Aim for FiveEnvironmental Science

Unit 6 · Topic 6.7

6.7 Energy from Biomass

Biomass is fuel from living or recently living things, from firewood and dung to ethanol and biodiesel. It is renewable if managed well and widely used, but burning it pollutes the air, overharvesting causes deforestation, and some biofuels, especially corn ethanol, return only a little more energy than they take to make.

Key terms

  • biomass
  • ethanol
  • biodiesel
  • deforestation
  • particulate matter

Solid biomass

Solid biomass includes wood, charcoal (wood heated with little oxygen so it burns hotter and cleaner), crop waste such as corn stalks, and dried animal dung. Billions of people, mostly in developing countries, cook and heat with these fuels because they are cheap or free.

Burning biomass releases CO₂, carbon monoxide (CO), nitrogen oxides, volatile organic compounds and particulate matter (tiny bits of soot and ash). When it is burned indoors on open fires or simple stoves, the smoke causes serious lung and heart disease (topic 7.5).

Gathering wood faster than forests regrow leads to deforestation, the clearing of forest. That causes soil erosion and habitat loss and removes trees that would otherwise absorb CO₂.

Is biomass carbon neutral?

Supporters call biomass carbon neutral: the CO₂ released when a plant burns was absorbed from the air while it grew, and new plants can absorb it again. That is only true if the land is replanted and the regrowth actually happens. It also ignores fuel used to harvest, process and ship the biomass. Clearing a mature forest for fuel releases carbon far faster than regrowth can take it back, so in practice biomass is not automatically carbon neutral.

Liquid biofuels: ethanol and biodiesel

Ethanol is an alcohol made by fermenting sugars, mainly from corn in the United States and sugarcane in Brazil. Yeast eats the sugar and makes ethanol, which is then distilled. Most US gasoline contains about 10 percent ethanol (sold as E10), and flex-fuel vehicles can run on E85.

Biodiesel is made from vegetable oils (such as soybean oil), animal fats or used cooking oil, and can be blended with or replace regular diesel.

Biofuels can replace some petroleum and use a renewable feedstock, but they have real costs:

  • Low net energy for corn ethanol: growing, fertilizing, harvesting and distilling the corn takes almost as much energy as the ethanol delivers. Sugarcane ethanol does much better.
  • Land and food: cropland used for fuel is not used for food, which can raise food prices, and new cropland may come from clearing forests or grasslands.
  • Farming impacts: fertilizer runoff (which feeds algal blooms, topic 8.5), pesticide use, soil erosion and heavy irrigation.
  • Ethanol holds about a third less energy per gallon than gasoline, so cars get fewer miles per gallon.

Worked examples

Try each one yourself first, then open the solution.

  1. Example 1Calculator allowed

    Net energy of a biofuel

    Making 100 units of energy in the form of corn ethanol requires 77 units of energy (fuel for tractors, fertilizer production, transport and distilling). (a) What is the net energy gained? (b) What is the energy return ratio (energy out ÷ energy in)? (c) A sugarcane ethanol process returns 8 units for every 1 unit put in. Which is the better energy investment?

    Show the solution
    1. Step 1: (a) Net energy = energy out − energy in = 100 − 77 = 23 units.
    2. Step 2: (b) Energy return ratio = 100 ÷ 77 ≈ 1.3. For every unit of energy invested, you get back about 1.3 units.
    3. Step 3: (c) Sugarcane's ratio of 8 is far higher than corn's 1.3, so sugarcane ethanol delivers much more usable energy per unit invested.

    Answer: (a) 23 units. (b) About 1.3. (c) Sugarcane ethanol, with a ratio of about 8 compared with 1.3.

  2. Example 2

    Reasoning chain: firewood and erosion

    A rural region switches from kerosene to firewood for cooking because kerosene prices rose. Describe a chain of effects on the local environment over the next decade.

    Show the solution
    1. Step 1: Demand for wood rises, so people cut trees faster than they regrow.
    2. Step 2: Tree cover falls, leaving soil bare. Roots no longer hold the soil, and rain hits the ground directly.
    3. Step 3: Soil erodes into streams, which raises turbidity (cloudiness) and buries fish spawning areas, while the land loses fertile topsoil.
    4. Step 4: People must walk farther for wood, and indoor smoke from wood fires raises rates of respiratory disease.

    Answer: Overharvesting causes deforestation, which leads to soil erosion and stream sedimentation, plus more indoor air pollution and health problems.

Common mistakes

  • Calling biomass automatically carbon neutral. It is only close to neutral when regrowth keeps up and processing energy is small.
  • Forgetting that corn ethanol has a low energy return. Its main drawbacks are low net energy, food-versus-fuel competition and farming pollution.
  • Leaving particulate matter and carbon monoxide off the list of biomass pollutants. They are the main health threats from indoor wood and dung fires.

On the exam

  • Expect questions asking for one advantage and one disadvantage of using corn for ethanol. Pair 'renewable, reduces gasoline use' with 'low net energy' or 'fertilizer runoff and food competition'.
  • Net energy and energy-return calculations can appear in the math free-response question; label which number is input and which is output.

Connected topics

Videos

Check yourself

4 questions on 6.7 Energy from Biomass. Pick an answer to see if you got it, and why.

Question 1 of 4Calculator allowed

Suppose making corn ethanol returns 1.3 units of energy for every 1 unit of energy put in (for fertilizer, farm machines, transport and distilling). If 1,000 MJ of energy is invested, what is the net energy gained?

Question 2 of 4

In many rural homes, families cook over open wood or dung fires indoors. Which health risk is most directly linked to this practice?

Question 3 of 4

Burning wood for energy is sometimes called "carbon neutral." Under which condition is this claim least accurate?

Question 4 of 4

A government requires that much more gasoline be blended with ethanol made from corn. Which environmental effect is most likely?

0 of 4 answered