A TECHNO-ECONOMIC AND ENVIRONMENTAL ASSESSMENT OF A TIRE PYROLYSIS PLANT
Mort Fels and Michael Pegg
Chemical Engineering; Dalhousie University
Halifax, NS; Canada
Abstract: An evaluation of two tire pyrolysis plants operating in Shanghai and Taiwan
were done. The tires are first shredded and heated to 425oC. The gas product used to
provide heat for the process is made up of CH4 (36%), C2–C4 (32%), H2 (20%), and some
CO2, N2 and CO. The oil contains multiple organic compounds and is physically similar
to a heavy diesel. It is sold to an oil refiner. The char is about 90 wt% C and is recycled
back to the tire manufacturer. Emissions of CO, NOX, SO2 were at least 100 times less
than Canadian standards. The major metallic emissions were in ―class 1‖ at 0.16 μg/m3
(stack); Canadian standards being 50. PCDD/F stack emissions were 0.005 ng/m3—an
insignificant amount (there is essentially no chlorine in tires). Economically, investment
capital would be $504/t-year of tires processed; operating costs would be $250/t. A 15%
rate of return would result if the oil could be sold at $575/t ($80/bbl) and 0 carbon value
or $180/t and $500/t carbon.
1. INTRODUCTION
Enormous quantities of scrap tires are generated worldwide (~ 5x106 t/y) and their disposal represents a significant
environmental and economic challenge. Annually, about 2x106 t are produced in Europe, 2.5x106 t in North
America, and 0.5x106 t in Japan [Boxiong et al., 2007]. China is by far the biggest consumer of rubber and the
second largest country to produce tires. Major techniques for the use of scrap tires are tire-derived fuel, road
construction and rubber crumb. Good analyses of the scrap tire markets are given by Pehlken and Essadiqi (2005)
for Canada and RMA (2006) for the US.
A Chinese company, Shanghai Greenman ECO, has proposed to use pyrolysis to convert the scrap tires into useful
products. To this end, the authors performed a comprehensive assessment of two plants which were processing
about 8,000 t/y of tires. This paper gives the results found.
2. PROCESS DESCRIPTION
A simplified process flow sheet for the pyrolysis of waste tires is shown in Figure 1.
Fig. 1. Tire Pyrolysis Simplified Process Diagram
Scrap tires are delivered by truck, weighed and stockpiled outside. The tires are cleaned with compressed air and
then conveyed into the shredder to produce chips of about 3 cm square. The shredded tires then are fed into the
pyrolysis reactor which is essentially an inclined rotating cylinder about 10 m long. During this time, the tire pieces
are heated to about 425 C by combustion of the pyrolysis gases outside the cylinder.
The solid residue that exits the pyrolysis reactor contains both carbon black and the steel from the tires. The steel is
separated magnetically, baled and shipped to recycle. The gaseous products exiting the reactor are cooled, whereby
the pyrolysis oil (TPO) is separated from the gas. The gas is compressed and used to provide the energy for the
pyrolysis reactions. The oil is then pumped to storage tanks. The yield (weight %) of products as measured at the
plant in Shanghai was as follows: Gas = 10%; Tire Pyrolysis Oil = 45%; Carbon = 35% and Steel = 10%.
3. PRODUCTS DESCRIPTION AND COMPOSITION
3.1 Tire Composition
Tires are made up of various rubber-like polymers. The most common is SBR (styrene-butadiene rubber). Other
rubber compounds are natural rubber (cis-poly isoprene), and cis-polybutadiene. Additives include carbon black to
strengthen the rubber, extender oil to soften and improve workability, and sulphur to cross-link the polymer chains
(vulcanization). A typical formulation (by weight) is given by Teng et al. (1995) as: SBR and natural rubber: 60–65
wt%; carbon black: 29–31%; zinc oxide: 2–3 %, sulphur: 1–2 %; extender oil and additives: 2 % each.
The elemental analysis of tires excluding the steel typically shows the following by weight: C = 80%; H = 8%;
O = 3%; S = 0.5%; ash = 8.5%. The HHV = 35 MJ/kg (coal = 32.3 MJ/kg).
3.2 Pyrolysis Gas
Elemental composition of the gas was mostly C (86%) and H (14%) by weight and consisted of the following
compounds (% by volume): CH4 = 24.3%; H2 = 17.6%; C4 = 14.2%; C2H6 = 13.3%; C2H4 = 9.4%; CO2 = 11.0%;
N2 = 6.2% and CO = 4%. The higher heating value was determined to be 32 MJ/kg.
3.3 Tire Pyrolysis Oil (TPO)
The elemental analysis of the oil gave the following (by weight): C = 87.0%; H = 8.4%; S = 1.3%; and N = 0.5%.
The higher heating value was 39.6 MJ/kg. As expected, there is a multitude of chemical compounds to be found in
the oil—alkanes, alkenes, aromatics and various cyclic compounds. A good appreciation of the large variety of
compounds in the oil can be seen in a study by Rofiqul et al. (2007) and Qu et al. (2006).
Physically, the oil is somewhat like a heavy distillate (about 70 wt%) with the remainder having light fractions
similar to gasoline (10%), kerosene (7%) and gas-oil (11%) (Mastral et al., 2000). Table 1 shows a comparison of
the major physical characteristics of TPO with #2 diesel.
Table 1 Comparison of Tire Pyrolysis Oil with Diesel
Property
Density, kg/m3
Viscosity, cs @ 40 C
Flash point, C
90% distillation point, C
TPO
968
3.88
15
555
Diesel
850
1.9–4.1
52
282–338
Therefore, this oil would, of necessity, be used as a feedstock for an oil refinery. Currently, an oil refiner in China is
purchasing the product from Shanghai Greenman.
3.4 Carbon Black Solid
The solid carbon product of tire pyrolysis is an important and valuable entity. Two main uses are to recycle it back
to the tire manufacturer (about 30 wt% of a tire is carbon black) and to process it further for use as activated carbon.
The solid from the plant was found to be (weight basis) 87% C, 1% H and 12% ash. Analysis of the carbon black
solid and its ash gave the results shown in Table 2. As can be seen, the major components were ZnO (used in the
vulcanizing process), and SiO2.
Table 2 Analysis of Carbon Black Solid (Major Components)
Carbon Black
Element
Conc (g/kg)
Zn
294
S
57
Ca
51
Fe
10
Al
8
K
8
Na
7
Pb
3
Ash
Oxide
ZnO
SiO2
CaO
Al2O3
Fe2O3
K 2O
Na2O
Content (wt %)
42.5
26.5
6.9
1.6
1.5
1.0
1.0
4. ENVIRONMENTAL ASPECTS
There are three sources of emissions to the environment of a pyrolysis plant: (1) wastewater from the cleaning of
tires prior to shredding; (2) air emissions from the combustion of the pyrolysis gas; and (3) fugitive emissions, that
is, volatiles given off from the oil collection, storage facilities and the plant itself.
4.1 Wastewater Emissions
Wastewater from the cleaning of tires would, of course, only occur if the tires were washed as a cleaning method. In
Shanghai, the tires are cleaned only with compressed air which is sufficient to remove any solid material which
adheres to the tires. On the other hand, for tires which have been exposed to road salt, it would be essential to use a
water wash to remove the salt. It is important to ensure that there is essentially no chloride content in the tires; this
element is necessary for the formation of dioxins and furans (PCDD/F) and could adversely affect the air emissions.
The amount of wastewater produced per month from general plant housekeeping was measured to be about 150
tonnes in the plant at Shanghai. The water is collected in a pond and is reused after any oil is removed by a standard
centrifugal separator. The amount of oil averages 0.26% (about 390 liters/month). This oil is sent to the main
burner for combustion.
There are several companies who make equipment designed for the purpose of removal of oil and solids from
wastewater resulting from petroleum operations. Either gravity or centrifugal separators can be used depending on
the particle size.
4.2 Emissions from Pyrolysis Gas Combustion
The pyrolysis of 1 tonne of tires (including the steel) produces approximately 0.1 tonne of gas. The composition of
the gas is mostly hydrocarbons and hydrogen. About 40% of the gas is burned to provide the energy for the
pyrolysis process, about 30% for drying and the remainder is burned in a flare stack.
Specific to the tire pyrolysis plant, one can divide the emissions into three classes:
Gaseous Emissions It is noted that there is a paucity of data on emissions from the gas combustion, because,
considering the very small quantity of molecules which would form pollutants (sulphur, chlorine, metals), these
precursors tend to remain either in the oil or in the solid carbon product. Table 3 gives some values measured in the
Shanghai facility (Tongji & ECU, 2008 ) compared with two others. The data are expressed in emission factors
(kg/t), stack concentrations (mg/m3) and ground-level concentrations (μg/m3).
Table 3: Emission Data from Gas Combustion
Ref
1
2
3
1
kg/t
4.9
0.64
0.75
SOX
mg/m3
2,030
265
311
CalRecovery (1995);
μg/m3
3.6
0.47
0.55
kg/t
5.6
0.16
0.51
2
NOX
mg/m3
2,330
67.7
210
Tongji & ECU (2008);
μg/m3
4.1
0.12
0.37
Kg/t
0.25
0.02
0.01
PM
mg/m3
105
8.4
2.5
μg/m3
0.19
0.02
0.005
kg/t
—
0.01
.003
THC
mg/m3
—
3.7
0.11
μg/m3
—
0.007
0.00
3
EPA (1993)
Metallic Emissions: Metal content of the 8.4 mg/m3 particulate sample reported for the test at Shanghai gave a total
amount of metal of only 26.8 × 10–4 mg/m3. This would indicate that the metal content of the ash is only 0.032%. It
is uncertain what other elements would be present; certainly oxygen from the metal, some sulphur (as sulfates) and
probably silicon dioxide.
Dioxins and Furans (PCDD/F): Three tests for dioxins were done on the combustion gas by the Environmental
Monitoring Group in Shanghai. The results are shown in Table 4. It should be noted that the emission levels
reported are a factor of 6 below the proposed Canadian standard of 32 pg/m3 (0.032 ng/m3) stack gas.
Table 4: Dioxin/Furan (PCDD/F) Emission Data from Shanghai [From Tongji & ECU, 2008]
Sample Number
1
2
3
Average
Emission Factor
μg/t
0.005
0.018
0.018
0.013
Stack Gas Emission
ng/m3
0.002
0.007
0.007
0.005
4.3 Fugitive Emissions
Estimated fugitive emissions from a ―generic‖ pyrolysis plant have been reported as 51.27 kg/day of VOC (see
Table 5 reproduced below from EPA, 1993). Using the dispersion formula, it can be determined that this quantity
would result in approximately 1.2 μg/m3 in the air.
Table 5: Estimated fugitive VOC emissions from a "generic" pyrolysis plan [After Table 8-6 in EPA, 1993]
Emissions source
Pipe flanges
Compressors
Open drains
kg/day
2.7
5.0
4.54
Emissions source
Pump seals
PRVs
Valves
kg/day
5.9
2.27
30.84
5. ECONOMIC RESULTS
An economic analysis was performed on the tire pyrolysis plant, base case of 10,000 tonnes per year, approximately
equivalent to one million tires annually (roughly the amount generated in Nova Scotia). Operating costs consist
essentially of labour and electricity to run the plant; revenue would be derived from the sale of the pyrolysis oil, the
carbon black, the steel, and payment for the tire disposal (tipping cost).
5.1 Capital Costs
For capital cost estimation, the equipment costs obtained from the Shanghai plant ($2.5 million) and that from the
Taiwan plant ($3.5 million) were used. Other direst costs such as installation, piping, electrical, etc. were obtained
from data from the actual plants ((Yeung-Chen, 2008). Working capital and contingency were calculated using
standard estimation practice (Peters et al., 2003) Table 6 shows a summary of the capital costs.
There are very few references to costs in the literature. The report by the University of California [UofC, 2006]
quotes a value of $628–828 per tonne/year for a plant capacity of 5 million tires/year.
Table 6 Capital Costs ($ million) for Tire Pyrolysis Plant; 10,000 t/year tires
ITEM
Equipment (E)
Other Direct costs
Working Capital
Contingency
Total Capital
BASIS
given
given
0.1 (capital)
0.08 (capital)
—
REFERENCE
Yeung-Chen (2008)
Yeung-Chen (2008)
Peters et al. (2003)
Peters et al. (2003)
VALUE, $106
3.00
1.13
0.50
0.40
5.04 (504/t/year)
5.2 Operating Costs
Direct operating costs essentially consist of labour and electricity to run the plant; revenue would be derived from
the sale of the pyrolysis oil, the carbon black, the steel, and payment for the tire disposal (tipping cost). All
operating costs were based on estimates by MedNova (Yeung-Chen, 2008) based on experience at the two operating
plants in Shanghai and Taiwan, and the labour rates in Nova Scotia and are shown in Table 7. The total number of
hours per year was estimated by plant personnel as 7680.
Table 7: Operating Revenue and Costs for 10,000 t/year Tire Pyrolysis Plant
OPERATING REVENUE
ITEM
VALUE
UNIT $
Oil sales
4500 t/year
$ 400/t
Carbon sales
3500 t/year
$ 300/t
Steel sales
1000 t/year
$ 150/t
Tipping fee
10,000 t/year
$ 80/t
Total Revenue
6
$10 /y
1.80
1.05
0.15
0.80
3.80
ITEM
Power
Labour
Maintenan
ce
Other
OPERATING EXPENSES
VALUE
UNIT $
2000 MWh/y
$0.05/kWh
45 people
$45,000/y
–
–
–
–
Total Expenses
$106/y
0.10
2.03
0.05
0.35
2.53
5.3 Return on Investment
The effect of the selling price of the pyrolysis oil and carbon on the rate of return is shown graphically in Fig. 2. At
a reasonable rate of return of 15%, it can be seen that the price one gets for the oil depends on the value of the
carbon, ranging from $450/t when the carbon has 0 value, down to $180/t if one can sell the carbon for $500/t.
6. CONCLUSIONS
It is concluded that the pyrolysis plant using the Shanghai Greenman technology is able to pyrolyse tires
satisfactorily and produce a product of tire pyrolysis oil, carbon black and recover high quality steel. The plant
operates in a safe and environmentally non-polluting manner. Emissions are well below Canadian standards. The
return on investment is dependent on the price obtainable for the oil and carbon products; conservatively with the oil
selling at the same price as crude, one should be able to realise at least a 15% return.
7. ACKNOWLEDGEMENTS
The authors wish to thank the group in Shanghai and Taiwan, especially Mr. Xu Junshi, Ms. Delphine Yu, Ms
Regine Hsu, Mr. Tom Wu and Dr. Yeh for their hospitality and help and guidance in explaining the details of the
process. The help and guidance of Ms. Wendy Yeung-Chen and Mr. Robert Spence of MedNova, Halifax is greatly
appreciated.
35
$500/t
30
$400/
t
$300/t
25
Rate of Return, %
$200/t
$100/t
20
$0/t
15
SELLING PRICE OF CARBON
10
5
0
100
200
300
400
500
600
700
800
Oil Selling Price, $/t
Fig. 2 Effect of Oil and Carbon Selling Price on Rate of Return
6.0 REFERENCES
Boxiong, S. Chunfei, W., Cai, L., Binbin, G. and Rui, W. (2007) Pyrolysis of waste tyres: The influence of USY
catalyst/tyre ratio on products J. Anal. Appl. Pyrolysis 78: 243–249.
CalRecovery, Inc. (1995) Environmental Factors of Waste Tire Pyrolysis, Gasification, and Liquefaction, Final Report
to California Integrated Waste Management Board, Sacramento, CA.
EPA (1993) (United States Environmental Protection Agency) Scrap Tire Technology and Markets - Chapter 8: Scrap
Tire Pyrolysis, Pacific Environmental Services, Noyes Data Corp/, Park Ridge, NJ.
Mastral, A.M., Murillo, R., Callen, M.S., Garcia, T., and Snape, E.C. (2000) Influence of Process Variables on Oils from
Tire Pyrolysis and Hydropyrolysis in a Swept Fixed Bed Reactor, Energy and Fuels, 14:(4) 739–744.
Peters, M.S., Timmerhaus, K.D. and West, R.E. (2003). Plant Design and Economics for Chemical Engineers, 5th ed.,
McGraw-Hill, New York, NY.
Pehlken, A. and Essadiqi, E. (2005) Scrap Tire Recycling in Canada CANMET-MCL
Qu, W., Zhou, Q., Wang, Y-Z, Zhang, J., Lan, W-W, Wu, Y-H, Yang, J-W, and Wang, D-Z (2006) Pyrolysis of waste
tire on ZSM-5 zeolite with enhanced catalytic activities, Polymer Degradation and Stability, 91: 2389–2395.
RMA (2006) - Rubber Manufacturers Association Scrap Tire Markets in the United States. Available from:
http://www.rma.org/
Rofiqul, I.M., Handiu, H. and Rafiqul, A.B.M. (2007) Limonene-Rich Liquids from Pyrolysis of Heavy Automotive Tire
Wastes, J. Environment and Engineering, 2: (4) 685–695.
Teng, H., Serio, M.A., Wojtowicz, M.A., Bassilakis, R., and Solomon, P.R. (1995) Reprocessing of Used Tires into
Activated Carbon and Other Products Ind. Eng. Chem. Res., 34: (9) 3102–3111.
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Pyrolysis, Gasification, and Liquefaction, Report to California Environmental Protection Agency, Integrated Waste
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Yeung-Chen Wendy, (2008), President, MedNova Tech, Halifax, NS, private communication