EMISSIONS FROM LABORATORY COMBUSTOR TESTS OF
MANUFACTURED WOOD PRODUCTS
Richard Wilkening, M.S., Graduate Student: Michelle Evans, B.S., Graduate Student; Kenneth Ragland, PhD, Professor; Department of Mechanical Engineering
University of Wisconsin-Madison Madison, WI 53706 USA and Andrew Baker, B.S., Chemical Engineer
U.S.D.A. Forest Products Laboratory
Madison, WI 53706 USA ABSTRACT
Manufactured wood products contain wood, wood fiber, and materials added during manufacture
of the product. Manufacturing residues and the used products are burned in a furnace or boiler
instead of landfilling. Emissions from combustion of these products contain additional compounds
from the combustion of non-wood materials which have not been adequately characterized to
specify the best combustion conditions, emissions control equipment, and disposal procedures.
Total hydrocarbons, formaldehyde, higher aldehydes and carbon monoxide emissions from aspen
flakeboard and aspen cubes were measured in a 76 mm i.d. by 1.5 m long fixed bed combustor as
a function of excess oxygen, and temperature. Emissions of hydrocarbons, aldehydes and CO
from flakeboard and from clean aspen were very sensitive to average combustor temperature and
excess oxygen. Hydrocarbon and aldehyde emissions below 10 ppm were achieved with 5%
excess oxygen and 1200°C average temperature for aspen flakeboard and 1100°C for clean aspen at
a 0.9 s residence time. When the average temperature decreased below these levels, the emissions
increased rapidly. For example, at 950°C and 5% excess oxygen the formaldehyde emissions were
over 1000 ppm. These laboratory tests reinforce the need to carefully control the temperature and
excess oxygen in full-scale wood combustors.
Presented
at:
First Biomass Conference of the Americas August 30-September 2, 1993 Burlington, VT To be published in Proceedings of the meeting.
INTRODUCTION
Manufactured wood products contain wood, wood fiber, and non-wood additives such as
adhesives. Manufacturing residues can be used to replace fossil fuels at the industrial site.
Eventually the manufactured products are discarded, and rather than going to a landfill, the wood
products may be burned in a boiler. Emissions from combustion of these manufactured wood
products have not been adequately characterized to specify the best combustion conditions,
emissions control equipment, and disposal procedures.
Primary wood product industries include lumber, plywood, composition board, and pulp and
paper. Primary residue is in the form of edge trimmings, sawdust, sander dust, shavings, and
fiber sludge. At the pulp and paper mills waste fiber containing additives and dyes accumulate.
During secondary manufacturing, adhesives, plastic overlays, paints, varnishes, lacquers, fillers,
strength additives. and dyes are applied to the wood products [Atkins and Donovan, 1992]. Wood
preservatives and fire retardant chemicals may also be added.
In this paper aldehyde, total hydrocarbon and carbon monoxide emissions from combustion of
flakeboard made from aspen are compared with emissions from "clean" aspen in a laboratoryscale, fixed bed combustor. Exterior grade flakeboard is made from thin flakes of wood and 3-7
% phenol-formaldehyde resin solids. Depending on the application, the phenol-formaldehyde
resin contains 1-7 % sodium hydroxide which acts as a catalyst, maintains resin solubility, and
helps the resin to penetrate the wood. In some flakeboard potassium hydroxide is substituted for
part of the sodium hydroxide. Approximately 1 % wax is added to the flakeboard as a water
repellant. Proximate analysis (Table 1) shows that the flakeboard used in this study has more fixed
carbon and less voiatiles than than the aspen. The ultimate analysis shows that flakeboard has
slightly more hydrogen and carbon and less oxygen. The mineral analysis was similar for the two
fuels except that the flakeboard had 608 ppm of sodium, whereas the aspen had only 6 ppm of
sodium. This raises separate issues concerning slagging and fouling due to the sodium.
Because of the phenol formaldehyde resin in the flakeboard, there is concern that the formaldehyde
emissions may be higher than in pure wood. Formaldehyde is considered a hazardous air pollutant
by the 1990 Clean Air Act. Under proper operating conditions a wood-fired, spreader-stoker
boiler has low formaldehyde emissions, however when the temperature and/or excess oxygen are
too low, the formaldehyde emissions can be very high [Hubbard, 1992, Atkins and Donovan,
1992]. Larson, et. al., 1992 measured hydrocarbon and formaldehyde emissions of clean
woodchips and woodchips impregnated with phenol formaldehyde resin in a rotary kiln combustor
which was fired with natural gas (35% gas and 65% wood on an energy basis). The products of
incomplete combustion were higher for the impregnated wood than for the pure wood.
Table 1. Analysis of the Flakeboard and Aspen
Proximate Analysis (as-rec. %)
Flakeboard
Aspen
6.56
6.64
moisture
77.44
81.57
volatiles
11.50
15.46
fixed carbon
0.57
0.29
ash
20,030
19,192
HigherHeating Value (kJ/kg)
Ultimate analysis (dry, ash free 9%)
Flakeboard
Aspen
6.47
6.21
hydrogen
51.12
50.77
carbon
41.89
42.23
oxygen
nitrogen
0.50
0.43
sulfur
0.02
0.05
TEST SETUP AND TEST PLAN
The combustor consists of a 76 mm i.d. by 1.5 m long, heated alumina oxide tube with a grate on
the bottom (Fig. 1). A Kanthal heating wire element controlled by a 220 V, 30 A variable-voltage
transformer was wrapped around this tube, and a moldable refractory board was wrapped around
the heating wire. Approximately 10 cm of high temperature Kaowool insulation surrounds the
refractory board and insulates the combustion chamber from a stainless steel tube which acts as the
outer most shell of the combustor. Flakeboard in the form of 6 mm cubes was introduced into the
top of the combustor by a specially designed variable-speed feeder. The cubes fall by gravity
down the combustion tube onto a bed of alumina oxide chips supported by a stainless steel grate.
Air is introduced from beneath the packed bed of chips (underfire air) and from two opposing jets
located 30 cm above the bed (overfire air), and is monitored by flow meters and controlled by
adjustable valves. As the combustion products exit the combustion chamber, they enter a 10 cm
diam. stainless steel exhaust duct An exhaust fan downstream in the duct pulls the exhaust gas
through a pyrex cyclone collector.
Temperatures, oxygen, and carbon monoxide were monitored continuously and recorded on a
computer every 5 s. Thermocouples were located 15 cm above the bed, at the exit of the
combustor tube, in the exhaust duct, and immediately after the particulate filter assembly. A 3 mm
stainless steel probe at the exit of the combustion tube was connected to carbon monoxide and
oxygen meters. Also at the exit of the combustor tube, a heated 1.5 mm stainless steel probe and
sample line were connected to a Hewlett Packard 5890 Series II gas chromatograph which used a
glass wool packed column and a flame ionization detector to measure total organic carbon (TOC)
emissions. In the exhaust duct, two 3 mm quartz probes were inserted - one probe is heated and
connected to a particulate filter assembly and impinger train, and the other is connected to a
separate impinger train and is used to capture aldehydes in a DNPH solution according to the
Boiler and Industrial Furnace (BIF) Method 011 [EPA, 1990]. The cyclone collector with a
removable flask collects flyash, and after a test run the underfire air is increased and the bottom ash
is blown into the cyclone. Further details are given in the M.S. thesis by Wilkening, 1993.
TEST RESULTS AND DISCUSSION
Combustion tests were run at nominal excess oxygen levels of 2.5 %, 5 % and 10 %
corresponding to excess air levels of 15 %, 40 % and 90 %. The calculated adiabatic flame
temperatures were 1860°C, 1600°C and 1070°C, respectively. The ratio of overfire air to underfire
air was about 2:l. The residence time in the reactor was held at 0.9 s by adjusting the fuel and air
flows. The fuel flow rate ranged from 0.33 g/s to 0.11 g/s, while the air ranged from 1.75 g/s to
2.5 g/s. Exhaust temperatures ranged from 500°C to 110°C. Lower temperatures required higher
air and fuel flows to maintain the 0.9 s residence time. To achieve constant conditions it was
important the feed the wood cubes at a constant rate; while this was achieved, there were some
fluctuations in the CO and O2 levels.
Total organic carbon emissions (TOC, Fig. 2) were very sensitive to temperature and excess
oxygen. The data correlated better with average combustor temperature (referred to as average
temperature) than with exhaust gas temperature. This is because the walls of the combustor were
heated, and thus the flame temperature and exhaust temperature were independent. The average
combustor temperature was defined as the weighted average of the lower combustor temperature
(measured 15 cm above the grate), the calculated flame temperature, and the measured exit
temperature. For both fuels the TOC was less than 10 ppm when the average temperature was
above 950°C with 10% excess oxygen, and above 1050°C with 5% excess oxygen. At 2.5%
excess oxygen the TOC was always above 10 ppm. As the temperature was reduced below these
levels, the TOC increased markedly. For the 5% and 10% excess oxygen levels the difference
between the pure aspen and the flakeboard was not significant. However, for 2.5% excess oxygen
the flakeboard had higher TOC emissions than the aspen. The maximum level measured was 4000
ppm at 920°C.
Formaldehyde emissions (Fig. 2) were also very sensitive to average temperature and excess
oxygen. The formaldehyde emissions were less than 10 ppm when the average temperature was
greater than 1200°C. When the average temperature dropped below 1200°C the formaldehyde
emissions increased rapidly. For example at 950°C and 5% excess oxygen the formaldehyde
concentration was over 1000 ppm. Excess oxygen levels of 10% allow a lower temperature to
prevent formaldehyde emissions. The flakeboard appears to require a slightly higher temperature
(about 50°C) than clean aspen to prevent formaldehyde emissions. Acetaldehyde emissions (Fig.
2) are similar to formaldehyde, but the levels were a factor of 2 to 10 lower. Acrolein and
propionaldehyde were observed at the 100 ppm to 1000 ppm level when the average temperature
was below 1000°C. No butylaldehyde was ever detected.
Carbon monoxide emissions (Fig. 2) were also very sensitive to average temperature and excess
oxygen. For CO there is a clear distinction between the clean aspen and the flakeboard. At higher
temperatures the CO emissions were at a low level which depended on the amount of excess
oxygen. As the average temperature dropped below 1200°C, 1100°C and 1000°C for excess
oxygen levels of 2.5%, 5%, and 10%, respectively, the CO level increased rapidly for the
flakeboard. These temperature regions were about 100°C lower (1100°C, 1000°C and 900°C) for
the clean aspen. Thus the flakeboard requires approximately 100°C higher temperature than the
aspen to burn cleanly. The burnout of CO at levels below 500 ppm appears to take more time than
the available 0.9 s.
CONCLUSION
Emissions of hydrocarbons, aldehydes and CO from flakeboard and from clean aspen were very
sensitive to average combustor temperature and excess oxygen. Hydrocarbon and aldehyde
missions below 10 ppm were achieved with 5% excess oxygen and 1200°C average temperature
for aspen flakeboard and 1100°C for clean aspen at a 0.9 s residence time. When the average
temperature decreased below these levels the emissions increased rapidly.
REFERENCES
Atkins, R.S., and Donovan, C.T., "Wood Products in the Waste Stream: Characterization and
Combustion Emissions", report prepared for the New York State Energy Research and
Development Authority, Albany, NY, under contract 1531-ERER-ER-91, 1992.
"EPA Methods Manual for Compliance with the BIF Regulations", U.S. EPA/530-SW-91-010,
Dec, 1990.
Hubbard, A., "Hazardous Air Emissions Potential From Wood-Fired Furnaces," Wisconsin Dept.
of Natural Resources, Bureau of Air Management report 1991.
Larsen, F., McClennen, W., Deng, X., Silcox, G. and Allison, K., "Hydrocarbon and
Formaldehyde Emissions from Combustion of Pulverized Wood Waste", Comb. Sci. and Tech.,
85, pp. 259-269, 1992.
Wilkening, R. T., "Emissions from the Combustion of Flakeboard in a Laboratory-Scale Fixed
Bed Combustor," M.S. Thesis, University of Wisconsin, Madison,WI, 1993.
LIST O F FIGURES
1. Schematic of test setup.
2. Emissions data
1. combustor tube 2. heating wire 3. refractory sheet 4.insulation 5. stainless steel tube 6. overfire airjets 7. thermocouple 8. stainless steel grate 9. air plenum 10. packed bed 11. underfire air 12. gc probe 13.thermocouple 14. aldehyde train probe 15. O2/CO train probe 16. exhaust duct 17.mirror 18. support screen 19. particulate probe 20. pitot tube 21. collection flask 22.cyclone 23. exhaust fan 24. feeder