"I don't necessarily want to burn coal during this time, but we have the 20-day option to appeal, so we will," he said.

He said he agreed to the state's ultimate demand that the coal-burning unit be retrofitted to use a different fuel or else abandoned by the end of 2010.

But he said he took issue with the results of a July emissions test, which found that particulate emissions - which show as a "plume" from the plant's stack - were too high.

That result had prompted the state's urgent deadline.

"We are appealing the use of that result," Isabella said, "because we found some deterioration of ductwork in the system."

That deterioration, he said, was caused by "sulphuric-acid-type conditions" caused by coal that had corroded the workings of the system, and had affected the result. He said the ductwork had been fixed, and the plume-emissions test was redone Thursday. Results are pending.

The city's appeal was not standing in the way of negotiations between utility staff and the state, Isabella said: "It's currently an ongoing discussion."

The state's recent deadline had come as Isabella seeks public support for building a new oil- and gas-burning unit, which could replace the coal-burning one.

Choromanski's letter abruptly brought forward the formal deadline for ditching coal, which had previously been stated as late 2012.

E-mail Juliet Fletcher:

JFletcher@pressofac.com