Radiative transfer models for rotary kilns - from fossil fuels to hydrogen gas and thermal plasmas

Licentiate thesis, 2025

Global warming caused by greenhouse gas emissions remains a serious challenge in our Society. Combustion of fossil-based fuels is common in thermal engineering processes within the industrial sector. Electrification of such processes is key to reducing related carbon dioxide and particle emissions. Industries that are aiming to electrify by adapting current processes to operate with alternative fuels, such as hydrogen gas or thermal plasmas, require a comprehensive understanding of the combustion processes, heat transfer mechanism, overall heat balances, and process optimization. Therefore, there is an urgent need to develop modeling tools and to conduct experiments to validate those tools.

This thesis investigates the effects on the heat transfer within rotating furnaces that occur when substituting fossil-based fuels for alternatives, such as hydrogen gas and electrically aided heating using thermal plasmas. The focus is on radiative heat transfer assessed in rotary kiln processes (high-temperature processes) that are used for iron ore and cement production.

Paper I present a heat transfer modeling study employing adiabatic flame temperature conditions, showing that the fuel shift from coal to hydrogen gas in rotary kilns for iron ore production significantly increases the local heat transfer rate to the inner surface of the kiln wall. Specifically, the heat transfer rate is affected near the burner region, achieving high levels for the hydrogen flame. Further investigations of the local effects of fuel shifting in rotary kilns require precision in terms of the heat transfer calculations, motivating the development of an updated gas radiation model, as presented in Paper II, and the application of such gas radiation models with computational fluid dynamics codes, as presented in Paper III.

By combining computational fluid dynamics codes with the updated gas radiation model, the present work concludes that, although using hydrogen gas as a fuel effectively reduces carbon dioxide emissions and particle formation, heat transfer during hydrogen firing is less efficient than during coal firing. Consequently, most of the heat transferred from fuel heat release exits with the flue gases during hydrogen combustion due to increased flame temperatures. This indicates a need for either coal-hydrogen co-firing conditions, as the presence of particles may enhance effectively the heat transfer rate within the kiln or alternative kiln designs.