An Ameren Corp. subsidiary plans to appeal a U.S. district court decision finding it violated the Clean Air Act for modifying a Missouri coal plant without the necessary permits.

Michael Moehn, chairman and president of Ameren Missouri, said, "We disagree with the ruling and believe the court misapplied the law and ignored the evidence presented at trial" and that it plans to "appeal at the appropriate time," according to an emailed statement.

In a Jan. 23 opinion, U.S. District Judge Rodney Sippel found that Ameren Missouri, known legally as Union Electric Co., had violated the Clean Air Act when it made boiler upgrades to Rush Island unit 1 in 2007 and unit 2 in 2010 without obtaining a preconstruction permit. The act's prevention of significant deterioration provisions require sources including power plants to obtain a permit and install state-of-the-art pollution controls when making major modifications to prevent air pollution increases.

Moehn defended the plant's progress in reducing SO2 emissions, which was a focus of discussions in the case.

"The Rush Island Energy Center is among the most efficient plants in the country and burns some of the lowest-sulfur coal available, and SO2 emissions are more than a third lower than when the projects at issue were performed many years ago. Air quality in and around the Rush Island facility is 64% below the current federal mandated level," Moehn said.

U.S. Department of Justice attorneys, which represented the U.S. Environmental Protection Agency, had argued that the modifications done at the two units resulted in significant net emission increases, triggering the need to upgrade SO2 controls. Both units are equipped with electrostatic precipitators and burn low-sulfur coal from Wyoming as means to lower SO2 emissions, S&P Global Market Intelligence data show.

The court found that the modifications do not qualify as routine maintenance, which have exempted it from needing a permit and installing start-of-the-art emissions controls, as required by the EPA. The court also found that Ameren failed to prove to that the projects were "necessary to respond" to increased electricity demand, according to the opinion, which cited Ameren's 2011 Corporate Social Responsibility Report.

"To the extent that system demand was growing, Ameren did not offer any evidence at trial to show how changes in system demand, if any, would or did specifically impact the operation of and emissions from the Rush Island units," Sippel said in the opinion.

The opinion mentions a status conference to be held Feb. 15, 2017. The court granted a motion to hold Ameren Missouri liable under the Clean Air Act. It also denied four motions from Ameren Missouri's attorneys and one motion from U.S. DOJ attorneys. The Court denied a motion from U.S. DOJ attorneys to exclude certain evidence from Ameren because they claimed the evidence was irrelevant. The court also denied a motion from Ameren to strike certain testimony from two experts called on by the prosecution that showed emissions increases from the modifications at the units.