NEW YORK, Jan 31 (Reuters) - A top investor in Green Plains Inc is calling for the biofuel producer to run a full sales process at a time when the company is facing increased risks and strategic buyers are eager to purchase renewable energy companies.

Ancora Holdings Group, which owns a nearly 7% stake and is Green Plains' second largest shareholder, sent a letter, seen by Reuters, to the board on Tuesday arguing a buyer would likely pay at least $50 a share for the company.

"Our analysis and diligence indicate that strategic buyers with considerable cash on their balance sheets could be interested in acquiring the Company at a significant premium to current trading prices," the letter said.

The company regularly reviews its strategy and ways to push up the share price, a spokeswoman said.

"We are confident in our ability to execute on our transformation plan, but we are always mindful of our alternatives," she added.

Green Plains' stock price climbed 13.9% to trade at $35.36 on Tuesday.

Thanks to its acquisition of Fluid Quip Technologies in 2021, Green Plains has new assets and technologies that are appealing to strategic buyers in the agricultural products and energy sectors, Ancora wrote.

The company has said it is working on having roughly 400 million pounds of renewable corn oil production capability by 2024. It is also in the process of commercializing its clean sugar ingredients, which could become a significant threat to industry leaders Archer-Daniels Midland, Ingredion, and Cargill.

"It is our belief that Green Plains' ties to the ethanol industry are masking the value of its strategic and highly competitive co-products," Ancora wrote.

Ancora noted its "positive dialogue" with the company and praised the board and management for moves that have helped Green Plains transition from an ethanol-dependent producer. The company said it appreciates its ongoing dialogue with Ancora and "seek(s) to maintain a constructive relationship with them."

But Ancora also warned that emerging risks, including reduced fuel consumption and political change are lurking and that "the macro environment presents a much greater challenge to Green Plains.

Faced with potential challenges, Ancora estimates Green Plains may only have a few months to act and should move decisively by hiring financial advisers who could run an organized auction process.

By running a comprehensive process, Ancora says Green Plains would be much more likely to catch the attention of potential buyers than by sitting back and waiting for someone to appear with an offer.

"Telling us that 'the Board is always open to fielding offers and exploring alternatives' is not a sufficient response," the letter said.

Ancora's hedge fund unit, led by James Chadwick, has become a top activist investor in recent years. In 2022, it secured board seats at C.H. Robinson Worldwide, Mueller Water and Berry Global while pushing auto salvage company IAA to sell itself. (Reporting by Svea Herbst-Bayliss; Editing by Lincoln Feast and Deepa Babington)