United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-98/013
4>EPA Project Summary
Products of Incomplete
Combustion from Direct Burning
Of Pentachlorophenol-treated
Wood Wastes
S.Y. Lee and J.C. Valenti
The report gives results of a study to
identify potential air pollution problems
from the combustion of waste wood
treated with pentachlorophenol
preservative for energy production in a
boiler. The study emphasized the
characterization of the products of
incomplete combustion (PICs) in the
combustion flue gas. The methodology
used was to compare the flue gas
concentrations of PICs prior to the air
pollution control device of a pilot-scale
combustor burning untreated wood and
burning wood treated with
pentachlorophenol preservative. The
tests showed that combustion is an
effective method of destroying the
pentachlorophenol in the treated wood,
with destruction efficiencies higher
than 99.99%. Differences in the flue gas
concentrations of various PICs from the
combustion of untreated and treated
wood fuels have been noted. The data
do not enable identification of the exact
cause of these differences in flue gas
concentrations. These differences are
possibly caused by the significantly
different chlorine content of the two
fuels. The difference in flue gas flow
rate required for the combustion of
these two fuels with different
combustion characteristics (moisture
content and heating value) may also
cause the differences in PIC formation
rates. These data are strongly
influenced by the design, configuration,
and operation of the combustor system
and may not be quantitatively
comparable to other combustors.
This Project Summary was developed
by EPA's National Risk Management
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
The use of waste wood as fuel for
producing energy is a promising
supplement to fossil fuels for many regions
of the country. In addition to recovering
energy and conserving landfill space,
burning waste wood fuels also mitigates
global warming by replacing fossil fuel
use. However, the environmental
consequences resulting from emissions
generated by combustion of waste wood
which contains paints, resins, or
preservatives are not well understood.
Combustion of these materials could
produce potentially hazardous products of
incomplete combustion (PIC) emissions
such as dioxins and furans.
Characterization of PICs from the
combustion of waste wood treated with
pentachlorophenol is reported in this study.
Utility poles and crossbars are typically
treated with a preservative such as
pentachlorophenol in orderto prolong their
service life. They are disposed of by land
filling after being taken out of service.
Burning such wood waste in boilers for
steam generation becomes an increasingly
-------
attractive waste management alternative
as it contains substantial energy values
and reduces land filling costs. Pilot-scale
combustion tests were conducted under
well controlled conditions in a 0.58 MW(2
million Btu/hr) combustor to characterize
PICs from burning pentrachlorophenol-
treated wood. The methodology used was
a side-by-side characterization of flue
gases from burning treated wood and
similar untreated wood. The gases were
sampled prior to the combustor's air
pollution control equipment to avoid the
effects of the control equipment. Sampling
and analyses for a wide variety of PICs,
including volatile organic compounds
(VOCs), semivolatile organic compounds
(SVOCs), dioxins, and furans, were
performed to assess the effect of
pentachlorophenol preservative present in
wood on PIC emissions.
Experimental
The combustion testing was conducted
using a multifuel combustor (MFC)
installed in the Environmental Protection
Agency's research facility located at
Research Triangle Park, North Carolina.
The MFC is a pilot-scale stoker
combustion system with 0.58 MW (2
million Btu/hr) maximum thermal output
which is sufficient for simulating the full
range of conditions that are typically
encountered in operating combustors. The
MFC facility contains a fuel storage and
feed system, a stoker combustor section,
radiant heating sections, convective
heating sections, a fabric filter baghouse,
and a caustic wet scrubber. The MFC is
capable of burning a wide variety of solid
fuels including municipal solid waste,
refuse derived fuel, biomass fuel, and coal
singly or in combination. The combustor is
constructed with modular sections, which
provides maximum flexibility for modifying
the combustor for research purposes.
Access ports are installed throughout the
entire combustor for sampling and visual
observation. The combustor was operated
in a spreader stoker firing mode during the
test period.
Two different wood fuels, untreated
wood and treated wood, were obtained
from a plant where wood poles are treated
with pentachlorophenol. The untreated
wood was pine poles that are typically
treated in the plant. The treated wood was
a mixture of recently treated poles, mostly
small pieces cut from the ends to meet
customers' length specifications, and aged
treated poles taken out of service. Both
treated and untreated poles were ground
into chips less than 7.6 cm (3 in.) in
diameter. The two fuels were characterized
by proximate, ultimate, and chlorine
analyses. The treated wood fuel is drier
with higher heating value, compared to the
untreated wood. The chlorine content of
the treated wood fuel is 10 times higher
than that of the untreated wood.
The only parameter investigated was the
difference in emissions when burning
treated vs. untreated wood fuels under
similar combustion conditions; no attempt
was made to conduct parametric tests for
this study. In general, the operating
conditions of the test were considered
optimal when the fuel was burning at the
designed heat release rate with nominally
160% excess air and a low level (< 100
ppm) of carbon monoxide (CO) emissions
in the flue gas.
The amount of fuel burned was not
measured continuously during a test. It
was estimated by weighing the fuel before
it was dumped into the loading hopper of
the MFC and measuring the time required
for its complete consumption to calculate
an average feed rate. The average feed
rate was used to estimate the amount of
fuel burned during a timed emissions
sampling period. The MFC is equipped
with continuous emission monitors (CEMs)
-for measuring oxygen (O2), CO, carbon
dioxide (CO2), nitrogen oxide (NO), sulfur
dioxide (SO2), total hydrocarbon (THC),
and hydrogen chloride (HCI) - and
equipment for continuously monitoring
process temperatures and pressures.
The emissions sampling point was atthe
duct which connects the convective section
of the furnace to the baghouse. The
selection of a sampling point prior to any
gas cleaning device was made to ensure
that any difference in flue gas pollutant
concentrations generated by combustion of
treated and untreated wood fuels would be
observed. Emission measurements made
after the flue gas cleaning device would be
expected to maskthis difference and would
reflect the effectiveness of the devices (i.e.,
baghouse and caustic scrubber) used.
The sampling and analysis program
focused primarily on organic emissions,
particularly those which are considered
potentially hazardous, from the
combustion of wood fuels. Sampling and
analytical protocols used were based on
existing EPA methods for measuring
organic constituents in flue gases.
Results and Discussion
Six combustion tests, three each for both
the untreated and the treated wood fuels,
were performed under similar combustor
operating conditions. The untreated wood
tests were performed first, followed by the
treated wood tests to avoid cross
contamination. Triplicate runs were made
to determine experimental repeatability.
Since it was not practical to measure fuel
feed rate continuously, the tests were
performed under constant excess air level
(nominally 160% excess air) and a
constant combustion gas temperature
(850°C) measured by a thermocouple
located 152 cm (60 in.) above the grate.
The heat release rate was set at 80% of
the maximum designed output of the MFC.
The fuel feed rate was adjusted manually
throughout all tests to maintain a constant
temperature of 850°C above the stoker
grate and keep CO emissions below 100
ppm, which were considered optimum
conditions for the tests.
It was difficult to produce identical
combustion conditions for all the tests,
mainly because the two wood fuels are not
homogeneous. The treated wood fuel
contains aged wood, which is drier, and
has 30% higher heating value than the
untreated wood fuel. Uneven fuel feed
rates resulting from the fairly frequent
manual adjustment of the fuel feeder could
have caused fluctuations in air-to-fuel ratio
and momentary excursions of CO
emissions, especially when burning
untreated wood. CEM data for CO and
CO2 and visual observations of fuel flow
into the stoker suggest that small
excursions occurred during the tests. The
nonhomogeneous nature of the fuels,
coupled with the practical difficulties to
burn such fuels under well defined
conditions, made the accurate estimation
of heat release rates difficult. The
measured combustion gas temperature
also may not have been always truly
representative of average temperature
across the combustor in a given axial
position due to a non-uniform gas
temperature profile. Tremendous gas flow
turbulence was generated above the
burning hot fuel bed when the cold
underfire air passed through the grate and
the cold overfire air was injected into the
furnace above the grate. Observations
through viewing ports indicated turbulent
-------
flow along the entire convective section
during the tests.
Pentachlorophenol Destruction
Efficiency
The concentration of pentachlorophenol
in the treated wood fuel averaged 4,100
mg/kg (ppmw). No pentachlorophenol was
detected in any of the SVOC samples
collected during the six tests. The
estimated pentachlorophenol destruction
efficiency (ORE) for each test has been
conservatively calculated based on the
pentachlorophenol analysis practical
quantitation limit (PQL). The PQL is based
on the lowest calibration concentration and
does not include factors such as percent
recovery or matrix effects. Burning of the
pentachlorophenol-treated waste wood in
a stoker combustor destroys the
pentachlorophenol efficiently with a ORE
higher than that required for hazardous
waste destruction (99.99%).
VOC Emissions
Analyses were performed for 50 VOCs
in each test sample. Only eight
compounds were found in concentrations
higher than the analytical detection limit in
at least one test. Higher levels of benzene
were found in the untreated wood test
samples, while bromomethane was found
at higher levels in the treated wood test
samples. Chloromethane, 1,3-butadiene,
iodomethane, acetone, chloroform, and
1,2-dichloroethane were found in both
untreated and treated wood test samples.
The trace levels of chlorinated VOC
emissions found in the untreated wood
tests may possibly be caused by small
amounts of naturally occurring chlorine in
untreated woods. Higher concentrations of
chlorinated VOCs were typically found in
the treated wood tests, probably as a result
of higher chlorine content in the treated
wood.
SVOC Emissions
Of the 87 SVOCs for which analyses
were performed in the SVOC samples,
only five targeted compounds were found
above the analytical detection limits:
phenol, acetophenone, naphthalene, 2-
nitrophenol, and phenanthrene. The
typically low SVOC emissions for both the
untreated and treated wood tests indicate
that good combustion conditions were
achieved during the tests. The slightly
lower SVOC concentrations for the treated
wood tests compared to those for the
untreated wood tests also suggest that the
combustion of the drier treated wood fuel
produced lower PICs. More moisture
released during the combustion of the
"green" untreated wood fuel may lower the
localized combustion zone temperature
and cause more PIC formation.
Dioxin/Furan Emissions
Total polychlorinated dibenzo-p-dioxin
and polychlorinated dibenzofuran
(PCDD/PCDF) concentrations [using the
International Toxic Equivalency (I-TEQ)
method of accounting fordifferenttoxicities
of PCDD/PCDF congeners] averaged
0.274 and 1.190 ng l-TEQ/dscm from the
untreated and treated wood tests,
respectively. These flue gas concentrations
are priorto any control devices. Particulate
control devices could potentially remove
part of this material from the gas stream,
depending on the fly ash collection
efficiency, PCDD/PCDF formation, and
solid-to-vapor-phase partition with the
device. The temperature of the flue gas at
the sampling point was between 150 and
160°C. It has been shown that the dioxin
and furan formation "temperature window"
in flue gases is between 200 and 450°C. It
is reasonable to expect that the formation
of dioxin and furan in the flue gas has
already been completed before the
sampling location. It is also likely that the
rates of formation and desorption of fly-
ash-associated PCDD/PCDF in a
subsequent particulate collection device
would be low at temperatures below
160°C. The PCDF concentrations are
significantly greater than PCDD
concentrations for both the untreated and
treated wood tests, indicating de novo
synthesis formation reactions rather than
condensation reactions. The PCDD/PCDF
congener distributions for the treated and
untreated tests also differ. In the untreated
wood tests, distribution is peaked at the
lower-chlorinated tetra-PCDD and di-PCDF
congeners; in the treated wood tests,
distribution is shifted to the higher-
chlorinated hexa-PCDD and penta-PCDF
congeners. The PCDD/PCDF
concentrations measured from both the
untreated and treated wood tests are
considered low when compared to the
stack concentrations from commercial
municipal waste combustors, which range
from 0.01 to 400 ng l-TEQ/dscm. The low
PCDD/PCDF concentrations measured
from the untreated wood tests are similar
to those from burning natural wood, which
range from 0.066 to 0.214 ng l-TEQ/dscm.
The PCDD/PCDF concentrations from the
treated wood tests are higher than those
reported (0.0359 ng l-TEQ/dscm) from a
waste-to-energy plant burning a mixture of
clean wood and pentachlorophenol-treated
waste wood.
The higher PCDD/PCDF concentrations
from the treated wood tests compared to
those from the untreated wood tests are
consistent with their higher HCI and
chlorinated VOC concentrations.
Approximately 200 ppm of HCI was
measured in concentrations from the
treated wood tests, while HCI was not
detected in the untreated wood tests. By
weighing and analyzing the front and back
halves of the PCDD/PCDF sampling train
separately, an indication of the
PCDD/PCDF associated with the
particulate material and as gaseous
concentrations was observed. The
percentage of PCDD/PCDF in the front half
catch is greater in the treated wood tests
than in the untreated wood tests. The
treated wood tests had a higher flue gas
flow rate as a result of the higher heating
value of the treated wood burned, which
would enhance particulate load at the
sampling point. This information suggests
that particulate carryover from the
combustor may affect the total amount of
PCDD/PCDF, especially in the treated
wood samples where a larger percentage
is in the front half catch. Combustor
operating conditions in addition to the
wood treatment may also contribute to
higher measured PCDD/PCDF
concentrations for the treated wood tests.
Conclusions
This study was conducted to identify
potential air pollution problems associated
with the combustion of utility poles treated
with pentachlorophenol preservative for
energy production in a boiler. The study
emphasized the characterization of the
PICs in the combustion flue gas. The
methodology used was to compare the flue
gas concentrations of PICs prior to the air
pollution control device of a pilot-scale
combustor burning untreated wood and
wood treated with pentachlorophenol
preservative as a fuel. The tests showed
that combustion is an effective method of
destroying the pentachlorophenol
3
-------
contained in the wood, with destruction
efficiencies higher than 99.99%.
Differences in the flue gas
concentrations of VOCs, SVOCs, and
PCDD/PCDFs from the combustion of
untreated and treated wood fuels have
been noted. The data do not enable
identification of the exact cause of these
differences in flue gas concentrations.
These differences are possibly caused by
the significantly different moisture content,
heating value, and chlorine content of the
two fuels. The difference in flue gas flow
rate required for the combustion of these
two fuels with different combustion
characteristics (moisture content and
heating value) may also cause the
differences in PCDD/PCDF formation
rates. These flue gas concentrations are
strongly influenced by the design and
operation of the combustion facility and
may not be quantitatively comparable to
other combustors.
-------
S. Lee and J. Valenti are with Acurex Environmental Corp., 4915 Prospectus Ave.,
Durham, NC 27713.
Robert V. Hendriks is the EPA Project Officer (see below).
The complete report, entitled "Products of Incomplete Combustion from Direct
Burning of Pentachlorophenol-Treated Wood Wastes," (Order No. PB98-
127731: Cost: $35.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
National Risk Management Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
------- |