What are coal combustion products?


Coal and Lignite are used by utility companies to fuel their boilers for the generation of electricity. These coals have a mineral content, varying from 5% to 30% of the weight of the coal, which forms a solid ash after the carbon in the coal is burned. The coals also have varying concentrations of sulfur, which forms Sulfur Dioxide gas when the coal is burned. Many utility generating units remove most of this Sulfur Dioxide gas by converting it to a solid material. These Coal Combustion Products (CCPs) from power production are collected and either sold or disposed. Texas produces over 13 million tons of these CCPs each year.

Fly Ash

Fly ash is the very small particle mineral residue that results from the burning of powdered coal in utility boilers. The individual particles are very small, like talcum powder, and are carried up and out of the boiler in the flow of air used in the boiler for burning coal. Hence the term “Fly Ash”. The Fly Ash particles are removed from the stack gasses using Electrostatic Precipitators or Bag Houses and are collected and stored dry for recycling. Fly Ash is a pozzolan: a silica, alumina, and calcium based material which, in the presence of water, will chemically combine with lime and produce a cementitious material with excellent structural properties. Some fly ash contains enough calcium compounds to be self-hardening when mixed with water. All Fly Ash can be used as a direct replacement for Portland cement in making concrete, in addition to many other applications.

Bottom Ash

Bottom Ash is the coarse, solid mineral residue that results from the burning of coal in utility boilers. The individual particles are much larger than Fly Ash particles and fall down through the air flow to the bottom of the boiler and are called “Bottom Ash”. The material is removed from the bottom of the boiler in a wet state and is transported to handling areas by conveyor or pipe. Bottom Ash has a similar chemical composition to Fly Ash, but is produced in size grades ranging from fine sand to large gravel. Because of the larger sizes, it does not have any cementitious properties. It is utilized in a variety of markets as an aggregate or filler material in construction projects and building products. One other product, Boiler Slag, is Bottom Ash produced by some older boilers, where the material is actually melted in the boiler, quenched in the bottom hoppers, and becomes hard and glassy. This material can serve as a low silica blasting grit.

FGD Material

FGD Material is the solid material resulting from the removal of Sulfur Dioxide gas from the utility boiler stack gasses in the Flue Gas Desulphurization process. The material is produced in the flue gas scrubbers by reacting slurried limestone or lime with the gaseous Sulfur Dioxide to produce Calcium Sulfate. FGD Gypsum is 90 to 95% pure Calcium Sulfate and has many uses. FGD Gypsum is used in many parts of the country in applications replacing natural gypsum, including wallboard production, agricultural fertilizer, soil amendment, and Portland cement production.

Coal Combustion Products: Recycling in Texas

What Are Coal Combustion Products?

Coal Combustion Products (CCPs) are the materials remaining from burning coal at electric power plants. It includes three primary materials: Fly Ash, Bottom Ash, and Flue Gas Desulphurization Material (FGD). Fly Ash is the fine particle residue removed from stack gases with various types of air pollution control equipment. Bottom Ash is the coarse, unburned solid matter that falls to the bottom of the boiler. FGD Material is the solid material produced in the air pollution control processes when stack gasses pass through pulverized limestone to remove sulfur dioxide.

How Much CCPs Are Generated In Texas?

Texas power plants produce over 13 million tons of CCPs each year. If this material was all loaded into dump trucks, the line of trucks would stretch from Austin to Boston and back. This is the state’s second largest potential waste stream, behind municipal solid waste, which amounts to over 18 million tons per years

Why Do We Want To Recycle CCPs?

CCPs have characteristics similar to virgin materials such as cement, lime, clay, sand, gypsum, and aggregates. As a result, CCPs can be used to make numerous construction products, from concrete blocks, to synthetic gypsum. Unfortunately, most CCPs are currently being disposed. CCPs are not only abundant, they are non-hazardous and are not considered to be a waste material when used in beneficial applications.

What Products Can Be Made From CCPs?

Fly ash is a material with a chemical composition similar to Portland cement and can replace a portion of the cement in concrete. Concrete containing Fly Ash is used widely in housing, office buildings, bridges, roads and highways, building blocks and concrete pipe, Bottom Ash is used as a substitute for sand, gravel, or limestone aggregates for a number of construction and building products, including concrete block, clay brick, and asphaltic concrete. FGD Material can be used to make a synthetic gypsum that is used in the production of wallboard for the building industry, in agricultural applications, and as a setting agent in the manufacture of cement. The material can also be used to make a road base to replace such products as sand and gravel.

What Benefits Accrue From Recycling CCPs?

Recycling CCPs into useful products can save utility customers millions of dollars a year. Unless they are recycled, the CCPs must be land filled, and the associated disposal expenses are passed along to the rate payers. Recycling also saves energy because CCPs replace products that require considerable energy to manufacture. Each ton of Fly Ash that replaces a ton of cement saves the equivalent of nearly one barrel of imported oil. Substituting CCPs for such products as limestone, clay, sand, and gravel also reduces depletion of these valuable resources.

What Are The Environmental Benefits?

Recycling CCPs results in reduced dedication of otherwise productive land to landfill usage. Recycling Fly Ash also means reduced emissions of air pollutants which result from making competing products such as lime and cement. Each ton of Fly Ash used to replace cement saves one tone of carbon dioxide (greenhouse gas) emissions. It also means fewer environmental problems, such as surface and groundwater problems, associated with mining virgin materials and reclaiming the land.