The importance of proper clean up and disposal of coal ash cannot be understated. It is imperative that this be done correctly to protect the environment. The power generation industry is spending thousands of hours studying data, building enhanced groundwater and surface water protection programs and identifying closure options that protect people and the environment in a cost-effective manner.
The team at Avid Solutions has unique knowledge and ability in this area. It has successfully designed and implemented pH controls for the ash pond outfall at a local power plant. The plant discharge initially cascaded through three ash settling ponds, with the final outfall to a nearby river. State pH limit for outfall is between 6 and 9. The first pond was set up to capture heavier metals and particulates. The center pond then captured most of the fine particulates, and the final small pond was used for water quality monitoring and reporting.
Due to the nature of the remote, inaccessible location, the control system used solar powered pH probes with wireless communication in the center pond and outfall flow rate as feedforward inputs to a chemical dosing predictive model. This model was developed by testing acid and caustic titration of five-gallon samples over a six-month span.
State permitting requirements made installation of a flow weir in the drainage manhole infeasible. So, the Avid Solutions team developed an accurate flow signal using an underwater static pressure bulb accurate to 0.01” wc mounted at the 3’ diameter overflow stand pipe discharging to the river. Model demand was trimmed by a simple pH control loop of the final pond and chemical dosing took place in the piping upstream of the final pond. The predictive model approach allowed instant response to changes in water chemistry and flow. The resulting system performance achieved an average +/- 0.5 pH from target and it was praised by the power company as being incredibly reliable.
This water treatment system implemented by Avid Solutions ran well for several years but required modification after the power company made the decision to retire the coal plant within the next five years.
The ash ponds at the coal plant were 150 ft. in elevation above a nearby river and close to an interstate and an airport. Because of this proximity, the NC 2014 Coal Ash Law classified this site as high risk, requiring the ash ponds to be drained and excavated within the next five years.
The power company decided to eliminate the center pond first to reclaim that land for building a new gas fired plant. The existing water treatment process for the coal ash would now only be able to use ponds one and three. The Avid Solutions team constructed a large water pumping station and filter skid to bypass the center pond. This required relocation of the triple pH probes used for the chemical dosing predictive model. One unexpected result was the impact that the algae in the center pond had made on the water chemistry. With this gone, the predictive dosing model for pH control required a completely new set of testing data and a rework. The team modified the Rockwell PLC interfaced with the existing DCS process control system to include high-level operator control of the pumping station. The existing Phoenix wireless radio network was expanded to include an extra node at the pumping station for transfer of operating data to the plant DCS via the pH controlled PLC. This system is currently working very well and will operate until the plant retires.
With help from independent experts such as Avid Solutions, the power company continues to make significant progress closing nonhazardous coal ash basins in ways that put safety first, protect the environment, minimize impact to communities and manage costs.