Biomass - Energy Policy Analysis: A Conceptual Framework - page 164

Air Emissions
Combustion of biomass produces carbon monoxide, nitrogen oxides, and particulates such as soot
and ash. The amount of pollution emitted per unit of energy generated varies widely by technol-
ogy, with wood-burning stoves and fireplaces generally the worst offenders. Newer, enclosed
fireplaces and wood stoves pollute much less than older, open fireplaces because they are more
efficient. Emissions from biomass power plants are generally similar to emissions from coal-fired
power plants, except biomass facilities produce little sulfur dioxide or toxic metals because most
biomass resources and natural gas contain far less sulfur and heavy metals than coal (Bain et al.
2003). Their most serious problem is particulate emissions, which must be controlled with special
air pollution control equipment (Brower 1992, 106).
Facilities that burn raw municipal waste present special pollution-control problems. Municipal
garbage often contains heavy metals, chlorinated compounds, and plastics, which generate harmful
emissions. This problem is much less severe in facilities burning refuse-derived fuel (RDF) from
which most inorganic material has been removed, leaving mostly shredded paper and paperboard
(Brower 1992, 107). However, RDF has considerably lower heat content than waste which has
not had high-Btu inorganic materials such as plastics removed. In general, the more one recycles
from municipal solid waste, the lower the heat content of remaining fuel.
Using methanol and ethanol distilled from biomass as vehicle fuels instead of conventional
gasoline could substantially reduce air pollution from automobiles. Both methanol and ethanol
evaporate more slowly than gasoline, reducing evaporative emissions of volatile organic com-
pounds (VOCs), which react with heat and sunlight to generate ground-level ozone, a compo-
nent of smog. In cars specifically designed to burn pure methanol or ethanol, VOC emissions
from the tailpipe could be reduced 85 to 95 percent, while carbon monoxide emissions could
be reduced 30 to 90 percent. Using blends of gasoline and ethanol reduces most air emissions
by proportionate amounts. However, emissions of nitrogen oxides, a source of acid precipita-
tion, would not change significantly compared to gasoline-powered vehicles without ethanol
(Brower 1992, 107).
Some studies have indicated that use of fuel alcohol increases emissions of formaldehyde and
other aldehydes, which are potential carcinogens. Other studies counter that these results consider
only tailpipe emissions, where VOCs, another significant pathway of aldehyde formation, are much
lower in alcohol-burning vehicles (Alson 1990). On balance, methanol vehicles would therefore
decrease ozone levels (Brower 2002). Thermochemical conversion of biomass using gasification,
liquefaction, and pyrolysis can produce carbon monoxide and VOCs. Standard pollution control
technology can neutralize or remove most of these before release into the atmosphere. Pyrolysis
and liquefaction also generate liquid and solid hazardous wastes such as residual tars, catalysts,
acids, char, and ash that must be disposed of properly, at some expense. More research is needed
on the environmental costs of controlling these air pollutants from biomass conversion (Brower
1992, 100, 108).
Greenhouse Gases
A major benefit of substituting biomass for fossil fuels is that, if done in a sustainable fashion, it
would greatly reduce emissions of greenhouse gases. The amount of carbon dioxide released when
biomass is burned is very nearly the same as the amount required to replenish the plants grown to
produce the biomass (Brower 2002). Thus, in a sustainable fuel cycle, there would be no net emis-
sions of carbon dioxide, although some fossil-fuel inputs may be required for planting, harvesting,