Products from Coal Combustion
The number of products that can be manufactured from ponded coal combustion ash is truly remarkable. The ash itself must first be recovered from the pond or landfill and then separated into its fundamental components. These are carbon, silicates and high-density iron rich materials.
Coarse Bottom Ash. The coarse glassy slag commonly referred to as "bottom ash" is from deposits that form in the furnace and are removed from the bottom of it. It often contains lots of voids from gas bubbles, and it is glassy, often tough, and durable.
Cement Blocks. A high value use for bottom ash is as a coarse aggregate in the manufacture of cement blocks (a.k.a. "cinder blocks"). The porous nature of the ash often qualifies the product for a "light weight" classification. At one time this was a very common use, however quality and supply problems have limited and reduced the amount of ash used in this product. The Coleman plant will produce a beneficiated coarse ash for block sand of high quality.
Road Aggregate. Bottom ash is used in asphalt as an aggregate. The most common application is as an "anti-skid" surface coat on asphalt roads. Bottom ash will retain its anti-skid characteristics for a long time, unlike limestone aggregate that polishes smooth rather quickly.
Fly Ash. Fly ash is the very small particles that are swept from the furnace with the combustion gases. It is collected in various combinations of electrostatic precipitators, air cyclones, and bag houses with overall efficiencies of greater than 99.9%.
Cement Additive. The most common and economical use for fly ash is as a pozzolanic replacement and additive for portland cement concrete. Adding fly ash to concrete increases its strength and durability. About 10 million tons of fly ash was used in this manner, generating several hundred million dollars of revenue. The manufacture of portland cement results in very high CO2 emissions. Every ton of fly ash added results in the reduction of one ton of CO2 emitted from portand cement manufacture.
Flowable fill. A mixture of fly ash with sand and small amounts of cement results in a material that flows freely and quickly sets up with the strength of compacted soil. This material, known as flowable fill has applications in construction where it reduces cost by cutting construction time.
Plastic Filler. The finest size portion of the fly ash can be used as a filler in plastic compounds. Fly ash reduces cost by replacing plastic resin and also increases the stiffness of the plastic. The surface of the fly ash can be modified to make it more compatible with the plastic or "funtionalized". This results in a new composite material with whole new properties. This use of fly ash is new and has a bright future.
Not all of the coal is completely burned in the furnace; small amounts escape combustion and are swept out with the fly ash. This carbon has several forms and interesting properties.
Fuel. Surprisingly the carbon which passed through the furnace will burn well on a second trip, as the reason it didn't get burned is simply chance. The carbon from the ash is found to be very high in surface area that compensates for its low volatile matter. Tests conducted by the University of Kentucky Center for Applied Energy Research found a 99% burn out rate for the carbon when it is reblended with the coal feed.
Adsorbent Material. The carbon is found to have a surface area and pore structure that makes it amenable to adsorb odors and organic pollutants. Research on organic dyes and waste oils found the carbon to have good activity for the application.
Ladle Carbon. One current commercial use for the carbon is in the lining of steel handling ladles. This application requires very low volatile content.
Iron ore. An iron rich magnetic fraction can be separated from the ash. This material has been used, on a local basis, as an iron ore.
Feed. The whole fly ash is also commonly used as a kiln feed for the alumino-silicate
fraction of portland cement. The low sulfur content helps to reduce cement
kiln dust (CKD) a difficult to handle waste material.
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