ENERGY RECOVERY FROM SCRAP TIRES
PROJECT NUMBER: HSW97–2
GEOFFREY D. SILCOX, UNIVERSITY OF UTAH
DAVID A. WAGNER, UNIVERSITY OF UTAH
PAULINO LUNA, CALIFORNIA INTEGRATED WASTE MANAGEMENT BOARD
ELIZABETH RAMÍREZ BARRETO, UNIVERSIDAD AUTÓNOMA DE BAJA
CALIFORNIA
NARRATIVE SUMMARY
INTRODUCTION
Discarded tires represent a major solid waste disposal problem in the U.S.-Mexican border
region. The disposal problem is particularly acute in Mexico because used tires are imported
from the United States, mounted on vehicles, and then discarded after just a few months of
use. This extra wear renders the tires unsuitable for retreading. Scrap tires are unsightly, pose
a fire hazard, disrupt landfills, and collect rainwater that then becomes a breeding ground for
mosquitoes that can transmit infectious disease.
This project included a survey of the number of scrap tires in El Cerro Centinela waste tire
pile, 15 kilometers west of Mexicali, and a testing program to characterize the use of
combustion grate technology for burning scrap tires to recover their energy content. The
testing program examined how temperature, underfire air flow rate, and tire piece size affect
generation of polycyclic aromatic hydrocarbons (PAH) in a grate fired reactor. The study
measured total PAH on a continuous basis using an EcoChem, PAS 2000 analyzer. The tire
survey is related to the combustion study because large stockpiles of tires are required for
energy recovery operations.
Uses for whole scrap tires include energy recovery (combustion), reefs and breakwaters,
playground equipment, and erosion control (Amos 1993). Energy recovery from scrap tires
may be particularly attractive for Mexico because energy costs are high and labor costs are
low. Tire collection, preprocessing (cutting), and feeding should be relatively inexpensive in
Mexico. Scrap tires can also be processed for use in various rubber products, as gravel
substitutes, and for sludge composting. Other options for utilizing scrap tires are limited.
Pyrolysis of the tires to recover carbon black, char, and oils may be economically viable if
markets for the low grade carbon black can be found. Pyrolysis produces carbon black that is
not of sufficient quality to be used in the manufacture of new tires. Roughly 80 percent of the
market for carbon black is in the manufacture of new tires. There is a potential market for
ground rubber as an additive to asphalt road aggregate. The resulting aggregate appears to be
more durable at high temperatures and it provides greater traction, decreased noise, and
reduced glare. Many of the roads in the U.S.-Mexican border region are unpaved and the
high summer temperatures that typically exist in this region soften the aggregate. Hence,
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there is a large potential market for adding ground rubber to aggregate for use in paving
roads. It is worth noting, however, that only two percent of the scrap tires produced in the
United States are used in asphalt aggregate.
Scrap tires are a potential source of energy. Combustion facilities that use tire derived fuel
(TDF) include power plants, cement kilns, pulp and paper plants, and tire manufacturing
plants (Lemieux 1993). Boilers that can burn whole tires require a design and construction
unique to the combustion of tires. These dedicated facilities also require large stockpiles of
tires and can have long payback periods. Many existing facilities, such as municipal solid
waste/waste to energy boilers, were not designed for the high temperatures that TDF can
generate. Tires can often be burned in utility boilers, cement kilns, fluidized bed boilers, and
spreader stokers with minimal retrofit and pretreatment. Burning TDF in these existing
facilities generally does not result in increased emissions if standard pollution control devices
are installed. Scrap tires are a valuable source of energy in many areas outside of the United
States. Regulatory oversight and monitoring in some of these areas can be less stringent than
in the United States or lacking altogether. The increased emissions from TDF and possible
lack of proper oversight underlies the importance of developing a general set of operational
guidelines for burning TDF, either alone or in conjunction with other fuels.
RESEARCH OBJECTIVES
The objectives included (1) a survey of the number of scrap tires in El Cerro Centinela waste
tire pile, 15 kilometers west of Mexicali, and (2) the characterization of how combustion
parameters, temperature, underfire air flow rate, and tire piece size, affect emissions of
polycyclic aromatic hydrocarbons (PAH) in a grate fired reactor. The acceptability of
combustion as a way of recovering energy from scrap tires is critically dependent on
minimizing emissions of hazardous compounds. PAH are a major concern in the combustion
of tires. The survey of El Cerro Centinela is related to the combustion study because large
stockpiles of tires are required for energy recovery operations.
RESEARCH METHODOLOGY
Investigators at the Autonomous University of Baja (UABC) characterized El Cerro
Centinela waste tire pile. The California Integrated Waste Management Board (CIWMB)
assisted in training the personnel at UABC. The area occupied by the pile was determined by
triangulation using a digital theodolite. The number of tires was determined from
measurements of pile depth and tire density (number of tires per m3) at 65 locations.
The combustion studies were performed at the University of Utah Combustion Center in
downtown Salt Lake City using a pilot scale 150 kW (0.5 x 106 BTU/h) grate fired
combustor. The reactor uses a grate system that can be water cooled, bare, or insulated to
determine how grate designs and operating conditions affect emissions of CO and PAH. We
investigated the effects of tire piece size, combustor temperature, and underfire air rate. No
overfire air was used in these experiments. They focused solely on grate region parameters.
The EcoChem PAS 2000 was used to continuously measure relative PAH concentrations.
Standard combustion gases were also continuously monitored.
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EQUIPMENT
The inner cavity of the reactor is three feet wide by five feet long. The heat release rate on
the grate was 500,000 BTU/(h ft2) (1.5 MWt/m2). Reactor temperature was measured in the
center of the lower section about 18 inches above the underfire air windbox with a
thermocouple and recorded automatically.
Gas samples and particles were drawn from the reactor through the lower port of the upper
reactor section. The sample passed through a condensate canister, after which the gas stream
was divided into two streams. A particle laden sample was diluted sample and then routed to
the PAS-2000 PAH analyzer. The measured PAH levels are qualitative, but they do show
relative values from one experimental condition to another. The sample was continuously
analyzed for CO, CO2, O2, NOX, and total hydrocarbons.
EXPERIMENTAL PROCEDURE
Three parameters were varied in the experiments as follows. The reactor temperatures were
1400°F, 1600°F, and 1800°F. Underfire air flow rates were 150 standard cubic feet per hour
(SCFH), 300 SCFH, and 450 SCFH. The tires were cut into one-, two-, and four-inch
squares. The reactor was operated in a batch mode. The charges of tire material had a mass of
300 g. The squares were cut with a band saw to avoid the surface area differences arising
from tire shredding processes. The steel was left in the pieces. A single, four-inch square
sample weighed 300 g. Adjusting the firing rate on the burners set the reactor temperature.
The reactor draft pressure was adjusted until the baseline O2 level was about eight to nine
percent by volume, on a dry basis. A 300 g sample was placed on the grate with a shovel.
The majority of the tire burned in approximately six minutes, leaving a pile of glowing char.
Complete burnout of the char occurred in about 30 minutes. Virtually all PAH was generated
during the initial six minutes of the burn.
RESEARCH FINDINGS
The tire pieces smoldered and released volatile matter for one to 30 seconds, depending on
reactor temperature, underfire air flow rate, and piece size. The sample then ignited and
burned vigorously. Once the flames subsided, the remaining char continued to burn for
approximately 30 minutes. The majority of PAH was generated from the time of ignition to
the point where the charge was reduced to a pile of glowing char. In most cases, this required
about six minutes. Combustion rates decreased with increasing tire piece size. This trend was
observed at all temperatures and underfire air flow rates examined. The differences in these
combustion rates underscore the important effect of surface area on combustion rates and
emissions.
Tire piece size had the greatest effect on emissions of PAH. PAH concentrations decreased
substantially as the tire piece size increased. This is due to the decreased rate of burning and
devolatilization of the larger pieces due to their lower surface-to-volume ratio.
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The effect of bed region temperature on emissions of PAH was small. The only condition
that exhibits a clear decrease of PAH with increasing temperature is the four-inch piece size.
Full scale, grate fired combustion of tires typically takes place at bed region temperatures of
about 1800 F (Clark et al. 1991)
Increasing the air flow rate to the grate tended to slightly increase emissions of PAH.
Moderate increases in PAH emissions occurred for most of the conditions studied due to the
increased cooling of the grate at higher flow rates. The cooling tends to quench oxidation
reactions.
CONCLUSIONS
As of March 1999, the volume of El Cerro Centinela waste tire pile was about 125,000 m3
(163,000 yd3). It covered 10.2 hectares (25.2 acres) and contained roughly 1.27 million tires.
There are two other major tire sites in the vicinity of the El Cerro Centinella waste tire pile.
The number of tires in both is not well known. The first pile (Tire Site No. 1 in the UABC
study) occupies roughly 50 acres. The tires originate primarily from the cities of Tijuana,
Ensenada, Tecate, and Mexicali. The estimated active tire inventory at the site in 1998 was
380,000. The tires are being shredded at the rate of 7,200 tires per day. The second pile (Tire
Site No. 2 in the UABC study) is near the first and occupies about 35 acres. It is estimated to
contain 500,000 tires. The area around Mexicali is estimated to contain five million tires
(Millman 1999).
The UABC investigators recommend that fire control and dumping policies be developed and
implemented at and around El Cerro Centinela waste tire pile. They also suggest that the
results of their measurements be used to develop a clean-up and remediation plan for the
area.
Pilot scale, grate fired combustion studies show that emissions of PAH are strongly affected
by tire piece size. This observation may be of limited practical importance if the scrap tires
are burned whole, as they are in many commercial facilities. Bed region temperature and
underfire air rates had minor effects on emissions of PAH. However, the underfire airflow
rate data suggest that there exits an optimum airflow which is high enough to minimize CO
emissions, yet low enough to minimize PAH emissions by avoiding excessive cooling of the
grate. Further work to optimize the underfire air rate is needed. In particular, additional
experiments in a well controlled reactor are needed to determine the optimum combustion
conditions that can be used to simultaneously minimize emissions of both NOx and PAH.
Grate region temperatures of 1000° C (1800° F) in conjunction with a tire piece size of 10
centimeters (four inches) dramatically reduce formation of PAH. Controlling the burning rate
of the fuel by increasing the piece size is the single factor that is most effective at minimizing
formation of PAH.
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RECOMMENDATIONS FOR FURTHER RESEARCH
The UABC investigators recommend that fire control and dumping policies be developed and
implemented at and around El Cerro Centinela waste tire pile. They also suggest that the
results of their measurements be used to develop a clean-up and remediation plan for the
area.
Further combustion studies are needed to understand the effects of underfire and overfire air
on PAH, CO, and NOx emissions under well controlled reactor conditions. Experiments in a
well controlled reactor are needed to determine the optimum combustion conditions that can
be used to simultaneously minimize emissions of both NOx and PAH.
ACKNOWLEDGEMENTS
Funding for this project was provided by the Southwest Center for Environmental Research
and Policy. The considerable assistance of Paulino Luna of the California Integrated Waste
Management Board and Elizabeth Ramírez Barreto of the Universidad Autonoma de Baja
California is gratefully acknowledged.
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