death.

Comparison of Fossil Fuels

The Tendency for Incomplete Combustion

Incomplete combustion occurs when there is an insufficient supply of oxygen. Instead of forming only carbon dioxide and water, the combustion may also produce carbon monoxide (CO), carbon (soot), or unburned hydrocarbons.

• Causes: Poor air supply, impure fuel, low combustion temperature.
• Products: CO, C (soot), sometimes toxic hydrocarbons.
• Consequences:
  • Carbon monoxide is toxic — it binds to hemoglobin, preventing oxygen transport in the blood.
  • Soot contributes to respiratory problems and environmental pollution.
  • Less energy is released compared to complete combustion, reducing fuel efficiency.

Good ventilation and correct fuel-to-air ratios are essential to ensure complete combustion and minimize these risks.

Evaluating CO₂ Emissions from Fuel Combustion

• Combustion of fuels produces energy along with carbon dioxide, a major greenhouse gas. Different fuels release different amounts of CO₂ depending on their molecular structure and carbon content.
• The combustion of hydrocarbons and alcohols releases CO₂ according to the number of carbon atoms in the fuel. 
• Each carbon atom typically forms one molecule of CO₂ in complete combustion.

Sample Equations and CO₂ Output per Mole

Factors Affecting CO₂ Emissions

• Carbon chain length: More carbon atoms = more CO₂ per mole.
• Combustion type: Incomplete combustion may produce CO or soot (C), not CO₂.
• Fuel efficiency: Fuels with higher hydrogen content (e.g. methane) produce more energy per mole of CO₂ released.

Key Formula (for enthalpy-related questions):

\[ \text{Mass of CO}_2 = \text{moles of fuel} \times \text{mol ratio} \times M_r(\text{CO}_2) \]

\[ \text{CO}_2 (g) = \frac{\text{mass of carbon in fuel} \times 44}{12} \]

The Greenhouse Effect and Carbon Dioxide

The greenhouse effect is a natural process where certain gases in the Earth’s atmosphere trap heat, maintaining a temperature suitable for life.

1. Short-wave solar radiation (UV and visible light) enters Earth’s atmosphere and is absorbed by the surface.
2. The Earth re-emits this energy as long-wave infrared (IR) radiation.
3. Greenhouse gases absorb this IR radiation and re-radiate it, warming the atmosphere.

Major Greenhouse Gases

CO₂ as a Greenhouse Gas

Carbon dioxide absorbs IR radiation in specific wavelengths (particularly at 15 μm), causing it to vibrate and re-emit energy in all directions. This contributes to atmospheric warming.

Rising CO₂ Levels and Climate Change

• Pre-industrial CO₂: ~280 ppm
• Current CO₂ (2024): >420 ppm
• Linked to global warming, ice melt, rising sea levels, and extreme weather patterns

Sources of Increased CO₂

• Combustion of fossil fuels (coal, oil, gas)
• Deforestation (reduces CO₂ absorption)
• Industrial processes (e.g., cement production)

Consequences of the Enhanced Greenhouse Effect

• Global warming: rising average surface temperatures
• Ocean acidification: CO₂ dissolves in seawater forming carbonic acid
• Melting ice caps and rising sea levels
• Biodiversity loss due to habitat disruption
• Extreme weather events: heatwaves, floods, droughts

Scientific Perspective

The IPCC confirms a strong correlation between anthropogenic CO₂ emissions and climate change. The contribution of each gas is measured using radiative forcing, where CO₂ has the largest net effect.

Solutions:

• Transition to renewable energy
• Reforestation
• Energy efficiency improvements
• Carbon capture technologies

Energy Released per Unit Mass

The energy efficiency of a fuel is often measured by the amount of energy it releases per gram when combusted. This is important for comparing different fuels in terms of cost-effectiveness and environmental impact.

• Hydrocarbons: Generally release high amounts of energy per gram due to their C–H bonds. Example: methane releases approximately 55.5 kJ/g.
• Alcohols: Release less energy than hydrocarbons because they already contain oxygen in their structure. Example: ethanol releases about 30 kJ/g.
• Coal: Releases variable energy depending on its grade (e.g., lignite vs. anthracite).

This data helps in selecting fuels for transport, heating, and industrial purposes where high energy density is desirable.