Page 587 - Environment: The Science Behind the Stories

Scales of production At small scales, farmers, ranch-
ers, or villages can operate modular biopower systems that
use livestock manure to generate electricity. Small household
biodigesters provide portable and decentralized energy pro-
duction for remote rural areas. At large scales, industries such
as the forest products industry are using their waste to gener-
ate power, and industrialized farmers are growing bioenergy
crops. In Sweden, over one-fifth of the nation’s energy supply
now comes from biomass, and biomass provides more fuel for
electricity generation than coal, oil, or natural gas. Pulp mill
liquors are the main source, but solid wood waste, munici-
pal solid waste, and biogas from digestion are all used. In the
United States, several dozen biomass-fueled power plants are
now operating, and several dozen coal-fired plants are experi-
menting with co-firing.

Benefits and drawbacks By enhancing energy effi-
ciency and recycling waste products, biopower helps move
our utilities and industries in a sustainable direction. The U.S.
forest products industry now obtains over half its energy by
FIGURE 20.15 Forestry residues are a major source of mate- combusting the waste it recycles, including woody waste and
rial for biopower in some regions. Here a Swedish logging liquor from pulp mill processing. Biopower also helps miti-
operation gathers woody residue. gate climate change by reducing carbon dioxide emissions,
and capturing landfill gas reduces emissions of methane, a
Bioenergy crops We are beginning to grow certain types potent greenhouse gas. Relative to fossil fuels, biopower also
of plants as crops to generate biopower. These include fast- benefits human health. By replacing coal in co-firing and
growing grasses such as bamboo, fescue, and switchgrass, direct combustion, biopower reduces emissions of sulfur diox-
as well as trees such as specially bred willows and poplars ide because plant matter, unlike coal, contains no appreciable
(FIGURE 20.16). Many of these plants are also being grown to sulfur content. In addition, biomass resources tend to be geo-
produce liquid biofuels. graphically widespread and well dispersed, so using them

Combustion strategies Power plants built to combust
biomass operate like those fired by fossil fuels; combustion FIGURE 20.16 These hybrid poplars, specially bred for fast
heats water, creating steam to turn turbines and generators, growth in dense plantations, are harvested for biopower.
thereby generating electricity. Much of the biopower pro- This is renewable energy, but it has environmental impacts: These
duced so far comes from power plants that use cogenera- monocultural plantations may replace natural systems over large
tion (p. 564) to generate both electricity and heating. These areas and do not function ecologically as forests.
plants are often located where they can take advantage of
forestry waste.
In some coal-fired power plants, wood chips, wood pellets,
or other biomass is introduced with coal into a high-efficiency
boiler in a process called co-firing. We can substitute biomass
for up to 15% of the coal with only minor equipment modi-
fication and no appreciable loss of efficiency. Co-firing is a
relatively easy and inexpensive way for utilities to expand
their use of renewable energy.
We also harness biopower through gasification (p. 555),
in which biomass is vaporized at extremely high temperatures
in the absence of oxygen, creating a gaseous mixture includ-
ing hydrogen, carbon monoxide, carbon dioxide, and meth-
ane. This mixture can generate electricity when used to turn
a gas turbine to propel a generator in a power plant. We can
also treat gas from gasification in various ways to produce
methanol, synthesize a type of diesel fuel, or isolate hydrogen
for use in hydrogen fuel cells (pp. 621–622). An alternative
method of heating biomass in the absence of oxygen results in
pyrolysis (p. 545), which produces a mix of solids, gases, and
liquids. This includes a liquid fuel called pyrolysis oil, which
586 can be burned to generate electricity.

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