US power plants withdraw about 136 billion gal per day (bgpd) of fresh water – second only to irrigation in terms of percentage of US national water use – while they consume about 3 bgpd, according to estimates for the year 2000. About 88% of the 136 bgpd withdrawn was used at plants with once-through cooling systems, which have very high water withdrawal requirements, but since nearly all of the water is returned to the source body, consumption is relatively low. On the other hand, recirculating wet systems have lower withdrawal requirements, but consumption, through direct evaporation, can be relatively high.

Fresh water availability is already a critical factor in electric power supply and will become increasingly so, with power generation in growing competition with other water uses, including food production and drinking.

Some recent headlines illustrate water-related impacts on power plant siting and operation: “Idaho may adopt moratorium on coal power due to water issues”; “Sempra Energy halts Gerlach project study”; “California’s efforts to end use of sea water to cool plants could jeopardise 24 GW”; “New power plants to dry up water supplies?”; “Feds order Susquehanna power plants and others to stop killing off fish”.

Such developments point towards a likely future of increased conflicts and competition for the water the power industry will need to operate its plants.

These conflicts will be national in scope, but regionally driven. It is likely that power plants in the west will be confronted with issues related to water rights, that is, who owns the water, and the impacts of chronic and sporadic drought. In the east, current and future environmental requirements, such as the Clean Water Act’s intake structure regulation (see below), could be the most significant impediment to securing sufficient water, although local drought conditions can also impact water availability.

The regulatory framework

Section 979 of the Energy Policy Act of 2005 speaks to the importance of water and energy issues and the Act instructs the Department of Energy to address issues related to adequate water supplies, optimal management, and efficient use of water and energy.

The US Environmental Protection Agency (EPA) has been charged with maintaining and improving the USA’s water resources. To accomplish this, EPA has issued several regulations under the Clean Water Act and the Safe Drinking Water Act that directly impact the discharge of pollutants from power plants to receiving waters as well as the intake of water for cooling and other power plant needs.

The Clean Water Act (CWA) provides for the regulation of discharges to surface waters. The CWA calls for a federal–state partnership in which the federal government sets the standards for pollution discharge and states are responsible for the implementation and enforcement. Initial emphasis was placed on “point source” pollutant discharge, but 1987 amendments authorised measures to address “non-point source” discharges, including storm water runoff from industrial facilities. Permits are issued under the National Pollutant Discharge Elimination System (NPDES), which designates the highest level of water pollution or lowest acceptable standards for water discharges. With EPA approval, the states may implement standards more stringent than federal water quality standards but they may not be less stringent. Certain sections of the CWA are particularly applicable to water issues related to power generation, notably:

Section 316(a), Water Thermal Discharge.

This requires the regulation of water thermal discharge from cooling water systems in order to protect shellfish, fish, and other aquatic wildlife.

• Section 316(b), Cooling Water Intake Structures.

Section 316(b) is arguably the most urgent water-related issue facing thermoelectric power generation in the near term. This section requires that the location, design, construction and capacity of cooling water intake structures reflect the best technology available for minimising adverse environmental impact, such as impingement or entrainment of aquatic organisms due to the operation of cooling water

intake structures. Regulations to implement Section 316(b) are being issued in three phases that cover different facility categories. The Phase I rule was issued in December 2001 and effectively requires all new thermoelectric power generation plants to install closed-cycle cooling systems due to standards for water intake capacity and velocity. The Phase II rule, issued in July 2004, applies to existing thermoelectric power generation plants that

withdraw more than 50 MGD of water and use at least 25% of the water withdrawn for cooling purposes only. Although the Phase II rule requires significant percentage reductions in both impingement and entrainment losses from uncontrolled levels, it also provides flexible compliance alternatives so that conversions of open-cycle to closed-cycle cooling water systems are not mandated. Regulations for Phase III were proposed in November 2004 and will apply to other industrial sources and new offshore and coastal oil and gas extraction facilities.

Also relevant is The Safe Drinking Water Act (SDWA), which serves to protect humans from contaminants in the nation’s public drinking water supply. Amended in 1986 and 1997, the law requires many actions to protect drinking water and its sources. In particular the SDWA requires EPA to set national drinking water standards and create a joint federal-state system to ensure compliance. While the provisions of the SDWA apply directly to public water systems in each state, the Act is relevant to thermoelectric power generation because waste streams may contain detectable levels of elements or compounds that have established drinking water standards. Under the SDWA, regulations that would require additional limits on mercury, arsenic, and other trace metals could also affect how power plants dispose of coal byproducts.

The IEP programme

DOE/NETL’s Innovations for Existing Plants (IEP) programme is an R&D effort directed at the development of advanced technologies that can enhance the environmental performance of the existing fleet of coal fired power plants.

In response to the growing recognition of the interdependence between fresh water availability and quality and electricity production, the IEP programme was broadened in 2003 to include research directed at coal fired power plant related water management issues. The overall goal of this effort is to reduce the amount of fresh water needed for power plant operations and to minimise potential impacts on water quality. Specifically, the hope is to reduce fresh water withdrawals and consumption by at least 5-10% by 2015.

The programme is built around four specific areas of research:

• non-traditional sources of process/cooling water;

• innovative water reuse and recovery;

• advanced cooling technology;

• advanced water treatment/detection technology.

A total of thirteen project awards have now been made under “innovative water management” solicitations, seven in November 2005 and five in August 2003. The November 2005 project awards can be summarised as follows:

Development and demonstration of a modeling framework for assessing the efficacy of using mine water for thermoelectric power generation – West Virginia University

Building on past studies (see below) demonstrating that mine water can be cost-effectively used for power plant cooling makeup water while improving cooling efficiency, the National Mine Land Reclamation Center at West Virginia University is developing a framework for assessing the costs, technical and regulatory aspects, and environmental benefits of using mine water for power generation. Researchers will conduct a field study at the proposed 300 MWe Beech Hollow power plant in Champion, Pennsylvania, to identify mine water sources able to supply 2000 to 3000 gallons of water per minute. Using the data and decision making processes derived during this study, as well as any appropriate data and information obtained from other power plants using mine water, a computer based design aid will be developed for estimating the cost of water acquisition and delivery to the power plant.

Non-traditional water sources such as coal mine discharges not only have the potential to reduce freshwater power plant cooling requirements, they also can improve the efficiency of the cooling process due to the lower water temperatures associated with deep mine pools.

• Recovery of water from boiler flue gas – Lehigh University

This project will combine laboratory and pilot scale experiments with computer simulations that will investigate the use of condensing heat exchangers to recover water from boiler flue gas. Researchers will conduct computational fluid mechanics analyses to aid in the design of the compact fin tube heat exchanger that will condense water vapour from flue gas. Experiments to determine the amount of water vapour condensation achievable as well as experiments to measure the heat transfer effectiveness of the fin-tube bundle will be conducted.

Analyses of the boiler and turbine cycle will be carried out to estimate potential reductions in heat rate due to recovering sensible and latent heat from the flue gas. In addition to water vapour recovery, other benefits of using a condensing heat exchanger include the simultaneous removal of sulphuric acid and an increase in power plant efficiency.

• Use of Air2Air technology to recover fresh water from the normal evaporative cooling loss at coal fired power plants – SPX Cooling Systems

SPX Cooling Systems will evaluate the performance of its patented Air2Air condensing technology in cooling tower applications at coal fired power plants. Researchers will quantify Air2Air water conservation capabilities with results segmented by season and time of day. They will determine the pressure drop and energy use during operation.

Additionally, SPX will develop a collection method for the recovered water, analyse water quality, and identify potential on-site processes capable of using the recovered water. The ultimate benefit to be explored will be the water savings potential of the condensing technology.

• A synergistic combination of advanced separation and chemical scale inhibitor technologies for efficient use of impaired water as cooling water in coal-based power plants – Nalco Company

The overall objective of this project, conducted by Nalco in partnership with Argonne National Laboratory, is to develop advanced scale control technologies to enable coal power plants to use impaired water in recirculating cooling systems. The use of impaired water is currently problematic technically and economically due to additional physical and chemical treatment requirements to address scaling, corrosion, and biofouling. Nalco’s research will focus on methods to economically manage scaling issues. The overall approach will be to use synergistic combinations of physical and chemical technologies with separations to reduce the scaling potential, as well as scale inhibitors extending the safe operating range of the system to maximise water utilisation efficiency and minimise waste discharge.

Reuse of treated internal or external wastewaters in the cooling systems of coal-based thermoelectric power plants – University of Pittsburgh

The overall objective of this study, conducted by the University of Pittsburgh and Carnegie Mellon University, is to assess the potential of three types of impaired waters for cooling water makeup in coal fired plants. The impaired waters to be studied include: secondary treated municipal wastewater; passively treated coal mine drainage; and ash pond effluent. Researchers will operate small pilot-scale cooling towers for side-by-side evaluation of impaired waters under different conditions and will assess the feasibility and relative importance of the three impaired waters by examining their availability at twelve power plant locations.

The ultimate goal is to provide alternative sources of water for cooling systems.

Reduction of water use in wet FGD systems – URS Group

This project team, consisting of URS Group, as prime contractor, the Electric Power Research Institute, Southern Company, Tennessee Valley Authority (TVA), and Mitsubishi Heavy Industries (MHI), will demonstrate the use of regenerative heat exchange to reduce freshwater use in coal fired power plants equipped with wet flue gas desulphurisation systems by minimising evaporative water loss in the FGD systems. Researchers will conduct pilot-scale tests of regenerative heat exchange to determine the reduction in FGD water consumption that can be achieved and will assess the resulting impact on air pollution control systems. During the demonstration, flue gas will be cooled enough to reduce the evaporation of water in the wet FGD system by about one-half.

• Application of pulsed electrical fields for advanced cooling in coal fired power plants – Drexel University

The goal of this research is to develop a scale prevention technology based on a novel filtration method and an integrated system of physical water treatment in an effort to reduce the amount of water needed for cooling tower blowdown. The filter will be a self-cleaning metal membrane, using pulsed electric fields to dislodge particles on the filter. Potential benefits from this research include the ability to operate at a higher cycle of concentration, which will reduce cooling tower blowdown water requirements.

The August 2003 projects

The five projects awarded in August 2003 can be summarised as follows:

Mine water West Virginia University’s Water Research Institute conducted an evaluation (now completed) of the use of water from abandoned underground coal mines to supply cooling water to power plants. The cost analysis concluded that depending on site conditions and water treatment requirements that utilisation of mine pool water as a source of cooling water makeup can be cost competitive with freshwater makeup systems.

Produced waters EPRI evaluated the feasibility of using “produced waters”, a byproduct of natural gas and oil extraction, to meet up to 10% of the approximately 20 MGD of make-up

cooling water demand for the mechanical draft cooling towers at Public Service of New Mexico’s 1800 MW San Juan Generating Station (SJGS) (see p14, upper diagram).

Most of the produced water in the region is collected in tanks at the wellheads and transported by truck to local saltwater disposal facilities. The SJGS evaluated a two-phased approach for transportation of produced water to the plant site. In the first phase, an 11 mile pipeline would be built to gather and convey nearby production. Existing unused gas and oil pipelines would then be converted to transport produced water in the second phase.

Cooling water currently used at the SJGS is withdrawn from the San Juan River and contains only 360 mg/l of TDS compared to produced water with a TDS concentration ranging from 5440 to 60 000 mg/l. Therefore, the produced water would have to be treated prior to use at the plant.

In addition to cooling tower make-up, produced water was also evaluated for use as bottom ash sluice water, fly ash wetting water, and FGD absorber make-up. It was determined that FGD absorber make-up would be the least costly use for treated produced water. The most economical treatment method identified was high efficiency reverse osmosis with a brine concentrator distillation unit that would process approximately 1100 gpm of produced water. Based on this, the cost for produced water treatment would include an initial capital cost of $14.1 million and operating costs of $2.98 million per year that includes approximately 2 MW of auxiliary power. The total project capital cost including collection, transportation, and treatment facilities was estimated to be $43.1 million.

Flue gas dehumidification The University of North Dakota’s Energy & Environmental Research Center (UNDEERC) along with Siemens worked on the development of a cost effective liquid-desiccant-based dehumidication technology to extract water vapour from coal-fired power plant flue gases (see lower diagram, p14). Prospects for commercial development of the process are encouraging.

Mussel elimination The New York State Education Department is completing an evaluation of the use of the naturally occurring bacterium, Pseudomonas fluorescens, to selectively eliminate invasive zebra mussels on cooling water intake systems of coal-fired power plants.

Passive treatment for trace pollutants TVA and EPRI are completing research on an extraction trench containing zero-valent iron for passive treatment to remove trace pollutants (mercury, arsenic, selenium, and NH3 “slip”) from fossil fuel power plant wastewater.

Other projects

A number of other DoE/NETL research projects have a bearing on power plant water use, including:

• Use of coal drying to reduce water consumed in pulverised coal power plants

Lehigh University is conducting laboratory scale testing to evaluate the performance and economic feasibility of using low-grade power plant waste heat to partially dry low-rank coals prior to combustion in the boiler. In this process heat from condenser return cooling water is extracted upstream of the cooling tower to warm ambient air that is then used to dry the coal. Lowering the temperature of the return cooling water reduces evaporative loss in the tower, thus reducing overall water consumption.

In addition, drying the coal prior to combustion can improve the plant heat rate, and in return reduce overall air emissions. Information from this project is being used to design a full-scale coal drying system at Great River Energy’s 546 MW lignite-fired Coal Creek power station, North Dakota. The Coal Creek project is being funded under DOE/NETL’s Clean Coal Power Initiative.

• An innovative fresh water production process for fossil fired power plants using energy stored in main condenser cooling water

The University of Florida is investigating an innovative diffusion-driven desalination process that would allow a power plant that uses saline water for cooling to become a net producer of freshwater. Hot water from the condenser provides the thermal energy to drive the desalination process. Saline water cools and condenses the low pressure steam and the warmed water then passes through a diffusion tower to produce humidified air.

The humidified air then goes to a direct contact condenser where fresh water is condensed out. This process has potential advantages over conventional desalination technology in that it may be driven by low-temperature waste heat. Cool air, a byproduct of the process, can also be used to cool nearby buildings.

• Hybrid cooling water system

In conjunction with the produced water feasibility study being conducted at the San Juan Generating Station (see above), EPRI is also conducting pilot scale testing of a hybrid cooling technology.

The Wet Surface Air Cooler (WSAC) – supplied by Niagara Blower – is a closed-loop cooling system coupled with open-loop evaporative cooling (see diagram opposite). Warm water from the steam condenser flows through tubes that are externally drenched with spray water. Heat is removed through the evaporative effect of the spray water. The tubes are always covered in water, hence the name “wet surface”. An attraction of the WSAC is that it can operate with degraded water. A high spray rate is used to ensure that the tubes are constantly flooded and helps the spray nozzles from becoming plugged.

Co-current flow of air and spray water eliminates dry spots on the underside of the tubes where fouling often occurs. The tubes have no fins and are spaced sufficiently far apart that solids or precipitates from the poor quality water are washed into the basin.

At SJGS this system is being used as auxiliary cooling for condenser cooling water. The spray water is blow down water from the existing cooling towers. The testing will determine to what extent the WSAC can concentrate untreated cooling tower blow down before thermal performance is compromised. It will be used as a pre-concentrating device for the cooling tower blow down that is typically evaporated in a brine concentrator or evaporation pond at this zero discharge facility.

• Enhanced performance carbon foam heat exchanger for power plant cooling

Ceramic Composites has partnered with SPX Corporation to develop high thermal conductivity foam to be used in an air-cooled steam condenser for power plants that could significantly decrease energy consumption associated with conventional dry cooling systems.

In addition, the availability of a more efficient dry cooling technology would offer an alternative to power plants for minimising adverse impacts such as organism intake, warm water discharge, and evaporative water loss associated with wet or wet-dry hybrid cooling technology. This project was due to be completed in July 2006.

• Demonstration of a market-based approach to the reclamation of mined lands in West Virginia

EPRI is demonstrating a market-based approach to abandoned mine land (AML) reclamation by creating marketable water quality and carbon emission credits. The project involves the reclamation of thirty acres of AML in West Virginia through the installation of a passive system to treat acid mine drainage, application of fly ash as a mine soil amendment, and reforestation for the capture and sequestration of atmospheric carbon dioxide.

Potential eco-credits include water quality credits due to decreased acid mine drainage and other benefits resulting from the soil amendment, as well as potential credits for CO2 sequestration due to more than 36 000 seedlings to be planted at the site.

• Novel anionic clay adsorbents for boiler blowdown water reclaim and reuse

The University of Southern California is conducting a feasibility study on using novel anionic clay sorbents for treating and reusing power plant boiler blowdown water, which can contain metals such as arsenic and selenium. Currently, it is difficult to economically and effectively clean this waste stream as flow rates are high and the metals concentrations are at trace levels. The goal of the project is to develop an inexpensive clay-based adsorbent that can be used to treat the high-volume, low-concentration wastewater stream.

Artificially created wetlands for treatment of scrubber wastewater

Clemson University has evaluated pilot scale constructed-wetland treatment systems for dealing with trace elements (arsenic, mercury, and selenium) in FGD waste water. The pilot scale system consists of a 6800 litre upstream equalisation basin followed by three parallel treatment trains, each train having four 378 litre cells: two wetland cells planted with bulrush, a gravel manganese oxidation cell, and a wetland cell planted with cattails.

(Left) Breakdown of US fresh water withdrawal (2000) and (Right) Approx breakdown of US fresh water consumption Pipeline system for transport of produced-water to San Juan Generating Station (SJGS) Conceptual design of UNDEERC/Siemens liquid desiccant process to recover water from flue gas Wet Surface Air Cooler (WSAC): the basic principles