A KINETIC STUDY OF SUPERCRITICAL STEAM DESULFURIZATION OF COAL AND MODEL COMPOUNDS (MODELING, ORGANIC SULFUR REMOVAL, PYRITIC)

SHIH-TIEN TU, Purdue University

Abstract

It has been observed that water above its critical temperature can selectively remove all three forms of sulfur from coal. An apparatus was developed for treating pulverized coal particles with steam over the pressure range of 100 to 4000 psia (0.7 to 31 MPa). The effects on the extent of desulfurization of three mid-western coals were studied for four process parameters: the duration of stripping, steam pressure, temperature, and quantity of steam. It was concluded that desulfurization of coal increased with increasing temperature and reaction time. Under the conditions we studied, pressure and quantity of steam did not have any significant effect on desulfurization. Steam reactions with model sulfur compounds representative of the sulfur constituents in coal were investigated in this work. The results indicated that steam reactions with dibenzothiophene and diphenyl sulfide were first order with respect to the individual concentrations. Pyrite was mixed with silica to form a mineral mixture for kinetic studies. The reaction of pyrite with steam was found to follow the unreacted shrinking core model with diffusion being the rate-determining step. Based on the model compound results and correlation of experimental data, we established a kinetic model for each form of sulfur in coal. The organic sulfur compounds follow a first order kinetics. The frequency factor of the rate constant is an adjustable parameter for each type of organic compound. The reaction of pyrite sulfur could be best described by the shrinking core model with diffusion control. The sulfatic sulfur removal by supercritical steam followed a simple first order kinetics indicating that sulfatic sulfur was uniformly embedded in the coal matrix. For each coal, values of the rate constant (sulfatic sulfur), diffusivity (pyritic sulfur), and the frequency factor (organic sulfur) were obtained that minimized the overall absolute deviation from the experimental data. Despite some scattering of data, the model was found to represent the desulfurization data of coals in a reasonable manner. Testing of model capability on a physically cleaned coal showed that the model was able to make quantitative predictions of the supercritical steam desulfurization phenomena.

Degree

Ph.D.

Subject Area

Chemical engineering

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