Bio1110 Chapter 29 Biofuels
  1. Biomass can be converted into biofuels such as bioethanol and biodiesel as a renewable alternative to fossil fuels.
    • Biomass is organic matter extracted from living organisms.

      Burning biomass crops instead of fossil fuels reduces net CO2 emissions, but deprives soil of nutrients that now do not cycle back into the ground.

      1. Firewood can be cut for burning; care should be taken that harvesting is sustainable.

      2. Waste such as cornstalks or manure can be burned.

      3. Composite "briquettes" made from compressed biomass provide portable fuel.

    • Sugar-rich crops such as corn and sugarcane can be fermented to produce ethanol that can be burned in car engines

      Corn is a major source of grain ethanol in the U.S.

      Other crops such as switchgrass and poplars, as well as agricultural wastes, are low in sugar content; their cellulose can be processed for cellulosic ethanol.

    • Grain ethanol is obtained from sugar-rich feedstock such as corn kernels.

      The kernels are ground up and treated with enzymes to break down starch into sugar subunits.

      After heating to kill bacteria, yeast is added to ferment the sugars, producing ethanol and CO2 as a byproduct.

      Distillation yields concentrated ethanol for fuel; the residues can be processed as animal feed.

    • Cellulosic ethanol is obtained from sugar-poor material such as switchgrass.

      The tough cellulose is harder to break down.

      The ground material is steamed and treated with special enzymes to break down cellulose into sugar subunits.

      Yeast is added to ferment the sugars, producing ethanol and CO2 as a byproduct.

      Another byproduct is indigestible lignin; this must be separated and can be used as biomass fuel.

    • Biodiesel can be made from waste oils and fats, or extracted from oil crops like sunflowers.

      Growing crops for this fuel have similar environmental impacts as growing corn for ethanol.

    • Feedstock for biodiesel can be vegetable oil, animal fat, or recycled grease. After filtering, a mixture of lye and methanol is added. Chemical reactions yield biodiesel and glycerin as a commercial byproduct; leftover methanol is reused. Washing with water removes impurities. The biodiesel can power diesel engines.
  2. Some of the trade-offs of current biofuel crops may be alleviated by LIHD crops.
    • • Growing corn and soybean monocultures requires energy for pesticides, fertilizers, and processing.

      One way to improve efficiency is to plant a diverse mix of low-input, high-diversity (LIHD) grassland plants.

      These non-food plants can be grown on degraded soil with minimal input of energy, potentially yielding more net energy output with less greenhouse gas emissions.

  3. Algae can be a very clean source for biofuel if costs can be lowered.
    • Algae produce oils that can be converted to biodiesel, and their carbohydrates can be fermented to make ethanol.

      Algae grow fast and can use wastewater, and has the highest oil production potential per acre.

      If production costs can be reduced, this technology may be a good competitor to fossil fuels.