Developments in waste tyre thermochemical conversion processes: gasification, pyrolysis and liquefaction

Fossil fuels, particularly crude oil, have proven to be a source of energy to households, transportation and power industries over the past decades. This natural reserve is diminishing at an alarming rate with crude oil having reserves to last the earth for the next half a century. As a result, researchers are constantly seeking remedial technologies to close this gap. Thermochemical conversion processes such as pyrolysis, gasification and liquefaction (PGL) offer an alternative solution to mitigating the world's high reliance on crude oil. These processes can be employed to provide energy, fuel and high-end value-added products. This paper aims to highlight all the research and development advancements and trends that have been made over the past three decades while employing waste tyres and other feedstock. In addition, the prominent countries and their associated researchers who have made novel discoveries in the field of thermochemical conversion are extensively discussed. The research findings show that significant research outputs such as the utilization of vast types of feed materials, the reaction mechanisms, the factors affecting the processes, and the application of the different end-products for thermochemical processes are well documented in the literature. Also, the collected data showed that significant advancements have been achieved in developing PGL technologies. The following conclusions were drawn: (i) PGL technologies show a generally increasing percentage interest from 1990 to 2020, (ii) many authors have identified the end-products obtained from waste feedstocks, such as; waste tyres, biomass, plastics, food waste, microalgae and animal manure to yield promising application benefits, (iii) China has shown the greatest interest in investing into waste to energy initiatives and has demonstrated the vast applications of waste derived products and, (iv) employing waste tyres as a feedstock has shown potential for producing high-end value products in their crude form or refined form. Some of the shortcomings identified during the study are the modest interest shown by most African regions as well as the lack of regulatory frameworks developed by certain countries.