Biomass and biofuel stores: Science and technology

Why living fuel piles rot, heat up and give off gas

What this tab covers The physics and chemistry: how the energy is stored, which materials it needs (scarcity, toxicity, recyclability) and what makes it age.

Biomass and biofuels store solar energy captured by plants as chemical bonds. Unlike fossil stocks, many of these stores are biologically active and slowly lose energy, heat up or give off gas while they wait.

Mechanism

Mixed

Wood piles, chips and pellets hold energy in cellulose and lignin; pellets are dried and compressed to about 8% moisture to make them stable and dense. Charcoal is wood heated with little oxygen, which drives off water and volatiles but wastes much of the original energy: traditional earth kilns convert only about 10% and efficient kilns about 30%. Ethanol and biodiesel are liquid fuels kept in tanks like petrol and diesel, though ethanol holds about a third less energy per litre than petrol (35% calculated from DOE heating values; DOE's handbook rounds this to about 30%) and biodiesel about 7.5% less than diesel. Biogas is usually held for hours to days at very low pressure in membrane domes over digesters.³⁴⁵⁸

Pellet moisture entering storage about 8%³ Charcoal kiln efficiency about 10% (traditional) to 30% (efficient)⁸ Ethanol energy per gallon 76,300 Btu vs 116,900 Btu for petrol⁵ Biodiesel (B100) energy per gallon about 119,550 Btu vs 128,488 Btu for diesel⁴

Material scarcity and toxicity

Mixed

No critical minerals are involved; the constraint is sustainable biomass supply and land. Stored pellets give off carbon monoxide as fatty acids in the wood oxidise, which has killed people entering storerooms. Damp chip piles, especially at 30-50% moisture, favour fungal growth. Fuel ethanol blends must be handled as toxic because of their petrol content, ethanol vapour in tank headspace can be flammable, and biogas contains hydrogen sulphide.¹²⁵

Pellet storage CO deaths in Europe at least 9 since 2002 (to 2012)²

Degradation and ageing

Unfavourable

Chip piles lose dry matter to microbes: monthly losses of 0.1-2.7% in published studies, averaging 1.1% for uncovered and 0.7% for covered piles, with most loss early on. Pellets self-heat as wood fatty acids oxidise, and spontaneous ignition can occur if the temperature reaches about 250 C. Biodiesel oxidises: fuel meeting the minimum specification should keep for up to a year in clean, dry tanks, but minimum-spec B100 without antioxidants should be tested every 1-2 months. Ethanol absorbs water, and in high blends the petrol part can separate out.¹³⁴⁵

Chip pile dry matter loss 0.1-2.7% per month; average 1.1% uncovered, 0.7% covered¹ Pellet self-ignition from about 250 C self-heating³ Biodiesel storage life at minimum specification up to 1 year in clean, dry storage⁴

Bio-energy stores compared¹²³⁴⁵⁸

Store	Typical storage time	Main loss or risk	Key safeguard
Wood chips	Months	Dry matter loss 0.1-2.7% per month, fungi	Cover piles, store drier chips
Wood pellets	Months	CO off-gassing, self-heating	Ventilation, dryness, confined-space rules
Charcoal	Months to years	Large loss at production (kiln 10-30% efficient)	Efficient kilns
Ethanol	Months	Water uptake, flammable vapour	Water checks every 1-2 months
Biodiesel	Up to about 1 year (minimum-spec fuel)	Oxidation, cold-weather gelling	Antioxidants, keep above cloud point
Biogas	Hours to days	Methane leakage, H2S	Gas-tight membranes, upgrading

Sources

Info-sheet 4: Storage of forest chips (SECURE-BIO-SUPPLY), S. Lieskoski Abo Akademi University, 2025
Safety alert: Risk of carbon monoxide release during the storage and transportation of wood pellets UK Health and Safety Executive, 2012
COFORD Connects note: storage and handling of wood pellets COFORD (Ireland), 2016
Biodiesel Handling and Use Guide, Sixth Edition (NREL/TP-4A00-86939) NREL (renamed National Laboratory of the Rockies in 2025) / US DOE, 2023
Handbook for Handling, Storing, and Dispensing E85 and Other Ethanol Blends US Department of Energy, 2016
Can woodfuel (charcoal and firewood) be modernized in Africa? (Njenga, Mwampamba, Mendum) ICRAF / Global Landscapes Forum, 2018