Based on the balance equations of mass, partial mass, momentum and thermal energy, the pyrolysis process of non-coking coals using a counter-rotating twin screw reactor (TSR) is modelled by means of commercial CFD code PhoenicsTM. In favour of good convergence of the mathematical solution, a structured, cylindrical-polar grid is chosen. Variable filling and intermeshing degrees are taken into account by blocking numerical cells in gas domain and intermeshing domain instead of performing two-phase calculations in a body-fitted co-ordinate system. Viscous behaviour of the granular coals is expressed by means of a generalised Newtonian fluid and chemical decomposition is implemented with aid of distributed activation energies.