5.1 Coal’s Environmental Significance and Technological Status

Coal is a fuel of immense strategic importance, yet it presents our greatest environmental challenge regarding sulfur emissions. While coal burning accounts for about one-quarter of the nation’s energy, it is responsible for approximately two-thirds of all sulfur dioxide emitted in the United States. Despite coal’s vast reserves, it is crucial to note that at the time this material was compiled, there were no sufficiently developed commercial desulfurization processes that had a significant impact on the industry. The following discussion, therefore, provides an overview of technologies that were under evaluation.

5.2 The Solvent Refined Coal (SRC) Process

The Solvent Refined Coal (SRC) process aims to create a superior, clean fuel from raw coal.

Product Characteristics

The product, Solvent Refined Coal (SRC), is a reconstituted material with highly desirable properties. It is:

• Free of water
• Low in sulfur
• Very low in ash
• Sufficiently low in melting point that it can be handled as a fluid when heated.

Process Flow

The process (Figure 11) begins with grinding coal and slurrying it in a solvent oil. The slurry is pressurized to 1,000 lb/sq.in. and passed through a heater to bring it to a temperature of around 450°C. A small amount of hydrogen is introduced, which serves to prevent repolymerization of the dissolved coal molecules and to convert the organic sulfur in the coal into hydrogen sulfide gas. After dissolution, the mineral matter (ash), including pyritic sulfur, is removed by filtration. The clean coal solution is then sent to a flash evaporator to recover the solvent for recycling. The remaining hot liquid residue is cooled to form a hard, brittle solid—the SRC product.

Performance and Economics

The SRC process dramatically improves the quality of the coal, but this improvement comes at a steep cost.

Table 4: Comparison of Raw Coal and Solvent Refined Coal (SRC)

As the data shows, the process achieves a remarkable 93% reduction in ash and significantly reduces sulfur content while increasing the heating value. However, the cost of the final product is substantially higher, making it more expensive than low-sulfur fuel oil at the time.

5.3 Devolatilization and Carbonization via Fluidized Bed

Another approach investigated by United Engineers and Company involves desulfurization in a fluidized bed. In this lab-scale process (Figure 12), ground coal is introduced into a heated, tubular reactor at a temperature of approximately 950°F. A flow of gas is adjusted to keep the coal particles suspended and behaving like a fluid. During this process, volatile matter is driven off, and some sulfur is removed. The resulting solid product is a char.

Analysis of this process revealed that the extent of desulfurization is directly affected by several key parameters: particle size, reactor residence time, reaction temperature, and the velocity of the fluidizing gas. As the graph in the source material indicates (Figure 13), desulfurization improves—meaning the ratio of sulfur in the char to the feed (ΔS) decreases—with longer residence times and higher temperatures. These conditions provide more energy and opportunity for the sulfur-releasing reactions to occur. Conversely, the relationship with fluidizing velocity and particle size is more complex, requiring careful optimization to balance reaction rates with elutriation and heat transfer.

5.4 Other Coal Cleaning Strategies

A variety of other coal cleaning concepts have also been explored.

Other Desulfurization Processes

Methods in various stages of development included causticized fluidized bed desulfurization, gasification of coal into high and low BTU gases, and the direct extraction of pyritic sulfur from raw coal.

Pyritic Sulfur Removal

The U.S. Bureau of Mines investigated several physical washing techniques aimed at removing pyritic sulfur, including centrifugation, flotation, and magnetic separation. However, these methods had a critical limitation: none had the potential to remove more than half the sulfur and each resulted in significant losses of the coal product.

Ash Removal

The benefits of low-ash coals, as demonstrated by the Detroit Edison Company, include reduced transportation costs, higher heating values, and improved boiler performance. Ash removal techniques primarily rely on physical separation based on differences in specific gravity. Coal has a specific gravity range of 1.1-1.8, whereas mineral impurities like carbonates and silicates are heavier (>2.0) and pyrites are much heavier (~5.0). Separation can be achieved using water, air, or dense media suspensions. It has been demonstrated that a combination of physical and chemical methods can produce “ultraclean coals” with ash yields as low as 0.1-1%.