Natural gas-fired generators contributed more than any other source to the nation’s electricity capacity last year, the U.S. Energy Information Administration’s (EIA) annual survey of electric generators found.

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Natural gas was used for 39% of electricity generation in 2019. The bulk of the fuel was used in combined-cycle technology, which surpassed coal-fired generators in 2018 to become the leading technology for electricity generating capacity in the country, EIA said.

Over the past four decades, the agency said, combined-cycle plants have gradually replaced aging steam turbines, which are relatively inefficient. The trend is expected to continue and further drive demand for natural gas.

“For steam turbines, boilers combust fuel to generate steam that drives a turbine to generate electricity,” researchers said. “Combustion turbines use a fuel-air mixture to spin a gas turbine. Combined-cycle units, as their name implies, combine these technologies: a fuel-air mixture spins gas turbines to generate electricity, and the excess heat from the gas turbine is used to generate steam for a steam turbine that generates additional electricity.”

Combined-cycle generators are designed to efficiently operate for extended periods, while combustion and steam turbines are now usually used only to meet peak demand loads.

“Not only are combined-cycle systems more efficient than steam or combustion turbines alone, the combined-cycle systems installed more recently are more efficient than the combined-cycle units installed more than a decade ago,” EIA said. “These changes in efficiency have reduced the amount of natural gas needed to produce the same amount of electricity. Combined-cycle generators consume 80% of the natural gas used to generate electric power but provide 85% of total natural gas-fired electricity.”

With the exceptions of Vermont and Hawaii, all states have at least one utility-scale natural gas electric power plant. Texas, Florida, and California, which lead the nation for the most electricity consumption, each have more than 35 GW of gas-fired capacity, EIA said.

In these and many states, most capacity is combined-cycle technology. However, one heavily populated state in particular, Illinois, has plenty of room to catch up, EIA said, as 67% of the gas capacity is made up of combustion turbines.