Process modeling and thermodynamic analysis of Lurgi fixed-bed coal gasifier in an SNG plant

This paper presents a comprehensive steady state kinetic model of a commercial-scale pressurized Lurgi fixed-bed dry bottom coal gasifier. The model is developed using the simulator Aspen Plus. Five sequential modules: drying zone, pyrolysis zone, gasification zone, combustion zone and overall heat recovery unit, are considered in the main process model. A non-linear programming (NLP) model is employed to estimate the pyrolysis products, which include char, coal gas and high-weight hydrocarbons/distillable liquids (tar, phenol, naphtha and oil). To accelerate solution convergence, an external FORTRAN subroutine is used to simulate the kinetics of the combustion and gasification processes which are formulated in terms of a series of continuous stirred-tank reactors. The model is validated with industrial data. The effects of two key operating parameters, namely oxygen/coal mass ratio and steam/coal mass ratio, on the thermodynamic efficiencies of the Lurgi gasifier and the gasification system as a whole are investigated via extensive simulation studies.