Federal approval for a pair of power-generating turbines in the Mississippi River near Vicksburg will take a bit longer than expected due to ongoing study, according to developers.

Boston-based Free Flow Power has applied to the Federal Energy Regulatory Commission for licenses to place the large propellers at 23 sites in Mississippi River between Kentucky and Louisiana. The process is in a pre-application stage of conducting studies, said Jon Guidroz, director of project development.

“I believe a prudent projection for FERC licenses would be toward the end of 2014 or beginning of 2015,” Guidroz said.

Potential environmental impact and a progress report on the firm’s application is the subject of meetings over three days, starting Wednesday in Memphis. A second begins at 9 a.m. Thursday in meeting room 2 at Vicksburg Convention Center. A third is Friday in New Orleans.

When first proposed, the projects were to be licensed this year. A demonstration at the Mississippi Welcome Center scheduled last year was put on hold when the river flooded.

Two sites in Warren County were identified in the firm’s preliminary plans, one north near the Brunswick community and another south of Davis Island.

Sites on the river were chosen based on flow volume and velocity, and proximity to power transmission facilities and potential customers. At each site, turbines on pylons at the bottom of the river would spin like propellers and transmit energy to the river bank.

Each turbine would produce about 40 kilowatts of power, or about the same as a gasoline or diesel-powered home generator. Pilot projects for turbine sites planned by Free Flow and other hydropower companies were funded heavily by green technology grants in the 2009 federal stimulus legislation.