Diesel engine performance and emissions with fuels derived from waste tyres

The disposal of waste rubber and scrap tyres is a significant issue globally; disposal into stockpiles and landfill poses a serious threat to the environment, in addition to creating ecological problems. Fuel production from tyre waste could form part of the solution to this global issue. Therefore, this paper studies the potential of fuels derived from waste tyres as alternatives to diesel. Production methods and the influence of reactor operating parameters (such as reactor temperature and catalyst type) on oil yield are outlined. These have a major effect on the performance and emission characteristics of diesel engines when using tyre derived fuels. In general, tyre derived fuels increase the brake specific fuel consumption and decrease the brake thermal efficiency. The majority of studies indicate that NOx emissions increase with waste tyre derived fuels; however, a few studies have reported the opposite trend. A similar increasing trend has been observed for CO and CO₂ emissions. Although most studies reported an increase in HC emission owing to lower cetane number and higher density, some studies have reported reduced HC emissions. It has been found that the higher aromatic content in such fuels can lead to increased particulate matter emissions.

Waste tyre – a source of alternative fuel

Waste tyres pose an environmental issue as they do not readily biodegrade and recovering their constituent components is quite difficult²⁰, an issue compounded by the vast quantity of waste tyres discarded annually⁶. Tyres are primarily made of rubber (45–65 wt. %), carbon black (21.5–35 wt. %), and steel (16.5–25 wt. %), but also consist of zinc, sulphur and additives²⁵. In addition, the composition varies according the application of the type e.g. a car tyre typically has 14% natural rubber and 27% synthetic rubber whereas truck tyres normally have more natural rubber (27%) and lesser synthetic rubber (14%)²⁶. The rest of the components such as fillers, chemical additives (e.g. sulphur), plasticisers and metals are the same for car and truck tyres²⁶. The rubber component is present as hydrocarbons compounded with fibrous materials⁷.
Most of the research related to waste tyres has focused on the production of fuel (TPO) which has been used for blending with diesel. In addition, researchers have also attempted to blend carbon black with diesel²⁷. Carbon black is the solid waste collected in the pyrolysis reactor upon completion of the pyrolysis of waste tyres²⁸. It has been found that ELTs from passenger cars have more sulphur and aromatic content compared to ELTs from heavy duty trucks²⁹. Table 1 gives the breakdown of tyre recycling and recovery processes in Australia.

Table 1 Summary of domestic tyre recycling and recovery markets

Form	Destination	Typical applications	Size	Proportion in market (%)
Whole tyres/shredded tyres	Tyre derived fuel	Energy recovery (e.g. pyrolysis)	>200 mm	8.7
Whole tyres	Stockpiling for future recycling	Pyrolysis, crumbing	Not applicable	31
Whole tyres/granules/shredded tyres	Civil engineering	Civil construction	10–60 mm	11
Granulated, crumbed or powdered	Rubber crumb, granules, landfill	Road construction, explosives, adhesives, disposal and flooring	1–10 mm	49.3

 

As seen in Table 1, the major portion of recycled rubber (49.3%) is granulated or powdered and disposed of in landfills. Crumb rubber can be used to replace the traditional polymer modified binder in spray seal pavements for its plasticity and waterproofing properties. Recycled rubber granulate can be used in a range of moulded products, flooring and matting, which can be used in sporting grounds and playgrounds. In 2013–2014, only 8.7% of the waste tyres were used to produce fuel²⁹. Thus, there is excellent potential for producing biofuel from ELTs via the pyrolysis process. As a value-add this will help reduce the financial burden on government bodies to treat waste rubber and prevent landfilling²⁹.

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