Project without external funding

Zur Modellierung der Steinkohlenpyrolyse im Doppelschneckenreaktor


Project Details
Project duration: 200703/2009


Abstract
Iron ore reduction is largely carried out in the blast furnace process which relies on the input of coke. Due to shortness of coking coals, expensive upgrading in the cokery and political conditions to lower emissions of carbon dioxide (CO2), several alternative processes based on smelting reduction and direct reduction are under way. In scope of the European Ultra-Low CO2 Steelmaking project (ULCOS), a continuous ore reducing process is being developed. This process incorporates partial pyrolysis of non-coking coals in a twin screw reactor (TSR) prior being fed into an ore smelting cyclone as a fuel and reducing agent. Direct reduction in a mixed iron-coal smelting bath is expected to lower specific coal requirement by 25% in comparison to common blast furnace processes. The purpose of the counter-rotating TSR is to preheat and upgrade the feed coals by partial pyrolysis, i.e. to increase the specific carbon fraction by lowering specific hydrogen and oxygen fractions, while heating the reactor by combustion of the pyrolysis gases. In order to determine preferable geometric layouts and working conditions of the twin screw reactor, occurring transport phenomena are investigated by means of a mathematical model using commercial CFD-Software Phoenicsâ„¢.

For modelling, a counter-rotating type TSR allows for assumption of a symmetrical velocity distribution between the rotating shafts. Therefore, the 8-shaped processing domain of the TSR has been reduced to one half, utilising a polar-cylindrical coordinate system. Variable spacing between the shafts and variable volumetric filling degrees are taken into account by blocking the numerical cells. Furthermore, shear stresses at the symmetrical plane left/right screw and at the interface coals/gas have been set to zero. For numerical reasons, the helical channel of the real screw is transformed into a series of mathematically coupled annuli in which the thickness of each annuli represents the pitch of the screw. Instead of moving numerical objects, i.e. relative movement between the screw and the housing, the velocities are formulated at the fluid/wall interfaces as if the screw rotates. On basis of this model, characteristic velocity and temperature distributions as well as pyrolysis kinetics have been calculated using the isothermal, no-slip boundary condition. So far, calculation results are physically reasonable and testing against selected experimental cases will be carried out.


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Last updated on 2017-11-07 at 13:58