9781422285534

33 Chapter Three: Using Coal

electricity to make the tiny particles clump into larger, heavier ones, which then fall and can be collected. Another method is to pass the emissions through a filter that traps the particles. The gaseous emissions from burning coal, and other fossil fuels—in fact, from any fuel that burns—are a significant problem. To cut down on these troubles, power generators in North America must have filters and scrubbers to remove the pollutants from the air. In a wet scrubber, fine droplets of a chemical are sprayed into the flue gases. This may be calcium carbonate (as in limestone), calcium hydroxide (as in lime) or a similar chemical. The droplets react with the gases to make solids that can be collected. In the spray-dry method, by contrast, jets of heated air turn the polluting chemicals into a dry, powdery substance. Metallurgic Coal The other main usage of coal is for making iron and steel. When coal is used in this way, it’s referred to as metallurgic or coking coal—that is, coal converted into coke. Coke is a hard black or grey substance created by heating bituminous coking coal to extremely high temperatures in an anaerobic environment. Because there is no oxygen, the coal can’t burn. Instead of burning, it is transformed by heat, in a process

called destructive distillation, into an array of substances, including coal tar, ammonia liquor, and coal gas. At the end of the process, all that remains is coke. All of these products are useful in industry. Coke contains more carbon (up to 95 percent) than the original bituminous coal. It burns more effectively and hotter, with a heat content similar to anthracite, at 12,000–13,500 BTUs per pound. Also, in terms of its chemistry, it acts as a reducing agent—a substance that removes oxygen. So it is both a primary fuel and reducing agent for smelting, which means

Leftovers Treating of 1 ton (0.9 metric tons) of coal by destructive distillation can produce: • 1,700 pounds (770 kilograms) of coke • 30 US gallons (115 liters) of coal tar • 15 US gallons (57 liters) of ammonia liquor • 14,000 cubic feet (400 cubic meters) of coal gas