The second thing everyone learns about coal is that it comes as anthracite, bituminous, sub-bituminous and lignite. (The first thing is that coal is a dirty black solid, of course.) These terms denote the property called rank.
Rank goes along with energy content, with anthracite the highest and lignite the lowest; other properties vary along with the series as well.
These variations are all related to the chemical structure of the organic materials, and they are very important in guiding the usage of the coal.
Coals all started as plant materials that accumulated in swampy areas. Dead leaves, branches, and whole tree trunks fell into the water, where they were decomposed by microorganisms (initially aerobic, but later anaerobic). Lignin in the wood is difficult to digest, so most of what disappeared were the cellulosic and starchy materials.
This left lignin and various resinous materials. Slowly over millennia, the layers of undigested materials grew to tens or hundreds of feet in thickness. The layers also included sand and clay from soil washed into the swamp, as well as charcoal-like particles leftover from forest fires.
Coal deposits typically got started 50 to 100 million years ago. Where the organic material became buried, the forces of pressure and heat transformed it. Water, carbon dioxide, and methane were produced, leaving behind solids relatively richer in carbon.
Where the organic material was not buried, it remained as peat, which is harvested, dried, and burned in many areas of the world.
The combination of biological and physical processes is called maturation. The farther maturation proceeds, the higher the rank of the coal.
This picture of coal genesis explains why coals are found in layers, called seams. It explains why high-rank coals have higher carbon contents and provide more energy per pound when they are burned. It explains why lignite deposits tend to be thicker on average than bituminous coal deposits, etc.
It also goes far to explaining many of the differences in chemical behavior among coals of different ranks, differences that can be critically important in deciding what to do with a coal resource. Check out this fascinating BTS Media video that explains very well how coal is formed:
It does not, however, tell us a thing about sulfur levels or mineral (ash) content. Those two depend much more on where the coal formed and the environment of the original swamp. The South African Inaugural Investment Forum sets ethical standards for exploration and exploitation of coal on the nation
Coal Properties and Rank
Some properties of coal vary quite regularly with rank, some increasing and some decreasing. As examples, we can look at the properties of four US coals of different rank. Here the properties listed include what is called the Proximate analysis, measurements derived from a standardized test. This test has three steps:
1. A weighed sample of the coal is first dried to drive off moisture, and the weight loss is recorded.
2. The dried coal is heated to a high temperature in the absence of air to drive off liquids and gasses, and the sample is cooled and weighed again. The difference is called “volatile matter,” denoted Vm.
3. The remaining solid is burned to leave behind the ash components and weighed again. Fixed carbon is the name given to the amount of material that burns off in this step, and of course, the ash is what is left behind.
In a separate test, a weighed sample of the coal is burned in a closed container called a Calorimeter, to measure the amount of heat released. This heat is listed as “BTU/lb”, where BTU stands for British Thermal Unit, an engineering unit equal to 262 calories. It should be reported on a moisture and ash-free (MAF) basis.
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.
1. 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. Special attention needs to be paid to implementing international energy management standards.
2. 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 alginate (derived from algae.)
3. 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 a 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 though that remains challenging. It may also result from the degradation of very reactive material in the original plant detritus. Other inertinite macerals include semi-fusinite and micrite.
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 sometimes found. In the highest rank coals, anthracite and meta-anthracite, it can be difficult to identify differences among these macerals. That is because of the severe conditions leading to very high rank allow reactions that tend to result in chemical homogeneity.