The Cholla coal-fired power plant is located in the Joseph City, Navajo Country, Arizona, US. Image courtesy of Alan Stark.
Unit two of the Cholla power plant was closed in 2015. Image courtesy of Travis Nep Smith.
The Cholla power plant is operated by Arizona Public Service (APS). Image courtesy of Alan Levine.

The Cholla power plant is a coal-fired power station located near Joseph City, in Navajo County, Arizona, US, which has been operating since 1962.

The facility was originally developed with four coal-fired units with a combined installed capacity of 1.02GW. The plant’s current generating capacity, however, stands at 767MW following the retirement of the 260MW unit two in 2015.

The Cholla power station is operated by Arizona Public Service (APS), a wholly-owned subsidiary of Pinnacle West Capital Corporation.

APS is also the owner of the first three units of the plant including the 116MW unit one and the 271MW unit three, while the 380MW unit four is owned by PacifiCorp.

The unit four is planned to be shut down permanently by the end of 2020, while units one and three will continue to be operational until 2025.

Cholla power plant development details

Construction on the Cholla power plant was started in 1961, while the unit one was commissioned in 1962. Units two, three and four were commissioned in 1978, 1980 and 1981, respectively.

In January 2015, APS and PacifiCorp agreed to close down the unit two by April 2016, install low-NOx burners and separated over-fire air (SOFA) for units three and four, and stop burning coal in any of the units by April 2025. The decision was taken in compliance with the state implementation plan (SIP) under the Clean Air Act (CAA) and the Regional Haze Rule (RHR) mandated by the US Environmental Protection Agency (EPA).

The unit two was permanently shut down in October 2015. APS completed a feasibility study to convert unit one to run on biomass in May 2019. If approved by the Arizona Corporation Commission, the biomass plant involving an estimated investment of £160m ($205m) can generate 84MW of electricity as early as 2022.

In January 2020, PacifiCorp announced its plan to close unit four by the end of 2020, citing the reason that coal-based power generation is no longer economical for its customers since electricity from alternative sources is available to them at a cheaper price.

Cholla power plant make-up

The Cholla power plant features an on-site coal handling facility including a coal preparation plant from where coal is fed into the pulverised coal-fired boiler of each operating unit of the plant. The steam generated by each boiler drives a steam turbine for electricity generation.

The power plant has a total of three flue stacks. Unit one has a tray tower scrubber with lime reagent for the removal of sulphur dioxide (SO2), while units three and four have spray tower scrubbers for controlling SO2 emissions.

All units of the plant are also fitted with fabric filters for controlling particulate matter emissions.

Infrastructure facilities

The other infrastructure facilities at the Cholla power station include three on-site process water impoundments, including the Cholla Lake, a sedimentation pond, and the West Area Retention Pond.

The Cholla Lake is used as the cooling pond for the power plant, while the sedimentation pond collects wastewater discharges and the West Area Retention Pond is used for collecting surface drainage.

The plant utilises a fly ash pond and a bottom ash pond, which are both located off-site. The 24m-high, 1.4km-long and 7.3m-wide fly ash pond occupies 420 acres, providing more than 22 million cubic metres of storage volume. The 22m-high, 1.28km-long and 3.6m-wide bottom ash pond occupies 80 acres and provides more than 2.8 million cubic metres of storage volume.

Power transmission

The electricity generated by the Cholla power plant is evacuated via an on-site 500kV switchyard. The electricity from the Cholla switchyard is transmitted through two 345kV lines connecting Pinnacle Peak and a 500kV line connecting Saguaro.

The Cholla switchyard is also interconnected by two 345kV transmission lines from the Four Corners power plant and a 500kV line from the Coronado power station.

Coal supply

The Cholla power plant burns up to 3.5 million tonnes (Mt) of coal a year, which was being supplied from the McKinely coal mine owned by Chevron near Gallup, New Mexico.

Following the closure of the McKinely mine in 2010, the power plant has been sourcing coal mainly from the Lee Ranch and El Segundo coal mines near Grants in New Mexico, which are owned by an affiliate of Peabody Energy.

Carbon capture initiative at the Cholla power plant

The US Department of Energy (DoE) awarded a $70.5m (£54.5m) grant under the American Recovery and Reinvestment Act (ARRA) to APS for an algae-based carbon mitigation demonstration project in 2009.

The demonstration project was intended to produce biofuel by feeding an algae pond with the carbon captured from the coal-fired facility.

Contractors involved

APTIM provides power maintenance services for the Cholla power plant.

Cholla Environmental Partners (CEP), a joint venture involving Burns & McDonnell and Zachry Construction, completed an engineering, procurement, and construction (EPC) contract for a turnkey emission control project for the power plant in 2009.

The scope of the contract included the replacement of an electrostatic precipitator (ESP) with a low-pressure, high-volume fabric filter (FF), as well as the replacement of the ash handling system of unit four. CEP also installed a FF for unit three and retrofitted a new wet lime flue gas desulfurization (FGD) system at the plant.

Babcock Power Environmental was subcontracted by CEP to provide the FGD system for the plant.

GEI Consultants was subcontracted by Lockheed-Martin, the contractor to EPA, for conducting site assessment of the coal ash impoundment facilities for the Cholla power plant in 2009.

Simpson Gumpertz & Heger (SGH) was selected by APS to assess the condition and develop repairs for the circulating water pipelines at the Cholla power plant. SGH completed the work in 2012.

Research-Cottrell installed the FGD unit for unit one in 1973, while the unit one boiler was designed and supplied by Combustion Engineering.

Electric Bond and Share Company (EBASCO), a company founded by General electric, provided the plant design and was engaged as a major construction contractor for the Cholla coal-fired facility. It also designed and constructed the dams for the fly-ash and bottom-ash ponds of the plant.