According to the U.S. Environmental Protection Agency (EPA), the two largest beneficial uses of encapsulated coal combustion residuals (CCR) -- commonly known as coal ash -- are as substitutes for portland cement in concrete, and for gypsum in wallboard. The EPA concluded that these uses of coal ash are most appropriate because they are comparable to the original materials, or below the agency’s health and environmental benchmarks. Concrete and wallboard account for nearly half of the total amount of coal ash that is reused. The extraction of raw minerals consumes energy, impacts the environment, limits the availability of natural resources and has a detrimental effect on water resources. The recycling and reuse of waste materials helps decrease these unfortunate impacts.
Coal ash is a by-product of coal combustion, and forms when coal is burned in boilers that generate steam for power generation and industrial applications. Burning coal produces three types of ash as coal combustion residuals. Fly ash is a fine powdery material carried up with hot flue and stack gases. Bottom ash is a much coarser sand-like material that is sluiced from the bottom of the boilers. Boiler slag is formed when the ash melts under the intense heat of combustion and collects at the bottom of the boiler and in exhaust stack filters. About 80-90 percent of CCRs are made up of basic non-radioactive minerals, such as silicon, iron, aluminum and calcium. Unfortunately, it is the remainder that gives coal ash its toxic reputation -- arsenic, mercury and lead.
Roughly 45 percent of the coal ash produced each year is recycled and beneficially reused. Fly ash has cement-like properties that make it useful in concrete and construction applications. Bottom ash can not only be used for concrete, but also as an additive to asphalt and for melting snow and ice. Boiler slag can mainly be used as blasting grit and roofing granules.
The largest EPA-approved beneficial reuse of CCRs is as an ingredient in concrete and wallboard, but it also sees use for a number of other purposes as well. It can be structural fill for highway embankments, building foundations and abandoned mines; as a permeable base for paving projects, such as the top layer on unpaved roads and subbase fill under paved highways; as a mineral filler in asphalt or a stabilizer during asphalt recycling; as an agricultural additive for soil stabilization; and it can even be used as the base material beneath athletic field tracks.
Most of these uses involve encapsulated coal ash, where it is bound within a product and contaminants have been prevented from leaking. In contrast, unencapsulated use is a loose or unbound particulate or sludge, and the EPA plans to develop a model for this type of reuse in 2015. Reuse protects air and water, and can even help lower greenhouse gas emissions.
About 59 percent of the coal consumed by electric utilities and independent power producers in the United States results in the generation of about 68 million tons of coal ash. More than half of the ash in the U.S. is disposed of in dry landfills and surface impoundments, and is one of the largest streams of industrial waste in the country. There are 676 units managing slurried coal ash at 240 facilities. These numbers include wet ash ponds, dry landfills at power plants, offsite dry landfills, inactive dumps and abandoned or inactive mines. The EPA's records show that 50 of these units at 32 different locations have been identified as High Hazard Potential, meaning that failure or mis-operation will probably cause economic and environmental damage, as well as the loss of human life.
Dry coal ash storage can be extremely hazardous, since it can be dispersed by wind as a carcinogen-heavy dust that can cause major health issues if inhaled. If the ash landfills are not properly lined, they can also be hazardous to groundwater.
Wet coal ash storage consists of ash mixed with water to form a slurry, and storing in large settling basins, known as tailings ponds or surface impoundments. Wet disposal is a more common disposal method than dry, and allows the water to drain and evaporate from the ash over time. The tailing ponds must be well-constructed and thoroughly lined, or the toxic substances in the coal ash can migrate into nearby water sources, dissolving and leaching into the ground. This microscopic stream of heavy metals and contaminants often finds its way into groundwater, where it is exposed to and injested by fish and wildlife.
With the correct drying, calcining and thermal desorption equipment, some of the hazards presented by coal ash can be severely mitigated. Heyl & Patterson manufactures several such solutions. Depending on the moisture content of the material and how fine it is, the company's dryer systems can dry coal ash and make it easier to transport and handle. This equipment can be custom designed and built, to best suit a customer’s specific application.
Heyl & Patterson fluid bed dryers are among the most efficient and cost-effective dryers on the market. Conventional designs are available for powders and granular materials, as well as unique designs for materials exhibiting characteristics not normally conducive to fluid bed processing, such as sludges, filter cakes and agglomerates. Fluid bed dryers were the subject of two recent articles on the H&P Blog, Fluid Bed Dryer Fundamentals and How a Fluid Bed Dryer Operates.
Heyl & Patterson rotary dryers are among the most versatile available, capable of handling almost any solid bulk material regardless of its conveyance and handling characteristics. These rotary dryers can be configured to meet a wide range of needs and applications. Factors such as starting and final moisture content, product temperature, drying air temperature, air velocity and retention time of the material in the dryer are considered in the specifications.
Heyl & Patterson rotary calciners are on the leading edge of thermal processing equipment and technology. Their versatility makes them an ideal choice for a wide range of specialty applications, including drying, calcining, chemical reactions and thermal desorptions. The separation of the heat source from the process environment confers several distinct advantages, including specific process atmospheres and no contamination from the heat source.
Applications and designs for fluid bed dryers, rotary dryers and rotary calciners can be investigated at Heyl & Patterson's pilot plant lab facility.
For more information about Heyl & Patterson dryers for coal ash, contact us or click here:
For more information about Heyl & Patterson calciners for coal ash, contact us or click here: