Can burning wood and crops be a clean way to make energy?
Topic 6.7 Energy from Biomass: describe how biomass and biofuels are used for energy and evaluate their benefits and drawbacks.
A focused answer to APES Topic 6.7, covering biomass and biofuels (wood, charcoal, dung, crop residues, ethanol, biodiesel), how they are used, their advantages and disadvantages, the carbon-neutrality debate, and a worked ethanol energy calculation.
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What this topic is asking
The College Board (Topic 6.7) wants you to describe how biomass and biofuels are used for energy and evaluate their benefits and drawbacks.
Biomass and biofuels
The carbon-neutral debate
Benefits and drawbacks
Why this matters
Biomass connects Unit 6 to the carbon cycle (Unit 1), to deforestation and land use (Unit 5), and to indoor air pollution (Unit 7 and 8). It is the classic example of a renewable that is not automatically clean: the AP exam rewards weighing its renewability against deforestation, food competition and emissions. A frequent free-response prompt asks you to evaluate replacing some gasoline with corn ethanol; the best answers acknowledge both sides, that ethanol is renewable and can cut net carbon dioxide and oil imports, but that growing the corn competes with food production, uses fertilizer, water and fossil-fuelled machinery, and may drive land clearing. Distinguishing crop-based biofuels (which compete with food) from waste biomass (crop residues, sawdust) that avoids that conflict is the kind of nuance that earns full credit.
Try this
Q1. Identify two examples of biomass used as fuel. [1 point]
- Cue. Any two of wood, charcoal, animal dung, crop residues, ethanol, biodiesel.
Q2. Explain why crop-based biofuels are controversial. [2 points]
- Cue. Growing crops such as corn for ethanol uses farmland, water and fertilizer that could grow food, so it competes with food production, can raise food prices, and may drive land clearing.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2021 (style)4 marksSection II (FRQ). (a) Identify two examples of biomass used as fuel. (b) Explain why biomass is sometimes called carbon neutral. (c) Identify one environmental drawback of relying on biomass for energy. (d) Describe one advantage of biofuels such as ethanol over gasoline.Show worked answer →
A 4-point FRQ on biomass energy.
(a) Identify (1 point): any two of wood, charcoal, animal dung, crop residues, or biofuels such as ethanol and biodiesel.
(b) Explain (1 point): the carbon dioxide released when biomass burns was recently absorbed from the air by the plants as they grew, so if the crop is regrown the net carbon added can be near zero.
(c) Identify (1 point): any one of deforestation, habitat loss, indoor air pollution from burning wood, soil depletion, or competition with food crops for land.
(d) Describe (1 point): ethanol is renewable (regrown) and can lower net carbon dioxide and dependence on imported oil compared with gasoline.
Markers reward two valid biomass examples, the recently-absorbed-carbon argument, a valid drawback, and a valid biofuel advantage.
AP 2018 (style)1 marksSection I (multiple choice). A major drawback of using crops such as corn to produce ethanol fuel is that it: (A) emits more carbon dioxide than coal per unit energy (B) competes with food production for land and water (C) cannot be regrown (D) requires no land at all. Justify your choice.Show worked answer →
A 1-point MCQ on biofuels. The answer is (B).
Growing crops for ethanol uses farmland, water and fertilizer that could otherwise grow food, so it competes with food production and can raise food prices. Ethanol does not emit more carbon dioxide than coal per unit energy (A), it is renewable and can be regrown (C), and it clearly requires land (D). The trap is forgetting the food-versus-fuel land-use conflict that makes crop-based biofuels controversial.
Related dot points
- Topic 6.3 Fuel Types and Uses: identify the major fuel types (coal, oil, natural gas, biomass) and describe their main uses and relative impacts.
A focused answer to APES Topic 6.3, covering the major fuel types (coal, crude oil, natural gas, biomass), the grades of coal, what each fuel is mainly used for, their relative energy density and emissions, with a worked combustion energy calculation.
- Topic 6.1 Renewable and Nonrenewable Resources: distinguish renewable from nonrenewable energy resources and explain why the distinction matters for sustainability.
A focused answer to APES Topic 6.1, covering the difference between renewable and nonrenewable energy resources, examples of each, the idea of potentially renewable resources, and why the distinction matters for sustainability, with a worked depletion calculation.
- Topic 1.4 The Carbon Cycle: describe the major reservoirs and fluxes of the carbon cycle and explain how natural processes and human activities move carbon between them.
A focused answer to APES Topic 1.4, covering carbon reservoirs and fluxes, photosynthesis and respiration, decomposition, combustion, the ocean as a carbon sink, and how fossil fuel burning alters the cycle, with a worked carbon-flux calculation.
- Topic 5.2 Clearcutting: describe clearcutting and explain its environmental consequences for soil, water and ecosystems.
A focused answer to APES Topic 5.2, covering clearcutting as a logging method, its economic appeal, and its consequences for soil erosion, water temperature and quality, flooding, habitat loss and biodiversity, with a worked erosion comparison.
- Topic 5.3 The Green Revolution: describe the methods and benefits of the Green Revolution and explain its environmental costs.
A focused answer to APES Topic 5.3, covering the Green Revolution, high-yield crop varieties, fertilizers, pesticides, irrigation and mechanisation, its benefits for food supply, and its environmental costs, with a worked yield-increase calculation.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)