The Environmental Protection Authority (EPA) has given Worsley Alumina approval to increase annual production from 3.7 million tonnes of alumina each year to 4.4Mtpa by building and operating two gas-fired cogeneration units.

The agency is still considering a separate Worsley Alumina proposal to use coal-fired cogeneration to drive the expansion.

But EPA chairman Dr Wally Cox said natural gas would provide a better result for the environment.

"The natural gas fired cogeneration option provides the best environmental outcome of the options being considered by Worsley for their expansion," Cox said.

"Significant energy efficiencies will be achieved by using the recovered heat from power generation in the alumina refining process.

"Also the gas alternative is predicted to result in a significant reduction in emissions of sulphur dioxide, particulates and carbon monoxide. By substituting natural gas for coal in the mixture of fuels used to generate energy, emissions of greenhouse gas per unit of alumina produced will decrease."

The EPA has said it wants additional gas turbine and heat recovery systems to be installed to meet the energy needs for this higher level of production. Surplus energy will be delivered to Western Australia’s public energy grid.