BY : Dr S V Zhubrin, CHAM Ltd
DATE : March, 2002
FOR : Demonstration case for CHAM-Japan.
A model for the simulation of the flow, heat and mass transfer in the combustion-driven moving bed of coal lumps is presented aimed at the simulation of some relevant physical phenomena taking place in a gas producer.
The chamber chosen for the present computations retains some major features of real-life industrial equipment: the air inlets are located at the lower part on the chamber front wall; air-pulverised coal particles (fines) are injected axially into the chamber; they are ignited; steady state combustion takes place devolatilizing the coal lumps; coal lumps enter the chamber from the top; combustion and volatile products flow through descending coal bed where their temperature is reduced and the temperature of coal lumps increase.
The chamber is axisymmetrical, and only half of it is simulated and represented.
The pulverized combustion of the fines is considered as one-phase phenomena, with combustible fines being treated as non-gas components which have the same velocity components and temperature as the gas. The processes accounted for are the volatilisation of fines material and gaseous exothermic oxidation of v latile products.
The flow of combustibles coal lumps goes in the opposite direction and the chemical transformation in both streams is governed by the rate of mass transfer between both gas and fines, and gas and bed lumps.
The independent variables of the problem are the three components of cylindrical polar coordinate system.
The main dependent (solved for) variables are:
The 7GASES model is used to simulate the combustion of pulverised-coal fines with the gas phase absorbing the carbon from both the fines and coal lumps.
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:
Reactions:
The gas-phase equilibrium-composition diagram, taking account of the elemental mass fractions of O, C and H, is used to calculate the gas product composition.
To simulate the relative motion between two different phases of gas mixture and coal lumps by means of one-phase algorithm, a "two-space" version of the Multiply-SharEd Space (MUSES) technique is used.
Specifically, the gases and the fines are treated as homogeneous mixture, with combustible fines being considered as non-gas components which have the same velocity components and temperature as the gas in the space-share 1, while the lump solids are treated as the phase of space-share 2.
The manner in which MUSES are implemented is as follows:
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 gas inlets, values are given of all dependent variables together with the prescribed flow rates.
The inlet flow rate of the coal lump flow is supposed to be governed by mass transfer between the gas and bed lumps. At coal inlet, the fixed pressure is imposed.
Fixed exit pressure of the gas mixture. As the gas 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 and lump temperature and velocities within the chamber.
Pictures are as follows :
