Environmental groups and residents are again challenging proposed changes to Alexandria's coal-burning power plant over concerns about pollution.

Virginia's Air Pollution Control Board scheduled a meeting Thursday to consider a plan by Mirant, the company that operates the plant. Mirant wants to start blowing the plant's exhaust higher into the atmosphere to disperse it over a greater distance.

Opponents have previously argued the plant either needs to lower its emissions with the latest technology called "scrubbers" or close. They have tried to use lawsuits and zoning rules to close the plant in the past.

Mirant says it operates within legal limits.

One new factor to consider is the plant is no longer needed to supply backup energy for an emergency in Washington. Two new lines built this summer now connect the city to the regional power grid.