AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/6/6-3)
Unit 6 · Topic 6.3
6.3 Fuel Types and Uses
Different fuels hold different amounts of energy and pollute in different ways. This topic covers wood, the stages of coal from peat to anthracite, natural gas, crude oil and tar sands, plus cogeneration, which gets more use out of each unit of fuel.
Key terms
- peat
- lignite
- bituminous coal
- anthracite
- natural gas
- cogeneration
Wood and peat
Wood is still a main fuel for cooking and heating in many developing countries, because it is cheap or free to gather. Burning it releases particulates and carbon monoxide, and collecting too much leads to deforestation (topic 6.7 has more).
Peat is partly decayed plant matter that builds up in waterlogged wetlands called bogs. Low oxygen in the bog slows decomposition, so dead plants pile up. Peat can be dried and burned, but it holds a lot of water and relatively little energy. It is also the first stage in the formation of coal.
How coal forms: peat to anthracite
When peat gets buried under more sediment, heat and pressure squeeze out water and other compounds and concentrate the carbon. Over millions of years it passes through a series of stages. Each stage is harder, has more carbon and less moisture, and releases more heat when burned.
| Stage | Carbon content | Energy per kg | Notes |
|---|---|---|---|
| Peat | Lowest | Lowest | Wet, spongy, not yet coal |
| Lignite | Low | Low | Soft brown coal, high moisture |
| Bituminous | High | High | The most common coal burned in power plants |
| Anthracite | Highest | Highest | Hard, shiny, rare; burns hottest and cleanest of the coals |
Natural gas and crude oil
Natural gas is mostly methane (CH₄). When it burns completely it makes mostly CO₂ and water, with far less sulfur dioxide, particulate matter and mercury than coal. It is called the cleanest fossil fuel, and a gas power plant releases roughly half the CO₂ of a coal plant for the same electricity. But methane that leaks before it is burned is itself a powerful greenhouse gas.
Crude oil is a liquid mix of many hydrocarbons (compounds of hydrogen and carbon). A refinery heats it so the different parts separate by boiling point, a process called fractional distillation. That turns crude oil into products for specific uses: gasoline for cars, diesel for trucks, jet fuel, heating oil, and heavy asphalt for roads. Oil is also the raw material for most plastics.
Crude oil can also come from tar sands (oil sands), a mix of sand, clay, water and a thick, tar-like oil called bitumen. Getting usable oil out of them takes large amounts of water and energy, as topic 6.5 explains.
Cogeneration
A typical fossil fuel power plant turns only about a third of the fuel's energy into electricity. Most of the rest leaves as waste heat, carried off by cooling water or into the air.
Cogeneration, also called combined heat and power, uses one fuel source to make both electricity and useful heat. The waste heat from generating electricity is piped to heat buildings, warm water or run an industrial process. Because less energy is thrown away, a cogeneration system can put well over half of the fuel's energy to use, so it burns less fuel and releases less pollution for the same total output.
Worked examples
Try each one yourself first, then open the solution.
- Example 1Calculator allowed
How much coal does a power plant burn?
A coal power plant produces electricity at a steady 500 megawatts (500 × 10⁶ J per second). It converts 35% of the coal's chemical energy into electricity. The coal contains 24 megajoules of energy per kilogram (24 × 10⁶ J/kg). How many metric tons of coal does the plant burn in one day? (1 metric ton = 1,000 kg)
Show the solutionHide the solution
- Step 1: Find the electrical energy made in one day. There are 24 × 60 × 60 = 86,400 seconds in a day. Electrical energy = 500 × 10⁶ J/s × 86,400 s = 4.32 × 10¹³ J.
- Step 2: Work backward through the efficiency. Only 35% of the fuel energy becomes electricity, so fuel energy = electrical energy ÷ 0.35 = 4.32 × 10¹³ J ÷ 0.35 ≈ 1.234 × 10¹⁴ J.
- Step 3: Convert fuel energy to mass of coal: 1.234 × 10¹⁴ J ÷ 24 × 10⁶ J/kg ≈ 5.14 × 10⁶ kg.
- Step 4: Convert to metric tons: 5.14 × 10⁶ kg ÷ 1,000 ≈ 5,140 metric tons.
Answer: About 5,100 metric tons of coal per day (5.14 × 10⁶ kg).
- Example 2Calculator allowed
Trap: applying efficiency in the wrong direction
A student needs 100 units of electricity from a plant that is 35% efficient. She multiplies 100 × 0.35 = 35 and says the plant needs 35 units of fuel energy. What went wrong, and what is the right answer?
Show the solutionHide the solution
- Step 1: Efficiency = useful energy out ÷ energy in. Here the useful output (electricity) is known, and the input (fuel) is unknown.
- Step 2: Rearrange: energy in = energy out ÷ efficiency = 100 ÷ 0.35 ≈ 286 units.
- Step 3: Sense check: the plant wastes most of its fuel energy, so the fuel input must be bigger than the electricity output, not smaller. The student's answer of 35 would mean the plant makes more energy than it uses, which is impossible.
Answer: She should divide, not multiply: about 286 units of fuel energy are needed.
Common mistakes
- Getting the coal order backward. Energy and carbon content rise from peat to lignite to bituminous to anthracite; moisture falls.
- Calling natural gas 'clean' with no qualification. It burns cleaner than coal or oil but still releases CO₂, and leaked methane is a strong greenhouse gas.
- Multiplying by efficiency when you are working from output back to input. If you know the useful output, divide by the efficiency to find the input.
On the exam
- Questions often ask which coal type has the highest energy content (anthracite) or the most moisture (lignite, or peat if it is offered).
- Efficiency and energy-content calculations show up in the math free-response question. Write each step with units so you can earn the setup point even if the arithmetic slips.
Connected topics
Videos
Check yourself
4 questions on 6.3 Fuel Types and Uses. Pick an answer to see if you got it, and why.
Which list orders these fuels from lowest to highest energy content per kilogram?
A university's power plant burns natural gas to make electricity, then pipes the leftover hot water to heat campus buildings. This is an example of
As peat is buried deeper and longer, it changes into lignite, then bituminous coal, then anthracite. Which change best explains why each step gives more energy per kilogram?
A utility replaces a coal-fired power plant with a natural gas plant that makes the same amount of electricity. Which outcome is most likely?
0 of 4 answered