The latest trial to get the green light at the steelworks could result in 400 tonnes a week of plastic being pushed into the coke ovens, while reducing the use of coal, the steelmaker said.

The Environment Protection Authority has approved a licence variation for the trial, which would involve adding 1 per cent by mass of polyethylene and polypropylene to coal to be burned into coke.

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BlueScope has trialled this before in 2020 but wants to triple the amount of plastic used in the next phase, adding it to three charge holes in an oven rather than one. The EPA said the previous trial results "identified no observable changes to the coking process, coke quality, or fugitive emissions".

The company said the coke ovens were "well monitored" and it is confident the trial will not cause more pollution. The EPA said BlueScope believes it could reduce greenhouse gas emissions by using plastic.

"The coke making process does not burn coal and so will not burn plastic," a BlueScope spokesman told the Mercury.

"Coke ovens use the process of pyrolysis, that is, heat in the absence of air, to thermally decompose coal.

"The coke making process converts coal to coke plus other valuable by-products like coke ovens gas, tar, and light oil."

Polypropylene is a common plastic used in everyday items including packaging, toys, cars and utensils, as is polyethylene, used largely in packaging.

The spokesman said the trial would test whether the coke ovens gas, which is collected for use elsewhere, maintained a similar composition.

The steelmaker has been embarking on a number of projects which could make minor, or larger, changes to production at Port Kembla, including trials of charcoal in the blast furnace to replace some coal, and adapting factory lines to produce wind turbine towers and steel to suit defence industries.

The EPA's conditions for the plastics trial state that BlueScope must make sure it takes what practical steps are necessary to prevent or minimise air pollution.