Engineering, Modeling, and Scale-Up of Rotary Kiln Reactors for Solid Waste Pyrolysis: A Review

Abstract

The rapid rise in solid waste generation and the environmental limitations of conventional disposal methods, such as landfilling and mechanical recycling, have intensified the need for sustainable thermochemical conversion technologies such as pyrolysis. Among the available reactor systems, rotary kilns are particularly attractive for large-scale applications due to their ability to continuously process heterogeneous feedstocks. However, despite their industrial maturity in sectors such as cement and mineral processing, the design and scale-up of rotary kilns for pyrolysis remain insufficiently understood from a chemical engineering perspective. This Review critically examines indirectly heated rotary kilns for solid waste pyrolysis, focusing on reactor configurations, operational challenges, and scale-up methodologies. Key engineering aspects governing scale-up, including material selection, reaction kinetics, reactor hydrodynamics, residence time distribution (RTD), solid transport behavior, and modeling approaches, are systematically discussed. Furthermore, particular emphasis is placed on the role of kiln internals, especially lifter configurations, in controlling heat transfer, solid mixing, process intensification, and mean residence time. Finally, a step-by-step engineering methodology for rotary kiln pyrolysis scale-up is proposed to support the development of safer, more efficient, and industrially reliable systems.