Pyrolysis
Waste tire pyrolysis involves the thermal degradation in the absence
of oxygen. The benefit of this application is the conversion of waste tires
into value-added products such as olefins, chemicals and surface-activated
carbon.
More than 30 major pyrolysis projects have been proposed, designed,
patented, licensed, or built over the past decade, but none have yet been
commercially successful1).
The primary barriers for this application are both economic and technical.
The capital cost is high, and the products from pyrolysis do not have sufficient
value and must compete with commodity materials. However, it is expected
that technological innovations may break through this barrier in the near
future. Developments of less costly techniques or processes for higher
value added products would enable pyrolysis to become a profitable alternative
for waste tire recycling. Pyrolysis is known for low emissions to the environment.
General Process
of Tire Pyrolysis:2),3)
Configurations differ slightly between different facilities, but the
basic process is common.
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| wt.% | Content | ||
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Hydrogen, CO2, CO Methane, Ethane, Propane, Propene, Butane, Other
hydrocarbons,
app. 1% of Sulfur |
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High aromatic
Mw 300 - 400 Low in sulfur (0.3 - 1.0%) Aromatics, Alkanes, Alkenes, Ketones, Aldehydes |
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>15 % of Ash (ZnO)
3 -5 % of Sulfur |
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Problems:
Low Product Price: The primary products are essentially low
molecular weight olefins and char.
Olefins: The pyro-gas prices are low in the current market.
Other chemicals are valuable, but the yield is low. High quality carbon
black is also valuable but there is no particular price advantage for the
same quality carbon from traditional processes.
Char: Surface activated carbon is a valuable product, but there
is no cost advantage compared to alternative methods (normal surface activated
carbon manufacturing).
High Process Cost: The valuable chemicals from pyro-gas or oil,
are generally high molecular weight substances. The purification of high
molecular weight substances is expensive.
New technologies:
There are two technological approaches to the problems discussed above.
(a) Higher Value Products from Pyrolysis. (high molecular weight
olefins)
The production of significant quantities of valuable high molecular
weight olefins to obtain curable and moldable olefins (Mw > 15,000) would
overcome current economic barriers. These are typically produced in small
quantities because the process temperature is high. At high temperature,
vulcanized rubbers are quickly decomposed to low molecular weight olefins
(Mw 300 -400). High molecular weight compounds can be generated by low
temperature pyrolysis.
However, lower temperature will require longer process times. New technological
breakthroughs will be necessary for the commercialization of low temperature
pyrolysis. Four new technologies are being developed.
Microwave pyrolysis :2: Lower process cost.
Microwaves can heat objects more uniformly than conventional heating methods. Microwave heating requires shorter heating times. Microwave pyrolysis will result in relatively high molecular weight olefins and a high proportion of valuable products such as ethylene, propylene, butene, aromatics, etc. The short process time also contributes to a reduction in the process cost. Moreover, for microwave heating, the shape of the tire chip is less important compared to the requirements of conventional heating. Whole tires or larger chips can be processed using microwave pyrolysis, which greatly reduces pre-processing cost.
Ultrasonic devulcanization:5),6)
Isayev has patented a method which minimizes heating and uses sonic energy to break down sulfur-carbon chemical bonds in tires. Chipped tires are heated to about 400 F, then subjected to 20,000 cycles per second of ultrasonic energy (just above the highest frequency the human ear can discern) at pressures up to several thousand pounds per square inch. The rubber is transformed from a solid to a highly viscous fluid within milliseconds. With additional curative agents the viscous material can be molded into new products. A prototype machine can handle approximately 50 pounds of tires per hour.
Supercritical fluid depolymerization:7),8)
Supercritical water can be used to controllably depolymerize the rubber compounds. This approach requires lower temperatures (approx. 750 F) and shorter processing times. Tire compounds are decomposed to high molecular weight olefins (Mw 1,000 - 10,000), or oils ( Max. 90 %).
The technique is being developed and has been tested only an an experimental scale. Because of the expensive supercritical water equipment, this application would require a relatively large initial cost.
Use of special catalysts:
Use of catalysts can reduce processing temperature or time. As shown in the above applications, reduced temperature and time can result in either higher molecular weight olefins or an increasing proportion of valuable substances. The advantage of catalysts is that no new equipment or knowledge is required. Therefore cost estimation and scale-up would be easy. Some research and pilot scale experiments have been conducted recently, but the types of catalysts are highly proprietary.
Reference Site
for Tire Pyrolysis:
Home Page of Hans Darmstadt at Laval
University, Quebec. C. Roy, H. Darmstadt, B. Benallal, A. Chaala and
A.E. Schwerdtfeger. Vacuum pyrolysis of used tires.
References