They don't need to go further into it as they state the answer right there in the article: We add fuel (our fuel is 97% natural gas). We only use fuel oil in th

They don't need to go further into it as they state the answer right there in the article: We add fuel (our fuel is 97% natural gas). We only use fuel oil in the dead of winter if there’s a natural gas shortage or emergency, otherwise we are natural gas all the time In other words they currently use gas and a bit of oil.