Experimental studies of heat transfer in cement rotary kilns and CO₂ plasma jets designed for future electrification

Licentiate thesis, 2025

Fossil fuel combustion remains the dominant source of heat in energy-intensive industries such as cement and glass manufacturing, contributing significantly to CO₂ and particulate emissions. To meet decarbonization goals, electrification of thermal processes, including thermal plasma and hydrogen combustion, is gaining momentum, as well as heating resistance elements for some applications. Experimental studies are needed to understand the heat transfer mechanisms, support model validation, and guide process adaptation for the implementation of electrification.
This thesis studies the heat transfer, energy efficiency, and radiative characteristics of rotary kilns during propane combustion and oxygen-enriched propane combustion, as well as the resistance heating elements that contribute to the development of low-emissions industrial systems. The work is based on three experimental studies.
In Paper I, three heating techniques for rotary kiln with distinct thermal signatures are investigated. Resistance heating produces a uniform axial temperature profile, reducing thermal gradients and minimizing flue gas losses. Oxygen-enriched combustion yields the lowest flue gas losses among the combustion cases and is the only method to achieve the high temperatures required for clinker formation. When these cases are scaled to industrial conditions using a validated heat balance methodology, the combustion systems benefited from reduced surface heat losses due to lower surface-to-volume ratios, while resistance heating retains its advantage of low flue gas losses through improved thermal control.
Paper II demonstrates that using crushed cement raw meal as the bed material facilitates the formation of a coating layer on the inner kiln wall. This layer acts as a thermal insulator, reducing surface heat losses and stabilizing the inner wall temperatures.
Paper III provides experimental insight into CO₂ plasma jets by mapping the axial radiative intensity profile under different arc currents and gas flows. A sharp radiative peak is observed near the burner outlet, followed by a rapid decline and stabilization downstream. This behavior highlights the transition from localized radiative emission to convective transport along the jet, offering valuable data for future modeling and integration of plasma torches into rotary kilns.