SEPTA's natural gas power generator in Nicetown is nearly finished.

Construction of the $26.8 million facility is 80 percent complete, SEPTA officials said, and the generator, being built on the same property as the Midvale Bus Depot near the Roosevelt Expressway and West Hunting Park Avenue, is scheduled to begin operating in January.

Yet, in a small municipal hearing room at 16th and Arch Streets on Tuesday, the project's near-completion was no obstacle to fierce opposition from a coalition of environmental and community activists. They were before the Licenses and Inspections Review Board appealing an air permit the facility was granted — the latest fight in two years of opposition to the generator. The opponents have had little success stopping or even slowing the project but insist they will prevail.

>> READ MORE: SEPTA approves natural gas power plant

"We'll take the issue to the EPA," said Peter Winslow, a member of 350 Philadelphia, an environmental group that has led the fight. "We'll take the issue to City Council. This is not going to end."

The generator's opponents, which also include the Center for Returning Citizens and Neighbors Against Gas Plants, are framing their opposition as a public health issue.

Henry Cole, an environmental consultant, testified at Tuesday's hearing that the vehicles using the Midvale Bus Depot and the two large roads nearby have already created a pollution problem in a predominantly poor minority neighborhood. The generator produces the same kinds of pollutants, including nitrogen oxide and carbon monoxide, as contained in exhaust from vehicles. In a June op-ed, City Councilwoman Cindy Bass noted that the neighborhood next to the bus depot has among the highest rates of children suffering from asthma in the city.

Cole also brought up concerns about ultrafine particles created by burning natural gas. The particles are small enough to pass through bodily membranes and may have associations with  heart disease and respiratory problems. That issue is one of concern to workers, too, said Matt Wang, a member of 350 Philadelphia and SEPTA's Local 234.

SEPTA did environmental studies that found the plant would produce virtually no detectable contaminants and would lead to a 41 percent reduction of greenhouse gases compared with what's generated by burning oil at the site now. SEPTA already had an air permit for the site, and the additional material expected to be generated by the plant would be so minimal it would keep emissions within what is allowed by the original permit, said Erik Johanson, SEPTA's director of business innovation.

The science behind the harm caused by ultrafine particles is unclear. Many studies conclude only that more studies are needed to determine the particles' effect on human health.

The fierceness of the opposition could suggest this sort of generator is new to the city. That isn't the case. There are 22 combined heat and power generators that, like SEPTA's, use natural gas in the city, according to a federal Department of Energy database. They include generators for hospitals, hotels, and office buildings. Most are small, generating a few hundred kilowatts of energy.

SEPTA's, which would generate 8,800 kilowatts, would be the third-largest in the city but is dwarfed by the 163,000-kilowatt generator in Grays Ferry at the Schuylkill Station. Also much larger is a 14,000-kilowatt station at the Northeast Water Pollution Control Plant operated by O'Brien Environmental Energy on the 3800 block of Richmond Street. Both are near residential areas.

Statewide, Pennsylvania has 78 combined heat and power installations that burn natural gas.

For SEPTA officials, the opposition to the plant is puzzling. Combined heat and power generators are considered more environmentally friendly than other types of power plants.

"They're framing this in such a way that this is a health risk," Johanson said. "We think this is a good project."

After Tuesday's hearing, Winslow acknowledged that the Nicetown generator has led to so much opposition in part because of what might follow.

"It's a precursor for additional projects that are similar," he said. "This is the initial step in a program that will expand over time."

Natural gas-burning generators present a challenge to groups concerned about the environment. They are much cleaner than coal-burning generators, and Pennsylvania's embrace of natural gas has led to a net decrease in the greenhouse gas emissions that cause climate change. As coal use has declined dramatically in Pennsylvania and natural gas use has grown, the amount of carbon dioxide, a key greenhouse gas, generated in Pennsylvania by 2015 was 41.0 million metric tons lower than it was in 2003.

Still, reduced emissions is not the same as zero emissions, and only Texas and California generated more carbon dioxide emissions as a result of energy production than Pennsylvania as of 2015, according to the U.S. Energy Information Administration. Natural gas, particularly its extraction, still creates the kinds of emissions that worsen climate change, just less of them.

"Right now, Pennsylvania does not have a goal it is working toward as far as greenhouse gas reductions," said David Masur, head of PennEnvironment, an advocacy group not directly involved in opposition to the Nicetown facility. "Quite to the contrary, we just seem to be blithely indifferent to what we need to be doing to avoid disaster."

After two hours of testimony and cross examination of Cole, the L&I board called an end to the day's hearing. No decision was made on the effort to revoke the air permit for the Nicetown plant, and the activists who filled the room vowed they would be back when the hearing continues Oct. 16.

Meanwhile, construction on the generator carried on.