Radiation models for CFD-simulations of hydrogen and coal flames in rotary kiln applications

Other conference contribution, 2024

This work applies a computational fluid dynamic (CFD) model with a detailed heat transfer model through a case study of a rotary kiln used in iron ore heat treatment, obtaining the heat transfer conditions within the kiln. The aim is to assess relevant gas radiation models for pure hydrogen and coal flames, addressing temperature limitations in existing models applied in CFD. With the heat transfer model, grey and non-grey assumptions are evaluated, and the analysis compares our latest weighted-sum-of-grey-gases model (WSGGM) developed for temperatures up to 5000 K to a WSGGM used in commercial CFD codes. Additional analyses include predicting the radiative source term in a cross-section of the kiln diameter using radial temperature and gas profiles from the CFD-simulated hydrogen flame, and applying a statistical narrow band model as reference. Comparing WSGGMs and grey/non-grey gas assumptions, the results show similar heat transfer conditions within the kiln for coal flames, whereas significant differences are observed between a grey/non-grey formulation for pure hydrogen flames. At temperatures exceeding 2400 K, the source term calculations reveal the high accuracy of our latest WSGGM in its non-grey form and the associated errors when applying the non-grey WSGGM from the CFD code. Employing a grey approximation at these temperature gradients fails to predict the source term for both WSGGMs. This work concludes that a grey gas assumption may effectively estimate the gaseous emission associated with coal-fired kilns while during hydrogen firing, where particle concentrations are lower, and temperature and water vapor gradients are higher, assuming a non-grey gas becomes critical.