Macerals

The various kinds of materials in the coal can be identified by analysis under the microscope. Specialists classify them into a variety of types, termed “macerals” by analogy to the term “mineral” used in inorganic rocks.

·       Most of the coal typically falls into a group of macerals lumped under the name vitrinite. This is a shiny material that is richer in oxygen than other parts of the coal. Geochemists classify vitrinites as Type III kerogens. When heated above about 400ºC (750ºF) to form cracked products (carbonization), vitrinite breaks down to form oxygen-containing pyrolysis products, such as phenolic compounds, CO₂, and pyrolysis water. Some vitrinites soften and liquefy as they are heated. In such coals, evolved gases can cause the melted materials to expand and swell; further heating leads to a dry product that appears to contain gas bubbles. This softening and re-hardening is called “caking” and is the basis of the manufacture of metallurgical coke. A measure of swelling and caking is called Free Swelling Index, sometimes referred to as “coke button.” Here, the small pieces of coal form larger and stronger chunks that can support the weight of iron ore in a blast furnace.

·       A second family of coal materials is classified as liptinites, which Van Krevelen called exinites. Geochemists classify these as Type I and II Kerogens, high in hydrogen and low in oxygen. These macerals are typically derived from spores, resins, and other non-woody materials in the forest. Liptinites are typically higher in hydrogen and lower in oxygen than the vitrinites. Amber is the extreme form of resinous materials found in the coal. During carbonization, liptinites are more reactive than the vitrinite, and coal chemists refer to them as “reactive” macerals. They are important contributors to the yields of liquids in carbonization, especially under conditions where a solvent is present to extract them during heating. They can also be important sources of reactive hydrogen in such processes. Liptinites include resinite (derived from resins such as pine tar), sporinite (derived from spores), and alginite (derived from algae.)

·       The third set of macerals is called inertinite. These are materials that undergo little change during carbonization – thus the name. They are typically low in both hydrogen and oxygen; geochemists classify these as Type IV kerogens. An important inertinite maceral is fusinite, which appears under microscopic examination to be not unlike charcoal. It may indeed be derived from charred material resulting from forest fires in the plants that formed the coal. It may also result from degradation of very reactive material in the original plant detritus. Other inertinite macerals include semi-fusinite and micrinite.

Physical separation of individual macerals from the whole coal can often be done, providing samples for analysis of carbon, hydrogen and oxygen. Some “typical” maceral analyses for a high volatile bituminous coal (typical of coals such as Illinois #6 and Kentucky #9) reveal results that provide the generalizations included in these descriptions.

In the lower rank coals, it is still possible to identify the original plant’s cellular structure, and individual blocks of woody material are sometime found. In the highest rank coals, anthracite and meta-anthracite, it can be difficult to identify differences among these macerals. That is because the severe conditions leading to very high rank allow reactions that tend to result in chemical homogeneity.