Characterization of Transient Puff Emissions from the Burning of
Carpet Waste Charges in a Rotary Kiln Combustor
Matthew J Realff, School of Chemical and Biomolecular Engineering, Georgia Institute of
Technology, 30332-0100, matthew.realff@chbe.gatech.edu
Paul Lemieux, US EPA Office of Research and Development, 109 T.W. Alexander Drive;
E305-01, RTP, NC 27711
Stephanie Lucero and James Mulholland, School of Civil and Environmental Engineering,
Georgia Institute of Technology, 30332-0512
Peter B. Smith, Lehigh Cement Company, 7660 Imperial Way, Allentown, PA 18195
Abstract
Transient puff emissions were characterized from burning carpet charges that were fed to a pilot-
scale rotary kiln combustor to assess the potential impact on emissions of using post-consumer
carpet as an alternative fuel in cement kilns. Carpet with polypropylene, nylon 6, and nylon 6,6
face fiber was cut in one to three inch square pieces and fed as 0.4 kg charges to a 73 kW natural
gas fired rotary kiln simulator. Gas emissions monitored included O2, CO2, CO, NO, NOx, N2O,
NH3, total hydrocarbons, and total polycyclic aromatic hydrocarbons. The charges required about
two minutes to burn, and the emission transient exhibited three phases. In the first phase lasting
about 30 seconds, fuel-lean combustion of volatiles occurred. In the middle phase lasting about
one minute, fuel-rich burning occurred and a CO spike was observed. In the final phase lasting
about 30 seconds, fuel-lean burning occurred. For nylon carpets, the fuel-lean combustion
periods were characterized by elevated NO emissions. Integrated over the duration of the
transient, the NO emission corresponded to a one to two percent conversion of fuel-nitrogen (i.e.
nitrogen in the nylon fiber) to NO. These tests demonstrate the feasibility of burning waste carpet
as an alternative fuel, that rapid volatilization of batch fed carpet can lead to emission transients,
and that NO emissions may result from the burning of nylon carpets.
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Introduction
What is carpet?
Carpet is a multilayer mixture of polymers, both thermoplastics and thermosets, and inorganic
fillers. The face fiber is composed of high-value polymers, such as nylons and polyethylene
terephthalate (PET) that provide the texture color and mechanical wear necessary for the carpet
to function as a flooring. These fibers are either tufted through a backing fabric of
polypropylene or woven through it in the "level loop" construction. The latex filled in varying
amounts with calcium carbonate provides the bulk of the backing and "glues" the face fibers into
the backing. The last layer is often another polypropylene backing.
Carpet is designed to be difficult to pull apart and to be resistant to mechanical wear - one reason
for the inherent difficulty in recycling it. The average weight of carpet is about 2.1 kg/m2 (3.88
lbs/yd2), with about 0.94 kg/m2 (1.75 lbs/yd2) of face fiber, and 1.16 kg/m2 (2.13 lbs/yd2) of
backing. The backing consists of 0.78 kg/m2 (1.44 lbs/yd2) of calcium carbonate, 0.2 kg/m2 (0.38
lbs/yd2) of polypropylene, and 0.167 kg/m2 (0.31 lbs/yd2) of latex. The distribution between
different face fibers has nylon 6,6 and nylon 6 comprising about 35 and 20 % of the carpet sold
and polypropylene comprising about 30% with the rest PET and wool. In addition to the large
stream of broadloom carpet, there is a small percentage of carpet tile, 10% of overall sales, much
of which is backed with polyvinyl chloride (PVC), although new non-PVC based backings are
now in production.
How much post-consumer waste carpet is generated?
It is estimated that between 1.8 and 2.3 million metric tons (4 to 5 billion pounds) of post-
consumer waste carpet is land filled annually in the United States, about 7.7 kg (17 lbs) per
person per year, and hundreds of million kilograms of post-industrial manufacturing waste is
similarly disposed of. This quantity is expected to grow over the next decade at a rate of 3% per
annum leading to approximately 2.3 to 2.7 billion kg (5 to 6 billion pounds) in a decade from
now. There is significant value locked up in this stream. The nylon in this stream would be
worth about $300 million, given a price of $0.44/kg ($0.20/lb) and the approximate percentage
of nylon face fiber carpets (55%) in the overall stream. However, the overall stream contains a
significant fraction of low value material, as well as including all of the backing components.
The question is how to realize the potential value in the material by building recycling
infrastructure that recognizes how to best put the different fractions of the stream to different end
uses.
What are the drivers for using carpet as a fuel?
The central issue for carpet recycling is the construction of cost-effective infrastructure that can
support the diverse set of production processes that will convert post-consumer carpet into useful
products. In order to build robust collection systems and infrastructure for high value uses for
recycled carpet we need to establish outlets for carpet that can absorb significant quantities of
material in a relatively undifferentiated form. A high volume use, such as a fuel, will enable a
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collector to dispose of those fractions of carpet, thus avoiding a high handling cost and disposal
fee.
Carpet consists of two basic material types, calcium carbonate and oil-derived polymers. Due to
the stringent performance requirements on carpet, the polymer materials are well characterized
and free from impurities, and hence represent a clean, oil based, fuel. The calcium carbonate is
an inert material that forms an ash. Due to the large percentage of the calcium carbonate in
carpet, if used as fuel in traditional power boilers, there would be increased generation of ash.
This suggests that finding a use for carpet as a fuel in which the calcium carbonate could be
incorporated into a product would be beneficial. Cement kilns are an example of such use,
although the quantity of calcium carbonate provided by carpet is very small compared to the feed
rates.
In more quantitative terms, carpet has an energy value of approximately 23200 kJ/kg (10,000
Btu/lb), whereas coal typically has a value of between 27900-32500 kJ/kg (12,000-14,000
Btu/lb). Carpet is essentially sulfur-free (0.1 wt%) and has very low halogen content (0.2 wt%),
as compared to coal which may have a sulfur content of the order of 1.2% and low halogen
content (comparable to carpet). The key element that is higher in carpet is nitrogen, because of
the nylon, which can render carpet as high as 6-8% nitrogen, compared to a value of 1.4% for
coal. It is not clear how much of this fuel nitrogen will be converted to NOx emissions, and if
nylon carpet is separated for use in higher value applications, what the fuel composition will be.
This is an area of active research, being undertaken in conjunction with the EPA and their test
kiln at Research Triangle Park.
Why is waste carpet proposed as an alternative fuel for use in cement kilns?
Cement kilns consume 5.31 gigajoules per metric ton (5.04 million Btu per ton) product (1999
figures) with 74% of this energy being provided by coal and coke combinations. 75,600,000
metric tons of finished cement was produced in 1999 and the industry has been on an upward
trend since these figures were released [1],
There are several specific reasons for cement kilns being a good outlet for carpet as a fuel.
1. Calcium carbonate is a raw material for cement production. The disadvantage of carpet
containing calcium carbonate and generating high ash loads if burned in boilers is thus converted
into an advantage in cement kilns.
2. Cement kilns are experienced at handling waste fuels. There are plants that have solid waste
feed systems that could handle carpet, if it were processed into the right form. 45 of 122 cement
kilns report using wastes as fuels and 10 of these report that waste is their primary fuel [1],
3. Cement kilns are energy-intensive furnaces and can utilize a large volume of waste fuels and
alternate materials sources, thus providing a good outlet for non-recyclable waste carpet
materials.
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To establish whether carpet could be an effective fuel for cement kilns, several experiments have
been performed at a pilot scale facility in EPA's Research Triangle Park facility. These
experiments enable a more precise quantification of the emissions from carpet than are possible
in a field trial where the background emissions and variability of operation are greater. The
specific issue that is addressed in this work is the effect on emissions of nitrogen species from
burning different types of carpet and, secondarily, different sized pieces of carpet.
Experimental
Rotary Kiln Simulator
Experiments were performed in EPA's 73 kW (250,000 Btu/h) Rotary Kiln Incinerator Simulator
(RKIS). A schematic is shown in Figure 1. The primary fuel is natural gas. A ram feeder is used
to inject small charges of waste (0.4 kg, nominal). The kiln rotates at a slow speed, typically 0.5
rpm. A secondary combustion chamber is available, but was not fired in this study. Exhaust
gases pass through a flue gas cleaning system consisting of an afterburner, spray quench,
baghouse, and wet scrubber. Emissions are monitored both at the exit of the primary combustion
chamber and the exit of the secondary combustion chamber, before the flue gas cleaning system.
This facility has been used to study transient puff emissions from combustion of charges of
several different types of wastes [2-4],
Figure 1
The RKIS is equipped with continuous emission monitors (CEMs) for oxygen (02), carbon
dioxide (CO2), carbon monoxide (CO), nitric oxide (NO), NOx (NO and nitrogen dioxide), and
total hydrocarbons (THCs). In addition to these conventional CEMs, CEMs for ammonia (NH3),
nitrous oxide (N2O), and polycyclic aromatic hydrocarbons (PAH) were used. The NH3 monitor
is a Perkin-Elmer MCS 100. N20 was monitored by on-line gas chromatography with an electron
capture detector. The PAH monitor was an Ecochem model PAS 2000.
Previous carpet burn study and a priori hypothesis
In fall 2002 we began carpet burn studies to support two initiatives. One is an initiative
supported by the carpet industry, the state of Georgia, and DOE to explore the feasibility of
burning post-consumer carpet in cement kilns. The second is a homeland security initiative at
EPA to study the disposal of contaminated carpet by incineration [5],
In a previous study conducted in August-October 2002 at the RKIS facility, we studied
emissions from the semi-continuous feeding of shredded carpet fiber and finely ground carpet
backing to the RKIS at rates of up to 30 percent of the total energy input [6], In these
experiments, there was almost no increase in CO emissions when carpet was co-fired, but carpet
nitrogen conversion to NO ranged from three to eight percent.
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As a follow-up to this work, experiments were designed to address emissions when carpet is co-
fired in 0.4 kg charges. The a priori hypothesis for the current study was that batch-fed carpet
would result in decreased NO emissions, but increased CO emissions.
Tests performed in this study
Sixty-two tests were performed at the RKIS facility between November, 2003, and February,
2004. Three types of carpet were tested: polypropylene, nylon 6, and nylon 6,6. An ultimate
analysis was performed on each type of carpet, with results summarized in Table 1. The major
difference in the elemental composition of the carpets is that the nylon carpets contained
significant amounts of nitrogen. The results are consistent with the fact that nylon contains 12
percent nitrogen and the fiber is approximately one-third of the total carpet weight. The
polypropylene carpet tested had a higher carbon content than the nylon carpet tested, and its heat
of combustion was significantly higher. For comparison, the composition of a typical bituminous
coal is shown [7], The ash content of carpet is higher due to the calcium in carpet backing. The
volatile content of carpet is significantly higher than that of coal. The sulfur content of carpet is
significantly less than that of coal.
Table 1
The carpet was provided in three different size squares: 1 inch, 2 inch, and 3 inch. Carpet
squares were banded together using polypropylene straps and batch fed in 0.4 kg (nominal)
charges every ten minutes. Because emission transients can vary between charges, depending on
how the material is released from the canister, at least five charges are necessary to provide
sufficient data to characterize this variability. We found that the size of the carpet squares in each
charge did not significantly influence emission transients. Therefore, results presented here
address differences in the emission transients from the three types of carpet tested. The kiln
temperature at the exit of the primary combustion chamber was 1000°C (nominal) for these
experiments.
Results
Transient puff duration
In Figure 2, typical emission transients for a nylon carpet charge are shown. During burning of
the charge, emissions of CO, THC and PAH peak at approximately two orders of magnitude
above the baseline. The NO/NOx profile is more complex. In all tests, the NOx and NO profiles
were identical, indicating thatN02 levels are negligible. When nylon carpet is co-fired in the
kiln, NO emissions increase during initial burning, decrease during peak burning when the
incomplete combustion byproduct emissions peak, and increase again in the final stages of carpet
burning. In Figure 3, a series of five CO and NO transients are shown to demonstrate the
repeatability of these puffs.
Figure 2 and Figure 3
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The duration of an emission transient was defined as the time for NO concentration to return to
within 25% of the baseline value. The average duration and standard deviation of durations for
carpets of different types and cut sizes are shown in Figure 4. As shown, there is no significant
difference in the burn duration of these charges which have the same mass but are cut in different
sizes and are composed of different types of face fiber.
Figure 4
Carbon emissions
Since the CO, THC and PAH transients all had similar shapes, only the CO results are presented.
These results are shown in Figure 5. The average CO concentration was obtained by integrating
over the emission transient duration, as defined above. For all three carpet types, peak CO
concentrations were two to three orders of magnitude above the baseline value of 10-15 ppm.
Figure 5
Nitrogen emissions
Peak NOx concentration and a concentration averaged over the transient duration are shown in
Figure 6. As previously mentioned, differences between NOx and NO emissions were negligible.
As expected because of its low nitrogen content, NOx emissions from polypropylene carpet were
small, with a slight increase observed that is likely due to a slight increase in flame temperature
and, therefore, thermal NO. Nylon carpet burning, on the other hand, resulted in a significant
increase in NO emissions above the 35-45 ppm baseline concentration. The total increase in NO
emissions was calculated by integrating over the transient. This value was then compared to the
total nitrogen input in each charge of nylon carpet. The conversion of the nylon nitrogen to NO
was found to range from one to two percent for these tests. This level of fuel nitrogen conversion
to NO is less than the three to eight percent found previously with the semi-continuous feeding
of shredded fiber and ground fines, which is consistent with a portion of the carpet combustion
occurring under oxygen-limited conditions in these batch-fed tests.
Figure 6
Emissions of other measured nitrogen species were small. NO2 emissions (see NO/NOx curves in
Figures 2 and 3), N2O emissions (Figure 7), and NH3 emissions (Figure 8) were not significant in
these tests.
Figure 7 and Figure 8
Conclusions
Transient puff emissions were characterized from burning carpet charges fed to a 73 kW pilot-
scale rotary kiln test combustor. The 0.4 kg charges required about two minutes to burn. CO
spikes of up to one percent were observed. Effects of carpet burning on thermal NO emissions
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were negligible, based on the polypropylene carpet tests. For nylon carpets, increased NO
emissions corresponded to a one to two percent conversion of fuel-nitrogen (i.e. nitrogen in the
nylon fiber) to NO. These tests demonstrate the feasibility of burning batch fed waste carpet as a
supplemental fuel, that rapid volatilization of batch fed carpet can lead to emission transients,
and that increased NO emissions may result from the burning of nylon carpets.
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References
[1] Portland Cement Association, U.S. and Canadian Portland Cement Industry: Plant
Information Summary, PC A 2000.
[2] Linak, W.P., Kilgroe, J.D., McSorley, J. A., Wendt, J. O. L., and Dunn, J. E. (1987a), On the
Occurrence of Transient Puffs in a Rotary Kiln Incinerator Simulator: I. Prototype Solid Plastic
Wastes, J. of the Air Pollution Control Assoc., Vol. 37, No. 1, pp. 54-65.
[3] Linak, W.P., McSorley, J. A., Wendt, J. O. L., and Dunn, J. E. (1987b), On the Occurrence of
Transient Puffs in a Rotary Kiln Incinerator Simulator: II. Contained Liquid Wastes on Sorbent,
J. of the Air Pollution Control Assoc., Vol. 37, No. 8, pp. 934-942.
[4] C.R. Stewart, P.M. Lemieux, B.T. Zinn, Application of Pulse Combustion to Solid and
Hazardous Waste Incineration, Combust. Sci. & Tech., Vol. 94, 1993.
[5] Lemieux, P. (2004), "EPA Safe Buildings Program: Update on Building Decontamination
Waste Disposal Area," EM, Vol. 29-33.
[6] Lemieux, P., E. Stewart, M. Realff, J. A. Mulholland (2004), Emissions Study of co-firing
waste carpet in a rotary kiln, Journal of Environmental Management, 70, 27-33.
[7] Bartok, W., Sarofim, A.F. (eds), Fossil Fuel Combustion, Table p.l, p. 835, Wiley, 1991.
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Table 1. Ultimate and proximate analysis results for three carpet types tested and a typical
medium-volatile bituminous Pennsylvania coal [7], All values are as received.
polypropylene
nylon 6
nylon 6,6
coal
Carbon (% mass)
56.93
42.25
45.59
81.6
Hydrogen (% mass)
8.47
5.47
6.13
5.0
Nitrogen (% mass)
<0.05
4.46
4.74
1.4
Sulfur (% mass)
0.07
0.11
0.11
1.0
Ash (% mass)
21.17
25.42
23.96
6.1
Oxygen (% mass, by difference)
13.36
22.28
19.46
4.9
Chlorine (ppm mass)
77
64
52
NA
Moisture (% mass)
0.21
0.85
0.58
2.1
Volatile matter (% mass)
69.11
61.90
65.57
24.4
Ash (% mass)
21.17
25.42
23.96
6.1
Fixed carbon (% mass, by difference)
9.51
11.83
9.89
67.4
Heat of combustion (MJ/kg)
28.10
17.17
18.81
33.26
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natural gas
burner
Continuous
Emissions
Monitors
ram
feeder
Figure 1. RKIS facility.
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200
180
160
140
60
40
20
0
2 3 4
Elapsed Tine (min)
2 3 4
Elapsed Time (min)
NO/NO*
Figure 2. Typical emission profiles for a nylon carpet transient puff.
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NO
CO
--
I
--
a
A» J
/Si
rS\- f
V A
A /s
(\
V
n
J \ N
'1 v J '
* ^ J "
¦J \
¦ 1
v
v
V ^1
V
V
0
0
10:34 10:36 10:38 10:40 10:42 10:44 10:46 10:48 10:50 10:52 10:54 10:56 10:58 11:00
Time
Figure 3. NO and CO transients from a series of five nylon carpet charges.
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(A
m
a>
E
200
= 150
a>
-------
100000
10000
E
Q.
Q.
O
o
J*
re
a>
CL
1000
100
10
T
T
I
I
I
baseline
polypropylene
nylon 6
nylon 6,6
250
200
E
Q.
a
O
O
0)
O)
re
150
g 100
0)
>
re
50
baseline
polypropylene
nylon 6
nylon 6,6
Figure 5. Peak and averaged CO emissions during carpet burning.
-------
250
200
E
9- 150
J*
re
a>
a.
100
50
baseline
polypropylene
nylon 6
nylon 6,6
E
Q.
Q.
n
100
75
50
25
baseline
polypropylene
nylon 6
nylon 6,6
Figure 6. Peak and averaged NOx concentrations during carpet burning.
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300
250
200
S"
Q.
— 150
O
CM
100
50
baseline polypropylene nylon 6
nylon 6,6
Figure 7. Averaged N20 emissions during carpet burning.
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25
20
Q- 15
Q.
TO 10
a>
Q_
0
baseline polypropylene nylon 6
nylon 6,6
Figure 8. PeakNH3 concentrations during carpet burning.
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