Thermodynamic comparison of pyrolysis and gasification of waste tyres

Abstract

Waste tyres are a particularly problematic pollutant; persistent, highly toxic, flammable, and difficult to process or store. However, waste tyres need not be viewed solely as a waste material, as they also offer promising properties as an energy material. Waste tyres have a higher energy density than coal, as well as lower ash content and favourable quantities of carbon and hydrogen. Extensive experimental research has demonstrated that thermochemical valorisation pathways including pyrolysis and gasification are viable for producing valuable chemical products from waste tyre. Despite this, there is as yet no established technology for waste tyre conversion. In this paper, fundamental thermodynamic and economic analysis is used to evaluate a range of process pathways to determine their economic favourability and environmental impact. The process performance targets derived in this way can serve as a basis for preliminary process design and provide estimates for the commodity value of waste tyre, informing long-range planning in both corporate and legislative settings. A range of pyrolysis and gasification pathways have been evaluated in terms of the fundamental thermodynamic metrics of carbon efficiency, atom economy, e-factor and chemical potential efficiency, and also their market-related revenue potential. It was found that pyrolysis pathways perform better in terms of thermodynamic efficiency and carbon footprint than gasification processes, which lose about 45% of the carbon feed to carbon dioxide. However, the gasification routes offer higher potential revenue, yielding as much as $625 per ton of waste tyre as compared to $205 from the pyrolysis route.