A revised air-quality permit allows Tucson Electric Power to install an array of smaller natural-gas engines at its H. Wilson Sundt Generating Station, above. The utility says the generators will result in a net reduction in most types of emissions.

Pima County has issued a revised air-quality permit allowing Tucson Electric Power Co. to install 10 natural-gas engines at its H. Wilson Sundt Generating Station on West Irvington Road, despite opposition from environmentalists.

The permit from the Pima County Department of Environmental Quality clears the way for TEP to remove two large natural-gas turbine generators and replace them with 10 gas-fired reciprocating internal combustion engines.

TEP says the new array of smaller generators will fire up more quickly than the current generators to meet peak power needs and will be used to help manage the intermittency of renewable solar and wind power on its grid.

Based on planned operations, TEP says the bank of gas engines will result in a net reduction in most types of emissions except for nitrogen oxides, and that increase would be within levels allowed by federal rules for new emission sources.

But the Sierra Club and other public commenters opposed the revised permit, saying the new gas engines will make Tucson’s air worse and jeopardize public health, and that TEP should add more renewable-energy sources instead.

“The air quality in Tucson has been terrible this summer, and because of this decision the pollution will get worse,” said Sandy Bahr, director of the Sierra Club’s Grand Canyon Chapter.

The Tucson area violated the federal standard for ozone for the first time on Aug. 2 when a high-pressure system trapped pollution over the city. Nitrogen oxides are contributors to ozone.

“Building more polluting power plants in Tucson is the last thing that should be done, especially when we could be building affordable clean energy infrastructure that takes advantage of all the free sunlight across this region,” Bahr said.

The Sierra Club and other commenters also challenged TEP’s plan on the basis of “economic justice” since low-income and minority residents in the area of Sundt are most affected by the plant’s emissions.

TEP spokesman Joe Barrios said the gas engines, together with major new renewable-energy and battery systems, will help the utility make renewables more efficient and the system more reliable by providing quick peak power when needed.

TEP added two 10-megawatt battery systems last year, including one linked to a 2MW solar array, and by the end of 2020 the utility plans to build a 30-megawatt battery system paired with a 100MW solar array, along with a 100- to 150MW wind project that could include storage.

“When all these projects are combined, they will complement each other,” Barrios said. “The goal is to bring more renewable energy online while maintaining reliability for customers.”

The utility stopped burning coal at the Sundt plant in 2015, two years earlier than planned, citing the lower cost of natural gas. The Sierra Club and other environmental groups had waged a campaign including street protests demanding that TEP stop using coal at Sundt.

After a series of public meetings, the PDEQ made some changes to the air permit in response to comments from the Sierra Club and local residents, including requiring that the old steam generators are shut down before the new engines are fired up, and requiring monthly and annual emissions reporting.

The PDEQ also noted that the new permit requires the new gas engines to be fitted with pollution-control devices, known as selective catalytic reduction systems, as an additional control for nitrogen oxides.

TEP also agreed to a lower cap on nitrogen-oxide emissions.

The utility’s initial application capped the net additional nitrogen-oxide emissions from the new plant at 39.4 tons per year, just shy of a threshold of 40 tons per year that would trigger additional review under EPA rules addressing “prevention of significant deterioration” of air quality.

In February, TEP acknowledged concerns over the new plant’s nitrogen-oxide emissions and formally asked PDEQ to reduce the cap on additional emissions to 30.4 tons per year.

Anyone who commented on TEP’s permit may file an appeal with the EPA Environmental Appeals Board within 30 days of the PDEQ’s issuance of the revised permit on Aug. 8, which would halt the permit pending a ruling.

The board may uphold the permit, modify the permit, or remand it back to PDEQ and EPA to add additional provisions.

Barrios said TEP already has received required approvals from the Arizona Corporation Commission and zoning approval from the city of Tucson.

A contractor has submitted the design of a new plant building to house the new engines at Sundt but is still awaiting building permits, he said.

Barring an appeal or other major delay, Barrios said TEP expects the massive, 18-cylinder gas engines to be delivered by Finnish supplier Wartsila in October and be up and running by the end of 2019.

TEP says the generation part of the project will cost an estimated $160 million, plus about $30 million to relocate a substation and upgrade transmission lines on the Sundt property.

Contact senior reporter David Wichner at dwichner@tucson.com or 573-4181. On Twitter: @dwichner. On Facebook: Facebook.com/DailyStarBiz

Senior reporter covering business and technology for the Arizona Daily Star/Tucson.com