Modeling a low pressure steam-oxygen fluidized bed coal gasifying reactor

Abstract

A comprehensive model accounting for the jetting region and homogeneous dilute phase reactions is developed for the adiabatic and continuous gasification of coal particles in a fluidized bed. The division of flows in the bed is determined by means of a modified two-phase theory which considers inlet gas jets, bubbles, free of particles, which develop at the top of the jets and grow in size as they rise, and an emulsion phase consisting of particles and the surrounding interstitial gas. The model describes the gasification of coal particles by pyrolytic devolatilization and three heterogeneous chemical reactions: oxidation by oxygen and steam, and reduction of carbon dioxide. Carbon monoxide and hydrogen produced by the heterogeneous reactions can be oxidized to carbon dioxide and steam by incoming oxygen within the dilute phase jets and bubbles. Furthermore, the water-gas shift reaction can occur in the dilute phase and interstitial gas. Simulations both with and without homogeneous reactions occurring in the jets and bubbles indicate that dilute phase homogeneous reactions have considerable influence on carbon conversion, bed temperature, and product gas composition. It has also been found that the jetting-emulsion mass and beat interchange has a substantial effect on overall bed performance and the temperature of the bed close to the inlet gas distributor. Results indicate that water-gas shift equilibrium is established rapidly and significant quantities of hydrogen and carbon monoxide and a nonuniform steam concentration are present within the combustion zone.