Block flow diagram of overall plant layout. Principal inputs are coal, water, non-carbon electricity, and make-up solvent. Outputs are clean middle-distillate liquids including a bio-oil component, vitrified slag, and sulfur. Schobert (2015) Click to enlarge.

The core of the process involves extracting coals with light cycle oil—a refinery byproduct; separating the residual solids; hydrotreating the liquid to remove sulfur; hydrogenating to eliminate aromatic compounds; and finally distilling to obtain the fuels of very low sulfur and low aromatics content, which represent the majority of the liquid yield.

As reported in the paper, the optimum yield of the raw extract after three stages of extraction is as high as 75%. The recovery of middle-distillate fuels from the hydrotreated and hydrogenated extract is up to 80%. A solvent stream would be recycled back to the front end of the process.

The liquid product is of particular interest as an aviation fuel as it appears to be suitable as a potential drop-in replacement for Jet A or JP-8 fuels. In addition, it has a much greater resistance to pyrolytic breakdown at high temperatures than either Jet A or JP-8.

The fuel could be considered for thermal management on high-performance aircraft such as the F-35. The middle-distillate is also fully compatible with conventional petroleum-derived diesel for light-duty applications such as in light road vehicles. It has not yet been tested in other diesel applications.

Background. Broadly, there are two approaches to coal-to-liquids: indirect and direct liquefaction. Indirect liquefaction processes gasify the coal to produce synthesis gas (carbon monoxide and hydrogen). The synthesis gas is then reacted over a catalyst to produce a mixture of liquid hydrocarbons (Fischer–Tropsch synthesis).

The synthesis gas for indirect liquefaction can be made not just from coal, but from any carbon feedstock that can be converted to synthesis gas. Depending on the choice of specific conditions for the synthesis reaction, it is possible to make hydrocarbons ranging from methane to C₄₀+ waxes. Although versatile, the approach also produces very high carbon dioxide emissions.

Direct liquefaction involves the hydrogenation of the macromolecular structure of coal, using hydrogen itself, various liquid compounds having easily donated hydrogen atoms, or both. The product is a synthetic crude oil, which contains vestiges of the molecular structural features of the coal (unlike the output of indirect liquefaction).

Direct liquefaction products require further downstream refining steps, such as sulfur reduction and aromatics saturation, to meet modern engine specifications and environmental standards.

Direct liquefaction also requires substantial quantities of hydrogen. Exact hydrogen consumption depends on the specific coal being processed, and the extent of conversion.

EMS Energy Institute process. Although coal conversion to liquid fuels has many potential environmental issues, the conceptual design of this EMS Energy Institute process augments the core processes of extraction, separation, hydrotreating and hydrogenation, and distillation with other operations selected to reduce total process emissions nearly to zero.

• Most of the hydrogen needed for the hydrotreating and hydrogenation steps would be produced by electrolysis of water using non-carbon electricity, e.g. from photovoltaics.

• The process heat necessary for the extraction, hydrotreating and distillation steps could be obtained from concentrated solar power or non-carbon electricity.

• Hydrotreating the extract inevitably produces hydrogen sulfide. New catalytic processes for H₂S splitting are recommended for destruction of this material, recycling the hydrogen back into the process, and recovering the sulfur as a marketable byproduct.

• Additional hydrogen could be produced by gasifying the residual partially extracted coal recovered after the final extraction step. Gasification at high temperature would melt the mineral constituents of the coal, allowing the resulting slag to be vitrified into a glassy product that could be used as, e.g. fill material for road construction.

• Heavy distillation residua or biomass material from CO₂ capture could also be co-fed to the gasification unit.

• Likely sources of CO₂ include emissions from fired process heaters and from the gasification unit, after shifting the raw gas to a CO₂-H₂ mixture and separating the desired hydrogen.

• The intended approach for carbon capture would use algae photobioreactors. Algae would be harvested for production of a bio-oil that could be blended with the coal-derived oil product. “Spent” algae would be consumed in the gasification unit.

Overall inputs to the process would be coal, water, non-carbon electricity, and make-up solvent for extraction. The outputs would be clean liquid fuel, with a bio-component, suitable for use in jet or diesel engines; sulfur for sale to various chemical applications, and a vitrified slag. Other emissions would be minimal.

The core of this process, involving solvent extraction, hydrotreating and hydrogenation, and distillation, has been shown to produce prototype fuel giving acceptable performance in several types of engines.

… The additional operations intended to make this a new and novel zero-emission coal-to-liquids process have not yet been tested in conjunction with this process. All have been tried and tested elsewhere, as indicated in the various citations to the literature. They need to be integrated with the extraction-through-distillation steps to demonstrate the emission-free operation. Much vigorous and excellent research is going on worldwide in incorporating renewable energy, and particularly biological-based operations, in hydrogen generation, H₂S splitting, and CO₂ capture, along with concentrated solar energy for heat production. Continuing developments in these fields offer further opportunities for revising the process flow schemes shown here, adding further novelty to this process.

—Schobert (2015)

This work was funded over a period of twenty years by the US Air Force, by the Air Force Research Laboratory at Wright-Patterson Air Force Base, and by the US Department of Energy, mainly through the National Energy Technology Laboratory.

Resources

• Harold H. Schobert (2015) “Toward the zero-emission coal-to-liquids plant” Technology doi: 10.1142/S2339547815400063