Possible Clues:

• Macbeth's query about his status, it would seem, is a gas
• Gas produced by decomposition of vegetable matter in wet conditions
• Name the cooking gas
• Biofuel option
• Gas: average energy including couple of therms?
• Dairy farm product
• It's gas for me, next to many a character in Macbeth
• Change the name of a fuel
• Flammable gas represented in 18-Across and 9-Down
• Fuel gas