Emission Control
Based on data from laboratory simulation1)
and field studies at industrial TDF combustion facilities, it can be said
that properly-designed solid fuel combustors can supplement normal fuels,
which typically consist of coal, wood, coke and various combinations of
these with 10 to 20% TDF and still satisfy environmental compliance emission
limits. Moreover, results from a dedicated tire-to-energy (100% TDF)
facility indicate that it is possible to control emissions to much lower
levels compared to existing solid fuel fired boilers (on a heat input basis)
using a specially designed combustor and add-on controls. See
also: Emissions from Open Tire Fires
for comparison.
It is very difficult to establish universal emission factors, or even
a range of emission factors for TDF, because of the limited amount of data,
and the differences between the design of combustors. However, the emission
data can be summarized by source type, such as boiler type, and industrial
usage.
Typical industries that have been successful in integrating TDF with
other fuels are:
This chapter will discuss the above applications including a summary of
the laboratory simulation data from the U.S.Environmental Protection Agency
(EPA).
Laboratory Simulations of TDF Emissions (Summary of
EPA Report)1)
Pilot-scale emissions testing of TDF was conducted in a 73 kW (250,000
Btu/hr) rotary kiln incinerator simulator in EPA's Environmental Research
Center in Research Triangle Park, NC (Lemieux, 1994). This size simulator
has been established as exhibiting the salient features of full-scale units
with ratings 20 to 40 times larger.
The TDF tested was wire-free crumb rubber sized to <0.64 cm (<1/4
in.) It was combusted at several combinations of feed rate, temperature,
and kiln oxygen concentration. The TDF was combusted with natural gas as
the primary fuel. Samples were taken to examine volatile and semi-volatile
organics, PCDD/PCDF, and metal aerosols. Data were collected to determine
the effects of feed rates, type of feeding, i.e., continuous versus batch,
and combustion controls on emissions. The data were taken in the exhaust
stream prior to any add-on air pollution control devices.
Based on this test program, it is concluded that with the exception
of zinc emissions, potential emissions from TDF are not expected to be
very different from other conventional fossil fuels, as long as combustion
occurs in a well-designed, well-operated and well-maintained combustion
device. If unacceptable particulate loading occurs as a result of zinc
emissions, an appropriate particulate control device would need to be installed.
Estimated Emissions of Metals
| Metals |
0% TDF (Natural Gas Only)
|
17% TDF (Steady State)
|
100% TDF (Estimated)
|
|
ng/J
|
lb./MMBtu
|
ng/J
|
lb./MMBtu
|
ng/J
|
lb./MMBtu
|
| Antimony |
7.72E-05
|
1.80E-07
|
9.05E-04
|
2.10E-06
|
5.32E-03
|
1.24E-05
|
| Arsenic |
4.80E-04
|
1.12E-06
|
1.59E-02
|
3.70E-05
|
9.35E-02
|
2.17E-04
|
| Beryllium |
ND
|
Not Detected
|
2.14E-05
|
4.98E-08
|
1.26E-04
|
2.93E-07
|
| Cadmium |
1.76E-04
|
4.09E-07
|
4.54E-04
|
1.06E-06
|
2.67E-03
|
6.21E-06
|
| Chromium |
2.78E-04
|
6.46E-07
|
1.66E-03
|
3.86E-06
|
9.76E-03
|
2.27E-05
|
| Lead |
3.45E-03
|
8.02E-06
|
2.83E-02
|
6.58E-05
|
1.66E-01
|
3.86E-04
|
| Manganese |
1.21E-03
|
2.81E-06
|
2.48E-03
|
5.77E-06
|
1.46E-02
|
3.40E-05
|
| Nickel |
3.00E-04
|
6.98E-07
|
1.50E-03
|
3.29E-06
|
8.82E-03
|
2.05E-05
|
| Selenium |
3.56E-04
|
8.28E-07
|
1.93E-03
|
4.49E-06
|
1.14E-02
|
2.65E-05
|
| Zinc |
1.23E-01
|
2.86E-04
|
15.21
|
3.54E-02
|
89.47
|
2.08E-01
|
Cement Kilns
Cement kilns account for the largest percentage of TDF use. These kilns
are very suitable for the burning of waste tires because the furnaces operate
at very high temperatures (around 2600 degrees Fahrenheit) and have long
residence times. High temperatures, long residence times, and an adequate
supply of oxygen ensure complete burnout of organics, which minimizes the
formation of dioxins and furans, a primary concern in solid waste
combustion. In addition, the cement production process can utilize the
iron contained in the waste tire's steel beads, belts and ply. These components
do not change the quality of the cement product, since large quantities
of iron ore are already present as one of the main ingredients.
Because of their suitability for tire burning, no technical difficulties
exist with the emission control techniques in the kilns. Moreover,
no modifications for TDF burning or special equipment for emission control
are necessary. The main considerations are the size of the TDF and the
timing of introduction. There are no general guidelines for operating technique,
since the above functions are highly dependent on individual cement kiln
design and requirements.
The only barrier for TDF burning in cement kilns is the economic feasibility
compared to other alternatives.
Boilers
In general, smaller-size and wire-free TDF is favored for boilers.
The fuel in boilers is primarily combusted by suspension firing or
by grate firing. Boiler types that combust fuel in suspension include fluidized
beds and cyclones. Combustion occurs primarily on the grate in underfed
stoker type boilers. Combustion occurs both in suspension and on the grates
in spreader stoker type boilers, depending on the fuel size and the grate
type, i.e., traveling, reciprocating, or chain.
TDF is difficult to burn in suspension because of its size and weight
compared to coal, oil and gas. Also, the metal contained in tires causes
operational difficulties in these boilers.
TDF has been successfully utilized as a fuel supplement in various
industries. Since TDF applications are more challenging for boilers than
for kilns, the type of boiler configuration and firing method significantly
affect the success of burning TDF. Therefore the discussion of emission
control techniques is based on the industry (or usage of the boiler) and
not on boiler type.
-
Utilities
Because of their configuration, large scale, and high operating temperature,
the boilers employed by utilities are expected to tolerate TDF burning.
Based on the experience and the emissions data from power plants burning
tires or TDF, cyclones and stoker type boilers work well, and thus the
use of tires or TDF as a supplemental fuel in that type of boiler is viable.
In many cases, the quality of the emissions actually improves with the
increased use of tires or TDF as a supplemental fuel.
Unfortunately, they are among the least common types of coal burning
utility boilers, with cyclone-fired boilers representing about 9% of all
coal-fired capacity and stoker-fired units even less.
Pulverized-coal boilers, the most common type in the industry, are
generally much more difficult to adapt to TDF burning than are cyclone
and stoker boilers, because the fuel must be pulverized into a fine powder
to be burned in suspension. Although tires can be shredded into fine granules,
such pieces are still larger than the coal particles, and are much harder
to burn completely. The design of the cyclone, although classified as a
suspension type unit, stokers are more tolerant of large fuel particles.
It is reported that some experiments to burn whole tires have been
successful. However, it is also reported that, to accept whole tires into
the boiler, some significant modifications to the fuel feeding system will
be necessary.
Although there is no evidence for emission problems with TDF in properly
designed utility boilers, their economical and technological scale create
considerable public opposition (NIMBY).
-
Pulp and Paper Mills
Compared to utility boilers, TDF burning at pulp and paper mills poses
several technical difficulties for control of emission levels.
Boiler sizes are smaller, and operating temperatures are lower. Thus, complete
burning of TDF particles in this kind of boiler is much more difficult.
Data have indicated that using TDF appears to deteriorate the emission
quality. Particulates in the emissions are increased with a corresponding
increase of TDF usage. Other criteria pollutants also increased in most
cases. Therefore, proper equipment or modifications to reduce emission
levels are required to burn TDF in these boilers. Several emission control
devices and techniques are known, and these have decreased emission levels
to within standards.
The emission control problem is the greatest single concern for burning
TDF at pulp and paper mills.
However, in order to minimize solid waste from these facilities, wood
waste is usually burned. This has a lower heating value (7,925-9,010
BTU) and high moisture content compared to TDF. Burning TDF in the boiler
improves the performance of the system. This aspect provides an attractive
advantage.
-
General Industrial Boilers
The basic design, configuration, and capacity of industrial boilers
is similar to those used in pulp and paper mills. Most of the problems
and advantages described for TDF burning at pulp and paper mills are essentially
the same.
There are very few reliable test data from these facilities in order
to evaluate emission issues.
References:
-
Joel I. Reisman, Paul M. Lemieux, Air Emissions from Scrap Tire Combustion,
EPA, Oct. 1997
-
Leslie
Lamarre, Tapping the Tire Pile, EPRI Journal, Sep-Oct 1995 v20 n5 p28(7)
-
Carla Rapoport, Fuel from old tires. (Elm
Energy builds and operates tire-burning power plants), Fortune,Nov 15,1993
v128 n12 p14(1)
-
Electric Power Research Institute
Subjects
Back to Top |
Go
to Next