Rim-ditch method overcomes space limitations for scrubber sludge storage
By Melissa A. Hedgecoth, Tennessee Valley Authority
TVA adds 25 years to its scrubber sludge storage capacity. Used in another industry, but unproved for sludge storage, a new system was successfully adapted
Well before the Clean Air Act Amendments of 1990, it was obvious that environmental rules and regulations were becoming more stringent. Power plants that now wish to remain competitive will have to constantly modify processes and procedures to satisfy the demands of environmental protection. In anticipation of more stringent future clean air requirements, the Tennessee Valley Authority (TVA) chose to install wet limestone flue gas desulfurization (FGD) systems on two 500-MW units at its Widows Creek Fossil plant (WCF) near Stevenson, Ala.
The FGD system uses a fluidized limestone slurry to scrub flue gas before it is released to the atmosphere. Calcium released from the crushed limestone extracts sulfur from the flue gas and combines to create calcium sulfate. The scrubber system has a venturi pattern that causes fly ash particulate to settle out and produce a fly ash/gypsum slurry. A scrubber system assures TVA that this plant will be in compliance with future clean air standards.
WCF?s scrubbers produce more than 800,000 tons of sludge each year. In an effort to develop 20 years of disposal capacity on-site, TVA considered several disposal options: (1) Wet sluice to disposal ponds and build new ponds as needed. (2) Wet sluice to pond, dredge and stack as dewatered gypsum in a final disposal area. (3) Dry sludge in a filter press and stack dry. (4) Sluice gypsum directly to final disposal site and wet stack ash by a rim-ditching method (Figure 1). With limited availability of on-site disposal acreage and the high cost of purchasing and developing additional land, the development of multiple disposal areas, as depicted in Options 1 and 2, was not economically or environmentally attractive. The solutions expanding the disposal area vertically instead of horizontally maximize the use of available disposal acreage.
Option 3 would require additional capital for a filter press installation. In addition, the operation and maintenance of the dry stacking method would require approximately six people, three pieces of heavy equipment, and three gypsum/fly ash hauling trucks. Option 4, which uses the wet stacking operation to place material in its final disposal area, would require no additional initial capital. Operation and maintenance of the wet stacking method was anticipated to be economical and require only two or three full-time employees and two pieces of heavy equipment.
It became obvious that the wet stacking method was the most economical choice and TVA selected an organization to design the system based on its experience in another industry. Ardaman & Associates Inc. of Orlando, Fla., had developed systems in the phosphate industry using a method known as rim-ditching. At WCF, the system is geared to handle FGD gypsum without fly ash or phoshogypsum. Little experience was available when TVA chose to pioneer this method to handle its gypsum/fly ash slurry. Extensive testing was done to characterize the engineering properties and evaluate the permeability and stability of the material. The data gathering, analysis and design phase took about one year.
Using the rim ditching theory and knowledge gained from extensive testing the contractors, engineers designed a 150 foot tall stack on the available 140 acre area to provide approximately 13,300 acre/foot of storage capacity. With the current production of more than 800,000 tpy, this will provide TVA with approximately 25 years of total storage life. The rim ditching process began with one month?s accumulation of material. At this point, TVA selected the Trans-Ash Co., Cincinnati, Ohio, to develop the gypsum/fly-ash stack.
The rim-ditching stacking method uses a combination of ditches and berms to make the gypsum/fly ash slurry flow along an elevated ditch around the inside of the stack perimeter dike (Figure 2). Coarse particles settle in the ditch around the rim and finer materials are directed to the center of the pond.
As the coarse material settles in the rim ditch, it is excavated from the ditch and continually used to construct the rim berm. After the rim berm is constructed, the rim ditch is dammed up to allow the ditch to fill with material. The operation then moves toward the center of the pond to develop another rim ditch and berm. This process continues as the material is stacked higher and closer to the center of the pond. The concept is successful only if the rim ditch stays elevated above the ponded water level in the interior of the pond. Elevating the ditch accelerates the dewatering of coarse particles and facilitates a more rapid excavation and construction of each new lift of rim ditches and berms.
One of the first problems that had to be overcome was the effect of fly ash in the mixture. The gypsum/fly ash mixture erodes more easily than conventional gypsum without fly ash. Also, fly ash tends to retain more water than gypsum, which hinders dewatering.
The system constructors found that material that is excavated as a wet slurry, dewatered and then compacted with a dozer is more stable than loosely placed and compacted moist material. This process is referred to as wet casting. After the wet cast material is sufficiently dewatered, it can be compacted in lifts by a dozer.
Successful management requires frequent monitoring of the stack operations as daily construction and maintenance of the ditches and berms continues. Filter fabric has been used selectively here to deter erosion. Decant structures also have been used to minimize the effect of erosion and to maintain the desired elevation differential between the rim ditch and the interior pond elevation.
The WCF rim-ditching operation has successfully functioned since October of 1991. As a result of the rim-ditching operation, the on-site disposal storage capacity was maximized while minimizing the disposal area footprint.
Due to the success of this operation, TVA has chosen to use the rim ditching method for FGD gypsum at two other plants. TVA would recommend the rim-ditching disposal method to other coal-fired plant engineers who might be searching for a way to handle their FGD scrubber byproducts. END
Melissa A. Hedgecoth works for the Tennessee Valley Authority. She holds a bachelor?s degree in civil engineering-environmental from Tennessee Techical University and a master?s in engineering management from the University of Tennessee, Chattanooga.