Much like what’s left in a grill or a fireplace at home, the coal ash produced by a power plant is the waste left after burning raw coal. Depending on the specific method used by the power or heating plant, about 10% of coal by weight is left as ash or other various forms of solid and liquid waste. With each plant producing 200,000 tons or more of ash per year, the sheer volume of waste makes it difficult to handle even if it was inert and harmless. Yet, coal ash is hazardous to human health and the environment, necessitating careful handling and containment. Understanding the various types of coal ash and their differences in composition is essential to planning a complete residue containment plan.
Types of Coal Ash
While each coal powered plant produces hundreds of thousands of tons of ash per year, this waste isn’t one homogeneous stream. The various waste products are separated into four main types of coal ash, and the amount produced of each type depends mostly on the coal source used and the exact combustion process.
Boiler Slag
Boiler slag generally makes up the smallest proportion of the coal ash produced by a power or heating plant. This type of by-product is solid and hard, with a glassy appearance that leaves it hard to process like other types of ash. These heavy pellets tend to settle to the bottom of any processing or holding pits, clogging up drains and other equipment. They can be useful if they’re sorted out by gravity and broken into a durable grit for sandblasting.
Flue Gas By-Products
Flue gas ashes are the second most common waste product by volume, and they’re often some of the most concentrated when it comes to hazardous content. These ashes and particles are created when the scrubbers installed on the vents are cleaned out. The scrubbers remove nitrogen oxides, sulfur oxides, and other hazardous gases that can contribute to acid rain issues. Yet, all those gases have to become a waste product when they’re trapped and cleaned out, and that becomes flue gas ash. A hydrated form of calcium carbonate, like limestone, is generally used in the scrubbers to absorb the tiny particles of the gases and heavy metals. That material can be reused as gypsum for producing sheet rock for construction or for enriching fields.
Fly Ash
Most of the coal ash residues and by-products created during the combustion process are fly ash. This ash is light and takes the form of a fine powder that’s hard to trap and easy to spread to the surrounding environment. When released accidentally, it can appear more like a smoke cloud rather than visible flakes of ash. Fabric and electrostatic filters are necessary to trap these tiny particles before they can blow out of the smokestacks with the vent air. The composition is remarkably similar to volcanic ash, so engineers were inspired by Roman use of that material to try using fly ash in modern concrete. About half of all new U.S. concrete contains fly ash, with some blends containing up to 40% ash by volume. This makes the material more resilient and environmentally friendly while making good use of the waste products from power generation.
Bottom Ash
Bottom ash is the only type of coal ash residue that resembles what most people think of as ash. This ash is too heavy to float in tiny particles like fly ash, but it’s lighter than slag. This ash isn’t quite as pozzolanic as fly ash, which means it can only be used in limited amounts in concrete without affecting the chemical reaction. It is somewhat useful in wallboard manufacturing as well. However, most bottom ash ends up being disposed of as a wet slurry or dry powder. It can still become dispersed in the air like fly ash if handled incorrectly, so it’s often mixed with water or liquid by-products to keep it under control.
Dry vs Wet Forms
Bottom and fly ash, in particular, both need careful handling to avoid being dispursed into the air where they’re almost impossible to recover. Dry handling allows for compaction and keeps the material as lightweight as possible while providing for disposal in a nearby hazardous waste landfill or a buried pit. Unfortunately, dry handling is riskier and requires more specialty equipment because of the ease of disturbing it. Adding water to make an ash slurry reduces the risks of it becoming airborne, but it increases the weight, introduces leak and seepage risks, and requires extra containment for spills. Each facility has to weigh the costs of both management methods before choosing one. Many power plants handle bottom ash as dry material and turn fly ash into a slurry due to the volume, but other facilities use the opposite arrangement instead.
Who Regulates Coal Ash Storage?
For many years, coal ash residues were only regulated at the state level. As with other by-products of the energy and fuel industry, the Environmental Protection Agency (EPA) excluded it from the general regulations applied to all hazardous waste. However, this changed in 2015. The EPA specifically instituted new federal regulations on coal ash storage and disposal. They enacted a number of regulations on coal ash released into open waters and re-use in 2008, but only in 2015 did they allow for both disposal-in-place and disposal-by-removal for solid coal ash and sludges. These EPA regulations are in addition to requirements set by the state and local agencies, so it’s essential to track them all before building new impoundments or closing existing ones.
What is Containment for These Kinds of Waste?
Materials like coal ash residues need two different types of containment. Primary containment is the pit, tank, impoundment, or other vessel used to hold the dry ash or slurry. This is the first stage of lining that is necessary to prevent environmental damage. But secondary containment is also important for coal ash residues and by-products, and this is slightly more complicated. Secondary containment measures are intended to control spills, leaks, and other accidental releases from any primary containers or transfer stations. Pumping stations, unloading pads, and processing units also need secondary containment in addition to all primary containment and storage areas. The same flexible liner products from BTL Liners can be used for both types of containment, as long as the materials are matched to the specific uses.
The Difficulties of Containment
New rules and regulations on coal ash disposal from the EPA are just one small part of why it’s challenging to contain this kind of waste product. Other issues include:
- The specific content of the ash that makes it hazardous to handle and difficult to keep in one place
- The sheer volume of the coal ash residues produced by the average coal-fired power plant
- The weight and spill risks of liquid slurries and similar by-products
- High costs for disposal, whether that’s in a dry landfill or through treatment and eventual discharge into nearby waters.
Transportation Issues
Finally, coal ash is particularly tricky to transport when the operator of the facility chooses to dispose of it off-site rather than through buried or closed pits. It’s lightweight if it’s left dry, but it’s easy to disturb or lose and is hard to compact into a smaller volume. Wet slurries are heavy and are hard to pump through standard equipment due to its abrasiveness and viscosity. It can also be hard to find a local disposal point for them. Yet, many states won’t allow for the on-site or buried disposal of coal ash, especially for power plants located close to residential areas. Using reliable and permanent lining solutions is usually the key to getting permission on all levels to keep disposal on-site and avoid these kinds of transportation issues.
Coal ash and all of its by-products and residues need constant containment to prevent environmental damage that could affect human and animal health. This material can be valuable rather than a waste product, but it needs careful handling to keep it pure enough to reuse or process further. Don’t underestimate the costs of cleaning up a spill or leak of coal ash slurry if you fail to invest in the right lining system. Work with us here at BTL Liners to protect all coal ash impoundments, pits, and ponds to save money on remediation and fines.
