Three large machines called turbines produce electricity at the Buck Combined Cycle Station in central North Carolina, near the town of Salisbury. Tall chimneys that once released smoke into the air all day and night are now unused. The last coal-fueled generators that operated at the energy center were closed a few years ago. Trains full of coal no longer arrive at the center, and large piles of coal no longer cover the ground. The center is owned by Duke Energy. Energy companies have had to sharply reduce the amount of coal they use for several reasons: Stronger government rules have reduced the level of carbon permitted to be sent into the air. And there is a growing demand for clean energy. The new energy center is fueled by natural gas. It is a much cleaner source of electricity than coal. Bill Wilson is the senior engineer of the Buck Combined Cycle Station. He says Duke Energy has closed about half of its coal-fueled energy centers in recent years and has replaced them with ones fueled by natural gas.