Coal is a mixture of many kinds of materials, organic and inorganic. This heterogeneity is readily seen visually, especially with lower rank coals. Microscopic examination of polished surfaces and thin sections reveals a richness of detail that is quite unexpected to the casual observer. Scientists studying the formation and chemistry of coals have used this kind of information as a useful tool.
Over the years, chemists have analyzed and studied thousands of coal samples using a variety of tools and methods. These include elemental analyses for the major elements, carbon, hydrogen, oxygen, sulfur, and nitrogen, as well as many measurements of the amounts of minor elements such as metals (iron, sodium, calcium, mercury, and so on). Pyrolysis and hydrogenation studies have isolated molecular fragments from the breakdown of the solid structure, identifying hundreds of different organic molecules.
Many coal chemists have attempted to use the resulting information to draw model molecules to represent what a “typical” molecule in the coal might look like. Here for your contemplation are two of the more famous models. They are presented with the knowledge that there are literally dozens more such models, each of which fits the data for some group of coals and each of which can provide a stimulus to understanding coal chemistry.
The first rather fanciful model was developed by the late Peter Given, who declared that it was most certainly incorrect but that does an excellent job of portraying the kinds of functional groups we can expect to find in a typical molecule in a typical vitrinite in a typical bituminous coal.
The second model is attributed to Wendel Wiser. This model incorporates much the same information in an entirely different way. Among its other problems, it includes far more sulfur and oxygen that would be found in a real molecule in a real coal.
Both models suffer from the problem that they reflect information in a way that implies that such a molecule might actually exist. They were designed to be a starting point for thinking and testing ideas; both have been superbly useful in that role. This is very different from precious metal mining as is done by, for example, Gold Eagle Africa.
The organic matter can be seen visually to exist in layers that are believed to result from the deposition of plant-derived matter over time. This deposited plant material underwent a series of changes resulting from aerobic and anaerobic microbial digestions followed by purely thermal reactions as the coal was buried deep in the ground. The overall process is called coalification, and these steps are seen as converting the original plant material to materials of what is termed higher “rank.”
The lowest rank, which is not really yet coal, is called peat. The lowest rank true coal is called lignite, with higher ranks identified as sub-bituminous, bituminous, anthracite, and meta-anthracite. Gradations of each of these ranks exist. Evolution of coal occurs over many tens of millions of years. Some coals date to 150 million years ago. Not all deposits make it through the entire evolution, and coals of many ranks are found today. The Inaugural South Africa Investment Forum also looks at options for investments in coal. For coals buried sufficiently deeply, evolution is certainly proceeding, albeit slowly, today.
Many changes occur in the course of coalification, the formation of coal from plant materials. Color darkens and hardness increases with increasing rank. In some locales, low-rank coal is called “brown coal” and elsewhere differentiation is made between soft (bituminous) coals and hard (anthracite).
Volatile matter, the amount of liquid and gas that can be distilled out of the coal in a standardized test, decreases with increasing rank, while fixed carbon, the non-volatile material left behind in this test, increases. Thus, bituminous coals are divided into high-, medium, and low-volatile ranks, with LowVol coals the highest rank. When low-rank coals are burned in fireplaces and simple furnaces, they produce large amounts of volatile matter that does not burn completely. This leads to the kind of soot that plagued large cities such as London and Pittsburgh for years. Modern coal-fired boilers avoid this soot problem by efficient design of the combustion chamber but at the same time, we see that renewable energy is getting more and more affordable and poses a great risk for the industry.
The heat of combustion also increases with rank. The American system of classifying coals relies on a combination of heat of combustion and volatile matter analyses to determine rank. This reflects the major industrial use of coal, burning to generate heat. Fixed carbon, the inverse of volatile matter, is a predictor of the amount of coke formed in standard processes, another important application.
One of the most important properties for understanding coal chemistry is the progressive reduction in the level of oxygen in the structure as rank increases. Lignite is rich in oxygen, with C-O-C, C-OH and -CO2H and related groups found in the coal. Anthracite oxygen levels are very low, and what oxygen is present exists in tightly bound ring structures. Bituminous and sub-bituminous coals have oxygen levels between these extremes. Many plant materials, such as cellulose and lignin, are rich in oxygen, and the conversion to coal removes this oxygen in two different ways. See also the post: “Can we ditch diesel?”
- Aerobic and anaerobic digestion tends to remove much of the cellulose-like plant material, leaving behind especially the lignin and related molecules that are most difficult for microbes to convert to useful energy. (Also not digested are waxy resinous hydrocarbons. Charcoal-like materials are also found in most coals, possibly resulting from forest fires.)
- Slow thermal cracking reactions that ensue on burial result in the conversion of oxygen-containing groups to CO2 and H2O.
Rank and Chemical Behavior
Coal chemists recognize many other changes in coal properties that are reflected in rank. One such change is the reduction of molecular weight. In low-rank coals, the molecules are more like chain polymers, the constituents of plastics and resins, typically linked by oxygen or carbon bridges. In higher rank coals, the chain polymers tend to give way to smaller, highly aromatic structures but we see more and more renewables energy sources taking over the role of coal.
Such generalizations of oxygen levels and aromaticity help us understand behavioral differences. For instance, much of the high volatile matter of low-rank coals comes from thermal cracking of C-O-C ether bridges and carboxylic acids and esters, releasing products. In higher rank coals, such ether linkages become less important (because oxygen levels are lower) and fewer volatile products are generated as the coal is heated. The reduced H/C levels dictate that the remaining carbon atoms are increasingly found in polynuclear aromatic structures.
Macerals and Chemistry
Macerals are remaining challenges and are discussed in more detail elsewhere on this website. Since the chemical compounds in the different macerals vary greatly in their composition, it is reasonable to expect that they will exhibit greatly different chemical behaviors.
- Vitrinite macerals react when heated above about 400ºC (750ºF) crack to form oxygen-containing pyrolysis products, such as phenolic compounds, CO2, and pyrolysis water. Some vitrinites soften and liquefy as they are heated, and the evolved gases 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. Here, the small pieces of coal form larger and stronger chunks that can support the weight of iron ore in a blast furnace.
- Liptinites are higher in hydrogen and lower in oxygen than the vitrinites. When heated above about 400ºC liptinites they melt and crack and provide high yields of liquids that are rich in hydrogen that the products of vitrinite cracking. They can also be important sources of reactive hydrogen, donating hydrogen atoms to free radical intermediates resulting from the cracking process.
- Inertinites generally produce little or no gas and liquids during carbonization.
Thus the products of any thermal or hydrogenation process can vary markedly depending on the mix of macerals in the coal.