By Cameron W. Hipwell, P.E.
Power Plant Water Services – 1
The conversion of a large array of source waters into high quality membrane purified water is critical for addressing applications in multiple industries ranging from industrial manufacturing, offshore oil and gas production, food and beverage production, pharmaceuticals, microelectronics, and power generation.
In particular, power generation from fossil fuels is highly water intensive because it consumes a significant portion of all treated water in developed countries.
Source waters ranging from rivers and lakes to municipal potable water, as well as seawater, must all undergo a series of steps to produce treated product water suitable for the end-use. Power generating stations can be located in coastal regions, inland near rivers and lakes, or at locations utilizing municipal potable water, groundwater, or biologically treated secondary effluent wastewater. Impurities in these various sources are total suspended solids (TSS), colloidal species such as silica, total dissolved solids (TDS), and dissolved organic matter. The nature and respective levels of these impurities determine the suitability of the water and the treatment steps needed for use in a power plant.
Membrane Process Systems Are Key
Key to producing purified water on-site is the utilization of membrane filtration process systems. The well-recognized filtration processes of microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO), shown in Figure 1, each provide discrete degrees of suspended or colloidal solids removal and, in the cases of nanofiltration and reverse osmosis, rejection of specific dissolved ionic species.
Ultrafiltration Pretreatment for Power Generation Production of Demineralized Water – 2
Each of these processes utilizes semipermeable membranes but in the case of MF can also include microporous depth filters.
Purified Water for Power Generating Stations
The production of high purity and ultrapure water is critical in addressing the requirements of boiler and turbine operation throughout the power generation industry.
Key water supply services within a power plant, as shown in Figure 1, include raw makeup water for the entire plant, boiler makeup water for the water/steam circuit, steam turbine condenser cooling water, and auxiliary cooling water. Other water services that may be required, depending on the specific plant, include makeup water for flue gas desulfurization systems, water for handling and disposal of ash, and, in the case of simple and combined cycle gas turbine plants, direct water fogging of the gas turbine inlet air (wet compression) for increasing power and emissions (NOx) control.
Purified water is characterized by specific limits depending on its end use. The most widely used general parameter for industrial and power generation, is total dissolved solids (TDS). In food and beverage manufacturing, for example, NF or RO permeate having TDS levels of 5-100 mg/liter is usually acceptable and for cooling tower makeup water TDS levels can be in the 100-500 mg/liter range.
However, for more severe pure water services such as demineralized water for power generation applications or medical device manufacturing, resistivity/conductivity values are most convenient. The ability for water to conduct electricity is determined by the concentration of dissolved ionized species. Deionized water has a low conductivity of 0.055 μsiemens compared to sea water having conductivity of >50,000 μsiemens.
For convenience, the ultrapure water parameter most cited is the reciprocal of conductivity expressed in megohms (MΩ), thus deionized water with conductivity of 0.055 μsiemens has a resistance of 18 MΩ. Ultrapure water with resistivity of 18 MΩ is typically specified as required for severe applications such as semiconductor manufacturing and production of makeup water for supercritical power boilers. For most demineralized water applications in power plants, water with >10 MΩ resistivity (0.1 microsiemens) is sufficient.
Producing High Quality Demineralized Water in Power GEN
By far the most critical water service for any power plant is maintaining the integrity and performance of the steam generator components. To minimize the transport of corrosion products to the steam generator, the makeup quality must be of sufficient purity. High purity water suitable for boiler makeup is typically characterized by contaminant levels of dissolved monovalent and multivalent ions and water resistivity, as indicated in Table 1.
Proper conditioning of the boiler makeup water can reduce corrosion-related operational problems and component failures. Prior to the emergence of membrane processes for the production of high purity water in power plants, pretreatment using conventional technologies was prevalent.
The predominant conventional pretreatment systems involved sedimentation clarifiers with chemical coagulants, multi-media filtration, then followed by ion exchange (IX) demineralization systems. A typical IX system consists of three resin beds – configured in series with a cationic exchanger followed by an anionic exchanger followed by a mixed bed exchanger. In the cationic exchanger, positively charged monovalent cations such as sodium (Na+) and potassium (K+) and divalent cations such as calcium (Ca++) and magnesium (Mg++) are exchanged with hydrogen (H+) that is attached to the cation resin. In the anionic exchanger, negatively charged chlorides (Cl-), sulfates (SO4–) and nitrates (NO3–), and CO2 are exchanged with hydroxide (OH-) that is attached to the anion resin. Water is formed as the resulting H+ and OH- ions released from the IX resin.
The final mixed bed ion exchanger contains an intimate mixture of cation and anion resins which act as a whole series of successive cation and anion exchange stages resulting in very high purity water as the end product. The mixed bed is much more complex compared to single resin vessels containing a single cation or anion exchanger. There must be provision for separating the two resins in the vessel prior to regeneration, and for sets of distributors for injecting and collecting two different regenerants. In addition there must be provision for remixing the resins prior to return to service.
Two-Pass RO Membrane Process for Production of Low TDS Makeup Water – 3
Ion exchange is a reversible process allowing the cation resins to be regenerated by passing strong acids (typ hydrochloric or sulfuric) through the cation resin bed followed by rinsing the resin beads of excess acids. Similarly, anion resins are regenerated using concentrated alkali solutions (typ sodium hydroxide).
Conventional deionization (DI) resin based systems are operated in batches until the resin is exhausted. The non-continuous process is inherently a less sustainable approach that utilizes large quantities of hazardous acid and caustic chemicals.
Modern water treatment processes for power plants involve more sustainable membrane technology in lieu of conventional pretreatment equipment and IX technology. Compact MF and UF membrane systems, as shown in Figure 2, have become an increasingly popular options as pretreatment steps for removing suspended and colloidal solids prior to RO. UF provides filtration to below 0.02 microns for high efficiency treatment of turbidity and suspended solids.
RO has advanced as the dominant technology for producing low TDS water. This is usually achieved using a double-pass RO membrane configuration as shown in Figure 3. TDS levels from the first pass can approach 5-300 mg/liter. Permeate for the first RO pass is fed to a second set of RO membranes for final TDS removal down to 1-20 mg/liter. Also, RO membranes can be used to directly remove TDS from both seawater and brackish water having TDS levels of up to 40,000mg/liter and 15,000 mg/liter, respectively. RO permeate from the seawater and brackish water feeds can typically be reduced to less than 10 mg/liter, in the second pass RO permeate.
RO permeate provides the ideal treated water quality for final polishing to produce >10 MΩ demineralized (demin) makeup water. For this final polishing step, continuous electrodeionization (EDI) with membrane (CO2) degasification is becoming a preferred choice over IX technology. Similar to IX technology, EDI removes ionized species from water using ion exchange resins and the process incorporates ion exchange membranes and a DC electric potential.
Feedwater entering an EDI membrane module (shown in Figure 4) flows to membrane compartments which contain ion exchange resins. The modules contains cation permeable and anion permeable membranes and the DC electric potential provides the driving force for passage of cations through the cation-permeable membrane and anions though the anion-permeable membrane. EDI feed water exits the EDI module as high purity EDI product water due to the removal of ions through the cation and anion membranes.
EDI systems can achieve >99 percent TDS removal and provide ultrapure water with up to 18 MΩ resistivity. EDI provides a sustainable ultrapure water solution by continuously regenerating without harsh acid and caustic chemicals that are required for conventional ion exchange (IX) systems.
EDI Membrane Module – 4
Utilizing EDI over IX resin bed systems avoids trucking hazardous chemicals, storage of chemicals, and disposal of acid & caustic chemicals. Environmentally engineered solutions incorporate high recovery RO systems with EDI for optimized performance and reduced operating costs (OPEX).
Unit Responsibility and True Turnkey Capability
Multi-step membrane process schemes incorporating pretreatment, UF, RO and EDI are thoroughly engineered to ensure reliable high efficiency production of membrane purified water.
Most often, providers of these systems procure individual components including membrane modules, pumps, instrumentation, controls, and interconnecting piping and then integrate them into a process system.
Several membrane filter media manufacturers do provide complete process systems using their membrane products, but outsource most peripheral items. The membrane manufacturer is seldom sufficiently vertically integrated to provide all components.
Case Study: South Texas CC Plant
A 1200-MW combined cycle natural gas turbine power plant shown in Figure 5 was experiencing high operational costs associated with the original conventional water treatment plant utilizing ion exchange resin based demineralization.
The plant recently took delivery of a 200 gpm ultrafiltration (UF) and brackish water reverse osmosis (BWRO) packaged system to replace the existing conventional clarifier, media filter, and cation/anion IX demin system.
The new membrane based system was fully wet-tested simulating onsite performance at Parker’s water purification facility in Los Angeles.
South Texas Combined Cycle Plant – 5
Membrane process systems have readily evolved into the preferred choice over conventional water treatment processes involving clarifiers, large media filters, cation and anion IX resin beds. MF and/or UF to remove suspended solids prior to RO and EDI are capable of removing suspended solids down to 0.02 microns in size which had previously caused fouling of downstream IX resin beds.
MF, UF, RO and EDI membranes are fully integrated into compact systems that are modularized for maximum flexibility. RO and EDI membrane process are also more environmentally favorable over IX as resin regeneration using hazardous chemical are avoided.
Pre-engineered Seawater RO (SWRO) systems, ideally suited for producing fresh water for seaside power plants seaside power plants, or any industrial application requiring ultrapure water produced on demand.
With a compact footprint and fully automatic controls, an SWRO unit can produce up to 1500 metric tons of RO water per day at the lowest energy consumption per ton of water produced. The RO produced water can then be polished using EDI membrane modules to provide ultrapure water with up to 18 MΩ resistivity.
Cameron Hipwell is a registered Professional Engineer with Parker Hannifin Filtration Group