Dense slurry: a solution in the pipeline for EPA’s CCR and ELG rules23 June 2014
Proposed EPA legislative initiatives generally referred to as the Coal Combustion Residuals (CCR) and Effluent Limitations Guidelines (ELG) rules are not yet finalised but will have a significant influence on the way ash and wastewater from US coal fired plants are handled. The Clean Water Act provides the federal guidelines for protection of groundwater but each state may regulate ground water differently. In addition the federal Mercury and Air Toxics Standards (MATS) rules that were finalised recently will also impact ash and wastewater chemistry, further bearing on the way ash and wastewater are dealt with.
In this context, GEA's Circumix Dense Slurry System for management and disposal of power plant coal ash and wastewater, offered in North America through NAES Corp, could prove an attractive option. It is employed at several power plants around the world and has been proven at commercial scale for some time, since 2003 in the USA. It is applicable to both new build and retrofit projects.
Circumix DSS employs a high-intensity mixing process that mixes minimal quantities of water (including wastewater) with coal combustion residuals to produce dense, pumpable slurry. Power plant byproducts such as gypsum from the FGD system, if it is not marketable, can be added to the slurry. The solid to liquid ratio is determined by the physical and chemical properties of the slurry ingredients and is regulated to maintain optimum viscosity and low-energy transport. The slurry is transported to the disposal facility via relatively small diameter pipelines, in which erosion is minimised due to particle suspension in the viscous slurry.
Transport of ash by open conveyor belts or trucks releases ash particles into the air regardless of whether it has been wetted or not. This cannot happen with transport by pipeline.
Transport of the ash to the dump by pump is also more energy-efficient than by conveyor belts or truck.
Operation of the Circumix DSS system is fully automatic and can be supervised from a central control station. Special facilities are not required at the dump, eg for distributing and smoothing out the Circumix DSS slurry material.
The Circumix DSS process maximises the availability of reactive ions in ash products and results in optimum use of the water contained in the slurry. Once discharged, the slurry hardens within 24 to 72 hours. When cured, the product exhibits low hydraulic conductivity, high compressional strength, no wastewater discharge, little or no fugitive dust emissions and enhanced sequestration of contained metals, says GEA.
The surface is more resistant to pressure than with other processes, and is so strong and compact that substantial wind-caused dispersion is highly unlikely. Even in the event of rupture of the peripheral embankment of the dump, the dried material is extremely firm and compact, which eliminates the risk of a sludge spill and improves ground water protection. Traditional dry ash management techniques do not result in a sufficiently solid deposit, and their hydraulic conductivity is much higher than that produced by Circumix DSS. This, combined with a more uneven surface, results in high rates of water infiltration and increased risk of fugitive dust.
Also, the high density reduces the volumes created by Circumix DSS for disposal.
There is no need to treat or to discharge transport water. The Circumix DSS concept of combining wastewaters for purposes of ash solidification and stabilisation results in an overall reduction of water consumption and reduces the generation of wastewater destined for treatment and/or discharge.
A primary intent of the proposed EPA ELG rule is to reduce or to eliminate the discharge of wastewater to the surface waters of the United States. Circumix DSS meets this requirement because it uses 90% less water, compared with lean slurry wet ash handling systems. The transport water is stabilised along with the ash in a solid end-product exhibiting favourable environmental properties. Leachate collected from dense slurry ash impoundments and/or landfills can be returned to the plant and used to produce additional dense slurry if appropriate. Therefore the risk of contaminating groundwater is significantly reduced.
Compared with conventional dry ash handling systems, GEA says Circumix DSS offers advantages including high sequestration of contained metals, elimination of the risk of dust generation, and the efficient use of disposal space.
From the perspective of environmental protection (especially surface water and air protection), GEA says Circumix DSS is superior to both wet 'lean ash' and 'dry ash' management systems, the latter being the recommended option in by EPA's proposed rules. In addition, says GEA, DSS is safer and less expensive than dry ash systems.
Even though the final CCR and ELG rules are yet to be finalised, wet lean slurry impoundments will be regulated out of existence, says GEA.
Several US plants are moving forward with plans to test Circumix DSS technology at their plants. NAES anticipates that GEA's Circumix DSS not only fulfills the requirements (eg, BAT and BADCT standards, among others), but also will be a recommended process in EPA's final CCR and ELG rules.
Dale Timmons, program manager, business development, at NAES Corporation, suggests that up to 80 plants might be candidates for Circumix DSS, but notes that the ash chemistry, wastewater chemistry, plant layout and other related factors are different for every plant, so it is not clear how many of these plants represent a match for the technology. Nevertheless, "preliminary assessments suggest that there are many", he says.