From dead tyres to liquid fuels

Australia generates around 450,000 tonnes or 56 million of end-of-life tyres on average, every year. Several options are available for managing the EOLT. These include: materials recovery such as for use in road surfaces, flooring, new rubber products; energy generation; and chemicals production from tyre-derived liquid fuel. Each option has several advantages and disadvantages.

Dr Mahmud Kibria, Professor Sankar Bhattacharya and Dr Mita Bhattacharya from Monash University have recently completed an experimental and techno-economic study, co-funded by the Tyre Stewardship, processing EOLT granules for liquid fuels and hydrogen production.

The researchers review literature on pyrolysis and gasification of tyres and conducted experimental work at three different scales, including a continuously fed large pyrolyser coupled with a distillation column, complemented with sample analysis involving gas chromatography, gas chromatography–mass spectrometry, scanning electron microscopy and X-ray fluorescence spectroscopy.

The results from the work — the yield and composition of the gas, oil and solid residues — were then used in the subsequent parts of the project. Based on the experimental work, process simulation and the analysis, the team believed in the technical viability of processing EOLT into liquid fuel which is currently explored for other uses.

A mathematical model simulating the pyrolysis process for a 60 T/day plant indicate that the gas generated has sufficient heat to sustain a pyrolysis process at 450–500°C even after considering a conservative 10% heat loss from the plant during operation.

The technoeconomic analysis carried out for a 60 T/day plant using the experimental data from the test program and quotation for key plant components estimated the total fixed cost to be $11m including land cost. Without the land cost, the cashflow was estimated to become positive in four years of operation.

The carbon black production cost was said to vary between $225 and $375/tonne and the total oil production cost between 22 and 38 cents/litre depending on the assumptions, according to the analysis. The calculated internal rate of return (IRR) was said to vary between 15% and 35% depending on the amount of bank loan taken and land ownership.

Gasification of EOLT was not part of this project. Even though gasification potentially allows for the production of key gases, CO and H2, for subsequent chemicals synthesis, the prospect of gasification of EOLT is too early to assess as insufficient gasification data is available from any large bench-scale continuous fed reactor. Pyrolysis uses no air or oxygen compared to gasification which requires sub-stoichiometric oxygen. This factor makes control of pollutant emission during pyrolysis less costly than gasification and certainly even less costly compared to incineration.

While Australia generates significant quantities of end-of-life tyres, the generation sources are spread over the country. A dedicated supply chain needs to exist to make pyrolysis plants viable at defined locations, according to Monash University researchers. Also, defined regulatory policies in support of pyrolysis processes will facilitate the introduction of pyrolysis plants which technically are not complicated, suggest researchers.

Image credit: ©stock.adobe.com/au/ kevinsday