Drying Processes for Coal Ash Reuse
With the correct equipment for coal ash drying, calcining and thermal desorption, some of the hazards presented by coal ash can be mitigated substantially.
The two largest beneficial uses of encapsulated coal combustion residuals (CCR) — commonly known as coal ash — are in concrete/concrete products/grout and in gypsum panel products. According to the U.S. Environmental Protection Agency (EPA), these uses of coal ash are the most appropriate because they are comparable to the original materials, and they fall below the agency’s health and environmental benchmarks. The extraction of new raw minerals consumes energy, impacts the environment, limits the availability of natural resources and has a detrimental effect on water resources. Recycling and reusing waste materials helps decrease these unfortunate impacts.
Coal ash is a byproduct of coal combustion. It forms when coal is burned in boilers that generate steam for industrial applications and power generation. 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.
Approximately 80 to 90 percent of coal combustion residuals are made up of basic nonradioactive 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 construction applications, and it can be used as a direct substitute for Portland cement in concrete.
- Bottom ash can be used for concrete, as an additive to asphalt and for melting snow and ice.
- Boiler slag mainly is used as blasting grit and roofing granules on asphalt shingles.
In addition, flue-gas desulfurization (FGD) material can be used as a replacement for mined gypsum in wallboard.
Concrete and wallboard account for nearly half of the total amount of coal ash that is reused. Aside from these EPA-approved beneficial reuses of coal combustion residuals, the material has a number of other uses. They include:
- 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.
- As the base material beneath athletic field tracks.
Most of these uses involve encapsulated coal ash. When encapsulated, the material is bound within another product, and any contaminants are prevented from leaking. (By contrast, unencapsulated use is a loose or unbound particulate or sludge. The EPA plans to develop a model for this type of reuse.) 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 United States is disposed of in dry landfills and surface impoundments. It is one of the largest streams of industrial waste in the country. There are 676 units managing slurried coal ash at 240 facilities. This includes 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 their failure or misoperation would likely cause economic and environmental damage as well as the loss of human life.
Dry coal-ash storage can be extremely hazardous because it can be dispersed by wind as a carcinogen-heavy dust that can cause major health issues if inhaled. If ash landfills are not properly lined, they also can be hazardous to groundwater.
Wet coal-ash storage consists of ash mixed with water to form slurry. The slurry is stored in large settling basins known as tailings ponds or surface impoundments. Wet storage is a common method of disposal. It allows the water to drain and evaporate from the ash over time. Tailing ponds must be well-constructed and thoroughly lined or the toxic substances in the coal ash can migrate into nearby water sources. Once the substances dissolve and leach into the ground, the microscopic stream of heavy metals and contaminants often finds its way into groundwater. There, fish and wildlife are exposed to the contaminants and ingest them.
Drying Equipment to Encapsulate CCR
With the correct equipment for coal ash drying, calcining and thermal desorption, some of the hazards presented by coal ash can be mitigated substantially. Industrial dryer manufacturers offer several such solutions. Depending on the moisture content of the material and how fine it is, some 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 specific application. Some suppliers can offer the services of a pilot-plant laboratory to test the proper applications and designs.
Some fluid-bed dryers can handle light, powdery material like fly ash or granular materials like bottom ash. Other fluid-bed dryer designs can handle materials such as sludges, filter cakes and agglomerates that exhibit characteristics not normally conducive to fluid-bed processing. Such designs would be ideal for tailing-pond sediment. Choosing the correct type involves a complex calculation that combines engineering considerations of specific product properties, available process requirements and economics.
A continuous-flow fluid-bed dryer removes moisture and produces dried bulk solids by processing materials in a heated flow of gas. The particles are suspended in a rising stream of heated gas moving with enough energy to cause the material to behave like a fluid. The natural characteristics of the fluidized mass place the extensive particle surface area in full, constant contact with the drying gas. The thorough exposure generates a high rate of heat and mass transfer for effective, efficient drying.
Rotary dryers also can be used to process CCR. Rotary dryers are capable of handling almost any bulk solid material regardless of its conveyance and handling characteristics. These dryers often can be configured to meet many needs and applications. Processes that require the drying of fine powders, lumpy solids, sticky semi-plastics, sludges, pellets, agglomerates or even a mixture — many of which describe wet or dry coal-ash storage — can be handled by this type of unit.
Ever since its initial development decades ago, the rotary dryer has been the workhorse of the drying industry. As with the fluid-bed, the rotary dryer is a direct heating technology resulting in high heat- and mass-transfer rates. Rotary dryers often have a conventional straight shell on a direct-heat dryer. The dryers typically are equipped with two tires rotating on four trunnion wheels and designed with internal flights to assist in showering the wet material through hot gases. Such dryers typically are sloped on an approximately quarter-inch-per-foot incline from the feed end to the discharge end. This helps convey the material along the shell. Factors such as starting and final moisture content, product temperature, drying air temperature, air velocity and material retention time in the dryer are considered in the design.
For a co-current rotary dryer, the majority of the surface moisture is removed at the feed end of the shell due to the large temperature difference between the gas and solids. Removal of any internal moisture is a longer process due to the resistance in the moisture’s path as it migrates from the inside of a solid particle to its surface. Typically, a larger dryer will be required.
Rotary calciners are in a class by themselves when it comes to the processing of ash. They do more than simply dry. Their versatility makes them a suitable choice for specialty applications such as calcining, chemical reactions and thermal desorptions.
Rotary calciners are an indirect heating technology. The separation of the heat source from the process environment confers several advantages. These include allowing specific process atmospheres and preventing contamination from the heat source itself. These advantages are invaluable in waste recycling such as with ash as well as with catalyst manufacturing, hydrocarbon processing and chemical production.
A rotary calciner is a cylindrical vessel that rotates inside a furnace. Within the cylinder, materials are heated rapidly and efficiently in a controlled environment. The material is fed continuously, rotated constantly and advanced in a steady plug-flow manner. Calciner applications tend to be highly specialized. The rate of heating the processed material, or temperature profiling, along the entire length of the furnace can be controlled within a few degrees. Ramping, reacting, decomposing, soaking, cooling, etc., can be achieved at any time in the process. Internal atmospheres can be oxidizing, reducing, hazardous, pressurized, inert, recycled or whatever is needed to achieve the desired product quality in a safe, controlled manner. The processing temperature can be high, allowing full benefit of radiative heat transfer and the production of ultra-high-purity materials.
In conclusion, the right industrial dryer manufacturer can provide users with any of these options, giving them the most appropriate equipment for the application. In the case of encapsulating coal ash, thermal processing can help make this waste product more suitable for its beneficial reuse in concrete, wallboard and other purposes.
Beneficial reuse of coal ash protects against catastrophic tailing-pond spills and the resulting contamination of soil and water. In addition, the recycling and reuse of this waste material decreases the amount of new energy expended to extract fresh raw minerals. Through the proper drying, calcining and thermal desorption of coal ash, there are some industries that can benefit from this industry’s waste.