*4.4. Steel from Waste Tyre*

Waste tyres also produce steel when converted by a thermochemical process. It is reported that the amount of steel recovered from waste tyres typically ranges from 10 to 15% by weight of the waste tyre [19]. The recovered steel can be reused by the tyre manufacturer or diverted to steel re-rolling mills.

#### **5. Diesel Engine Performance and Exhaust Emission Using Tyre Oil**

According to various researchers [95,104], the properties of waste-tyre pyrolysis are similar to those of diesel and gasoline. In today's world, the diesel engine is the most widely used internal combustion engine. Increased demand for diesel fuel, combined with limited resources, has prompted a search for alternative fuels for diesel engines, such as alcohol, LPG, biodiesel, and compressed natural gas (CNG) [55]. The results of studies on engine testing with tyre oil in the literature vary due to the different properties of the test fuels and different test-engine technology [102]. In an engine-emissions analysis, many variables must be controlled, such as engine speed, fuel composition, and load condition. Tyre fuel has proven to be one of the most important and useful research outputs. However, funding for the use of tyre-derived pyrolytic fuel or diesel-blend fuel has been limited because the effects on overall engine performance and emissions have not been sufficiently confirmed. As a result, additional research focusing on diesel engine emissions using oil from waste tyres is expected to have a positive impact in alternative industries. Furthermore, it could be a promising option in the search for low-emission energy sources.

Several researchers have conducted tests on diesel engine performance with tyre oil in recent years. Table 4 summarises their findings. Vihar et al. [43] experimentally analysed the combustion characteristics and emission of tyre pyrolysis oil in a turbo-charged sixcylinder compression ignition engine using 100% TPO as fuel. They found a stable diesel running throughout the experiment with an almost similar thermal efficiency and specific fuel combustion. However, due to the higher density of TPO compared with diesel which has a direct link with fuel spray to the cylinder, the ignition delay (ID) of combustion and cylinder peak pressure (CPP) were found to be higher. Engine exhaust emission NOX, CO, SO2 and HC was found to be significantly higher (2–50%), whereas smoke emission was found slightly lower while running the engine with 100% TPO compared with diesel. Similar results were reported by Žvar Baškoviˇc et. al. [105] when conducting an experiment in a 1.6-litre multi-cylinder common-rail diesel engine running with 100% pure TPO. Tudu et al. [42] examined the effect of diethyl ether in a diesel engine running on a tyre-derived fuel-diesel blend. They blended 40% tyre-pyrolysis oil with diesel and simultaneously 4% diethyl ether to improve the CN of the blended fuel. It was reported that those blended fuels reduce the NOX emission by approximately 25% with respect to diesel operation at full load [42]. Cumali and Huseyin [16] carried out an experimental investigation of fuel production from waste tyres using a catalytic pyrolysis process and tested it in a 0.75-litre single-cylinder diesel engine. This study ran the engine with blends of 5%, 10%, 15%, 25%, 35%, 50%, and 75% TPO with diesel and 100% TPO as fuel. It was reported that 50%, 75%, and 100% tyre-oil blends significantly increase CO, HC, SO2, and smoke emissions compared to diesel emissions and are therefore not suitable for direct use in commercial diesel engines without engine modification. Hossain at al. [106] also reported a small changes in engine combustion performance running a 5.9-litre, six cylinder turbo-charged diesel engine with 10% and 20% of TPO. However, this study found a significant change in brake-specific emission of NOX, CO2, CO, and particle emission. The brake-specific NOX reduced by 30%, whereas the CO emission increased by 10% with tyre oil blends, as shown in Figure 8.

**Table 4.** Diesel engine emission and performance with TPO.


**Figure 8.** Brake-specific NOx (**a**) and CO (**b**) emission with TPO. (100D—100% diesel; 90D10T—90% diesel + 10% TPO; 80D20T—80% diesel + 20% TPO).

## **6. Discussion and Synthesis**

The idea of waste tyre management is to find the best way to reduce the environmental impact produced by this waste. Waste tyre pyrolysis technology is proven as an efficient method in waste tyre management in recent decades. High-quality fuels from scrape tyre can be produced through pyrolysis, which will eventually replace non-renewable fossil fuels. Despite the fact that there has been a lot of research interest in waste tyre thermochemical conversion to fuel in recent decades, the commercialisation of TPO as an automotive engine fuel technology is still a long way off. It is necessary to fully recognise the current development stage as well as many technical and economical hurdles that need to be overcome for further development of waste tyre to energy technology and upgrade the technology on an industrial scale. There is minimal study regarding the industrial cost of tyre pyrolysis. It is essential that the financial and environmental benefits of the tyre pyrolysis have been thoroughly researched, and the cost has been decreased further for a large-scale commercial application to be viable in the long term. To realise the full potential of waste tyre pyrolysis technology, further research and development are needed, and some of the future challenges are described below:

