BY : Dr S V Zhubrin, CHAM Ltd

DATE : November, 2000

FOR : Demonstration case for PHOENICS 3.3.1

A model for the simulation of the flow, heat and mass transfer in the rotary kiln is presented aimed at the simulation of the relevant physicochemical phenomena taking place in refuse particle combustion.

The rotary kiln in question is an idealised one retaining, however, the major features of a real-life system. It consists of a steel well insulated rotating cylinder inclined to horizontal.

One part of the fuel (refuse) solid particles are injected into the airstream of the main burner located at the center of the front wall of the rotary kiln. The second part of refuse particles of higher volume fraction is fed into the sludge burner located just beneath the main burner. In the sloping chamber of sludge burner the particles are mixed with the air before entering the kiln at the angle to the side wall in the gravity direction.

The thermal design task is to calculate the field distribution of phase velocities, gas/particle volume fractions, gas mixture composition and phase temperatures.

The independent variables of the problem are the three components of cylindrical polar coordinate system.

The main dependent (solved for) variables are:

- Three velocity components of gas flow,
- Three velocity components of particulate flow,
- Pressure,
- Volume fractions of gas and fines,
- "Shadow" volume fraction,
- Kinetic energy of gas turbulence and its dissipation rate,
- Specific gas enthalpy,
- Specific particle enthalpy and
- Mixture fraction of gas composition.

The 7GASES model is used to simulate the combustion of refuse particles with the gas phase absorbing the carbon from the particles.

The effective (ie laminar-plus-turbulent) diffusion coefficients of the gaseous species are all taken as equal, and the reaction rates are supposed diffusion-limited; consequently all gas species concentrations depend on carbon mass fraction, in piecewise-linear fashion.

The oxidation of carbon is presumed to proceed in two stages, viz:

- to create CO2 and H2O, and then
- to create CO and H2, as more fuel is added.

Reactions:

- C (solid) + 0.5 O2 -> CO
- CO + 0.5 O2 -> CO2
- C (solid) + CO2 -> 2CO
- C (solid) + H2O -> CO + H2
- H2 + 0.5 O2 -> H2O

The gas-phase equilibrium-composition diagram, taking account of the elemental mass fractions of O, C and H, is used to calculate the combustion product composition.

The gas density is computed from the local pressures, gas temperatures and local mixture molecular masses.

The specific enthalpies are related to gas and fines temperatures.

At all inlets, values are given of all dependent variables together with the prescribed flow rates.

Fixed exit pressure. As the fluid is assumed incompressible, this pressure is set equal to zero and the computed pressures are relative to this pressure.

The smooth-wall 'wall functions' are used to provide the non-slip conditions for momentum equations.

It is assumed that there is no heat exchange to the wall, ie. an adiabatic boundary conditions are employed.

The plots show the flow distribution, mixture composition as represented by the model, gas temperature and velocity within the rotary kiln.

Pictures are as follows :

- Rotary kiln geometry.
- Gas velocity vectors at the middle plane.
- Particle velocity vectors at the middle plane.
- Gas velocity vectors near the rotating wall.
- Particle volume fractions at the cross sections.
- Particle volume fractions at the rotating wall.
- Gas temperature contours.
- Temperature of the rotating wall.
- Particle temperature contours.
- Mass fractions of carbon monoxide, CO.
- Mass fractions of carbon dioxide, CO2.
- Mass fractions of water vapour, H2O.
- Mass fractions of nytrogen, N2.
- Mass fractions of oxygen, O2.
- Mass fractions of hydrogen, H2.
- Turbulent viscosity.
- Gas density.
- Turbulence energy.
- Reciprocal time scale.
- Pressure contours.

All model settings have been made in VR-Editor of PHOENICS 3.3.1.

The relevant Q1 file can be inspected by clicking here.