Coal desulphurisation is a field of immense strategic importance and considerable technical difficulty. Although coal combustion accounts for only about a quarter of the United States’ energy supply, it is responsible for approximately two-thirds of the nation’s sulfur dioxide emissions. Despite great interest in developing effective pre-combustion cleaning methods for coal, no processes are yet sufficiently developed, either technically or commercially, to have a significant impact on the industry.

4.1 The Forms of Sulfur in Coal

A primary challenge lies in the complex nature of sulfur within the coal itself. Sulfur exists in two main forms:

• Inorganic Sulfur: Primarily as pyritic sulfur (iron pyrite) and some sulfates.
• Organic Sulfur: Chemically bound to the carbon structure of the coal.

While physical cleaning methods can remove a portion of the inorganic pyritic sulfur, they have shown little success in reducing the organically bound sulfur, which requires chemical transformation.

4.2 The Solvent Refined Coal (SRC) Process

One of the more prominent chemical approaches under evaluation is the Solvent Refined Coal (SRC) process. This technology produces a reconstituted coal product that is low in sulfur and ash, free of water, and can be handled as a fluid at elevated temperatures. The process involves grinding coal into a fine powder and slurrying it in a solvent. This slurry is then heated to approximately 450°C and pressurized to 1,000 lb/sq. in. in the presence of hydrogen, which dissolves over 90% of the coal’s carbon content and prevents repolymerization. The un-dissolved mineral matter, including ash and pyritic sulfur, is then filtered out. Finally, the solvent is recovered via flash evaporation, leaving a hard, brittle, refined solid fuel.

The SRC process demonstrates a marked improvement in fuel quality, but at a significant cost.

The SRC process is technically highly effective, reducing ash by over 90% and sulfur content by approximately 71%. However, this purification comes at a prohibitive cost, increasing the price per ton by 200%. The resulting fuel is substantially more expensive than the raw coal feedstock and even costlier on an energy basis than low-sulfur fuel oil, highlighting the immense economic barrier to the commercial adoption of pre-combustion coal cleaning.

4.3 The Devolatilization/Carbonization Process

Another approach under investigation involves desulfurizing ground coal in a fluidized bed reactor. In this devolatilization process, the efficiency of sulfur removal is governed by a set of key operational parameters:

• Particle size
• Reactor residence time
• Reaction temperature (approaching 950°F)
• Fluidizing velocity

4.4 Other Coal Treatment Concepts

Other concepts for coal cleaning are in various stages of development. Gasification combined cycle systems offer an advanced approach that integrates pre-combustion cleanup with high-efficiency power generation. Physical washing methods—including centrifugation, flotation, and magnetic separation—have been explored to remove pyritic sulfur. However, these physical methods have limited potential, as they are generally unable to remove more than half of the total sulfur and often lead to significant product losses. The limited maturity of these coal-focused technologies necessitates a broader strategic view of fuel cleaning and blending.