Radiative Heat Transfer in Suspension-Fired Systems

Doktorsavhandling, 2019

Radiation is the dominating heat transfer mechanism in most furnaces, and the radiative properties of combustion products represent important aspects regarding its suitability as a heat source. However, fuel and energy markets are undergoing rapid changes due to increasing concerns related to the security of supply and increased global warming. Industrial processes that have traditionally relied on a specific fuel must increase their flexibility in terms of their energy supply. Therefore, an increased understanding and improved modeling capacity of radiative heat transfer are required to facilitate a more rapid evaluation and implementation of novel fuels in large furnaces that have reduced emissions and sustained efficiency. For such studies, digital twins of processes may be a useful and economical alternative.

This PhD thesis on radiative heat transfer focuses on: (i) the application to one industrial process, i.e., rotary kilns for heat treatment of iron ore pellets; and (ii) radiative heat transfer from soot particles in various propane flames. In-flame measurements of the combustion conditions, radiative intensities and radiative heat fluxes were performed during several measurement campaigns with different burners, furnace geometries and fuels, including gaseous, coal, and co-firing fuels. The radiative heat transfer was modeled using a discrete transfer model and a 3D-modeling tool that applies a discrete ordinates method, with the latter being developed within this thesis.

The 3D-modeling tool was used to study the heat transfer conditions within the rotary kiln, as well as the heat treatment of the bed material, and a first attempt to validate the model in relation to actual measurements was made. Radiation was shown to account for more than 80% of the total heat transferred to the bed material, and the flame radiation was dominated by the particles present. Nevertheless, the possibility of using co-firing, including up to 30% biomass, was found to be feasible, and was not expected to have any significant impact on the radiative heat transfer within the process.

The soot volume fraction and the radiative properties of soot particles were measured following gas extraction and using a laser-induced incandescence system for the various propane flames, while altering the combustion conditions and oxidizer composition. The modeled radiative intensities of the flames correspond well with the measured values, indicating that the soot volume fraction was accurately measured by either technique. Furthermore, very high soot volume fractions were observed when there were high concentrations of oxygen in the oxidizer, revealing the potential for promoting the radiative heat transfer in such furnaces.