Emissions Controls, Changing Usage Widen Heat Rate Chasm Between Coal and Gas Power Plants

Between 2006 and 2015, annual average heat rates from the nation's natural gas-fired power plants plunged 7%, while only decreasing 1% for coal plants, the Energy Information Administration (EIA) revealed on August 21.

Heat rates, which are measured in British thermal units per kilowatt-hour (Btu/kWh), refers to energy conversion efficiency, calculated based on the amount of energy used by a power generator to produce a unit of electricity that is supplied to the grid. Net generation accounts for all power consumed by the plant to operate the generators and other equipment, such as a fuel feeding systems, boiler water pumps, cooling equipment, and pollution control devices.

Lower heat rates generally indicate more efficient generation, because less fuel is needed to produce a kilowatt. (For more about heat rates, boiler efficiency, turbine efficiency, and electrical efficiency, see POWER's 2015 feature "Understanding Coal Power Plant Heat Rate and Efficiency").

The EIA, which collects heat rate data from the nation's fossil fuel-fired and nuclear power plants, said that the heat rate for all natural gas-fired generation averaged 8,471 Btu/kWh in 2006, which was about 18% less than the average heat rate of 10,351 Btu/kWh for coal-fired generation.

By 2015, the average heat rate for natural gas-generation (7,878 Btu/kWh) was about 25% lower than the average heat rate for coal generators, which had gone up to 10,495 Btu/kWh.

Meanwhile, the data shows heat rates for nuclear plants remained relatively stable during the period, increasing only slightly from 10,436 Btu/kWh in 2006 to 10,458 BTU/kWh in 2015.

Operating heat rates of coal- and natural gas-fired electricity generation. Source: EIA

Emissions Controls Hamper Efficiency

According to the EIA, the small rise in average operating heat rate for coal-fired generation may be attributed to the addition of new emissions control equipment, which was installed on almost 205 GW of coal capacity from 2006 to 2015-or about 73% of the coal-fired generator fleet that was operating in 2016. "These emissions-control measures often require more on-site usage of electricity, which involves consuming fuel but not necessarily producing electricity output," the agency said.

The increase in coal generation heat rates was partially offset by the net effects of adding about 19.5 GW of new, more-efficient coal generating capacity, and by the retirement of 43.1 GW of less-efficient coal capacity, the EIA noted. Coal units installed between 2006 and 2015 had a weighted-average design heat rate of 9,665 Btu/kWh, compared with the coal units that retired over this period, which had a weighted average design heat rate of 10,343 Btu/kWh.

Usage, Technology Are Other Factors

Emissions-control investments were also made at about 37.5 GW of natural gas-fired generators, or about 9% of the natural gas-fired fleet during that same period. "However, relative to the effects on coal-fired generation, these investments have not been a significant influence on average operating efficiency trends for natural gas-fired generation," it added.

Significantly, the agency pointed out that changes in usage patterns of coal and natural gas generation could affect their heat rates. "Plants that are cycled on and off more frequently-as opposed to being operated more continuously-may consume more fuel to produce electricity, especially during ramping periods (times of increasing demand for electricity)," it explained.

The main factor for natural gas generation's improved heat rates has been a surge in capacity from combined cycle systems, which are more efficient than simple cycle systems.

Almost 58 GW of combined cycle capacity, with a weighted-average design heat rate of 7,029 Btu/kWh, was added between 2006 and 2015. Nearly 34 GW of natural gas capacity, with a weighted average design heat rate of 11,218 Btu/kWh, retired during that period, the agency said.

Natural gas-fired combined cycle technology accounted for about 75% of the nation's gas-fired fleet in 2006, a share that grew to 85% by 2015. By 2015, it added, combined cycle plants operated at an average heat rate of 7,340 Btu/kWh. In contrast, simple cycle natural gas-fired generators, which encompass several distinct technology types (gas turbines, internal combustion engines, and steam turbines), operated at a consumption-weighted average heat rate of 9,788 Btu/kWh.

-Sonal Patel is a POWER associate editor (@sonalcpatel, @POWERmagazine)

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