Coal's formation is always taught as the successive decomposing of organic material (vegetation) under extreme pressure. In certain geologic ages, there were huge lakes and swamps where mountains now stand. The Appalachian Mountains are a perfect example. During the Pennsylvanian period of the Paleozoic Era, many millions of years ago, plants and trees grew in a great shallow sea and then fell and began to slowly decompose. Then, over millions of years, sediment covered this "stew" and the weight of thousands of feet of sediment began compressing it into a carbonized layer known to us as coal. The Pittsburgh coal is from this geologic age. The mountains, on the other hand, did not rise until millions of years after the coal formed.
There may be more to the formation of coal than this simplistic "composting" theory. Some scientists1 have proposed that bacteria played a very important role in the development of all fossil fuels, whether coal, oil, or natural gas. This is not so far-fetched as it sounds because we now use bacteria to "digest" crude oil from a tanker spill and render it into non-toxic waste. Another type of bacteria could just as easily turn organic matter into oil or coal. At least one prominent scientist, Thomas Gold, has proposed that these creative bacteria, or bacteria-like life, form a whole universe of hydrocarbon life deep within the Earth's mantle. In the future, our knowledge of geology may change radically as we explore the depths of the planet.
As you will read, there are many different grades of coal, all with varying properties. The main properties of coal that we rate are heat value, moisture, ash, sulfur content, and volatiles. We mine different grades of coal for different reasons. The coal from the Powder River Basin (Wyoming) was essentially worthless until air pollution emissions from power plants became a concern. Then, the market for PRB coal surged as it is very low in sulfur. Even when you factor in that you have to burn two tons of sub-bituminous PRB coal to equal the heat of a ton of bituminous coal, it is still cost-effective in reducing SO2 emissions. The Pittsburgh coal is valued for many reasons; a relatively high heat value, volatiles for chemical and tar making, and desirable coking qualities. Other coals are valued as being "steam coals"; they provided a good flame characteristic and have less ash. Steam coals fired industrial boilers, home furnace boilers, and train engines for over a century. Whereas electric power plants preferred to burn one type of steam coal, they now blend different coals to reduce emissions.
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