By Teresa Hansen, Senior Editor
Water issues are becoming a huge factor in U.S. economic development activities and the power generation industry is feeling the effects. The U.S. Department of Energy’s (DOE’s) report titled “Freshwater Needs for Thermoelectric Generation,” published in August 2006, reported that energy-water issues exist for all thermoelectric generating plants. And, according to Christian Larsen, vice president and chief nuclear officer of the Electric Power Research Institute (EPRI), water issues now rank as high as financing issues among companies planning to build new nuclear power plants. Concern doesn’t end there, however, energy-water issues are highly-visible and high-ranking concerns for regulators, Congress, DOE, media and the general public.
To complicate matters, water availability issues are being intensified by population increases that are occurring in many places where water is least available. Figures 1 and 2 show that many high-growth states are also located in areas where precipitation is low, especially in the west and southwest. Recently, California Gov. Arnold Schwarzenegger issued a drought declarationthe state’s first since 1991. In addition, portions of the southeast U.S. have experienced severe to extreme drought for several years. Because many of the areas experiencing water shortages and drought are also some of the country’s most heavily populated, it makes sense that these are also areas with the highest growing demand for electricity. Therefore, gaining water permits for new power plants is difficult and isn’t expected to get easier.
Thermoelectric power plants require significant amounts of water to operate, with the largest amount needed for cooling water. The 2006 DOE report on freshwater demand for power generation said that a 500 MW coal-fired plant uses more than 12 million gallons of water each hour for cooling steam turbine exhaust. Of course, this entire amount is not actually consumed. The DOE report, among other sources, said that while the power industry is responsible for almost 40 percent of freshwater withdrawals, it is responsible for only about 3 percent of the freshwater consumed in the United States. (The DOE defines freshwater consumption as the quantity of water withdrawn from a water body that is not returned to the source but is lost to evaporation. Water withdrawal is defined as the total quantity of water removed from a water source.)
In addition to water availability, water quality is also a major energy-water issue. For many years, power plant owners/operators have been required to operate within the National Pollutant Discharge Elimination System (NPDES) permit program, which is part of the Clean Water Act. The NPDES program controls water pollution by regulating the sources that discharge pollutants (effluent water) into waters of the United States, including streams, rivers, lakes, storm drains and even sewer systems. Since the mid-1990s, it has become increasingly difficult for power plant owners to obtain NPDES permits.
According to Jeff Schroeter, Genova Power Solutions LLC’s managing director, this is often because the body of water available to take a power plant’s discharge has already reached its load limit making it impossible for the plant to add discharge water, no matter how clean, and still keep the water body below its load limit. So to obtain a permit, many power plants must install zero liquid discharge (ZLD) systems to eliminate all effluent water discharge. ZLD systems also enable plant developers to forgo the expense and time involved in obtaining a NPDES permit.
“ZLD is often the default choice for a power plant,” Schroeter said. “The main reason ZLD is used is because it was too costly or difficult to get an NPDES permit.” A third, less common, reason ZLD is used is because the stream into which the power plant is going to discharge water is designated as requiring the discharge water to be so clean that the expense of installing and operating a ZLD is no greater than the expense to clean the water and discharge it.
Bruce Larkin, a senior chemical engineer in Black & Veatch’s energy business, echoed Schroeter’s statement. Situations that dictate the use of ZLD would include plants in arid areas, plants that need to shorten project permitting time or those located in areas with very stringent discharge requirements.
Using ZLD as a water conservation and recycling measure seems to make sense. After all, the plant is removing water from its waste stream, reusing it and thus reducing its water consumption. However, Dan Sampson, an industry technical consultant for Nalco Co.’s Power Business Unit, warns that ZLD should never be installed solely as a water conservation measure.
“In a water-cooled plant, about 80 percent of the water is lost into the atmosphere, mostly from evaporation in the cooling tower, leaving only about 20 percent to be recovered,” he said.
Larkin agreed, saying that saving fresh water is a secondary consideration because the amount saved by ZLD is a relatively small percentage of the plant’s overall demand.
Although neither expert recommends ZLD as a water conservation measure, they do agree that a properly-sized and efficiently-operating ZLD system can result in significant water savings.
Figure 3 illustrates a traditional water balance diagram that includes all of a plant’s water cycles. In this case, total makeup water flow into the plant is 1,902 gallons per minute (gpm) while total wastewater flow (blowdown) from the plant is 420 gpm. According to Sampson, a ZLD system will allow this plant to reuse about 400 gallons of blowdown, decreasing makeup water demand to 1,502 gallons per minute. In this example, the roughly 20 gallons of blowdown not used for makeup water is converted to concentrated brine by the ZLD system and can be hauled away for disposal. Figure 4 illustrates this, showing that the total blowdown from the plant is reduced from 420 gpm to about 20 gpm.
Even though a ZLD system provides a plant with the ability to save a fairly substantial amount of water, Sampson recommends the system be used only as a last resort. He said that while in many cases ZLD is the only way a plant can obtain an NPDES permit, it is a “poor solution for permitting issues.”
According to Sampson, ZLD systems are complex and wrought with operational and maintenance problems. “I have seen and worked on many ZLD systems and I have yet to see one pass a performance test,” he said. “I would do almost anything to get a plant off of ZLD.” For one thing, a ZLD system greatly affects a power plant’s operating costs. Increased capital and maintenance costs, additional operations manpower requirements, decreased reliability and equipment cleaning costs are the main culprits, he said.
Larkin also considers ZLD a last resort for power plants. “It seems trivial to say that ZLD is cost effective when it is the lowest cost option. But, some situations can make ZLD the most cost effective option.”
Although some ZLD operating and reliability issues are difficult to eliminate, Sampson said there are two major issues that, if properly addressed, can mitigate the problems associated with ZLD systems: A properly sized system and an adequate number of dedicated operations and maintenance personnel. Unfortunately, most power plants have neither.
Undersized systems cause many ZLD problems, according to Sampson. When specifying a ZLD system, designers/engineers determine how much waste water will be discharged. They then typically select the lowest-priced system designed to handle the required flow.
The main problem with this approach is that no system will be able to handle 100 percent capacity all of the time, Sampson said. As a result, ZLD systems rarely meet their nameplate rating.
For example, Sampson said if 100 gpm capacity is needed and the plant is sized with a 100 gpm ZLD system, problems will occur. Not only will the system not be 100 percent reliable but its reliability will degrade as it ages.
A single-train 100 gpm system can be counted on to be mechanically reliable 80 percent to 85 percent of the time. The system will degrade with agethe rate depends on how much the water properties deviate from the original design settingand capacity will diminish (about 20 percent between system overhauls and cleanings), Sampson said. Thus, the system’s true capacity is closer to 65 percent to 70 percent of its 100 gpm rating.
In addition, the system will have to be brought down on occasion for maintenance. During this time, a redundant system is needed; otherwise, the plant owner will be required to haul off all its wastewater. A second system is also needed during peak waste water production when the flow rate is 100 percent of the expected flow.
Simple ZLD systems, such as those relying only on a brine concentrator, are available around 90 percent of the time, Sampson said. In these cases he recommends installing two 60 percent to 70 percent capacity trains.
For complex systems, which include reverse osmosis to concentrate the wastewater stream before it flows to a crystallizer or other downstream equipment, Sampson recommends installing two identical trains, each capable of handling 100 percent of the required flow.
“Almost no one has this,” Sampson said. “And, almost everyone who has a ZLD system has had to haul waste water off site.”
Most new systems being specified today are still undersized. Because a ZLD system has a long lead time (about two years) the system is usually ordered and installed long before the plant staff is even selected. This means that no one with hands-on knowledge of ZLD system operations is involved in the initial specifications, Sampson said. “It is important to have an operations person involved during the design process to ensure a large enough system is specified and that redundant systems are installed.”
According to Sampson, the worst case for ZLD specification often occurs when the owner issues an EPC contract for the design and construction of the plant and/or ZLD system. In this case, the plant owner has little or no input, meaning the likelihood increases that the ZLD system will be incorrectly specified.
By contrast, the best case is often when the owner selects a partner to work with on plant design. This approach eliminates much of the competitiveness from the process and allows the partner to provide better information about the systems’ abilities and what is really needed, Sampson said.
Another common reason ZLD systems fail is due to inadequate manpower to properly operate the system.
“A ZLD system is a completely separate process,” Sampson said. “It’s a chemical plant in its own right that should not be rolled into the power plant. It needs a dedicated staff separate from the power plant operations staff.” He recommends a staff of at least six and as many as 14 dedicated personnel for a ZLD system. A small, simple system needs five operators to work a rotating shift (ensuring an operator is on duty 24/7) and one full-time manager. A larger, more complex system needs two operators on each shift (10 individuals). Because it only takes a few minutes to foul up water chemistry and create a problem requiring several days to correct, one experienced person is required to constantly monitor operating parameters and perform chemistry duties while a second person should be available to promptly respond to and correct operating problems.
In addition to the operators, a complex ZLD system needs one to three full-time maintenance technicians (mechanics, instrumentation and control technicians, and so on) and one full-time manager.
“Few plants have enough dedicated staff,” Sampson said. Usually, a pro forma plan does not include additional dedicated staff for the ZLD system. Instead, it assumes that the plant’s operations and maintenance staff will handle the ZLD system. “This is not a good plan,” he said.
Operating a ZLD system is nothing like operating a power plant. The system is designed assuming a specific design water analysis (water chemistry, ion count, and so on), which is performed before the system is specified and installed. The system design is based on this analysis, but the actual ZLD feedwater properties are uncertain until the system is actually up and running. Frequently, the actual water properties are different than the design analysis, which almost always results in capacity loss. The more unbalanced the water properties, the more rapidly the equipment in the system will degrade, requiring more cleaning and maintenance, thus more time off-line.
Although there is little that can be done to change this, as water properties will vary with many uncontrollable conditions, the effects of changing water properties can be mitigated with a dedicated staff, thus increasing availability, Sampson said. This is because the staff simply becomes more familiar with the system and how it operates. With more experience, the staff is better able to diagnose and correct problems.
“A dedicated operations staff is the best way to increase availability,” Sampson said. Bottlenecks can often be overcome only through operator knowledge. Even with dedicated staff it still takes operators about two years to fully understand the system. “These first two years are by far the worst, but the learning curve can be reduced with dedicated staff.”
Maximizing peak flow storage is another way to mitigate the effects of uncertain water properties. Sampson recommends a plant install at least 96 hours’ worth of maximum peak flow storage. This will allow the operators to normalize the water’s chemistry before it goes into the ZLD system. It also allows operators to control the flow rate into the ZLD system. “The waste water storage is necessary so we can separate the operation of the ZLD system from the operation of the power plant.” Sampson said.
Sampson said his recommendations are expensive and few, if any, power plants have opted to follow them. Consequently, nearly all ZLD plants have been forced to haul wastewater off-site, curtail plant operation, or both. Each plant’s designer and owner must assess the risks and determine if a ZLD system is the correct choice and, if so, what type and size system will work best.
“Use of ZLD is a plant-specific decision,” Larkin said. Engineers and designers are looking for the most cost-effective and simplest water and wastewater management plan for their clients, he said. “Sometimes that involves ZLD, other times use and management of a discharge is most desirable.”
The industry seems to be trending toward increased use of ZLD systems. Discharge requirements are becoming more stringent and schedules, especially for combined cycle plants are typically short, Larkin said. After all, “water is becoming more scarce and poorer quality water supplies are being used and considered.”