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
See also Tire Derived Fuel Characteristics
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.
See also Economics of TDF in Cement Kilns


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.

See also Economics of TDF in Boilers
See also Economics of TDF
See also Economics of TDF
See also Economics of TDF


 References:

  1. Joel I. Reisman, Paul M. Lemieux, Air Emissions from Scrap Tire Combustion, EPA, Oct. 1997
  2. Leslie Lamarre, Tapping the Tire Pile, EPRI Journal, Sep-Oct 1995 v20 n5 p28(7)
  3. Carla Rapoport, Fuel from old tires. (Elm Energy builds and operates tire-burning power plants), Fortune,Nov 15,1993 v128 n12 p14(1)
  4. Electric Power Research Institute
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