Jul 23rd 2018
Flue gas desulfurization (FGD) systems in coal-fired power plants are used to remove sulfur dioxide (SO2) air emissions. FGD systems create wastewater that is often saturated with calcium sulfate while containing both metals and chlorides. FGD water is recycled internally and then purged out of the system when chloride concentrations exceed a set level. High chloride concentrations inhibit SO2 absorption from the flue gas and create corrosion concerns with wetted equipment. Chloride blowdown levels range with operations from 10,000 mg/L to as high as 30,000 mg/L.
Most FGD installations already include some wastewater treatment, typically referred to as ‘triple box’, wherein heavy metals and fluorides are removed via moderate pH (~pH 9) precipitation, combined with polymer and coagulation, and followed by a filter press. Historically, the filtrate is released; however, chloride, selenium, and other constituents that make up total dissolved solids (TDS) are not removed. The industry is facing new regulations that force TDS removal.
Saltworks has reviewed and tested multiple options in order to inform an appropriate path for future development. The top three contending options, summarized below, are based on industrially practiced technology. However, leading innovation is applied to options 1 and 3. The results show that monovalent electrodialysis reversal (mEDR) holds the greatest promise to provide a step change in reducing costs of FGD wastewater treatment.
All options assume an upstream ‘triple box’ treatment step is already in place. All options produce a final brine reject that is either combined with fly ash for solidification and landfill disposal or sent to a ZLD system. The recovery and brine reject volume for each option is included in the analysis; however, the final disposal costs of said brine is excluded and assumed to be roughly equivalent, or at least not impact the final conclusions drawn for next steps.
Simplified process flow diagrams for FGD wastewater treatment options 1 and 2 are shown in Figures 1 and 2 below. They differ in their final concentration step: Option 1 includes ultra-high pressure reverse osmosis (UHP RO), which can produce a brine reject at 130,000 mg/L TDS; Option 2 includes an evaporator, which produces a more concentrated brine reject at 180,000 mg/L TDS, but at a higher cost than UHP RO.
Both options produce treated water that is suitable for environmental discharge and leverage an upstream seawater reverse osmosis (SWRO) step, which is low cost and widely available. Based on the FGD waters tested by Saltworks, the chemical softening soda ash must be employed with RO and evaporators. Its cost accounts for almost 50% of the treatment total cost of ownership (capital plus operating cost; see Table 2 below).
Monovalent electrodialysis (mEDR) selectively pulls out chlorides to increase internal recycle rate and decrease wastewater volume. See Figure 3.
mEDR builds upon traditional electrodialysis technology, the world’s second-most practiced membrane desalination technology, and is enabled by recent advances in monovalent ion exchange membranes. These are manufactured from a ductile and highly conductive ion exchange polymer. Under an electrical field, these membranes selectively allow monovalent anions, such as chloride, to pass through the membrane and concentrate up in the brine reject stream while blocking multivalent ions, such as sulfate (see Figure 4). Similar to how kidneys remove waste from the human body, mEDR removes chloride in the wastewater and recycles the low chloride water back to the FGD system for reuse.
Since mEDR only moves chlorides and calcium to the brine reject, it produces a non-scaling brine comprising primarily sodium and calcium chloride. Since sulfates are prevented from entering the brine, scaling products do not form and high membrane system brine concentrations can be achieved without the need for expensive soda ash softening.
The mEDR process has been tested on a micro-pilot (see Figure 5) and Saltworks’ micro-stack with real FGD wastewater from a power plant to prove feasibility and provide initial performance data. Full-scale stacks are available in two sizes, also shown in Figure 5, and results scale-up linearly based on a series of past projects.
Figure 5. Micro-pilot and full-scale EDR stacks.
Tests show that mEDR can achieve over 90% recovery and remove more than 90% of the chlorides. The brine reject produced consisted mainly of calcium, magnesium, and sodium chloride, and greater than 150,000 mg/L TDS. The analytical results of the inlet FGD wastewater, processed water and brine reject are shown in Table 1 below. Figure 6 shows the relative removal of various ions, including the extremely high rate of chloride removal vs sulfate.
Table 1. Water chemistry data for FGD wastewater, processed water and reject brine.
The test plant operated reliably without the need for chemical softening. The membranes demonstrated excellent monovalent anion selectivity—chlorides were reduced by more than 90%, while 94% of sulfates remained in the FGD water. On a molar basis, this means for every 200 chloride ions that were removed from the FGD wastewater, only one sulfate ion passed through the monovalent anion exchange membrane. This high monovalent selectivity is critical for realizing chloride removal from FGD wastewater and concentrating calcium/magnesium/sodium chloride in the brine reject without scaling. No irreversible organic or inorganic fouling occurred, and preventive oxidant cleaning followed by a sodium chloride wash every 30 days will maintain performance.
Table 2 below compares capital and operating costs of all three options, based on real FGD water, test data, and vendor prices, assuming 300 m3/day capacity (12.5 m3/hr). Costing is based on test data and water chemistry, as shown in Table 1. The total cost of ownership is calculated, assuming capital costs depreciate over a 20-year lifespan at 8% while adding in operating costs such as energy, chemicals, and membrane replacements. Installation, operating labour, and taxes are excluded. The mEDR installation desalting to 1,500 mg/L chlorides requires six Saltworks E200 stacks, while desalting to 500 mg/L requires twelve stacks. Flow rates higher than 300 m3/day are readily achievable by adding more stacks.
*Costs assume pre-filtration and balance of plant built in China to Saltworks’ specifications; Saltworks to supply membranes, stacks, process engineering (P&ID), and control PLC with program embedded
*Costs do not include installation, labour, VAT; assume equal impact to all options
*Costs assume economies of scale in orders and production
Table 2. Comparison of FGD wastewater treatment costs.
The results show potential, where:
Full-scale plants can be implemented as the foundation technology (electrodialysis); this is not new, and production systems are in placed as shown in Figure 7 below.
Standard mEDR skids are produced consisting of six FlexEDR E200 stacks, each with a hydraulic treatment capacity of 200 m3/day per stack. Each stack will remove roughly 4,000 mg/L TDS per pass and multiple passes are required for higher removal. The monovalent ion exchange membranes are essential to the solution and Saltworks Ionflux membranes are recommended due to their high monovalent selectivity, robustness, and oxidant tolerance, balanced with their lower cost.
Saltworks owns a series of mobile pilot plants that are available to demonstrate any of the three options on site: RO, EVAP, or mEDR. The pilots are containerized (Figure 8) and re-create full-scale processes that include automation and controls.
mEDR technology, based on industrially available electrodialysis, shows much promise and compelling economics for FGD wastewater treatment at up to 50% cost savings. Field pilots in specific applications are recommended to determine the following:
After a field pilot, full-scale plants can be rapidly delivered as production is in place and electrodialysis has a long heritage of implementation. This article is intended as general guidance. Specific project needs, water chemistry, and site requirements will change economics. Contact us for further details.
Saltworks Technologies is a leader in the development and delivery of solutions for industrial wastewater treatment and lithium refining. By working with customers to understand their unique challenges and focusing on continuous innovation, Saltworks’ solutions provide best-in-class performance and reliability. From its headquarters in Richmond, BC, Canada, Saltworks’ team designs, builds, and operates full-scale plants, and offers comprehensive onsite and offsite testing services with its fleet of mobile pilots.
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