The Port of Seattle is attempting to make Seattle-Tacoma International Airport even more environmentally friendly with a new proposal to make it the first U.S. airport entirely heated by renewable natural gas.

The Port put out a request for proposals last month, which call for renewable natural gas service for the airport’s boilers and bus fueling system. These two areas account for more than 80 percent of the port’s emissions. If the plan goes through, it will replace all of the current fossil natural gas used.

Renewable natural gas is also known as biomethane. It’s a natural byproduct of the decomposition of organic matter, typically produced by landfills, wastewater treatment plants, and food and animal waste digesters.

“The Port can play a major role in creating a renewable natural gas market because we offer a stable, long-term use of gas.” said Arlyn Purcell, director of aviation environment and sustainability, Port of Seattle. “If we can attract a project developer to supply the airport, this will spur more opportunities to feed the current gas pipeline with RNG rather than have landfills or digesters flare the gas on-site or allowing their methane emissions to escape into the air.”

The Port expects to award a contract by late 2019.