The amount of generating capacity from natural gas-fired combined-cycle (NGCC) plants surpassed coal-fired plants in 2018 as the technology with the most electricity generating capacity in the United States.

As of January 2019, U.S. generating capacity at NGCC power plants totaled 264 gigawatts (GW). That compared with 243 GW at coal-fired power plants.

According to the Energy Department's Energy Information Administration (EIA), total capacity for generating power in the U.S. across all types of natural gas-fired technologies passed coal as the primary capacity resource more than 15 years ago. However, different natural gas-fired generating technologies are used differently.

The 721 MW FGE Texas I combined cycle power plant. Credit: FGE PowerSteam turbines (which can also be powered by oil or coal) burn fuel to generate steam, which is used in a steam turbine to generate electricity. Combined-cycle units heat up fuel and use the fuel-air mixture to spin gas turbines and generate electricity. Waste heat from the gas turbine is used to generate steam for a steam turbine that generates additional electricity.

Natural gas-fired combustion and steam turbines are less efficient and more expensive to run, EIA said. That means they are typically used only during periods of peak electricity demand. Similarly, almost all coal plants burn coal to generate steam, with little opportunity for efficiency improvements.

Efficiency gains

Earlier in 2019, EIA said that economics and efficiency gains over the past five years drove a significant increase in the average size of a natural gas-fired combined-cycle power block. EIA said that the average size installed between 2002 and 2014 was about 500 megawatts (MW). After 2014, power block capacity increased, reaching an average of 820 MW in 2017.

EIA said that power blocks have increased in size as the performance of combined-cycle units has continued to improve, and as current and projected natural gas prices and supply offer a competitive advantage for the technology.

In its most recent analysis, EIA said that as of the end of 2018 NGCC power plants accounted for about half of all U.S. natural gas-fired generating capacity. However, they provided almost 90% of total natural gas-fired generation. Capacity factors for NGCC plants, which reflect their actual output as a percentage of their capacity, are nearly equal to coal plants and are typically in the 50% to 60% range. By contrast, natural gas combustion and steam turbines have capacity factors of around 10%.

The 583 MW Rhode Island Energy Center, a combined cycle natural gas fired power plant. Credit: EntergySince the beginning of 2015, about 40 GW of coal-fired capacity has retired, EIA said. No new coal capacity has come online in the United States. During that same period, NGCC net capacity has grown by about 30 GW. Electricity generation from these additions, along with output from new wind and solar generators, has largely offset the coal retirements.

Decline in coal consumption

In early December, the Energy Department said that coal consumption in the U.S. was on track to fall to 691 million short tons (MMst), a 4% decline from 2017 and the lowest level since 1979.

U.S. coal consumption has been falling since its peak in 2007. DOE forecasts that 2018 coal consumption will be 437 MMst (44%) lower than 2007 levels, mainly driven by declines in coal use in the electric power sector.

The electric power sector is the nation’s largest consumer of coal, accounting for 93% of total U.S. coal consumption between 2007 and 2018. The decline in coal consumption since 2007 is the result of both the retirements of coal-fired power plants and decreases in the capacity factors, or utilization, of coal plants as increased competition from natural gas and renewable sources have reduced coal’s market share.

In terms of electricity generation, EIA said that NGCC plants have recently begun providing more electricity than coal plants. Electricity generation from NGCC power plants first surpassed coal-fired generation on a monthly basis in December 2015 and again in the first half of 2016. Those period were marked by relatively low natural gas prices.

Higher natural gas prices reversed the crossover until February 2018, when NGCC generation again surpassed coal generation. EIA said that as more NGCC plants come online and coal plants continue to retire, NGCC-powered electricity generation should consistently rank as the most prevalent source of electricity generation.