A mainland clean energy company that owns 25 percent of Aiea-based ClearFuels Technology Inc. said it plans to increase its ownership in the company to a majority stake.

Los Angeles-based Rentech Inc. said yesterday it will merge one of its subsidiaries into ClearFuels, with the latter continuing as the surviving company. The transaction is the result of an option agreement signed by the two companies in September 2010.

Rentech said the merger is expected to close within a month. The companies said they expect no interruption of the project and technology development activities of ClearFuels.

Privately-held ClearFuels is developing potential biomass-to-energy projects in Hawaii, the Southeastern U.S., and internationally. ClearFuels is in active negotiations with Hawaiian Electric Co. for a long-term contract to supply biodiesel for energy generation at the utility.

Rentech’s shares closed down 1 cent at $1.14 yesterday on the American Stock Exchange.