MELBOURNE, Nov 17 (Reuters) - Fortescue Metals Group said on Tuesday it was assessing prospects to build an ammonia plant in the southern island state of Tasmania, as part of its push to become a major renewable energy producer.

The project would include a 250-megawatt hydrogen plant at the Bell Bay Industrial Precinct with green ammonia production capacity of 250,000 tonnes per year for domestic and international export, Fortescue said in a statement.

Bell Bay is about 41 km (26 miles) northwest of the city of Launceston.

The project is targeted for an investment decision by the Fortescue Board in 2021, the company said.

Fortescue founder and chairman, billionaire Andrew Forrest last week outlined plans for the miner's wholly owned Fortescue Future Industries (FFI) unit to build a renewable energy business, aiming to compete with oil companies to provide low-cost green energy globally.

Already used in the fertiliser industry, ammonia is expected to play a role as a clean power source, potentially as a chemical store for excess renewable energy, and in the shipping industry as a fuel that only emits water and nitrogen.

One way of making green ammonia is through using hydrogen from water electrolysis and nitrogen separated from the air in a process powered by renewable electricity.

The plant has the potential to be one of the world's biggest green hydrogen projects and will be powered by Tasmanian renewable energy, Fortescue said.

FFI's plans to have 235 gigawatts (GW) of installed energy capacity although did not provide a timeline. (Reporting by Melanie Burton; editing by Christian Schmollinger)