How Biomass Electricity is Counted in Carbon Accounting

Background briefing for the Week 3 assumption audit exercise.

The rule

Under international carbon accounting rules (the UNFCCC framework), CO₂ released when biomass is burned for electricity is counted as zero.

This is an accounting convention rather than a measurement. The chimney at a biomass power station releases roughly 1000 kg per MWh of CO₂, slightly more than coal (910 kg per MWh). But the accounting rules assign this a value of zero.

The reasoning

The logic behind the rule is a carbon cycle argument:

  1. Trees absorb CO₂ from the atmosphere as they grow.
  2. When the wood is burned, that CO₂ is released back.
  3. New trees grow in their place and reabsorb the CO₂.
  4. Over a full cycle, the net emission is zero.

Therefore, the argument goes, emissions from burning biomass do not need to be counted in national greenhouse gas inventories.

Where the rule applies

The UK follows this framework. Biomass electricity (mainly wood pellets burned at Drax power station in North Yorkshire) is classified as renewable and zero-carbon in official energy statistics.

In 2024, bioenergy accounted for roughly 40 TWh of UK electricity generation — about 14% of the total. Under the accounting rules, none of the CO₂ from burning this fuel appears in the UK’s reported emissions.

What the accounting includes and excludes

Included in “zero”

  • CO₂ absorbed by trees during growth
  • CO₂ released at the power station chimney

Not included in “zero”

  • Time delay before regrowth absorbs the CO₂
  • CO₂ from harvesting, chipping, and drying the wood
  • CO₂ from shipping pellets across the Atlantic
  • CO₂ from processing raw wood into pellets
  • Changes to the forest’s ability to absorb carbon if it is not replanted
  • Differences between fast-growing plantations and natural forest

Key numbers

Stage Approximate CO₂ (kg per MWh)
Combustion (what the chimney releases) 1000
Harvesting + processing + drying 30–50
Ocean shipping (US East Coast → UK) 15–25
Total lifecycle estimate 1050–1075
Regrowth of forest −1000
Official accounting value 0

For comparison: coal ≈ 910 kg/MWh; gas ≈ 360 kg/MWh; wind ≈ 11 kg/MWh (lifecycle).

Where the 1000 comes from. A tonne of pellets releases about 1700 kg CO₂ when burned and contains about 17 GJ of energy, so the fuel emits 100 kg CO₂ per GJ. A power station converting heat to electricity at about 36% efficiency needs 3.6 / 0.36 = 10 GJ of fuel per MWh of electricity. 10 GJ per MWh × 100 kg CO₂ per GJ is 1000 kg CO₂ per MWh.

Why it is worse than coal. Wood contains more C–O bonds than coal (mostly C). The extra O atoms add mass, and mean the C is already partly oxidized – so burning the same mass of fuel releases less energy.

The carbon payback question

Even if new trees are planted, they take time to reabsorb the CO₂ released at combustion. Estimates of this “payback period” vary widely:

  • Managed fast-growing plantation (SE United States): 20–40 years
  • Natural hardwood forest: 40–100+ years
  • If the forest is not replanted: payback is never achieved

The UK’s net zero target is 2050. If the carbon payback takes 40–100 years, biomass burned today will not be carbon-neutral within the policy-relevant timescale.

Your task

Discuss as a desk. Using this summary:

  1. List the assumptions the “zero at combustion” accounting rule makes. (Aim for at least five.)

  2. For each assumption, ask: Is this testable? What evidence would challenge it?

  3. Which assumptions, if wrong, would most change the conclusion? Rank your top three.

You will use these answers in the application session, where you’ll test what happens when the assumptions change.