In 2007, the United States produced over 131 million tons of coal combustion products from the nation’s 1,308 coal-fired power plants. This 131 million tons of residuals must go somewhere, and has to be handled in a precise, controlled manner – especially ever since the EPA, under the Obama administration, began to tighten regulations and create new restrictions on coal ash.
Coal ash handling systems are created to safely and efficiently transfer ash from boilers to storage units, where they are then finally disposed of in a responsible manner. There are several types of solids that constitute ‘coal ash’, and several ways of handling residuals from the burning of coal.
Here is a brief primer on coal ash, two types of handling systems, and the industry’s shift from wet coal ash handling systems to dry coal ash handling.
An Overview of Coal Ash
Coal ash is a part of what are called coal combustion residuals (CCR), primarily because the residuals from burning coal are more than ash; they also include solid materials. In this context,
CCR consists of four different solids: fly ash, bottom ash, boiler slag, and flue gas desulfurization material. Fly ash is fine ash that is generated by burning coal and is pushed out of the boiler by flue gases. This is typically removed from chimneys before it reaches the open air by particle filtration equipment.
Bottom ash is the ash that accumulates in the bottom of a boiler. It is non-combustible and accumulates in ash hoppers below the boilers. Bottom ash, unlike fly ash, is typically in solid form that must be ground down.
Both bottom ash and fly ash have to be safely and efficiently removed from the boiler system to a disposal system that adheres to regulations. Due to the unique nature of each, separate systems have been created to handle each.
Fly Ash Handling Systems
The challenge of a fly ash handling system is to remove fly ash before it enters the outside air. Fly ash systems are constructed from heavy-duty, abrasion-resistant steel for reduced wear and maintenance. The ash is directed horizontally or vertically using vacuums to a filter receiver that then processes the ash into lockhoppers and bins. The ash is then fed via feeders and pumps to beneficiation equipment that helps translate the ash into recyclable material that is often used in cement, cinder blocks, and other building materials.
Eventually, the ash is sent to storage domes before being diverted to loadout silos that load ash onto vehicles for transport to external storage (or to material processing plants for inclusion in concrete and other materials).
Bottom Ash Handling Systems
Bottom ash can be more complex than fly ash due to the physical nature of it versus fine particulate ash. The combustion residuals at the bottom and sides of a boiler are scraped or moved into an ash hopper located below the boiler. They are then fed into a conveyor system that uses air (or in some cases, water) and a conveyor/drag chain assembly to carry the solids along in an airtight space to a primary crusher. The crusher reduces the solids into a more manageable size. This is then carried via conveyor to a secondary crusher to further reduce the particulates before being located into a bottom ash storage bin or silo. From there, the ash is often carried away for external transport using loadout silos.
Shifting from Wet to Dry Coal Ash Handling
Prior to the mid-2000’s, coal ash handling systems were primarily wet. Bottom ash conveyors were usually submerged in water, and water was also used in fly ash handling. This creates environmental concerns, though; the process generated slurry that was stored in ash ponds across the nation that have the potential to leak into groundwater, which is what has happened in North Carolina with several Duke Energy installations.
Dry ash handling was invented in the 1970’s as a way to avoid these problems. It has been steadily phased in, although the process had to be accelerated by the EPA beginning with the Obama administration. Currently, two-thirds of plants that use ash ponds that dry fly ash handling systems. Most bottom ash handling systems remain wet. Many plants intend to eliminate ash ponds, but not necessarily eliminate water from bottom ash handling.
With new regulations from the EPA, however, more and more plants are moving toward completely eliminating water from the process altogether.
For those who still use wet bottom ash handling, recirculation systems can convert wet sluice assemblies into dry ash systems in a relatively short amount of time. The recirculation system is intended to reduce the moisture content of the ash while re-using the water for additional cycles. There are also other systems that make use of water-less ash handling.
Wet-to-dry conversion can help reduce costs by reducing water use by millions of gallons a year per plant, and can be a more efficient and compliant way of handling ash in the future.
ProcessBarron Coal Ash Handling Systems
ProcessBarron has decades of experience in constructing modern, technologically-advanced coal ash handling systems for use throughout North and South America. Our systems are designed to reduce wear and tear, lower the need for maintenance, and improve efficiency all while complying with all existing regulations.
Contact ProcessBarron for more information on modern coal ash handling system construction and conversion for a facility.