Which fossil fuel tends to produce the least carbon dioxide per unit of energy released?

• A. Coal
• B. Natural gas
• C. Oil
• D. Peat

Answer: (B)

The key idea is CO2 emitted per unit of energy. Fuels differ in how much carbon they hold relative to the energy they release: fuels with less carbon per unit energy burn cleaner in terms of CO2. Natural gas is mainly methane, which has a high hydrogen-to-carbon ratio, so when it’s burned it yields a lot of energy from hydrogen with relatively less carbon to form CO2. That makes its CO2 per unit energy smaller than the other fossil fuels. Coal contains a lot of carbon and less energy per unit mass, so it produces more CO2 for the same amount of energy. Oil sits between coal and natural gas in terms of carbon content per energy. Peat is often moist and has low energy content, so to get the same energy you burn more material, which generally leads to higher CO2 emitted per unit energy than natural gas. So natural gas tends to produce the least CO2 per unit of energy released.