Fossil Fuels
Energy from the Past
Explore the formation, extraction, and environmental impact of coal, oil, and natural gas: the carbon-based energy sources that power modern civilization.
AP Environmental Science
What Are Fossil Fuels?
Fossil fuels are hydrocarbons formed from the remains of ancient organisms buried under heat and pressure over millions of years. They store chemical energy in carbon-hydrogen bonds that releases as thermal energy during combustion.
The Three Major Types
Coal
Solid hydrocarbon formed from ancient plant matter in swamps. Highest carbon content per unit. Used primarily for electricity generation.
Petroleum (Oil)
Liquid hydrocarbon from marine organisms. Refined into gasoline, diesel, and jet fuel. Most versatile fossil fuel for transportation.
Natural Gas
Primarily methane (\(\mathrm{CH}_4\)). Cleanest-burning fossil fuel. Used for heating, cooking, and electricity. Often found with oil deposits.
How Fossil Fuels Form
Coalification and petroleum maturation are distinct pathways
Plant-rich peat can progress through coal ranks with burial and heating. Organic-rich aquatic sediment can instead form source-rock kerogen that generates oil and gas, which may migrate into porous reservoir rock beneath a seal.Distinct coal and petroleum formation
Plant-rich peat can progress through coal ranks with burial and heating. Organic-rich aquatic sediment can instead form source-rock kerogen that generates oil and gas, which may migrate into porous reservoir rock beneath a seal.Fossil Fuels and the Carbon Cycle
Combustion Chemistry
1
Coal (carbon): \(\mathrm{C} + \mathrm{O}_2 \rightarrow \mathrm{CO}_2\). Produces the most \(\mathrm{CO}_2\) per unit of energy released.
2
Petroleum (octane): \(2\mathrm{C}_8\mathrm{H}_{18} + 25\mathrm{O}_2 \rightarrow 16\mathrm{CO}_2 + 18\mathrm{H}_2\mathrm{O}\). Moderate \(\mathrm{CO}_2\) emissions per joule.
3
Natural gas (methane): \(\mathrm{CH}_4 + 2\mathrm{O}_2 \rightarrow \mathrm{CO}_2 + 2\mathrm{H}_2\mathrm{O}\). Least \(\mathrm{CO}_2\) per unit energy among the three.
Coal Rank and Quality
Conventional vs Unconventional Extraction
Conventional
Oil and gas trapped in porous reservoir rock, accessed by drilling wells. Flows naturally or with pumps. Relatively lower cost and environmental disruption per barrel.
Unconventional
Oil sands, oil shale, and tight gas requiring hydraulic fracturing (fracking) or surface mining. Higher cost, greater water use, larger environmental footprint.
Extraction risks arise from distinct surface and subsurface pathways
Surface mining can expose sulfides to oxygen and water, enabling acid generation. At a shale well, surface spills and failures of casing or cement create distinct potential contamination pathways.Conditional extraction pathways
Surface mining can expose sulfides to oxygen and water, enabling acid generation. At a shale well, surface spills and failures of casing or cement create distinct potential contamination pathways.Environmental Impacts
1
Climate change: \(\mathrm{CO}_2\) and \(\mathrm{CH}_4\) emissions trap heat in the atmosphere, raising global temperatures.
2
Air pollution: \(\mathrm{SO}_2\), \(\mathrm{NO}_x\), and particulate matter from burning coal and oil cause smog, acid rain, and respiratory disease.
3
Water pollution: Oil spills, acid mine drainage, and fracking fluid contaminate waterways and groundwater.
4
Habitat destruction: Mountaintop removal mining, pipeline construction, and offshore drilling damage ecosystems.
\(\mathrm{CO}_2\) Emissions by Fuel Type
How to Analyze Fossil Fuel Problems
1
Identify the fuel type: Coal, oil, or natural gas? Each has different emissions profiles and environmental impacts.
2
Determine the extraction method: Surface mining, underground mining, drilling, fracking? Each has distinct ecological costs.
3
Trace the pollutants: What emissions result? \(\mathrm{CO}_2\), \(\mathrm{SO}_2\), \(\mathrm{NO}_x\), particulates, methane leaks?
4
Evaluate the scale: Compare energy output (in joules or BTUs) to environmental cost. Use \(\mathrm{CO}_2\) per GJ as a standard metric.
5
Consider alternatives: Could renewable sources replace this use case? What are the trade-offs?
Example: Emissions Calculation
A power plant burns 500 tonnes of coal per day. Coal produces approximately 2.86 tonnes of \(\mathrm{CO}_2\) per tonne of coal burned. How much \(\mathrm{CO}_2\) does the plant release annually?
1
Daily emissions: \(500 \times 2.86 = 1{,}430\) tonnes of \(\mathrm{CO}_2\) per day
2
Annual emissions: \(1{,}430 \times 365 = 521{,}950\) tonnes of \(\mathrm{CO}_2\) per year
3
Convert: \(\approx 522{,}000\) tonnes = 0.522 million tonnes \(\mathrm{CO}_2\)/year
Example: Fuel Switching Savings
If the same power plant switches from coal (96 kg \(\mathrm{CO}_2\)/GJ) to natural gas (56 kg \(\mathrm{CO}_2\)/GJ) while producing the same energy output, what percentage reduction in \(\mathrm{CO}_2\) emissions is achieved?
1
Difference: \(96 - 56 = 40\) kg \(\mathrm{CO}_2\)/GJ reduction
2
Percentage reduction: \(\frac{40}{96} \times 100 \approx 41.7\%\)
Example: EROI Analysis
Energy Return on Investment (EROI) measures how much energy is gained for every unit invested in extraction. A higher EROI means more net energy.
Conventional Oil
EROI: ~15:1. For every 1 joule invested, you get 15 joules back.
Oil Sands
EROI: ~5:1. Much more energy needed for extraction: mining, heating, processing.
Natural Gas
EROI: ~10:1. Moderate efficiency. Fracked gas has a lower EROI than conventional.
Common Mistakes
✕ "Natural gas is clean energy"
Natural gas is cleaner-burning than coal, but it still emits \(\mathrm{CO}_2\) and methane: a potent greenhouse gas. It is not a renewable or "clean" energy source.
✕ Confusing reserves with resources
Reserves are economically recoverable with current technology. Resources are the total estimated amount in the ground. Resources are always larger than reserves.
✕ Ignoring methane leakage
Methane (\(\mathrm{CH}_4\)) has ~80x the warming potential of \(\mathrm{CO}_2\) over 20 years. Leaks during extraction and transport can significantly offset the emissions advantage of natural gas.
Fossil Fuel Comparison
| Property | Coal | Oil | Natural Gas |
|---|---|---|---|
| State | Solid | Liquid | Gas |
| Primary use | Electricity | Transportation | Heating / Electricity |
| \(\mathrm{CO}_2\) per GJ | ~96 kg | ~73 kg | ~56 kg |
| EROI (conventional) | ~18:1 | ~15:1 | ~10:1 |
| Key pollutants | \(\mathrm{SO}_2\), \(\mathrm{NO}_x\), ash | \(\mathrm{NO}_x\), VOCs | \(\mathrm{CH}_4\) leaks |
| Extraction | Mining | Drilling / Fracking | Drilling / Fracking |
Question 1 of 5
Which fossil fuel produces the most \(\mathrm{CO}_2\) per unit of energy when burned?
Coal emits approximately 96 kg of \(\mathrm{CO}_2\) per GJ, which is the highest among the three major fossil fuels. This is because coal has the highest carbon-to-hydrogen ratio.
Question 2 of 5
What does EROI measure?
Energy Return on Investment (EROI) compares the energy obtained from a fuel to the energy required to extract it. A higher EROI means more net energy for society.
Question 3 of 5
A factory switches from coal to natural gas, producing the same amount of energy. Approximately what percentage reduction in \(\mathrm{CO}_2\) emissions can it expect?
Coal emits ~96 kg \(\mathrm{CO}_2\)/GJ and natural gas emits ~56 kg \(\mathrm{CO}_2\)/GJ. The reduction is \(\frac{96 - 56}{96} \approx 42\%\), which is approximately 40%.
Question 4 of 5
Why is burning fossil fuels different from biological respiration in terms of the carbon cycle?
Biological respiration recycles carbon that is already in the active carbon cycle. Burning fossil fuels introduces carbon that was locked underground for millions of years, increasing the total amount of \(\mathrm{CO}_2\) in the atmosphere.
Question 5 of 5
Which of the following is an environmental concern specific to hydraulic fracturing (fracking) that is NOT a major concern with conventional drilling?
Fracking involves injecting high-pressure fluid containing chemicals into rock formations. This creates unique risks of groundwater contamination and has been linked to induced earthquakes: problems not associated with conventional drilling at the same scale.