Dr. Dmitri Kopeliovich
Waste tires account about 2% of the global solid waste.
Tires are almost not biodegradable.
More than 1 billion of scrap tires are generated every year in the world. Most of them are dumped into landfills contaminating the environment, providing breeding sites for mosquitoes and presenting a fire risk.
However the environmental aspect is not the only reason to recycle the scrap tires. Dumped tires store a large amount of energy, which may be recovered from them in form of solid, liquid and gaseous fuel.
The energy may be recovered from scrap tires directly by using them as a fuel in incinerators.
This technique is called combustion. It allows to recover maximum amount of the energy: 12,000-16,000 BTU/lb (27,900-37,200 kJ/kg). However the direct tire combustion produces black contaminated smoke, which should be cleaned.
The alternative approach of scrap tires energy recycling is a decomposition of the tire material by a thermal conversion process.
There are two types of thermal conversion of carbon containing materials: pyrolysis and gasification.
Pyrolysis is the thermal decomposition of an organic substance in the absence of Oxygen (inert atmosphere).
In pyrolysis large hydrocarbon molecules of the substance break down into smaller molecules. Generally three products form as a result of pyrolysis: a fuel gas, liquid (pyrolytic oil)- up to 60% and solid residue (char)- 35-40%.
The solid char is composed of carbon black, which may be used for manufacturing tires and other rubber products, as a pigment in inks, paints and toners, as active carbon and for fabrication electrodes and cell batteries cores.
The liquid fraction consists of a mixture of oils, which may serve as a fuel or a raw material in oil refinery process. The combustion heat of the liquid fraction is 18,000 BTU/lb (42,000 kJ/kg).
The gaseous product is composed of the gases: CO, CO2, H2, CnHm.
The liquid/gas ratio depends on the temperature. The higher the temperature, the lower the liquid/gas ratio.
The typical temperature range of pyrolysis is 750-1800°F (399-982°C).
Another type of the thermal decomposition is gasification.
Gasification is the thermal conversion of an organic substance into a combustible gas in the presence of an oxidant (air, steam).
The oxidant (gasification agent) is supplied to the feedstock (e.g. shredered tires) where a series of heterogeneous reactions takes place:
Pyrolysis: Feedstock → C (carbon black) + volatile fraction
C + 1/2O2 → CO
C + O2 → CO2
C + CO2 → 2CO
C + H2O → CO + H2
C + 2H2 → CH4
nC + m/2H2 → CnHm
There are three general types of gasification process: fixed bed, fluidized bed and entrained flow (see the picture):