By Katariina Majamaa, Steve Rosenberg and Bill Carlin
|Water treatment technologies for a coal power plant.|
Increasing demand, evolving energy sources and environmental drivers are shifting the global power landscape. With growing populations in emerging markets, like Asia and Pacific, comes growing demand for power and new installations. The shifting global energy landscape means the global water landscape is also shifting, as water is used to propel the turbines that run the generators that produce power for communities around the world. The power generation industry relies on quality water to complete the processes necessary to generate electricity. Thermal cycle plants also require large amounts of water for steam, cooling and condensing. In the U.S. alone, the National Renewable Energy Lab notes that electricity production from fossil fuels and nuclear energy requires 190,000 million gallons of water per day, accounting for 39 percent of all freshwater withdrawals. These demands for water are shaping the shift in water treatment in power generation from freshwater sourcing to wastewater treatment and reuse. For example, strong environmental drivers in China and the U.S. are increasing flue gas desulphurization (FGD) and minimum liquid discharge (MLD) applications. Typically the upgrades to advanced water treatment are done in phases, starting with the lower hanging fruits such as increasing the cooling tower cycles or treating the cooling tower blowdown for a reuse.
Powering the Water-Energy Nexus
In order to meet the demand for power generation across markets, and also meet increasingly stringent environmental regulations, operators can use treatments to protect assets and improve plant operating efficiency. Reducing energy usage, chemical costs and water footprints, while increasing reclamation of non-conventional water sources like wastewater, are key industry focal points. Nuclear power plants worldwide face the added challenge of reducing their radioactive waste sustainably and cost effectively. The water-energy nexus, the concept that water and energy are interconnected – energy is needed to create power and power is needed to create water, has become a major driver of innovation in the industry, as high water quality is of paramount importance to protect investments and ensure equipment runs efficiently and reliably. Effective water treatment technologies on the market today including ultrafiltration (UF), reverse osmosis (RO) and nanofiltration (NF) and ion exchange (IX), help ensure reliable, efficient, more sustainable operations in a range of power plants.
UF: Reducing the impact of poor water on boiler and cooling system management
Ultrafiltration reduces colloids, particles and bacteria in a continuous system to deliver higher reliability and lower maintenance with a smaller footprint. Used as a pretreatment for downstream units like RO, UF helps reduce the impact of poor or variable make-up water on boiler and cooling system management.
UF in Action: Northern China is home to one of the largest fossil power plant in Asia. The plant’s source water is a combination of cooling tower blowdown wastewater with high levels of salts and alkalinity, and pretreated water from the Huanghe River, which has high levels of mud, sand and suspended solids. This highly-variable source water requires a treatment technology that functions effectively across various temperatures and pressures. These demanding operational requirements can lead to increased fouling, a contaminant build-up on the filter, corrosion and equipment malfunction. When operators expanded the plant’s system to reclaim additional cooling tower blowdown through two additional generators, they also upgraded the treatment system to include conventional filtration and outside-in UF technology.
RO and NF: Removing contaminants reliably
RO and NF provide a balance between efficient contaminant removal, demineralization and reliability. RO and NF elements can cost-effectively remove a broad range of molecules and ions that can cause scaling and fouling. By using RO and NF technologies, operators can increase the number of cooling tower cycles and facilitate the use of alternative water sources to address local water scarcity challenges.
Cost Comparison – 1
|Cost comparison of MLD vs ZLD showing that MLD processes enable users to achieve up to 95 percent liquid discharge recovery at a fraction of ZLD’s costs.|
UF and RO: Reusing Difficult Waters
The Gaojing Power Plant in Northern China is an excellent example of how large, mature facilities can retrofit their water footprint to comply with stricter environmental standards. This 40 year old power plant was first upgraded to reuse cooling tower blowdown as feed water to boilers. Dual membrane technology (UF and RO) together with pretreatment and chemical dosing, helped Gaojing Power Plant to achieve reuse of cooling tower blowdown of more than 70 percent. Followed by a change in cooling tower water source from surface water to secondary effluent from the nearby municipal wastewater treatment plant, the Gaojing Power Plant was able to realize a double reuse of water. This project provided a true test of UF and RO enabling the reuse of difficult waters. Cooling tower blowdown is highly turbid and has high hardness. An extreme challenge for many technologies but reliably handled here. As a result of a successful first phase retrofit/expansion, additional phase expansions with UF and RO were developed.
RO in Action: A captive power station for a major brewer in North America turned to Dow Water & Process Solutions to help combat challenging local source water. By upgrading the system with a next-generation configuration using DOW FILMTEC™ ECO RO elements, the brewery has increased uninterrupted beer production and reduced the energy required to treat feed make-up water. The brewery is also meeting its electrical energy and process steam requirements. The new water treatment system has reduced the power station’s energy consumption by 10-20 percent and improved peak flow performance.
IX: Efficiently removing ionic impurities
By removing ionic impurities, ion exchange resins produce softened, partially or fully demineralized water reaching to ultrapure water quality. IX resins can be highly specialized to remove a narrow or wide range of contaminants and tailored for specific fossil and nuclear power applications to soften, demineralize and polish make-up water, or enable FGD. IX resins are also the technology of choice for condensate polishing applications designed to protect multi-million dollar assets such as boilers, steam generators, turbines and nuclear reactors. IX resins also can help improve the quality and clarity of reactor water, fuel pool water, and re-use or discharged water.
MLD in Action – 2
|A multistage MLD process to treat challenging FGD wastewater at a power plant in China.|
IX in Action: Conemaugh Station in Pennsylvania required treatment of FGD wastewater to meet discharge limits for boron. Dow’s boron selective IX resin was installed in 2012 and has successfully allowed Conemaugh Station to meet discharge limits. As the first installation of boron selective resin in FGD wastewater treatment, it has seen highly variable feed water and many other challenges. Through all of these difficulties, the system has continued to meet discharge limits. It proves to be an effective, reliable technology in removing boron from FGD waste water.
A Cohesive System
UF, RO, NF and IX work cohesively throughout a power plant to optimize operations and minimize unscheduled outages. For example, make-up water may need pretreatment (UF), demineralization (RO) and polishing (IX) to help reduce corrosion, scale, fouling and other asset-limiting factors. IX in deep bed or powdered resin beds serve as the backbone of condensate polishing for boilers, steam generators and turbines to help remove dissolved purities like chloride, sulfate, silica, and sodium along with particulate iron and copper corrosion products. Treating strator cooling water with an IX mixed bed polisher will help remove soluble contaminants to prevent generator failure. To help minimize corrosion and reduce ionic hideout, blowdown water can be treated with ion exchange for reuse or discharge. The shifts in water treatment needs for the power industry also drive the innovations in water treatment technologies at individual component level as well as in integrated systems.
Environmental regulations have driven many fossil-fueled power plants to install FGD systems, requiring the treatment of FGD blowdown water prior to discharge. Highly selective ion exchange resins are helping operators meet discharge regulations by removing trace contaminants, like arsenic and mercury.
As freshwater sources grow increase scarce across the globe, many industries, including power, are moving away from a “take, make, dispose” approach to a circular strategy where raw materials, including water, are reduced, reused and reclaimed. A circular economy model employs advanced water technologies that help enable facilities to reduce their intake of freshwater by sourcing reclaimed water for operations and increasing the number of cycles for which the water is used. The water can then be treated and upgraded before discharge for alternate use.
Operators are feeling pressure from environmental regulations, sustainability trends and the rising price of water and discharge mitigation costs to reduce their impact on the environment. Sustainability has become a boardroom issue, with many recognizing the need to improve their water footprint. Minimum liquid discharge (MLD) is enabling up to 95 percent liquid discharge recovery at a fraction of the cost of solutions aiming for zero liquid discharge (ZLD). While the term MLD might be new, the processes on which it is based rely on advanced UF, RO, NF and IX filtration technologies. MLD can help plants make solid, substantial gains in minimizing liquid discharge while also minimizing their capital and operating costs, since achieving the final 3 to 5 percent liquid elimination for ZLD can be very costly. Since the greatest challenges to ZLD are economic because ZLD systems require higher capital and operating expenditures and have greater technical challenges, it can nearly double a user’s costs.
MLD shows we can achieve significant gains in sustainable water management at a lower cost than ZLD. MLD solutions can be tailored for individual needs. For example, MLD can be used in the wet wastewater stream of FGD or for cooling tower blowdown.
MLD in Action: More strict discharge requirements and water scarcity in China over the past few years have fueled the consideration of zero-liquid discharge to manage flue gas desulfurization wastewater. Because thermal ZLD processes are both capital and energy intensive, membrane based minimal liquid discharge technology was applied to reduce the volume of water undergoing thermal processing and thus reduce costs. In close partnership with an original equipment manufacturer in China, DOW completed 6 months of membrane piloting and demonstrated a membrane application capable of reliably reaching high water recovery despite the challenging FGD wastewater. The cascading treatment process from selective ion separation membrane followed by brine concentration using high pressure membranes were the key membrane treatment strategy successfully employed. The proven performance during piloting provided the OEM confidence to design and build a full scale system which is currently in commissioning stage at a power plant operated by one of China’s largest national power groups.
The Future of Power
Power operators are being squeezed on both ends in terms of water sourcing and water discharge, which has elevated water to a business factor, and, on a broader scale, an economic development factor. Despite the huge market, the belief persists that investing in advanced water treatment technologies is too costly. But for the power industry, it is costly to not invest in advanced water treatment technologies. Power providers are seeing the impact the cost of water and the cost of interrupted supply and operations can have on continuous power generation. In most stringent cases whether or not to invest on advanced water treatment becomes a question of having the license to continue operation. From ensuring optimal operating efficiency and protecting assets, like generators and cooling towers, and lowering the cost of water and energy through reuse, advanced water treatment technology is ensuring we can keep the lights on and the economy humming.
Katariina Majamaa is a global strategic marketing manager at Dow Water & Process Solutions. Steve Rosenberg is a research fellow for Dow Water & Process Solutions. Bill Carlin is a senior technical service engineer for Dow Water & Process Solutions.