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Biofuels and Carbon Emissions

Explain sugarcane bioethanol as a renewable fuel, follow carbon through its production and combustion, and evaluate emissions across its life cycle.

  • SEC G3 Pure Chemistry 2027

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Burning bioethanol releases carbon dioxide. Why, then, might it reduce net emissions compared with a fossil fuel? The answer depends on where the carbon came from and which stages you count.

A renewable fuel from sugarcane

A biofuel is made from biological material, or biomass. Sugarcane can supply sugars used to make bioethanol, which is ethanol used as a fuel.

New sugarcane can grow on a human timescale, so this source is renewable. Crude oil takes millions of years to form, so it is non-renewable. Renewable describes replacement of a resource; it does not mean that using it causes no pollution or uses no land.

Follow carbon through production and use

Sugarcane absorbs atmospheric carbon dioxide during photosynthesis and stores carbon in sugars. Yeast enzymes convert glucose to ethanol and carbon dioxide during fermentation in warm conditions without oxygen:

C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(g)

The dilute ethanol solution must then be separated and concentrated. That processing needs energy. See Alcohols for the production conditions and chemistry in more detail.

Complete combustion of ethanol also releases carbon dioxide:

C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)

This equation shows water collected as a liquid after cooling. In a hot flame, water is vapour.

Track all six carbon atomsPhotosynthesis uses atmospheric carbon dioxide to make glucose containing six carbon atoms. Fermentation makes two ethanol molecules containing four carbon atoms in total and releases two carbon dioxide molecules. Complete combustion of the two ethanol molecules releases four carbon dioxide molecules. Two plus four equals six carbon atoms returned. Real life cycles can have additional emissions.Recent plant carbon does not disappearAtmospheric CO₂Carbon taken up duringphotosynthesisOne glucose molecule6 carbon atomsFermentationTwo ethanol molecules4 carbon atoms in totalTwo CO₂ molecules2 carbon atoms releasedComplete combustionFour CO₂ molecules4 carbon atoms released
Carbon accounting for one glucose molecule in an idealised pathway. Six recently absorbed carbon atoms return as carbon dioxide: two during fermentation and four during combustion. This excludes extra farming, processing, transport and land-use emissions.

Follow one glucose molecule’s six carbon atoms. Fermentation sends two into carbon dioxide and four into two ethanol molecules. Burning both ethanol molecules releases those remaining four as carbon dioxide. The six carbons are conserved; they do not disappear because the fuel is renewable.

What does “carbon neutral” mean?

Carbon neutral means that carbon-dioxide uptake balances carbon-dioxide release over the stated system and time period. In an idealised crop-carbon cycle, growing plants take up carbon dioxide and production and combustion return that recent plant carbon. Regrowing the crop can take up carbon again.

Burning fossil fuels returns carbon that was stored underground for millions of years. It adds carbon to the active atmosphere–organism cycle without the corresponding recent uptake used to grow a biofuel crop.

That comparison explains the possible benefit of bioethanol. It does not establish the emissions of every real biofuel project.

Widen the boundary to the whole life cycle

A life cycle includes cultivation, processing, transport and use. Ask what would happen at each stage:

StageQuestion to ask
Growing the cropHow much carbon dioxide is taken up? Is the crop regrown?
Farming and transportIs fossil fuel burned in machinery and vehicles?
Fermentation and separationWhere does carbon go, and what supplies the processing energy?
Fuel useHow much carbon dioxide is released for the useful energy supplied?
Land useIs forest cleared, releasing stored carbon or reducing future uptake?

A fair fuel comparison supplies the same useful energy, rather than assuming that equal volumes of two fuels do the same job. Land, food production and habitats also matter; a carbon calculation alone does not settle every environmental question.

Two claims to keep separate
  • “Sugarcane bioethanol is renewable” concerns replacing the crop.
  • “This bioethanol has zero net carbon-dioxide emissions” requires evidence about uptake, release and the full life cycle.

Work through a carbon-neutrality claim

Guided practice 1

Bioethanol “carbon neutral” explanation

About 7 min

Evaluate the claim

A supplier describes sugarcane bioethanol as “carbon neutral”. Explain the scientific basis for the description and one reason the claim may not be fully true in practice.

Consider uptake, release and production

CO2 uptake occurs during
CO2 is released during

Hints

Hint 1: carbon cycle
Compare uptake during crop growth with release during production and combustion.
Hint 2: wider boundary
Include farming, processing, transport or land-use change, not only combustion.
View solution step by step

  1. Explain the carbon-cycle basis

    Method

    Identify recent plant carbon and follow it through production and combustion.

    Reason

    Sugarcane absorbs CO₂ while growing, while burning its ethanol releases CO₂.

    Working

    For the idealised glucose pathway, six carbon atoms are absorbed, two are released during fermentation, and four during ethanol combustion. Regrowth can take up carbon again.

  2. Test the real-life boundary

    Method

    Identify an additional source of net emissions.

    Reason

    Farming machinery, processing and transport may burn fossil fuels; land clearance can also release carbon and reduce uptake.

    Working

    An idealised crop-carbon cycle can balance uptake and release, but this does not prove that the whole production life cycle has zero net emissions.

Try without prompts

Mind stretcher 1: Use a carbon budget to test the claimExtension

For a model project, growing a crop takes up 100 kg of carbon dioxide. Fermentation and burning its bioethanol together release 100 kg. Farming, processing and transport add another 25 kg from fossil fuels. Assume no land-use change and count only the listed carbon-dioxide flows.

  1. Calculate the net carbon-dioxide release for the whole project.
  2. A fossil-fuel alternative supplies the same useful energy and releases 80 kg in total. Compare the two projects.
  3. Does this evidence support the claim “bioethanol produces no carbon dioxide”? Explain the difference between lower net emissions and no emissions.
Show answer
  1. Total release is 100 + 25 = 125 kg; net release is 125-100 = 25 kg.
  2. The bioethanol project has 80-25 = 55 kg less net release for the same useful energy. These model values describe this comparison, not all biofuels.
  3. No. Production and combustion release carbon dioxide. Crop uptake offsets the 100 kg of recent plant-carbon release in this model, but the additional 25 kg remains. Lower net emissions do not mean no emissions, and this calculation does not include other greenhouse gases or unlisted land-use effects.

Bring the ideas together

Bioethanol from sugarcane is a renewable alternative to fossil fuels. Its carbon was recently absorbed during plant growth, but carbon dioxide is released during fermentation and combustion. Judge a carbon-neutrality claim using the whole life cycle and compare fuels for the same useful energy.

Practise and check

Use the Organic Chemistry topic check to practise and check your understanding.

Syllabus and review details

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