DULUTH, Minn. (AP) -- Minnesota Power says it shouldn't be rushed into closing two older, smaller coal-fired power plants, saying more research is needed on the environmental benefits and economic impacts.

The Duluth-based utility filed its comments Monday with the Minnesota Public Utilities Commission.

Minnesota Power is asking the PUC for more time for a more comprehensive study to weigh the costs of closing the plants against keeping them open by switching them to natural gas or adding new pollution controls so they can keep burning coal.

The plants are the Laskin Energy Center in Hoyt Lakes and the Taconite Harbor Energy Center on the North Shore.

The Duluth News Tribune reports environmental groups and the state Department of Commerce have called for closing the plants and switching to cleaner or cheaper energy sources.