The plant, located in the city's business park, is currently allowed to emit 100 pounds of nitrogen oxide per hour during a maximum four-hour period when its turbines are firing up, under guidelines set by the California Energy Commission in 2003.

CHARLIE NEUMAN / Union-Tribune More smog-producing emissions could be released from the Palomar Energy plant, seen in October 2005.

SDG&E said the plant's annual nitrogen-oxide emissions will not exceed its limit of 124 tons. That yearly cap was recently increased from 104 tons after the company paid about $2 million as an “offset” cost intended to eliminate pollution sources elsewhere in San Diego County in exchange for increased emissions from Palomar.

Nitrogen oxide results in ozone when it reacts with other compounds in air and sunlight. To put 225 pounds in perspective, in 2005, power plants and boilers in San Diego County produced an average of 2.6 tons of nitrogen oxide per day, according to the state Air Resources Board. That year, county skies absorbed an average of 72,000 tons of the gas, which is also produced by vehicles and numerous other sources.

Just how much additional nitrogen oxide could be released in the air over Escondido if SDG&E's petition is granted is unclear. It depends on how often SDG&E starts the plant, said Michael Lake, associate director of the county Air Pollution Control District.

“It would make a big difference if the plant is started up one or two times a year, as opposed to 50 or 60 times,” Lake said.

Lake said SDG&E is not alone in the state to request relaxing caps for nitrogen-oxide emissions for plants that require a longer startup period compared to older ones.

Palomar Energy's Escondido plant is one of numerous plants approved in California after the energy crisis of 2000-2001 that use a new and more complicated energy-saving technology. These plants take longer to rev up their turbines to operating speed, Lake said. They are unable to meet air pollution control standards of older plants, which take two to four hours to start up, depending on how long they have been idle.

The Palomar plant recycles energy by using natural gas to power two turbines and generators. Excess heat from those is then captured and used to produce steam to turn a third turbine and generator. As a result, the plant produces 45 percent more energy than older plants and significantly less pollution, according to SDG&E officials.

“It's a longer process, but a cleaner process,” Van Herik said.

The longer startup, however, produces more nitrogen oxide. About two-thirds of the plant's annual nitrogen-oxide emissions will occur during the plant's startup periods, Jim McCrank, a Sempra vice president, said in the air pollution district's report.

However, thanks to the plant's new technology, the overall amount of emissions, compared to power plants built 20 years ago, is significant. For example, with air pollution control standards in place, most existing power plants emit 20 parts-per-million of nitrogen oxide, Lake said. Newer plants including Palomar emit two parts per million.

During the past six months, as the Palomar plant was being tested, Sempra requested that the Air Pollution Control District grant seven “variance reliefs,” which means the company was not meeting emission standards outlined in its state permit. The district is not alarmed, Lake said. “You can't bring that kind of large equipment in total compliance at get-go,” he said.