AP® Environmental Science · Unit 6 · Topic 6.7
Energy from Biomass
Biomass energy comes from burning or fermenting recently living material like wood or corn. It counts as renewable because the plants can regrow on a human timescale, but whether it's actually carbon neutral depends on replanting and net energy math.
Biomass is organic matter from plants or animals burned or converted into fuel: wood, crop waste, animal manure, or corn turned into ethanol. Coal is also plant matter originally, but it took millions of years to form, so once burned it is gone for any timescale that matters to people. A tree can regrow in decades. That difference in regrowth speed, not the carbon atoms themselves, is why wood counts as renewable and coal does not.
Turning corn into ethanol takes energy at every step: planting, fertilizing, irrigating, harvesting, and fermenting the kernels into alcohol, then distilling it. Each of those steps burns fuel or electricity, so you have to spend energy to get energy. Net energy is what is left after subtracting that investment from what the fuel delivers. If a process returns 1.3 units of energy for every 1 unit invested, the net energy gained is 0.3 units of energy for every 1 unit invested — the extra fraction beyond break even. Invest 10 units and you net 3 units back, on top of recovering the original 10.
Biomass is often called carbon neutral because growing plants absorb CO2, balancing out what burning them releases. But that balance only holds if the harvested plants are actually replanted and allowed to regrow on a matching timescale. Burning a tree releases its stored carbon in minutes; regrowing a replacement forest to reabsorb that same carbon takes decades. If land is cleared and not replanted, or replanted too slowly, the carbon dioxide stays in the atmosphere and the process behaves like fossil fuel burning. This connects biomass to the unit's land use theme: devoting corn acreage to ethanol competes with food production, tightening supply and pushing up food prices.
Key terms
- Biomass
- Organic matter from recently living plants or animals used as a fuel source.
- Net energy
- The energy delivered by a fuel minus the energy invested to produce it.
- Carbon neutral
- A process where carbon released equals carbon reabsorbed, keeping atmospheric CO2 unchanged overall.
- Energy return ratio
- The amount of energy a process returns per unit of energy invested, such as 1.3 units out for 1 unit in.
Watch it explained
- How it worksWalk me through a corn kernel becoming ethanol and the energy spent along the way.
- Worked exampleFind the net energy of an ethanol process that returns 1.3 units for every 1 unit invested.
- Common mistakeWhy isn't biomass automatically carbon neutral?
- Going furtherWhat happens to food prices and land use when a quarter of a corn crop goes to fuel?
Check yourself
End of topic.