9781422285534

16 Coal

For comparison, gasoline has a heat content of 20,400 BTUs, diesel is nearer 19,300 BTUs, and seasoned wood has a value of 5,000–8,000 BTUs. Of course, these are all averages. Each source of coal is tested and graded for its mineral and microscopic structure, based on structures called macerals , its energy yield, and for various contents such as sulfur, water, and traces of substances such as mercury and arsenic. Coal Times and Places Much North American coal was formed during the Carboniferous period, but almost every time span saw coal formation after the first small, moss-like plants invaded the land. Some sub-bituminous coal in the western United States date to the Triassic and Jurassic periods (252–201 and 201–145 million years ago, respectively). Some Canadian coal formed more recently during the Cretaceous period (145–66 million years ago). Large amounts of coal in Wyoming date to the Cretaceous and early Paleogene periods, making it 130–50 million years old. No matter when or where coal formed, it is sometimes called “buried sunshine,” because it contains carbon-based substances that got their energy from sunlight. When coal burns today, it is releasing that prehistoric light energy, partly in the form of light again, and also as heat. Macerals A high-power microscope shows that most rocks are made of mineral crystals and grains. In coal, similar tiny particles from the original living plant matter are called macerals. There are several main kinds, and their numbers and proportions give the coal its features. • Vitrinite macerals are broken-down remains of harder plant parts like trunks, stems, and roots. • Liptinite macerals come from softer plant parts such as leaves, buds, saps, and resins. • Inertite is the fossilization product of charcoal, or plants that had burned in natural wildfires that swept through the ancient forests.