Characteristics of Coal

What is Coal? The word coal describes many different types of rocklike materials, which contain complex hydrocarbons and organic compounds in varying mixtures. It is important to understand the types that exist, and the benefits and pitfalls associated with each type. Selecting the right type or mixture of coal for use in power plant or industrial boilers will help to maximize profits while maintaining emissions within set standards.

The various types of coal used in power plant or industrial boilers was formed from the remains of peat, ferns, and other plants that grew on land. Fossilized remains of trees and plants are found in many coal beds. The deposits were subjected to different amounts of heat and pressure over time. Different types of coal were formed depending on how much the organic material from the plants was changed. Coals that underwent the most change have a different molecular structure from the organic material they came from. Those coals that underwent the least change more closely resemble the molecular structure of the organic material they came from.

There are four different types of coal:

• Anthracite – contains between 86% and 98% carbon by weight and it burns in boilers slowly
• Bituminous – contains between 69% & 86% carbon by weight
• Sub-bituminous – contains less carbon, more water and is a less efficient source of heat for boilers
• Lignite – contains up to 70% water by weight. Emits more pollution from boilers than other coals

Anthracite forms from bituminous coal when great pressures developed in folded rock during the creation of mountain ranges. This occurs only in limited geographic areas – primarily the Appalachian region of Pennsylvania. Anthracite has the highest energy content of all coals & is used for making coke, a fuel used in steel foundry ovens.

Great pressure results in the creation of bituminous coal. Bituminous coal is commonly used in power plant boilers for generation of electric power. Bituminous coal has a higher boiler heating value than either sub-bituminous or lignite, but less than that of anthracite. Bituminous coal is mined chiefly in the Midwest and Appalachian regions.

Plant chemists conduct analyses of the coal as it is received. Knowing the properties of the coal, boiler operators can adjust the procedures to obtain the maximum heat from the coal and minimize the negative effects of poor combustion. The following content is analyzed:

• Moisture is the water contained in the coal. It does not provide any heat. Instead, heat must be supplied by the fuel to evaporate the water when the fuel is burned. This heat is lost in the flue gas. The heat loss due to moisture in the fuel can be large.
• Volatile matter indicates how well the coal will burn. Volatile matter consists of hydrocarbons and other gases that are produced when a sample of dry coal is heated. Coals with less than 15% volatile matter may be difficult to burn and require special procedures for efficient combustion.
• Fixed Carbon is the combustible matter left after the volatile matter in coal is driven off as gas. The amount of fixed carbon indicates how much of the fuel must be burned as a solid. Coals with a high percentage of fixed carbon usually burn more slowly because the solid carbon burns slower than hydrogen and other volatiles. A high fixed carbon content usually means that the coal must be ground in the pulverizer to a smaller particle to improve the speed of burning. Fixed carbon is calculated by subtracting the percentage of ash, moisture, and volatile matter from 100%.
• Ash is the residue left after all the combustible material in the coal is burned. Ash is an undesirable part of the coal. Ash increases maintenance costs and can reduce plant reliability. It does not provide any heat, adds weight to the fuel, and increases shipping and disposal costs.
• Heating Value is the amount of heat produced, expressed in BTU’s when the coal is burned completely. When the heating value is determined it can be reported in two different ways, as the Higher Heating Value (HHV) or Lower Heating Value (LHV).

Another major concern with coal is its sulfur content. Most coal has sulfur trapped in it (chemically bound), similar to the carbon and hydrogen; the sulfur will burn. The burning of sulfur creates oxides of sulfur, commonly known as SOx. SOx from boiler stacks contribute to environmental pollution through acid rain. SOx emissions can be reduced in three ways: by removing sulfur from fuel, dry desulphurization and wet desulphurization. All of these methods increase the cost of burning coal in boilers.

Many power plants now burn a mixture of coal in their boilers to minimize the costs associated with burning a high sulfur content coal from the Appalachians with that of a low sulfur content coal from the Powder River Basin (PRB). The PRB sub-bituminous coal has a low heating value with low sulfur content, while the Appalachian bituminous coal has a high heating value with high sulfur content. The PRB coal produces less heat per pound but costs less on the back end when environmental considerations are added. The Appalachian coal generates more heat in the boilers, at the cost of expensive emissions curtailing equipment.

Plant Management must use industry best practices for burning coal in their boilers along with cost calculations associated with transport, handling, burning and emissions curtailment associated with the chosen coal source.