Radiative Heat Transfer in Oxy-Fuel Steam Generators

Dissertation von Sebastian Rehfeldt
Universität Stuttgart, 2015

Oxy-fuel combustion is regarded as a promising technology for the mitigation of greenhouse gas emissions from fossil fuel fired power plants. The purpose of this work is to assess the impact of oxy-fuel combustion on radiative heat transfer in coal-fired steam generators and its calculation. Radiative transfer in a lignite-fired 30 MW oxy-fuel furnace is modeled on the basis of measured gas temperature and concentration profiles of major radiating gas species. Heat transfer by radiation is modeled by Monte Carlo ray tracing, with the intent of model calibration. Simulation results are evaluated and compared to measured values of incident radiative heat flux at the furnace wall.

This work contributes to the evaluation of radiative heat transfer in large furnaces with respect to optical path length. In industrial applications, heat radiation is often considered as being gray in order to reduce the computational effort. Emissivity correlations for gray gases are widely used for heat transfer simulation. Adaptation of a gray emissivity model is presented and compared to other emissivity correlations fitted to elevated concentrations of radiating gas species. Recommendations on coupling gray emissivity and radiation models with spectrally averaged absorption coefficients are given. Finally, the adapted emissivity model is applied to CFD simulation of the regarded furnace in order to estimate its influence on gas temperature and radiative heat flux.

This thesis should be of interest to process and combustion engineers, in the fields of fossil fuel fired power generation and carbon dioxide capture technology. It should also be of interest to software engineers, and practitioners of combustion and radiative heat transfer simulation.

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