INDIANAPOLIS (AP) – An Indianapolis Power and Light Co. plant that has produced electricity for more than 80 years is preparing to burn its last load of coal next spring and switch to natural gas. 

The massive conversion will cost $70 million. It's a move that Indianapolis Power and Light says is the utility's best option to meet clean-air regulations.

The Indianapolis Star (http://indy.st/1d3s7Jj ) reports that natural gas will be used to drive turbine generators and generate electricity by heating water into steam.

The company says the conversion will tack on about $1 a month to the average customer's electric bill.

Indiana has outpaced the nation in relying on coal for decades. The state still uses coal-fired power plants to produce about 80 percent of its power.