At stake is finding an answer to one of the most urgent problems caused by coal-fired power production – preventing migration of contaminants (e.g., heavy metals) from the combustion waste products. These environmentally problematic materials – called coal combustion residuals (CCRs) – are made up of ash left after coal is burned and can include concentrated quantities of lead, arsenic, selenium, and mercury amongst others. CCRs are typically made up of fly ash (smaller, lighter particles) and bottom ash (heavier, coarser particles).
CCRs are generally stored under water in retention ponds, which historically was the easiest and most economical option especially when the pond was located on the plant’s premises and near the waste product production area. These storage ponds have come under increasing public and regulatory scrutiny, largely because of older facilities that are unlined. Unlined facilities can have contaminant seepage leaving the ponds and migrating into surface water or groundwater sources. In addition, a growing recognition of the risk around dam failures is pushing slurried facilities towards investigating drier disposal options.
Paste technology helps safely lock CCR constituents away
Our solution, which has been applied at CCR test sites with good results, is applying a technology that has been effectively used for decades in a different field.
Hard-rock mining produces tailings, a similarly fine-grained material, which, like coal ash, contains constituents of concern that must be isolated from surface water and groundwater receptors. One increasingly common solution that WSP applies is dewatering tailings and then combining with binder (e.g., cement) if required, as well as other reagents. The resulting “paste,” much like toothpaste in consistency, can be safely disposed of on the surface or underground. In some cases, the fly ash component of CCR materials already exhibits cementitious properties and this can be advantageously used in lieu of additional binders.
One of the advantages of this paste is that, once transported to a disposal site by pipeline, it cures with little to no water bleed into a firm mass that locks metals and other problematic materials in place. After it cures, the paste mass can be rehabilitated using typical dry disposal rehabilitation techniques. Some US states have enacted laws that require companies to remove all CCRs from ponds and deposit them in purpose-built landfills. Paste made from CCRs can also be deposited in these landfills, meeting standards such as the Paint Filter Liquids Test and other regulatory requirements.
Underground paste disposal prevents surface subsidence and underground collapse
In hard-rock mining, paste is frequently pumped underground to fill a mine’s worked-out voids and tunnels. This prevents collapse of those voids, allowing mining to continue in adjacent rock. Another benefit is that the paste mass stops the flow of groundwater through the mine (known as “mine water”), which might pick up metals and salts present in the rock and eventually contaminate groundwater and surface water.
These two benefits – support of abandoned workings and preventing the flow of mine water – are also possible through paste made with coal ash. To reduce transportation costs, many power plants are located close to the mines where their coal originated. This means it could be practical to take paste made from the power plant’s waste products and pump it underground, into the abandoned coal mine workings. In addition to stopping the flow of mine water, paste can mitigate the risk of collapse in the workings and any potential propagation to surface from the shallow voids. This will ultimately reduce or eliminate the risk of sinkhole development, which is of concern to many members of the public in coal country.
A more sustainable treatment for liquid waste from coal-fired power generation
In addition to metal-containing solids, coal combustion produces liquids such as flue gas desulfurization (FGD) wastewater, which is water that was used to remove sulfur from the plant’s exhaust stack, and which contains many constituents of concern (COCs). Moreover, FGD wastewater is particularly difficult to treat to a standard that meets regulatory requirements.
One of the aspects of paste production, which can be turned to advantage, is that it requires water to mix with the constituents to create the paste. Using FGD wastewater can help produce a paste product that exhibits many of the properties of “Roman concrete”, i.e., it keeps getting stronger over time through secondary mineral formation, which in turn locks away the constituents of concern in the wastewater. This approach also lessens the environmental impact associated with disposal of this wastewater and minimizes the draw on clean source water.
Overcoming legal and regulatory hurdles to CCR paste
Paste technology is mature and reliable, proven through decades of use worldwide by the mining sector and several examples in the coal-fired power industry as well. There is a wide array of vendors able to provide the necessary equipment and consumable inputs. Our work with multiple sites has demonstrated that virtually any coal ash and FGD wastewater can produce an effective paste product. This means that from a technical viewpoint, there are no significant hurdles preventing the use of paste technology to address CCRs.
Conversations with members of the power industry, as well as presentations at industry conferences, reveal that the main impediment to implementation of the CCR paste solution is regulatory uncertainty, combined with concerns about legal ramifications. As regulators become more familiar with paste technology and recognize how it can address the impact of CCRs, power producers will have a new option to efficiently deal with their waste products and protect surface water and groundwater.