The company, Constellation Energy, says it is an issue of fairness. A little more than two years ago, it completed an $885 million installation that has vastly reduced emissions from two giant coal-burning units at its Brandon Shores plant here, within view of the city’s downtown office towers.

The goal was to comply with a Maryland law, but the company also anticipated that the federal Environmental Protection Agency would adopt similar limits. The agency followed through last year, completing a rule on sulfur and nitrogen emissions that was due to take effect on Sunday.

But last Friday, a three-judge panel of the United States Court of Appeals for the District of Columbia issued a stay of the regulations, ceding to challenges filed by several major coal-burning utilities, the State of Texas, the National Mining Association and the International Brotherhood of Electrical Workers. They argued that the deadline was draconian, among other objections.

The court said it hoped to hold a hearing on the case in April.

Having invested the $885 million — nearly as much as it cost to build the two generating units in 1984 and 1991 — Constellation argues that laggard plants should also have to comply with the emission limits or shut down. Otherwise, it argues, the utility will be operating at a big disadvantage: simply running the retrofitted plant requires 40 megawatts of electricity, enough to keep a small town humming.

“When we started making plans for this project, we did it with the expectation that there would be a federal regime, and we still have that expectation,” said Paul Allen, the company’s vice president for environmental compliance.

On a press tour of the plant on Thursday, engineers showed off the extensive new construction, including the replacement of twin 700-foot smokestacks with a new one that belches steam produced in the process of scrubbing out acid gases. The old stacks, still standing but capped, are now “hood ornaments,” said Heather Lentz, the general supervisor of operations.

“It’s a premier clean coal plant,” she said.

Depending on the demand for electricity, barges bring anywhere from two million to three million tons of coal up the Patapsco River each year for burning at Brandon Shores. With the new technology, the coal-burning produces 90 percent less nitrogen oxide, an ingredient of smog; 95 percent less sulfur, which causes acid rain; and vastly lower fractions of other pollutants.

The rule that was to come into effect on Jan. 1, known as the Cross State Air Pollution Rule, is intended to address the longstanding inability of some states to meet federal air pollution standards because of contaminants that blow in from other states, mostly from power plants.

Before they were cleaned up, Brandon Shores and five other coal-burning plants in Maryland were part of the problem: the state’s nurses association commissioned a study that found that in 2006, emissions from the plants caused 700 deaths per year nationwide, including 100 in Maryland.

Pointing out that it took only three years to install the scrubbing technology, completing construction in 2009, Constellation argues that other utilities could have been getting ready, too.

Its criticism of other utilities is part of “a very clear, longstanding split” between companies that made the leap and those that deferred the investment or even challenged the rules in court, said John Walke, a coal expert at the Natural Resources Defense Council. But the laggards, he said, should have seen it coming.

Constellation’s competitors see it differently, saying that they cannot build for rules that do not yet exist. American Electric Power, one of the utilities that sued to block the new rule, knows how long it probably takes to build a scrubber, said Pat Hemlepp, a spokesman, because it has already built 25 of them at plants it operates.

But for companies in traditionally regulated states like most of those where A.E.P. operates, the utility needs authorization from a public service commission to collect the cost from ratepayers, which takes about a year, in addition to time for engineering work and obtaining numerous permits, he said.

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