This report describes research on coal combustion processes and pyrite oxidation processes. Work focuses on the oxidation of iron pyrite to form the non-slagging magnetite.

This document is the third quarterly status report on a project conducted at the High Temperature Gasdynamics Laboratory at Stanford University, Stanford, California and concerned with enhancing the transformation of iron pyrite to non-slagging species during staged, low-NO(subscript x) pulverized coal (P.C.) combustion. The research project is intended to advance PETC's efforts to improve our technical understanding of the high-temperature chemical and physical processes involved in the utilization of coal. The work focuses on the mechanistic description and rate quantification of the effects of fuel properties and combustion environment on the oxidation of iron pyrite to form the non-slagging species magnetite. The knowledge gained from this work is intended to be incorporated into numerical codes that can be used to formulate anti-slagging strategies involving minimal disturbance of coal combustor performance.

The information presented constitutes the report for the period July 1 to September 30, 1996. Characterization of intraparticle mass transport limitations during pyrite oxidation was embarked upon. The effort was intended to confirm that intraparticle transport limitations are negligible. Samples of 20 micron pyrite particles extracted from the flow reactor after oxidation at 1550 K in 1% oxygen level were analyzed. The samples has been extracted after reaction times of 42 ms, 52 ms, 77 ms, and 146 ms. For these samples, the bulk product compositions previously determined by X-ray diffraction analysis consisted of varying proportions of FeS2, Fe{sub 1-X}S, FeO, and Fe3O4. The particles were analyzed to determine if the iron compounds previously identified by bulk X- ray diffraction analysis (XRD) were well mixed within individual particles. The extracted pyrite particles, epoxied and sectioned, were subjected to a variety of analytical techniques using the microprobe (JEOL 733 Superprobe). Secondary electron and backscatter electron imaging was performed. Iron, sulfur, and oxygen elemental X-ray maps were generated. Energy dispersive spectrometry was used for qualitative elemental analysis of selected particles. These particles were subsequently subjected to qualitative elemental analysis by wavelength dispersive spectrometry (WDS) using Fe2O3 and FeS2 as standards. During WDS analysis, micron-radius hemispherical volumes bisected by sectioning plane were sampled. The microprobe analyses of oxidized pyrite showed that, generally, particles could be modeled as well-stirred, having negligible compositional gradients of the scale of the particle radius. In all four samples of pyrite analyzed, there were particle edge effects. Furthermore, there were finger-like projections of different phases in mixed-phase particles. Nevertheless, compositional gradients were of concern only in the 77 ms samples.

This paper presents results of investigations on the transformation of iron pyrite to non-slagging species during staged combustion of pulverized coal. Work focuses on the oxidation of iron pyrite to magnetite.

This document is the eighth quarterly status report on a project that is conducted at the High Temperature Gasdynamics Laboratory at Stanford University, Stanford, California and is concerned with enhancing the transformation of iron pyrite to non-slagging species during staged, low-NO(subscript x) pulverized coal (P.C.) combustion. In general, the project has the following objectives: (1) the characterization of the various mechanisms of intraparticle mass transfer and chemical reaction that control overall pyrite combustion rates and (2) the synthesis of the reaction rate resistances of the various mechanisms into a general rate expression for pyrite combustion. The knowledge gained from this project will be incorporated into numerical codes and utilized to formulate slagging abatement strategies involving the minor adjustment of firing conditions. Ultimately, the benefit of this research program is intended to be an increase in the range of coals compatible with staged, low-NO{sub X} combustor retrofits.