Arizona, USA coal-fired power plant (1974–2019)
Navajo Generating Station was a 2.25-gigawatt (2,250 MW), coal-fired power plant located on the Navajo Nation, near Page, Arizona, United States. This plant provided electrical power to customers in Arizona, Nevada, and California. It also provided the power for pumping Colorado River water for the Central Arizona Project, supplying about 1.5 million acre feet (1.85 km3) of water annually to central and southern Arizona. As of 2017 permission to operate as a conventional coal-fired plant was anticipated until 2017–2019, and to December 22, 2044, if extended. However, in 2017, the utility operators of the power station voted to close the facility when the lease expires in 2019. In March 2019, the Navajo Nation ended efforts to buy the plant and continue running it after the lease expires.
On November 18, 2019, the plant ceased commercial generation. Full decommissioning of the site was projected to take approximately three years. On December 18, 2020, the three smokestacks were demolished.
History.
In the 1950s and 1960s, there was a need for new electric generation in the Southwest to supply power to growing populations in southern California, Arizona and Nevada. The US Bureau of Reclamation also needed a large source of power for running the pumps of the planned Central Arizona Project (CAP).
A number of power projects were considered to fill these needs, including Bridge Canyon and Marble Canyon hydroelectric dams on the Colorado River. However, the proximity of the proposed dams to the Grand Canyon raised opposition, initially from the National Park Service and then more vigorously from a coalition of environmental groups which promoted construction of a thermal or nuclear power plant as an alternative. As a result, the proposed dams were abandoned in favor of the Navajo Power Project, consisting of the Navajo Generating Station (NGS) along with the Kayenta mine, Black Mesa &amp; Lake Powell (BM&amp;LP) Railroad, and of 500 kV transmission lines.
The site selected for the new power plant was about six miles (10 km) east of Glen Canyon Dam and three miles (5 km) south of Lake Powell on of land leased from the Navajo Nation. The site was close to a source of competitively priced fuel and a reliable source of surface water for cooling. The nearby city of Page and U.S. Highway 89 provided infrastructure to support construction and operation of the project. The contract for engineering and construction was awarded to Bechtel Corporation, which began construction in April, 1970. Generating units 1, 2, and 3 were completed in 1974, 1975, and 1976 respectively at a total cost of about $650 million.
Between 1977 and 1990, following Clean Air Act amendments for assessing and protecting visibility in national parks and wilderness areas, the EPA, Salt River Project and other entities cooperated in visibility studies to assess possible effects that NGS emissions might have on those areas.
The series of studies culminated in the Winter Haze Intensive Tracer Experiment (WHITEX) and the Navajo Generating Station Visibility Study (NGSVS). Those studies indicated that controlling sulfur dioxide (SO2) emissions might improve winter visual range in the Grand Canyon by 2% to 7%, which prompted the EPA to propose a rule requiring NGS to reduce SO2 emissions by 70%.
NGS and environmental groups, however, negotiated an approach that would achieve a greater degree of improvement at lower cost. They agreed to, and recommended a requirement of a 90% reduction in SO2 emissions based on an annual average, with installation to be completed in 1999. The EPA accepted the recommendation and implemented those conditions in its final rule.
The technology selected for flue gas desulfurization (FGD) were wet SO2 scrubbers with forced oxidation. Stone &amp; Webster, the architect-engineering firm for the scrubber project, began construction in 1994 and completed the work on Units 3, 2, and 1 in 1997, 1998, and 1999 respectively. The cost of the scrubber project was about $420 million.
During spring overhauls in 2003 through 2005, the electrostatic precipitators were completely gutted and rebuilt for reliability and optimum performance. The original collecting plates and weighted wire electrodes were replaced with improved collecting plates and rigid discharge electrodes. Control equipment was replaced with upgraded automatic voltage controls and rappers.
In 2007, an analysis of nitrogen oxide (NOx) emissions and control options was conducted for SRP in support of Clean Air Act regional haze reduction efforts, although there were no (NOx) reduction requirements on NGS at the time. The analysis concluded that Low NOx burners with separated overfire air (SOFA) would provide the Best Available Retrofit Technology (BART) alternative in accordance with EPA BART guidelines. Consequently, the plant voluntarily installed low NOx-SOFA burners on units 3, 2, and 1 during eight-week outages in February through March 2009, 2010, and 2011 respectively.
The Los Angeles Department of Water and Power withdrew from the project in 2016. After the rise of shale gas in the United States, natural gas power prices ($32/MWh) fell under the cost of coal power for NGS ($38/MWh), and NGS production decreased. The owners require a potential new owner (such as the Navajo Nation) to assume responsibility of future cleanups.
Design and specifications.
The plant had three identical 750 MW steam electric generating units. Major components of each unit included a boiler, turbine, generator, closed-cycle cooling system, and environmental control equipment.
The boilers were tangentially fired, balanced draft, reheat type steam generators, supplied by Combustion Engineering. Designed for supercritical operation, each boiler supplied up 5,410,000 pounds of steam per hour to the turbines at 3500 psi (241 bar) and 1000 °F (537 °C).
The main turbines are General Electric (GE) tandem compound reheat turbines, each directly coupled to a generator and designed to rotate at 3600 rpm.
Steam discharged from the turbines entered a condenser where water circulating through tubes cools and condensed it, producing a vacuum that maximized the pressure drop across the turbine. The heat picked up by the circulating water was removed by evaporation in six induced-draft cross flow cooling towers (two per unit). 
The plant used about of water per year from Lake Powell, or about 1/2 gallon (1.9L) per kWh, mainly for cooling water and scrubber operation; see section #Wastewater handling.
The plant consumed about 8 million tons of low sulfur bituminous coal each year, supplied by Peabody Energy's Kayenta Mine near Kayenta, Arizona. The coal was hauled 75 miles from mine silos to the plant by the Black Mesa and Lake Powell Railroad that is owned and operated by the plant. Characteristics of the coal in 2011 included a sulfur content of 0.64%, an ash content of 10.6%, and higher heating value (HHV) of 10,774 Btu/lb.
The plant had three 775 ft (236 meter) tall flue gas stacks, listed among the tallest structures in Arizona. The stacks were constructed of reinforced concrete, supporting an internal metal liner. The plant's original stacks were demolished in the late 1990s after being replaced by larger diameter stacks of the same height, resulting in the plant having up to six stacks visible for a time.
The new stacks were required to accommodate cooler flue gas saturated with water vapor that resulted when wet SO2 scrubbers were added.
Performance.
The plant had a net nameplate capacity of 2250 MW or 750 MW net per unit, the rated power output leaving the plant through the transmission lines. The gross capacity was 2409.3 (MW) or 803.1 MW per unit, the rated power output at the generator that includes power used internally in the operation of the plant.
Annual net energy generation in 2011 was 16.9 terawatt-hours (TWh), with a net capacity factor of 86%. Annual gross generation was 18.3 TWh. Fuel consumed (primary energy) in 2011 provided of heat input, resulting in a net heat rate of 34% or .
In 2014, generation fell to 72% of capacity, and to 61% in 2016. In the first 11 months of 2016, the plant consumed and produced 10.7  TWh of power, giving 33% efficiency.
Environmental controls.
Flue gas emissions.
Fly ash particulate was removed from the flue gas by hot-side electrostatic precipitators (ESPs) and SO2 scrubbers. The ESPs, constructed as part of the original plant, removed 99% of particulate. Scrubbers removed an additional 50% of what remained in the flue gas after passing through the ESPs.
Sulfur dioxide (SO2) emissions were controlled by forced oxidation wet scrubbers. The scrubbers consisted of two countercurrent–flow absorber vessels on each unit, using limestone slurry sprays to remove more than 92% of SO2 from the flue gas. The three scrubbers in total consumed about 24 MW of power, and used 130,000 tons of limestone and of water per year, producing 200,000 tons of gypsum per year.
Prior to installation of the scrubbers, SO2 emissions were about 71,000 tons per year.
Nitrogen oxide emissions were controlled in the combustion process by the use of low-NOx SOFA burners. Prior to 2009 when installation of the new burners began, NOx emissions were about 34,000 tons per year. The new burners reduced NOx emissions by about 14,000 tons per year, or more than 40%.
Fine particulate measuring 2.5 micrometers or less (PM2.5), of concern for its potential effect on health and visibility, primarily results from reactions of SO2 and NOx in the atmosphere to form sulfate and nitrate aerosols. NGS combined permit limits for these precursors has been , lower than 94% of all U.S. coal steam units, while the actual NGS rate in 2011 was .
Carbon dioxide (CO2) emissions from NGS were seventh highest of all U.S facilities in 2015, largely a result of the amount of energy it produced. In the same year, however, its CO2 emissions per unit of energy generated were lower than 75% of all U.S. coal fired power plants. The plant's low CO2 emissions per electric output, compared to other coal fired plants, were attributable to a relatively low heat rate combined with exclusive use of bituminous coal, which creates less CO2 per heat output than other types of coal.
Mercury emissions in 2011 were or ().
Air quality effects.
Northern Arizona and the Colorado Plateau have consistently met National Ambient Air Quality Standards (NAAQS) established to protect public health.
The EPA's Air Quality Index (AQI), showing air quality on a daily basis, lists no unhealthy days for the general population in all of northern Arizona and southern Utah counties. Unhealthy days for ozone-sensitive groups – those with asthma or lung disease – are also rare in these counties despite high natural background levels in the Intermountain West. Coconino County, with the highest occurrences, averages less than two unhealthy days per year, these occurring between March and June, suggesting a correlation with spring stratospheric ozone intrusions. NGS emissions are unlikely to have contributed to high ozone occurrences, as the plant is located at the northern border of Coconino County, and prevailing southwesterly winds in the spring blow toward counties to the north and east, which have not reported any days with unhealthy ozone levels.
Moreover, Coconino County's two ozone monitoring sites – Grand Canyon Hance Camp and Page – show significant differences within the county (the sites being 70 miles apart). During periods when ozone readings have been highest, peak concentrations in Page were noted to be more than 10 percent lower than those at the Grand Canyon.
Nitrogen dioxide (NO2) levels in the Page area average about 3 ppb, 94 percent lower than the NAAQS standard of 53 ppb. Carbon monoxide (CO) levels have also been far below the standards.
Fine particulate (PM2.5) levels in the Grand Canyon region have been among the lowest in the nation since before NGS completed installation of SO2 scrubbers and low NOx-SOFA burners.
Annual mean levels for PM2.5 in the Page area are about 3 micrograms/cubic meter (μg/m3) – one-quarter of the NAAQS standard of 12 μg/m3 and as low or lower than the cleanest U.S. cities listed by the World Health Organization.
Visibility in the Grand Canyon region has also been among the best in the nation since before NGS installed scrubbers and low NOx-SOFA burners.
Within Coconino County, visibility is generally better in the Page/Glen Canyon area at the north end of the county than at the Grand Canyon and areas further south (Sunset Crater, Walnut Canyon, Wupatki National Monuments). Visibility in the Page/Glen Canyon area averages 3.5 deciviews above natural background conditions, within less than one deciview of the best visibility listed in the contiguous U.S. (The DV scale is near zero for a pristine atmosphere, with a change of one deciview being a just noticeable change.)
In 2012, a fish consumption advisory was issued for striped bass in lower Lake Powell, raising concerns about mercury emissions from the plant.