About 160,000 tons of coal are now stacked up at Tucson Electric Power’s H. Wilson Sundt Generating Station on Irvington — enough for four to five months of coal burning.

Kelly Presnell / Arizona Daily Star

A deal that was in the works for months is now official: The last coal-fired unit at Tucson Electric Power’s south-side power plant will run totally on natural gas by the end of 2017.

But plans are much murkier for the short-term use of coal vs. gas at the Sundt Generating Station’s Unit 4, on East Irvington Road just east of Alvernon Way, which the EPA says has no pollution controls.

About 160,000 tons of coal are now stacked up on the plant’s east side — enough for four to five months of coal burning. Right now, the plant’s not burning coal, although it was for most of the year until June 1.

With environmentalists continuing to push for a coal-free plant, TEP says it may or may not use coal off and on until 2017.

The Environmental Protection Agency has signed off on the plan to get Sundt Unit 4 off coal by the end of 2017. The plan will soon be published in the Federal Register.

TEP says the prices of coal and gas will be the key factor in what Unit 4 burns until then.

“That’s the short of it — cost is the primary driver between coal and gas ... because the cost (of either) is passed along to the customer,” TEP spokesman Joe Barrios said this week. “The Sundt plant is an important part of our delivering service to our customers. We have to keep it running for reliability purposes. We’ve looked at the issue from many different angles and heard from many stakeholders in the community.

“Is it possible that we would stop using coal before the end of 2017? Yes, but right now under the plan using coal remains an option.”

The utility will be under pressure from environmental groups to stop burning coal sooner, however, out of concern for sulfur-dioxide and carbon-dioxide emissions and potential water pollution associated with coal burning. Sulfur dioxide is linked to respiratory problems in people.

Among those advocating a permanent halt to coal use is the Sierra Club, which would prefer that the plant ultimately switch to renewable energy although it prefers gas to coal.

“We think that Tucson Electric Power working with EPA to ban coal combustion in Tucson is a great step forward. In terms of addressing the problems with water pollution and air pollution and public health problems, it’s the most cost-effective short-term solution,” said Dan Millis, the Sierra Club’s Beyond Coal program coordinator.

“We feel that TEP should embrace it sooner, by pledging to their fellow members of the community not to burn any more coal in the meantime. If they want to burn their stockpile up, we can’t stop them from doing that but they shouldn’t purchase any more coal,” Millis said.

The entire Sundt plant furnishes about 21 percent of TEP’s total, internal power-generating capacity, not counting what the utility buys from outside power sources. Unit 4, which went online in 1967, provides about 7 percent of the utility’s total capacity.

Unit 4’s switch from coal to gas this year continues a longstanding pattern, dating to 2009, in response to recurring price changes for both commodities, particularly gas.

Generally, natural gas prices fell from about 2008 into early 2012, then rose until early 2014, when they began to drop once more, data from the federal Energy Information Administration show.

Sometime this fall or winter, TEP may switch Unit 4 back to coal because its officials expect natural gas’ cost to rise as winter approaches, Barrios said. But if the price of gas drops in that period, the utility may burn coal only for three months, then return to gas, he said.

“Or we may never switch to coal in the next six to nine months, if (natural gas’) cost stays low,” Barrios said.

Millis says financial costs aren’t the only numbers TEP should be considering. He cites EPA documents showing that Sundt Unit 4 would emit about 1,000 pounds of sulfur dioxide an hour if it keeps burning coal, compared to 1.9 pounds per hour for natural gas.

Coal also produces significantly more greenhouse-gas emissions than gas, although TEP’s total CO2 emissions — not just from the Sundt plant — have dropped 15 percent from 2000 to 2012.

Millis also says that mercury produced by coal-fired power plants can pollute waters. At the same time, he says, natural gas causes water pollution problems from fracking, the practice of injecting fluids into shale beds at high pressure to free up gas or oil. Industry officials dispute that fracking causes pollution — an issue being debated from coast to coast.

Contact reporter Tony Davis at tdavis@azstarnet.com or 806-7746.