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NC Woody Biomass
“Nature’s renewable energy!”
http://www.ces.ncsu.edu/fore
stry/biomass.html
Extension Forestry
Campus Box 8008
NC State University
Raleigh, NC 27695-8008
Air Quality and North Carolina
Wood Energy
High fossil fuel costs and concerns about climate change have
thrust low-cost, home-grown renewable fuels, like wood, into
the energy spotlight. The enactment of North Carolina’s
Renewable Energy Portfolio has increased the interest and
opportunities to burn wood fuel to make electricity, heat, and
steam. This factsheet reviews the air quality impacts of
supplementing fossil fuels with woody biomass and current
regulation on emissions from wood-fired plants.
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Burning wood in a power
plant or industrial operation
is among the cleanest solid
energy sources currently
available. Complete and high
temperature combustion
under controlled conditions
ensures low emissions from
wood energy. Regulations
require the monitoring of
stack gases and particulate
emissions to limit and reduce
the amount of pollution.
Some people may envision
smoky wildfires, wood stoves
and residential fireplaces
when they think of wood
energy. But today’s wood
burning facilities are light
years ahead of the 1970’s
version wood stoves,
standard fireplaces and wood
fires which can pose
problems for local air quality.
Variations in wood
combustion efficiency result
from age of equipment,
design and variations in
moisture, fuel size,
combustion temperature and
air mixture. When wood
burns incompletely or slowly
it emits smoke, ash, odors
and windborne particulates
that can impact regional air
quality and human health.
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Industry is by far the largest
user of wood energy,
including power plants,
factories, industrial facilities
(especially paper mills).
Fossil fuel conversion to
wood energy has been
motivated by cost-savings
and availability of woody
residues. Most wood-processing
facilities fully
utilize their internal residue
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stream of wood sources for steam,
heat and electric energy production.
Wood’s cost-saving benefits may get an
even greater boost with renewable
energy and climate change policy that
avoids traditional fossil fuel energy
sources. Regardless of the motivation,
wood is a valuable fuel and feedstock.
Current and future users will be
motivated to fully combust and extract
the maximum amount of energy (and
value), while minimizing pollution.
Clean Air Standards:
All industrial fuel users must comply
with clean air standards and
regulations regardless of the fuel type.
Industrial combustion facilities are
regulated by the United States
Environmental Protection Agency and
NC Division of Air Quality. These
agencies manage air quality, through
regulation, permits, and enforcement
to ensure that new or existing
facilities minimize air quality
degradation. Regulations apply to all
major combustion facilities, including
specific emissions related to wood
fuel combustion. It is important to
note that emission standards are
strictest for larger-scale facilities.
Figure 1 illustrates that a wood-fired
power plant with modern emission
control equipment emits 1/500 to
1/280 of the emissions of a wood
stove, for the same amount of wood
fuel input.
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There are several air pollutants that
any industrial facility must control
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Figure 1. Total Particulate Matter from Various Wood-combustion Systems (lbs./ million
Btu output)
Adapted from http://www.biomasscenter.org/information/emissions.html
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regardless of fuel used. The most
common regulated pollutants for oil,
gas, coal and wood are nitrous oxides
(NOx), carbon monoxide (CO, a product
of incomplete combustion), sulfur
dioxide (SO2), mercury (Hg), and
particulate matter (very small, airborne
particles). Greenhouse gases too have
become a significant concern in recent
years. In this section we will discuss
several fuel sources in the context of
each of these pollutants. The following
table provides a comparison among
various fuel emissions.
Natural gas is considered the cleanest
industrial fuel because it can be burned
to generate low levels of NOx, CO, SO2,
and particulate matter, in comparison
to other fossil fuels. However, natural
gas has become very expensive and
future supplies are expected to be
tight.
The use of oil as industrial fuel in the
United States has declined because of
very high cost; it is included here for
comparison sake. Oil is available in
different grades, and while the NOx and
CO are usually low, sulfur levels in oil
can be high, requiring methods to
control SO2.
Wood as a supplement to coal shows
promise. NOx emissions from wood are
generally less than from coal, with
studies reporting that mixing wood with
coal reduced overall NOx from the
facility. SO2 is much lower with wood
than coal. Depending upon where the
coal comes from, coal can contain 75
times more sulfur than wood on a per
Table 1. Particulate Matter Emission Limit for North Carolina Wood-burning Indirect Heat
Exchangers (lbs/ Million Btu by Output)
Maximum Heat Output in Million Btu
/Hour
Allowable Emission Limit for
Particulate Matter in lbs/ Million Btu
Up to and Including 10 .70
100 .41
1,000 .25
10,000 and Greater .15
(Source:15A NCAC 02D.0504, History Note: G.S. 143-215.3(a)(1); 143-215.107(a)(5))
Table 2. Comparing Uncontrolled Emissions from Different Fuel Sources U.S. EPA 2006
Fuel Type
NOx
SO2
Hg
Particulate
Matter
Greenhouse
Gases
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High
Medium Low High
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unit heat value basis. Therefore, while
sulfur removal is a large part of the
effort in coal-fired facilities, wood
energy facilities generally require little
effort is to meet SO2 emissions
standards. There is technology and
equipment available to effectively
control the NOx, CO and SO2 from both
coal and wood energy plants.
Wood combustion for energy results in
negligible mercury emissions when
compared to coal sources.
Coal and wood present additional air
quality impacts compared to oil or
natural gas. As solid fuels, coal and
wood vary greatly in quality, energy
value, moisture and ash. Variation in
fuel quality makes handling the fuel,
ash and emissions more difficult.
The control of fine particles is required
for all solid fuel energy facilities as they
are swept up in the combustion air
stream and out the stack. While fine
particles may not be toxic, per se,
breathing extremely fine particulate
matter has deleterious health impacts.
Wood’s distinct advantage is that it
contains about 90% less ash than coal.
Effective methods of control of
particulate matter have been developed
for both coal and wood combustion
facilities that remove most particulate
matter from the exhaust air. The
particulate matter from either a coal
plant or a wood plant is higher than a
natural gas plant, but particulates from
both types of facilities can be controlled
to minimize air quality impacts using
best available control technology
(BACT).
Particulates and Health
Impacts from Wood
Combustion
Particulate matter is the solid matter or
very fine droplets that result from
combustion or through secondary
physical and chemical conversions in
the atmospheric. Visible at larger sizes
and invisible in its smallest form,
particulates are the air pollutant of
greatest concern when considering
wood combustion. Particulate matter is
a generic term and is defined without
regard to chemical form in the Clean Air
Act (Section 109). Small particulates
range in size from 10 micrometers to
2.5 microns (PM10, PM2.5) and less in
diameter. The smallest particles cause
greatest human health concern, since
small particles can lodge deep within
the lungs when inhaled and remain air-born
for longer periods and distances.
Many wood-fired boilers employ a “tall”
stack of sufficient height to disperse
emissions and reduce ground-level
concentrations of particulate matter
(and other pollutants) to mandated
levels. Elaborate cyclonic and
electrostatic precipitators ( dry and
wet) scrub the exhaust and collect
particulates in the modern wood-fired
operations. Therefore, most references
to older wood burning systems may
overstate wood combustion emissions
relative to corresponding gas and oil
systems of the same period.
Specific wood-fired operations use the
best available control technology
(BACT) to ensure that they meet
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emission limits, prevent pollutant and
associated deterioration of air quality.
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Wood energy is the only traditional
industrial fuel that does not
substantially add to atmospheric carbon
dioxide in the long term, if wood supply
comes from sustainable sources and
forest acreage is maintained. In the
context of greenhouse gases, wood
presents an advantage over all of the
other fuel sources discussed in this
article. While the combustion of wood
and fossil fuels result in carbon dioxide
and water vapor, over time the carbon
stored in solid wood, roots, limbs and
soil can offset the carbon release in
energy production. With wood
combustion there is a release of carbon
to the atmosphere, which is later taken
up in subsequent tree growth.
Carbon dioxide release from fossil fuel
combustion—oil, natural gas and coal—
represent ancient carbon deposits
(millions of year old), the carbon from
wood combustion differs in that it is
derived from recently living plant
matter. Therefore, the carbon emitted
from burning wood is reabsorbed as
new trees grow. As long as trees are
replanted at least as fast as wood is
burned, wood-to-energy power plants
will not significantly increase
greenhouse gas levels. Fossil fuels
emissions, in contrast, cannot be
compensated by the same tree growth
since there is not enough land to plant
in trees necessary to compensate for
the worldwide burning of fossil fuels.
Since fuel and transportation costs
comprise the largest operational costs
associated with combustion-based
steam electrical generation plants,
wood’s rather low-energy content
(compared to fossil fuels) puts it at a
disadvantage in competitive markets.
Admittedly, there is not enough wood
supply to supply the total energy needs
of society, so wood will likely be an
interim solution as other renewable
technologies and low-carbon energy
sources become economically viable.
Adapted from: 2007. Schroeder, R. and
M.A. Monroe. Impacts on Air
Quality.Wood to Energy fact sheet. 3 p.
http://edis.ifas.ufl.edu/pdffiles/FR/FR19
000.pdf
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www.ces.ncsu.edu/forestry/biomass.html
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Distributed in furtherance of the acts of Congress of May 8 and June 30, 1914. North Carolina State University and North Carolina A&T State University commit themselves to positive
action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, veteran status or disability. In addition, the two Universities welcome all persons
without regard to sexual orientation. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
Distributed in furtherance of the acts of Congress of May 8 and June 30, 1914. North Carolina State University and North Carolina A&T State University commit themselves
to positive action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, veteran status or disability. In addition, the two Universities
welcome all persons without regard to sexual orientation. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local
governments cooperating.
copies of this public document were printed at a cost of or per copy.
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Prepared by
Mark Megalos
Extension Forestry Specialist
NC State University
WB-0010/2009
