Duke Energy says the cost of the coal-gasification power plant it's building in southwestern Indiana has risen by $530 million, boosting the project's estimated price to nearly $2.9 billion — or about twice the original estimate.

The company submitted paperwork explaining the cost increase Friday with state utility regulators who must approve Duke's request to pass onto customers the higher cost for the plant being built near Edwardsport.

The plant's opponents have said Duke's original plant cost estimate of between $1.3 billion and $1.6 billion was low and that the final price would be considerably higher.

Duke told regulators last fall that construction costs for the 630-megawatt plant expected to go online in 2012 were rising significantly as engineering on the project progressed.