(58e) CFD Modeling of Rotary Kilns for Pyro-Processing Applications

A rotary kiln is a pyro-processing device used to raise materials to high temperature in a continuous process.  Rotary kilns are commonly used to destroy waste materials (incineration) or to create new materials (materials processing).  The focus of this paper is on materials processing applications, although many of the concepts and methods discussed also apply to incineration.

Materials processing applications in rotary kilns include cement calcining, trona calcining, phosphate ore nodulizing, soda ash bleaching, laterite ore reduction, petcoke calcining, and lime sludge calcining.  Creating a phase change (e.g., the calcinations of limestone) is an energy intensive process.   Energy input is needed to heat up the kiln and raw processing material to the required processing temperature to produce the desired quality product.  Coal, petroleum coke, waste fuels, natural gas, and oil are used in the rotary kiln to generate the energy needed.

Pyro-processing in the rotary kiln is a complicated process and the control of bed temperature profile is the key to a good quality product.  A detailed understanding of process not only can improve the kiln efficiency, but also can improve the product quality.  Three-dimensional computational fluid dynamic (CFD) modeling of the rotary kiln provides unique and detailed information about processes that take place within the rotary kiln, which helps the operator to operate the kiln more efficiently.

Reaction Engineering International (REI) has developed a propriety CFD code, GLACIER, which has been used to model rotary kiln systems in a number of different materials processing fields. The general REI two-phase reacting flow model employs a combination of Eulerian and Lagrangian reference frames.  The flow field is assumed to be a steady-state, turbulent, reacting continuum field that can be described locally by general conservation equations.  The governing equations for gas-phase fluid mechanics, heat transfer, thermal radiation and scalar transport are solved in an Eulerian framework.  The governing equations for particle-phase mechanics are solved in a Lagrangian reference frame. The overall solution scheme is based on a particle-source-in-cell approach.

For the rotary kiln application, a submodel for the solid bed material has been coupled to the gaseous flow in GLACIER.  The solid bed material is tracked along the length of the kiln; kinetics of reactions in the bed (e.g., calcinations) are included in the solid bed model and an energy balance is performed along the length of the kiln to determine the temperature and composition of bed material. The temperature and composition of the solid bed are assumed to vary only along the kiln length in the current model.   Each application requires a different set of heterogeneous kinetics, specific to the material being processed.

In addition to specific process chemistry, rotary kiln applications have different heat and mass transfer challenges that must be implemented within the CFD model.  REI has developed and tested a variety of physical submodels, including: angle of repose, shell-bed heat exchange, dust radiative heat transfer, lifters/particle showering, chain heat transfer. 

The effect of lifters in the kiln on dust generation and the heat exchange between the gas phase and solid bed material are included through solid material showering in the current model.   The effect of dams on the bed solid material residence time can also be accommodated in the model.  Similarly, the effect of chains on heat transfer and dust generation can also be included in the model. Figure 1 illustrates how the model tracks particle trajectories in an ore-processing kiln, with emphasis on the heat and mass transfer model. 

Figure  SEQ Figure \* ARABIC 1.  Trajectories of 58 micron ore particles in kiln, colored by extent of reaction, showing details of mass and energy balance.

The results of applying this unique CFD model to rotary kilns in a number of different pyro-processing applications will be presented in this paper and comparisons made between the predictions and field experience.  Areas of emphasis have included process product quality optimization, kiln temperature control, NOx and CO pollutant emissions, fuel switching, fuel efficiency, and kiln equipment optimization.