Navajo Generating Station, Navajo Reservation, Page, Arizona

June 16, 2008 at 2:01 am | Posted in Power Plant | Leave a comment

Operated by Salt River Project, Tempe, Arizona

The past year has seen several major utilities sign agreements with the U.S. EPA to install scrubbers after years of discussion and several lawsuits. Environmentally proactive Navajo Generating Station (NGS) made a commitment to add sulfur dioxide (SO2) scrubbers over 10 years ago and has emerged as a leader in low SO2 emissions. Another NGS success story is the plant’s waste minimization effort. By reducing its hazardous waste output by 97% over the past 12 years, NGS has earned a variety of environmental awards.

By Robert K. Talbot (Plant Manager), Robert B. Candelaria (O&M Manager), and Marc McKenna (Environmental Engineer/Scientist), Salt River Project/Navajo Generating Station

The Navajo Generating Station (NGS), the eighth-largest coal-fired power generating station in the U.S., generates a net 2,250 MW from its three units (Figure 1). It is owned by a consortium of western utilities and power providers: the U.S. Bureau of Reclamation (24.3%), Salt River Project (21.7%), Los Angeles Department of Water and Power (21.2%), Arizona Public Service Co. (14.0%), Nevada Power Co. (11.3%), and Tucson Electric Power Co. (7.5%). Salt River Project (SRP) operates NGS, which occupies 1,786 acres of land in northern Arizona and has 520 full-time employees.

1. Navajo Generating Station.

The Navajo Generating Station, the eighth-largest coal-fired power generating station in the U.S., generates a net 2,250 MW. At full capacity, the plant burns 25,000 tons/day of 0.5% to 0.65% low-sulfur coal.

Courtesy: Salt River Project

NGS, completed in 1976 at a cost of $650 million (of which $200 was spent on air- and water-quality control equipment), was built with environmental concerns in mind. For example, the plant’s water system is a zero-discharge process; all cooling tower blowdown, all wastewater, and most stormwater are recovered, recycled, and reused (Figure 2).

2. Environmentally conscious.

Navajo Generating Station, completed in 1976, occupies 1,786 acres of land in northern Arizona near the Grand Canyon. NGS uses a zero-discharge water treatment process; all cooling tower blowdown, all wastewater, and most storm water are recovered, recycled, and reused in plant processes.

Courtesy: Salt River Project

The plant’s commitment to the environment is ongoing and further illustrated by a six-year, $420 million SO2 scrubber project (Figure 3). The new scrubbers, completed in 1999, remove more than 90% of the SO2 that NGS produces. They enable the facility to meet the newly established SO2 emission limit of 0.1 lb/mmBtu. NGS routinely ranks as the lowest coal-fired power plant SO2 emitter in its class.

3. New scrubbers.

The six-year, $420 million scrubber project completed in 1999 reliably reduces SO2 emissions by over 90%—well below the 0.1 lb/mmBtu limit. The old stacks were demolished after the scrubbers were commissioned.

Courtesy: Salt River Project

NGS receives its coal from the Kayenta Mine located at Black Mesa on the Navajo Reservation. The coal, mined by Peabody Coal Co., is a bituminous low-sulfur fuel with a sulfur content of 0.5% to 0.65%. The Black Mesa and Lake Powell electric railroad—a dedicated 50-kV “line”—averages three 140-mile round trips daily, shuttling coal to three 750-MW (net) units that together burn 25,000 tons per day at full capacity.

Negotiation the key to success

The scrubber project was the solution arrived at after difficult negotiations involving NGS’ impact on visibility in and around the Grand Canyon. SRP, the EPA, the U.S. National Park Service (NPS), and various environmental groups, acting through the Grand Canyon Trust, agreed that NGS should have a 90% SO2 reduction rate (0.10 lbs/mmBtu). The EPA had first proposed a system that would remove 70% of the SO2, while environmentalists sought a 95% rate, according to a source close to the negotiations. Additionally, there were differences in the manner proposed for measuring emissions. The final agreement allowed SRP to use an annual rolling average basis rather than a short-term daily average.

Environmental groups pushed the retrofit in the belief that it would preserve air quality in a pristine region and establish a precedent that would force a handful of similar coal-fired units in the West to add scrubbers. More recently, environmentalists have focused their attention on the quality of air in Eastern states. As a result, there have been few scrubber projects on the scale of Navajo’s in the West over the past 10 years.

Prime property

It’s easy to see why NGS is such a high-profile power plant. Its 775-ft-tall stacks dominate the high desert skyline just outside the small town of Page on Lake Powell’s southern shore. Grand Canyon National Park is 12 miles to the southwest. Within a 100-mile radius are Zion, Bryce Canyon, Capitol Reef, Canyonlands, and Arches National Parks, and Wupatki and Canyon De Chelly National Monuments.

What led to the agreement was the National Park Service’s concern about Grand Canyon haze. During a joint research effort by the EPA, NPS, and SRP that focused on NGS’ impact, sensors within the canyon detected a unique tracer gas injected into one of the plant’s units, according to representatives of the Environmental Defense Fund (EDF). Although the power plant met the Clean Air Act’s SO2 emissions guidelines, the EDF and the Flagstaff-based Grand Canyon Trust sued, based on a visibility provision of the law. Plant owners countered that the potential for impact attributable to NGS would occur less than 8% of the time due to the nature of typical meteorological conditions.

Following another major research effort conducted by SRP and reviewed by the National Academy of Sciences, SRP stated that the primary source of pollutants impacting the canyon was not NGS, but rather and more broadly the American West and Southwest. Realizing that it would be difficult to reconcile two strong differing opinions on the science, SRP and the owners of NGS decided that negotiation would be a better way to solve the problem than litigation. They reached an agreement with the EPA in August 1991 and the scrubber project was born.

The EPA is pleased because its original goal was a 70% reduction in SO2 emissions from NGS. The owners said that, if they were allowed to average emissions on a 365-day basis instead of a 30-day average, then they would shoot for a 90% reduction. Performance of the systems has been exceptional, given that the plantwide compliance emissions are typically about 0.04 lbs/mmBtu—compared with the emission limit of 0.100 lbs/mmBtu based on a rolling average of 365 days of boiler operation.

Phased construction pays

SRP agreed to have the scrubbers up and operating on a phased basis; the target dates called for having one unit on-line by November 1997, a second by November 1998, and a third by August 1999. Also specified in the binding agreement were start-up testing dates, as well as dates for contracting with engineering firms to begin construction. SRP also changed its maintenance schedule from early spring to mid-winter, thus reducing emissions during winter (the time of concern identified by the EPA). As a result of the negotiations, the capital cost of compliance was reduced from an estimated $510 million to $420 million.

The schedule gave the owners bargaining room for the budget, but the higher SO2-removal standard limited the choice of scrubber technologies—the 90% standard limited the options to a wet scrubber system. Stone & Webster Engineering Corp. and Asea Brown Boveri teamed up to design forced oxidation flue gas desulfurization (FGD) systems for each of NGS’ three units.

The wet process selected for NGS had a proven track record; many coal-fired power plants used the technology. At NGS, limestone is dumped into a hopper, 60 ft below grade, that feeds two storage silos via two 700-ft-long conveyors. Two ball mills crush 36 tons/hr of limestone to a nominal 225-mesh powder, maximizing its surface area. Each scrubber is composed of two absorber vessels constructed of special, corrosion-resistant C-276 steel. Banks of 1,000-hp pumps spray calcium carbonate slurry (limestone and water) into a closed chamber.

Flue gas enters at 275 to 325F and is pushed through the absorber modules by four 8,000-hp induced-draft fans, triggering the chemical reaction. SO2 combines with the calcium carbonate mist and forms a calcium-sulfate-gypsum slurry at the bottom of the scrubber modules. The slurry’s solids content is about 10% at this point, but about 90% of its moisture is reclaimed as the slurry passes through banks of cyclones and belt presses. The scrubber requires 130,000 tons/yr of limestone to remove approximately 70,000 tons/yr of the pollutants from the flue gas. Its output is approximately 200,000 tons/yr of low-grade gypsum. Byproducts not marketed are landfilled 1.5 miles from the power plant.

Minimizing waste

In the early 1990s, NGS realized that multiple long-term benefits would accrue to the plant’s owners and employees if NGS could reduce or eliminate chemicals identified by the EPA and OSHA as creating environmental and health risks. In 1992, the plant’s environmental division initiated an aggressive program to target and eliminate materials identified by OSHA and the Resource Conservation and Recovery Act (RCRA) as hazardous materials or hazardous waste. A waste management team representing all levels of the organization was formed and charged with evaluating all processes and plant operations generating any waste. All options to minimize the health and environmental impact of these processes were considered while keeping focused on the goal: Reduce the amount of hazardous wastes generated by NGS.

A side benefit of reducing or eliminating hazardous wastes from the work environment was the plant’s achievement of the status of a conditionally exempt small quantity generator (CESQG), as defined by the EPA and RCRA. A CESQG generates less than 220 lb/month of hazardous waste and therefore has significantly less burdensome RCRA recordkeeping, training, and reporting requirements.

Teamwork pays dividends

Some of the changes implemented by the waste management team include:

  • A mechanism for reviewing and approving all materials being considered for use at NGS. All chemicals used by NGS and its contractors must be preapproved prior to use.
  • The education of all plant personnel about plant policies regarding materials procurement and waste management.
  • A contractor orientation program, which makes all contractors partners in waste minimization and waste management activities at NGS.
  • The targeting for review and elimination (wherever possible) of chemicals fitting the RCRA definition for hazardous waste. As it turned out, many of these materials—especially RCRA F Type wastes—could be eliminated and replaced with far less toxic substitutes.
  • The elimination of chlorinated solvents.
  • The replacement of most oil-based paints used at the plant by water-based coatings.
  • The substitution of water-based paints for paints with high lead and chrome content.
  • Scrutiny of parts washing, with the objective of eliminating or drastically reducing the use of conventional solvents. Some parts washers were eliminated; filter systems were installed on all remaining parts washers to prolong the life of the solvent; and hot water washers replaced some of the eliminated parts washers. The users of parts washers were taught methods for prolonging the life of parts washer solvent. Since these changes were implemented, NGS solvent usage has been reduced and parts washers routinely go 18 months between changes.
  • The elimination of the use of all chlorinated cutting oils in the machine shop.
  • An evaluation/review of the character of laboratory waste and practices and methods for analyzing and disposing of it. In many cases, old analytical methods were replaced with newer technology solutions such as continuous online instrumentation. Changes were made in other wet method chemistry processes to reduce the volume, character, and toxicity of generated waste. In some cases, the processes were eliminated entirely. As a result, a philosophy of conserving resources, waste minimization, and recycling is now part of NGS’ culture.

These waste-minimization efforts have been quite effective and must be considered remarkable based on a comparison of the amount of waste products the plant generated in the past versus the present. In 1991, NGS generated 39,037 pounds of hazardous waste. Last year, it generated only 1,387 pounds (a new record low) and no more than 220 pounds in any month, earning the plant status as a CESQG. Now that attention to waste has been ingrained in NGS’ culture, it’s likely that its record will continue to be broken year after year.


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