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Waste Management & Research (1993) 11, 463-480
POTENTIAL TO USE WASTE TIRES AS SUPPLEMENTAL
FUEL IN PULP A N D PAPER MILL BOILERS, CEMENT KILNS
A N D IN ROAD PAVEMENT
Morton A. Barla%*,William E. Eleazer, 11' and Daniel J. Whittle?
'
The objective of this paper is to evaluate the recycling potential of waste tires as an
energy source and for use in road pavement. North Carolina, U.S.A., is used as a case
study. Scrap tires may be burned for supplemental fuel in pulp and paper mill boilers
and cement kilns. Five pulp and paper mill boilers in North Carolina could consume
over 90% of the 6 million tires generated annually in the state. Cement kilns located
within 400 km of North Carolina population cen:lerscould consume about 6.6 million
tires annually. Based on the quantity of pavement laid in North Carolina, nonproprietary and proprietary versions of asphalt ruhher concrete have the potential IO
consume 1.8 and 7.2 million tires, respectively. Rubber modified asphalt concrete has
the potential to consume up to 16.5 million tires. However, technological and
economic limitations suggest that large scale implenientation is unlikely for the short
term. Environniental considerations pertainills to each alternative are discussed.
Estimates of this nature are critical as planning regions formulate solid waste
management plans which include recycling.
Key Words-Asphalt, energy. incineration, recycling. rubber, solid ivaste, tires.
I . Introduction
Historically, there have been two main methods for the disposal of wiiste tires:
stockpiling and landfilling. I t is estimatcd that S2% of the 234 million scrap [ires
generated annually in the U.S.A. are landfilled. stockpiled or illegnlly dumped, 9% arc
used in energy recovery processes, 4% arc exported, 2% are used i n asphalt pavement
and 2% are recycled into new rubber products (Robinson 1991). Tire stockpiles range
from backyard storage of a few tires to massive tire piles containing thousands and
sometimes millions of tires. Stockpiling is both unsightly and a potential hcalth hazard.
The combination of rainwater, windblown pollen and dust trapped within discarded
tires creates an environment which can increase the breeding rate of disease carrying
mosquitoes by a factor of 4000 (Grady 1987). Tire piles also present a fire hazard. Once
ablaze, tire stockpiles are very difficult to extin_guish. can release toxic smoke and can
contaminate groundwater near a site. A fire i n Ontario, Canada i n 1990 burned 14
million tires and released over 600,000 I of oil to the surrounding soil. When placed i n a
landfill with municipal solid waste, whole tires d o not compact. In addition to reducing
landfill capacity, whole tires can rise to the surface of landfills, causing damage to the
final cover of a closed landfill. By 1991, 32 states had enacted legislation to regtilate tire
disposal including 18 states which banned land disposal of whole tires (Malcolm Pirnie
1991). Burial of sliccd tires i n landfills is still permitted its slicing minimizes the potential
of B tirc to rise to the surface.
0734-242X,'93/060463 + I8 SOX.0O;O
(
1993 ISWA
,
464
M.A . Barlaz et al.
In order to utilize scrap tires as a resource there must be proven. cost cffective and
environmentally sound alternatives in place. In addition, there must bc adequate
manufacturing capacity for processing scrap tires and demand for a product that can be
produced from such tires. The objective of this study is to evaluate the potential to use
tires ( I ) for energy recovery in industrial boilers and (2) for incorporation into asphalt
road pavement.
1.1 Use of rires.for siippler~ictita1Jiiel
To evaluate the use of tires as supplemental fuel, we focus on two industries i n which
there has been experience with the use of tire derived fuel (TDF); the pulp/paper and
cement industries. Pulp/paper production is an energy intensive process and pulp/paper
mills typically have their own boilers and turbines to meet their electrical needs. The
wood scrap and bark that remain after the usable part of a tree has been converted to
pulp is referred to as "hog" fuel and is often burned in pulp/paper mill boilers. The
American Paper Institute reports that 351 pulp and paper mills in the U.S.A. burned 414
trillion kJ of hog fuel in 1989 (Kearney 1990). Gas, oil or coal are frequently needed to
supplement hog fuel and stabilize boiler operations. Thc nccd for supplementary fucl
provides an opportunity for the utilization of tires. Thc use of T D F by the pulp and
paper industry has gradually increased since its initial use in the mid-1970s. Recent
estimates indicate that nearly 13 million tires are used by at least 12 mills in the U.S.A.
(Kearney 1990).
There are over 200 cement kilns in the U.S.A. with an energy requirement of
approximately 3200 trillion kJs a year (estimated from data in Table 3). A large portion
of scrap tires generated in Europe, including about 30% of scrap tires generated in
Germany, are burned as fuel by the cement industry (OMOE 1991, Anon. 1992). By
1992, there were at least nine cement kilns in the U.S.A. using tires as supplementary fuel
and this number is predicted to increase (Getz & Teachcy 1992).
Other alternatives for energy recovery from tires include utility boilers and dedicated
tire incinerators. Electric utility boilers consume massive amounts of fossil fuel and, in
some cases, have the potential to utilize tires as an energy source. Historically, utilities
have been reluctant to accept non-traditional fuels for use in boilers for fear of
disrupting power generation. Thus, electric utility boilers were not considered in this
study. Nevertheless, it should be noted that by 1991, eight power utilities had conducted
trial burns so the use of TDF in utility boilers may increase (Malcolm Pirnie 1991). The
Oxford Energy dedicated tire incinerators in California, U.S.A. and Connecticut, U.S.A.
represent viable and environmentally sound technology. However, only a limited
number of such incinerators are likely to be constructed, and these, in areas with large
supplies of tires and high electricity rates. State agencies cannot control construction of
additional dedicated incinerators as part of their solid waste managet~ientplanning.
Thus, such incinerators were not considered in our estimates of recycling capacity.
1.2 Use of tires in asphalt road p r r ~ w i i e t i r
Asphalt pavement is composed of mineral aggregate held together by asphalt cement, a
residue of petroleum. There are two general methods for using tire rubber in asphalt
mixes. In asphalt rubber concrete, ground tire rubber is mixed with asphalt cement for
use a$ a binder for the mineral aggregate particles in the asphalt mix. Tire rubber may
also be used in rubber modified asphalt concrete (RUMAC). Here, ground rubber chips
'I
I
Wasie tires as suppIemeiua/fil
465
replace a portion of the mineral aggregate. At least 12 states haveexperimented with the
use of tires in ARC or RUMAC.
North Carolina is used as a case study to illustrate the methodology required to
estimate the potential to reuse a component of the waste stream. In this paper, we focus
on scrap tires which represent one component of municipal solid waste. The mcthodology and analysis employed here to estimate potential tire use may be applied to other
technologies, other constituents of municipal solid waste and other states or planning
regions. The framework presented here has been used for an assessment of the recycling
potential of post consumer plastic waste (Barlaz er 01. 1993).
2. Scrap tire generation in North Carolina
An assessment of whether there is adequate manufacturing capacity to recycle a
particular waste requires an estimate of the quantity of the waste produced as this
represents the maximum quantity available for recycling. The scrap tire generation rate
for the State of North Carolina was estimated using three methods. The first estimate is
based on the commonly used generation rate of one tire per person per year (Kearney
1990). Using the 1990 census, this suggests a generation rate of 6.6 million tires per year
in North Carolina. This number is the upper end of generation rate estimates. For
example, Florida uses a generation rate of 0.75 tires per person per year (Ruth 1991).
The second estimate is based on the number of registered vehicles in North Carolina (5.5
million), the number of tires per automobile (4) and the estimated life span of a tire (4
years) (Estakhri 1990). This leads to an estimated generation rate of 5.5 million tires per
year. This number may be conservative because it neglects trucks which have more than
four tires. The third estimate is based on proceeds from the 1% scrap tire disposal fee in
North Carolina. This fee generated 83.52 million in 1990. Thus, $352 million were spent
on tires in North Carolina in 1990. Assuming an average retail price of $60 per tire, the
scrap tire generation rate is 5.9 million tires per year. The average of these three
estimates is 6 million which will be used for this study. In addition to tires generated
annually, it is estimated that there are between 9 and 12 million tires stockpiled in North
Carolina (Eggers pers. comm.).
A typical passenger tire weighs 9.1 kg of which 5.45 kg is rubber, 1.83 kg is steel, and
1.83 kg is fiber (Estakhri 1990). Thus, North Carolinians produce 33,000 metric tonnes
of scrap tire rubber, as well as 11,000 metric tonnes each of steel and fiber each year.
3. Energy value and chemical composition of tires
Coal yields between 26,000 and 31,000 kJ kg-', while whole tires and tire derived fuel
chips yield 30,000-35.000 kJ kg-' (OMOE 1991). The following heating values are used
in this paper; whole tires, 31,000 kJ kg-', shredded tires (larger than 5 x 5cm),
31,000 kJ kg-' and dewired shredded tires (smaller than 5 x 5 cm), 34,000 kJ kg-I. The
sulfur, nitrogen and chlorine contents of T D F and coal are presented in Table 1. The
sulfur content of TDF is lower than that of eastern coal but higher than that of western
coal. The nitrogen content of T D F is lower than that of coal while the chlorine content
of T D F is higher. The implications of these data on potential emissions are discussed
below.
466
*
M . A. Barlaz et al.
Waste tires as supplemental fuel
4. Results
TABLE I
Sulfur, nitrogen and chlorine production from combustion of tire derived fuel (TDF)
’
and coal
The use of tires as supplemental fuel in pulp and paper mill boilers and cement kilns is
evaluated in this section. Their potential use in asphalt road pavement is evaluated in
section 5. Evaluation criteria include modifications required to incorporate scrap tires
into the process, environmental, performance, economics and an estimate of the number
of tires which could be consumed by each alternative.
4.1 Use of tire derivedfirel in the pirlp and paper i t i h r s t r j
4 . I . I Pulp arid papper r i d 1 boiler techiiology
Multifuel boilers that contain grates are the only type of boiler in the pulp and paper
industry which can accept TDF. Historically, grate system boilers have been easily
adapted to use a variety of fuels. A brief description of the hog fuel combustion process
and how it can be modified to incorporate T D F is presented in this section.
Bark and other wood waste is placed on a conveyor which discharges into a
pulverizer. The pulverizer grinds wood waste into easily combustible chips referred to as
hog fuel. Hog fuel is placed on an elevator conveyor which discharges into a live bottom
hopper from which it is metered into the combustion chamber. The combustion chamber
stoker, typically a mechanical throwing device or an air-fed blower, distributes hog fuel
across a chamber grate. Heat released during combustion is used to produce steam
which powers a turbine.
The only modification to the hog fuel combustion process required for T D F use is an
addition to the fuel feed system. A method for metering tire chips and thoroughly mixing
them with the hog fuel is required. This usually involves installation of a live bottom
hopper for discharge of tire chips to the hog fuel conveyor at a metered rate.
Paper mills generally purchase T D F from a local shredder. The optimum T D F
particle is dewired and 5 cm x 0.16 cm %
(: nenes er al. 1990). Dewiring is necessary for two
reasons. First, it reduces metal concentrations in boiler ash. Second, dewired T D F
prevents adherence of metal slag to the boiler grate system. The level of dewiring
required is dependent on the grate and ash disposal systems. Champion Paper in
Bucksport, Maine requires 95% wire removal and this appears to be typical among mills
which use T D F (Harrison pers. comm.). ,
4.1.2 Environmental considerations
The two major areas of environmental concern in pulp and paper mill boilers are air
emissions and ash disposal. The major air pollutants released during multifuel boiler
operation are sulfur oxides (SO,), nitrous oxides (NO,<)and particulate matter which
could include heavy metals. The sulfur, nitrogen and chlorine contents of T D F and coal
are compared in Table 1. Based on these data, we calculate that sulfur production from
the combustion of western coal is comparable to that of T D F while T D F produces less
sulfur than eastern coal. Calculations also predict reduced NO, emissions when T D F is
substituted for coal as tires have 11% of the nitrogen content of coal on a kJ basis (Table
1 and OMOE 1991). Chlorine emissions are predicted to increase. There are only limited
data available on actual emissions from boilers using T D F . In general, these data show
reduced SO, and NO, emissions, but not without exception (Malcolm Pirnie 1991). I n
cases where SO., did increase, it was still within permitted values. Emissions data are
highly dependent on the air pollution control equipment in place, operating conditions
and the fraction of the energy input of a boiler contributed by TDF.
1
_ _
Energy value (kJ kg-‘)
_
Sulfur (%)
SulTur production (kg x 10” kJ):
Nitrogen (%)
Nitrogen production (kg x IO6 kJ)$
Chlorine (%)
Chlorine production (kg x IO6 kJ)$
~
TDF
34,000*
1.2t
0.35
0.24t
0.07
0.1SIl
0.04
467
Energy source
coal
(Eastern U.S.A.)
27,000*
2*
0.74
1.76tS
0.65
0.08$4
0.03
Coal
(Wcstern U.S.A.)
27,000*
0.P
0.3
I .76t$
0.65
.
O.OSlI5
0.03
* OMOE (1991).
Kearney (1990).
$ Calculated from energy value and composition data.
,$No difference between eastern and western coal.
/I Ronchak (1990).
f EPRI (1987).
Available data indicate that release of particulate matter increases when tires are
substituted for coal. However, emissions data from test burns at seven pulp and paper
mills showed that while particulate emissions increased, releases were still within
permitted values (Malcolm Pirnie 1991). As above, the fraction of T D F fed to a boiler,
operating conditions and pollution control equipment all vaned in the data sct. Mills
with electrostatic precipitators or baghouses are considered to be the best equipped to
control particulate emissions. Mills with electrostatic precipitators are able to burn up to
10% T D F on a kJ basis (Gray pers. comm.). Mills with other particulate removal
systems such as mechanical collectors in combination with wet scrubbers may have to
limit T D F input to 2.5% in order to comply with emission requirements (OMOE 1991).
Emissions of chromium, cadmium and lead generally decreased when T D F was used
in pulp and paper mill boilers (Malcolm Pirnie 1991). However, zinc emissions incrcascd
in each of the six trial burns for which data were reported. Particulate matter is likely to
be enriched in zinc as zinc is added to rubber in the tire rubber manufacturing process.
The concentration of zinc in chipped tires containing the cord and wire and in dewired
tire rubber is 1.4 and 1.53%, respectively (Ronchak 1990). This represents 100 to 1000
times more zinc than typical concentrations in coal (Malcolm Pirnie 1991).
Organic emissions were measured in five T D F trial burns as either total hydrocarbons
or polynuclear aromatic hydrocarbons. Emissions increased in two tests, decreased in
one and exhibited no change in two tests (Malcolm Pirnie 1991).
I n summary, the use of T D F in pulp and paper mill boilers will lead to increases in
some pollutants and decreases in others. Actual emissions will depend on operating
conditions, air pollution control equipment and the amount of T D F burned. Mills
considering the use of T D F would have to conduct test burns to determine the amount
of T D F which can be burned while remaining within permitted emission levels.
An additional consideration regarding air emissions is whether a mill is currently
protected from ,an emission standard by a “grandfather” clause. Such a clause would
allow for a variance from air pollution regulations where such regulations were enacted
after the mill was constructed. Typically a mill is given a certain length of tinic to meet
M. A . Barlaz et al.
Wasre tires ns supplenlentalJuel
current standards. In order to implement a TDF use process that represents a revision of
permitted operating procedures, a mill would have to renegotiate its air emission permit
and possibly surrender its grandfather protection prematurely. While this is likely to be
beneficial with respect to particulate emissions, the regulatory burden tnay discourage a
mill from implementing a T D F use program, thus reducing the rate at which til-es are
used as a supplemental fuel.
The metal content of ash collected at the bottom of the combustion chamber (bottom
ash) and by the particulate matter removal system (fly ash) is also of concern whcn T D F
is substituted for traditional fuels. Ash produced during the combustion of T D F may
contain as much as 1500% more zinc than if coal is used as the stabilizer fucl (Kearne!
1990). However, concentrations of cadmium, chromium and lead in ash are all reduced
when T D F is used (Kearney 1990). Generally, mills must demonstrate that they can
properly dispose of ash with elevated zinc concentrations before they are allowed to burn
TDF. The concentration of zinc in ash is not high enough to permit its economic
recovery.
TABLE 2
North Carolina pulp and paper mills with the potential to use tire derived fuel
468
4.1.3 Economic characteristics
The capital investment needed to incorporate T D F in multifuel boilers is typically
limited to the cost of T D F storage and metering equipment and the cost of environmental permit modifications. Total capital investment is estimated to be between S 150.000
and 5350,000 in 1990 terms for plants processing 1.5-3 million tires per year (Kearney
1990). The cost of T D F relative to coal depends on several factors including the niill’s
current fuel purchase agreements, the cost o f T D F and the location of the mill relative to
T D F markets. The average cost to a paper mill for dewired T D F is between 80.95 and
PI .61 per million kJ. The cost of coal to paper mills is generally between S 1.52 and S I .90
per million kJ (Kearney 1990). Thus, there are cases where T D F would be an
economically viable alternative fuel. As pressure to reduce land disposal of tires
increases. the fee paid to a T D F producer for accepting tires is likely to increase. This
should then decrease the cost of T D F to a user such as a paper mill.
4,1.4 Potential constimption of tires in North Carolina p ~ i l poud p p e r iiiill hoi1cv.T
Criteria which must be met in order for a pulp and paper mill to consider the use o f T D F
are:
( I ) The boilers must have grates in the combustion chamber.
(2) The boiler must have a particulate matter reinoval system capable of handline the
increased particulate emissions which occur when T D F i s burned.
(3) The mill must not be adversely affected by the need to renegotiate its air emission
permit.
(4) The mill must have access to competitively priced TDF.
( 5 ) The mill must have a disposal alternative for ash containing elevated zinc
concentrations.
Analysis of every pulp and paper mill in North Carolina to determine its exact
potential for using T D F was beyond the scope of this study. The seven mills in North
Carolina with the greatest potential for using T D F are listed in Tdbk 2 in descendin_g
order of likelihood. This order was determined by considering each plant’s current fuel
source, air pollution control systems, size and proximity to a population center. The
status of air pollution permits was not considered although this could alter the order i n
Table 2. Abitibi-Price is ranked first because they have already performed a test burn
Distance from
major city*
Company. location
Abitibi-Price, Roaring River
Weyerhauser, New Bern
Winston-Salem, 96 k m
Raleigh. 176 k m
Champion Paper. Roanoke Rapids Raleigh, 144 km
Federal Paper. Rieglewood
Wilmington. 40 km
Weyerhauser. Plymouth
Raleigh, 200 km
Champion Paper, Canton
Asheville. 40 kni
Jackson Paper, Sylva
Asheville, 80 km
Total
469
Current Particulate Potential
fuelst
control: tire uses
ww,o
MC,WS
MC,EP
MC,WS
MC,WS
MC,EP
MC,WS
MCWS
0.H
0S.H
H,C,O.G
ww,c
H.C.0
H
0.27
1.1
0.46
0.53
4.0
0.21
0.05
6.65
~~
~~
~
*The nearest major populatian center lo a mill and the distance bctween the mill and the city (Dyer 1990).
All cities are in North Carolina.
t WW, wood waste; 0, oil: C . Coal; H , hog luel; G . gas.
$ MC, mechanical dust collector; WS, wct scrubber; EP, electrostatic precipitator.
5 Expressed as millions of tires. Potential annual tire use calculated by multiplying multifuel boiler aVCrdge
intake in kJ/year by assumed percentage o l TDF use. and divided by average TDF energy content
(34,000 kJ kg-’) and the weight o l TDF in one tire alter shredding and dewiring (7.3 kg). The assumed
percentage ofTDF use is 2.5% lor boilers equipped with a wet scrubber and 8% Tor boilers equipped with a n
clcctroslatic precipitator.
with T D F and demonstrated that they can remain within their regulated SO,, NO, and
particulate emission levels. Trace emissions of .heavy metals occurred during the test
burn and the plant was studying ways to correct this problem in 1991.
The number of tires which each paper mill has the capacity to burn is estimated in
Table 2. Estimates assume that boilers equipped with either electrostatic precipitators or
wet scrubbers could accept 8 % or 2.5% of their kJ intake in TDF, respectively. Actual
values would be dctertiiined i n trial burns. The data in Table 2 show that the paper mills
evaluated could accept bctwecn 80,000 and 4 million tires per year. At present, paper
mills actually burning T D F have capacities of at least 500,000 tires per year (Kearney
1990). Assuming this as a n approximate lower limit of economic feasibility suggests that
actual scrap tire-use capacity i n North Carolin:i is lower than the 6.65 million reported in
Table 2. Elimination of mills with the capacity to burn less than 500,000 tires per year
(Price. Champion Paper in Roanoke Rapids and Canton. and Jackson Paper) puts scrap
tire-use capacity in North Carolina pulp and paper mills at about 5.6 million tires per
year. Seventy-three per cent of this capacity resides with one mill (Weyerhauser) which is
200 km from a major population center. Geographical differences between centers of tire
utilization and centers of scrap tire generation will impede actual scrap tire utilization in
pulp and paper mills where transportation costs exceed the value of the scrap tire. Of
course. the value of a tire as fuel will fluctuate with the price of energy and scrap tire-use
processes which are not economical in 1993 may be economical in the future.
4 . 2 Usc
of ~ l t elire cleriiwi~fuclin ceitwnt kilns
4.2. I The centetii procliicrion process
Raw materials used for cement production are limestone, clay or shale. and iron ore or
iron waste. These materials are pulvcrized and then mixed in either a wet or dry process.
In the wet process, raw materials are ground and mixed in slurry form while in the dry
470
M.A . Barlai et al.
process, the grinding and mixing are performed with dry materials. After grinding and
mixing, raw materials are discharged into a kiln where they are exposed to temperatures
between 1400'C and 1650°C for 1
4h. The heating process causes the formation of
1.25-cni chunks of cement referred to as clinker. Finally, the clinker is pulverizcd to an
extremely fine powder and a small amount of gypsum is added (Kosniatka 1988).
Over the past 20 years, the cement production process has been modified by the
construction of prc-heatcrs or prc-calciners integrated with a shorter dry kiln. These prcheaters provide about 85% calcination before the feed enters the kiln. Thc prc-hcatcrs
can be divided into two sections, one section draws heat from the kiln providing about
40% calcination and II second section, it flash furnace. increases calcination to 85%
(Kosmatka 1988). I'rc-heating technology is used for :I majority ofcCment producccti i n
the U.S.A. I t is installed on any new kiln and many older plants have been modificti to
use short kiln technology.
The only modifications to the cement production process required for thc use of T D F
are in the fuel feed system. Kilns with prcheaters can accept whole tires for up to 20Yn of
their fuel intake on a kJ basis ( U M O E 1991).Tire storage and mechanical conveying and
metering equipment must be installed to transport tires from a storage bin to the
preheater. Whole tires are typically supplied by local brokers and the cement company
may collect a tipping fee for accepting tires.
Kilns without pre-heaters require that
n this case, T D F
accept T D F for about 5% of their energy
is blown into the bottom end of the kiln as a substitute for powdered coal. Required
equipment typically consists of storage bins, conveying and metering equipment and a
pneumatic blower. The shredded T D F is generally between 5 and I O cm'.
4.2.2 Etivirontiietitul cotisideerations
The primary pollutants produced during fuel combustion are bottom ash, fly ash, and
sulfur and nitrous oxides. Cement kilns are generally equipped with baghouses that oKcr
excellent particulate control. Fly ash recovered by the baghouse is typically remixed in
the cement as a raw material. Cement kilns d o not produce bottom ash as other types of
incinerators, but rather non-combustibles in the fuel are incorporated into the cement.
The non-combustible fraction of T D F will contain a large amount of ferrous (iron) slag
which is a raw material in cement. Thus, the iron content of T D F is advantageous.
There are no known reports of emission tests for SO,, NO, or hydrochloric acid on
kilns burning tires. However, theoretical considerations are useful for an initial analysis.
Sulfur released during the combustion of TDF o r coal is expected to be incorporated
into the calcining limestone to form gypsum, a raw material of cement. Thus sulfur
emissions are not a concern in cement production. The alkaline environnient of a cement
kiln is expected to neutralize the majority of the hydrochloric acid generated (Mantus
1992). Finally, the data in Table 1 and reported elsewhere (OMOE 1991) predict that
production of nitrous oxides will be reduced when TDF is substituted.
There are no reported data on organic emissions when tires are burned in cement
kilns. However. data show no substantial increases in organic emissions when hazardous
organic wastes are burned in cement kilns (Mantus 1992).
In addition to air emissions, another potential concern is the incorporation of metals
from tires into cement followed by release of these metals from cement to the
environment. To the author's knowledge, no leaching tests of cement produced i n kilns
burning whole tires or T D F have been reported and such data are needed for a thorough
!
Waste tires as supplementalfuel
47 1
evaluation of this technology. A recent review of the incineration of hazardous waste in
cement kilns provides some useful information (Mantus 1992). It has been shown that
99% of the antimony, arsenic, barium, beryllium, cadmium, chromium, lead, nickel,
silver, vanadium and zinc fed to a kiln were retained in the cement kiln dust plus clinker.
The toxicity characteristic leaching procedure (TCLP), which is used by the USEPA to
determine the potential of a material to release harmful constituents, has been applied to
both ccnient kiln dust and clinker produced i n kilns receiving hazardous waste. Data on
antimony, arsenic, barium, beryllium, cadmium. chromiuni, lead, mercury, nickel,
selenium, silver and thallium show that there is no correlation between the metal
concentration in cement kiln dust and the m o u n t of nietal released by the TCLP. The
concentration of zinc, that is present in elevated concentrations in tires relative to coal,
was not measured. The TCLP was also applied directly to cements from 97 kiln systems
including some which burn hazardous waste. None of the average nietal concentrations
in TCLP extracts exceeded health based standards. Again, zinc concentrations were not
measured.
In summary, metals release tests on ccmcnt produced in kilns burning tires have not
been conducted to date. The available data on hazardous waste conibustion in kilns
suggest that the release of metals from cement produced in kilns accepting tires is likely
to be minimal. However, the TCLP may not be the most appropriate measure of the
potential for metal release from cement.
4.2.3 Ecotiotnic comiderutiotis
The use of whole tires as a supplementary fuel source in kilns with pre-heaters requires
the purchase of storage equipment and mechanical conveying and metering systems. A
complete tire handling and feed system for a throughput of 1.5 million tires per year is
estimated to cost $250,000. Purchase of a pneumatic blower system for a kiln which uses
T D F IS estimated to cost $60,000 to $100,000 (Kearney 1990) for the same throughput.
This cost does not include production of T D F which is typically purchased from a third
party.
Though the capital cost for modification of a plant without a pre-heater is generally
less, the use of tires iskener_ally considered more attractive to kilns with pre-heaters
whe'e-whole tires rather than shredded T D F can be used. A plant using whole tires may
be able to cfiafgea-fipping fee, or at leasi receive tireswithout cost. It is estimated that
for tires to be an economical alternative fuel source, whole tires must be supplied at a
total cost of323 to 53l/metric tonne (about $0.25 per tire) and T D F needs to be supplied
at a price slightly less than coal which costs approximately $38.50 to $49.50/metric tonne
(Kearney 1990). Kilns with local access to a large supply of tires and which can charge a
tipping fee for the disposal of tires could be potentially very profitable.
The presence of iron in tires reduces the need to purchase iron when tires are used as
supplemental fuel. Similarly, gypsum formed during tire combustion reduces the need to
purchase gypsum as a raw material
_
_
_
_
I
-
4.2.4 Cuse studies
The use of T D F in two cement kilns in the U.S. is discussed here. Calaveras Cement of
Redding, California, uses a dry process kiln equipped with a pre-heater. The plant uses
whole tires in the pre-heater section and blows shredded T D F into the kiln. The plant
has used 25% T D F on a kJ basis for over 5 years. This amounts to approximately 2
million tires per year (Jacinta pers. comm.). In addition to energy recovery, the plant has
,
M . A. Barlaz et al.
472
Waste tires as sirpplrtneiitaljitei
TABLE 3
Potential tire use by cement kilns bordering North Carolina
the use of 18-26'1/0 rubber which has been ground 10 pass a 0.84Imm screen opening. The
proccss is protected by a patent to 1993 and has been used by the pavement industry for
more than I5 years. The UltralineT" process is a nun-proprietary ARC process. 11 LISCS
5% tire rubber ground to pass a 0.297"
screen opening for a dense graded surface mix
;ind 15% tire rubber passing a 0.595 mni opening for an open graded surface mix (Ruth
Capacity
Potential
City, distance (km) type? ( I O 6 metric tonsiyear): tire uses
Company, location*
.
Santee, Holly Hill. SC
Giant, Harleyville. SC
Blue Circle, Harleyville, SC
Blue Circle, Atlanta, GA
Southwestern, Knoxville, T N
Signal Mt., Chattanooga, TN
Tarmac-Lonestar, Cloverdale, VA
Total
473
Charlotte, 242
Charlotte, 242
Charlotte, 242
Charlotte, 403
Wet
Wet
0.96
0.9
1991).
0.74
Dry (pc)
Dry
i t i n either the stirpace or structural layers, is referred to as recycled asphalt pavement,
Asheville, 201
Dry (pc)
0.59
0.56
0.55
0.4 I
0.7
I .7
0.4
Asheville, 322 Wet
Win.-Sal. 161 Dry (pc):l
* Kiln location, type and capacity as reported
I.o
The practice of excavating old pavement, adding rejuvenating agents, and rciipplying
I .5
0.4
I.s
7.4
by the Portland Cement Association (1990).
t Type of kiln, either wet process or dry process, (pc) kiln includes a precalciner.
J Annual clinker production
Expressed as millions of tires per year. Potential tire use calculated as follows:
( I ) dry process: assume 4 MMkJ/metric tonne product, 5 % use of tires on a kJ basis.
(2) dry process with precalciner: assume 4 MMkJjmetric tonne product. 20% utilization or lircs on
a
kJ
basis.
(3) wet process: assume 5.2 MMkJ/metric tonne product, 5% use of tires on a kJ basis.
(4) 31,000 kJjkg tires and 9.1 kg/tire.
/I 50% of clinker is produced in a kiln with a pre-heater and 50% in a kiln with no prc-hcalcr.
realized a 50% reduction in the cost of iron ore (Kearney 1990). Arizona Portland
Cement of Rillito, Arizona, also operates a kiln equipped with a pre-heater. The kiln
uses 10% TDF on a kJ basis and consumes approximately 3 million tires per year (Bittle
pers. comm.).
I
/
4.2.5 Poteniial uiilizaiion of tires by regioncil cemcni kilns
There are no cement kilns in North Carolina so the state will have to look to kilns i n
neighboring states to disposc of its waste tires. Therc arc seven kilns within 400 k n l of
North Carolina population centers. An estimate of the potential tire use of these seven
kilns is presented in Table 3. The data in Table 3 indicate that seven kilns located i n
South Carolina, Tennessee and Virginia have the potential to consume approximately
7.4 million tires per year. At present, kilns actually burning TDF have capacities of 0.5-3
million tires per year (Kearney 1990). Assuming 0.5 million as an approximate lower
limit of economic feasibility would lower the estimated utilization capacity in Table 3.
Elimination of mills with the capacity to burn less than 500,000 tires per year (Blue
Circle and Signal Mountain) puts scrap tire use capacity at about 6.6 million tires per
year. While this exceeds scrap tire generation in North Carolina, it does not address
scrap tire generation in the resident state of each kiln. While there may be regulatory or
political factors which will restrict implementation of this alternative for use of North
Carolina's scrap tires, cement kilns in the southeast represent a viable alternative for
resource recovery of tires produced in the region.
5. Use of tires in asphalt rubber concrete
Asphalt rubber concrete (ARC) is a hot mix asphalt concrete in which a mixture of
asphalt and ground rubber is used as a binder. Use of asphalt rubber concrete permits
and is becoming increasingly popular. North Carolina uses more than 20% recycled
pavement i n its full depth asphalt concrete projects and this pcrcentage is increasing. The
recycl;ihility of ARC has yet to be proven and i t has heen sugested that incorporntion
of ground tire rubber should be limited to 5% until such an assessment occurs (Roberts
1989). A crumb rubber asphalt road is scheduled to be replaced i n California i n 1992
(Blunienthal 1991).
A special blending unit is needed to mix asphalt and rubber for ARC. The blending
equipment is typically supplied by a contractor for temporary use at a local mixing plant.
Slightly higher heating temperatures and longer mix times arc required to blend the
asphalt rubber and mix in the mineral aggregate. Hauling and placement temperalures
are also slightly higher and compaction should only be performed by steel wheeled
rollers (Roberts 1989). A slightly longer time is required before tralfic is permitted on
A R C pavcment relative to conventional asphalt concrete (Turgeon 1989).
The Florida Department of Transportation receives ground tire rubber from Rouse
Industries i n Vicksburg, Mississippi. Currently. an agreement is being arranged whereby
Rouse Industries will accept as many of Florida's whole tires as the State buys in ground
tires. Rouse Industries is also developing a portable grinder capable of grinding rubber
to pass ;I 0.297mm screen. They plan to transport the grinder to a particular asphalt
plant and grind tires as needed for the plant's asphalt rubber projects (White pers.
.
C O I l l l l l ).
.r. I
Pzr.~;lr.llrtrlltu~
There have been over 35 pro,jccts in 12 states i n which ;isphalt rubber has heen used ;IS:I
hinder i n hor mix asphalt concretes (Estakhri 1990). Overall. asphalt rubber concretes
have performed :is well or better t h x n control sections. ARCs tend I O be superior to
conventional asphalt mixes with respect to crack resistance but seldom perform better in
the areas of skid resistance and roughness. ARCS are significantly blacker because of the
high carbon content in tires. This reduces glare on wet pavenienis (Kobelt 1990).
Field trials of ARCS have yielded mixed results. Data from a 1984 field trial of ARC
core samples in Minnesota indicate that ARC had higher penetration and lower resilient
moduli values than the control section. However, inspection of the road after 7 years of
service revealed no apparent physical difference between the rubberized and control
sections (Turgeon 1991).
A field trial of ARC in Connecticut in 1980 was performing well after 8 years of
service. The ARC section is in better condition with regards to cracking; however,
roughness and skid resistance values are similar to the control section (Estakhri 1990).
Tests performed in Florida in 1990 using the UltrafineTMprocess have not provided
conclusive results to date. However, few problems were reported during pavement
construction. The Florida tests are expected to yield ARCs superior to the conventional
sections (Ruth 1991).
414
M . A . Barlaz et al.
Waste tires as supplentental fuel
5.2 Cost
process. After mixing is complete, the mix should be maintained close to discharge
temperature until placement. This is particularly difficult because of the higher temperatures involved relative to conventional asphalt pavement mixtures.
The same paving equipment used for conventional hot mix asphalt concrete can be
used for RUMAC. However, paving machines should be equipped with full width
vibratory screens to aid compaction (Takallou & Hicks 1988).
The placement cost of ARC, using 18-26% rubber, is 50-100% higher than conventional
hot mix asphalt concretes. This is attributed to the cost of the ground tire rubber, the
increased mixing time, the limited number of licensed ARC paving companies and the
experimental nature of the process. ARC using 5 and 15% rubber contents as used in
Florida are reported to cost only about 10% more than conventional asphalt concrete
(Ruth 1991). The reduced cost relative to the patented technique is attributed to the use
of less ground rubber and the absence of a royalty payment. In addition, increased
mixing times are not required due to the smaller size and percentage of rubber in the mix.
5.4.I
5.3 Poreniiul tire iisc in North Curolinu
The North Carolina Department ofTransportation laid about 3 million metric tonnes of
plant mix asphalt in 1990 for road resurfking. Assuming a binder content of 6.5%, the
use of 5% ground tire rubber in the binder (UltrafineTM),and 5.45 kg rubber per tire,
potential tire use in North Carolina is 1.8 millions tires per year. This assumes that
asphalt rubber concrete is incorporated into all of the state's pavement projects.
Assuming the use of 20% ground tire in the binder (proprietary process), up to 7.2
million tires could be consumed. While it is unlikely that all of the state's pavement
projects will switch to ARC immediately, the data d o indicate that ARC has the
potential to consume a significant fraction of the state's scrap tires.
5.4 Rubber modified asphalt concrete
Rubber modified asphalt concrete (RUMAC) uses ground rubber to replace 2-5% of the
mineral aggregate in hot mix asphalt concrete. A process patented as PlusRideT" is the
most well-known and field tested RUMAC. This method uses an aggregate gradation
known as gap grading. Aggregate gradations are such that very little mineral aggregate
remains between 0.841 and 6.3 mm. Rubber chips between 0.841 and 6.3 mm are used to
replace the missing mineral aggregate. A non-proprietary variation of PlusRideT",
which uses smaller rubber particles to produce a denser aggregate gradation has also
been developed (Takallou & Hicks 1988). As is the case with ARC, RUMAC
recyclability remains unknown.
The major construction modification required to produce RUMAC is the need for a
method of combining the rubber and mineral aggregate in proper proportions. Continuous. dryer drum, and batch mixing plants can all be used to prepare RUMAC. Batch
plants are generally preferred because the manual bag count method of introducing
rubber into mineral aggregate offers superior quality control (Esch 1984). [ t is also
suggested that for PlusRideTMapplications, two mineral aggregate stockgiles be maintained, one with material larger than the specified gap size and the other containing
material smaller than the recommended gap size (Allen & Turgeon 1990). A common
problem with RUMAC is the inability to obtain sufficient fine content in the mix. This
causes the pavement to have an unacceptably high air void content. Mineral filler such as
fly ash, limestone dust, and electrostatic precipitator dust (Cottrell flour) are recommended when achieving adequate fine percentages is a problem (Takallou & Hicks
1988).
Higher mixing temperatures, longer mix times, and i-1.5% more binder is needed to
thoroughly coat the mineral aggregate and rubber chips with asphalt in the RUMAC
'
475
Per/:fbr.nlclllc.e
Experiences of Plus RideT" in Alaska, Minnesota, California, and Virginia are discussed
here. Field trials in Alaska demonstrated that PlusRideTMoffered superior stopping
distancc over conventional pavement, cspecially under icy road conditions. The average
stopping distance at 46 km per hour on four projects in Alaska was 20.4 in on the
PlusRideTMpavement section and 27.7 m on conventional pavement. These tests were
performed at subfreezing temperatures and show a 25% reduction in stopping distance.
Improved stopping distance was attributed to the crumbling of surface ice as rubber
particles in a pavement flex under traffic weight. The benefit of rubber particles was
especially noticeable on high speed and high traffic areas or where maintenance
personnel could remove snow from the pavement. Roads with accumulated snow will
not show improved skid resistance because the rubber particles will be prevented from
flexing (Esch 1984).
Other states in which PlusRideTMhas been tested have had mixed results. Of two field
trials in Minnesota in 1984, performance on one test section was equivalent to the
control with respect to cracking and skid resistance. A second section performed poorly
and had to be repaved within 1 year (Allen & Turgeon 1990). A trial in California
produced a pavement which did break up surface ice. However, the asphalt rubber
pavement did not last as long iis conventional pavements (Duenno pers. conim.). Tests i n
Virginia with RUMAC containing 3% rubber in 1983 provided disappointing results.
The test section broke up and had to be removed after about 2 months (Hughes 1985).
All three of these states attributed the Failure of RUMAC to problems in construction.
The PlusRide'." system. as with non-proprietary RUMAC, is not forgiving with rcspect
to m i x and construction specifications (Hughes 1985).
5.4.2 Cost
The in place cost of PlusRidelh1is two to three times that of conventional asphalt. The
increased cost is attributed to the cost of the rubber, increased mixing costs, increased
attention needed during mixing and construction and the limited number of proficient
RUMAC paving companies. As more RUMAC field trials are conducted and contractors become more confident in the product, the cost of this pavement should
decrease. Similarly, increased data on the performance of generic versions of RUMAC
should result in the availability of a cheaper alternative to the proprietary version.
5.4.3 Poretitid lire i i s c iii North Caroliiio
The North Carolina Department of Transportation (NCDOT) laid about 3 million
metric tonnes ofhot mix asphalt in its resurfacing program in 1990. Assuming the use of
3% ground tire rubber in the aggregate, 16.5 million tires could be consumed if all of the
state's road pavement projects used RUMAC. However. the high cost and difliculties
M.A . Barlaz et at.
Waste tires .as supplemental fuel
The considerations applicable to asphalt rubber concrete are equally applicable to
RUMAC and to a company which collects tires for use as supplemental fuel. There may
be a net saving to society if tires are used as fuel. This could justify a government policy
which directs scrap tires to energy users and assures that they receive a tipping fee
sufficient to make the combustion of tires economical. Less governmental input would
be needed if the tipping fee is less than that of other disposal options.
In addition to economics, there are other factors that may drive the recycling of scrap
tires. There is a public relations benefit to any industry which convinces the public that it
is using a waste stream in a beneficial way. There is also the risk that the public will
protest the use of waste. Where a paper mill is under pressure to reduce sulfur emissions
and threatened with either fines or the cost of more pollution control equipment, the
effort required to convince the public that burning tires which are lower in sulfur is
acceptable may be worthwhile. In the absence of such pressure or the potential to gain a
competitive advantage in a market, it is not clear whether there is any incentive for an
industry to voluntarily begin burning waste, regardless of its environmental safety. In
the absence of such incentive, government policy may be required to encourage
beneficial reuse of a waste.
I n the devclopmcnt of a policy to iniplcnicnt ;I scrap tire niiinageiiieiit alternalivc, thc
environmental implications and risk of all alternatives must be considered. It is unlikely
that there is a management alternative with no risk so the objective must be to minimize
risk and cost. Risks associated with the generation of ash which is high in zinc must be
compared with the potential release of metals from cement or road pavement, and also
compared with land use required for burial or stockpiles. The outcome of such a risk
analysis will depend on what alternatives are available in a given region. While North
Carolina has a large paper industry, combustion of tires in paper mill boilers may not be
an affordable alternative in other regions.
Two methods for incorporation of scrap tires into asphalt road pavement which have
the potential to use significant quantities of tires are asphalt rubber concrete and rubber
modified asphalt concrete. The proprietary and non-proprietary versions of ARC have
the potential to use 7.2 million and 1.8 million scrap tires, respectively. Though it
generally provides a superior pavement, the l50-200% increase in cost of the proprietary
process limits its widespread use. While the cost of the non-proprietary process is only
slightly greater than that of conventional asphalt, the process has not been fully tested
and this limits its widespread use. RUMAC has the potential to use 16.5 million waste
tires. However, construction problems coupled with high cost will limit the use of rubber
modified asphalt concrete for the foreseeable future.
The major factor which seems to be limiting the use of more tires in asphalt pavement
is cost. As patents expire, more data become available on non-proprietary processes and
more contractors gain experience in working with asphalt rubber in all forms, the use of
tires in asphalt pavement can be expected to increase. However, increases are likely to be
incremental. One unanswered question concerning all rubberized asphalt concretes is the
issue of recyclability. A second unanswered question is the release of metals from rubber
asphalt pavcnicnt. Hcrc too, further study is necdcd.
The assessment performed here is useful for states or other planning regions as thcy
develop solid waste management plans that include recycling. Recycling rcquircs the
presence of proven and cost effective technology, sufficient manufacturing capacity and
a demand for a product manufactured from the recycled waste. The methodology
employed here can providea planning region with a firm estimate of the true potential to
recycle a particular component of the solid waste stream. The methodology may also be
useful for development of policies designed to facilitate implementation of a desired
solid waste management strategy.
7. Summarj and conclusions
Acknowledgements
478
We estimate that six million scrap tires per year are generated i n North Carolina.
Relative to some coals, tires have a higher energy content, less sulfur and nitrogen, and
often cost less, making them an attractive alternative fuel. The combustion and air
pollution control technology exists to burn scrap tires safely in pulp and paper mill
boilers. The fraction ofTDF fed to a boiler must be limited so that particulate emissions
are within acceptable criteria. Preliminary trial burns are needed to optimize T D F
combustion conditionsand more data are needed to verify the actual performance of
pulp and paper mill boilers using TDF. Use of T D F by North Carolina paper mills has
the potential to consume over 90% of the state's annual scrap tire generation. In
practice, the amount consumed will be reduced by geographical and economic limitations.
There are no cement kilns in North Carolina. However. there are seberal within a
reasonable distance of North Carolina population centers. The potential tire use by
these kilns is approximately 6.6 million tires per year. Should cement companies in the
southeastern U S A . begin to use tires as supplemental fuel, it is likely that priority will
be given to the collection of tires generated in the resident state ofeach kiln. Most of the
waste products from tire combustion in cement kilns are incorporated into the cement
product and tire combustion in cement kilns appears to be enviroqmentally sound.
Again, there are no reported data on actual air emissions when T D F is burned in cement
kilns and no data on leaching of metals from cement produced in kilns which burn tires.
479
This work was supported in part by the Center for Waste Minimization and Management of the U.S. Environmental Protection Agency.
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