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Understanding tire pyrolysis and the process

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Tire recycling is not new.

Currently, the standard method for tire recycling is rubber granulation. However, rubber granulate applications aren’t environmentally friendly or circular and don’t recycle 100 per cent of the materials in waste tires.

Tire pyrolysis, an alternative method of tire recycling, offers the automotive industry circularity and the possibility of reusing materials beyond recycling.

What is tire pyrolysis?

Tire pyrolysis is a form of chemcycling where ground tire waste undergoes thermochemical decomposition at high temperatures in an inert oxygen-free atmosphere to yield recovered Carbon Black (rCB), recovered steel, pyrolytic oil, and pyrolytic gas.

Without oxygen, the polymers in tire wastes don’t burn. The heat catalyses chemical reactions to break down the vulcanising bonds in the rubber granulates. Since 85 per cent of tire components are petroleum- or polymer-based, pyrolytic oil and gas are two major products generated.

All pyrolytic products from tires have circular uses.

  • The rCB is a valuable alternative material that can be used to manufacture new tires instead of medium-grade virgin Carbon Black (vCB) produced from fossil fuels.
  • The recovered steel can be used again in tire manufacturing.
  • The high-quality pyrolysis oil can be used as vehicle fuel or to produce fine-grade vCB for tire manufacturing.
  • The pyrolytic gas can run a facility plant and reduce the carbon footprint of all the pyrolytic products produced there.

Tire pyrolysis adapts the well-known process to specifically recycle end-of-life tires (ELTs) since each feedstock needs a different temperature and treatment time.

Three main types of pyrolysis exist — slow, fast, and flash.

Each uses different temperature ranges, heating rates, and residence times to give various products. The distinction between the types is not clear-cut. Furthermore, reactors can be batch or continuous types and use various kinds of beds.

Tire pyrolysis is the most environmentally friendly and safe method to dispose of tires. It produces few poisonous pollutants and has a low carbon footprint compared to other methods — landfilling, incineration for energy, open burning, or rubber granulate civil engineering applications.

Figure 1: Systems boundaries, Buadit et al. 2020. (Image credits: Life Cycle Assessment of Material Recovery from Pyrolysis Process of End-of-Life Tires in Thailand)

How does pyrolysis work?

The products, yield, and quality will all be different depending on the pyrolysis reactor. However, all pyrolysis methods share basic processes, which can be grouped into three steps.

Figure 1 explains the various steps involved in the basic pyrolysis process.

Phase 1: Feedstock Preparation

The processes before actual pyrolysis are crucial and influence the quality of the pyrolytic products. The ELTs are shredded to separate steel and fabric from the rubber components.

Mechanical primary and secondary shredders cut the rubber down to produce 10-50 mm rubber granulates stored in silos.

Shredding tires leads to better quality products than entire tires, as they can be heated faster and more evenly. The separated steel can be recycled. Our process at Contec involves sourcing and using the best quality feedstock during this phase.

Phase 2: Pyrolysis Process

Before the rubber granules are fed into the pyrolysis reactor, the chamber undergoes inertisation to protect the process and staff from combustion.

The oxygen content of air is reduced from 21 per cent by volume to less than 13 per cent by pumping in nitrogen, an inert gas. Not all pyrolysis processes use this step, and by not opting for this step, they risk explosions. Inertisation is crucial to the safety of the process.

Next, the rubber granulates are fed into the reactor and heated to temperatures between 400 and 700°C. Some pyrolysis methods use high pressure and catalysts to aid this process.

The heat leads to various decomposition and volatilization reactions like cracking, dehydration, isomerization, aromatization, dehydrogenation, and condensation. The solid tire waste is converted to volatile gases, steel, and char.

Phase 3: Post-Processing

After passing through the condenser, most of the gas liquefies into pyrolytic oil rich in aromatic compounds, and the un-condensed gas is used as fuel to run the pyrolysis process, which is energy intensive.

The char comes out mixed with finer steel bits and inorganic salts. The solids go under a magnetic separator to remove all traces of steel, which is recyclable. The char is refined and powdered to produce rCB, then pelletised to meet market demand. 

Is the process safe?

The traditional pyrolysis process, despite its benefits, has some risks and is prone to explosions and fire. Historically, the equipment can be damaged, and people have been injured and even killed due to explosions in pyrolysis plants.

Problems can occur because the gases produced from tire decomposition are combustible. If excess oxygen gets into the system because of a mishap or flaw and comes in contact with the gases at high temperatures, the gas can ignite and cause an explosion.

Explosions are a result of a lack of inertisation and proper process control. Therefore, modern tire pyrolysis processes have introduced many security measures to eliminate or diminish the chances of such explosions.

Contec has incorporated stringent safety measures while planning and constructing its protected tire pyrolysis plant to ensure the process is safe for its people, the neighbourhood, and the environment:

  1. Contec uses an expensive inertisation process, even though it isn’t legally required.
  2. Contec avoids gas pressure buildup by taking the following precautions:
    1. Cleaning pipes even when the plant is in process.
    2. Using two sensors to check pressure each second so that personnel can take steps to correct pressure changes.
    3. Using a relief pipe to divert excess gas.
    4. Carrying out a thorough inspection of pipes, pressure, and gas odour before the start of any run.
  3. Contec maintains complete and instant control over the temperature of molten salts, used as a heating medium. These circulate in a different jacket, which makes the system safer. The disposal of molten salts also causes no environmental problems, as their chemical composition is similar to that of fertilisers.

Tire pyrolysis at Contec

Contec has improved upon older tire pyrolysis processes in collaboration with the Warsaw University of Technology and has been involved from the planning stage in setting up the pilot plant in Szczecin, Poland.

The protected Contec tire pyrolysis process uniquely uses molten salts as a heat transfer medium.

Molten salts, historically used to store solar energy, have recently been incorporated into tire pyrolysis. Contec heats the molten salts and pumps them into a jacket where they circulate and remain in a loop around the reactor holding the ELTs’ rubber granulates. An auger inside the reactor rotates to ensure that each rubber particle is evenly heated for the optimum duration so that the rCB is consistently high quality. In addition, staff checks rCB quality every 1-3 hours, so 90 per cent of the product meets stringent quality criteria.

Contec also introduced engineering elements and security steps to ensure that staff has complete control over the molten heating process and that strict safety requirements are met. Less than 15 per cent of all stops in the plant have been non-scheduled stops due to equipment failure. Moreover, molten salts need less energy to maintain their temperature than other methods and can be reused, making the entire tire pyrolysis process more efficient, economical, and eco-friendly.

In addition, the inertisation process ensures that explosion risks are minimised.

The Contec process has ensured that our rCB’s carbon footprint is 80 per cent less than conventional vCB and that our products are circular. Thus, we manage the growing ELTs disposal problem to provide circular rCB, steel, and fuel, to tire manufacturers and the automotive industry to help them become more sustainable and attain their climate-neutral goals.

Get in touch to learn more about our sustainable solutions.

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