Abstract

In the last 10 years the growth of America’s natural gas fueled, gas turbine power plants have flourished in its lower 48 states. They are replacing older coal-fueled steam turbine power plants, ending a century’s-old dominance of King Coal, for the nation’s electricity production.

Article

In the last 10 years the growth of America’s natural gas fueled, gas turbine power plants have flourished in its lower 48 states. They are replacing older coal-fueled steam turbine power plants, ending a century’s-old dominance of King Coal, for the nation’s electricity production.

The first simple cycle gas turbine (GT) electrical power plant, rated at 4 MW output, ran in Switzerland in 1938. Subsequently, in the relatively short span of 80 years, gas turbines are now a dominant means of electrical power production in the United States. Fueled by clean burning, low-carbon natural gas and aided by technology adopted from aviation jet engine advances, gas turbine power plants now have low capital costs and by far, the highest thermal efficiencies ever recorded.

In the form of gas turbine combined cycle plants (CCGTs), they are approaching thermal efficiencies of 65%. In a CCGT, which are generally in the 100 to 800 MW range, the hot exhaust of the Brayton cycle electric power gas turbine is used to produce steam to drive a Rankine cycle electric power steam turbine. Thus, a CCGT uses one unit of fuel (generally natural gas) to supply two sources of electric power.

Sustainment by Natural Gas

Underlying their introduction and operation, simple cycle GT and CCGT plants in the U.S. have been fueled by the world’s most extensive natural gas pipeline system, a network of some 300,000 miles of interstate and intrastate gas pipelines. This network has an ample supply of domestic natural gas, recently supplemented by the development of fracking, which has created America’s shale gas industry.

Ten years ago, when I started writing this column [1], Energy Information Administration (EIA) data [2], showed that 23% of the U.S. electrical power in 2009 was generated by natural gas as a fuel. In 2019, that number has almost doubled to 38%, with coal (formerly dominant in 2009 at 44%) dropping to 23%, nuclear at 20%, wind and solar at 9%, hydro at 7% and biomass and geothermal at the remaining 3%.

For the leading 38%, most of the natural gas was fuel for new Brayton cycle gas turbine power plants (either simple cycle or combined cycle) replacing coal plants, with the remainder used in existing, Rankine coal plants converted to natural gas. All told, EIA data [3] shows that more than 100 U.S. coal-fired power plants had been replaced by gas turbine plants or converted to natural gas since 2011.

This recent replacement of coal-fueled plants in the U.S. has been by far, the quickest route to lower the world’s carbon emissions. The combined effects ofhalvingthe fuel’s content through the use of natural gas, and the higher record-breaking CCGT thermal efficiency (~60%) cuts carbon emissions per unit of electricity by potentially as much as 75%. In 2018, which had a same level of total annual net electricity as 2005, carbon dioxide power plant emissions in the U.S. dropped by 27% from 2005. This is a record-breaking reduction, nearly equal to the total CO2 2017 emissions of Germany, the world’s fourth largest economy [4],

The Pipeline is the Lifeline

Let me highlight how a natural gas pipeline system is a key element in the operation of a gas turbine power plant, by giving an account of our own GT power plant, here on the Storrs campus of the University of Connecticut (UCONN) [5].

U.S. coal-fired capacity retired or repurposed to natural gas by conversion type (2011-2019) [3] gigawatts

U.S. coal-to-natural gas plant conversions by conversion type and capacity (2011-2019) [3]

We have a 25 MW cogeneration, combined cycle power plant that has served our 20,000-student campus since 2006. It has three 7 MW Solar Taurus gas turbines burning natural gas, with 300,000 gallons of fuel oil as a backup fuel. The Solar units drive water-cooled generators for campus electrical power, and supply exhaust gas heat to generate steam for campus heating, for a 4.6 MW steam turbine electric generator set, and for steam turbine driven chilled water refrigerator units for campus building air conditioning.

The UCONN power plant natural gas fuel comes from the nearby interstate 1129 mile, 3.2 billion cubic feet of gas per day, Algonquin Gas Transmission pipeline. Such major large diameter pipelines (typically 26-42 inch) are typically buried unobtrusively underground, immune to natural disasters, such as winter storms or hurricanes. They are the most efficient means of bringing fuel to gas turbine power plants. As Smil [6] notes, gas pipelines can have power transmission capacities of up to 10-25 GW (electrical transmission lines leading away from power plants are at least an order of magnitude lower). The heart of the Algonquin pipeline in Connecticut are three compression stations (gas turbine powered) along its length, that maintain high gas pressurization and adequate gas flow rate.

The UCONN power plant is connected to the Algonquin pipeline with a 2.5-mile 8-inch diameter 500 psig rated pipeline, owned by the University. Safe fuel supply is critical for this underground high-pressure pipeline running through the heart of the campus. The pipeline is pressure tested every five years, and is cathodically protected against corrosion, with thermal soil sensors in place where it is in proximity to underground steam piping.

The UCONN pipeline operates at a 380 psig delivery pressure from the higher-pressure Algonquin pipeline. From a pressure standpoint, it is advantageous to have a gas turbine 100 MWLMS100 units.

Thus, the high-pressure natural gas supplied by a major pipeline allows fuel to be injected directly into a gas turbine combustor, without the need of a fuel gas boosting compressor unit. For instance, if UCONN was just supplied by a city natural gas pipeline (typically at 15 psig) it would require a separate fuel gas boosting unit, along with an expensive safety system to prevent and deal with a possible fire (or explosion). I estimate that such a compression unit would consume about 1 MW to boost city gas pressure fuel for injection into the Solar combustors, representing about a 4% reduction (from 25 MW) in available power for the campus. Connection to major natural gas pipelines is a definite advantage for a gas turbine power plant.

Natural gas, transported by pipelines, is being used to fuel record-breaking, thermally efficient gas turbine plants in the United States. The current result is more efficient electricity production and a significant reduction in the nation’s carbon dioxide emissions.

Correction to GGTNSeptember 2020’s ATTT #43

Error in Equation (2) on page 55. The corrected equation is:

η=111+γ1/2M02PRγ1/γ

References

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