Simulation of NZ Coal Gasification for an IGCC Application
The aim of this study was to investigate coal gasification and its relevance to NZ lignite and sub-bituminous coals. A simple method of coal characterisation was developed for simulation purposes. NZ lignite and sub-bituminous coal were compared in an Integrated Gasification Combined Cycle (IGCC) application and associated preliminary economics determined.
The method of coal characterisation involved breaking down the fixed carbon and volatile matter in coal from proximate analysis data into carbon, methane and water, and calibrating against the gross calorific value. Ash was not considered. The gasification process was modelled using Gibb's free energy minimisation. Data from the U.S. Department of Energy's Shell Gasifier Base Cases using Illinios #6 coal was used to verify both the gasifier and the IGCC flow-sheet models. The results were further fine tuned by decreasing the H:C and O:C ratios in the coal in order to match the gasifier output composition and temperature.
The IGCC power output and other key operating variables such as gas turbine inlet and exhaust temperatures were kept constant for study of comparative economics. The results indicated that 16% more lignite than sub-bituminous coal was required. This translated into a larger gasifier and air separation unit, but smaller gas and steam turbines. The gasifier was the largest sole contributor (30%) to the estimated capital cost of the IGCC plant.