April 15, 2008

Focus on O&M (April 2008)

WATER TREATMENT
Tag-teamed seawater cleanup

The scarcity of water resources is causing big problems in China, both directly and indirectly. The lack of potable water directly impairs the health and well-being of millions of people and restricts agricultural and industrial activity. The shortage of water also causes a secondary problem by constraining the country’s ability to expand electric power production to meet the needs of its rapidly growing economy.

 

To produce electricity, power plants require large volumes of high-quality water for boiler makeup and for cooling. However, in most locations, no surplus water is available, and the government will not allow new plants to place additional demands on already scarce water supplies. To gain government approval, a new plant must either use more-available kinds of water—such as seawater and wastewater—or it must pay very high fees for surface or ground water.

At the new power plant in the Liaoning Province coastal city of Zhuanghe, seawater was a natural choice. The plant supports the region’s thriving economy, which is driven in large part by the governing sub-provincial city of Dalian, a prosperous industrial center with the third-largest port in China and the country’s northernmost ice-free seaport.

The construction of the Zhuanghe plant (Figure 1) was undertaken in two phases. The first phase, completed in the fall of 2006, provided 1,200 MW; the second phase completed the 3,200-MW plant at the end of last year. During the first phase, the plant drew surface water from a reservoir located 12 miles away. With completion of the second phase, the plant switched over to seawater for all cooling water and boiler makeup needs.

 

 

UF, then RO. “The Zhuanghe plant has been designed as a model facility to showcase the best available technology,” according to Mr. Zhang, plant manager. “The first large saltwater desalination plant in China was commissioned in 1999, and it has since been well-established that reverse osmosis is an incredibly economical process, with lower operating costs and a smaller footprint than thermal distillation.”

The reverse osmosis (RO) system at the Zhuanghe plant also has the advantage of using a two-stage process that can be tailored to meet different requirements for particular applications. Only the first stage, seawater RO, is required for the cooling water used by accessory equipment. Boiler makeup requires higher-quality water that must also pass through the second stage, a brackish water RO system.

To optimize the performance of RO systems and protect them from fouling, an effective pretreatment system is required. The Zhuanghe plant chose ultrafiltration (UF) as the pretreatment solution because UF occupies a small footprint and provides higher-quality permeate than conventional pretreatment systems. Fully automatic control and relatively low investment costs were also important factors in the plant’s selection of UF technology. UF pretreatment for RO systems is an increasingly common combination in desalination plants and other large-scale RO systems in China and around the world.

Beijing Lucency Enviro-Tech Co., Ltd., one the largest providers of industrial and municipal water filtration systems in China, was responsible for designing and installing the UF pretreatment system.

The Zhuanghe plant chose Targa-10 UF cartridges from Koch Membrane Systems Inc. (KMS) after plant officials visited two other power plants that employ Targa cartridges for very similar RO pretreatment applications. These cartridges use a proprietary semi-permeable polysulfone hollow-fiber membrane that has been successfully deployed at municipal and industrial water treatment plants in China and elsewhere for more than a decade. In China alone, Targa cartridges treat more than 132 million gallons per day of water from a variety of sources.

The hollow fibers are true ultrafiltration membranes. Their nominal molecular weight cutoff of 100,000 daltons (a dalton is one-twelfth the mass of a Carbon-12 atom) results in the removal of particulates and larger molecular weight components. The KMS fibers have demonstrated the ability to reduce turbidity to less than 0.1 NTU (nephelometric turbidity units) and SDI (silt density index) to between 1.0 and 3.0, making them an ideal pretreatment step for spiral RO membranes.

Two trains of UF were commissioned in October 2006, and three additional trains were commissioned at the end of 2007 for the second phase of the plant’s construction. The trains operate in parallel, and each is equipped with 44 cartridges, resulting in a train capacity of 61,000 gallons per hour (Figure 2).

 

 

Prior to the ultrafiltration, the seawater is pretreated by coagulation and sedimentation, both performed in the same tank. FeCl₃ or Poly FeSO₄ is used as coagulant, and if needed, anionic PAM (anionic polyacrylamide) is used as a coagulant aid. To control biological growth, sodium hypochlorite is added to the feed of the sedimentation tank. The level of controlled free chlorine ahead the UF is about 0.3 to 1.0 mg/l. A 100-μm screen pre-filter that can be automatically backwashed is installed ahead of the UF to remove larger particles.

Live long and filter. “The most important reason that ultrafiltration was selected for seawater pretreatment is the high quality of the permeate,” said Mr. Zhang. “The high permeate quality results in fewer RO cleaning passes and ensures longer RO element life. But another key reason is that, with our new ultrafiltration system, we are able to cost-effectively tap limitless seawater and avoid draining our scarce surface water resources.”

For more than two millennia, the Dalian region has derived fame and fortune from its strategic coastal location. Now, with UF and RO technology, the sea will support the region’s growth in yet another way, by providing a sustainable supply of water for the new power plant.

—Contributed by Koch Membrane Systems Inc. (www.kochmembrane.com).

New cooling towers to improve river’s health

Big benefits to the Chattahoochee River ecosystem are expected from the start-up of two new cooling towers (Figure 3) at Georgia Power’s Plant McDonough in Smyrna. The first tower began commercial operation this March, and the second was expected to follow suit a month later.

 

 

The $96 million cooling towers represent the end product of an agreement between Georgia Power and the state Environmental Protection Division in 2000 to lessen the environmental impact of water discharged into the Chattahoochee River. “The towers will help to enrich the river’s habitat by improving the dissolved oxygen levels in the river and by allowing the river to better assimilate treated wastewater that is returned to it from upstream sewage plants,” said Chuck Huling, Georgia Power’s VP of environmental affairs.

Georgia Power has installed state-of-the-art plume abatement technology on the towers to reduce evaporation losses and to minimize fogging and misting in nearby areas during cold, damp weather. Each cooling tower is 550 feet long, 73 feet wide, and 55 feet high. Each is designed to lower the temperature of 137,000 gallons of water per minute by 20 degrees F.

“Reducing the temperature of the water discharges from the plant will greatly reduce its thermal effect on the river,” said Tony Tramonte, Plant McDonough’s manager. “Installing these cooling towers was the right thing to do for the river and the region.”

Plant McDonough has two coal-fired units with a total generating capacity of 540 MW. They are scheduled to be replaced in 2012 by three natural gas–fired combined-cycle units that will use the new cooling towers. The new gas units will supply 2,520 MW—more than four times the plant’s current capacity.

—Contributed by Georgia Power (www.georgiapower.com)

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