Chemical Cleaning of Coal

Abstract

Between the expensive coal conversion processes and the relatively inexpensive flotation and magnetic methods for pyrite removal are the chemical methods for desulfurizing coal. Chemical cleaning removes more sulfur than physical separations, less than coal conversion, at costs and investment which are intermediate. Fuel value losses are small for chemical methods, comparable with the best of the physical separation procedures. In addition, certain chemical treatment methods effect useful changes in coal composition and properties. The principal chemical desulfurization methods involve treatment, at elevated temperature, with either basic media or oxidizing systems. Aqueous sodium hydroxide, at 225°C, removes almost all of the pyrite from coal, while subsequent washing with dilute acid removes most of the remaining mineral matter. At 300°C, similar treatment can remove, in addition, up to 40 percent of the organic sulfur from coals and also destroy caking properties. Oxidative desulfurization, using steam and air, also removes most of the pyritic sulfur and 20–40 percent of the organic sulfur, under milder conditions than those required for alkali treatment. The steam-air desulfurization process is presently the simplest and cheapest concept for removing organic sulfur from coal, making much eastern and midwestern coal environmentally acceptable as boiler fuel at realistic cost.