The carbon footprint of pellet fires is sometimes claimed to be carbon neutral (or even carbon negative) but the reality is that all pellet fires are at least slightly carbon positive in their emissions.
The level of emissions from pellet fires is heavily affected by pellet sources.
There are two main sources of wood used for pellet fuel:
- Purpose Harvested Wood : Purpose harvested wood is wood from tress and plantations specifically grown for the purpose of being felled and turned into pellet fuel.
- Waste Residue Wood : Sawmill residue (sawdust) and wood sourced from plantation pine waste (pruned branches, left over wood from felling)
Of the two, waste residue wood is less damaging as using it for pellet fuel removes it from decomposing and emitting carbon dioxide and methane. Methane is also a greenhouse gas and its ability to trap heat is much higher than carbon dioxides. (
link) Methane’s heat trapping ability is about 80 times that of carbon dioxide in the first 20 years of its emission. (
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The other reason pellet fuel cannot be considered carbon neutral is its processing. Harvesting, drying, pelletising and transport all use fossil energy.
So why would you choose a pellet fire?
Pellet fuel as a heating source produces less emissions than fossil fuels so gas, petrol, diesel, heat sources and electrical hearing powered by coal and other fossil fuels, produce far more emissions than pellet fires. Studies differ in the difference but consistently cite green house gas emissions of lpellet fires to be half that of fossil fuels. As an example pellet fuel produces 135-165 grams/kilowatt hour, compared to 357 grams per kilowatt hour if using heating oil.
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What about the ash left over after burning. If we bury that aren’t we sequestering carbon.
Unfortunately not. Ash is mostly minerals, not carbon. Good pellets leave less than 1% of their weight as ash. A Hearth.com user who weighed the ash from a 40 lb bag worked it out at .9375%. That ash is mostly calcium, potassium and magnesium compounds that came from the soil, not from the air. In one boiler test, the unburned combustibles left in the ash pan came to just 0.09–0.22% for wood pellets of what went in. Wood is about 50% carbon, so in practice more than 99% of it leaves the flue as CO₂.
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Don’t emissions calculations depend on if you grow a tree to burn or grow a tree to make up for burning fuel?
Pay it forward vs pay it back. Back-accounting looks at one tree: burn it now and wait decades for regrowth. That gap is the “carbon debt.”
Forward or landscape accounting looks at a rotational plantation where planting comes before harvest. In that case the total stored stock stays steady, so there’s no debt.
NZ pine plantations with residue pellets fit the forward case well.
If a tree is specifically grown for turning into pellet fuel, then felling the tree and burning it means you decades of growth required to absorb the amount of carbon released when the fuel is burned. However
Not negative: Nothing is permanently stored. Carbon negative would need capture and storage, as with BECCS or biochar.
“Neutral” only at the chimney: The CO₂ from burning is treated as neutral on the assumption that regrowth of vegetation captures and stores carbon that already exists in the atmosphere. That holds only under specific conditions, particularly when its production and management follow sustainable practices. aimspresspreprints
Net emitter over the full lifecycle: Harvesting, drying, pelletising and transport all use fossil energy. One full-lifecycle study found pellet heat produced 165 grams/kilowatt hour initially and drops to 135 grams/kilowatt hour by year 50, compared with 357 grams of greenhouse gas/kilowatt hour for heating oil. woodworkingnetworkwoodworkingnetwork
The neutrality claim is disputed: Many question whether it is, indeed, carbon neutral. The main argument is “carbon debt”: CO₂ is released immediately, but regrowth takes decades to absorb it. woodworkingnetwork
Pellets made from sawmill residue score better than pellets made from purpose-harvested logs. In NZ, most pellets come from plantation pine waste, which is the more favourable case. On electricity, though, a heat pump will usually beat pellets on emissions in NZ because the grid is mostly renewable.
