Ameren and the alliance submitted preliminary cost and design reports to the Department of Energy late this summer, pegging the total cost of the project at $1.65 billion.
But St. Louis-based Ameren, which in recent weeks has taken various steps to prune capital expenditures and operating costs, decided not to continue to participate in the development beyond the end of December, the alliance said in a news release.
FutureGen was initially conceived nearly a decade ago. But the first iteration of the project was scrapped in 2008 after years of rising costs and political wrangling.
Last summer, Energy Secretary Steven Chu announced that Meredosia was selected as the centerpiece of a revamped FutureGen program to demonstrate the potential of capturing greenhouse gas emissions from coal-fueled power plants.
Ameren was a partner in the project with the FutureGen Alliance, a consortium of some of the world's largest coal producers.
The alliance said it will seek the Energy Department's approval of whatever agreement is ultimately reached with Ameren.
While Ameren won't be a partner in the project anymore, the company will continue to help the alliance obtain permits and maintain the plant so it can be retrofitted as planned.
"Because this project is important to the state and Morgan County economies and America's energy future, we have pledged our continued support through the ongoing development phase," the company said in a statement.
The updated cost estimate includes $1.1 billion to repower an oil-fired generating unit at Meredosia and $550 million for a carbon dioxide pipeline and underground storage.
The alliance also said it has identified several hundred million dollars in "potential cost reduction opportunities" that will be studied in the coming months.
"Contingent on DOE's approval, we have an opportunity to lead the entire program, build on potential cost savings, and bring the power plant on-line in 2016 as planned," Ken Humphreys, the alliance's chief executive, said in a statement.