The complexity of coal structure and chemistry has important implications for conversion technologies and catalysis. Coals are inhomogeneous on the macroscopic, microscopic, and molecular levels. They are insoluble, opaque, macromolecular systems composed of a mixture of organic and inorganic constituents. While knowledge of coal's physical and chemical structures remains rudimentary, knowledge and understanding of coal reactivity are even more limited. Most of the available tools for determining chemical structure are designed to work with systems of pure compounds and either do not work when applied to coals or become much more complex in their application. The efficacy of solid catalysts when used with solid coals decreases very significantly compared to their effectiveness in fluid systems. Opportunities exist to develop entirely new catalysts that will contact coals and effect desired reactions. The committee identified a role for DOE in supporting advanced research on coal conversion and catalysis to ensure the cleanest and most efficient utilization of coal, consistent with the goals of the advanced fuels and power systems programs, and to compensate for the absence of significant industrial research in this field.
In reviewing current DOE coal advanced research programs, the committee particularly noted the decline in efforts devoted to coal liquefaction technology. Given the likely growth in importance of coal liquids in the mid and long-term, as described in the committee's strategic planning scenarios (see Chapter 4), the committee identified coal liquefaction as an important area for advanced research within the DOE coal program. Industrial transformations of fossil fuels are catalytic, and the creation of new and improved catalysts and better reactors to use those catalysts has been a central thrust of fuel chemistry for almost a century. The use of catalytic chemistry with coals presents unique and difficult problems. Since coal is a solid, it cannot move around into contact with a catalyst surface. Thus, the use of immobile solid catalysts typical of oil and gas processing is not possible with coal. It is necessary either to render the coal fluid, to use catalysts of extraordinarily high dispersion, or to use catalysts that are themselves mobile fluids. All three approaches have been used with some success, and there has been a fairly continuous improvement in catalysts used. Further enhancements can be anticipated based on a mix of applied and fundamental studies on topics such as highly dispersed catalysts, diffusion in coals and coal-catalyst contacting, and effective mobile catalysts. Both lower-temperature catalysts and more selective chemistries have the potential to reduce costs.
Research opportunities can be conveniently divided into two major categories: improvements in current processing chemistry and technology and liquefaction processes based on new chemistries. Possible improvements in chemistry and technology (see also Chapter 6) include: