End-of-Life Tire: A Popular Feedstock for Thermochemical Conversion Processes

The continuous increase of ground transportation has led to an important generation of end-of-life tires (ELTs) worldwide. Generally, the lifespan of the tire is four years. After this lifetime, the tires are released into the environment as waste, which occupies a massive amount of land. ELTs are extremely resistant to biological degradation and pose a potential threat to the environment. For this reason, the methods of ELTs treatment are becoming more and more widespread today.
Currently, several methods are adopted by industry to reduce discarded tires from the environment. Most of these methods convert tires from their own physical form to a different physical form but do not let them dissolve completely from the environment. Tire retreading, reclaiming, and grinding are the most common forms of physical transformation of dumped tires considered as recycling and reuse. Retreading involves the removal of the remaining worn-out tread and the application of the new tread. After retreading, the tire regains its original properties. Reclaiming of ground tire rubber (GTR) involves the destruction of the cross-linked structure involving cross-link scission and degradation of polymeric chains. Due to the changes in the physical and chemical structure of GTR, the obtained reclaimed rubber can be used in the manufacturing of new rubber compounds. Another physical conversion of tire is crumb rubber; a shredded/ground version used in rubber compounds such as various engineering rubber parts, and for secondary applications such as artificial turfs and roads.
Each of the methods mentioned above has its advantages and disadvantages. For example, the production cost of rubber powder is high, and the demand is limited. On the other hand, the production process of reclaimed rubber has some problems such as low profit, high energy consumption, and serious pollution. In addition, rubber products can no longer be used to produce new rubber products after being recycled two to three times, so they must be disposed of eventually.
Waste tire as one of the municipal solid wastes has a relatively high heating value (38–45 MJ/kg) relative to other waste materials such as biomass (22–28 MJ/kg), food waste (3–9 MJ/kg), animal manure (12–15 MJ/kg) and plastics (24–44 MJ/kg) as well as common fossil fuels such coal lignite (~17 MJ/kg) and subbituminous (33 MJ/kg). In this sense, the high energy-containing waste tire could be an intriguing feedstock to be utilized for producing value-added energy.
Generally, thermochemical conversion decomposes ELTs chemically to produce heat and fuel. Among all the thermochemical conversion technologies, pyrolysis is one of the most attractive processes to tackle the waste tire disposal problem while allowing energy recovery. When compared to other thermochemical conversion processes of ELTs, pyrolysis presents several advantages in terms of operational, economic, and environmental aspects. For instance, pyrolysis exhibits higher energetic efficiency and lower emissions of particulate matter and air pollutants (e.g., CO, CO2, NOx, SOx, and polycyclic aromatic hydrocarbons (PAH)) in comparison to incineration.
Pyrolysis enables the recovery of both energy and materials from ELTs, yielding valuable gas, liquid, and solid fractions. The distribution and composition of these products mainly depend on the pyrolysis conditions and reactor features.
It has been reported that tire pyrolysis oil has a High Heating Value (HHV), with a range of 40 − 44 (MJ/kg), which could make it useful as a replacement for conventional liquid fuels. However, the main disadvantage of using tire pyrolysis oil as fuel is its strong acrid smell and its low flash point, as compared with other conventional liquid fuels. The flash temperature of a fuel is an important factor because it gives an indication of the hazards of fire during storage and usage of the fuel. The produced pyrolysis carbon black also has a High Heating Value (HHV) of around 30-35 (MJ/kg), which could also encourage its usage as a solid fuel. Carbon black could also be used as activated carbon, printers’ ink, etc. The pyrolysis gas obtained from waste tires mainly consists of light hydrocarbons and has a high calorific value (approximately 30 MJ/kg), which can meet the process requirement of energy. Hence, the tire pyrolysis gas has enough energy not only to drive the pyrolysis process but also to produce heat and electricity.