As has widely been reported, the U.S. EPA signed the final coal combustion residuals or CCR rule on December 19, 2014. The rule affects coal-fired power plants of electric utilities and independent power producers that use landfills or surface impoundments for the storage of their CCRs. CCRs include fly ash, bottom ash, and flue gas desulfurization (FGD) byproducts. The CCR rule institutes minimum criteria for the design, operation, closure, and post-closure of these disposal facilities. Although the rule does not require surface impoundments to close, many plants may elect closure by converting any existing wet CCR systems to systems that produce a dry or dewatered material. See related Power Engineering articles: “The New CCR Rule,” March 2015; “The Coal Ash Rule: How the EPA’s Recent Ruling Will Affect the Way Plants Manage CCRs,” February 2015.
Many plants have surface impoundments for temporary or permanent storage of wet CCRs: sluiced bottom ash, sluiced fly ash, and wet FGD slurries or sludge. Temporary storage facilities are used for settling and dewatering. Often, more than one impoundment (i.e., a “pond”) is used so that the sluiced CCRs are directed to one pond while the second is inactive. When the material has settled, the plant dredges the material for final disposal via trucks in an on- or off-site landfill.
For plants that decide to close their on-site ponds, possible plant modifications include:
- Bottom ash: water removal using dewatering bins or remote submerged flight conveyors (RSFC)
- Fly ash: conversion to a dry pneumatic system
- FGD slurry: addition of dewatering equipment
Because ash options have been featured in past articles, I will discuss FGD slurry options.
Although most of the focus on plant modifications for CCRs has been on bottom ash and to a lesser extent fly ash, there are over 50 FGD byproduct ponds or other facilities (based on EPA data) in the U.S. for coal-fired power plants. Most of the FGD systems that transport slurry to these ponds are calcium based (lime, magnesium lime, or limestone), but a few use sodium as the reagent. The sodium FGD systems produce a soluble liquor that often is treated in evaporation ponds. These plants are located in semi-arid regions with low annual rainfall such as Wyoming. This column will focus on the more common calcium-based FGD systems using lime or limestone reagent that dewater or store the byproduct in ponds. The slurry from the SO2 absorber is typically sent to one or more outdoor dewatering or storage ponds.
Elimination of ponds handling calcium-based FGD slurry requires dewatering equipment such as thickeners, hydrocyclones, and vacuum filters. Limestone forced oxidation FGD systems produce a stable calcium sulfate (gypsum) slurry that is easily dewatered. Other FGD systems produce a mixture of gypsum and calcium sulfite which is difficult to dewater. Studies of alternatives can be done to develop a reliable low capital cost system. If fly ash from the station is available, a mixture of fly ash and FGD slurry after primary dewatering (hydrocyclones) may be feasible. The fly ash could come from other units at the same station, or nearby stations if trucking the ash is reasonable. For lime FGD systems, thickeners and vacuum drum or horizontal belt filters may be required as the FGD slurry is difficult to dewater. If mixing with fly ash is not suitable, vacuum filters on a limestone forced oxidation FGD process can accomplish full dewatering to a byproduct that can be landfilled. The recent Ghent CCR project included a full dewatering system for the gypsum slurry.
Dewatering the FGD slurry requires the evaluation of impacts to the existing absorber system. Although most plants recycle the decanted water from their settling ponds, the existing water balance is not as closed as it will be when a dewatering system is added. For example, rain adds water to the pond. Outfalls from the pond act as a blowdown stream to decrease dissolved solids. On the other hand, dewatering could result in higher chlorides in the absorber recycle slurry which may require a new or increased FGD wastewater blowdown stream. The corrosion resistance of the absorber and piping materials to the expected higher chlorides needs to be considered. Addition of dewatering equipment may also increase the concentration of fines in the recycle slurry because of the small solid particles in the hydrocyclone overflow that are returned to the absorber reaction tank. The fines reduce the average crystal size, and could adversely affect absorber and vacuum filter performance.
If eliminating an FGD CCR impoundment is required, careful consideration of the alternative techniques to dewatering FGD slurry is needed. The impact of these techniques on the existing system is also required, but can be managed.