The behavior of iron-sulfur species in fluidized-bed gasification on coal

Abstract

The behavior of iron-sulfur species derived from pyrite, especially that present in the coal feed as particles of free pyrite, is of concern in the operation of fluidized-bed coal gasifiers. Work conducted at the Institute of Gas Technology (IGT) has shown that the decomposition of pyrite in a U-GAS® pilot plant has yielded a liquid phase that spreads on the surface of ash particles. Formation of a liquid phase in decomposition of pyrite in an inert atmosphere has also been observed in connection with the investigation of boiler deposits. To aid in understanding the behavior of iron-sulfur species, we have prepared an Fe S phase diagram at temperatures relevant to coal gasification and with equilibrium sulfur potentials exhibited, for the most part, as hydrogen sulfide to hydrogen mole ratios; in a fluidized-bed gasifier, the latter can easily be obtained from the composition of the reactor product gas, which closely approximates that in the back-mixed fluidized bed. To show, in addition, the effect of oxygen potential in the bed, we have also prepared phase stability diagrams at 100°F intervals from 1500 to 2000°F (816 to 1093°C). According to the Fe S diagram, formation of an iron-sulfur liquid in the gasifier or in an inert atmosphere cannot be explained on the basis of equilibrium considerations. Instead, a kinetic explanation, based on increased concentration of iron in the exterior regions of the decomposing pyrite particle, as sulfur is removed from the surface, is much more plausible. Phase stability diagrams indicate that a liquid composed of iron, sulfur, and oxygen could be formed in steam-oxygen gasification of high-sulfur coal at temperatures above 1850°F (1010°C). However, no evidence of its formation was found by optical petrography in the examination of solids from the U-GAS pilot plant in which steam-to-hydrogen mole ratios ranged up to about 2, but formation of a submicroscopic liquid layer on iron sulfide particles could have been responsible for deposition of these particles in the hot cyclone of the U-GAS pilot plant. Its formation in steam-air gasification, where the steam-to-hydrogen mole ratio typically is about 0.6, is less likely.