Biomass: How energy resides

Why wet wood carries so little energy per kilo

What this tab covers The physical form in which the energy is held or carried, where it comes from, the chain of transformations that put it there, and how concentrated it is.

Chemical energy in the carbon-hydrogen bonds of plant matter, made by photosynthesis from sunlight, carbon dioxide and water. It is a stock (standing forests and soils) that is refilled by a flow (net primary production).

Form Chemical energy in carbohydrates, lignin and oils Origin Indirect solar

How it got there

Fusion in the Sun Sunlight Photosynthesis in leaves and algae Sugars, cellulose, lignin and oils Wood, crops, residues, dung

How concentrated it is

Measure	Value	Note
Heating value of oven-dry wood	18-20 MJ/kg⁷	Falls to about 8 MJ/kg for chips or sawdust at 50% moisture (FAO: 8.2-8.3).
Average land productivity (ice-free land)	426 g of carbon per m2 per year¹	Oceans average 140 g C/m2/yr.
Implied power density of land plant growth	about 0.5 W/m2³	Calculated here: 426 g C/m2/yr at about 39 MJ per kg of carbon (2,200 EJ of land NPP divided by 56.4 Pg C).

Plants capture a small fraction of the sunlight falling on them and store it in sugars, cellulose, lignin and oils. Net primary production (NPP) is what remains after the plants' own respiration: the new matter available each year to animals, decomposers and people. Burning or digesting that matter releases the energy and returns the carbon to the air.

Biomass is energy-dense once harvested and dried (18-20 MJ per kilogram of dry wood), but the flow that creates it is thin: averaged over land it amounts to roughly half a watt per square metre, against hundreds of watts of incoming sunlight. That is why bioenergy needs very large areas, and why water content, collection and transport often decide its usefulness more than the energy in the plant itself.

Biomass: How energy resides

How it got there

How concentrated it is

Sources