The Olive Pit’s Second Life as Fuel

An olive mill produces rivers of waste — skins, pulp, water, and a great heap of cracked stones. For centuries that pit pile was a nuisance. Today it is one of the quiet success stories of the industry: a clean, dense biomass fuel that heats houses and runs the very mills that make it.

The olive stone is hard, woody and surprisingly energy-rich. After milling, the crushed pits are separated from the pulp, dried, and burned as biomass — loose, or pressed into uniform pellets for stoves and boilers. Weight for weight they give a steady, hot burn that compares well with wood pellets, and in olive-growing regions they are cheap and abundant because the mills generate them anyway. What was once a disposal headache, piling up behind the mill, has become a product sold by the tonne.

From nuisance to fuel

The numbers are the reason it works. Dried olive stone carries a calorific value in the region of 17–19 megajoules per kilogram — close to a good wood pellet — while burning notably cleaner, with ash content down around 0.7% against roughly 1–2% for wood. That low ash means less clinker in the burner and less sweeping. Spain, the world’s largest producer, turns out enormous quantities of stone fuel each season, and the trade in it has grown from a local curiosity into a real biomass market.

	Olive stone (hueso)	Wood pellet	Olive pomace (orujillo)
Calorific value	~17–19 MJ/kg	~18–19 MJ/kg	~16–18 MJ/kg
Ash content	Low (~0.7%)	~1–2%	Higher
Source	Crushed pits, dried	Compressed sawdust	Milling residue, dried
Best for	Stoves, boilers	Stoves, boilers	Industrial boilers
Note	Must be dried & de-oiled	Widely standardised	Bulkier, dustier

A neater circle

The appeal is partly green and partly practical. Burning a by-product that already exists, on site, displaces fossil fuel and saves the cost and pollution of hauling waste away — some mills run their own driers and presses on the heat of their own pits, closing the loop neatly. It is not a flawless fuel: burning anything releases emissions, and the stones must be properly dried and cleaned of oily residue to burn well. But as second lives go, it is a good one. The same fruit that gives you oil for the table quietly helps heat the building it was pressed in.

What it is	Crushed, dried olive stones burned as biomass, loose or pelletised
Calorific value	Roughly 17–19 MJ/kg — near a good wood pellet
Ash content	Low, around 0.7% — cleaner than typical wood pellets
Main source	Spain (the largest producer), and olive regions generally
Requirements	Must be dried and cleaned of oily residue to burn cleanly
Appeal	Cheap, local, low-ash; closes the mill’s waste loop

If you burn biomass at home

• Ask a local mill about stone pellets — often cheaper than wood and hot-burning.
• Buy them dry and store them dry — damp stone fuel smoulders and clogs a burner.
• Expect low ash — less clinker and less sweeping than many fuels.
• Check your burner’s spec — not every stove is rated for stone or pomace.
• It is an honest, local fuel made from something that would otherwise be a heap of spat-out pits.

Olive stone fuel: common questions

Can you really burn olive pits for heat?

Yes. Dried, crushed olive stones are a well-established biomass fuel, burned loose or as pellets in stoves and boilers, with a calorific value close to wood.

Is olive stone better than wood pellets?

It is comparable in heat and notably lower in ash — around 0.7% versus roughly 1–2% for wood — so it burns cleaner. Availability and price depend on being near olive country.

Where does most stone fuel come from?

Spain, the world’s largest olive-oil producer, generates enormous quantities each season, though it is made wherever olives are milled in volume.

Does it need special preparation?

Yes — the stones must be dried and cleaned of oily residue to burn cleanly and avoid gumming up a burner. Buy it dry and keep it dry.

Is it a green fuel?

It reuses a by-product on site and displaces fossil fuel, which is genuinely useful, but burning anything releases emissions. It is a sensible second life, not a zero-impact one.

Drawn from olive-mill by-product and biomass fuel research (calorific value and ash content of olive stone).