FGD Wastewater Evaporation Pilot Project at a Large Power Plant
By Benjamin Laurent
In the fall of 2015, the U.S. Environmental Protection Agency (EPA) promulgated revised effluent guidelines for steam electric power generating units (EGUs). The 2015 Effluent Limit Guidelines (ELGs) final ruling (1) is now in place. While there is now some uncertainties as to the effective start date for the new guidelines, the tightening discharge limits will certainly impact a number of electric power plants across the United States. Particularly challenging wastewaters in this context include flue gas desulfurization (FGD) purge water and coal combustion residual (CCR) pond waters.
As a result, EGUs face stringent discharge limits for selenium, mercury, arsenic, and nitrite/nitrate in FGD wastewater in the coming years. Table A summarizes FGD wastewater effluent limits under the 2015 regulations.
One of the main methods used to treat these challenging waste streams and addressing the new regulation is volume reduction through evaporation often as part of a zero liquid discharge (ZLD) treatment train.
Evaporation Technology Solutions
Traditional evaporation solutions for treating these wastewaters have often proved to be cost-prohibitive, operationally challenging, and/or resource-intensive for the following reasons:
L High capital cost because of the need for large amounts of exotic materials of construction.
L Scaling in heat exchangers and other process equipment requires excessive downtime for cleanings.
L Pretreatment systems are required to alter water chemistry to reduce maintenance burdens. These may require additional chemicals, processes, and waste disposal.
L Operation is very sensitive to changes in chemistry and requires significant monitoring of chemistry and laboratory resources to limit process downtime because of plant upsets.
L The need to connect an additional piece of large capital equipment (a crystallizer) to the evaporation system in order to achieve ZLD.
The Electric Power Research Institute (EPRI) conducted a study using a different evaporation technology installed at the Water Research Center at Southern Co.’s Plant Bowen, located in Cartersville, GA. The purpose of this project (2) was to evaluate the efficacy of an adiabatic evaporator for FGD wastewater treatment/concentration at a 952-megawatt (MW) coal-fired power plant. The system used flue gas from the power station as the source of thermal energy for evaporation.
Produced concentrated FGD slurry and other process liquids were analyzed and characteristics of dissolved and suspended constituents were determined for subsequent ZLD-type treatment and possible environmentally acceptable disposal. These pre-full-scale demonstration tests assessed process efficiency, extent of fouling, and performance degradation over an extended period of time.
The study revealed that the adbiatic concentrator technology, which uses a direct contact evaporation process, can be a particularly appealing option for sites implementing a ZLD treatment train and without many of the disadvantages experienced by more traditional technologies. Furthermore, there is potential for producing a concentrated slurry that can be stabilized (or solidified) for disposal without the need to add a crystallizer stage to the process. The study validated that the adiatic concentrator offered an alternative to reducing plant wastewater volumes and facilitating efficient capture and disposal of water contaminants in an environmentally responsible manner.
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