The proposal is exercisable within 48 months, starting from 10 October, and includes an option to lease an area for a construction compound.
Lease of the site will be subject to planning permission.
The site has quayside access and benefits from a commercial agreement for port support from the Bristol Port Company.
John Seed, managing director of Helius Energy, said: ‘We are delighted to have secured this highly desirable site for a proposed 100MWe biomass power plant following last month’s sale of our 65MWe Stallingborough project.
‘Helius Energy remains committed to becoming a leading developer and operator of dedicated biomass power plants.’
Helius will be involved in the construction, implementation and initial phase of the project, with RWE Innogy investing approximately £205m in the development of the new plant.