Biomass Energy
People have used biomass energy—energy from living things—since the
earliest “cave men” first made wood fires for cooking or keeping warm.
Biomass is organic, meaning it is made of material that comes from living
organisms, such as plants and animals. The most common biomass materials
used for energy are plants, wood, and waste. These are called biomass
feedstocks. Biomass energy can also be a non-renewable energy source.
Biomass contains energy first derived from the sun: Plants absorb the sun’s
energy through photosynthesis, and convert carbon dioxide and water into
nutrients (carbohydrates).
The energy from these organisms can be transformed into usable energy
through direct and indirect means. Biomass can be burned to create heat
(direct), converted into electricity (direct), or processed into biofuel (indirect).
Thermal Conversion
Biomass can be burned by thermal conversion and used for energy. Thermal
conversion involves heating the biomass feedstock in order to burn,
dehydrate, or stabilize it. The most familiar biomass feedstocks for thermal
conversion are raw materials such as municipal solid waste (MSW) and scraps
from paper or lumber mills.
Different types of energy are created through direct firing, co-firing, pyrolysis,
gasification, and anaerobic decomposition.
Before biomass can be burned, however, it must be dried. This chemical
process is called torrefaction. During torrefaction, biomass is heated to about
200° to 320° Celsius (390° to 610° Fahrenheit). The biomass dries out so
completely that it loses the ability to absorb moisture, or rot. It loses about
20% of its original mass, but retains 90% of its energy. The lost energy and
mass can be used to fuel the torrefaction process.
During torrefaction, biomass becomes a dry, blackened material. It is then
compressed into briquettes. Biomass briquettes are very hydrophobic,
meaning they repel water. This makes it possible to store them in moist areas.
The briquettes have high energy density and are easy to burn during direct or
co-firing.
Biomass can also be co-fired, or burned with a fossil fuel. Biomass is most
often co-fired in coal plants. Co-firing eliminates the need for new factories for
processing biomass. Co-firing also eases the demand for coal. This reduces the
amount of carbon dioxide and other greenhouse gases released by burning
fossil fuels.
Pyrolysis
Pyrolysis is a related method of heating biomass. During pyrolysis,
biomass is heated to 200° to 300° C (390° to 570° F) without the presence
of oxygen. This keeps it from combusting and causes the biomass to be
chemically altered.
Pyrolysis produces a dark liquid called pyrolysis oil, a synthetic gas called
syngas, and a solid residue called biochar. All of these components can be
used for energy.
Pyrolysis oil, sometimes called bio-oil or biocrude, is a type of tar. It can be
combusted to generate electricity and is also used as a component in other
fuels and plastics. Scientists and engineers are studying pyrolysis oil as a
possible alternative to petroleum.
Syngas can be converted into fuel (such as synthetic natural gas). It can also
be converted into methane and used as a replacement for natural gas.
Incineration
Incineration is a thermal waste treatment technique that can be understood as
a controlled combustion process with the primary objective of volume
reduction and energy recovery from the waste stream. Incineration is the most
popular WTE technique, whereby heat produced from combustion can be
recovered and converted to electric power. The organic content of waste is
combusted and heat is produced, whereas the inorganic content contributes
to the formation of ash. End products of incineration include ash, heat, and
combustion gases. Fig. shows the overall process of incineration.
Gasification
Biomass can also be directly converted to energy through gasification. During
the gasification process, a biomass feedstock (usually MSW) is heated to more
than 700° C (1,300° F) with a controlled amount of oxygen. The molecules
break down, and produce syngas and slag.
Syngas is a combination of hydrogen and carbon monoxide. During
gasification, syngas is cleaned of sulfur, particulates, mercury, and other
pollutants. The clean syngas can be combusted for heat or electricity, or
processed into transportation biofuels, chemicals, and fertilizers.
Slag forms as a glassy, molten liquid. It can be used to make shingles, cement,
or asphalt.
