Peat is partially decomposed plant material that accumulates in waterlogged soils (bogs and mires). It’s basically the first, low-grade step in coalification—over time peat can become lignite and then higher-rank coals. Because peat still contains a lot of carbon and combustible organic matter, people can dry and burn it for heat, so it’s used as a fuel in some places (EK ENG-3.C.2). Limits to know for the AP exam: peat has high moisture and lower energy density than coal, so it’s less efficient and produces CO2 (and sometimes smoke) when burned. It’s considered a nonrenewable resource on human timescales because peat forms very slowly. For a quick Topic 6.3 review, see the Fiveable study guide (https://library.fiveable.me/ap-environmental-science/unit-6/fuel-types-uses/study-guide/gSCAb3LVL5rkRp2HrqFp) and try related practice questions (https://library.fiveable.me/practice/ap-environmental-science).

Lignite, bituminous, and anthracite are the three main coal ranks formed by coalification—more heat, pressure, and deeper burial increase rank and quality (EK ENG-3.C.3). Key differences: - Lignite: lowest rank, high moisture, low carbon and energy density, “brown coal.” Burns dirty and produces less heat per mass, so it’s usually used close to where it’s mined in power plants. (low carbon ~25–35% by mass) - Bituminous: mid-rank, higher carbon and energy content, commonly used for electricity generation and to make coke for steel. More efficient and hotter-burning than lignite. (mid carbon range) - Anthracite: highest rank, highest carbon content and energy density, low impurities and smoke—burns hottest and cleanest of the three. Used for residential/commercial heating and where a higher-grade fuel is needed. (high carbon ~86–97%) For the AP exam, know that rank relates to heat, pressure, and depth of burial and the typical uses (EK ENG-3.C.3). Review Topic 6.3 on Fiveable (study guide: https://library.fiveable.me/ap-environmental-science/unit-6/fuel-types-uses/study-guide/gSCAb3LVL5rkRp2HrqFp) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).

Natural gas is called the cleanest fossil fuel mainly because it's mostly methane (CH4, EK ENG-3.C.4) and, when burned, it releases less air pollution and less CO2 per unit of energy than coal or oil. Combustion of natural gas produces about 50–60% less CO2 than coal per unit of heat and far lower amounts of sulfur dioxide, mercury, and particulate matter, so it causes fewer smog and health problems. That said, natural gas still emits CO2 and its main component, methane, is a potent greenhouse gas if leaked during production or transport—so leaks can cut into its climate advantage (CED notes methane and other GHGs have GWP > 1). For quick AP review, check the Topic 6.3 study guide (https://library.fiveable.me/ap-environmental-science/unit-6/fuel-types-uses/study-guide/gSCAb3LVL5rkRp2HrqFp) and Unit 6 overview (https://library.fiveable.me/ap-environmental-science/unit-6); practice questions at (https://library.fiveable.me/practice/ap-environmental-science) help reinforce these concepts for the exam.

Heat, pressure, and depth control coal “grade” through coalification. Plant material first becomes peat (low carbon). With increasing burial depth, temperature, and pressure over geologic time, peat turns into lignite (young, low-energy coal), then bituminous (more carbon, higher heat value), and finally anthracite (highest carbon content, highest energy, least volatile matter). Greater heat/pressure drives off water and volatile compounds and reorganizes carbon into denser forms, so deeper burial = higher grade coal. This is exactly what the CED lists for EK ENG-3.C (peat → lignite → bituminous → anthracite; coalification). For quick review, check the Topic 6.3 study guide (https://library.fiveable.me/ap-environmental-science/unit-6/fuel-types-uses/study-guide/gSCAb3LVL5rkRp2HrqFp) and grab practice questions at (https://library.fiveable.me/practice/ap-environmental-science) to prep for exam-style wording.

Tar sands (oil sands) are mixtures of sand, clay, water, and very heavy oil called bitumen (CED: crude oil from tar sands). To get usable oil you either strip-mine the sands or use in-situ methods. For surface deposits, operators dig up the sand, separate bitumen with hot water and mechanical separation, then upgrade the thick bitumen into lighter crude that refineries can process. For deeper deposits, in-situ techniques like steam-assisted gravity drainage (SAGD) inject steam to heat the bitumen so it flows to production wells. After recovery, the bitumen is upgraded (hydrogen added, molecules cracked) into synthetic crude oil, then refined into gasoline/diesel. Tar-sands extraction is energy-intensive and causes big land disturbance and higher greenhouse gas emissions than conventional oil—good to note for AP questions on fuel types and impacts. For a quick review, see the Topic 6.3 study guide (https://library.fiveable.me/ap-environmental-science/unit-6/fuel-types-uses/study-guide/gSCAb3LVL5rkRp2HrqFp) and more unit practice (https://library.fiveable.me/practice/ap-environmental-science).

Cogeneration (combined heat and power) is when one fuel source produces both electricity and useful heat instead of wasting that heat. For example, a natural gas turbine makes electricity and the hot exhaust is captured to heat buildings or run industrial processes. Because typical power plants lose ~60% of fuel energy as waste heat, cogeneration can push overall efficiency to 70–90% by using that heat productively. That means less fuel burned for the same energy services, lower CO2 and pollutant emissions per unit of useful energy, and often lower costs. On the AP exam, cogeneration is the classic example of improving energy efficiency in Topic 6.3 (ENG-3.C.7). For a quick review, check the Topic 6.3 study guide on Fiveable (https://library.fiveable.me/ap-environmental-science/unit-6/fuel-types-uses/study-guide/gSCAb3LVL5rkRp2HrqFp) and more Unit 6 resources (https://library.fiveable.me/ap-environmental-science/unit-6).

Because wood (firewood and charcoal) is cheap, local, and doesn’t need electricity or pipelines, many people in developing countries rely on it as their main fuel (EK ENG-3.C.1). It’s often the easiest-accessible biomass: nearby trees, brush, or agricultural waste can be gathered or made into charcoal with simple tools. Limited income, weak infrastructure for fossil fuels or natural gas, and rural cooking/heating traditions keep demand high. That convenience comes with trade-offs: using wood at high rates can cause deforestation, and burning it indoors leads to serious indoor air pollution and health risks. On the AP exam, remember to connect the fuel type to uses and impacts (use terms like firewood, charcoal, biomass). For a quick Topic 6.3 review, check the Fuel Types and Uses study guide (https://library.fiveable.me/ap-environmental-science/unit-6/fuel-types-uses/study-guide/gSCAb3LVL5rkRp2HrqFp). For broader Unit 6 review and practice problems, see (https://library.fiveable.me/ap-environmental-science/unit-6) and (https://library.fiveable.me/practice/ap-environmental-science).

Charcoal is wood that’s been heated with little or no oxygen (partial combustion/pyrolysis) until most volatile compounds and water are driven off, leaving mostly carbon. Regular firewood is unprocessed wood with higher moisture and volatile organics. Key differences: charcoal has higher energy density and burns hotter and more steadily, produces less smoke and fewer volatile gases, and lights faster; firewood gives more smoke and lower heat per mass and needs more oxygen to burn well. For APES, remember EK ENG-3.C.1: both are common wood fuels, often used in developing countries because they’re accessible, but charcoal’s production can cause deforestation and air pollution if made inefficiently. Want more on fuel types and uses? Check the Topic 6.3 study guide (https://library.fiveable.me/ap-environmental-science/unit-6/fuel-types-uses/study-guide/gSCAb3LVL5rkRp2HrqFp) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).

Crude oil is a mix of hydrocarbons that gets turned into usable fuels by heating and separating it by boiling point (fractional distillation). In a refinery, crude is heated; light fractions (like gasoline) vaporize at ~40–200°C and are collected near the top, heavier fractions (diesel, heating oil) lower down. Those fractions are then chemically processed: cracking breaks large molecules into smaller ones (makes more gasoline), reforming rearranges molecules to improve octane, and blending mixes additives to meet fuel specs. The result: gasoline for cars, diesel for trucks, and heavier fuels for heating or industry—this is exactly how fossil fuels become specialized fuel types (CED: ENG-3.C.6). For AP review, know “distillation → cracking/reforming → blending” and that refineries optimize yields based on demand. Want a quick refresher? Use the Topic 6.3 study guide (https://library.fiveable.me/ap-environmental-science/unit-6/fuel-types-uses/study-guide/gSCAb3LVL5rkRp2HrqFp), the Unit 6 overview (https://library.fiveable.me/ap-environmental-science/unit-6), and plenty of practice questions (https://library.fiveable.me/practice/ap-environmental-science).

Bitumen is a very thick, sticky form of crude oil—a heavy, viscous hydrocarbon that’s basically too dense to flow at surface temperatures. In the AP CED it’s the hydrocarbon component in tar sands (also called oil sands): tar sands are mixtures of sand, clay, water, and bitumen (EK ENG-3.C.5). Because bitumen won’t flow, it must be extracted by surface mining or heated/injected in place, then upgraded or diluted before it can be refined into fuels like gasoline or diesel. Extraction is energy-intensive, releases more CO2 than conventional oil, and can cause large landscape disturbance and water contamination—useful things to mention on the exam when asked about fuel types, uses, and environmental tradeoffs. For more on this topic review the Unit 6 study guide (https://library.fiveable.me/ap-environmental-science/unit-6/fuel-types-uses/study-guide/gSCAb3LVL5rkRp2HrqFp), the Unit 6 overview (https://library.fiveable.me/ap-environmental-science/unit-6), or try practice questions (https://library.fiveable.me/practice/ap-environmental-science).

Methane (CH4) is the main component of natural gas because of how fossil fuels form and migrate. When organic matter is buried and subjected to heat and pressure (coalification and petroleum formation), thermal and microbial processes break large organic molecules into smaller hydrocarbons. Methane is the simplest, lightest hydrocarbon, so it’s produced in the greatest proportion and can migrate upward into reservoir rocks more easily than heavier molecules. Shallow, low-temperature deposits often get biogenic methane (microbial decomposition), while deeper, higher-temperature settings produce thermogenic methane. Because it’s mostly methane, natural gas burns cleaner than coal or oil (EK ENG-3.C.4). For AP review, this matches Topic 6.3: Fuel Types and Uses—check the Topic study guide (https://library.fiveable.me/ap-environmental-science/unit-6/fuel-types-uses/study-guide/gSCAb3LVL5rkRp2HrqFp) and more Unit 6 resources (https://library.fiveable.me/ap-environmental-science/unit-6). Practice questions are at (https://library.fiveable.me/practice/ap-environmental-science).

Crude oil is turned into car and plane fuels by refining and chemical processing. First, fractional distillation heats crude so compounds separate by boiling point into fractions (gasoline, kerosene/jet fuel, diesel). Then refiners upgrade fractions: catalytic cracking breaks large molecules into smaller ones (more gasoline), reforming rearranges molecules to raise octane, and hydrodesulfurization removes sulfur. Final fuels are blended and treated with additives (antioxidants, detergents, anti-icing agents) to meet performance specs: gasoline needs high octane for engines, diesel needs high cetane for compression ignition, and jet fuel (kerosene-based, e.g., Jet A) must have consistent freezing point and energy density. These steps match EK ENG-3.C.6 (fossil fuels made into specialized fuels) and are the kind of content tested in Unit 6 (see unit overview: https://library.fiveable.me/ap-environmental-science/unit-6 and topic study guide: https://library.fiveable.me/ap-environmental-science/unit-6/fuel-types-uses/study-guide/gSCAb3LVL5rkRp2HrqFp). For extra practice, check AP-style problems at (https://library.fiveable.me/practice/ap-environmental-science).

Yes—peat is basically the “baby stage” of coal. It’s made of partially decomposed plant material (wet, low-oxygen environments like bogs) that accumulates and is only mildly compressed. Because it’s not been subjected to long-term heat, pressure, and deep burial, peat has high moisture and relatively low carbon content, so it burns poorly compared with true coals. With increased burial time, heat, and pressure (the coalification process), peat can transform into lignite, then bituminous coal, and eventually anthracite—each stage has higher carbon content and energy density (see EK ENG-3.C.2 and EK ENG-3.C.3). Peat is listed in the CED as a fuel but is less efficient and dirtier per unit energy than higher-rank coals. For AP review, that distinction—“partially decomposed” vs. progressively coalified forms—is what they expect you to know (Topic 6.3 study guide: https://library.fiveable.me/ap-environmental-science/unit-6/fuel-types-uses/study-guide/gSCAb3LVL5rkRp2HrqFp). For more practice, check Fiveable’s unit page and practice questions (https://library.fiveable.me/ap-environmental-science/unit-6 and https://library.fiveable.me/practice/ap-environmental-science).

Wood, coal, and natural gas each have tradeoffs you should know for APES (ENG-3.C). - Wood (firewood/charcoal): Advantage—readily accessible in many developing countries, renewable if harvested sustainably; low tech. Disadvantages—burns inefficiently (more particulate pollution), causes deforestation and habitat loss if overused, and releases CO2 and particulates. (EK ENG-3.C.1) - Coal (lignite, bituminous, anthracite): Advantage—energy-dense and cheap for electricity generation; many types from coalification give different qualities. Disadvantages—highest CO2 and air-pollutant emissions (SOx, NOx, particulates), mining impacts (habitat loss, acid mine drainage), and ash disposal issues. (EK ENG-3.C.3) - Natural gas (mostly methane): Advantage—cleanest fossil fuel (lower CO2 and fewer particulates per kWh), flexible for heating and power. Disadvantages—still emits CO2; methane leaks are potent greenhouse gases; extraction (fracking) can contaminate groundwater and cause earthquakes. (EK ENG-3.C.4) For exam prep, link these points to trade-offs and solutions (cogeneration, switching to renewables). Review the Topic 6.3 study guide for summaries (https://library.fiveable.me/ap-environmental-science/unit-6/fuel-types-uses/study-guide/gSCAb3LVL5rkRp2HrqFp) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).

Cogeneration (combined heat and power, CHP) uses one fuel source to make electricity and capture the waste heat for useful heating—so you get more energy out of the fuel. A fuel (natural gas, biomass, coal, or oil) burns to spin a turbine or run an engine that makes electricity; instead of dumping the hot exhaust, heat exchangers capture it for space heating, hot water, industrial processes, or district heating. That boosts total efficiency from ~35% (typical power plant) to as much as 70–80%. Real-world examples: hospital and university CHP plants that run on natural gas to power lights/medical equipment while heating buildings; district heating + power systems in Scandinavian cities using biomass or waste incineration; industrial CHP in paper mills where steam powers processes and generates electricity. This fits the AP CED: cogeneration is specifically listed under EK ENG-3.C.7. For a quick topic review check the Topic 6.3 study guide (https://library.fiveable.me/ap-environmental-science/unit-6/fuel-types-uses/study-guide/gSCAb3LVL5rkRp2HrqFp) and practice problems (https://library.fiveable.me/practice/ap-environmental-science).