Page 239 - Synthetic Fuels Handbook

FUELS FROM BIOMASS 225

postconsumer waste stream; it seems appropriate for them to be included in the tertiary
biomass category. Vegetable oils derived from processing of plant components and used
directly for bioenergy (e.g., soybean oil used in biodiesel) would be a secondary bio-
mass resource, though amounts being used for bioenergy are most likely to be tracked
together with fats, greases, and waste oils.
Many different types of biomass can be grown for the express purpose of energy produc-
tion. Crops that have been used for energy include: sugar cane, corn, sugar beets, grains,
elephant grass, kelp (seaweed), and many others. There are two main factors which deter-
mine whether a crop is suitable for energy use. Good energy crops have a very high yield
of dry material per unit of land (dry metric tons/hectare). A high yield reduces land require-
ments and lowers the cost of producing energy from biomass. Similarly, the amount of
energy which can be produced from a biomass crop must be less than the amount of energy
required to grow the crop. In some circumstances like the heavily mechanized corn farms
in the U.S. Midwest, the amount of ethanol which can be recovered from the corn is barely
larger than the fuel required for tractors, fertilizers, and processing.
The simplest, cheapest, and most common method of obtaining energy from biomass
is direct combustion. Any organic material with a water content that will not interfere with
sustained combustion of the material can be burned to produce energy. The heat of com-
bustion can be used to provide space or process heat, water heating, or, through the use of
a steam turbine, electricity. In the developing world, many types of biomass such as dung
and agricultural wastes are burned for cooking and heating.
Thus, almost all crops, whether grown for food, animal feed, fiber, or any other purpose,
result in some form of organic residues after their primary use has been fulfilled. These
organic residues, as well as animal wastes (excrement) can be used for energy production
through direct combustion or biochemical conversion. Current worldwide production of
crop residues is very large; but an increased scale of use for fuel may have significant envi-
ronmental impacts, the most serious being those of lost soil fertility and soil erosion.
Most crop residues are returned to the soil, and the humus resulting from their decom-
position helps maintain soil nutrients, soil porosity, water infiltration, and storage, as well
as reducing soil erosion. Crop residues typically contain 40 percent of the nitrogen (N), 80
percent of the potassium (K), and 10 percent of the phosphorous (P) applied to the soil in the
form of fertilizer. If these residues are subjected to direct combustion for energy, only a small
percentage of the nutrients are left in the ash. Similarly, soil erosion will increase. Estimates for
the United States indicate that 22 percent of crop residues could be removed without causing
substantial soil erosion, providing energy equivalent to 5 percent of U.S. needs.
Biomass is a renewable energy source, unlike the fossil fuel resources (petroleum, coal,
and natural gas) and, like the fossil fuels, biomass is a form of stored solar energy. The
energy of the sun is captured through the process of photosynthesis in growing plants. One
advantage of biofuel in comparison to most other fuel types is it is biodegradable, and thus
relatively harmless to the environment if spilled.
Many different biomass feedstocks can be used to produce liquid fuels (Worldwatch
Institute, 2006). They include crops specifically grown for bioenergy, and various agricul-
tural residues, wood residues, and waste streams. Their costs and availability vary widely.
Collection and transportation costs are often critical.
Sugarcane, sugar beet, corn, and sweet sorghum are agricultural crops presently grown
commercially for both carbohydrate production and animal feeds. Sugarcane, corn, and
sweet sorghum are efficient at trapping solar energy and use specific biochemical pathways
to recycle and trap carbon dioxide that is lost through photorespiration. Sugar beets are
efficient because they store their carbohydrate in the ground. Sugarcane was the basis for
the World’s first renewable biofuel program in Brazil (Bourne, 2007). Corn is the basis for
the present renewable ethanol fuel industry in the United States.